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We present experiments investigating nonlinear optics in the terahertz (THz) range related to the 1s–2p intra-excitonic heavy-hole transition in GaAs/AlGaAs multiquantum wells (MQWs). Driven by intense THz fields of the Dresden free-electron laser the system exhibits two different phenomena: (i) we observe efficient near-infrared (NIR) sideband generation as a perturbative effect in the transmission of a narrowband NIR laser, and (ii) we present unambiguous evidence for the intra-excitonic Autler–Townes splitting, a non-perturbative effect.

In this paper we present two-color GaAs/AlGaAs quantum cascade lasers with separate optical confinement of the two laser stacks. The laser stacks, each comprising cladding layers and an active region, are grown on top of each other by molecular beam epitaxy on GaAs substrates. The lasers operate at 10.4 µm and 9.1 µm wavelength and have built-in second-order intersubband nonlinearities. Due to coupling between the optical fields of the respective waveguides, not only the second-harmonic of each wavelength, but also the sum-frequency of the two lasers is measured. This concept can be extended by independently biasing the two laser stacks, whilst maintaining overlap between the two optical modes.

A GaAs/AlGaAs quantum well infrared photodetector was combined with a deeply etched three-fold PhC structure. This makes the detector sensitive to mid-infrared radiation incident via the sample surface. The geometrically tunable peak responsivity is polarization independent when operated at normal incidence. Changing the angle of incidence a structure of several peaks is observed. In contrast to normal incidence illumination where the single peak can be identified as one particular PhC mode at the Γ-point near the second order Bragg condition, the multiple peaks apparent under an arbitrary angle refer to all PhC modes which lie within the spectral responsivity of the detector. The measurement therefore represents a direct characterization procedure for band structure mapping of photonic crystals. A study of the polarization dependence shows that TM polarized waves excite even modes while TE waves couple to modes with odd symmetry. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report second-harmonic (SH) and sum-frequency generation in GaAs-based quantum-cascade lasers. A doping dependence study of the second-order susceptibility in one of the investigated structure is shown. We also demonstrate that grating-coupled surface emission is a highly efficient way to couple out the SH radiation.

A high density plasma process was developed for reactive ion etching (RIE) of GaAs/AlGaAs based devices which can be applied for the processing of advanced resonator designs. We present diffractive resonators fabricated using inductively coupled plasma (ICP) RIE with SiCl"4 chemistry. A sidewall protection is formed during the process, which allows for steep vertical and smooth profiles. The technology therefore improves the fabrication of sophisticated QCL cavities and results of the first laterally coupled distributed feedback (DFB) QCLs are presented together with our progress made in etching photonic crystal QCLs.

We monitor the near‐infrared (NIR) transmission of a GaAs/AlGaAs multi quantum well while illuminated with intense terahertz (THz) pulses. We present the first clear evidence of the intra‐excitonic Autler‐Townes effect when pumping the 1s‐2p transition of the heavy‐hole exciton. We discuss our findings on the basis of a simplified two‐level model and find good qualitative agreement on resonance up to a Rabi energy of 0.6 times the 1s‐2p transition energy.

The near-infrared transmission of a semiconductor multiple quantum well is probed under intense terahertz illumination. We observe clear evidence of the intraexcitonic Autler-Townes effect when the terahertz beam is tuned near the $1s\mathrm{\text{\ensuremath{-}}}2p$ transition of the heavy-hole exciton. The strongly coupled effective two-level system has been driven with terahertz field strengths of up to $10\text{ }\text{ }\mathrm{kV}/\mathrm{cm}$ resulting in a Rabi energy of $\ensuremath{\approx}0.6$ times the transition energy. The induced near-infrared spectral changes at low intensities are qualitatively explained using a basic two-level model.

We describe the fabrication and operation of quantum cascade ring lasers providing grating‐coupled surface emission. The devices exhibit tunable far fields, ranging from spot‐ to ring‐shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry‐Perot lasers ‐ demonstrates the compatibility of reduced beam divergence and two‐dimensional integrability, resulting in an attractive light source for spectroscopic applications.

A novel approach for the tuning of quantum cascade lasers is presented. The system is based on an organic polymer which undergoes spectral changes as a function of the atmospheric pH. Deposition of the polymer on the air‐waveguide of an InGaAs/InAlAs QCL induces a reversible tuning of the emitted light by 7 cm−1. This is caused by inducing variations in the losses of the laser due to changes in the absorbance of the polymeric cladding. This system is a first example for reversible tuning in air at ambient temperature.

We investigate nonlinear optics related to the intra-excitonic 1s-2p heavy-hole transition in GaAs/AlGaAs quantum wells. Tuning intense terahertz (THz) light around this resonance we report (i) efficient second order sideband generation and (ii) first clear evidence of the Autler-Townes effect.

