Ultrabroadband detection of terahertz radiation from 0.1 to 100THz with photoconductive antenna Itho lab M1 Taisuke katashima 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 What is a THz wave ? THz region:0.1~10THz Microwaves Visible X-ray γ-ray Frequency(Hz) 103 killo Example 106 mega Radio 109 giga Radar 101 2 tera 101 5 peta Optical Medical ・Frequency: 1THz=1012Hz ・Wavelength: 1THz ・Wavenumber 1018 exa 300µm 33cm-1 1021 zetta Application of THz ・THz Time-domain Spectroscopy (TDS) This system can detect the waveform of electric field. We can obtain the amplitude and phase of electric field by Fourier transform. Complex refractive index is able to be calculated The principle of THz-TDS THz wave At x=d n0d Ei ( ) E0 expi c n~( )d Et ( ) E0 expi c Et ( ) T ( ) Ei ( ) 2 Refractive index: n0 E t () ~n Sample 0 Ei () d x ( ) t ( ) i ( ) c c ln T ( ) ( ) n0 Extinction cofficient: ( ) Refractive index: n( ) 2d d Complexrefrctiveindex : n~( ) n( ) i ( ) THz –TDS setup splitter mirror 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. 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. 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. Experimental setup 2 Ti:Sapphire Laser delay stage probe BS pump chopper Si BS GaSe emitter PC antenna I/V amp lock-in amp computer Experimental setup 1 Ti:Sapphire Laser probe BS pulse width: 10fs, center wavelength: 800nm repetition: 78MHz pump chopper A THz radiation (a) Si BS (a) (b) Gap size: 5µm (b) I/V amp lock-in amp computer PC antenna Low-temperature grown GaAs substrate (LT-GaAs), Au electrodes Experimental results 1 Amplitude (a.u.) 100 10 Reflection at the boundary between an LT-GaAs substrate and the air -1 10-2 10-3 0 5 10 15 20 25 Frequency (THz) 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. 30 Experimental results 2 8 phonon absorption of CdSe Log intensity (a.u.) Intensity (a.u.) 6 4 2 0 -2 -4 -6 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Time (ps) absorption of CO2 0 20 40 60 80 100 120 140 160 Frequency (THz) Temporal waveform of the THz electric field Fourier-transformed power spectrum Frequency component up to 100THz was observed. Response function of monochromator and MCT detector Experimental setup Si BS Monochromator MCT detector Black-body furnace at 500℃ Response function of this system is calculated. Response of the system (a.u.) Intensity (a.u.) Black-body radiation CO2 absorption This system has a sensitivity up to 95 THz. Response function 95THz 60 70 80 90 100 110 Frequency (THz) THz radiation spectrum detected with monochromator and MCT detector. 30 Experimental setup Si BS Monochromator Intensity (a.u.) 25 THz radiation 20 15 10 95THz 5 0 GaSe crystal MCT detector 40 50 60 70 80 90 100 110 Frequency (THz) THz radiation up to 95 THz was detected with monochromator and MCT detector. Comparison of calibrated spectrum with experimental spectrum We calculate the calibrated spectrum with the response function. Log intensity (a.u.) Calibrated spectrum PC antenna detection 0 20 40 60 80 100 120 140 160 Frequency (THz) The calibrated spectrum and experimental one are alike. Smooth response of the PC antenna detector Estimating the sensitivity from a model calculation ・The photocurrent by incident THz radiation at a time delay t J (t ) e E (t ' ) N (t 't )dt' J ( ) E ( ) N ( ) Fourier transform ・The currier number N (t ) G( ) D( t )d N ( ) G ( ) × D( ) Fourier transform ・The photocurrent is described by noted above equations. J ( ) E ( ) D( ) G( ) 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 Sensitivity of the PC antenna detector d : width of probe beam 101 Experimental data 10-1 τd= 5fs Sensitivity (arb. units) Experimental data is consistent with calculated one. 100 10-2 The pulse width is similar to that of this experiment laser. τd= 15fs τd= 10fs 10-3 Noise level 10-4 0 20 40 60 80 100 120 140 160 Frequency (THz) Sensitivity of the PC antenna detector If more broad and strong THz radiation is obtained, PC antenna has the sensitivity over 130THz. d : width of probe beam 101 100 Experimental data 10-1 τd= 5fs Sensitivity (arb. units) Log intensity (a.u.) Summary 10-2 τd= 15fs τd= 10fs 10-3 Noise level 10-4 0 20 40 60 80 100 120 140 160 Frequency (THz) 0 20 40 60 80 100 120 140 160 Frequency (THz) Sensitivity of the PC antenna detector ・THz radiation up to 100THz was observed with GaSe crystal emitter and PC antenna detector. ・PC antenna is capable of detecting THz radiation beyond 100 THz. . 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
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