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\twocolumn[
\Large 
\centerline {\bf Project Description}
\normalsize
\vspace{0.5cm}
]


%\noindent {\bf THz Radiation Production and Amplification Using a New
%Generation of Electron Beam Devices} 

\section{ Introduction }

Spurred by the need for
compact \& broadband telecommunications systems, detection of
hazardous materials, and 
non-invasive medical imaging, interest in the TeraHertz (THz) portion
of the electromagnetic spectrum has grown dramatically over the past
few years.  One major impediment to the practical realization of THz
technology is the limited availability of continuous wave signal
sources.  Such sources are needed, for example, in the vast majority
of THz receivers and transmitters.
The two most widely used signal sources at THz frequencies
are far-infrared (FIR) lasers and solid state frequency
multipliers. The FIR laser is capable of producing milliwatts of
power, but only at specific frequencies determined by the molecular
structure of the lasing gas and the mid-infrared pump laser. Recent
advances in solid-state frequency multiplier technology has pushed
their upper frequency limit to just over 2~THz. However, at these
frequencies multipliers can produce harmonics with only a few
nanowatts of power. {\bf A breakthrough in the availability of
  tunable THz sources is needed for the true potential of this
  spectral regime to be realized.}


Here we propose to conduct a series of experiments to demonstrate 1)
the use of the Smith-Purcell (SP) effect to generate coherent THz
radiation and 2) a traveling-wave tube (TWT) amplifier capable of
operating at THz frequencies. These investigations will utilize a
modified scanning electron microscope (SEM) as a convenient source of
$\sim$30~keV electrons required for these experiments. The work is
part of an ongoing collaboration between the University of Arizona and
Raytheon Inc. to develop technologies that will help realize the
potential of this important spectral regime.  

\section{ Device Theory and Operation}

\subsection{Smith-Purcell Radiation}

Smith-Purcell radiation is produced when an energetic electron beam
passes over a conductive, periodic structure (e.g. a diffraction
grating) at a fixed distance close to the surface.  The effect was
first predicted by Frank (1942) and observed at visible wavelengths by
Smith and Purcell (1953).  In their work Smith and Purcell utilized a
250-300~keV electron beam. The Coulomb field of the electrons in the
beam induce currents in the underlying periodic conducting
structure. The oscillating currents generate a dipole moment on the
grating surface that gives rise to the observed radiation.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.35,angle=0]{./figures/SPFig1.eps}
\caption{\label{spfigure} {\small \it Schematic representation of the
Smith-Purcell (SP) effect, edited from Kube (2003).}} 
\end{figure}

The generation of SP radiation is illustrated in
Figure~\ref{spfigure}, where the
diffraction grating is approximated by a series of narrow conducting
strips on a flat plane. An electron passing overhead induces an
oscillating image current on the grating surface. The frequency (or
wavelength) of the emitted radiation is set by the periodicity of the
grating ($D$) and the electron's velocity ($\beta =v_o / c$). To the
observer the
perceived grating periodicity will be a function of viewing angle,
hence the observed wavelength of SP radiation is a function of viewing
angle. A figure illustrating SP emission from a 3-D grating is shown
in Figure~\ref{spfigure2}.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.38,angle=0]{./figures/SPgrating.eps}
\caption{\label{spfigure2} {\small \it Schematic representation of the
Smith-Purcell (SP) effect from a 3-D grating. }} 
\end{figure}


The wavelength of the SP radiation can be expressed in terms of the
grating equation (Smith \& Purcell 1953) as:  

\begin{equation}
\lambda = \frac{D}{|n|} \left ( 1/\beta - \cos \theta \; \sin \phi
\right ).
\end{equation}

\noindent where $\lambda$ is the wavelength of the emitted radiation, $D$ is
the grating period,  $n$ is the diffraction order, $\beta = v/c$ is the
reduced electron velocity, and $\phi$ \& $\theta$ are the emission angles
defined in Figure~\ref{spfigure2}.  

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.46,angle=0]{./figures/grating_period.eps}
\caption{\label{spwavelength} {\small \it Verification of the
    Smith-Purcell effect in dependence of the grating period, from
    Urata et al. (1998). }} 
\end{figure}


The wavelength dependence of THz SP radiation has been experimentally
verified by Urata et al.(1998) at THz frequencies for a variety of
grating periods and beam voltages (ex. Figure~\ref{spwavelength}). 

