Simple strategy for enhancing terahertz emission from coherent longitudinal optical phonons using undoped GaAs/n-type GaAs epitaxial layer structures

Transcription

1 Presented at ISCS21 June 4, 21 Session # FrP3 Simple strategy for enhancing terahertz emission from coherent longitudinal optical phonons using undoped GaAs/n-type GaAs epitaxial layer structures Hideo Takeuchi and Junichi Yanagisawa Department of Electronic Systems Engineering, The Univ. of Syuuichi Tsuruta and Masaaki Nakayama Department of Applied Physics, Graduate School of Eng., Osaka City Univ. Hisashi Yamada and Masahiko Hata Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd #Outline (A) Motivation The reason why we focus our attention on terahertz (THz) waves. (B) Strategy for producing the THz waves from the longitudinal optical (LO) phonon. (C) Experimental results. (D) Summary.

2 THz spectroscopy: Why is THz spectroscopy attractive? #. THz waves frequency range between infrared light and microwave. high sensitivity for the water concentration in materials. Shifting from the research stage to the industrial/commercial stage: e.g., imaging technologies. THz transmittance images of the two leaves: freshly cut leaf and the same leaf after 48 hours B. B. Hu and M. C. Nuss Opt. Lett. 2 (1995) Clear difference in freshness between the two leaves Imaging application Fig. 1 Spatial resolution?

3 Resolution in conventional THz imaging systems Fig. 2: M. Herrmann et al., in Terahertz Optoelectronics ed. K. Sakai (Springer, 25) p.339 (Fig. 5). Spatial resolution: ~ 1 mm at most Possible factors (1) Diffraction limit of the terahertz wave (2) Chromatic aberration What is the origin?

4 Responsible factors hidden in conventional THz emitters #. Conventional THz emitter: Dipole antenna - μmgap e + Fig. 3 Antenna pattern formed on a low-temperaturegrown GaAs epitaxial layer #. Emission mechanism of the THz wave Excitation of the femtosecond laser pulses Generation of the surge current of photogenerated carriers j(t) Fig. 4: Typical spectral profile (dipole antenna). M. Tani et al., Appl. Opt. 3 (1997) 7853 Peak position: ~ 1. THz = 3 μm. Diffraction limit E THz ( t) t j( t) Broad spectral profile Chromatic aberration Solution for improving resolution: High frequency monochromatic emitter

5 The earlier strategy #. THz emitter with use of coherent optical phonons (»1 THz) In general, THz waves from coherent phonons are weak in bulk crystals. # The earlier proposal Application of (GaAs) m /(AlAs) m multiple quantum wells (MQWs). M. Nakayama et al., Appl. Phys. Express 1 (28) 124. Room Temperature #. Strategy Symmetry breaking of the GaAs-like LO phonons at each GaAs/AlAs interface. Impulsive interference of the heavy-hole (HH) and light-hole (LH) excitons. Driving force of the coherent LO phonon. #. Restriction The photon energy of the pump beam should be tuned the center energy between the HH and LH exciton energy spacing. Fig. 5: THz-wave spectra from the MQWs. Limiting the pump photon energy.

6 The present strategy i-gaas/n-gaas epitaxial layer structures.8 i-gaas(5 nm)/n-gaas sample #. Sample structure.6 i-gaas n-gaas i-gaas (d nm)/n-gaas(3μm, cm -3 ) Simple structure. Energy (ev).4.2 Surface Fermi level pinning Features: Upward band bending caused by the surface Fermi level pinning ~E g / Distance from the Surface (nm) Fig. 6: Potential structure of the i-gaas/n-gaas sample. Liner potential slope: Uniform built-in electric field. Two advantages: (1) An increase in initial displacements of the constituent atoms. Enhancement of the THz emission of the coherent LO phonon. (2) Sweeping-out effects on carriers. Reduction of the free-carrier absorption of the THz wave.

7 We are the experimental researchers! Confirmation using photoreflectance (PR) spectroscopy Lamp Monochromater Probe beam Pump beam Pump Beam on off LASER ΔR Chopper Photodiode Lock-in Amp. Reflectivity, R Lapsed Time Time ΔR R DC Voltmeter Fig. 7: Schematic view for the PR measurement apparatus. Photoreflectance (PR) spectroscopy Measurement of the reflectivity change ΔR induced by the pump-beam illumination.

8 Confirmation of the formation of the built-in electric field Photoreflectance (PR) measurement (a) RT 8 Appearance of the Franz-Keldysh oscillations (FKOs). ΔR/R (normalized) i-gaas layer thickness d = 5 nm d = 8 nm d = 12 nm Evidence for the presence of the built-in electric field. Estimation of the built-in electric field from the FKOs Photon Energy (ev) Fig. 8: PR spectra of the i-gaas/n-gaas samples at room temperature (RT). The j-th extrema position, hω j, is expressed by the following equation: 3π hω j = hθ j /3 + E g with hθ e hθ 2 2 h F 2μ E g : Fundamental transition energy; : Electro-optical constant; μ: interband reduced effective mass (μ =.556 m for GaAs); F: Built-in electric field. 2 1/3

9 Built-in electric field in the i-gaas layer Photon Energy (ev) 1.5 (b) d = 5 nm d = 8 nm d = 12 nm Table I: Electric field in the i-gaas layer Sample d = 5 nm d = 8 nm d = 12 nm F (kv/cm) * F (kv/cm) * *1 : from the FKOs (experimental value) *2 : from the simulation (calculation value) ξ = [(3π/4) (j-1/2)] 2/3 Fig. 9: Plots of the extrema of the FKOs from the i-gaas(d nm)/n-gaas samples as a function of quasi-index ξ. : H. Takeuchi et al., JAP 97 (25) The built-in electric field in the i-gaas layer is enhanced with a decrease in d. Slope of the solid line = hθ 2 2 e h F 2μ 2 1/3 The built-in electric field is controllable.