We present a coherent coupling scheme for mid‐infrared quantum cascade lasers. Based on far field analysis, fundamental lateral modes are realized in Y‐coupled resonators, which are locked in phase. The concept enhances the output power of a single facet coherent emitter, while maintaining good beam quality due to modal control. High flexibility in the choice of material system, emission wavelength and waveguide design combined with mature processing techniques make quantum cascade lasers an ideal candidate to gain insight into coherent spacial beam combining for high power laser applications.

Y-coupled cavity quantum cascade lasers have been processed from GaAs/AlGaAs and InP/InGaAs/AlAs/AlInAs wafers. Farfields of these samples were investigated. A phase coherence between the two coupled ridges was observed which results in the farfield pattern of a double slit experiment. Coherent coupling gives perspectives for high power laser arrays.

The authors present the effect of injector doping concentrations between 0.7 × 1016 cm−3 and 3.9 × 1017 cm−3 on the performance of short-wavelength InP-based quantum-cascades lasers (QCL) emitting around 3.8 μm. For the maximum operation current an almost linear dependence on the doping is obtained. However, the maximum emitted optical power, the highest wall-plug efficiency as well as the maximum operating temperature are found for a doping density of 1.7 × 1017 cm−3. Characterization of the lowest doped sample indicated an unstable operation of the QCLs.

We present a novel approach for the reversible switching of the emission wavelength of a quantum cascade laser (QCL) using a halochromic cladding. An air-waveguide laser ridge is coated with a thin layer of polyacrylic acid. This cladding introduces losses corresponding to the absorption spectrum of the polymer. By changing the state of the polymer, the absorption spectrum and losses change, inducing a shift of 7 cm(-1) in the emission wavelength. This change is induced by exposure to acidic or alkaline vapors under ambient conditions and is fully reversible. Such lasers can be used as multi-color light source and as sensor for atmospheric pH.

The authors use a quantum well intersubband photodetector fabricated into a two dimensional photonic crystal to investigate the optical defect modes of a single missing hole defect. The modes appear as a local enhancement in spectral photocurrent due to an increased in-coupling of surface incident light when a defect mode is present. The frequencies of these localized modes are tracked as they are varied by the defect geometry and compared to simulations.

We present a novel method for the reversible switching between different emission wavelengths for a grating-free mid-IR QCL utilizing the absorbance changes of a pH-responsive cladding under ambient conditions.

We demonstrate a quantum cascade laser employing two different injector regions and matched 4.6 mum optical transitions for low-voltage-defect operation. The laser has a pulsed wall-plug efficiency of 19% at 80 K and operates pulsed at 300 K.

A monolithic coupling scheme in which two active waveguides merge into a single waveguide to form a Y-shaped resonator is demonstrated for mid-infrared quantum cascade lasers. Lasers with emission wavelengths of 10.5 μm and 4.2 μm were processed from lattice-matched GaAs/AlGaAs and strain-compensated InP/InGaAs/AlAs/AlInAs structures. Phase-locking is observed in the laser cavities, resulting in coherent interference of the emitted radiation. Far fields were recorded on both sides of the devices and analyzed in respect to their radiative origin. By matching the recorded far field intensity profiles to corresponding near field distributions, the lateral mode distribution within the resonator is derived. Depending on the length of the coupling section, even or odd cavity modes evolve. Moreover, a comparison between the fabricated devices shows the emission wavelength's impact on the coupling performance of the Y-junction. The results demonstrate the feasibility of coherent laser resonators with prospective applications in interferometric sensing and high power laser arrays.

We demonstrate a quantum cascade laser that uses two alternating injectors with matching 4.6 mum active regions that operates in pulsed mode up to room temperature with a peak wall-plug efficiency of ~10 %.

We report surface emission of second‐harmonic radiation generated in the cavity of GaAs/AlGaAs quantum cascade lasers. Intersubband nonlinearities in the active region of the quantum‐cascade laser lead to second‐harmonic generation. A distributed‐feedback grating etched into the top of the semiconductor surface provides for both single‐mode fundamental laser operation and for surface‐emission of the frequency‐doubled light. The optical peak power from the surface is approx. 130 μW of SH light at 5.35 μm wavelength for ∼ 1 Watt of fundamental optical power (10.7 μm) at liquid‐nitrogen temperatures.

We report second-harmonic and sum-frequency generation in GaAs based quantum cascade lasers. Different waveguide designs and active regions were investigated as well as a doping dependence study of the second-order susceptibility in one of the investigated structures is shown. We present farfield measurements which give information about the modal behavior depending on the waveguide design and dimensions. We also demonstrate that grating-coupled surface emission is a highly efficient way to couple out the second-harmonic radiation.

