Detection and Homodyne Mixing of Terahertz Gas
Laser Radiation by Submicron GaAs/AlGaAs FETs
Dmitry Veksler, Andrey Muravjov, William Stillman, Nezih Pala, and Michael Shur
Department of Physics and ECSE
Rensselaer Polytechnic Institute
Troy, NY 12180, USA
veksld@rpi.edu
I.
INTRODUCTION
Dyakonov and Shur proposed to use nonlinear properties
of plasma excitations in a 2D gated electron gas for terahertz
detectors, mixers, and THz radiation sources in 1993 [1]. The
basic idea of detection can be formulated as follows:
Electromagnetic radiation excites plasma waves in the
channel. The nonlinear properties of such waves and
asymmetric boundary conditions at source and drain of the
channel lead to the radiation-induced constant voltage drop
along the channel [1,2], which is the detector response. A
short FET channel of a given length, acts as a resonant cavity
for plasma waves. Experimental exploration of the subject
has begun long time ago. See, for example, a series of
publications [3,4,5,6,7], reporting observation of the farinfrared detection in short channel high-electron-mobility
transistors (HEMTs) fabricated from different materials and
in Si MOSFETs. More resent publications [8,9]
demonstrated that plasma wave electronic devices offer
reasonable sensitivity, comparable with traditional
commercial THz detectors, and fast temporal response. The
preliminary calculations performed by Kachoroskiy and Shur
[10] based on the solution of the hydrodynamic equations
show that plasma wave electronic detectors can respond to
THz and sub-THz radiation with modulation frequency well
above the cutoff frequency of the transistor.
At the same time an advantage of gated plasma wave
devices, such as FETs, is controllable sensitivity and
possibility of fast external switching. The goal of this work is
experimental demonstration of THz heterodyne mixing on
the plasma wave FETs. These devices are expected to enable
The work of W. Stillman and D. Veksler was supported by the NSF
(Grant No. 0333314)) and by the NSF Connection One I/UCRC at RPI.
The work was also partially supported by the ONR.
numerous terahertz detection applications in radio
astronomy, biomedical imaging, homeland security, and
explosive detection.
II.
EXPERIMENT
The experiment was performed using Fujitsu FHX06X
AlGaAs/GaAs HEMT [11]. Radiation of optically pumped
CW terahertz gas laser SIFIR-50 FPL, operating on the
wavelength 118.83 μm (2.52 THz), was focused on the
0.25x200 μm2 FET with a parabolic mirror. A 3D stage with
micrometric screws was used to align the transistor in the
focus of the mirror. A biasing box powered by batteries was
used to apply gate and drain bias to the device. The box had
a variable load resistance in the drain circuit. The signal from
the load resistance was fed into Tektronix TDS 3052
oscilloscope. Radiation intensity was modulated by a
mechanical chopper.
In order to simulate heterodyne mixing of two terahertz
frequencies, we tuned the laser to multiple mode operation
by adjusting the main terahertz laser cavity length. At the
certain cavity setting the laser generated simultaneously two
or more cavity modes, those fell inside the width of the
0.1
a)
0.0
-0.1
-0.2
Response, mV
Abstract— We demonstrated detection and homodyne mixing
of laser modes of optically pumped terahertz gas laser at 2.52
THz by submicron AlGaAs/GaAs field-effect transistors
(FETs). The mechanism of the detection of terahertz radiation
is based on the phenomena of non-linear excitations of plasma
waves in a channel with 2D electron gas. According to
theoretical models, the response time associated with this
detection mechanism can be as fast as picoseconds, which
makes these plasma wave devices attractive for real time
detection and heterodyne mixing applications in the terahertz
frequency range.
-0.3
-0.4
-0.5
0.1
0.0
-0.1
-0.2
-0.3
-0.4
-0.5
b)
0
1
2
3
4
5
6
7
8
9
T im e, m s
Figure 1. Transistor response to 2.52 THz radiation. (a) Laser is in
single mode regime. (b) Mode mixing in transistor. Rectangular show
the fragment of the waveform, expanded in Fig. 2.
III.
SUMMARY
We demonstrated heterodyne mixing of two laser modes
on plasma wave FET terahertz detectors at room
temperature.
-75
a)
-80
Signal
-85
Signal, dBm
active gas gain spectrum, typically ~ 10 MHz. The response
signal was measured as a change of the voltage drop on drain
and source of the transistor at constant current. Captured
waveforms on Fig. 1 correspond to single mode (a) and
double mode (b) laser operation. Zero level in Fig. 1 is
observed when the laser radiation is blocked by the chopper
wheel. The negative drain-to-source voltage induced by the
incident radiation was in the mV range. Slight change of the
main cavity length allowed for two different transversal laser
modes to be excited simultaneously. As the result, the mode
beatings were registered by the transistor, as a broadening of
the waveform in Fig 1 (b) at the open chopper condition. The
rise and decay times of the waveforms in Fig. 1 are not
related to the device temporal response, and are determined
only by the chopper speed.
-90
-95
noise floor
The beatings are shown on the insert in Fig. 2 in greater
details. The Furrier transform revealed a beating frequency
to be approximately 1.2 MHz.
-100
1
0.1
-75 5 0
0.0
-80
55
60
65
70
75
80
85
90
95
10 0
b)
-85
-0.1
0.00
10.00
Time, μs
Signal, dBm
Response, mV
Amplitude (a. u.)
-105
20.00
-90
-95
-100
0
0
6
1x10
6
2x10
6
3x10
-105
6
4x10
-75 5 0
Frequency (Hz)
Figure 4 shows the device response to THz radiation in
direct and homodyne detection modes versus drain current.
Both responses increased monotonically with increasing
current, though the increase of the homodyne response is less
pronounced.
60
65
70
75
80
85
90
95
10 0
c)
Figure 2. Spectrum of laser mode beating. Insert shows a fragment of
waveform from Fig. 1.
-80
-85
Signal, dBm
We also evaluated the most powerful line of the SIFIR50 FPL, namely 1.63 THz line. For this experiment we
employed Agilent 4396B spectrum analyzer. The output
resistance of the detector circuit was matched to 50 ohm of
the analyzer input resistance. The signal was coupled to the
analyzer through 1μF capacitance. Figure 3 demonstrates the
evolution of the beatings frequency while adjusting the main
cavity length trying to excite different spatial modes of the
cavity. The highest observed beating frequency was 8.76
MHz.
55
-90
-95
-100
-105
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10.0
Frequency, MHz
Figure 3. Spectra of SIFIR-50 FPL mode beatings. Lines in figures (a),
(b), and (c) correspond to different lengths of the laser’s main cavity. Laser
frequency is f=1.62 THz. The resolution is 3 kHz for all the spectra.
Presented data demonstrate that the terahertz response of
the transistor is at least faster than 10-7 sec., while theoretical
expectations extend this time down to 10-11 sec.
These novel types of THz detectors are promising for
numerous applications in radio astronomy, biomedical
imaging, homeland security, and explosive detection.
f=1.6 THz, Vgs=-0.45 V
f m = 200 Hz
Signal, mV
2.0
1.5
1.0
0.5
beatings at 8.76 MHz
0.0
0
1
2
3
4
Id, mA
Figure 4. Dependence of FET response in direct (sircles) and homodyne
(squares) detection modes vs drain current. Curves are drawn to guide the
eye. Laser frequency f=1.62 THz.
ACKNOWLEDGMENT
Authors are grateful to Prof. Albert Redo and Prof. X.-C.
Zhang for assistance in experiments.
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