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\begin{document}

\noindent {\bf \Large SUMMARY OF PERSONNEL AND WORK EFFORTS}
\bigskip

\large

\noindent
{\bf PI Walker:} The proposed effort will be a major component 
of the PI's research over the next three years. He will
have overall responsibility for the project and coordinate
activities between the various groups.
He will spend $\sim$3 months per year working on the project.
However, due to the PI's salary being maintained by his 
academic appointment, only 1 month of salary 
per year is being requested.

\smallskip

\noindent
{\bf Co-I Prober} will be responsible for the HEB device development
associated with the proposed work. He will administer the
subcontract at Yale and work closely with the University
of Virginia group in making the array devices. He requests
1 month of salary support per year.

\smallskip

\noindent
{\bf Co-I Lichtenberger} leads the team at the University of 
Virginia and will combine the HEB device expertise from 
the Yale group with their on processing expertise to 
make high quality arrays of HEBs. He requests 5\% support
per year for these efforts.

\smallskip

\noindent
{\bf Co-I Chin} is providing the FIR laser LO system for use
with the prototype arrays we will be developing. His 
participation in the construction of the LO was funded
by an SBIR grant. The LO system will be operational by
the start date of the proposed research. 

\normalsize

\clearpage

\medskip
\centerline {\Large \bf Development of Large-Format Integrated Heterodyne Arrays} 
\centerline {\Large \bf for Far-Infrared Astronomy} 
  
\section{Introduction}
           
The advent of large format ($\sim$100 pixel) spectroscopic imaging cameras 
in the far-infrared will fundamentally change the way 
astronomy is performed in this important wavelength regime. While the 
possibility of such instruments has been discussed for more than two
decades \citep{gil79},
only recently have advances in mixer technology, device fabrication, 
micromachining, digital signal processing, and telescope design made the 
construction of such an instrument possible and economical. Here we propose
to implement these technologies to develop the first integrated, far-infrared
heterodyne array. The array will utilize the latest Nb and Nb-Au
diffusion cooled 
Hot Electron Bolometers (HEB) in an efficient, micromachined, waveguide
mount. HEB devices are inherently broadband, require very little local 
oscillator (LO) power (diffusion cooled HEBs require $\sim$30 times less 
LO power than SIS tunnel junctions), and do not require a magnetic field 
for operation (Prober 1993). They are
also relatively simple structures, making fabrication easier than SIS
and device-to-device performance more uniform. These properties make
HEBs ideal for array applications. The noise performance of HEBs starts
to become competitive with SIS mixing devices at $\sim 1$~THz and HEBs are
the most sensitive mixer technology above 1.4~THz. It is in this
frequency range where laser micromachining can be used to fabricate
efficient, compact arrays of feedhorns and waveguide structures
\citep{wal98}. The goal of the proposed effort is to
demonstrate the viability of large format arrays in the FIR by 1) constructing 
a prototype 1x10 array at 1.5~THz and 2) showing that the same
technology can be used to make arrays up to 10~THz. Once an
integrated,  1x10~THz heterodyne array
has been demonstrated, a technological roadmap to achieving the goal of
large format, far-infrared (FIR) heterodyne arrays will become clear.

Arrays only reach their true potential if the performance of each
pixel approaches that of an optimized individual detector. We will
therefore begin by constructing prototype single channel versions
of the array pixels using the best available Nb HEBs. We will then 
upgrade to Nb-Au HEBs. The development of HEB devices will be led by 
Co-PI Prober's group at Yale. In a complementary effort, Arthur Lichtenberger's 
team at the University of Virginia will focus on how best to take these 
optimum HEB devices and realize them in an array format that can be coupled
efficiently and reproducibly to the micromachined horns. The micromachined 
waveguide mounts, IF/bias electronics, cryogenics, optics, and IF processor 
will be the responsibility of the Arizona group. All three groups will work 
together on instrument characterization; email, teleconferences, and
postdocs and grad students occasionally working in a Co-I's lab for
extended periods will facilitate this necessary spirit of cooperation. 

The 1.5~THz array will be designed for use on the 1.7~m
AST/RO telescope at the South Pole. AST/RO is the {\it only} telescope
currently in operation
that can access the astrophysically important 200 micron atmospheric
window. The PI and his collaborators have built and deployed a 4-channel,
810~GHz SIS array (PoleSTAR) on AST/RO \citep{wal01}. This coming November,
he will be in charge of the integration of a single pixel, 1.5 THz
HEB receiver (TREND; Sigfrid Yngvesson, PI) on AST/RO. The
experience gained in building PoleSTAR and working with TREND will be 
fully utilized in the development of the proposed integrated array. 

Our hope is that this work will lead to the availability of relatively
low-cost, sensitive, FIR heterodyne arrays that could be used to maximize
the scientific pay-off from ground, stratospheric, suborbital, and,
ultimately, space based telescopes.  

\section {Scientific Motivation}

\subsection {High Resolution, Spectroscopic Imaging Arrays in the Far-Infrared}
 
In the wavelength regime between 300 and 60 microns there are a number
of atomic and molecular emission lines that are key diagnostic probes
of the interstellar medium. These include transitions of [\cii],
[\nii], [\oi], HD, OH, 
CO, and H$_2$O. In Giant Molecular Clouds (GMCs), evolved star envelopes, and
planetary nebulae, these emission lines can be extended over many arc minutes
and possess complicated line profiles that can only be disentangled using 
high resolution (${\rm R} > 10^6$) spectroscopy. Observations of 
these lines are crucial to understanding the chemistry and energy balance of 
the ISM. It is the cooling provided by these lines that partially
regulates the collapse of molecular cloud cores into stars and traces
the formation of molecules in evolved stars that ultimately
replenish the ISM. Due to the hot (T $\ge$ 1000K) and dense ($n_{\rm
H} > 10^{12}\ {\rm cm}^{-3})$)
conditions expected in many protoplanetary disks, emission lines in the
FIR will also play a pivotal role in cooling and molding young planetary
systems. In order to separate the emission of the disks from that of
the ever present molecular outflows and probe velocity fields, instruments 
incorporating both high spectral resolution and imaging capabilities are 
again required.  
   
The only instruments capable of realistically providing the required 
spectral resolution and spatial coverage for these studies are heterodyne 
array receivers. Unfortunately, none exist. Figure 1 compares the capabilities 
of SOFIA \& Herschel's 
first light instruments in terms of number of pixels versus spectral 
resolution. When viewed in this way, there is a clear, urgent need for large 
format heterodyne instruments. Fortunately, recent breakthroughs in detector 
technology, micromachining, local oscillators, amplifier technology, and
backend spectrometers now make the construction of large arrays of
heterodyne receivers possible. The proposed 1x10 array for AST/RO
will not only provide a technological roadmap for large format
spectroscopic imaging cameras, it will also serve to demonstrate how
such arrays can maximize the scientific return from single aperture
telescopes such as AST/RO, the South Pole Submillimeter Telescope 
(SPST), SOFIA, and SAFIR. In terms of SARA program guidelines, the 
proposed work will 1) increase array format size, 2) reduce detector noise,
3) provide new fabrication and formatting techniques, and 
4) extend wavelength coverage. 
 

\begin{figure}[htb]
\centering
\begin{minipage}{7in}
\parbox{3.8in}{\includegraphics[angle=0,scale=0.85]{overview-white.eps}}
\parbox{2.6in}{\caption{\small \label{nii} Overview of SOFIA/Herschel 
Terahertz-frequency
spectroscopic instruments in development.  The proposed instrument
will provide a technological roadmap to making heterodyne arrays with
100+ pixels.}}
\end{minipage}
\end{figure}

\subsection {1.5~THz Science on AST/RO}

South Pole site testing suggests that the 200 micron
wavelength atmospheric window has a median winter transparency
of order 10\%. 
The atomic and molecular transitions accessible in this spectral range
are important diagnostics of a variety
of astronomical phenomena including planetary atmospheres,
star-forming regions, young stellar objects, circumstellar 
envelopes, planetary
nebulae, starburst galaxies, and molecular clouds. 
Deployment of these technologies on a ground-based telescope 
is a path of technological development that has exciting
prospects.  On AST/RO, the $\sim 35^{\prime\prime}$ beam size
and high spectral resolution ($\sim 0.4$~ \kms) of provided by
the array will allow the study of galactic star-forming
regions and large-scale studies of nearby galaxies.
In the future, an expanded, 100 pixel version of the array could be 
used on the South Pole Submillimeter Telescope (SPST; see attached
letter from A. Stark), an 8-meter 
telescope which would have a beamsize of $\sim 7''$.
The following sections describe several science objectives for
the proposed array on AST/RO.

