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In this paper, we give an overview of quantum cascade detector technology for the near- and mid-infrared wavelength range. Thanks to their photovoltaic operating principle, the most advanced quantum cascade detectors offer great opportunities in terms of high detection speed, reliable room temperature operation, and excellent Johnson noise limited detectivity. Besides some important features dealing with their fabrication and their general characteristics, we will also briefly present some possibilities for performance improvement. Elementary theoretical considerations adopted from photoconductive detectors confirm that optimization of such devices always involves various trade-offs.

We report on investigations of far-field intensity patterns and angle-resolved lasing spectra emitted fromquantum cascade (QC) lasers at wavelengths around Lamda = 7.7 µm in pulsed operation. Measurements of the angle-dependent intensity noise in the far-field reveal that small driving currents lead to an incoherent superposition of multiple lateral cavity modes. Elevated currents result in a phase-coupled superposition of the light fields arising from the excited lateral modes. This superposition leads to beam steering and strong pulse-to-pulse intensity fluctuations. We present for both the incoherent and the phase-coupled superposition, a model which allows us to explain the observed far-field intensity patterns and their noise properties. The angle-resolved lasing spectra of the investigated QC lasers show distinct sets of longitudinal modes with distinctively different dependences of the lasing spectrum on the far-field angle. They can be assigned to the incoherent and the phase-coupled superposition of the lateral modes.

We present a comprehensive analysis of the wall-plug efficiency of mid-infrared (λ ∼ 4–10 μm) quantum cascade (QC) lasers, by taking into account the transition efficiency of the active region, the light out-coupling efficiency from the emission facet, the voltage efficiency across the superlattice, and the current efficiency including the leakage and thermal backfilling current densities at threshold. All relevant parameters including their interdependencies have been taken into account and analyzed in order to optimize the wall-plug efficiency of practical QC lasers. Expressions for optimizing the extraction energy Δinj and the mirror loss αm are deduced, which are then used to maximize the wall-plug efficiency of QC lasers. Finally, predictions for the wall-plug efficiency of realistic QC lasers are made both at room temperature and at low temperature. Good agreement is found with corresponding experimental data from the literature.

In this paper, we report the design, fabrication, and characterization of an InP/InGaAs heterojunction bipolar transistor (HBT) employing a lattice-mismatched In(0.53)Ga(0.47)As/GaAs strained-layer-superlattice (SLS) base structure. The performance of the SLS-base device is also compared with that of an In(0.53)Ga(0.47)As uniform-base structure. The digitally graded-base devices exhibit a slightly lower gain at low bias voltage and a higher current gain at high currents. While the offset voltage remains comparable in the two structures, the graded-base InP/InGaAs HBTs, with built-in fields of - 66 kV/cm, have typically a maximum dc current gain of 18% larger than that of transistors with uniform base.

A midinfrared quantum cascade detector with a spectrally broad (ΔE/E = 27.3%) response is designed, fabricated, and tested. This detector consists of 26 differently designed active region stages in order to cover a wavelength region from 4.7 to 7.4 μm. The device could be operated above room temperature and showed peak responsivities of 13 mA/W at 10 K and 1.25 mA/W at room temperature. A background limited detectivity of 1.55×1010 Jones was seen up to a temperature TBLIP of 110 K.

We present a comprehensive analysis of an external-cavity (EC) quantum cascade (QC) laser system, which is based on a coupled-mode system with the length of the EC set equal to that of the laser chip and accommodating the large difference in physical cavity length by introducing an effective speed of light for light propagating in the EC. By analyzing the rate equations and discussing the cross sections and losses of the EC QC lasers, numerical results as well as suggestions for a further optimization of the spectral tuning range of the EC system are presented. Besides, the delayed onset of the EC modes with respect to that of the internal Fabry–Perot modes in pulsed operation has been simulated using the rate equations, yielding a good agreement with the experimental data.

We report on the temporal and spectral evolution of the lasing spectrum of an external-cavity quantum-cascade (QC) laser operated in pulsed mode. Time-resolved lasing spectra, recorded on nanosecond time scale, revealed that after turn-on of the 100 ns current pulse multiple-mode lasing starts at the gain maximum of the QC laser controlled by the cavity formed by the chip facets. Depending on the spectral tuning after a time delay of 15–35 ns, the QC laser couples to the external cavity as seen from a change in lasing wavelength to that defined by the external grating as wavelength selective element.

