1. Ultrabroadband detection of THz radiation and the sensitivity estimation of photoconductive antenna Itoh lab Michitaka Bitoh H. Shimosato et al. Ultrafast Optics V, (Springer, Berlin, 2007) pp. 317-323

2. Outline Introduction THz region THz time-domain spectroscopy (THz-TDS) THz emission THz detection Motivation Experimental results and discussions ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna Summary

3. Terahertz region 3mm30mm300mm3mm30mm 1THz = 10 12 Hz ⇔ 300mm ⇔ 4.1meV THz region 0.1~100THz 1THz10THz100THz0.1THz10GHz light ・ intermolecular vibration ・ lattice oscillation ・ superconducting gap etc In THz region 6K 17.5K 24K 27K 30K 33K30K<T c <33K In THz region, a depletion of the real part of the conductivity which correspond to the opening of a superconducting gap can be observed like this. Real part of conductivity σ 1 (ω) for the MgB 2 film normalized to its normal state value σ 1N (40K). electric wave

4. THz Time-domain Spectroscopy (THz-TDS) fs pulse laser Beamsplitter Delay stage THz emitter Sample THz detector Pump pulse Probe pulse Complex refractive index From n and κ, many optical coefficient can be obtained. Amplitude( 振幅 ) phase( 位相 ) Fresnel coefficients t vs, t sv

5. THz emission from PC antenna fs laser pulse Bias transient current J (t) Forward THz wave Backward THz wave Decay time of J : ~100fsE THz (ω):~10THz (LT-GaAs substrate)

6. THz emission via DFG DFG (Difference-frequency generation) ・・・差周波発生 THz wave I(t,ω,Δω) E THz (t,ω THz ) NL crystalfs laser pulse ∬ Intensity 0 Frequency spectrum of fs pulse laser frequency 0 Intensity Δω Frequency spectrum of THz wave from NL crystal Differential frequency generation ω 3 =0 ⇒ optical rectification Δω Integration of different frequency frequency

7. THz detection using PC antenna fs laser pulse THz wave A t = t1 t = t2 > t1t = t3 > t2 t : time delay transient current J(t)

8. Motivation Authors demonstrated ultrabroadband detection from 0.1 to 100 THz using the PC antenna detector, in two generation method. Investigation of the sensitivity of the PC antenna for mid- infrared region. THz region 0.1~100THz 1THz10THz100THz0.1THz10GHz electric wavelight MgB 2 :Δ=3.4THz YBCO:Δ=4.9THz superconducting gap lattice vibration spectral range of THz-TDS

9. Outline Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation Experimental results and discussions Ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna Summary

10. Setup A 5um 400um LT-GaAs substrate emitterdetector 10fs

11. The PC antenna emission and detection Water vapor absorption The reflection of THz radiation by phonon (between LT-GaAs and the air) THz radiation from 0.1 to 25 THz was observed monotonously (except around 8 THz)

12. DFG generation and the PC antenna detection Phonon absorption of GaSe crystal Absorption by CO 2 THz radiation was observed up to 100THz (except for the frequency range from0.1 to 10 THz)

13. Outline Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation Experimental results and discussions ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna Summary

14. The PC antenna sensitivity (experimental date) Response function of this system This system can detect the frequency up to 95 THz. The response around 70 THz is distorted by the absorption by CO 2.

15. The PC antenna sensitivity (experimental date) Calibrated power spectrum of the THz radiation Extracted power spectrum from… Because of the absorption of CO 2, this region is unreliable. Calibrated power spectrum of the THz radiation was obtained. From (a) and (b), the sensitivity of PC antenna can be calculated.

16. The PC antenna sensitivity (model calculation) The current by the incident THz radiation at delay time t is described as Correspond to the sensitivity According to the convolution theorem of the Fourier transformation: I(t): the shape of probe laser pulse R(t): time response of carriers The carrier number N(t) is described by the following equation: This is transformed into the following equation: E(t): the incident electric field N(t): number of photocarriers e: the elementary electric charge μ: the electron mobility σ: optical conductivity

17. The PC antenna sensitivity (model calculation) Authors assumed the shape of probe pulse as the following equation τ d : the pulse width R(t) is estimated from the transient reflection measurement. I(t) and R(t) are transformed into I(ω) and R(ω) respectively 10fs τ d =10(fs)

18. Experimental date and calculated one Good agreement between experimental data and calculated one with τ d =10fs. Noise level This results indicates the feasibility of ultrabroadband detection over 130 THz using the PC antenna if broader and stronger THz radiation obtained. By using 5fs pulse laser, the more sensitive and broadband detection is expected with the PC antenna.

19. Summary By using the PC antenna for detection, spectrum response from 0.1 to 100 THz except around 8 THz was obtained using a combination of PC antenna emitter and a GaSe crystal. The sensitivity of the PC antenna from a model calculation shows that the PC antenna is capable of detecting ultrabroadband THz radiation beyond 100 THz with an advanced setup.

20. My study By using 5fs pulse laser, the more sensitive and broadband detection is expected with the PC antenna. Time (fs) Intensity (a.u.)

21. My study DAST (4-N,N-dimethylamino-4’-N’- methyl stilbazolium tosylate)

22. My study Absorption of DAST itself THz radiation from 100 to 170 THz was obtained

23. My study There is good agreement between experimental data and model calculation when the pulse duration is 5fs. Future plan is ・・・ ・ To detect THz wave at broader frequency range with this system ・ To measure something with this system