Ultrafast Coherent Electron Transport in Semiconductor Quantum Cascade Structures

Eickemeyer, Felix T.; Reimann, K.; Woerner, M.; Elsaesser, Thomas; Barbieri, Stefano; Sirtori, Carlo; Strasser, G.; Müller, Thomas; Bratschitsch, Rudolf; Unterrainer, K.

doi:10.1103/physrevlett.89.047402

Cited by 63 publications

(44 citation statements)

References 18 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Although this is measured for a radiative transition and not resonant tunneling, the dephasing times should be similar. This value for T * 2 is also consistent with experimentally measured dephasing times of T 2 ∼ 0.5 ps for resonant tunneling injection in a quantum cascade structure [103]. These figures illustrate the transition from incoherent to coherent transport, which occurs for large ∆ 0 and τ , and is characterized by τ ef f ≈ 2τ .…”

Section: (B)supporting

confidence: 74%

Terahertz quantum-cascade lasers

2007

View full text Add to dashboard Cite

The development of the terahertz frequency range (1-10 THz, λ ≈ 30-300 µm) has long been impeded by the relative dearth of compact, coherent radiation sources of reasonable power. This thesis details the development of quantum cascade lasers (QCLs) that operate in the terahertz with photon energies below the semiconductor Reststrahlen band. Photons are emitted via electronic intersubband transitions that take place entirely within the conduction band, where the wavelength is chosen by engineering the well and barrier widths in multiple-quantum-well heterostructures. Fabrication of such long wavelength lasers has traditionally been challenging, since it is difficult to obtain a population inversion between such closely spaced energy levels (hω ≈ 4-40 meV), and because traditional dielectric waveguides become extremely lossy due to free carrier absorption.This thesis reports the development of terahertz QCLs in which the lower radiative state is depopulated via resonant longitudinal-optical phonon scattering. This mechanism is efficient and temperature insensitive, and provides protection from thermal backfilling due to the large energy separation (∼36 meV) between the lower radiative state and the injector. Both properties are important in allowing higher temperature operation at longer wavelengths. Lasers using a surface plasmon based waveguide grown on a semi-insulating (SI) GaAs substrate were demonstrated at 3.4 THz (λ ≈ 88 µm) in pulsed mode up to 87 K, with peak collected powers of 14 mW at 5 K, and 4 mW at 77 K.Additionally, the first terahertz QCLs have been demonstrated that use metalmetal waveguides, where the mode is confined between metal layers placed immediately above and below the active region. These devices have confinement factors close to unity, and are expected to be advantageous over SI-surface-plasmon waveguides, especially at long wavelengths. Such a waveguide was used to obtain lasing at 3.8 THz (λ ≈ 79 µm) in pulsed mode up to a record high temperature of 137 K, whereas similar devices fabricated in SI-surface-plasmon waveguides had lower maximum lasing temperatures (∼92 K) due to the higher losses and lower confinement factors.This thesis describes the theory, design, fabrication, and testing of terahertz quantum cascade laser devices. A summary of theory relevant to design is presented, 3 including intersubband radiative transitions and gain, intersubband scattering, and coherent resonant tunneling transport using a tight-binding density matrix model. Analysis of the effects of the complex heterostructure phonon spectra on terahertz QCL design are considered. Calculations of the properties of various terahertz waveguides are presented and compared with experimental results. Various fabrication methods have been developed, including a robust metallic wafer bonding technique used to fabricate metal-metal waveguides. A wide variety of quantum cascade structures, both lasing and non-lasing, have been experimentally characterized, which yield valuable information about the transport ...

show abstract

Section: (B)supporting

confidence: 74%

Terahertz quantum-cascade lasers

2007

View full text Add to dashboard Cite

show abstract

“…The question arises whether coherent optical effects after ultrafast optical excitation can be observed in the opposite regime, where coherences were shown not to be important in the stationary transport. Recently, optical pumpprobe measurements in mid-infrared 29,30,31,32 and THz QCLs 33 have shown an oscillatory behavior attributed to (coherent) resonant tunneling through the injection barrier. In this section, this effect is investigated with respect to the relation between the tunnel coupling and the level broadening.…”