\paragraph{[N$\;$II]: A Probe of the Warm Ionized Medium.}

COBE all-sky observations demonstrate that the [\cii~]\ 158 $\mu$m line is
the dominant interstellar coolant at long wavelengths
\citep{wright91,bennett94}.  Galactic H~I clouds, Photodissociation
Regions (PDRs), and the Warm Ionized Medium (WIM) are all expected to
contribute significantly to the total [\cii~] luminosity of the
Galaxy.  Because of the ubiquity of the [\cii~]\
line in both neutral and ionized 
regions and its importance as an interstellar coolant, 
detailed understanding of its excitation in the Galaxy is a
prerequisite for understanding the structure, energy balance, and
evolution of molecular clouds in the Milky Way and star forming
ensembles in external galaxies.  

Since the ionization potential of nitrogen is 14.5 eV, the 
1461.1319~GHz = 205.176~$\mu$m \citep{Brown94} line of [\nii~] must arise
in regions where hydrogen is largely ionized, and thus serves as an
extinction-free probe of the WIM. 
There is an observed nonlinear relationship between 
[\cii~]\ and [\nii~]\ emission \citep{bennett94} on large
scales. By observing a variety of GMCs, isolated globules, and planetary
nebulae in the 1461 GHz [\nii~] line at $\approx$ 700 times the
spectral {\it and} angular resolution of COBE, the [\cii\ ]/[\nii\ ]\ intensity
ratio can be used in detail as a powerful probe to determine how
much of the Galaxy's [\cii~]\ emission arises in ionized gas. 
We will finally gain a clear understanding of the relative
importance of the WIM, PDRs, and diffuse H I to the production of [\cii~]. 

Coupled with the 2460 GHz = 122~$\mu$m [\nii~] line 
routinely observed by ISO toward \hii\ regions and planetary nebulae
\citep{Fong01,Giannini00,Lorenzetti00,Unger00}, the 1461 GHz [\nii~]\
line also offers tantalizing scientific prospects in its own right.
Its low critical density ($40\ {\rm cm}^{-3}$ at $T=8000$K) provides a
unique way to assess the electron density and nitrogen abundance in
low-density ionized regions where most optical-wavelength analyses
break down \citep{Rubin94,Colgan93}. These far-infrared lines also
offer the opportunity to study ionized regions that are otherwise obscured by
dust, such as (ultra)compact \hii\ regions, highlighting a unique way
to explore the intricate and dynamic interplay between hot stars and the
molecular clouds which form them. 

How bright will the line be in 
the AST/RO beam?  COBE showed that the 1461 GHz [N$\;$II] line is the
second-brightest emission line from the Galaxy at long wavelengths,
about 10\% as bright as the 158 $\mu \rm m$ [C II]
\citep{wright91} and about $4~\times$ greater than the \jeqf\
line of CO
\citep{wright91}, which is routinely and easily observed at
AST/RO with antenna temperatures exceeding 10 K.  A synthetic spectral
line map of an embedded \hii\ region is demonstrated in
Figure~\ref{trend}, with line strengths modeled via the photoionization
code {\tt CLOUDY} \citep{Ferland98}. 
Using receiver noise temperatures characteristic of 1.5 THz
HEB receivers (see Section 3.1), the
expected {\it median} transparency (5.5\%) at the frequency of the
[\nii~] line, an rms noise level of $\sim 1$~K can be achieved in a single
3 hour period.

\begin{figure}[htb]
\centering
\includegraphics[angle=0,scale=1.1]{TRENDsimul.eps}
\caption{\small \label{trend}{\bf Synthetic 1461~GHz [\nii~] spectral map and
1370~GHz H$_2$D$^+$ spectrum from AST/RO (South Pole).} The [\nii~] map
has 20$''$ grid spacing for a spherical, ionization-bounded compact
\hii\ region of density $n = 100\ {\rm cm}^{-3}$, excited by a star of
$T_* = 40,000\ {\rm K}$, observed at D $\approx$ 2~kpc.  An HST/WFPC2
image of planetary nebula NGC~2440 is included for scale. The
H$_2$D$^+$ spectrum is modeled towards IRAS
16293-2422, with an assumed  H$_2$D$^+$ abundance of $10^{-11}$.  
Both observations will take 2 days each to achieve the
modeled $\sigma_{\rm RMS}$ in good winter weather.} 
\end{figure}


\paragraph{H$_2$D$^{+}$: A Key to Testing Gas-Phase Chemistry.}

The molecular ion H$_3^{+}$ plays a pivotal role in modeling the
gas-phase chemistry of molecular clouds and star forming regions
\citep{herbst73}.  H$_3^{+}$ is directly formed from
the cosmic ray ionization of H$_2$, and initiates the chain of
ion-molecule reactions that form a substantial fraction of the 121
observed species of interstellar molecules.  After many unsuccessful
searches, H$_3^{+}$ has finally been
detected in absorption toward a small sample of diffuse and dense
molecular clouds \citep{Geballe96,Mccall99,Kulesa01} and towards the
Galactic Center \citep{Geballe99}.  

Unlike H$_3^{+}$, its deuterated form, H$_2$D$^{+}$, has a nonzero
dipole moment and conventional rotational spectrum, with transitions
occurring at submillimeter wavelengths. Besides serving as a 
tracer of H$_3^{+}$, H$_2$D$^{+}$ is directly
responsible for the high abundances of deuterated molecules observed
in cold molecular clouds
\citep{dalgarno73,watson73,millar89,Butner95}, which preserve
a record of a star forming cloud's thermal history. Like H$_2$,
H$_2$D$^{+}$ exists 
in both ortho and para forms, with the para species expected to be 
more abundant for $12 < T < 40$ \citep{pagani92a}.
After many attempts to detect the ground-state 
$1_{10}-1_{11}$ ortho transition at 372.421 GHz 
\citep{phillips85,pagani92b,vandishoeck92}, detection 
has finally been achieved towards one source \citep{stark99}. 
The lower excitation requirements (65~K versus 104~K) and
expected higher abundance led \cite{boreiko93} to search
for the $1_{01}-0_{00}$ para transition at 1370.0853~GHz using the
KAO, and a tentative detection of the transition was reported in
absorption toward Orion IRc2.

With the proposed array on AST/RO, it will be possible to perform
sensitive searches for the favorable 1370~GHz transition of 
H$_2$D$^{+}$ toward a variety of Galactic sources, including NGC 6334,
Eta Carina, and the Galactic Center. The high
critical density of the 1370 GHz transition ($\geq 10^8\ {\rm
cm}^{-3}$) implies that searching for the H$_2$D$^+$ line in absorption
will be much more productive.  A synthetic absorption spectrum of
H$_2$D$^+$ towards IRAS 16293-2422 in the $\rho$ Ophiuchus cloud is
provided in Figure~\ref{trend} as an illustrative example of this
point; by comparison, 
the emission intensity is less than $10^{-2}\ {\rm K}\
{\rm km}\ {\rm s}^{-1}$.  The Galactic Center is especially promising
since it provides a bright infrared continuum, a large expected column of
H$_2$D$^+$ \citep{Geballe99}, and it is ideally placed in the
sky for observations with AST/RO. 
%Using the {\it median} atmospheric transparency (10\%) at 1370~GHz, 
%the rms noise level of the
%\cite{boreiko93} detection will be reached in a single 24 hour period.  



\paragraph{CO $J = 11\rightarrow 10\;,\; J = 13 \rightarrow 12$: A Probe of High Excitation Gas.}

One of the major goals of ground, airborne, and space-based
observational astronomy is the identification and study of
protostellar objects.  A study of protostars provides insight into
the initial conditions required for star formation, protostellar
evolution, and the formation of solar systems like our own. 
In the past \citep{walker86,walker93,zhou93,narayanan98}, studies of
protostellar outflow and infall have been 
most sensitive to cold gas in the extended envelopes of these
objects. The 1.5~THz array on AST/RO will be able to probe the
physical conditions of the dense, hot gas located much closer 
to the protostar.  In fact, the infall modeling of \cite{ceccarelli96} 
predicts a correlation between CO linewidth $\Delta v$ and $J$, since
the  higher $J$ transitions are generated in the warmer gas closer 
to the protostar and is therefore falling faster. 
We will search for such linewidth correlations between the CO
$J>10$ lines and the CO ~ \jeqs ~ and ~
\jeqf ~ transitions currently observable with AST/RO. 

