Generation of THz Radiation by Excitation of InAs with a Free Electron Laser Page: 3 of 3
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relatively wide pulse width of the FEL and the strong absorption for high-frequency components of THz
radiation in the black plastic plate (- 99% at 1 THz).
The maximum power directly detected with the bolometer was estimated to be about 1 pW for a 10 W FEL
power. Considering the reflection and absorption loss due to the Si (70% loss) and plastic plates (60% loss), the
generated power was estimated to be about 8 pW. This suggests the efficiency of the THz radiation from the
InAs emitter is on the order of 10-7. The observed efficiency for excitation with a mode-locked Ti: sapphire
laser (X ~ 800 nm, pulse energy ~ 10 nJ/pulse) was on the order of 10-6. The reason for the low emission
efficiency with FEL excitation can be explained by the saturation effect. Fig. 3 shows the FEL intensity
dependence of the bolometer signal measured by using a variable optical attenuator. The unit for the FEL
intensity was normalized by the intensity without the attenuator. As clearly shown in Fig. 3, the THz radiation
power is strongly saturated at higher intensities. The THz radiation power expected at the maximum pump
intensity (without the attenuator) is estimated to be about 30 times higher than the measured value when we
assume a quadratic dependence of the THz radiation power on the FEL intensity (shown by a dashed line in
Fig. 3) as observed at lower intensities (un-saturated regime).
100 - - '
10-
10
0.1
0.01 0.1 1
Normalized Intensity
Fig. 3. Intensity dependence of THz power signal from bolometer. The
intensity was normalized to that without any optical attenuator. The dashed
line indicates a quadratic dependence (slope 2) on the excitation intensity.
Although we didn't apply any magnetic field bias to the InAs emitter in this experiment, it was reported that the
THz emission efficiency could be improved by a factor of 20 when a strong magnetic field (- 2T) is applied to
InAs [1]. Therefore, by reducing the saturation effect (using a larger excitation area) and by applying a strong
magnetic field, we believe it is quite feasible to achieve 1 mW level THz radiation with the femtosecond FEL
excitation of the InAs emitter.
The authors acknowledge the support from the Telecommunications Advancement Organization of Japan
through the grant for promotion of international research collaborations. The Rensselaer team was supported by
the U.S. National Science Foundation, and the JLab team by the U.S. DOE under contract DE-AC05-84-
ER40150.
Reference:
[1] Sarukura et al, J. Appl. Phys. 84, 654 (1998).
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Tani, Mashiko; Kono, Shunsuke; Gu, Ping; Sakai, Kiyomi; Usami, Mamoru; Shinn, Michelle D. et al. Generation of THz Radiation by Excitation of InAs with a Free Electron Laser, article, January 1, 2001; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc718446/m1/3/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.