We report second-harmonic generation in GaAs-based quantum cascade lasers due to intersubband nonlinearities. Using a distributed feedback grating we obtain single-mode second-harmonic surface emission, where no fundamental light is emitted via the surface.

In this paper we present single mode quantum cascade lasers (QCLs) based on the GaAs and the InP material systems. We show results for first- and second-order distributed feedback (DFB) QC lasers with surface gratings. The InP based lasers are grown by metalorganic vapor phase epitaxy (MOVPE) and show single mode continuous wave emission up to 200 K. In pulsed operation we achieved single mode surface emission peak output powers exceeding 1 Watt at room temperature. The presented GaAs/AlGaAs laser features an air/AlGaAs waveguide, combined with a second-order distributed feedback grating. That laser shows 3 Watts of single mode output power via the surface at 78 K.

We present an experimental study on efficient second order sideband generation in symmetric undoped GaAs/AlGaAs multiple quantum wells. A near-infrared laser tuned to excitonic interband transitions is mixed with an in-plane polarized terahertz beam from a free-electron laser. The terahertz beam is tuned either to the intraexcitonic heavy-hole 1s-2p transition or to the interexcitonic heavy-hole light-hole transition. We find strong evidence that the intraexcitonic transition is of paramount influence on n=+-2 sideband generation, leading to an order-of-magnitude resonant enhancement of the conversion efficiency up to 0.1% at low temperature. At room temperature, the efficiency drops only by a factor of 7 for low terahertz powers.

Photonic crystal (PC) devices require high fabrication accuracy for on demand positioning of resonances. We describe post-fabrication fine-tuning of a PC quantum well infrared photodetector (QWIP) by sidewall-deposition of silicon nitride. The PC resonance was shifted over a bandwidth of 43 cm−1. From photoresponse measurements we calculated a tuning coefficient of ∂ν/∂dSiN=−0.06 cm−1/nm. The QWIP responsivity did not suffer from nitride absorption while the PC resonance increased by a factor of 1.6. This shows that post-fabrication tuning by dielectric deposition with, e.g., silicon nitride is a feasible method to achieve precise implementations of PC devices.

Non-bandgap defect modes, defect modes showing up in photonic crystal (PhC) structures which do not exhibit a complete photonic bandgap, are investigated experimentally by means of a QWIP (quantum well intersubband photodetector) incorporated within a two-dimensional (2D) triangular photonic crystal of air holes. The modes appear as a local enhancement in the photocurrent spectrum due to an increased in-coupling of surface incident light when a defect mode is present. Such delocalized non-bandgap defect modes and the influence of the entire PhC on their energetic positions are experimentally determined and show good agreement with (2D as well as three-dimensional (3D)) simulations.

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95μm with a side mode suppression ratio of 25dB. A linear tuning coefficient of 0.13cm−1∕K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of ∼3°. Based on these characteristics the presented compact coherent light source may find its way into today’s midinfrared spectroscopy applications.

Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shallow second order grating between a continuous surface-plasmon layer and the active region. The combined effect of an air cladding and a metallic layer on the opposite sides of the waveguide increases the overlap with the grating region resulting in calculated coupling coefficients up to 100 cm(-1). The waveguide design was implemented by Au thermo-compression bonding after grating formation and subsequent backside processing of ridges with air claddings. Lasers as short as 176 microm show single-mode behavior with a side-mode-suppression-ratio of 20 dB and thresholds (10 kA/cm(2)) as well as output powers (150 mW) close to Fabry-Pérot device performances are reached for 360 microm long devices.

We report on the fabrication and operation of quantum cascade ring lasers providing grating-coupled surface emission. The devices exhibit tunable far fields, ranging from spot- to ring-shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry–Perot lasers—demonstrates the compatibility of reduced beam divergence and two-dimensional integrability, resulting in an attractive light source for applications in midinfrared spectroscopy and imaging.

Second-harmonic (SH) emission in the form of Cerenkov-type phase-matched radiation has been measured for quantum-cascade lasers (QCLs) with built-in nonlinearities. The QCLs operate at 10.9μm wavelength and, due to material dispersion, show a high collinear phase mismatch, resulting in low SH external conversion efficiencies on the order of 2μW∕W2. Due to our waveguide design, we were able to couple out and measure the generated SH light in the form of Cerenkov-type phase-matched radiation from the substrate of the device. The SH power collected from the Cerenkov beam leads to an increased external conversion efficiency of ∼50μW∕W2.