\cite{ceccarelli98}
observed the $J=14\rightarrow13$ to 
$21\rightarrow20$ rotational transitions of CO towards the
protobinary system IRAS 16293-2422 using the LWS onboard $ISO$. 
Combining the LWS results with ground-based $J=6\rightarrow5$
observations, they conclude the CO arises in C-shock heated
gas ($\sim 1500$~K) associated with the outflow. The large 
$ISO$ beam ($\sim 85''$) prevented detailed maps 
from being made. With the $\approx 30''$ beam of AST/RO
at 1.3-1.5 THz, the
observer will be able to make detailed maps of the shocked regions
associated with outflow sources. These maps can be compared to
CO $J=4\rightarrow3$ maps made simultaneously with the existing
suite of receivers.  These points all represent crucial tests of theory.


\section {Technical Description}

In the past the fabrication of integrated arrays of heterodyne mixers
at submillimeter wavelength was prohibited by several factors:

\begin{itemize}
\setlength{\topsep}{-1ex}
\setlength{\partopsep}{-1ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}

\item Sensitive mixing devices either did not exist or were difficult to
   fabricate.

\item When mixers were available, their performance would often vary
   significantly from device to device.

\item There was insufficient LO power to efficiently pump more than
   one or two detectors at any given time.

\item Stacking more than a few mixer blocks together in the focal plane
   with their associated backshorts (if necessary), IF amps, magnets,
   bias lines, etc. was mechanically complex and could overload the
   cryogenic system.

\item The cost of the frontend components and the required backend
   spectrometer were prohibitive.

\end{itemize}

However, through the arduous efforts of many researchers in the
field, most of these hurdles have now been overcome. In the following
sections, we will describe plans for an instrument that combines a number 
of leading edge technologies to make a compact, sensitive 
submillimeter heterodyne array.      

\subsection {\it HEBs for Array Applications}

	Submillimeter heterodyne receivers have advanced dramatically in
the past decade, and these advances will be incorporated, and further
developed, in new missions such as SOFIA and Herschel/FIRST.  Up to 1~THz,
almost all (cryogenic) receivers use niobium (Nb) based SIS mixers.  SIS mixers
will likely be used to 1.2~THz in the future, with a likely upper limit of
about 1.4~THz due to film losses and junction gap frequency for known film
materials and devices (Zmuidzinas, private communication). The LO power is of order 1~$\mu$W.  Below 
0.7~THz, the receiver noise (DSB) is approximately (2 to 4) $\times T_Q$ with 
$T_Q =hf/kB = 48~K \times f$(THz) (e.g. $T_Q = 40~K$ at
0.8~THz). Above 0.8 THz, 
the noise rises to a larger value than this formula, due to losses in the film
circuit used to resonate out the SIS junction capacitance.  Schottky
mixers are presently employed at frequencies above 1~THz \citep{gai00}. 
Hot Electron Bolometer (HEB) mixers with much lower noise and $P_{LO}$ are now
ready for use in this higher frequency range \citep{wys99}. 

       The HEB mixer consists of a narrow, thin superconducting 
microbridge contacted with thick normal-metal films (see Figure 3a and 3b).
The simplicity of their construction does in fact minimize the device-to-device
variation in performance often observed in more complicated structures such
as SIS junctions. HEB mixers use the heating of electrons at the superconducting
transition temperature, $T_c$ to accomplish the mixing.  The resistance oscillates 
at the IF due the combined LO and signal power [1].  
The conversion efficiency is 
typically good (-8 to -10~dB), and there is no capacitance (as there
is for SIS) 
and no gap frequency limit.  Thus, coupling to broadband planar antennas or 
waveguide structures is
feasible to 10~THz or higher.  Two mechanisms of electron cooling are
used:  hot-electron outdiffusion (first proposed and studied by co-PI
Prober) \citep{pro93} and phonon cooling \citep{ger90}. Both diffusion cooled Nb HEBs and 
phonon cooled NbN HEBs have achieved comparable low receiver noise 
temperatures ($T_R$) above 2~THz with large IF bandwidth, to 9~GHz for
diffusion-cooled devices.
The limit on $T_R$ appears to be set by thermal fluctuations
\citep{kar96,bur99}  
and this argues that devices which operate at a lower temperature -
with lower $T_c$ should have improved performance. We find this to be
the case for diffusion cooled devices. For phonon-cooled devices there are, at
present, no materials which would allow this $T_c$ reduction, given the required 
IF bandwidth of 1-2~GHz.

\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4.3in}{\includegraphics[scale=0.35,angle=0]{heb3.ps}
\includegraphics[scale=0.3,angle=0]{heb2.ps}}
\parbox{2.1in}{\caption{\small Morphology of an HEB. {\bf (a)}
Schematic of an HEB. {\bf (b)} Scanning Electron Micrograph (SEM) of a
Nb-Au HEB produced at Yale.}}
\end{minipage}
\end{figure}

HEB mixers promise to achieve noise of a few times $T_Q$.
At 2.5~THz, $T_R$ = 1800~K (DSB)
was measured with a large IF bandwidth, 9~GHz, and mixer noise was $T_M$ = 350~K.
An 810~GHz, phonon-cooled HEB receiver built by the Center for Astrophysics 
has been in operation on the Arizona/MPIfR 10m Heinrich Hertz Telescope
(HHT) for several years \citep{kaw01}. The double sideband noise temperature 
of the receiver is ~800~K. With a Nb-Au diffusion cooled HEB, the receiver 
noise temperature is expected to be less than $\sim$1/3 this value, comparable to 
what can be achieved with Nb SIS devices. However, an HEB utilizes 10-100 
times less LO power, requires no magnetic field, and has a much broader 
bandwidth. These characteristics make HEBs an excellent choice for focal
plane arrays above $\sim$1~THz.

	The Yale group has studied (with JPL) diffusion cooled Nb HEBs
\citep{ska95, kar96, bur99}. 
Devices were produced at JPL.  $T_c$ was 5 to 6~K.  Microwave mixing at 20-40
GHz was studied at Yale, and mixing at 0.6, 1.2 and 2.5~THz was studied at
JPL.  The microwave studies can be done quickly, with no background noise,
and with known coupling.  The Yale findings correspond reasonably well
with those at JPL, once the THz losses are accounted for. These
Yale-JPL studies confirm most of the basic predictions for diffusion
cooled HEBs: IF bandwidth is determined by the out-diffusion time and thus
controlled by the length; the noise is dominated by thermal fluctuations.  The
noise is thus expected to scale with $T_c$.  Studies at Yale and JPL
first tested aluminum HEBs, with $T_c = 1$ to 2~K.  These indeed have the low
noise predicted, but the dynamic range is quite limited, due to output
saturation.  This is partly due to the lower $T_c$ (1~K for the Yale
devices), but also due to the long coherence length of Al.

