Takeya Unuma

Intersubband absorption linewidth in GaAs quantum wells due to scattering by interface roughness, phonons, alloy disorder, and impurities

We calculate the intersubband absorption linewidth 2Γop in quantum wells (QWs) due to scattering by interface roughness, LO phonons, LA phonons, alloy disorder, and ionized impurities, and compare it with the transport energy broadening 2Γtr=2ℏ/τtr, which corresponds to the transport relaxation time τtr related to the electron mobility μ. Numerical calculations for GaAs QWs clarify the different contributions of each individual scattering mechanism to the absorption linewidth 2Γop and transport broadening 2Γtr. Interface roughness scattering contributes about an order of magnitude more to the linewidth 2Γop than to the transport broadening 2Γtr, because the contribution from the intrasubband scattering in the first excited subband is much larger than that in the ground subband. On the other hand, LO phonon scattering (at room temperature) and ionized impurity scattering contribute much less to the linewidth 2Γop than to the transport broadening 2Γtr. LA phonon scattering makes comparable contributions to ...

Effects of interface roughness and phonon scattering on intersubband absorption linewidth in a GaAs quantum well

We experimentally and theoretically study the effects of interface roughness and phonon scattering on intersubband absorption linewidth in a modulation-doped GaAs/AlAs quantum well. Quantitative comparisons between experimental results and theoretical calculations make it clear that interface roughness scattering is the dominant scattering mechanism for absorption linewidth in the temperature range below 300 K. Even at room temperature, phonon scattering processes contribute little to linewidth, while polar-optical phonon scattering limits electron mobility.

Femtosecond acceleration of electrons under high electric fields in bulk GaAs investigated by time-domain terahertz spectroscopy

We have investigated terahertz emission from undoped bulk GaAs photoexcited by femtosecond laser pulses under strong bias electric fields, F, up to 300kV∕cm. The initial positive signal in the terahertz waveforms, which is usually interpreted as electron acceleration in the Γ-valley, is found to increase with increasing F for F<50kV∕cm, but to start decreasing gradually above 50kV∕cm. The observed behavior suggests that the effective acceleration mass of electrons significantly increases with increasing F. The mass enhancement is due most likely to band mixing between the Γ- and the higher satellite valleys under high electric fields.

Intersubband transition energy and linewidth modified by a submonolayer AlAs insertion into GaAs quantum wells

We demonstrate a modification of the energy and linewidth of intersubband transitions by the hetero-insertion of a submonolayer into semiconductor quantum wells (QWs). Different changes in the transition energy and absorption linewidth are observed with systematically varied positions of a 0.92-monolayer AlAs insertion into 7.0-nm-wide GaAs QWs. The experimental results are well reproduced by theoretical calculations taking into account energy shifts and scattering processes in electron subbands due to the hetero-insertion. Our findings suggest that the combination of the insertion position and well width can be used to separately tune the transition energy and absorption/emission linewidth for optoelectronic devices.

Terahertz complex conductivities of carriers with partial localization in doped polythiophenes

We have investigated the complex conductivity spectra σ(ω) of two p-doped polythiophenes—poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene)—with various carrier densities by using terahertz time-domain spectroscopy. The real part of σ(ω) is found to gradually decrease with decreasing frequency ω and to approach a finite value for ω→0 unlike the Drude conductivity behavior, suggesting that carriers in polythiophenes have a partially localized nature. By reproducing both the measured real and imaginary parts of σ(ω) with the Drude–Smith model, we show that carriers become less localized with increasing carrier density up to ∼1.8×1020 cm−3.

Dephasing of Bloch oscillating electrons in GaAs-based superlattices due to interface roughness scattering

We have investigated dephasing mechanisms of Bloch oscillating electrons in GaAs-based superlattices (SLs) by time-domain terahertz electro-optic sampling method. It was found that dephasing time τr for GaAs∕AlAs SLs in the Wannier-Stark regime rapidly becomes shorter with decreasing well width Lw as τr∝Lw3−5. The observed strong Lw dependence of τr indicates that the dominant dephasing mechanism is interface roughness scattering, which was indeed confirmed by quantitative comparison between theory and experiment. It was also found that alloy disorder scattering is negligibly weak even in the case of GaAs∕Al0.3Ga0.7As SLs.

Power dissipation spectra and terahertz intervalley transfer gain in bulk GaAs under high electric fields

We have investigated terahertz radiation emitted from electrons photoexcited by femtosecond laser pulses in bulk GaAs under strong bias electric fields. Power dissipation spectra of electrons for step-function-like input electric fields have been obtained by calculating Fourier spectra of the measured terahertz traces. The cutoff frequency νc for negative power dissipation (i.e., gain) due to intervalley transfer is found to gradually increase with increasing electric fields below 50 kV/cm and saturate at ∼1 THz above 50 kV/cm at 300 K. From the temperature dependence of νc, it is found that νc is governed by the emission of optical phonons.

Direct observation of carrier delocalization in highly conducting polyaniline

We investigate the complex conductivity spectra σ(ω) of doped polyaniline, prepared as a typical conjugated polymer with various dc electrical conductivities up to 1000 S/cm, by using terahertz transmission spectroscopy. With the increase in dc conductivity, the imaginary part of σ(ω) is found to definitely change its sign from negative to positive throughout the measured frequency range. This shows that the optoelectronic nature of carriers in conducting polyaniline shifts from the localized regime to the delocalized regime. The real and imaginary parts of σ(ω) can be simultaneously reproduced by an extended Drude model, giving the degrees of carrier (de)localization.

Dispersion relations for evaluating the complex refractive index of medium without the information of its thickness

A general method to obtain the complex refractive index of a medium from absorbance, or alternatively from optical path length data, without knowing the sample thickness is proposed. The method can be formulated in any spectral range and it is here applied particularly in the terahertz spectral range to both simulated and experimental data. The key idea is the derivation of nonconventional dispersion relations that partly resemble traditional Kramers-Kronig relations. The method is shown to work well in extracting the complex refractive index of a drug system and a precipitated calcium carbonate.

Determination of the time origin by the maximum entropy method in time-domain terahertz emission spectroscopy

We have developed a scheme for determining the time origin by the maximum entropy method (MEM) in time-domain terahertz (THz) emission spectroscopy. By applying the MEM to trial damped sinusoidal waveforms, we confirmed that the MEM gives true phase shifts across the resonance features and that its inherent uncertainty in determining the time origin is +/-15 fs for 100-fs-class excitation/sampling optical pulses. Furthermore, when the MEM was applied to a THz waveform recorded experimentally with a finite sampling interval for the Bloch oscillation in a semiconductor superlattice, a misplacement of the time origin was indeed detected with an accuracy limited by the worse of the MEM inherent uncertainty and the sampling interval.