C. Gmachl

Stability of pulse emission and enhancement of intracavity second-harmonic generation in self-mode-locked quantum cascade lasers

We report the observation of stable pulse emission and enhancement of intracavity second-harmonic generation (SHG) in self-mode-locked quantum cascade (QC) lasers. Down-conversion of the detector signal by heterodyning with an RF signal allows the direct observation of the pulsed laser emission in the time domain and reveals a stable train of pulses characteristic of mode-locked lasers. The onset of self-mode locking in QC lasers with built-in optical nonlinearity results in a significant increase of the SHG signal. A pulse duration of /spl sim/12 ps is estimated from the measured increase of the SHG signal in pulsed emission compared to the power expected for the SHG signal in CW emission. This value is in good agreement with the pulse duration deduced from the optical spectral width.

Fabrication technologies for quantum cascade photonic-crystal microlasers

In this paper we describe the technological and fabrication methods necessary to incorporate both photonic and electronic-band engineering in order to create novel surface-emitting quantum cascade microcavity laser sources. This technology offers the promise of several innovative applications such as the miniaturization of QC lasers, and multi-wavelength two-dimensional laser arrays for spectroscopy, gas-sensing and imaging. This approach is not limited to light-emitting devices, and may be efficiently applied to the development of mid- and far-infrared normal-incidence detectors.

Surface-plasmon quantum cascade microlasers with highly deformed resonators

We report the demonstration of surface-plasmon microcylinder quantum cascade lasers with circular and deformed resonators. An improved self-alignment fabrication technique was developed that allows the use of wet etching, necessary to achieve smooth and clean surfaces, in combination with the deposition of the surface-plasmon-carrying metal layer up to the very edge of the resonator, where the optical mode is mostly located. The diameter of the microcylinders ranges from 75 to 180 /spl mu/m while their deformation coefficient /spl epsiv/ ranges from /spl epsiv/=0 to /spl epsiv/=0.32. Circular microcylinder lasers show a reduction of /spl sim/50% of the threshold current density with respect to devices with standard ridge-waveguide resonators. On the other hand, highly deformed microcylinder lasers exhibit a complex mode structure, suggesting the onset of chaotic behavior.

Fabrication methods for a quantum cascade photonic crystal surface emitting laser

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.

Cavity ringdown spectroscopy of NO with a cw single frequency quantum cascade laser

Summary form only given. Cavity ringdown spectroscopy (CRS) enables the measurements of very weak absorption introduced.into a high-finesse optical cavity. This method is based on measuring a decay time of the cavity mode. In this contribution we shall report the ultrasensitive detection of NO by its fundamental absorption at 5.2 /spl mu/m by CRS technique. A continuous wave quantum cascade distributed feedback (QC-DFB) laser was used as the spectroscopic source.

Wideband frequency modulation spectroscopy using the quantum cascade laser

Summary form only given. The quantum cascade (QC) laser is a promising new source for tunable, infrared laser radiation and the spectroscopic capabilities of the QC laser were recently demonstrated. In this report, we extend previous work by demonstrating wideband frequency modulation (FM) of the QC laser and the detection of FM spectroscopic signals from a weakly absorbing sample of molecules.

Frequency stabilization, line narrowing and modulation studies in quantum cascade lasers

Summary form only given. Some of the applications of stabilized laser sources (mid-IR sub-Doppler spectroscopy, precision laser ranging) require reduction of the high frequency spectral noise components, resulting in fine narrowing and an increase in the coherence length of the laser. In order to achieve this, considerable servo bandwidth is required. For this reason, the phase transfer function behavior of the laser stabilization system is crucial, time delays and inductive effects being detrimental. In this paper, we report on our latest developments in QC laser stabilization, including some elegant electronics that have alleviated a number of common control theory issues including resonances and variability in the transfer function.

Miniband electronic distribution in superlattice quantum cascade structures

Summary form only given.Assessment of the excited state electronic distribution is crucial for a quantitative description of population inversion and optical gain in quantum cascade lasers either with coupled quantum wells or superlattice (SL) active regions. So far, a simple 4 level model has been used for an approximate description of experimental results. Characteristic of intersubband emission is the negligible dependence of the emitted photon wavelengths from the excess energy of electrons injected in the upper subband. In contrast, the photon energy associated with interminiband transitions are strongly dependent from the initial state energy, owing to the superlattice k/sub z/ energy dispersion. Hence, the electronic distribution in the excited miniband of a finite superlattice can be directly extracted from the analysis of the fine structure at energy higher than the minigap in the mid-infrared electroluminescence spectra.

Spectral noise measurements of a 8.5 micron quantum cascade laser with implications for stabilization to 1 kHz

Summary form only given. The quantum-cascade (QC) distributed feedback laser operating in the mid-infrared promises to be an excellent tool for remote sensing applications as well as in situ monitoring of trace gases. In addition to the high cw power, the QC laser exhibits excellent intrinsic frequency stability. We report here on the characterization of the spectral noise density of a 8.5 /spl mu/m QC laser operating cw at liquid nitrogen temperatures.

Quantum cascade lasers based on guided modes at a metal-semiconductor interface

Planar waveguides for long-wavelength quantum cascade (QC) lasers require thick cladding layers. This prolongs and complicates the growth and processing procedures of these lasers. In this paper we demonstrate that optical waveguiding for a laser in this wavelength range can also be achieved using electromagnetic surface waves at a metal-semiconductor interface directly above the active region of the laser. No cladding layer is needed. The interface-guided modes are transverse-magnetic modes existing at the interface between a metal and a semiconductor, which have dielectric constants opposite in sign. Therefore, the metal layer, which is usually deposited on the top surface of a semiconductor laser for the purpose of an electric contact, can be also exploited to provide the waveguide interface.