Midinfrared quantum cascade lasers with monolithically integrated Y-junctions are investigated. Two different emission wavelengths of 10.5 and 4.2μm were realized in two different material systems, lattice-matched GaAs∕AlGaAs on GaAs and strain-compensated InGaAs∕InAlAs∕AlAs on InP. With identical Y-junction dimensions, phase locking is observed in both structures. In GaAs based devices, fundamental lateral modes are present in the coupled waveguides, which are coherently synchronized at the Y-junction. In InP based devices, modes of higher order are excited, which originate from coupling. The generation of multiple modes yields an out-of-phase fraction which reduces the level of coherence.

The effect of doping concentration on the performance of short-wavelength quantum-cascade lasers based on the strain-compensated InGaAs/InAlAs/AlAs heterostructure on InP, emitting at 3.8 μm, is investigated for average doping concentrations between 0.3 and 3.9×1017 cm−3 (sheet densities between 1.6 and 20.9×1011 cm−2). Although the threshold current density is rather independent of doping concentration, the maximum current density increases with doping and exhibits a saturation for the highest doping level. Other important performance characteristics such as differential quantum efficiency, peak optical emission power, slope efficiency, and maximum operating temperature are observed to be maximized for structures with an average doping of 2−3×1017 cm−3, corresponding to a sheet density of about 1.5×1012 cm−2.

A monolithic coupling scheme in which two active waveguides merge into a single waveguide is presented for a GaAs∕AlGaAs quantum cascade laser. The evolving fields interfere and a constant phase is observed in the Y-shaped laser cavity, resulting in a far field pattern of a double slit. The mode distribution is comprehensively derived by matching the far field profiles to simulated values and shows a weak current dependence. The device demonstrates the feasibility of coherent laser resonators with prospective applications in interferometric sensing and high power laser arrays.

An inductively coupled plasma reactive ion etching process was developed for transferring patterns from a thin intermediate mask consisting of Ni or SiNx into GaAs. Smoothed out etch floors and sidewalls can be achieved under an approximately 200V bias by switching between an anisotropic etch phase and a deposition phase by gas chopping. The deposition of Si–O based film that protects the sidewall from chlorine attack is promoted by the addition of O2 to an SiCl4∕N2 gas mixture. The total achieved etch depth was approximately 20μm in this work, but the process can in principle be adopted for a larger depth and other chlorine-etchable III-V semiconductors. SiCl4 is shown to act as a suitable deposition gas as well as an etch gas.

We demonstrate an In(0.635)Al(0.356)As/In(0.678)Ga(0.322)As strain compensated quantum cascade laser that employs heterogeneous injector regions for low voltage defect operation. The active core consists of interdigitated undoped and doped injectors followed by nominally identical wavelength optical transitions. The undoped injector regions are designed with reduced voltage defect while the doped injectors are of a more conventional design. The measured average voltage defect is less than 79 meV. At 80 K, a 2.3 mm long, back facet high reflectance coated laser has an emission wavelength of 4.7 mum and outputs 2.3 W pulsed power with a peak wall-plug efficiency of 19%.

The authors report on a quantum well infrared detector embedded in a surface-plasmon waveguide and processed into a deeply etched photonic crystal structure. The device was characterized by collecting the polarization dependent response spectra at different angles of incidence. With this method it is possible to map the photonic band structure by directly detecting the modes of the photonic crystal. It therefore represents a new and direct characterization procedure for photonic crystals. The device shows a strong mixing between TE and TM polarized modes, which is caused by the asymmetric vertical waveguide design.

Polarization dependent band structuring mapping was performed which showed a strong polarization mixing for the surface-plasmon waveguide used in photonic crystal mid infrared photodetectors. The response spectra of the detector for unpolarized, TM and TE polarized light were obtained.

Mid-infrared beam shaping is demonstrated by using grating coupled surface emitting quantum cascade ring lasers. The devices allow for far-field tuning, ranging from highly symmetric spot- to ring-shaped beam patterns, depending on the grating period.

Quantum cascade lasers (QCLs) have been established as versatile light sources in the mid-infrared. On the one hand their multilayer heterostructure offers great flexibility in emission wavelengths and allows for the integration of nonlinear light conversion mechanisms. On the other hand the bigger waveguide dimensions, when compared to band-gap lasers, make advanced cavity designs, such as various distributed feedback designs, photonic crystals and microcavities more readily accessible. We present surface emitting QCLs operating around 10mum with optical peak power levels in the Watt range. By nonlinear light generation in the laser cavity, we generate frequency-doubled light that is extracted from the surface of the device by grating coupling, while the fundamental radiation is blocked.

The photonic crystal enables response to surface incident radiation for intersubband-based QWIPs. The angular and polarization dependence of the spectral photocurrent is used to map the photonic band structure and to investigate polarization conversion effects.