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A.Schneider et al. OPTICS EXPRESS 5376/Vol.14,No.12(2006) Itoh Lab. Michitaka Bito

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

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A.Schneider et al. OPTICS EXPRESS 5376/Vol.14,No.12(2006) Itoh Lab. Michitaka Bito

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  1. 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

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

  3. THz region 0.1~10THz 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. 1GHz 10GHz 0.1THz 1THz 10THz 0.1PHz 1PHz electric wave light visible ray 300mm 30mm 3mm 300mm 30mm 3mm 300nm Telecommunication wavelength 1500~1560nm 1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV

  4. THz Time-domain Spectroscopy (THz-TDS) Beamsplitter Probe pulse • 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 Pump pulse Delay stage THz emitter THz detector Sample refractive index extinction coefficient complex dielectric constant absorption coefficient optical conductivity Various optical constants are obtained easily !

  5. Generation of THz pulse DAST (4-N ,N-dimethylamino-4’-N’- methyl stilbazolium tosylate) Difference-frequency generation (DFG) Pump pulse DFG Nonlinear optical crystal Gigantic nonlinear optical coefficient d (The highest of all known materials) dDAST:290 dZnTe :90 dGaAs:54 w3 =｜w1 - w2｜ w3 w1 w2 Intensity DFG An efficient emitter of THz pulse with DFG Frequency

  6. w3 Intensity w1 w2 Frequency Coherence length lc Coherence length lcis the parameter of velocity-matching between the optical and the THz pulse. velocity-matching condition is given by l: Optical wavelength ng(l): refractive index of group velocity n(l): refractive index l: length of nonlinear crystal w1,w2: angular frequency of optical pulse w3: angular frequency of THz pulse Coherence length lc is given by When lc is greater than l, THz pulse is generated efficiently.

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

  8. Contour plot of the coherence length lc(n,l) Coherence length lc in DAST Coherence length lc in ZnTe 750nm 1500nm 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. Combination of DAST and ZnTe Red:DAST (no coloring lc > 1.5 mm) Green:ZnTe (no coloring lc > 0.75 mm) The broadband THz frequency range is accessible with a fundamental wavelength near 1500 nm.

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

  10. Detection with DAST crystal Source crystal (DAST): 0.6mm thick Detection crystal (DAST): 0.69mm thick The oscillation persists for t > 1 ps in time-domain and the numerous dips in frequency-domain ・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. Influence of water vapor absorption This shows the spectrum from 1.3 to 4.8 THz is velocity-matched to optical pulses with 1.5 mm as theoretical calculation in DAST.

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

  12. 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.

  13. Detection of THz wave THz-induced lensing (TIL) Electro-optic (EO) sampling Probe pulses Photo Diodes Waveplate THz pulses EOC PBS Solid/dashed line: Probe beam diameter without/with THz field. Anisotropy of refractive index by THz pulses. (Pockels effect) 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。 Polarization of probe pulses vary from time variation of THz electric field. Analysis of polarization variation Time-domain spectrum of THz pulses

  14. Nonlinear dielectric polarization

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