The angular distribution of the emitted radiation is given by Francia (1960):

\vspace{1cm}

\begin{eqnarray}
\frac{dP_n}{d\Omega} &  =  & \frac{e \; I \; n^2 L}{2 D^2 \epsilon_0} \frac{\sin^2
\theta \sin^2 \phi}{(1/\beta - \cos \theta \; \sin\phi)^3} \; |R_n|^2 \nonumber
\\
& & \times \exp \left ( - \frac{d}{h_{int}} \sqrt{1 + (\beta \gamma
\; cos \phi)^2}\right )
\end{eqnarray}


\noindent where $e$ is the electron charge, $I$ the beam current, $L$
the grating length, $\epsilon_0$ the permittivity of free space and
$d$ the distance of the beam above the grating. The Lorentz factor $\gamma$
is given by $\left ( 1 - \beta^2 \right )^{-1/2}$.  
  
The radiation factor $|R_n|^2$ is analogous to the reflection coefficients of
optical gratings and is a measure of grating efficiency.  Radiation
factors can be computed following the modal expansion method of Van
den Berg (1973) and Haeberle' et al. (1994) and are a function of
beam energy and observation geometry. In the THz experiments being
proposed here the radiation factors are $\sim$3.4 for a metallic grating
and a 30 keV beam (Urata et al. 1998). 

In our experiments we will use an electron beam generated by an
SEM. At the brightness levels typical of such beams the SP radiation
is in the {\it spontaneous} regime, meaning that the radiation from the
grating does not significantly impact the electron beam itself. The
observed SP intensity is equal to the incoherent sum of the radiation
induced by the interaction of electrons in the beam with the grating
and is proportional to the beam current. Even at modest beam brightness
the observed SP intensity can be an order of magnitude higher than
that of a black body radiator at 3000~K (Kapp et. al. 2004). As
described by Kapp et al., under certain conditions the image currents
induced on the grating can interact with the beam causing the
electrons to ``bunch''  along its length with the period length $D$ of
the grating. This electron bunching is similar to what occurs in a
free electron laser (FEL) and in TWTs. In an FEL the bunching is
induced by alternating transverse magnetic fields. In TWTs the
bunching is induced by interaction of the electron beam with a slow
moving electromagnetic wave (see below). As an electron bunch moves
down a FEL or TWT its charge density continues to increase. 
Under these conditions
a SP device is said to operate in the {\it amplified} or {\it super-radiant}
regime, with the growth rate per unit distance given by (Kapp et
al. 2004): 

\begin{equation}
\mu_g = {{2\pi} \over {(\gamma \beta)^2}} \left ( I \over{\lambda \;
b \; I_A} \right )^{1/2}
\end{equation}

For centimeter size gratings and beam currents of $\sim$1~mA, the power gain
expected at THz frequencies is of order $10^6$ (60 dB) (Kapp et
al. 2004) over the (nW) power level obtained in the spontaneous emission
regime. Therefore, in the super-radiant regime  SP effect
devices should be capable of producing output power levels of $\ge$1~mW,
comparable to an FIR gas laser and orders of magnitude greater than
solid state devices. To date no groups appear to have unambiguously
detected super-radiant emission using  SP-effect devices at THz
frequencies, most likely due to limited beam current and grating
length (Urata et al. 1998;  Knapp et al. 2004).  {\bf The detection and
characterization of the super-radiant regime of an SP-effect device at
THz frequencies will be a major goal for the proposed study}. The
custom gratings needed for the investigations will be fabricated
using a high precision milling machine in the PI's lab. 

Recently, it has been observed (Smith et al. 2004) 
that enhanced diffraction occurs in the natural gratings that are
formed by negative 
index material (NIM) prisms used to study NIM metamaterials. Smith et
al. (2004) have used a
split-ring resonator (SRR) structure embedded in negative-$\epsilon$
material in their 
work. Their observations suggest that SP-effect
experiments with a NIM grating would uniquely enhance the SP-effect radiation. 
From the work of Yamaguti et al. (2002)
it appears an order of magnitude improvement in output
power may occur if photonic crystals are used.  An
illustration of the generation of SP-effect radiation using a NIM
grating is shown in Figure~\ref{nim}. 
{\bf We will systematically move from the reproduction of
the classical experiment of Smith \& Purcell (1953) to investigate 2-D and 3-D
photonic crystal implementations of both positive and negative index
material gratings.} 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.3,angle=0]{./figures/nim.eps}
\caption{\label{nim} {\small \it Generation of enhanced SP-effect
radiation from passing an electron beam to within close proximity of a
negative index material (NIM) prism. }} 
\end{figure}