Section: Ultrafast Optical Dynamicsmentioning

confidence: 99%

“…29,30,31,32,33,34,35 An ultrafast nonlinear pump pulse resonant on the laser transition excites the sample, leading to a gain saturation at the laser energy, and a subsequent weak probe pulse tests the laser transition as a function of the delay time between the two pulses (see Fig. 8).…”

Section: Ultrafast Optical Dynamicsmentioning

confidence: 99%

See 1 more Smart Citation

Density-matrix theory of the optical dynamics and transport in quantum cascade structures: The role of coherence

2009

View full text Add to dashboard Cite

The impact of coherence on the nonlinear optical response and stationary transport is studied in quantum cascade laser structures. Nonequilibrium effects such as pump-probe signals, the spatio-temporally resolved electron density evolution, and the subband population dynamics (Rabi flopping) as well as the stationary current characteristics are investigated within a microscopic density-matrix approach. Focusing on the stationary current and the recently observed gain oscillations, it is found that the inclusion of coherence leads to observable coherent effects in opposite parameter regimes regarding the relation between the level broadening and the tunnel coupling across the main injection barrier. This shows that coherence plays a complementary role in stationary transport and nonlinear optical dynamics in the sense that it leads to measurable effects in opposite regimes. For this reason, a fully coherent consideration of such nonequilibrium structures is necessary to describe the combined optical and transport properties.

show abstract

“…Our approach to studying the coupling of transport and laser dynamics is to time-resolve the dynamics of the gain recovery following an impulsive perturbation of the intra-cavity field. Pump-probe methods have previously been used to explore the possibility of coherent transport in resonant-tunnelling QC structures [7] under non-lasing conditions [8]. Here, we explore the gain dynamics of operating QCL's below and above threshold, and the measured dynamics is used to develop a model for the QCL level populations coupled to the cavity photon density rate equation.…”

Section: Introductionmentioning

confidence: 99%

Ultrafast Mid-Infrared Dynamics in Quantum Cascade Lasers

Norris¹

2010

View full text Add to dashboard Cite

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. This report summarizes the entire research period. In this program, we performed experiments using a femtosecond mid-infrared pump-probe system implemented for QCL samples operating at 4.6 and 5.3 µm. We employed femtosecond time-resolved pump-probe measurements to probe the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery was determined by the time-dependent transport of electrons through the cascade heterostructure; as the laser approaches and exceeds threshold, the gain recovery ABSTRACTThis report summarizes the entire research period. In this program, we performed experiments using a femtosecond mid-infrared pump-probe system implemented for QCL samples operating at 4.6 and 5.3 µm. We employed femtosecond time-resolved pump-probe measurements to probe the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery was determined by the time-dependent transport of electrons through the cascade heterostructure; as the laser approaches and exceeds threshold, the gain recovery shows a dramatic reduction due to the onset of quantum stimulated emission. Since the electron transport through each state in the cascade is determined by the state lifetime, the transport in a cascade laser is driven by the photon density in the cavity. The gain recovery is qualitatively different from that in conventional atomic, molecular and interband semiconductor lasers due to the superlattice transport in the cascade. We also studied the effects of pulse propagation in the laser, including group velocity dispersion and coherent pulse reshaping due to ultrafast Rabi flopping of the gain medium.

show abstract

Ultrafast Coherent Electron Transport in Semiconductor Quantum Cascade Structures

Cited by 63 publications

References 18 publications

Terahertz quantum-cascade lasers

Terahertz quantum-cascade lasers

Density-matrix theory of the optical dynamics and transport in quantum cascade structures: The role of coherence

Ultrafast Mid-Infrared Dynamics in Quantum Cascade Lasers

Contact Info

Product

Resources

About