In the past year the Yale group has developed a lower-$T_c$ Nb HEB,
using a superconducting Nb-Au bilayer \citep{sid01}. 
The 10-nm Au thickness reduces $T_c$
to 1.6~K. {\it Fabrication was done at Yale}.  The IF bandwidth is 1.3~GHz, and
can be much larger with a shorter device.  Device properties are
summarized in Table 2. The Nb-Au HEB mixers display very low $T_M$ and for
these, the quantum noise, $T_Q$, will contribute at THz frequencies.  Of
course, that is the ideal limiting situation.  Because of the short
coherence length in Nb-Au, the dynamic range is much larger than for the
Al HEBs \citep{sid2-01}. 
The required LO power ($P_{LO} = 2$~nW) is also larger than 
for Al, which reduces or eliminates problems with saturation.
For the device studied, with $T_c$ = 1.6~K, no output
saturation is expected, and input (rf) saturation is also not expected
with moderate rf bandwidth, 200~GHz, since the rf background is 
$<<P_{LO}$.  For larger rf bandwidths ($> 0.5$~THz), an increased $T_c$ would 
be used, with thinner Au, to obtain larger $P_{LO}$, which scales
\citep{bur97}  
as $T_c^2$ for a given material. Nb-Au has larger $P_{LO}$ for the same 
$T_c$, compared to Nb, but still low noise. Thus, the Nb-Au system is 
appropriate for a large range of future THz applications. The low required 
$P_{LO}$ and low noise make these Nb-Au HEBs ideal for sensitive array 
applications, into the few THz range at least [2].

\begin{table}[!hbt]
\centering
\begin{tabular}{|ccccc|}
\hline
 & $T_c$ & $T_M$ & $T_R$ & $P_{LO}$ \\
\hline \hline
Nb  ( 2.5 THz)  & 5-6 K  &  350 K  &   1800 K & 20-40 nW \\
Nb  (30 GHz)    &  5 K   &  200 K  &   \dots & 6 nW \\
Nb-Au (30 GHz)  & 1.6 K   & 50 K    &   \dots &   2 nW \\
\hline
\end{tabular}
\caption{\label{put-label-here} {\small Summary of HEB properties}}
\end{table}

%(In REFs =Superconducting bilayers with much lower Tc are used for
%single-photon and direct detection; for that application diffusion cooling
%is minimized; these are known as TESs.)
%Ta HEBs with $T_c = 2$~K may also function well in low noise
%applications (Ref - boris article).


\subsection {Micromachined Waveguide Mounts}

HEBs have been successfully used in both quasi-optical and waveguide
mixer mounts \citep{bur99, kaw01}. When waveguide and quasi-
optical mixers are made at the same frequency with comparable mixing devices, 
laboratory measurements show waveguide mixers outperform quasi-optical 
mixers both electrically and optically \citep{koo98,koo94,wal92}. 
This occurs for two
reasons: 1) because
waveguide has an intrinsically lower loss than the microstrip
structures used in
planar designs, and 2) waveguide feedhorns can couple
to the fundamental Gaussian mode with a ~98\% efficiency, compared
to the $\leq$ 89\% efficiency of dielectric lens-planar antennas
\citep{gol98}. 
 
At wavelengths shorter than 400~$\mu$m (800~GHz), rectangular waveguide
structures, feedhorns, and backshorts become extremely difficult to
fabricate using standard machining techniques. We have used a new laser
milling technique to fabricate high quality, THz waveguide components and
feedhorns. Once metallized, the structures have the properties of standard
waveguide components \cite{wal98}. With this process, waveguide
components of varying height and width can be machined quickly
and at low cost to an accuracy of $\sim 1$~$\mu$m.  
{\it By combining this new laser micromachining process with HEBs, 
the construction of high performance, large format, waveguide 
imaging arrays at THz frequencies becomes possible}.

As a demonstration of the impact of laser micromachining technology, 
proof-of-concept 1.5~THz
corrugated feedhorns and 1x10 arrays of 5~THz `Potter' horns were fabricated 
using the laser micromachining system at the University of Arizona. Figure 4 
is an electron microscope image of a 1.5 THz corrugated feedhorn,
with $\lambda$/4 waveguide matching to full-height rectangular
waveguide. 
Figure 5a is an optical microscope image of two of the horns in the
1x10, 5 THz array. A white light, interferometric microscope image 
of the horn cross-section is shown in 5b. The horns are designed as split-block
structures. The images are of one of the mating halves. To our knowledge,
the 5 THz horns shown in Figure 5 are the highest frequency, dual-mode
feedhorns ever made. The entire array was etched in about 1.5 hours. 
A single corrugated feedhorn at 1.5~THz takes a similar amount of time
to etch.  
 
\begin{figure}[!htb]
\begin{minipage}{7.0in}
\parbox{3in}{\includegraphics[scale=0.9,angle=0]{1500ghz_corrugated.ps}}
\parbox{3.4in}{\caption{\small SEM of portion of
a prototype 1.5 THz feedhorn and waveguide structure. The feedhorn and matching
waveguide are together 2~mm long.  The structure was etched using 4.3
Watts of laser power focused into a 6 $\mu$m spot in 200 Torr of
chlorine gas.  The laser beam was scanned at 5~cm/s and incrementally
moved 2~$\mu$m  between line scans.  Under these conditions,
nominally 1~$\mu$m shavings are removed per pass of the laser over the
surface.  The total etch time is one hour, not including the overhead
time for pattern generation and stage motion.}}
\end{minipage}
\end{figure}

As a test of our fabrication and assembly process, we performed beam
pattern measurements on a laser micromachined 2~THz corrugated
feedhorn \citep{wal98}. The differences between the theoretical and measured beam
profiles were within the resolution limit and amplitude sensitivity of
the test set-up.

Waveguide surface roughness values measured with atomic force
microscopy are typically on the order of 200 nm RMS. This surface
quality is already sufficient to provide low-loss waveguide
performance to $\ge 10$~THz.  The RMS surface roughness can be reduced
even further, to under 25~nm, using standard polishing etches based on
HF-HNO$_3$-HC$_2$H$_3$O$_2$ solutions.

Our initial investigations into the application of laser micromachining
were conducted under a Collaborative Research and Development Agreement (CRDA) 
between the PI (at the University of Arizona) and Lincoln Laboratory.
One purpose of the CRDA is to transfer technology, such as the 
micromachining process described here, out of national laboratories into
the private sector. Based upon the success of this collaboration,
the Engineering Division of the National Science Foundation, NASA
SARA program, and, most recently, Army Research Office have awarded
funds to construct a laser micromachining system
optimized for the fabrication of THz waveguide and optical components
at Steward Observatory. This system is now fully operational and we are
ready to use it for the development of imaging arrays.

\subsection{ \it Focal Plane Array Concept}

All waveguide components
for the arrays will be micromachined out of silicon and 
assembled into an integrated package. The array design and 
fabrication can be readily scaled for use at shorter wavelengths.
To illustrate how 2D arrays can be fabricated with this technology,
a conceptual design for a $4\times 4$ array of micromachined
fixed-backshort, waveguide mixers is shown in Figure 6. The
focal plane array consists of a ``Horn Block'', ``Bolometer Block'',
and ``Backshort Block''.

\begin{figure}[!htb]
\begin{minipage}{7in}
\parbox{4.25in}{\includegraphics[scale=0.5,angle=0]{horn.eps}}
\parbox{2.2in}{\caption{\small Optical microscopy of two
laser-machined 5~Thz Potter horns in a $1\times10$ array; a
holographic cross section demonstrates the accuracy of the Arizona
micromachining system.}}
\end{minipage}
\end{figure}

\vspace{-0.2in}

\subsubsection{Horn Block}

The Horn Block is assembled from four, $1\times 4$ corrugated feedhorn
subarrays (e.g. Figure 5). Each subarray is formed by bonding two gold
plated, silicon
wafers in which mirror images of the corrugated feedhorns have been
laser micromachined. Alignment between the two halves of a horn
subarray is insured by posts and corresponding holes wet-etched in
each half before laser machining. Assembly of the package
will be done with the aid of an infrared microscope and micromanipulator
system designed originally for flip-chip bonding. 
Each horn is micromachined with a circular-to-rectangular waveguide
transition to match the input impedance of the HEB.


\subsubsection{Bolometer Block}

At high frequencies ($\ge$650~GHz) it becomes nearly impossible to manually 
mount detectors across small waveguide structures. An alternative approach 
is to fabricate the mixing elements on $\sim$1~$\mu$m thick silicon nitride membranes
which subsequently are integrated with the horn structure. To date, this 
approach has been used successfully with SIS junctions up to frequencies of 
$\sim$850~GHz \citep{koo98}. As in this successful design, we will fabricate 
the mixing devices on membranes several times larger than the waveguide
aperture. These windows are formed by etching a pyramidal hole from
the backside of the silicon wafer supporting the membrane (see
Fig. 7). The original membrane mounted 850~GHz mixer worked well both 
in the lab and at the CSO. However, if during assembly, the nitride membrane 
was not made flush against the backshort block, significant signal could
be lost due to the generation of unwanted substrate modes. In the 
array design, we have reduced this effect dramatically by using a
suspended stripline RF choke design similar to that of Blundell \& Tong
and micromachining a novel RF choke structure \citep{hes01} at the
Horn and Junction Block interface. With these chokes in place, more
than 1~mil (milli-inch) separation can be easily tolerated between 
the Horn, Bolometer,
and Backshort Blocks. The choke and waveguide probe designs were optimized
at both 1.5 and 5~THz by collaborator Kooi using Hewlett-Packard's High Frequency 
Structure Simulator (HFSS). 