\subsection{Traveling Wave Tubes}

Traveling wave tube amplifiers are currently in use from below 1~GHz
to $\sim$100 GHz and are capable of generating power levels from watts to
megawatts. They are used in a wide variety of applications; including
radar, communications satellites, and electronic countermeasure
systems.  Amplification occurs through an interaction between an
electron beam and a nearby radio frequency (RF) circuit carrying the
signal to be amplified. Through the interaction some of the kinetic
energy of the electrons in the beam is transformed into RF
radiation. For this to occur the velocity of the RF signal flowing
through the circuit must be slowed down to match the speed of the
electrons in the beam. The possibility of such an interaction was
recognized as early as 1933 (Haeff 1933).  Lindenblad (1940) and later
Kompfner (1942) were the first to describe using a helical
transmission line wrapped around an electron beam as a tube
architecture that meets these requirements. The helix serves as the
slow-wave structure that matches the axial propagation velocity of the
RF signal to that of the electron beam. The basic design
of such a TWT is shown in Figure~\ref{gilmour1} (Gilmour 1994). 
Today slow-wave structures
composed of coupled cavities also serve this purpose. 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.3,angle=0]{./figures/Gilmour1.eps}
\caption{\label{gilmour1} {\small \it Schematic of an experimental helical
traveling wave tube (TWT), from Gilmour (1994). }} 
\end{figure}

When an electron
beam is injected along the axis of the helix axial electric field
components of the RF signal accelerate some electrons while
decelerating others. This interaction causes the electrons in the beam
to bunch-up in a manner similar to that observed in the super-radiant
regime of the SP-effect. As an electron bunch travels down the center
of the helix each interaction with the RF signal increases the charge
density in a given bunch. The fields produced by the bunching
electrons in the beam will induce corresponding electric currents in
the helix, adding constructively to the electric field of the signal
it is carrying. As the beam-wave interaction continues, the induced
waveform rapidly becomes larger than the initial signal and
amplification occurs (Gilmour 1994). A figure illustrating the
relationship between electron bunch charge density and RF circuit
voltage with distance along the tube axis is shown in
Figure~\ref{gilmour2} (Hess 1960). 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=2.5,angle=0]{./figures/Gilmour2.eps}
\caption{\label{gilmour2} {\small \it Illustration of the interaction
    between electron beam and RF signal in a TWT, resulting in RF
    amplification. Adapted from Hess (1960) in Gilmour (1994).}} 
\end{figure}

Note that near the end of the tube the charge density in an
electron bunch becomes large enough for space-charge forces to begin
to disperse it, leading to destructive interference and field saturation.

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.9,angle=0]{./figures/meandering1.eps}
\caption{\label{meandering} {\small \it 3-D rendering of the geometry of a
    meandering waveguide slow-wave structure. }} 
\end{figure}

Today the two most common types of slow-wave structures used are the
venerable helix and coupled waveguide cavities. The helical design can
generate tens to hundred of watts over as much as two octaves of
frequency. The
coupled-cavity type TWT is narrower band (10-20\%), but is capable of
producing megawatts of power. The practical high frequency limit of
TWT's was continuously pushed to higher frequencies until the early
1990's when high frequency, high electron mobility transistors (HEMTs)
became available. Today, HEMT-based amplifiers have been built to
$\sim$200~GHz, beyond which limitations in photolithographic processing make
them extremely difficult (or impossible) to produce. {\bf The high
frequency limit of TWTs is set principally by losses in the slow
wave structure.  Using modern machining and photolithographic
techniques TWT operation to several THz should be possible. 
This plus the fact that TWTs
have been shown to be capable of very low-noise operation ($<$0.2 dB,
Hammer and Wen 1964) provides a strong incentive to push TWT
technology to higher and higher frequencies. }

\begin{figure}[!hbt]
\centering
\includegraphics[scale=1.9,angle=0]{./figures/meandering2.eps}
\caption{\label{splitblock} {\small \it 3-D rendering of the proposed
    meandering waveguide structure to be implemented 
    in a copper split-block first
    at 350~GHz and then at 1.5~THz.}} 
\end{figure}

