Dispersive Terahertz Gain of a Nonclassical Oscillator: Bloch Oscillation in Semiconductor Superlattices

Sekine, Norihiko; Hirakawa, Kazuhiko

doi:10.1103/physrevlett.94.057408

Cited by 87 publications

(74 citation statements)

References 25 publications

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“…Here, THz signals were governed by electron motion because the miniband width for heavy holes is very small and the optical absorption involving light holes is relatively weak. 8,10,11) The THz electric field was detected in the time domain through the Pockels effect of a 0.1-mm-thick (110) ZnTe crystal, which provided flat sensitivity up to ∼3.5 THz. We performed this measurement at various sample temperatures ranging from 80 to 298 K. Considering the known temperature-dependent band gaps of the host materials, 17) we adjusted the central photon energy of optical pump pulses so that electrons would be injected into nearly the same energy positions of the conduction first miniband at all temperatures.…”

mentioning

confidence: 99%

Capacitive response and room-temperature terahertz gain of a Wannier–Stark ladder system in GaAs-based superlattices

Naka

Hirakawa

Unuma

2016

Appl. Phys. Express

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We investigate the phase and terahertz (THz) gain of Bloch oscillations in GaAs-based superlattices at various temperatures of T = 80-298 K by using THz emission spectroscopy under bias electric fields. The transient current is found to start from its maximum nearly as damped cos ω B t (ω B /2π: Bloch frequency) throughout this temperature range, having only a small initial phase even for kT > ħω B (k: Boltzmann constant) and dephasing time shortening with increasing temperature. A spectral analysis indicates inversionless THz gain that originates from the capacitive nature of a Wannier-Stark ladder system with broadened energy levels at room temperature. © 2016 The Japan Society of Applied Physics I n the terahertz (THz) region, there has been a great need for gain media that allow for wide tunability and room-temperature operation of compact solid-state sources. 1) It is well known that kT c ≃ ħω (k: Boltzmann constant) provides the temperature T c above which the population inversion relevant to THz gain at photon energy ħω cannot be easily achieved in nanostructures such as quantum cascade lasers. 2,3) In a typical case where ω=2π ∼ 2 THz, T c is estimated to be as low as ∼100 K. Several theoretical studies have predicted that a Wannier-Stark ladder system in semiconductor superlattices (SLs) will have voltage-tunable THz gain without population inversion at not only low temperatures (kT < eFd) but also high temperatures (kT > eFd) up to room temperature, when carriers have moderate scattering rates under dc bias electric field F and SL period d. [4][5][6][7] Here, eFd is the energy separation between neighboring Wannier-Stark levels. This type of THz gain was also supported experimentally 8,9) and has since been demonstrated more directly by measuring the complex conductivity spectra of GaAs-based SLs at low temperatures. [10][11][12] In a previous study, 11) we revealed that the unique phase of transient Bloch oscillations (coherent quantum beats) observed at 10 K reflects the translationally symmetric distribution of electrons onto a Wannier-Stark ladder and their capacitive response to an optically switched stepfunction-like bias electric field; these features are equivalent to the existence of inversionless THz gain in the steady state, where Bloch oscillations are fully dephased. The capacitive response reported 11) produces a cos ω B t-like current for t > 0, with ω B =2π equal to eFd=h, under the step-function-like bias electric field. Although Bloch oscillations in themselves can be observed at higher temperatures as well, [13][14][15] it has never been examined how the oscillation phase and hence the THz gain in semiconductor SLs behave when kT goes beyond eFd with increasing temperature up to room temperature.In this paper, we report phase-sensitive THz measurements of Bloch oscillations in GaAs-based SLs at various temperatures ranging from 80 to 298 K. The transient current induced by the femtosecond optical excitation of electrons under bias electric field F was found to start from its maximum ...