\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4in}{\includegraphics[scale=1,angle=0]{Fig7-color.eps}}
\parbox{2.4in}{\caption{\small Assembly Drawing of the 1.5~THz Array Mixer Block}}
\end{minipage}
\end{figure}

\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4.5in}{\includegraphics[scale=1,angle=0]{Fig8-color.eps}}
\parbox{2in}{\caption{\small Exploded view of an individual array mixer assembly.}}
\end{minipage}
\end{figure}

The impedance of an HEB ($\sim$50~$\Omega$) is purely resistive and does 
not vary significantly with frequency. This property allowed us to design 
a very broadband waveguide probe for use with HEBs. The widest
bandwidth was achieved using an asymmetric probe (see Figure 8a) similar 
in principle to that described by \citet{ker92}. Since the
probe is one-sided, the `hot' IF/bias line loops back and becomes 
a coplanar waveguide (CPW) transmission line on the groundplane. 
Figure 8b shows the HFSS calculation
of the return loss ($S_{11}$) of the probe as a function of frequency.
The probe is found to work well from 1.2 to 1.8~THz! The 
addition of an iris at the input of the feedhorn enhances the probes
performance. Such an iris is simple to implement using laser 
micromachining. If indeed this bandwidth is achieved, we would increase 
the $T_c$ of the HEBs slightly to avoid input saturation. We find
this would increase $T_m$ by only 10-20\% and require a readily
accommodated increase in LO power. 

An exploded view of an individual mixer block is shown in Figure 7. 
The feedhorns and waveguide backshorts will be micromachined and gold plated 
by the Arizona group using existing facilities. The fabrication of the 
array Bolometer Block, including the HEB, waveguide probe, and IF output lines,
will be a joint effort between Yale and UVa. There is nothing in the design of 
the array that prevents it from being scaled to 5~THz.

For heterodyne arrays efficient telescope illumination can be achieved with a
$2f\lambda$ separation ($\sim 3\omega$) between feedhorns in the
telescope focal 
plane. At the Nasmyth focus of AST/RO this separation is $\sim 3.6$~mm.
Since all the mixers share a common 
Bolometer Block, crosstalk between array pixels could be a concern. 
Using HFSS, we have carefully constructed a 2x2 simulation of an array
with only $\sim$1~mm separation 
and found the crosstalk to be down below the -35~dB level across 
the band. 
%This results of the simulation are shown in Figure 13. 
The separation between array elements in the proposed
array is three times as great, suggesting crosstalk in the Bolometer
Block will not be a significant problem.

The IF output of each mixer is ultimately conveyed to wire bonding pads
on the periphery of the bolometer block by CPW lines. 
The IF impedance of HEBs provides a good, broadband match to 50~$\Omega$ 
transmission line, so no sophisticated IF impedance transformers are required. 

\begin{figure}[!htb]
\centering
\includegraphics[scale=0.85,angle=0]{Fig9a.eps}
\includegraphics[scale=0.95,angle=0]{Fig9bc.eps}
\caption{\small HEB waveguide probe. a) 3-D CAD drawing. b) HFSS simulation of
probe return loss (S11 \& S21).  The plots indicate that the probe
should work well from 1.2-1.8~GHz.}
\end{figure}

\subsubsection{Backshort Block}

The fixed rectangular backshort is laser micromachined on a pyramidal
structure designed to fit the cavity behind the membrane. The
pyramidal structures can be readily made by wet-etching silicon
through an SiN mask evaporated on the wafer \citep{reb87}. Once
etched, the Backshort Wafer is gold plated. With this design
the performance of the array can be optimized in the lab by trying
different backshort lengths.

\subsection{IF/Bias Distribution system}

\subsubsection{Choice of IF Frequency and Bandwidth}

Unlike the situation often encountered in conventional single pixel
and array receivers, in the proposed system it is the choice of the IF 
frequency and bandwidth that determines the size and cost of the array camera. 
In the proposed design the waveguide mixers are micromachined out of silicon
using computer controlled laser micromachining and wet etching techniques.
With this approach it costs the same to make $\sim$10 mixers as to
make just one. In both SIS and HEBs fabrication runs, typically 100's of 
devices are made at once on a single wafer. Our concept of having a
single, common bolometer block or wafer does not add significantly
to the cost or complexity of device fabrication. Since HEBs require
$\sim$30 times less LO power than SIS devices, a commercially available
FIR laser system can easily drive the entire array.

The additional cost per pixel is driven by the cost of the 
IF components and the spectrometer needed to process the signal.
Fortunately these costs are also dropping. For the [\nii], H$_2$D+,
and CO 11-10 transitions discussed earlier, strong laser lines 
yielding IF frequencies of 4.6 (Yngvesson, private communication), 
7.64 (Borieko \& Betz 1993), 
and 5.2 GHz (Chin 1980) are available. IF frequencies up
to 9~GHz have been demonstrated to yield low noise response in
diffusion-cooled HEBs (McGrath et al., 2002). 
Wideband, low-noise amplifiers are now available with sufficient
bandwidth to cover all three IF frequencies.  TRW Aerospace has
expressed interest in providing us amplifiers for the prototype array.
Ultimately, we want to expand
the 1x10 array to a 10x10 array. For this project we would have 
a wafer run at TRW that would yield several hundred amplifier chips.
The cost per chip would then be $\sim$ \$500. This price is more than
an order of magnitude cheaper than standard IF amplifiers purchased
one at a time.   

AST/RO has 6 Cologne AOSs, each of which can provide 1 GHz of bandwidth.
We will build an IF processor that will take the 10 IF outputs from
the array and downconvert them so that two of them will share a single
AOS. The resulting bandwidth of each pixel is 500 MHz at 1 MHz resolution.
This translates into 100 km/s of velocity coverage with a resolution 
of 0.2 km/s. This bandwidth and resolution are suitable for a wide variety of
Galactic astronomy projects; from large scale mapping of molecular
clouds to searching for emission from protoplanetary disks. 
When we expand the array to 100 pixels, the AOSs will no longer
be sufficient. By that time 2-bit/4-level correlator chips each
with up to 2~GHz of bandwidth and 128 lags will be commercially
available. A ``turn-key'' correlator system of this type is expected 
to cost as little as ~\$2K per channel, potentially making very 
large format arrays tractable (Timoc 2002).


\subsubsection{Array IF Amplifier and Bias Boards}

Running bias wires and cabling to even a small number of mixers can
be an arduous task. Bias wires are required for the mixing device
and IF amplifier. Coaxial cable must be used between the mixer,
1st IF amplifier and the outside world.
Since the mixer is typically at or below 4~K, the IF amplifier at
$\sim$15~K, and the outside world is at 290~K, the materials from
which the cables are made and how they are heat strapped are critical
to the cryogenic performance of the system. To simplify this situation
for large format arrays, we are investigating a different approach.