\bigskip

The implementation of a helical slow-wave structure becomes more
difficult with frequency. Meandering waveguide has been successfully
used at high frequencies as an alternative slow-wave structure 
(Figure~\ref{meandering}). 
Here interaction between beam and wave is maintained by
periodically folding the waveguide back into the beam path.  Sharp
corners in the bends should be avoided in order to reduce reflection
which reduce gain.  The waveguide cross section should be retained in
the bends for the same reason. Electron-signal interaction occurs
where the particle beam passes through the waveguide. The degree of
electron-beam interaction that takes place at each encounter is
described by the gain parameter $C$, given by (Pierce 1950): 

\begin{equation}
C = \left ({Z_{\circ} \over 4\,V_{\circ}/I_{\circ}} \right )^{1/3}
\end{equation} 

\noindent where $Z_{\circ}$ is the interaction impedance and $V_\circ/I_\circ$
is the beam impedance. Waterman (1979) successfully constructed a 46
GHz TWT using the meandering waveguide structure shown in above. For
this structure he derives a value of $Z_\circ = 4.65 \Omega$.  For our
proposed investigations we will use a 1/8$^{\rm th}$ scale of Waterman's design
implemented in a copper ``split-block'' (J. Waterman has agreed to
collaborate with us on this project, see Support Letter). A 3-D
rendering of the machined structure is shown in
Figure~\ref{splitblock}. At the
design frequency of 368 GHz the characteristic dimensions of the
meandering structure are $G = 79.5 \; \mu$m, $W = 457 \; \mu$m, $L = 177
\; \mu$m , and $H=346 \; \mu$m, with a $127\mu$m diameter beam hole.
With our electron microscope beam source (see discussion below),
typical values of beam current and voltage are $V_\circ = 25$ keV and
$I_\circ = 1$~mA. Substitution into the above equation then yields a
value of $C= 0.0036$. The power gain $P_G$ of a TWT can be expressed
as  (Gilmour 1994): 

$$P_G = 47.3 \,C N\,{\rm dB}$$

\noindent where $N = {\rm tube \; length}/L$.  
To achieve a value of $P_G$ of 10 dB will
require a $N\sim 59$ and an overall tube length of $\sim
1$~cm. Empirical measurements (Kapp et al. 2004) of the growth of a
similar electron beam over this distance yielded an average beam
diameter of $\sigma = 42$~$\mu$m. This value of sigma is less than
1/4$^{\rm th}$
the diameter of the beam hole, indicating the beam will remain
sufficiently collimated throughout the structure, obviating the need
of an additional collimating magnetic field. Clearance of the beam
through the tunnel will be monitored by precisely measuring the
temperature of the waveguide block.  An accidental encounter of the
beam with the block will raise the block's physical temperature. Using
the precision milling machines available in the PI's lab (see below),
the slow-wave structure can be fabricated with surface finishes
approaching optical quality, minimizing waveguide loss. 
We have modeled the full length of the slow-wave structure in CST
Microwave Studio and found the total loss (including the effects of
finite conductivity, surface roughness and leakage through
the beam tunnel) to be $\le 2$~dB across the operating range of the
tube (Figure~\ref{loss}). 

\begin{figure}[!hbt]
\centering
\includegraphics[scale=2.35,angle=0]{./figures/loss.eps}
\caption{\label{loss} {\small  \it CST Microwave Studio simulation of the
    proposed design demonstrates low losses across the full
    operating range of the TWT.  }}  
\end{figure}

\begin{figure*}[!hbt]
\centering
\includegraphics[scale=0.5,angle=0]{./figures/schematic.eps}
\caption{\label{scheme} {\small \it a) The Amray\texttrademark 1000a
    scanning electron microscope in the PI's lab.  b) Cutaway diagram
    of the SEM.   c) Image of the branch-line coupler in
    Figure~\ref{coupler} taken with the Amray\texttrademark\ SEM. }} 
\end{figure*}

\bigskip

Predictions of $C$, $Z_\circ$, and $P_G$ will be tested by real
circuits and beams in experiments with the modified SEM.
Experimentally-determined coupling impedances will then be used in
large-scale TWT simulations (from Raytheon) to design optimized
TWT circuit lengths and parameters.  TWTs below 30~GHz routinely
have 60~dB of power gain, so there is good reason to hope for a
successful outcome. Once a TWT has been successfully demonstrated
(i.e. exhibited a predictable amount of gain) at
368~GHz, a THz version of the device will be designed and
constructed. 