The authors report on short-wavelength In0.53Ga0.47As/AlAs0.56Sb0.44 quantum cascade detectors (QCDs). At room temperature, one device detects at 505 meV (2.46 mu m) with a responsivity of 2.57 mA/W, while a second QCD is sensitive at 580 meV (2.14 mu m) with a responsivity of 0.32 mA/W.

We report on the experimental study of the structural, electronic and thermal properties of state-of-art Sb-based quantum-cascade lasers (QCLs) operating in the range 4.3-4.9 mu m. This information has been obtained by investigating the active region band-to-band photoluminescence signals, detected by means of an GaInAs-array detector. This technique allowed to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. We demonstrate that electron transport in QCLs based on Sb-ternary barriers may be insufficient, thus affecting the tunneling of electrons and the electronic recycling and cascading scheme. Finally, we present the first measurement of the electronic and lattice temperatures and the electron-lattice coupling in Sb-based QCLs based on a quaternary-alloy. We extracted the thermal resistance (R-L = 8.9 K/W) and the electrical power dependence of the electronic temperature (R-e = 11.7 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx structures operating at 4.9 mu m, in the lattice temperature range 50 K = 80 K. The corresponding electron-lattice coupling constant (alpha = 10.8 Kcm(2)/kA) reflects the reduction of the electron-leakage channels associated with the use of a high conduction band-offset.

The authors extracted the thermal resistance (R-L=9.6 K/W) and the electrical power dependence of the electronic temperature (R-e=12.5 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx quantum-cascade lasers (QCLs) operating at 4.9 mu m, in the lattice temperature range of 60-90 K. The low electron-lattice coupling constant alpha=10.4 K cm(2)/kA can be related to the beneficial effect of the high conduction band offset, peculiar to the GaInAs/AlGaAsSb material system, on the electron leakage. The authors found an active region cross-plane thermal conductivity value k(perpendicular to)=1.8 +/- 0.1 W/(K m), which is approximately three times larger than that measured in QCLs with GaInAs/AlInAs heterostructures. (c) 2007 American Institute of Physics.

Short-wavelength (lambda < 4 µm) GaInAs-AlAsSb quantum cascade (QC) lasers have been demonstrated using a "bound-to-continuum" design for the purpose of reducing the electric injection power density. As a result, we have reduced the low-temperature electric injection power density of the lasers by 40%, compared to that of GaInAs-AlAsSb QC lasers emitting at the same wavelength but adopting a triple-quantum-well design. The lasers in the present report can operate up to room temperature (300 K) in pulsed mode, emitting at short-wavelength lambda about 3.7 - 3.9 µm.

We report on the demonstration of continuous-wave (CW) operation of GaInAs-AlGaAsSb quantum cascade (QC) lasers. By placing a 2.5-/spl mu/m-thick gold layer on both sides of the laser ridge to extract heat from the active region in the lateral direction, together with mounting the device epilayer down, we have achieved CW operation of GaInAs-AlGaAsSb QC lasers composed of 25 stages of active/injection regions. The maximum CW operating temperature of the lasers is 94 K, and the emission wavelength is around /spl lambda//spl sim/4.65 /spl mu/m. For a device with the size of 10/spl times/2000 /spl mu/m/sup 2/, the CW optical output power per facet is 13 mW at 42 K and 4 mW at 94 K. The CW threshold current density is 1.99 kA/cm/sup 2/ at 42 K, and 2.08 kA/cm/sup 2/ at 94 K, respectively.

Trap centers with an energy level positioned 1/3 of the band gap below the effective conduction band edge are observed in the electroluminescence spectra of InAs/(Galn)Sb superlattice photodiodes with a cutoff wavelength of 11 µm. The trap centers are recognized by simulating the low-temperature current-voltage characteristics of the diodes. Excellent quantitative agreement on both, the l- V characteristic and the differential resistance between the experimental data and the theoretical prediction is achieved. The quantitative simulation of the I- V characteristics shows, that the 77 K performance of InAs/(Galn)Sb photodiodes is dominated by generation-recombination processes even at long wavelengths. Above 50 K, tunneling currents are not of importance.