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M1 colloquium 2013/10/9 Coherent transition between shallow acceptor levels in 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
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Contents Background Introduction Previous work Sample ( Ge;Ga )
・optical responses in two-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
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Background light-matter interaction in a two level system Past Now
Optical absorption 光吸収 Past Coherent transition is occurred by Resonant photoexcitation electron Ε=hν Δ= hν Stimulated emission 誘導放出 共鳴光による励起 Few-cycle & ultrashort pulse generation technology Now Ε≠hν Δ= hν Non-resonant photoexcitation brew up coherent transition 非共鳴光による励起
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Carrier Envelope Phase(CEP)
Phase difference between Electric Field & Envelope CEP:𝜑 包絡線 Envelope 𝑣 𝑔 𝑬 𝟎 𝒆𝒙𝒑 − 𝒕 𝟐 𝝉 𝟐 𝐜𝐨𝐬(𝝎𝒕−𝝋) 𝝋= 𝝅 𝟐 𝝋=𝟎 sin型 cos型 電場 Electric Field 𝑣 𝑝 Figure of CEP Few-cycle pulse shape is characterized by CEP CEP influence Non-linear phenomena introduction
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Phase-dependent transition between two-level energy system
P=Re[m21r12] resonant frequency = 2.0THz Response vary by phase of incident pulse even if frequency component are the same. introduction
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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)
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THz spectroscopy THz spectroscopy introduction 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 Superconducting gap in Superconductors Charge carrier dynamics in Semiconductors Soft mode in Dielectric materials Sciences in THz region Intermolecular vibration Electromagnon in Multiferroics etc... introduction
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Sample ( Ge;Ga ) Absorption by free carrier (Drude response)
ideal material to study coherent phenomena using THz pulses THz absorption of Ge;Ga THzパルスのスペクトル D C VB (7-0) (8-02) (8+01) ・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) L-H T(10K) Sharp absorption near 2THz ⇒caused by acceptor level Rydberg atom
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Motivation we observe coherent transition
Take notice of two-level system of Ge;Ga → observe non-linear transmittance of Ge;Ga single crystal by using intense few-cycle THz pulse. we observe coherent transition and non-perturbative response of 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
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Experimental set up (WGP) EO detector (ZnTe) P5 P4 P3
LiNbO3 emitter (Generation of THz wave) Ti:sapphire laser (1kHz 0.5mJ 200fs) P2 P4 P5 (WGP) Sample Ge:Ga t=0.5mm 10K Filter for controlling pulse shape (Detection of THz wave)
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Control of THz pulse shape
cos-like sin-like multi-cycle ① ② ②を挿入 ①を2枚挿入 (reference) Bandpass filter(2THz) for multi-cycle pulse Parallel plate stainless for CEP control We insert these filter before sample to control shape of excitation pulse
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Time domain spectroscopy
Transmitted Pulse Incident Pulse Ein(t) Etra(t) Sample Out of the sample
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Re-emitted pulse Itra Iin IRe ー ＝ Re-emitted pulse
分極を誘起 Fresnel loss Fresnel loss Induce polarization Incident Pulse Transmitted Pulse ＋ ＋ ＋ － － － Re-emitted pulse interfere incident pulse ＋ ＋ － － Power Spectrum Sample Reemitted pulse Itra Iin IRe absorption ー ＝ Reemitted pulse = Transmitted pulse – Incident pulse
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multi-cycle excitation pulse
Incident pulse Re-emitted pulse Reemission spectra results
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multi-cycle excitation pulse
Simulated Reemission The experimental results were almost reproduced by this calculations Simulated temporal 𝜌 22 Rabi-flopping （ 𝜌 22 =1） r22 r11 P=Re[m21r12] results
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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
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mono-cycle excitation pulse
Reemission spectra 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
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mono-cycle excitation pulse
Simulated Reemission Maximum amplitude 8E0 : 3.2 KV/cm : 6.4 KV/cm 16E0 >8E0 These results contradict the experimental results. →This simulation can’t be used. <8E0 No difference between these data. discussion
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Tunnel ionization by intense Electric Field
Blossey’s critical field for donors in Ge 8.9 kV /cm \ Potential energy Without external electric field With external electric field By intense Electric Field (6.4 kV /cm for 16E0) Potential distort Carriers cannot exist as bound state but ionize. 電場がかかっている場合には分子のポテンシャルは電場により大きく歪みます。図では左側のポテンシャルが上がり、右側のポテンシャルが下がっています。となると右側のポテンシャル障壁を透過して電子が飛び出しイオン化します。これをトンネルイオン化と呼んでいます。さらにポテンシャルが歪むと、障壁が電子のいるポテンシャルより下がります。こうなると電子はこぼれ出すようにイオン化してしまいます。 transition between shallow acceptor levels disappear (Ref) Rotsaert E, Clauws P, Vennik J, 1989 J. Appl Phys. 65, 730 discussion
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Summary Mono-cycle excitation pulse
According to simulation, optical responses different by the two excitation pulses(sin and cos like) under 3.2KV/cm incident 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-2p transition 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.
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Future plan ・Ge;Sb (P± line : f=0.21) ・Coupled Quantum well
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 ーLog(T) 1s 2p m=±1 2p m=0 CB 2p± Experimental result of linear spectra of Ge;Sb （cos like excitation pulse） 2P±
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補足
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Rabi oscillation Excited state Ground state Optical absorption
resonant frequency Excited state ラビ振動数 v Ground state Optical absorption Stimulated emission
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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
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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.
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