In the implementation of a new TWT amplifier undesired backward wave
oscillation (BWO) may occur, turning the amplifier into a source of
coherent radiation. BWO's can be significantly reduced or eliminated
by severing the slow-wave circuit. The two halves of the slow-wave
circuit remain reconnected by the electron beam which then induces an
amplified version of the input signal in the second half of the
circuit. If undesired BWO's are found to be present in the output of
the 368 GHz tube, we will add a sever. However, as described above,
there are many applications where a clean source of coherent THz
radiation is desirable. Therefore, as part of our investigation, we
will optimize a tube design to act as a BWO. 


\section{Experimental Approach}

In the past, development efforts in SP-effect and TWT devices have been
out of reach of most university investigators due to the lack of a
flexible electron beam source. John Walsh's group at Dartmouth were
the first to use an SEM to characterize electron beam devices (Urata
et al. 1998).  Both proposed SP-effect and TWT test set-ups will
utilize a recently refurbished, Amray\texttrademark 1000a Scanning 
Electron Microscope. A picture of the unit in the PI's lab is shown in
Figure~\ref{scheme}a. Figure~\ref{scheme}b is a schematic of the SEM's
magnetic lensing 
system. Figure~\ref{scheme}c is an image obtained with the SEM of a 1.45 THz
waveguide structure machined in the PI's lab. The Amray 1000a can be
configured either as a narrow beam, high magnification SEM or as a
somewhat broader ($\Omega \sim 50 \; \mu$m) higher beam current ($\sim 1$~mA)
source. The configuration change can be made in a matter of minutes.   
 
\noindent {\bf The availability of an SEM as a flexible e-beam source
  is an invaluable tool for the proposed work.}


\begin{figure*}[!hbt]
\centering
\includegraphics[scale=3.3,angle=0]{./figures/SP3D.eps}
\caption{\label{sp3d} {\small \it Experimental schematic for
    characterizing Smith-Purcell effect devices. }} 
\end{figure*}

\subsection{Smith-Purcell}

The experimental set-up for characterizing SP-effect devices is shown
in Figure~\ref{sp3d}. The SP-effect grating is oriented in the sample chamber
such that the e-beam travels just above and transverse to the grating's
grooves. Even with apertures removed in the focusing optics to permit
higher current operation, the SEM can provide images of sufficient
resolution to orient the beam relative to the grating. The beam focus
is placed midway along the grating. The test set-up must provide a
means of coupling SP-effect radiation from the grating to outside the
SEM's sample chamber. In a series of experiments resulting in the
measurement of spontaneous SP-effect emission,  Kapp et al. (2004)
used a hollow brass pipe to convey some fraction of the emitted
radiation to a vacuum window. We will initially adopt a similar
approach here. The emerging radiation will  be synchronously detected
by an Infrared Lab's 4$^\circ$K bolometer system. Power measurements
will be made both as a function of beam impedance and grating
orientation. As part of the device characterization, the power pattern
of the emergent beam will be measured. The temperature grating block
will be monitored with the goal of detecting when the e-beam
inadvertently strikes the grating. Temperature monitoring will have
the added benefit of allowing the blackbody contribution to the
emitted radiation to be estimated.  The grating will also be
electrically isolated so the beam current striking it can be
measured. Based upon measurement results, the grating design will be
re-optimized, machined, and characterized in the PI's lab.    


\begin{figure*}[!hbt]
\centering
\includegraphics[scale=3.1,angle=0]{./figures/TWT3D.eps}
\caption{\label{twt3d} {\small \it Experimental schematic for
    characterizing traveling wave tube (TWT) devices. }} 
\end{figure*}

