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Generation of Terahertz Radiation by using Ultrafast Laser Pumping and an Enhancement Cavity (Ken-ichiro Maki)

Background and the purpose of this research

Many kinds of sources in terahertz (THz) region have been developed for measurements such as spectroscopy and imaging. Photo-conductive antennas, in which THz radiation is produced from a semiconductor irradiated by an ultrafast laser (or femto-second laser, 1 femto second = 1 x 10^-15 second), are widely utilized as a broadband THz source suitable for spectroscopy. More powerful THz sources, however, are required for the measurement of thick and/or highly absorptive materials and objects at far field with sufficient signal to noise ratio.
The purpose of this study is the development of high-power THz pulse source. One of the principles of THz pulse sources is based on the nonlinear effect in optical crystals pumped by the ultrafast laser. In this research, an enhancement cavity is introduced to this method in order to enhance the pumping intensity in the crystal, and increase the intensity of THz pulses and the conversion efficiency [1].
The source with the method can be applied to THz time domain spectroscopy or THz-TDS and be expected as the novel THz pulse source taking the place of the conventional photo conductive antenna.

Principle of the generation and the enhancement

The principle of producing THz pulses from a nonlinear optical crystal is differential frequency mixing between wavelength components in the broad spectrum of a ultrafast laser [2]. The bandwidth and the pulsewidth of the THz radiation from a lithium niobate, one of the nonlinear optical crystal, are 0.5 ? 3 THz and ~1 pico second (10^-12 second), respectively [3].
The intensity of THz radiation increases as the intensity of the laser is enhanced. The intensity of THz radiation is proportional to the square of the laser intensity because the differential frequency mixing is one of the second order nonlinear optical effects. This intensity of the laser can be enhanced by an enhancement cavity. The laser pulses are overlapped in the cavity and the peak intensity of the pulse is enhanced when the repetition rate of the laser pulse (76 MHz) matches the cavity length (4 m). We can obtain high-power THz radiation from the crystal in the cavity because of the enhanced pumping energy.
The optimal condition of the enhancement is that the transmittance of the input coupling mirror of the cavity is equal to the round-trip loss of the laser in the cavity. The enhancement factors of the intensity compared to that without cavity are expected to be 14 for the pumping, 196 for the THz radiation when both the transmittance and the loss are 7 %.

PExperiment of the measurement of the THz radiation

The experimental setup for the generation of THz radiation with the enhancement cavity is shown in Figure. 1. The pumping system was a Titanium:sapphire ultrafast laser with a center wavelength of 735 nm. The pulse duration and the average power of ultrafast laser were 160 fs and 0.35 W, respectively. The pump laser was modulated with a frequency of 100 Hz. A ring enhancement cavity was built consisting of flat mirrors and two concave mirrors. A MgO-doped LiNbO3 (MgO:LN) was used because its damage threshold is higher than that of non-doped LiNbO3. The crystal, with a length of 5 mm, was placed in the beam waist of the cavity. A silicon prism coupler was attached to the surface of the MgO:LN crystal and serves as output coupler. The average power of the THz radiation was measured with a 4 K Si bolometer [4].
The bolometer signals with and without enhancement cavity are shown in Figure 2. We confirmed that the power of the THz radiation with the cavity enhanced 16 times compared to that in single-pass pumping. The signal looks like the modulation waveform of a continuous wave because the response time of the bolometer is longer than the duration of the THz pulse. The frequency of the THz radiation is estimated to be 0.5 ? 3 THz according to the measurement results of single-pass pumping. The average power of the THz radiation is assumed to be 2.5 μW according to the sensitivity of the bolometer (2 x 10^6 V/W). This power is 4 ? 36 times higher than that of the conventional photo-conductive antenna (0.07 ? 0.6 μW) [5]. The enhancement, however, is not achieved to the theoretical factor yet. This is expected to be solved by using cavity control system and the dispersion compensation for the ultrafast laser pulse in the cavity.


Figure 1. Experimental setup with the enhancement cavity

Figure 2. Waveforms of the THz intensity measured by a bolometer. (red: with the cavity, blue: without the cavity)




[1] G. McConnell, A. I. Ferguson, and N. Langford, “Cavity-augmented frequency tripling of a continuous wave mode-locked laser”, J. Phys. D: Appl. Phys., 34, 2408-2413 (2001)
[2] C. Weiss, G. Torosyan, J.-P. Meyn, R. Wallenstein, and R. Beigang, “Tuning characteristics of narrowband THz radiation generated via optical rectification in periodically poled lithium niobate”, Opt. Exp., 8, 497-502 (2001)
[3] A. G. Stepanov, J. Hebling, and J. Kuhl, “Efficient generation of subpicosecond terahertz radiation by phase-matched optical rectification using ultrashort laser pulses with tilted pulse fronts”, Appl. Phys. Lett., 83, 3000-3002 (2003)
[4] M. Theuer, G. Torosyan, C. Rau, R. Beigang, K. Maki, C. Otani, and K. Kawase, “Efficient generation of Cherenkov-type terahertz radiation form a lithium niobate crystal with a silicon prism output coupler”, Appl. Phys. Lett., 88, 071122 (2006)
[5] M. Tani, S. Matsuura, K. Sakai, and S. Nakashima, “Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs”, Appl. Opt., 36, 7853-7859 (1997)