Ultrabroadband terahertz generation using DAST single crystal APPLIED PHYSICS LETTERS 97, 021105-1 I.Katayama, R.Akai, M.Bito, H.Shimosato, K.Miyamoto, H,Ito and M.Ashida Ashida lab. Takuya Iwasa

Contents Introduction Motivation Experimental setup Results & discussion Summary My work

THz Spectroscopy System Time Domain Spectroscopy（THz-TDS） THz-TDS uses short pulses of broadband THz radiation. The transmitted THz electric field is measured coherently, which provides both high sensitivity and time-resolved phase information. t THz wave Probe laser E Time delay THz-TDS pump probe system Principle of THz-TDS

Electromagnetic wave Wavelength 30mm 3mm 300μm 30μm 3μm 300nm Submilimeter wave Far infrared Near infrared Micro wave optical 10GHz 0.1THz 1THz 10THz 100THz 1PHz Frequency broader THz region THz radiation is from 1011 to 1013Hz approximately. If we generate broader radiation, we can measure from THz region to optical region in continuity.

THz emitter A 400-μm-thick DAST crystal was used as nonlinear crystal. Difference frequency generation A 400-μm-thick DAST crystal was used as nonlinear crystal. A DAST crystal is an organic compound crystal and has large nonlinear optical coefficient.(非線形光学係数） Nonlinear crystal fs laser THz wave Fourier transform Intensity Intensity ω2 ω1 ω３ frequency frequency DAST crystal

THz detector Photoconductive antenna Current meter A μ: mobility of electron N: the number of carrier fs laser THz wave

Motivation To generate the terahertz wave using a DAST crystal by considering the refractive index dispersion. To detect the THz wave generated by a DAST crystal using monochromator and THz-TDS.

Experimental setup (Spectral Phase Interferometry for Direct Electric-field Reconstruction)

BaF2 (a positive dispersion medium) pulse image Positive chirping Short wavelength Positive dispersion medium Long wavelength Time lag Group velocity dispersion (郡速度分散)

Results & discussion The data of laser phase spectra observed using SPIDER. BaF2 thickness ―　0mm ―　2mm ―　4mm 0mm-thick BaF2 2mm-thick BaF2 0mm-thick BaF2 Long wavelength Short wavelength Negative chirping Positive chirping

Results & discussion BaF2 thickness ―　0mm ―　2mm ―　4mm The best of these data is the one of negative chirping.

Experimental setup Chirp mirror Ti:Sapphire laser BS Chopper Pump pulse BaF2 plate DAST 400mm Delay stage Band-pass filter PC antenna Probe pulse I/V amp lock-in amp PC

Results & discussion In this graph, very high frequency part is present.

Results & discussion BaF2 thickness ―　0mm ―　2mm ―　4mm The Fourier transform of the electric waveform. (dotted line)

Results & discussion DAST The energy is absorbed a lot. 0mm-thick BaF2 (Large positive dispersion) Short wavelength Short wavelength Long wavelength Long wavelength The region of generating High-frequency THz wave The energy is absorbed a lot.

Summary In the DAST crystal, the group velocity dispersion is important for the generation of the ultrahigh-frequency terahertz wave. A waveform is detected by using photoconductive antenna.

My work I try to measure a wave form by using this optical system when there is a sample in front of PC antenna.

Monochromator Concave mirror Diffraction grating Slit Slit HgCdTe and InAs Detector THz pulse

SPIDER法 (Spectral Phase Interferometry for Direct Electric-field Reconstruction) Sum frequency w (t) I (t) W W I (w) w Intense strongly-chirped pulse from Ti: sapphire amplifier Nonlinear Crystal t I (t) Interferogram Replica2 Replica1 Pulse to be measured and its duplication I (w) SPIDER interferogram w