Ultrabroadband detection of terahertz radiation from 0.1 to 100THz with photoconductive antenna

1. Ultrabroadband detection of terahertz radiation from 0.1 to 100THz with photoconductive antenna Itho lab M1 Taisuke katashima

2. Contents ・Introduction of THz wave ・experiment experimental setup and results (detection of ultrabroadand THz radiation) experimental setup and results (estimating sensitivity of PC antenna) ・summary

3. What is a THz wave ? THz region:0.1~10THz γ-ray X-ray Visible Microwaves Frequency(Hz) 103 1018 106 109 1012 1015 1021 zetta peta exa killo mega giga tera Radar Optical Medical Radio Example ・Frequency: 1THz=1012Hz ・Wavelength: 1THz 300µm 33cm-1 ・Wavenumber

4. Application of THz ・THz Time-domain Spectroscopy (TDS) This system can detect the waveform of electric field. We can obtain the amplitudeand phase of electric field by Fourier transform. Complex refractive indexis able to be calculated

5. The principle of THz-TDS THz wave At x=d Refractive index Sample x 0 d Extinction cofficient: Refractive index:

6. THz –TDS setup splitter mirror

7. The background of this study The material whose character changes in the ultrabroad frequency range needs the system which can spectroscope this in the same range. (example: La2-xSrxCuO4) It is essential to generate and detect ultrabroadband THz radiation to realize THz-TDS in ultrabroad frequency range.

8. Purpose of these experiments Previous experiments Kono et al. reported THz radiation over 60 THz with a ZnTe emitter and a PC antenna. This experiments The purpose is to detect THz radiation up to 100 THz with a GaSe emitter and a PC antenna detector and to investigate the sensitivity of PC antenna.

9. Experimental methods Experiment 1 generation: PC antenna detection: PC antenna Experiment 2 generation: GaSe crystal detection: PC antenna Investigating the sensitivity of PC antenna Measuring black-body radiation at 500℃ by using monochromator and MCT (HgCdTe) detector Comparing the experimental spectrum with the Planck distribution Response function of monochromator and MCT detector.

10. I/V amp lock-in amp Experimental setup 2 Ti:Sapphire Laser delay stage probe BS pump chopper Si BS PC antenna GaSe emitter computer

11. I/V amp lock-in amp A Experimental setup 1 pulse width: 10fs, center wavelength: 800nm repetition: 78MHz Ti:Sapphire Laser probe BS pump THz radiation chopper (a) (b) Si BS Gap size: 5µm (b) (a) computer PC antenna Low-temperature grown GaAs substrate (LT-GaAs), Au electrodes

12. Experimental results 1 Reflection at the boundary between an LT-GaAs substrate and the air Fourier-transformed power spectrum of the THz electric field Many dips are caused by the vapor absorption. Monotonously decreasing spectral distribution from 0.1 to 25 THz was observed.

13. Experimental results 2 phonon absorption of CdSe absorption of CO2 Temporal waveform of the THz electric field Fourier-transformed power spectrum Frequency component up to 100THzwas observed.

14. Response function of monochromator and MCT detector Black-body radiation Experimental setup Si BS Intensity (a.u.) Monochromator CO2 absorption Response function MCT detector Black-body furnace at 500℃ Response of the system (a.u.) 95THz Response function of this system is calculated. This system has a sensitivity up to 95 THz.

15. THz radiation spectrum detected withmonochromator and MCT detector. Experimental setup THz radiation Si BS Monochromator 95THz GaSe crystal MCT detector THz radiation up to 95 THz was detected with monochromator and MCT detector.

16. Comparison of calibrated spectrum with experimental spectrum We calculate the calibrated spectrum with the response function. The calibrated spectrum and experimental one are alike. Smooth response of the PC antenna detector

17. w µ w × w N ( ) G ( ) D ( ) Estimating the sensitivity from a model calculation ・The photocurrent by incident THz radiation at a time delay t Fourier transform ・The currier number Fourier transform ・The photocurrent is described by noted above equations. D(ω)・G(ω) corresponds to sensitivity of PC antenna E(t) ：THz electric field, N(t)： the number of photocarriers G(t) ：shape of probe laser pulse, D(t) ： time response of carriers

18. d: width of probe beam Experimental data τd= 5fs τd= 10fs τd= 15fs Noise level Sensitivity of the PC antenna detector Sensitivity of the PC antenna detector Experimental data is consistent with calculated one. The pulse width is similar to that of this experiment laser. If more broad and strong THz radiation is obtained, PC antenna has the sensitivity over 130THz.

19. d: width of probe beam Experimental data τd= 5fs τd= 10fs τd= 15fs Noise level Sensitivity of the PC antenna detector Summary ・THz radiation up to 100THzwas observed with GaSe crystal emitter and PC antenna detector. ・PC antenna is capable of detecting THz radiationbeyond 100 THz.

20. . We obtained a monotonously decreasing spectral response of the PC antenna from 0.1 to 100 THz of PC antenna. First experiment: using PC antenna emitter and PC antenna detector Second experiment: using GaSe crystal and PC antenna detector