Just as integration of the mixer array leads reduces complexity,
we must also integrate the IF amplifiers and bias lines.
In Figure 9 we show a 3D CAD rendering of how the cryogenic
portion of the 1x10 array will be realized. There are a total
of 4 boards, each operating at a different temperature. The 
first board is at 0.3~K and contains the micromachined array of
HEB mixers. The IF output signals are conveyed to the edge of the
board by microstrip lines. The second board contains the first
stage IF amplifiers and 4-wire, HEB bias networks. The amplifiers
are the TRW devices discussed earlier. The amplifier and HEB
bias enter the board through standard micro-D connectors located
on each end. The interconnection between the two boards is
achieved with a removable microstrip `ribbon' cable being
developed jointly between the UofA, Yale, and UVa. The cable
is fabricated using Zyvex, a new, low dielectric 
material from Rogers Duroid. The material is only 2 mils thick,
has good thermal properties, and is flexible. On Zyvex, 50$\Omega$
microstrip lines are only 6 mils wide, permitting high packing
densities. With an 18 mil spacing between lines, the width of
the cable is only 0.375 in. With this spacing, the crosstalk
between lines is negligible. The cable `connector' is a rectangular
block of Rexolite with interconnecting strips of conductive material
deposited on the bottom with the same width and spacing as the
microstrip lines. When the Rexolite bar is secured, the conductive
strips bridge the space between the cable microstrip lines 
and the microstrip lines on the board. To increase thermal isolation,
the Zyvex microstrip lines between the focal plane array (0.3~K) and 1st 
IF amplifier board (4~K) will be made of niobium. The microstrip lines 
between the other stages will be made of copper. A second IF amplifier
board is at attached to the 77~K radiation shield. A 4th board mounted
to the inside of the 300~K vacuum vessel serves to breakout the 
IF signals into conventional semi-rigid, coaxial cable. 
If the Zyvex microstrip cable does not perform up to specifications,
it is still possible to use  (for arrays of $\sim 100$ pixels)
either miniature niobium or stainless steel coaxial cable to interconnect 
the various IF stages. 


\subsubsection{Array Bias Control System}

A computer controlled array bias system has already
been developed by the PI's group at Arizona and is currently being
used with their 4-channel, SIS 810~GHz array (PoleSTAR) on the AST/RO
telescope at the South Pole and a 7-channel, 345~GHz SIS array (Pleiades)
soon to be installed on the HHT. The system allows manual tuning of each 
mixer and amplifier, plus automated bias point selection. 
All voltages (and currents) are controlled and monitored
through a flexible graphical user interface (GUI) written in C and
controlled by a single PC. 
The control system is already extensible to 100 or 
more channels. For the proposed 10 channel system, the bias cards
will be modified to work specifically with HEBs. The system fits
in two standard 5" high, 19" racks.
 
\subsection{Local Oscillator}


The laser LO system for the array will be supplied by NASA/GSFC as a result
of an SBIR development awarded to DeMaria
ElectroOptics Systems (DEOS, now part of Coherent), of Bloomfield, CT, for 
the design and construction of a waveguide CO$_2$-pumped submillimeter local
oscillator suitable for use on SOFIA.  

%An engineering drawing of the
%FIR laser system is presented in Figure 16.
%
%                       Enter Figure 16 here
%
%\begin{figure}[htb]
%\epsscale{0.65}
%\plotone{rect.ps}
%\caption[Engineering Drawing of FIR Laser System]{Engineering Drawing
%of FIR Laser System}
%\end{figure}

This FIR laser utilize the same
ultra-long-life sealed-off pump laser technology which DEOS has
implemented on a number of military flight programs, combined with an
innovative absolute pump-frequency locking technique and a compact
folded FIR laser design. DEOS staff have demonstrated the requisite
pump laser technology with sealed-off shelf lives in excess of 11
years and operating lives of $>$25,000 hours. Unlike conventional FIR
lasers, this Stabilized Integrated Far-InfraRed (SIFIR) laser system
will have an autonomous master controller and will operate completely
sealed-off. The user of the SIFIR does not have to be knowledgeable
about FIR lasers to operate the SIFIR.  The design paradigms for the
SIFIR are intended to reduce its operation to that of a scientific
``appliance'', as opposed to a finicky instrument. The SIFIR laser 
is only ($\sim$6"W x 6"H x 24"L) and weighs $\sim$35 lbs.
These innovations are key to meeting the
deployability requirements of ground-based/airborne THz
radiometers. Further, DEOS has employed a thermally self-compensated
design approach for the FIR laser. With this design approach, DEOS
staff have demonstrated long-term frequency stability of 35~kHz at
1.6~THz over periods of days, and short-term linewidths of 25~kHz
integrated over 3 minutes. With this laser system, the array could 
potentially achieve a
velocity resolution of $\sim 4$~m/s! Such high velocity resolutions
may prove useful in studying such objects as planetary atmospheres and
activity in proto-planetary disks.  The high output power of the SIFIR
laser will provide ample LO power to pump HEB arrays of $>$100 pixels.


%\begin{figure}[!htb]
%\centering
%\begin{minipage}{7in}
%\parbox{4in}{\includegraphics[scale=0.4,angle=0]{Fig10b-color.eps}}
%\parbox{3in}{\caption[Array Cryostat]{\small The cryostat has two 4He 
%\includegraphics[scale=0.8,angle=0]{Fig10a-color.eps}
%\includegraphics[scale=0.8,angle=0]{Fig10b-color.eps}
%\caption[Cryogenic Subsystem]{\small a) Four-board HEB and IF
%assembly consists of the HEB array at 0.3~K, 1st IF amplifier board at
%4~K, second IF amplifier board at 77~K, and breakout board at
%300~K. Interconnections between the four boards is provided by Zyvex
%microstrip lines and Rexolite ``connectors''.  b) The cryostat has two 4He 
%cans, a LN$_2$ can, and a 3He stage. The inner 4He can is used 
%to liquify the 3He. The outer 4He can cools the 1st stage IF
%amps. The 3He stage will be essential for testing the Nb-Au
%devices. The 2nd stage IF amps are heat sunk to the 77~K
%radiation shield.}
%\end{minipage}
%\end{figure}

\begin{figure}[!htb]
\begin{minipage}{7in}
\parbox{4.5in}{\includegraphics[scale=0.93,angle=0]{Fig10a-color.eps}}
\parbox{1.9in}{\caption[Cryogenic Mixer Assembly]{\small  Four-board HEB and IF
assembly consists of the HEB array at 0.3~K, 1st IF amplifier board at
4~K, second IF amplifier board at 77~K, and breakout board at
300~K. Interconnections between the four boards is provided by Zyvex
microstrip lines and Rexolite ``connectors''.}}
\end{minipage}
\end{figure}

\begin{figure}[!htb]
\begin{minipage}{7in}
\parbox{4.5in}{\includegraphics[scale=1.1,angle=0]{Fig10b-color.eps}}
\parbox{1.9in}{\caption[Array Cryostat]{\small The cryostat has two 4He 
cans, a LN$_2$ can, and a $^3$He stage. The inner 4He can is used 
to liquify the $^3$He. The outer 4He can cools the 1st stage IF
amps. The $^3$He stage will be essential for testing the Nb-Au
devices. The 2nd stage IF amps are heat sunk to the 77~K
radiation shield.}}
\end{minipage}
\end{figure}


\subsection{Array Cryostat}

Before building the 1x10 array, we will conduct performance
tests on single channel versions of diffusion cooled Nb and
Nb-Au HEBs. Microwave measurements at 30-40~GHz will be done
at Yale. The 1.5~THz evaluations will be conducted at Arizona.
The Nb HEBs can be tested in standard $^4$He cryostats. The Nb-Au
HEBs work best at $\sim 0.5$K, and require $^3$He systems. Both
Yale and Arizona already have (or soon will have) cryostats 
suitable for this work.

A new, larger cryostat capable of $^3$He operation will be needed for 
housing the 1x10 array. Our ultimate goal is to expand the array
to a 10x10 format. Heat load estimates show that the biggest
increase in capacity for the array expansion is needed in the
4 and 77~K stages, since it is here that the heat from the IF
amplifiers is dissipated. This suggests that with some forethought,
the same $^3$He/$^4$He core could be used in both the 1x10 and
10x10 arrays. The additional capacity at 4 and 77~K needed by
the full array could be provided by either a closed-cycle 
refrigerator or by adding toroidal $^4$He and nitrogen vessels.
In consultation with Infrared Laboratories, we have developed
a conceptual design for such a cryostat. In Figure 10 we show
a 3D CAD drawing illustrating how a 100 pixel, heterodyne 
array receiver would look mounted in the cryostat. In the 
100 pixel array, the amplification is evenly divided between 
4 sets of boards. The $^3$He stage would need to be
recycled every 1-2 days. Other cryogens would need to be added
on 2 to 7 day intervals, depending on whether a closed-cycle
refrigerator or cryogens are used in cooling the IF 
amplifiers.
 

\begin{figure}[!htb]
\centering
\begin{minipage}{7in}
\parbox{4in}{\includegraphics[scale=1,angle=0]{Fig11.eps}}
\parbox{2in}{\caption[Integration of the Array on AST/RO]{\small
AST/RO optical path to the proposed hetrodyne receiver array at the
Nasymth focus}} 
\end{minipage}
\end{figure}

\vspace{-0.1in}

\subsection{Optics}


The array will be bolted on the Nasmyth focus of AST/RO as shown in
Figure 11. This position was chosen because of the greater field of
view it provides. From the warm receiver room (located just below
the telescope) optical constraints limit the size of
the array to 3x3. Although the proposed 1x10 array has only one
more pixel, it has the advantage of being extensible to 100 or
more elements. The instrument will be completely enclosed in a 
housing that will keep it at a suitable operating temperature.