\subsection{TWT}

The experimental set-up for TWT characterization is provided in
Figure~\ref{twt3d}. The meandering waveguide, slow-wave structure is
oriented 
vertically in the sample chamber so the e-beam can enter the beam
tunnel. The e-beam focus is placed midway down the structure. Images
will be made with the SEM to help align the TWT with respect to the
beam. Input and output signals are coupled to the TWT via waveguide
flanges. At 368~GHz, a solid-state source comprised of a Gunn
oscillator and frequency multiplier provides
the input signal to be amplified.  An existing Coherent-DEOS laser will provide
the 1.5~THz input signal for the second-generation TWT devices.  
Two directional couplers
with power detectors are used on the input and output in order to
provide an accurate measurement of power gain.  Both input and output
signals will be coupled via waveguide to the sample cell.  
As in the case of the
SP grating, the waveguide block will be both electrically isolated and
instrumented with a sensitive temperature sensor in order to detect
the charge or heat of the electron beam if it strikes the block. The
TWT's performance will be measured as a function of frequency and beam
impedance. New slow-wave structures will be designed and fabricated as
needed. The test set-up of Figure~\ref{twt3d} can also be used to characterize
a TWT structure designed to operate as a BWO. In this case the BWO
output port is actually near the beam entrance; the other waveguide 
waveguide board would be terminated.  

\begin{figure}[htb]
\includegraphics[angle=0,scale=0.3]{./figures/branch_coupler.eps}
\caption{\small \label{coupler} \it SEM imaging of a 1.5~THz branchline
  coupler based on the Atacama Large Millimeter Array (ALMA)
  design, fabricated using the laser
  micromachining system at the University of Arizona.} 
\end{figure}

\section{Precision Micromachining}

Both the SP-effect gratings and the TWT slow-wave circuits have
structures that are small compared to the design wavelengths. The ability to
machine such structures at the required size scale and accuracy has
been a major hurdle in scaling well--characterized structures from the
microwave to THz spectral regimes. With the aid of funds from the NSF,
NASA, and Army Research Office over the past decade, the PI's group at
the University of Arizona has established a micromachining capability
uniquely suited to making high quality waveguide and quasi-optical
components to (sub)micron accuracies. The lab contains two
custom-designed laser micromachining systems. One system is optimized
for making waveguide devices from $\sim 0.8$ to 5~THz. The second
system is used to make similar devices from $\sim 5$ to 30 ~THz. In
these systems the beam from an argon-ion laser is focused onto a
silicon wafer residing in a 200 Torr chlorine ambient. The incident
laser beam has enough power (typically several watts) to vaporize a
small volume ($\le 1$~$\mu$m$^3$) of silicon that reacts with the
chlorine to make SiCl$_4$, which remains in the gas phase. Under
computer control the laser beam can be steered to create 3-D
structures such as gratings or meandering waveguides (d`Aubigny et
al. 2004). Once the etching is complete, the etched silicon is gold
plated as needed to make it behave like normal waveguide or
quasi-optical structures. An example of a 1.5~THz waveguide power
combiner fabricated with the system is shown in Figure~\ref{coupler}. 
A picture
of one of the laser micromachining systems to be used for the THz
portion of the proposed work is shown in Figure~\ref{laser}.  

\begin{figure}[htb]
\includegraphics[angle=0,scale=0.33]{./figures/lmc.eps}
\caption{\small \label{laser} \it Laser micro-milling machine in the PI's
  laboratory, used for fabrication of devices at frequencies $>$1~THz. } 
\end{figure}

\begin{figure}[htb]
\includegraphics[angle=0,scale=0.36]{./figures/kern.eps}
\caption{\small \label{kern} \it Kern micro-milling machine in the PI's
  laboratory, used for fabricating devices at frequencies $<$~1~THz. } 
\end{figure}



At frequencies below 1~THz the size of the structures to be etched
often require volumetric removal rates beyond the capabilities of the
laser micromachining systems. In order to fabricate such structures
the PI's group has recently obtained funds from the NSF's MRI program
to procure a state-of-the-art micromilling machine (KERN MMP 2522, see
Figure~\ref{kern}) that can machine metallic and ceramic structures to 1
micron precision and accuracy. This system will be used to machine
prototype 345 and 368~GHz SP-effect gratings and TWT slow-wave
structures for the proposed effort, whereas the laser micromachining
system will concentrate on fabrication of the 1.5~THz structures.

