Mattias Beck

GaAs/AlxGa1−xAs quantum cascade lasers

A unipolar injection quantum cascade (QC) laser grown in an AlGaAs/GaAs material system by molecular beam epitaxy, is reported. The active material is a 30 period sequence of injectors/active regions made from Al0.33Ga0.67As/GaAs-coupled quantum wells. For this device a special waveguide design, which complies with a GaAs heavily doped substrate and very short Al0.90Ga0.10As cladding layers, has been optimized. At a heat-sink temperature of 77 K, the laser emission wavelength is 9.4 μm with peak optical power exceeding 70 mW and the threshold current density is 7.3 kA/cm2. The maximum operating temperature is 140 K. This work experimentally demonstrates the general validity of QC laser principles by showing laser action in a heterostructure material different from the one used until now.

Bound-to-continuum and two-phonon resonance, quantum-cascade lasers for high duty cycle, high-temperature operation

Recent advances in quantum-cascade (QC) laser active-region design are reviewed. Based on a rate equation model of the active region, we show why new gain regions. based on a two-phonon resonance or a bound-to-continuum transition exhibit significantly better performance than the traditional design based on a three-quantum-well active region. Threshold current densities as low as 3 kA/cm/sup 2/ at T=300 K, operation with a peak power of 90 mW at 425 K, single-mode high-power operation up to temperatures above 330 K at /spl lambda//spl ap/16 /spl mu/m and continuous wave operation up to T=311 K are demonstrated. QC lasers able to operate at high duty cycles (50%) on a Peltier cooler were used in a demonstration of a 300-MHz free-space optical link between two buildings separated by 350 m.

Quantum-cascade lasers based on a bound-to-continuum transition

Laser emission at about 3.5 THz from quantum cascade lasers based on a bound-to-continuum transition is reported. Maximum pulsed operation temperature is above liquid nitrogen (90 K). CW operation reaches 55 K with powers up to 15 mW at 10 K.

External cavity quantum cascade laser tunable from 7.6 to 11.4 μm

We present the development of a broad gain quantum cascade active region. By appropriate cascade design and using a symmetric active region arrangement, we engineer a flat gain and increase the total modal gain in the desired spectral range. Grating-coupled external cavity quantum cascade lasers using this symmetric active region are tunable from 7.6 to 11.4 μm with a peak optical output power of 1 W and an average output power of 15 mW at room-temperature. With a tuning of over 432 cm−1, this single source covers an emission range of over 39% around the center frequency.

High-temperature operation of distributed feedback quantum-cascade lasers at 5.3 μm

High-temperature operation of a low-threshold 5.3 μm quantum-cascade distributed feedback laser is presented. The emission spectrum was single mode with more than 20 dB side mode suppression ratio for all investigated temperatures and up to thermal rollover. For 1.5% duty cycle and at 0 °C, the laser emitted 1.15 W of single mode peak power; at 120 °C, a value of 92 mW was seen. For a 3 mm long device, we observed a room-temperature threshold current density of 3.6 kA/cm2. This remarkable performance is mainly due to a 4 quantum-well active region using a double phonon resonance for the lower laser level.

Ultrastrong coupling of the cyclotron transition of a 2D electron gas to a THz metamaterial

Quantum Hall Meets Metamaterial Controlling and tuning light-matter interaction is crucial for fundamental studies of cavity quantum electrodynamics and for applications in classical and quantum devices. Scalari et al. (p. 1323) describe a system comprising an array of metamaterial split-ring resonators and a series of two-dimensional electronic gases (2DEG) formed in GaAs quantum wells. In a magnetic field, the electrons in the 2DEG performed cyclotron orbits and formed Landau levels. Strong coupling was observed between photon and magnetic cyclotron modes, producing a tunable semiconductor system for studying the light-matter interaction of two-level systems. A system of terahertz resonators coupled to two-dimensional electron gases presents a tunable test bed for the study of two-level physics. Artificial cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems because of the possibility of manipulating the cavity quantum electrodynamic ground state with controllable physical properties. We report here experiments showing ultrastrong light-matter coupling in a terahertz (THz) metamaterial where the cyclotron transition of a high-mobility two-dimensional electron gas (2DEG) is coupled to the photonic modes of an array of electronic split-ring resonators. We observe a normalized coupling ratio, Ωωc=0.58, between the vacuum Rabi frequency, Ω, and the cyclotron frequency, ωc. Our system appears to be scalable in frequency and could be brought to the microwave spectral range with the potential of strongly controlling the magnetotransport properties of a high-mobility 2DEG.

Broadband tuning of external cavity bound-to-continuum quantum-cascade lasers

A quantum-cascade structure based on a bound-to-continuum design exhibiting a broad gain curve is presented. The full width at half maximum of the measured luminescence spectrum is 297 cm−1 at room temperature. Grating-coupled external cavity lasers using this active region could be tuned over 150 cm−1 (1.45 μm), which is equal to 15% of the free running wavelength (λ≅10 μm), in pulsed mode at room temperature. Time resolved spectra showed a single-mode operation with a 30 dB side mode suppression ratio after the first 12 ns of the pulse.

Low-loss Al-free waveguides for unipolar semiconductor lasers

A promising waveguide design for midinfrared (λ=5–20 μm) unipolar semiconductor lasers is proposed and demonstrated in (Al)GaAs quantum cascade structures. In the latter, the active region is embedded between two GaAs layers, with an appropriate doping profile which allows optical confinement, with low waveguide losses and optimal heat dissipation. Low internal cavity losses of 20 cm−1 have been measured using different techniques for lasers with emission wavelength at ∼9 μm. At 77 K, these devices have peak output power in excess of 550 mW and threshold current of 4.7 kA/cm2.

Far-infrared (λ=88 μm) electroluminescence in a quantum cascade structure

Intersubband electroluminescence has been investigated in a quantum cascade structure based on vertical transition designed for far-infrared (λ=88 μm) emission. A narrow luminescence peak with a full width at half maximum of 0.7 meV is measured at low excitation currents (30 A/cm2) and low temperature (T=5 K). The electroluminescence efficiency exhibits a strong temperature and current dependence, consistent with an interplay between electron–electron and optical phonon scattering.

Continuous wave operation of a 9.3 μm quantum cascade laser on a Peltier cooler

High average power quantum cascade lasers at 9.3 μm using InP top cladding layers and both junction up and junction down mounting are presented. A 3 mm long, junction up mounted device emitted 54 mW average power at 30 °C and 11.5% duty cycle with a threshold current density of 3.72 kA/cm2. A similar, but only 1.5 mm long device with high reflection coating on both facets was mounted junction down and tested at even higher duty cycles. At −27 °C, we achieved continuous wave operation with a threshold current density of 3.3 kA/cm2.