10 Experimental Procedure & Samples #. Condition Ti:sapphire pulse Laser Pulse-duration time: 7 fs Lock-in detection Room temperature Scan range: -2 to 8 ps #. Sample i-gaas/n-gaas structures d = 5, 8, and 12 nm Gate pulse Off-axis parabolic mirror Reflected pulse THz wave Pump pulse Sample Power: 4 mw Photon energy: 1.57 ev Off resonant condition. Si filter Optically gated dipole antenna w/o the calibration of frequency-dependent sensitivity. Off-axis parabolic mirror Fig. 1: Setup of the optical components for the THz emission measurement..

11 THz waves from the i-gaas/n-gaas samples 1 i-gaas layer thickness d = 5 nm Monocycle oscillation around the time delay of ps the first burst Amplitude (pa) 5 d = 8 nm d = 12 nm The first burst is followed by the oscillation patterns with a period of 113fs. 113 fs 8.8 THz Frequency of the GaAs LO phonon Time Delay (ps) Fig. 11: THz waveforms from the samples. 8.8 THz: Corresponding wavelength = 34 μm. Improvement of the spatial resolution. The amplitude increases with a decrease in the i-gaas layer thickness d. A decrease in the i-gaas layer thickness d Enhancement of the built-in electric field The present strategy is reasonable.

12 Fourier power spectra of the THz waveforms Intensity (arb. units) d = 5 nm d = 8 nm #. Coherent GaAs LO phonon band The peak intensity of the coherent GaAs LO phonon band increases with a decrease in d. In d = 5 nm. the peak intensity exceeds that of the first burst band. d = 12 nm 5 1 Frequency (THz) Fig.12: Fourier power spectra of the THz waveforms shown in Fig. 11. In general, the peak intensities of optical phonon bands are much weaker than that of the first burst band. Taking account of the fact that the sensitivity of the detector (dipole antenna) drastically drops in the frequency range higher than several THz, The present strategy leads to the development of the intense monochromatic THz emitter.

13 Dynamical aspect of the THz waves from the LO phonon Intensity (arb. units) d = 5 nm 5 1 Frequency (THz) Time Window [τ ps, 8 ps] [-2 ps, 8 ps] [ ps, 8 ps] [1 ps, 8 ps] [2 ps, 8 ps] Fig. 13: Time-partitioning Fourier transform spectra of the i-gaas(5 nm)/n-gaas sample. #. Time-partitioning Fourier Transform 8ps I ( ω, τ ) = A( t) exp( iωt) dt τ Useful way to estimate the decay time. #. The first burst band Rapidly decay before ~ 1. ps. #. The coherent LO phonon band Still remains at 2. ps. 2 The duration time of the terahertz wave from the coherent LO phonon is much longer than that of the terahertz wave from the first burst.

14 Estimation of the decay time of the coherent LO phonon #. Decay rate of the LO phonon band ~.5 ps -1, regardless of the sample structure. Peak Intensity (arb. units) d = 5 nm d = 8 nm d = 12 nm τ (ps) Fig. 14: Peak intensities of the LO phonon band plotted as a function of the time delay τ. Comparison with the earlier work Cho et al., PRL 65 (199) 764. #. Time-partitioning Fourier Transform 8ps I ( ω, τ ) = A( t) exp( iωt) dt τ Square of the amplitude of the THz wave. Decay time of the THz wave from the coherent LO phonon 2. ps Much longer than that of the THz wave from the first burst band. The components of the terahertz-wave signals can be divided by the control of the time delay. 2 The same value The decay time is no related with the epitaxial layer structure. Leading to the novel concept for setting up the following THz imaging system with highly spatial resolution.

15 Proposal: THz imaging system with highly spatial resolution Optical delay line Femtosecond-pulse laser system i-gaas/n-gaas THz emitter λ: 34 μm THz wave Sample Polarizer Analyzer CCD Camera EO crystal High sensitivity up to ~ 1 THz. Fig. 15: Schematic view of the proposed THz imaging system. Scan range of the optical delay line within the range that the coherent LO phonon appears. Enables highly spatial resolution (~.1 mm).

16 Summary We have investigated the feasibility of enhancing the THz wave from coherent LO phonon with use of the i-gaas(d nm)/n- GaAs simple epitaxial layer structures. The THz wave from the coherent GaAs LO phonon is enhanced by the decrease in d leading to the enhancement of the built-in electric field. Intensity (arb. units) d = 5 nm d = 8 nm In d = 5 nm, the intensity of the GaAs LO phonon band exceeds that of the first burst band. The key findings in the present work. d = 12 nm 5 1 Frequency (THz) Fig. 17: Fourier power spectra of the THz waveforms shown in Fig. 11. We, therefore, conclude that the present simple strategy opens the way to the realization of the THz imaging system with highly spatial resolution.