The telescope beam first reflects off a secondary and tertiary 
mirror before it passes through a silicon beam splitter (Walker et
al., 2001). The beam
splitter injects the local oscillator beam from the FIR laser
(located in the warm receiver room) into the focal plane array. 
The horns are already spaced to provide adequate coverage of the
focal plane. Only a dielectric lens is required to make the 
final match between the telescope beams and the array feedhorns.


\section{Observing Time on AST/RO}

AST/RO welcomes proposals from the astronomical community. 
Once the proposed array has been commissioned it will become
available to all observers. The array will go into operation
whenever the automated tipping radiometer indicates conditions
are favorable in the 200 micron atmospheric window. At that
time a queued program schedule will be implemented. 

\section{Pushing the Boundaries: 3 and 5~THz HEB mixers}

Currently, whisker-contacted, corner-cube, Schottky diode 
mixers are used in astronomy at high ($\sim 5$) THz frequencies.  
However, device physics suggests HEBs are capable of low noise operation
to at least $\sim 5$~THz (Prober 1993). Also with laser micromachining, 
it is possible to fabricate waveguide mixer mounts at these frequencies
(see Figure 5). Using realistic dimensions and mechanical tolerances, 
we have simulated a scaled version of the mixer mount of
Figure 7 to 5~THz. HFSS results indicate the loss in the waveguide
structure to be $<$1dB. As part of our array development work,
we propose to fabricate 3 and 5~THz waveguide mounted, HEB mixers and
characterize their performance. This work would be performed in
the last 1.5 years of the array project when we will have gained
extensive experience assembling this style of mixer. One beauty of
the proposed approach is that the design, fabrication, and assembly
of mixers is essentially the same at 1.5 THz as it is at 5 THz.
Indeed, the 5~THz HEBs can be fabricated on the same 0.75~$\mu$m
nitride layer as the 1.5~THz devices. In terms of laser micromachining,
5~THz structures are actually more straightforward to make
than at lower THz frequencies since there is less silicon to
remove. For the performance tests, we will use the same
LO source, cryostats, IF amps, and test set-up used in testing the 
single 1.5~THz devices. In short, the 3 and 5~THz mixer development is
a natural extension of the array work and can be performed at
little additional cost. The demonstration of 3 and 5~THz HEB mixers
in array waveguide mounts would set the stage for future 
large format, FIR heterodyne arrays. 

\section {Educational Impact of Proposed Research}

\subsection{University of Arizona}

Almost all the instrumentation designed and fabricated in 
the Steward Observatory Radio Astronomy Lab (SORAL) was done 
by students under the supervision of the PI. Past Ph.D. grads
from the lab include Jason Glenn (now a faculty member at
U.C. Boulder) and Gopal Narayanan (faculty member at U. Mass.).
Currently, there are 4 grad students and 2 undergrads 
working in the lab on a wide range of instrumentation projects. 
When appropriate, the students use the instruments they 
have built toward their undergraduate or graduate degrees.
Even though the salary of only an Arizona postdoc is requested
in the proposal, all the students will benefit from the project.
We feel the complexity of the proposed project requires the
focused attention a postdoc can provide.  

The PI is now in the process of organizing an `Astro-Technology'
initiative at the University of Arizona. The goal is to build
a cross-disciplinary program that facilitates
the exchange of faculty members, postdocs, students, ideas,
and physical resources between departments (e.g.
Astronomy, Planetary Sciences, Optical Sciences, and Electrical
Engineering) interested in applying leading-edge technology to the
solution of astrophysical problems. The proposed instrument will
serve as a model for this initiative, illustrating how diverse 
technologies can be fused to provide a new, powerful instrument 
for observational astronomy.

\subsection{Yale}

In D. Prober's laboratory at Yale, there are typically one or two 
undergraduates at any time working on projects.  In the past 5 years, 
4 students have done year-long senior projects, and all of these 
students have gone on to Ph.D. programs in Physics or Applied 
Physics.  A senior undergrad is doing his project with I. Siddiqi 
this year, and will present his results at a THz conference this 
fall, and also submit them for publication, likely as the first 
author.   Yale undergraduate majors are well equipped to contribute 
to, and benefit from, these research experiences.

The two graduate students who have worked on HEB mixers, Peter Burke 
and Irfan Siddiqi, have both done outstanding work.  Siddiqi will 
finish his Ph.D. in the coming academic year.  Burke won a Millikan 
Postdoctoral Fellowship at Caltech in 1997.  A postdoc who previously 
worked on the HEB program, Robert Schoelkopf, is now a faculty member 
at Yale in Applied Physics with his own research group.

\subsection{University of Virginia}

The subcontract to the University of Virginia will be used to
partly fund a graduate research assistant. The student will
work with the Dr. Lichtenberger and other team members to
develop the optimum strategy for fabricating and assembling
the micromachined components into a working mixer.


\section{Plan of Action}

\begin{table}[!hbt]
\centering
\begin{tabular}{|l|c|c|c|}
\hline
 & {\bf \large Arizona} & {\bf \large Yale} & {\bf
\large UVa} \\
\hline
{\large \bf YEAR 1} & & & \\

\hline

$\bullet$ Refine fabrication of Nb HEBs on SiN substrate & & \bigX & \\

$\bullet$  \parbox{5in}{Fabricate single, 1.5 THz Nb and Nb/Au HEBs on SiN for DC testing of
IV curves in a geometry compatible with laser machined mixer blocks} & & \bigX & \\
$\bullet$  Procure helium-3 array cryostat & \bigX & & \\

$\bullet$  Design and prototype 1st and 2nd stage (1x10) IF amplifier boards  & \bigX & & \\

$\bullet$  Design and prototype 290K, IF breakout board  & \bigX & & \\

$\bullet$  Fabricate and test microstrip `ribbon' cable  & \bigX & \bigX &  \\

$\bullet$  Optimize procedures for assembling the mixers   & \bigX & & \bigX \\

$\bullet$  Micromachine single, 1.5 THz waveguide mixer horns and backshorts  & \bigX & & \\
  
$\bullet$  Wet-etch nitride windows and backshort pyramidal structures & & &
\bigX \\

$\bullet$  \parbox{5in}{Characterize performance of a single 1.5 THz Nb and Nb/Au HEBs in
laser micromachined mixers using existing cryostats and GSFC FIR laser
system}  & \bigX & \bigX &  \\
\hline
{\large \bf YEAR 2} & & & \\
\hline 

$\bullet$  Fabricate prototype 1x10, 1.5 THz Nb HEBs on SiN & & & \bigX \\

$\bullet$  Fabricate prototype 1x10, 1.5 THz Nb/Au HEBs on SiN  & & \bigX &  \\

$\bullet$  Build array IF Processor and bias control hardware  & \bigX & & \\

$\bullet$  Configure array cryostat to support a 1x10 mixer array  & \bigX & & \\

$\bullet$  Micromachine 1x10, 1.5 THz mixer feedhorns and backshorts  & \bigX & & \\

$\bullet$  Assemble 1x10, 1.5 THz mixer array  & \bigX & & \bigX \\

$\bullet$  Characterize array performance  & \bigX & \bigX & \\

$\bullet$  Make detailed designs of array telescope mount and optics  & \bigX & & \\
\hline
{\large \bf YEAR 3} & & & \\
\hline 

$\bullet$  Fabricate additional 1x10, 1.5 THz HEB arrays on SiN as needed  & &
\bigX & \bigX \\

$\bullet$  Complete telescope mount and optics  & \bigX & & \\

$\bullet$  Perform full system tests  & \bigX & \bigX & \\

$\bullet$  Prepare array for shipment to the Pole at the end of 2005  & \bigX & & \\

$\bullet$  Fabricate single 3 and 5 THz HEBs on SiN  & & \bigX & \\

$\bullet$  Micromachine single waveguide mixers for 3 and 5 THz.  & \bigX & & \\

$\bullet$  Assemble 3 and 5 THz micromachined mixer blocks  & \bigX & & \bigX \\

$\bullet$  Characterize the performance of single pixel FIR mixers at
3, 5 THz in lab cryostats  & \bigX & \bigX & \\
\hline
\end{tabular}
\end{table}



\clearpage

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\item[1.] When HEBs are used in direct detection applications, they are
usually known as Transition-Edge Sensors (TESs).  The operation is the
same, though the bandwidth requirement for direct detection is far
less demanding than for heterodyne detection.

