Terahertz Conductivity and Possible Bloch Gain in Semiconductor Superlattices

Shimada, Yozo; Hirakawa, Kazuhiko; Odnoblioudov, Maxim; Chao, K. A.

doi:10.1103/physrevlett.90.046806

Cited by 89 publications

(72 citation statements)

References 13 publications

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“…As mentioned earlier, the ω Bresonant capacitive response of electrons distributed onto the Wannier-Stark ladder without population inversion has been observed at 10 K with an initial phase of α = 0. 11) In the present case, this nature of the Wannier-Stark ladder system turned out to be preserved up to room temperature with sufficiently small initial phase, which is comparable to the value of ω B δt = ±0.22 rad given by the temporal uncertainty δt = ±15 fs in determining the time origin.Because electrons were subjected to an optically switched step-function-like bias electric field in our measurement scheme, 8,11) we can obtain the spectral shape of complex conductivity by performing the Fourier transform of the observed THz waveforms (Fig. 2).…”

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confidence: 99%

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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%

“…Because electrons were subjected to an optically switched step-function-like bias electric field in our measurement scheme, 8,11) we can obtain the spectral shape of complex conductivity by performing the Fourier transform of the observed THz waveforms (Fig. 2).…”

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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“…10 Recently, nonlinear dynamics involving electron transport in semiconductor superlattice driven by an intense terahertz electromagnetic irradiation has been a central focus of theoretical and experimental studies. [11][12][13][14][15][16][17][18] Keay et al 13 observed the photon-assisted tunneling and oscillatory dependence of the photon induced currents on terahertz electric field. It was reported that current through a biased GaAs/ AlAs superlattice is reduced when exposed to an intense terahertz field.…”

Section: Introductionmentioning

confidence: 99%

“…It was reported that current through a biased GaAs/ AlAs superlattice is reduced when exposed to an intense terahertz field. 14 Moreover, terahertz emission 15 and possible terahertz gain 16 can be achieved in miniband superlattices. The recent development in terahertz quantum cascade lasers is based on the nonlinear transport and electron-phonon interaction in superlattices [19][20][21] These studies are motivated by the promising properties offered by superlattices for terahertz optoelectronic device applications.…”

Section: Introductionmentioning

confidence: 99%

Noise temperature spectrum of hot electrons in semiconductor superlattices

Wang

Cao

Zhang

2009

Journal of Applied Physics

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The small signal response and thermal noise spectra in miniband superlattice are investigated. The properties of hot electron differential mobility, velocity fluctuation, and noise temperature are determined around a stationary condition. The field and frequency dependent drift velocity, electron energy, effective mass, and electron temperature are obtained. At low frequencies, noise temperature increases rapidly with the electric field. Our calculated noise temperatures for miniband superlattice are in good agreement with the experimental results, with the sample thickness estimated to be around 4 m.

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“…The THz pulse was observed in mirror reflection geometry. This concept was used to study physical concepts like Bloch oscillations 42 or quantum beats in optical transitions to neighboring Wannier-Stark levels and Bloch gain in artificial superlattices. 43,44 Other groups studied the "velocity overshoot" in the coherent ballistic transport of carriers simultaneously generated by fs-laser pulses, 45 or the transient current generated in the surface layer of III-V semiconductors due to surface band bending, 46 or due to the Dember effect.…”

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confidence: 99%

Tunable, continuous-wave Terahertz photomixer sources and applications

Preu

Döhler

Malzer

et al. 2011

Journal of Applied Physics

431

293

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Realization of gigahertz-frequency impedance matching circuits for nano-scale devices Appl. Phys. Lett. 101, 053108 (2012) Synchronization of a renewable energy inverter with the grid J. Renewable Sustainable Energy 4, 043103 (2012) Monolithic high-temperature superconducting heterodyne Josephson frequency down-converter Appl. Phys. Lett. 100, 262604 (2012) Generation of pure phase and amplitude-modulated signals at microwave frequencies Rev. Sci. Instrum. 83, 064705 (2012) Additional information on J. Appl. Phys. This review is focused on the latest developments in continuous-wave (CW) photomixing for Terahertz (THz) generation. The first part of the paper explains the limiting factors for operation at high frequencies $ 1 THz, namely transit time or lifetime roll-off, antenna (R)-device (C) RC rolloff, current screening and blocking, and heat dissipation. We will present various realizations of both photoconductive and p-i-n diode-based photomixers to overcome these limitations, including perspectives on novel materials for high-power photomixers operating at telecom wavelengths (1550 nm). In addition to the classical approach of feeding current originating from a small semiconductor photomixer device to an antenna (antenna-based emitter, AE), an antennaless approach in which the active area itself radiates (large area emitter, LAE) is discussed in detail. Although we focus on CW photomixing, we briefly discuss recent results for LAEs under pulsed conditions. Record power levels of 1.5 mW average power and conversion efficiencies as high as 2 Â 10 À3 have been reached, about 2 orders of magnitude higher than those obtained with CW antenna-based emitters. The second part of the paper is devoted to applications for CW photomixers. We begin with a discussion of the development of novel THz optics. Special attention is paid to experiments exploiting the long coherence length of CW photomixers for coherent emission and detection of THz arrays. The long coherence length comes with an unprecedented narrow linewidth. This is of particular interest for spectroscopic applications, the field in which THz research has perhaps the highest impact. We point out that CW spectroscopy systems may potentially be more compact, cheaper, and more accurate than conventional pulsed systems. These features are attributed to telecom-wavelength compatibility, to excellent frequency resolution, and to their huge spectral density. The paper concludes with prototype experiments of THz wireless LAN applications. For future telecommunication systems, the limited bandwidth of photodiodes is inadequate for further upshifting carrier frequencies. This, however, will soon be required for increased data throughput. The implementation of telecom-wavelength compatible photomixing diodes for down-conversion of an optical carrier signal to a (sub-)THz RF signal will be required.

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Terahertz Conductivity and Possible Bloch Gain in Semiconductor Superlattices

Cited by 89 publications

References 13 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

Noise temperature spectrum of hot electrons in semiconductor superlattices

Tunable, continuous-wave Terahertz photomixer sources and applications

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