\noindent {\bf The availability of these systems will permit the rapid
  prototyping and optimization of the gratings and waveguide
  components needed for this project, from microwave to THz spectral
  regimes.} 

\section{ Realizing Practical Devices}

The primary goal of the proposed work is to demonstrate TWT and
SP-effect devices at THz frequencies.  Using the e-beam of an SEM 
provides the most flexible and readily-available 
means for achieving this goal. However, to make these
devices practical they must be designed to operate in compact,
efficient, self-contained packages. Once we have experience operating
these devices at THz frequencies, we will be better able to optimize
their operating characteristics.  The main task will then be to
design a new cathode based on low-power, low-temperature  field
emitters such as carbon nanotubes.  Cathodes employing carbon
nanotubes have already been demonstrated in a Northrop/Grumman TWT at
4.5 GHz (see Figure~\ref{milne}) and require only 250V of gate voltage. At THz
frequencies the size of a unit such as this can be reduced by an order
of magnitude, making them very attractive for space-based applications
as well.  Carbon nanotubes may 
have the additional advantage of having exemplary noise
performance.  In 1964, Hammer \& Wen found that high-B fields 
applied in the vicinity of the cathode reduced
the transverse velocity component in the e-beam, significantly
improving the noise performance of the TWT.  Therefore nonthermionic
emitters such as carbon nanotubes may intrinsically provide better
noise performance.  
{\bf In the third year of the proposed effort we will perform a
detailed design study of an SP-effect and TWT device that is optimized
for use with field emission devices. }

\begin{figure}[!hbt]
\centering
\includegraphics[scale=0.6,angle=0]{./figures/milne_twt.eps}
\caption{\label{milne} {\small \it Milne et al. (2003) demonstration of
    compact TWT and electron 
    sources at 4.5~GHz using carbon nanotube technologies.  This
    example showcases the kind of
    packaging we will explore at 350~GHz and 1.5~THz.}} 
\end{figure}


\section{Plan of Action}

\noindent {\it Year 1}

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

   \item Upgrade SEM to operate at high currents \& voltages
   \item Complete assembly of test bed electronics
   \item Design and fabricate SP-effect grating and coupling optics
     optimized for 350~GHz
   \item Design and fabricate TWT slow-wave structure optimized for 350~GHz
   \item Perform initial experiments with SP-effect grating in
     spontaneous emission regime
   \item Perform initial experiments with TWT slow-wave structure
\end{enumerate}

\medskip
\noindent {\it Year 2}

\begin{enumerate}
\setlength{\topsep}{-1ex}
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   \item Make necessary design changes to SP-effect grating \& optics
   and TWT structure based upon Year 1 results
   \item Repeat performance tests using reoptimized structures
   \item Design SP grating structures using photonic crystal and NIM
     material 
   \item Test performance of SP-effect grating in super-radiant regime
   \item Test performance of TWT in backward-wave oscillator (BWO)
     mode
\end{enumerate}

\medskip
\noindent {\it Year 3}

\begin{enumerate}
\setlength{\topsep}{-1ex}
\setlength{\partopsep}{-1ex}
\setlength{\itemsep}{-0.4ex}
\setlength{\parsep}{-0.4ex}
    \item Test performance of photonic band gap and NIM-based SP grating
    \item Design, fabricate and test SP-effect grating for operation
    at 1.5~THz
    \item Design, fabricate and test TWT slow-wave structure at 1.5~THz
    \item Perform a design study for implementing THz SP-effect signal sources
    and TWT amplifiers as standalone, compact units
\end{enumerate}

\section{ Educational Impact}

The Astronomy, Optical Sciences, Electrical Engineering, and Physics
Departments at the University of Arizona and Raytheon Inc. are teaming
together to create a Center of Excellence in THz Science \&
Technology. The purpose of the center is to 1) contribute to
breakthroughs in the THz spectral regime and 2) train students in the
multidisciplinary approach to the formulation and solving of
problems. Students must learn this approach if they are to play a
leadership role in today's research environment. The proposed
investigation is an ideal match to these goals.  

The design, fabrication, and test of SP-effect and TWT devices will be
the focus of the dissertation work of at least two graduate students
and the senior research projects for two undergraduates. In addition,
2 or more engineers and/or physicists from Raytheon Inc. are expected
to participate in the proposed research and use it to help fulfill
requirements for advanced degrees. As part of our collaboration,
Raytheon is interested in employing undergraduates for summer intern
positions in related fields. The exposure students will receive to the
corporate research environment will help them in making career choices
in the future.  The PI has faculty appointments in Optical Sciences
and Electrical Engineering, as well as Astronomy. These appointments
enable him to serve as the graduate advisor to students in all three
departments.  Currently he is the thesis advisor to 3 Ph.D. students
in Astronomy, 2 Ph.D. students in ECE, and 1 Ph.D. student in Optical
Sciences. Three undergraduate (1 Physics, 2 Optical Sciences) are now
employed in the PI's lab. 