\item[2.] The use of a Ta HEB with $T_c=1.6$~K might provide mixing
performance comparable to the Nb-Au microbridges of Siddiqi et al. (2001)



\end{thebibliography}

\clearpage

\noindent{\Large \bf FACILITIES, EQUIPMENT \& OTHER RESOURCES}

\large 
\subsection*{ \it University of Arizona, Steward Observatory}

In 1992 the PI established a laboratory
(the Steward Observatory Radio Astronomy Laboratory, SORAL) for
the development of state-of-the-art submillimeter-wave receiver systems.
The laboratory contains (or has ready access to) all the equipment
(spectrum analyzers, network analyzers, vacuum pumps, cryogenic
support facilities, etc.) needed for the development of receivers.
SORAL has $^4$He and $^3$He cryostats available for testing single pixel Nb
and Nb-Au HEB mixers.

The NSF Division of Astronomical Sciences awarded
the PI funds to construct a 7 element array receiver at 345 GHz.
This array will begin operation on the HHT in October 2002.
In the past, SORAL has produced facility 230 and 492 GHz receivers
for the HHT and, recently, was the lead group in the design and
construction of a dual channel 492/810 GHz receiver and 4 pixel
810 GHz array receiver (PoleSTAR) for AST/RO at the South Pole.
The 492/810 GHz receiver has been in operation for 4 years and is the primary
facility receiver for the telescope. The 810 GHz array was commissioned
in November 2000 and is the first array to go into operation in this
important atmospheric window.  This summer SORAL will be working
very closely with Sigfrid Yngvesson at UMass in preparing a
single pixel, 1.5 THz HEB receiver for integration onto AST/RO.
The experience
SORAL personnel have gained in the design and deployment of these
receiver systems will be fully utilized in the construction of the
proposed prototype systems.

As a result of its successful collaboration with Lincoln Laboratory
in applying laser micromachining technology to the fabrication
of THz components, the Physical Foundations of Enabling Technologies
Program in the Engineering Division of the NSF awarded the PI
a grant to construct a laser micromachining facility at Steward Observatory.
This system is now fully operational.
The design of this facility is based on the successful Lincoln
Laboratory system and optimized for fabricating the mixer arrays described
in this proposal. The availability of a state-of-the-art laser micromachining
system at Steward Observatory provides the community with a {\it powerful,
new} ability to develop a wide variety of needed THz components.
Photolithographic facilities are also available at Steward Observatory
and the Electrical Engineering Department for fabricating
the wet-etched portions of the focal plane arrays.

Through a grant through the Defense University Research Instrumentation
Program (DURIP), SORAL has recently purchased a new laser for the
micromachining system, an e-beam evaporator for metallizing micromachined
components, and a thin film deposition system for depositing nitride
and other films.

SORAL also has licenses for
both Hewlett Packards High Frequency Structure Simulator (HFSS) and
Microwave Design System (MDS) software packages. These programs
are used to accurately model and optimize mixers and other
crucial receiver components.


\subsection*{\it Yale University}

The PI's laboratory has extensive microwave testing capability from
0.1 - 40 GHz, developed for the testing of mixing in Nb-based HEBs to
temperatures T = 0.2 K.  The equipment includes low-noise IF
cryogenic amplifiers: a 1.5 GHz center frequency with 0.4 GHz
bandwidth very low noise IF amplifier (TIF = 5 K with integrated
isolator), 0.1 - 8 GHz and 0.1 - 20 GHz amplifiers, for broadband low
noise testing of IF response, and cryogenic couplers and octave-wide
circulators.  The new 3He system is outfitted with special rigid
coaxial cables and 2.4 mm connectors which provide excellent
broadband response to 50 GHz, and have teen tested in place to
achieve broadband coverage to 40 GHz, the limit of our HP 8722D
synthesizer source.  This HP unit is a vector network analyzer, and
facilitates careful testing of all components, in addition to serving
as the LO or signal source.  A second microwave source is used for
tests of coherent mixing.  Testing of the Nb-Au HEB devices will take
place in this new cryogenic system, which can test device mixing from
0.2 to 6 K.

The Yale Microelectronics Clean Room allows us to fabricate the Nb-Au
HEB devices, using electron-beam lithography on a JEOL 6400 SEM,
adapted for e-beam writing with a Nabity control system.  Nb-Au HEB
devices are now routinely produced at Yale, as are other similar
devices with 0.1 micron dimensions.  A bilayer PMMA resist system has
been developed to allow the deposition of Nb and Au films without
breaking vacuum between depositions, to ensure superb alignment with
the double-angle deposition method, and to ensure clean interfaces
between the Nb and Au films.  Our results of device properties prove
out both these expectations.  A new AFM is used for inspecting HEB
devices, as shown in the micrograph in the proposal body.  Deposition
systems, including a multisource Varian evaporator and a Lesker
multitarget sputter/evaporation system with in-situ ion beam cleaning
are managed by the PI's group in the Clean Room.  The Nb-Au devices
were made in the Lesker system.  A new electron-beam lithography
system, with 2 nm beam size, will be procured during the next year,
and available to D.Prober's group.  This system will allow extremely
uniform production of HEB devices.


\subsection*{\it University of Virginia}

The Superconductor Device Laboratory (SDL) operates within the Applied 
Electrophysics Laboratory (AEpL) in the Department of Electrical 
Engineering at the University of Virginia. The primary facility 
consists of a 6,000 square foot laboratory, of which roughly two-thirds 
is clean room space. This facility is indicative of the
institutional support that the SDL enjoys as a result of our long term
commitment to excellent research in solid state technology, and also to
education. The AEpL program involves graduate and undergraduate
students, approximately thirty-five of which are currently involved
in ongoing research. 

The laboratory is fully equipped for the investigation and fabrication 
of complex Nb superconductive circuits. These processing capabilities include: 
UHV loadlocked 3" DC magnetron sputtering of aluminum, 
niobium and gold in a dedicated trilayer system, high vacuum loadlocked 
3" magnetron sputtering of insulators and metals, UV and deep UV contact 
photolithography with sub-micron resolution, a  Karl Zeiss 982 field 
emission scanning electron microscope with 1 nm resolution, two fluorine 
and a chlorine based reactive ion etchers, plasma etching,  a multisource 
Temescal E-beam vacuum evaporation, dedicated ion milling station,
and electrochemical plating and etching.  The laboratory also has two 
high speed dicing saws, a computer controlled Tencor surface profiler, 
an FSM laser based thin film stress measurement system, and a Westbond 
wirebonder. 

The SDL also benefits greatly from cooperative research efforts with
other departments. In particular the Materials Science and Electrical 
Engineering Departments share an FEI-200 focused ion beam system (\$500K). 
Also, a cooperative research program with the Department of Physics has 
led to the establishment of our Far Infrared Receiver Laboratory. This 
facility is fully equipped to design, assemble and evaluate millimeter 
and submillimeter wavelength mixers and multipliers. Sources include 
two submillimeter wavelength gas laser systems (300~GHz - 4.5~THz) and a 
variety of millimeter wavelength sources, multipliers and amplifiers from 
Millitech, J.E. Carlstrom and Radiometer Physics GmbH. A Bruker IFS 66V 
Fourier Transform Infrared Spectrometer (120~GHz - 225~THz) is available 
for materials and component evaluation, as well as HP network analyzers 
to 100~GHz (8510 and 8720s), spectrum analyzers to 110~GHz, 
and a variety of power meters, microscopes and probe stations.
Computer systems include two HP 735 Apollo Workstations each running
Hewlett-Packard's high frequency structure simulator and microwave design 
software.
\vfill
\eject


\end{document}




