When ground-state atoms are accelerated through a high Q microwave cavity, radiation is produced with an intensity which can exceed the intensity of Unruh acceleration radiation in free space by many orders of magnitude. The reason is a strong nonadiabatic effect at cavity boundaries and its interplay with the standard Unruh effect. The cavity field at steady state is still described by a thermal density matrix under most conditions. However, under some conditions gain is possible, and when the atoms are injected in a regular fashion, squeezed radiation can be produced.
Conventional quantum cascade (QC) lasers are intrinsically edge-emitting devices with mode confinement achieved via a standard mesa stripe configuration. Surface emission in edge emitting QC lasers has therefore necessitated redirecting the waveguided laser emission using a second order grating. This paper describes the methods used to fabricate a 2D photonic crystal (PC) structure with or without a central defect superimposed on an electrically pumped QC laser structure with the goal of achieving direct surface emission. A successful systematic study of PC hole radius and spacing was performed using e-beam lithography. This PC method offers the promise of a number of interesting applications, including miniaturization and integration of QC lasers. 2003 American Vacuum Society.
The quantum-cascade laser can be used as an infrared source for a small portable photoacoustic trace gas detector. The device that we describe uses a quantum-cascade laser without collimating optics mounted inside an acoustic resonator. The laser is positioned in the center of a longitudinal resonator at a pressure antinode and emits radiation along the length of the resonator exciting an axially symmetric longitudinal acoustic mode of an open-ended cylindrical resonator. Experiments are reported with an 8-mum, quasi-cw-modulated, room-temperature laser used to detect N2O. (C) 2003 Optical Society of America.
Experimental results using the amplified spontaneous emission spectroscopy of a type-I quantum-cascade laser are presented. Using the Hakki-Paoli method, the optical gain spectra of the laser are extracted for the wavelength of 8.2 mum at various subthreshold current levels. The change in refractive index with increased bias current is obtained from the peak wavelength shifts of the Fabry-Perot spectrum. A low value of -0.5 for the linewidth enhancement factor is found. A group index of around 3.47 has also been determined from Fabry-Perot modal spacings. (C) 2003 American Institute of Physics.
We report on the midinfrared emission from electroluminescent devices with quantum cascade active regions based on InGaAs/InP heterostructures. We observe emission at lambdasimilar to12 mum from two different structures and compare their emission characteristics based on the different band structure designs. Their relevance in view of the realization of InP-based quantum cascade lasers with aluminum-free waveguides is discussed. (C) 2003 American Institute of Physics.
We have optimized the design of the broadband quantum cascade laser for cw operation. The improved design leads to a gain ripple of only about 4 cm(-1) over more than a 0.5-mum spectral range. Simultaneous cw emission at several wavelengths spanning the range between 6.7 and 7.4 mum has been achieved in a temperature interval from 20 to 77 K. (C) 2003 American Institute of Physics.
Optimized second-harmonic generation (SHG) in quantum cascade (QC) lasers with specially designed active regions is reported. Nonlinear optical cascades of resonantly coupled intersubband transitions with giant second-order nonlinearities were integrated with each QC-laser active region. QC lasers with three-coupled quantum-well (QW) active regions showed up to 12 muW of SHG light at 3.75 mum wavelength at a fundamental peak power and wavelength of 1 W and 7.5 mum, respectively. These lasers resulted in an external linear-to-nonlinear conversion efficiency of up to 1 muW/W-2. An improved 2-QW active region design at fundamental and SHG wavelengths of 9.1 And 4.55 mum, respectively, resulted in a 100-fold improved external linear-to-nonlinear power conversion efficiency, i.e. up to 100 muW/W-2. Full theoretical treatment of nonlinear light generation in QC lasers is given, and excellent agreement with the experimental results is obtained. For the best structure, a second-order nonlinear susceptibility of 4.7 x 10(-5) esu (2 x 10(-4) pmN) is calculated, about two orders of magnitude above conventional nonlinear optical materials and bulk III-V semiconductors.
We combine photonic and electronic band structure engineering to create a surface-emitting quantum cascade microcavity laser. A high-index contrast two-dimensional photonic crystal is used to form a micro-resonator that simultaneously provides feedback for laser action and diffracts light vertically from the surface of the semiconductor surface. A top metallic contact allows electrical current injection and provides vertical optical confinement through a bound surface plasmon wave. The miniaturization and tailorable emission properties of this design are potentially important for sensing applications, while electrical pumping can allow new studies of photonic crystal and surface plasmon structures in nonlinear and near-field optics.
We demonstrate an efficient intracavity nonlinear interaction of laser modes in a specially adapted quantum cascade laser. A two-wavelength quantum cascade laser structure emitting at wavelengths of 7.1 and 9.5 mum included cascaded resonant optical intersubband transitions in an intracavity configuration leading to resonantly enhanced sum-frequency and second-harmonic generation at wavelengths of 4.1, 3.6, and 4.7 mum, respectively. Laser peak optical powers of 60 and 80 mW resulted in 30 nW of sum-frequency signal and 10-15 nW of second-harmonic signal, both in good agreement with theoretical calculations.
We have investigated the behavior of a terahertz quantum cascade laser in an external magnetic field. A reduction of the threshold current density and a simultaneous enhancement of the laser emission intensity are observed. Although several mechanisms can induce this effect, the suppression of nonradiative Auger-intersubband transitions through Landau quantization of the in-plane electron motion is the most probable candidate. In addition, the injection rate via resonant inter-Landau-level transfer and the waveguide properties are modulated by the field. We also observed clear shifts of the emission spectra when the external magnetic field is applied, while operating the device at constant voltage or current. (C) 2003 American Institute of Physics.