Extended Abstracts of the 2009 International Conference on Solid State Devices and Materials, Sendai, 2009, pp814-815
E-4-1L
Monte Carlo Analysis of Optical Pulse Response of
Plasmon-Resonant Terahertz Emitter
Kentaro Kubota*1
*1
Eiichi Sano*1
Yahya Moubarak Meziani*2
Taiichi Otsuji*2
Research Center for Integrated Quantum Electronics, Hokkaido University,Sapporo, Japan
*2
Research Institute of Electrical Communication, Tohoku University, Sendai, Japan
1. Introduction
under the negatively-biased gates (G2). Conventional
scattering processes were included in the Monte Carlo
simulations [6]. The Ramo-Shockley theorem was used to
calculate the electron current induced on the electrodes (both
sides of the analyzed region). Three simulation runs with
different initial random numbers were carried out to remove
numerical errors. For comparison, a conventional HEMT
was also analyzed with the same method.
The application of terahertz(THz)waves to engineering,
e. g., imaging, information and communication technologies,
has been investigated. Developing compact, tunable, and
coherent sources that operate at THz frequencies is essential
for innovation in these fields. We have found that THz
emitters based on the plasmon resonance meet the
requirements [1-2]. To enhance the THz emission power
caused by inherent gain, we previously described a
plasmon-resonant THz emitter with a dual-grating-gate
high-electron-mobility transistor (HEMT) structure [3]. The
THz radiation from the emitter was measured by using
reflective electro-optic sampling (REOS) [4] and a Si
bolometer [5]. In addition to the experimental
demonstrations, theoretical work is needed to elucidate the
physical mechanism of THz emission. We report the first
simulation results using Monte Carlo analysis of optical
pulse responses for our proposed plasmon-resonant THz
emitter.
4.Result
Snapshots of particle energy distributions and electron
concentration in the InGaAs layer are shown in Fig. 2. The
gate lengths LG1 and LG2 were 100 and 300 nm, and the gate
bias VG1 and VG2 were 0 and -1.5 V, respectively. A clear
modulation of electron concentrations caused by the electric
field was observed. Transient current responses were
Fourier-transformed to obtain frequency responses. The
current spectra simulated for the HEMT and THz emitter
along with THz spectra measured with REOS [7] are shown
in Figs. 3(a) and 3(b), respectively. The simulated spectra
were in good agreement with the measured ones. The
frequency response for HEMT monotonously decreased as
the frequency increased. Peaks can be distinguished for the
THz emitter. These peaks may have been caused by the
oscillatory motion of electrons in the plasmon cavities and
its harmonics. The conditions needed for plasmons to
self-excite will be examined using the present simulation
method to fully elucidate the emitter operation.
2 .Device structure and operation principle
A cross section of our THz emitter [3] is shown in Fig. 1.
The key features of the emitter are doubly interdigitated
grating gates (G1 and G2) and a vertical cavity structure
between the top grating plane and a terahertz mirror at the
back. The dual-grating gates periodically localize the
two-dimensional (2D) plasmon in stripes (plasmon cavities),
which are of the order of 100 nm, with micron-to-submicron
intervals. When an optical pulse illuminates the emitter, the
electrons generated under the negatively-biased gates (for
example G2) are asymmetrically injected into the adjacent
plasmon cavities (under G1). This injection may also excite
plasmons in the plasmon cavities. A 15-nm thick undoped
In0.2Ga0.8As layer was used for the 2D plasmon layer. A
60-nm thick In0.48Ga0.52P layer with a 17-nm
carrier-supplying layer acts as a barrier.
5.Conclusion
We have simulated the optical pulse responses for a
plasmon-resonant THz emitter using the Monte Carlo
method. The simulated Fourier spectra for our THz emitter
and a conventional HEMT were in good agreement with
those measured with reflective electro-optic sampling. The
Monte Carlo simulation results show that there is a
distinguished difference between the Fourier spectra
measured for these devices. This analysis method will be a
powerful tool for elucidating the operation of THz emitters.
3 .Simulation model
The unit structure shown in Fig. 1 was analyzed to find a
way to reduce the memory size and computation time of the
emitter. To obtain steady-state solutions, we numerically
solved Poisson and current continuity equations for electrons
and holes within the drift-diffusion approximation. Twenty
thousand particles were placed in accordance with the
steady-state electron distribution, and then Monte Carlo
analysis [6] coupled with Poisson’s equation calculation was
performed. A 70-fs (FWHM) optical pulse was illuminated.
Particles were assumed to be generated in the InGaAs layer
Acknowledgment
This work was financially supported in part by the
SCOPE Program from the MIC, Japan, and by the Grant in
Aid for Scientific Research(S) from the JSPS, Japan.
Reference
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PIERS, 393 (2008)
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87, 052107 (2004).
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Uno, and E. Sano, Appl. Phys. Lett. 89, 263502 (2006).
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Simulated region
Fig. 1. Cross section of plasmon-resonant THz emitter.
1
Current intensity (a.u.)
2
1.5
1
0.5
0.6
0.4
0
G1
G2
-0.5
(a)
0
200
400
600
Simulated
0.2
0
0
2
G2
800
4
6
8
10
Frequency (THz)
(a)
1000
0.6
Position (nm)
Current intensity (a.u.)
Energy (eV)
Measured
0.8
(b)
log10 (concentration)
Fig. 2. (a) Particle energy distribution at 2 ps.
(b) Electron concentration in InGaAs layer.
0.4
0
6
0.2
0
(Vertical scale is enlarged.)
Measured
Simulated
0
1
2
3
4
5
6
7
8
Frequency (THz)
(b)
Fig. 3. Frequency responses of (a) HEMT and (b) THz emitter.
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