Coherent transition between shallow acceptor levels in Ge using the shaped THz pulses

1. M1 colloquium2013/10/9 Coherent transition between shallow acceptor levelsin Ge using the shaped THz pulses 未来物質領域 芦田研究室 M1 掃部 豊 指導担当教員 永井 正也、松原 英一 蓑輪 陽介、芦田 昌明

2. Contents Background Introduction ・optical responses intwo-level system ・CEP ・THz spectroscopy Previous work Sample ( Ge;Ga ) Motivation Experimental method ・Set up ・CEP control of THz pulse ・Re-emitted pulse Results & Discussion Summary & Future work

3. Background light-matter interaction in a two level system Optical absorption 光吸収 Past Coherent transition is occurred by Resonant photoexcitation Ε=hν 非共鳴光による励起 Δ= hν 誘導放出 Stimulated emission 共鳴光による励起 Few-cycle & ultrashort pulse generation technology electron Now Ε≠hν Non-resonant photoexcitation brew up coherent transition Δ= hν

4. Carrier Envelope Phase(CEP) Phase difference between Electric Field & Envelope 包絡線 Envelope cos型 sin型 電場 Electric Field Figure of CEP Few-cycle pulse shape is characterized by CEP CEP influence Non-linear phenomena introduction

5. Phase-dependent transition between two-level energy system r22 resonant frequency = 2.0THz Response vary by phase of incident pulse even if frequency component are the same. r11 introduction P=Re[m21r12]

6. Previous work Rb原子のラジオ波による2準位原子の遷移 Li et al. PRL (2010) Ti:Saffireレーザーによる原子ガスからの高次高調波発生 Baltiska et al. Science (2002)

7. THz spectroscopy THz spectroscopy Electronics Photonics THz region 1GHz 10GHz 100GHz 1THz 10THz 100THz 1PHz 10PHz 100PHz 1m 100mm 10mm 1mm 100μm 10μm 1μm 100nm 10nm 1nm 0.1nm microwave FIR MIR NIR UV X-ray γ-ray VIS 1THz ⇔wavelength：300μm ⇔energy ：4.1meV ⇔temperature：48K Sciences in THz region Charge carrier dynamics in Semiconductors Soft mode in Dielectric materials Superconducting gap in Superconductors Electromagnon in Multiferroics etc... Intermolecular vibration introduction

8. Sample ( Ge;Ga ) ideal material to study coherent phenomena using THz pulses ・Acceptor level binding energy = 12meV ・Energy diagram has been assigned by analogy with that of a Rydberg atom ・Technology of CEP control of intense THz pulse R T(300K) Absorption by free carrier (Drude response) VB (7-0) (8-02) C L-H T(10K) D (8+01) Sharp absorption near 2THz ⇒caused by acceptor level Rydberg atom THzabsorptionof Ge;Ga THzパルスのスペクトル

9. Motivation • Take notice of two-level system of Ge;Ga • → observe non-linear transmittance ofGe;Ga single crystal • by using intense few-cycle THz pulse. we observe coherent transition and non-perturbativeresponseof Ge;Ga by using systematically-varied intensity and shape of incident pulse. • we want to discuss interaction between • few-cycle electromagnetic pulse and matter purpose

10. Experimental set up Sample Ge:Ga t=0.5mm 10K (WGP) Ti:sapphirelaser (1kHz 0.5mJ 200fs) (WGP) EO detector (ZnTe) P5 P4 P3 P2 P1 LiNbO3 emitter (Generation of THz wave) (Detection of THz wave) Filter for controlling pulse shape

11. Control of THz pulse shape We insert these filter before sample to control shape of excitation pulse cos-like (reference) ②を挿入 sin-like Parallel plate stainless for CEPcontrol ①を2枚挿入 Bandpass filter(2THz) for multi-cycle pulse multi-cycle ② ①

12. Time domain spectroscopy Transmitted Pulse Incident Pulse Ein(t) Etra(t) Sample Out of the sample

13. Re-emitted pulse Fresnel loss Fresnel loss 分極を誘起 Induce polarization Incident Pulse Transmitted Pulse ＋ ＋ ＋ － － － Re-emitted pulse interfere incident pulse ＋ ＋ － － Reemitted pulse Power Spectrum Sample Itra Iin IRe absorption ー ＝ Reemitted pulse = Transmitted pulse – Incident pulse

14. multi-cycle excitation pulse Incident pulse Re-emitted pulse Reemission spectra results

15. multi-cycle excitation pulse Simulated Reemission The experimental results were almost reproduced by this calculations Simulated temporal Rabi-flopping （1） r22 r11 results P=Re[m21r12]

16. mono-cycle excitation pulse Parallel plate For the incident field amplitude of 16E0, the slow component is dominant. This experimental results contradicts the simulation (discussed later) results

17. mono-cycle excitation pulse Reemissionspectra for sin & cos like excitation pulse sin cos Reemission at 2.04 THz decreased with the amplitude of the incident pulse and the reemission component below 0.5 THz increased. results

18. mono-cycle excitation pulse Simulated Reemission • Maximum amplitude : 3.2KV/cm : 6.4 KV/cm 8E0 16E0 >8E0These results contradict the experimental results. →This simulation can’t be used. <8E0 No difference between these data. discussion

19. Tunnel ionization byintense Electric Field Blossey’scritical field fordonors in Ge8.9 kV /cm Potential energy By intense Electric Field (6.4 kV /cm for 16E0) Potential distort Carriers cannot exist as bound state but ionize. With external electric field Without external electric field transition between shallow acceptor levels disappear \ (Ref) Rotsaert E, Clauws P, Vennik J, 1989 J. Appl Phys. 65, 730 discussion

20. Summary Mono-cycle excitation pulse According to simulation, optical responses different by the two excitation pulses(sin and cos like) under 3.2KV/cmincident electric field. This contradicts the experimental results. competition between coherent transitions and ionization oscillator strength (probability of transitions between energy levels ) of D line of Ge:Ga（f=0.095) is very small (1s-2ptransition of hydrogen atom : f=0.42), → the ionization of the shallow acceptors occurred before the Rabi flopping. → So, Drude absorption caused by free carriers of low-frequency component became dominant.

21. Future plan Using intense incident electric field, an ionization of the shallow acceptors tend to occur before Rabi flopping. So, following samples are ideal. ・Ge;Sb (P± line : f=0.21) ・Coupled Quantum well ・large oscillator strength ・Rabi flopping occur before ionization Experimental result of linear spectra of Ge;Sb （cos likeexcitation pulse） 2P± ーLog(T) CB 2p m=0 2p m=±1 2p± 1s

22. 補足

23. Rabi oscillation resonant frequency Excited state ラビ振動数 v Ground state Stimulated emission Optical absorption

24. CEP control of THz pulse d TE Cos-like THz波 10mm 50×10×0.1(mm) Parallel plate stainless Sin-like We used these two pulses. 平行平板導波路のTEモードは パルスの群速度位相速度 ⇒パルスの位相をシフトさせる Ref. Nagai, Minowa, Ashida otst 2013(Kyoto) introduction

25. マルチサイクル励起光の場合 再放射波の計算結果は実験結果を再現している。 the amplitude of the reemission normalized by the incident field amplitude was gradually suppressed when the incident pulse intensity was increased. Furthermore, another reemission appeared at an amplitude of 16E0, which was in phase with the incident pulse. This indicates that complete Rabi flopping occurred for the multi-cycle pulses.