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confidence: 99%

Capacitive response and room-temperature terahertz gain of a Wannier–Stark ladder system in GaAs-based superlattices

Naka

Hirakawa

Unuma

2016

Appl. Phys. Express

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“…In Ref. [22], the position of t = 0 was determined empirically by choosing a position which did not cause artificial jumps in the phase of the Fourier spectra of (THz-TDS). To determine the position of t = 0 more accurately, we adopted a newly developed method, the maximum entropy method (MEM) [23−25] .…”

Section: Methodsmentioning

confidence: 99%

Terahertz electromagnetic waves emitted from semiconductor investigated using terahertz time domain spectroscopy (Invited Paper)

Zhu¹,

Zhuang²

2011

中国光学快报

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Ultrafast electromagnetic waves radiated from semiconductor material under high electric fields and photoexcited by femtosecond laser pulses have been recorded by using terahertz time domain spectroscopy (THz-TDS). The waveforms of these electromagnetic waves reflect the dynamics of the photoexcited carriers in the semiconductor material, thus, THz-TDS provides a unique opportunity to observe directly the temporal and spatial evolutions of non-equilibrium transport of carriers within sub-picosecond time scale. We report on the observed THz emission waveforms emitted from GaAs by using a novel technology, the time domain THz electro-optic (EO) sampling, which has a bipolar feature, i.e., an initial positive peak and a subsequent negative dip that arises from its velocity overshoot. The initial positive peak has been interpreted as electron acceleration in the bottom of Γ valley in GaAs, where electrons have a light effective mass. The subsequent negative dip has been attributed to intervalley transfer from Γ to X and L valleys. Furthermore, the power dissipation spectra of the bulk GaAs in THz range are also investigated by using the Fourier transformation of the time domain THz traces. From the power dissipation spectra, the cutoff frequency for negative power dissipation (i.e., gain) under step electric field in the bulk GaAs can also be obtained. The cutoff frequency for the gain gradually increases with increasing electric fields up to 50 kV/cm and achieves saturation at approximately 1 THz at 300 K. Furthermore, based on the temperature dependence of the cutoff frequency, we find that this cutoff frequency is governed by the energy relaxation process of electrons from L to Γ valley via successive optical phonon emission.

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“…Finally, we transform [15] the denominators in (8) and rewrite ∆ǫ ⊥ ω = ǫ ⊥ ω − ǫ through the spectral density function [13,16]…”

Section: Basic Equationsmentioning

confidence: 99%

“…In contrast to this, for the wide miniband BSL, with the bandwidth 2T ≫ ε B , a negative differential conductivity, i.e. gain due to Bloch oscillations, was studied theoretically starting the 70s [5] (see further results and references in [6]) and demonstrated in recent experiments [7,8]. At the same time, a similar behavior of the THz response, including a crossover from gain to absorption regime with detuning energy shifted through the resonance, was reported for the BSL with tight-binding electronic states [9].…”

Section: Introductionmentioning

confidence: 99%

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Complex permittivity of a biased superlattice

Hernández‐Cabrera

Aceituno

Vasko

2008

Journal of Applied Physics

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Intersubband response in a superlattice subjected to a homogeneous electric field (biased superlattice with equipopulated levels) is studied within the tight-binding approximation, taking into account the interplay between homogeneous and inhomogeneous mechanisms of broadening. The complex dielectric permittivity is calculated beyond the Born approximation for a wide spectral region and a low-frequency enhancement of the response is found. A detectable gain below the resonance is obtained for the low-doped GaAs-based biased superlattice in the THz spectral region. Conditions of the stimulated emission regime for metallic and dielectric waveguide structures are discussed. The appearence of a localized THz mode due to BSL placed at the interface vacuumdielectric is described.

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Dispersive Terahertz Gain of a Nonclassical Oscillator: Bloch Oscillation in Semiconductor Superlattices

Cited by 87 publications

References 25 publications

Capacitive response and room-temperature terahertz gain of a Wannier–Stark ladder system in GaAs-based superlattices

Capacitive response and room-temperature terahertz gain of a Wannier–Stark ladder system in GaAs-based superlattices

Terahertz electromagnetic waves emitted from semiconductor investigated using terahertz time domain spectroscopy (Invited Paper)

Complex permittivity of a biased superlattice

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