High efficiency generation and detection of terahertz pulses using laser pulses at tele- communication wavelengths A.Schneider et al. OPTICS EXPRESS 5376/Vol.14,No.12(2006) Itoh Lab. Michitaka Bito

Outline Introduction Terahertz region THz Time-domain Spectroscopy (THz-TDS) Generation of THz pulse Coherence length l c Experimental results and discussion Contour plot of the coherence length l c (  ) Detection with DAST crystal Detection with ZnTe crystal Summary

Terahertz region There are many properties in THz region, for example intermolecular vibration, lattice oscillation, superconducting energy gap and so on. THz wave is absorbed by water. 3mm 30  m THz region 0.1~10THz 300  m3m3m 300nm30mm300mm 1THz10THz0.1PHz1PHz0.1THz10GHz1GHz visible ray 1THz = Hz ⇔ 300  m ⇔ 4.1meV electric wavelight Telecommunication wavelength 1500~1560nm

THz Time-domain Spectroscopy (THz-TDS) THz-TDS can be carried out at room temperature and ambient air. Temporal waveform is detected. The complex refractive index of the sample can be estimated without Kramers-Kronig analysis. fs pulse laser Beamsplitter Delay stage THz emitter Sample THz detector Various optical constants are obtained easily ! Pump pulse Probe pulse refractive indexextinction coefficient complex dielectric constant absorption coefficient optical conductivity

Generation of THz pulse Difference-frequency generation (DFG) Pump pulseDFG Nonlinear optical crystal Frequency Intensity 11 22 33  3 = ｜  1 -  2 ｜ DFG DAST (4-N,N-dimethylamino-4’-N’- methyl stilbazolium tosylate) Gigantic nonlinear optical coefficient d (The highest of all known materials) An efficient emitter of THz pulse with DFG d DAST :290 d ZnTe :90 d GaAs :54

Coherence length l c Coherence length l c is given by : Optical wavelength n g ( ): refractive index of group velocity n( ): refractive index l: length of nonlinear crystal  1,  2 : angular frequency of optical pulse  3 : angular frequency of THz pulse Coherence length l c is the parameter of velocity-matching between the optical and the THz pulse. velocity-matching condition is given by 11 22 33 Frequency Intensity When l c is greater than l, THz pulse is generated efficiently.

Motivation DAST has high conversion efficiency than ZnTe for generation. ZnTe is more versatile than DAST for detection because organic crystals like DAST suffer from fast degradation and limited thickness. The velocity-matched generation with DAST and detection with ZnTe at the wavelength 1500nm theoretically and experimentally Why DAST for generation? Why ZnTe for detection? Why 1500nm? Telecommunication wavelength range is from 1.5 to 1.56  m.

Contour plot of the coherence length l c (  ) Coherence length l c in DASTCoherence length l c in ZnTe 1500nm 750nm Telecommunication wavelength are suited for generation and detection with DAST. Frequency-doubled pulses is suited for velocity-matching at the 2 to 3 THz range in ZnTe. Red:DAST (no coloring l c > 1.5 mm) Green:ZnTe (no coloring l c > 0.75 mm) Combination of DAST and ZnTe The broadband THz frequency range is accessible with a fundamental wavelength near 1500 nm.

Experimental setup For EO sampling (ZnTe)For THz-induced lensing (DAST) DAST A fundamental wavelength is 1500 nm. At room temperature. Source crystal (DAST): 0.6mm thick Detection crystal (DAST): 0.69mm thick Source crystal (DAST): 0.6mm thick Detection crystal (ZnTe): 0.5mm thick

Detection with DAST crystal The oscillation persists for t > 1 ps in time-domain and the numerous dips in frequency-domain Influence of water vapor absorption ・ There are resonance frequencies of DAST at 1.1THz and 5.2THz. ・ A continuous spectrum is obtained in the central part of the spectrum from 1.3 to 4.8 THz. This shows the spectrum from 1.3 to 4.8 THz is velocity-matched to optical pulses with 1.5  m as theoretical calculation in DAST. Source crystal (DAST): 0.6mm thick Detection crystal (DAST): 0.69mm thick

Detection with ZnTe crystal ・ l c < l (above 3.5THz) ・ Absorption of ZnTe strongly increases above 4THz. The spectrum extends only to 4 THz. A continuous spectrum from 1.3 to 4 THz is observed. Source crystal (DAST): 0.6mm thick Detection crystal (ZnTe): 0.5mm thick

Summary Authors demonstrated The velocity-matched generation with DAST and detection with ZnTe at the wavelength 1500nm theoretically and experimentally. The broadband THz frequency range is accessible with wavelength of 1500 nm. (theoretically) A continuous spectrum from 1.3 to 4 THz is obtained by EO sampling. (experimentally) An agreement was obtained between theoretical calculation and experimental results.

Detection of THz wave THz-induced lensing (TIL)Electro-optic (EO) sampling Solid/dashed line: Probe beam diameter without/with THz field. THz 波 ( ガウシアン ) で屈折率が変化。 それが平凸レンズみたいな役割。 透過プローブ光の直径が変化。 解析して時間波形。 Probe pulses THz pulses Photo Diodes PBSEOC Waveplate Anisotropy of refractive index by THz pulses. (Pockels effect) Polarization of probe pulses vary from time variation of THz electric field. Analysis of polarization variation Time-domain spectrum of THz pulses

Nonlinear dielectric polarization