\section{ Results from Prior NSF \\  Support}

\begin{itemize}

\item A NSF Young Investigator Award (AST-9457445) was made to 
the PI for the period 7/1/94 - 7/1/99
of \$62,500 per year (matching funds have been obtained for all 
5 years). This award was used to support research and teaching
efforts. Research included further development of the laser 
micromachining waveguide technology, the construction of submillimeter
receiver systems and/or components at 230, 345, and 490~GHz for the 
Heinrich Hertz Telescope (HHT), a 492/810~GHz receiver system
for the AST/RO telescope at the South Pole, the funding
of graduate and undergraduate students for the study of protostellar evolution,
and the construction of a submillimeter polarimeter and student radiotelescope.
The receiver systems described above are (except for the polarimeter)
facility instruments. We estimate $\sim$30 papers from the HHT and 20 papers
from AST/RO contain observations made with these instruments. Papers
describing the instruments are Groppi et al. 2000; Walker et al. 2001).

\item A grant (AST-9622569) from the NSF ATI program was made to the PI
to construct the 7-channel, 345~GHz array (DesertSTAR I) for the HHT.
(formal title: ``An 870 micron Array Receiver for the Heinrich Hertz Telescope'')
The award period was from 8/1/96 - 7/31/99 for a total of \$380,931. 
DesertSTAR has been a successful collaboration between the University of
Arizona (PI: system design and construction), NRAO (John Payne: J-T cooler), 
the University of Massachusetts (Gopal Narayanan and Neil Erickson: mixers and LO), 
and the University of Virginia (Art Lichtenberger: SIS junction fabrication).
It will be the first array receiver to operate in the 870 micron atmospheric window (Groppi et al. 2000; 2003).

\item An award (ECS-9800260) from the NSF Physical Foundations of 
Enabling Technologies Program was made to the PI  
to construct and utilize a laser micromachining system at the 
University of Arizona (Drouet d'Aubigny et al. 2001). 
The award period was from 6/1/98 - 5/31/00 
for a total of \$280,000. The system is fully operational 
and has been used to make waveguide components for integrated array
development and prototype quasi-optical structures for NASA's
{\it Terrestrial Planet Finder}. 

\item A grant from the NSF ATI program was awarded to the PI and Co-I Daniel
Prober (Yale University) to develop ``An Integrated 370 micron Heterodyne Imaging Array''. 
This is an ongoing effort to combine laser micromachining technology
with recent developments in Hot Electron Bolometer (HEB) mixers to develop
the first truly integrated submillimeter-wave heterodyne imaging arrays
(Groppi et al. 2002).
The arrays are being designed to permit scaling to higher frequencies
and formats. The award amount (\$450K) is being divided over a three year
period (02/01/02 to 01/31/05) between the participating institutions.

\item A subaward of \$99,348 was provided to the PI from the NSF ATI program for his 
contribution to ``A 1.5 THz Common User 
Receiver System'' (S. Yngvesson- PI). Over the two year award period (01/01/00 to
12/31/02) the PI's group designed and built the cryogenic, electronic, and optical
system for the TREND 1.5 THz receiver system now on the AST/RO telescope at the
South Pole. The laser LO system and high frequency mixer were provided by the
University of Massachusetts (Gerecht et al. 2003).  

\item A subaward of \$290K from the NSF Office of Polar Programs was made to
the PI for his group's work in ``Continuing Operation of the Antarctic Submillimeter
Telescope and Remote Observatory'' (A. Stark- PI: OPP-0126090). The PI's group built most
of the facility receivers for AST/RO. This 3 year award 
(02/01/02 to 01/31/05) supports the continued maintenance and upgrade of these
systems (Stark et al. 2001).

\item An MRI award of \$1,742,356 was made to the PI from the ECS
  division to construct ``SuperCam: A SuperHeterodyne Camera for the
  Heinrich Hertz Telescope'' (AST-0421499). The PI's group is in the
  first year of a 3 year proposal to build the world’s first large
  format, submillimeter-wave heterodyne receiver system. 
  SuperCam will contain 64 independent receiver
  pixels. The instrument will reside on the Heinrich Hertz Telescope
  and be used to map the plane of the Milky Way and nearby galaxies in
  a variety of important astrophysical emission lines. The duration of
  the award is 09/01/04-8/31/07. 
\end{itemize}

\clearpage

\pagestyle{empty}

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\end{document}
