M. Bahriz

Design of mid-IR and THz quantum cascade laser cavities with complete TM photonic bandgap

The use of a connected honeycomb lattice for creating 2D photonic crystal QC laser structures is presented in this study. Full three-dimensional finite-difference time-domain simulations are used to analyze the properties of the honeycomb lattice in the two cases of realistic mid-IR and THz QC laser structures. A surface plasmon QC laser structure and a metal-metal waveguide geometry are considered in the mid-IR and the THz range, respectively.

Room-temperature operation of λ≈7.5μm surface-plasmon quantum cascade lasers

We report the pulsed, room-temperature operation of λ≈7.5μm quantum cascade lasers (QCLs) in which the optical mode is a surface-plasmon polariton excitation. Previously reported devices based on this concept operate at cryogenic temperatures only. The use of a silver-based electrical contact with reduced optical losses at the QCL emission wavelength allows a reduction of the laser threshold current by a factor of 2 relative to samples with a gold-based contact layer. As a consequence, the devices exhibit room-temperature operation with threshold current densities ∼6.3kA∕cm2. These devices could be used as all-electrical surface-plasmon generators at midinfrared wavelengths.

A micropillar for cavity optomechanics

We have designed a micromechanical resonator suitable for cavity optomechanics. We have used a micropillar geometry to obtain a high-frequency mechanical resonance with a low effective mass and a very high quality factor. We have coated a 60-μm diameter low-loss dielectric mirror on top of the pillar and are planning to use this micromirror as part of a high-finesse Fabry-Perot cavity to laser cool the resonator down to its quantum ground state and to monitor its quantum position fluctuations by quantum-limited optical interferometry.

Demonstration of air-guided quantum cascade lasers without top claddings

We report on quantum cascade lasers employing waveguides based on a predominant air confinement mechanism in which the active region is located immediately at the device top surface. The lasers employ ridge-waveguide resonators with narrow lateral electrical contacts only, with a large, central top region not covered by metallization layers. Devices based on this principle have been reported in the past; however, they employed a thick, doped top-cladding layer in order to allow for uniform current injection. We find that the in-plane conductivity of the active region - when the material used is of high quality - provides adequate electrical injection. As a consequence, the devices demonstrated in this work are thinner, and most importantly they can simultaneously support air-guided and surface-plasmon waveguide modes. When the lateral contacts are narrow, the optical mode is mostly located below the air-semiconductor interface. The mode is predominantly air-guided and it leaks from the top surface into the surrounding environment, suggesting that these lasers could be employed for surface-sensing applications. These laser modes are found to operate up to room temperature under pulsed injection, with an emission spectrum centered around l (1/4) 7:66 mum.

Direct imaging of a laser mode via midinfrared near-field microscopy

Fabry-Perot standing waves inside a midinfrared quantum cascade laser have been imaged using an apertureless scanning near-field optical microscope. The devices emit at λ≈7.7μm and they feature air-confinement waveguides, with the optical mode guided at the semiconductor-air interface. A consistent portion of the mode leaks evanescently from the device top surface and can be detected in the near field of the device. Imaging of the evanescent wave across a plane parallel to the device surface allows one to directly assess the effective light wavelength inside the laser material, yielding the effective index of refraction. Imaging across a plane perpendicular to the device surface allows one to directly measure the electric field decay length, which is found in excellent agreement with the numerical simulations.

High temperature, single mode, long infrared (λ = 17.8 μm) InAs-based quantum cascade lasers

We demonstrate quantum cascade lasers in the InAs/AlSb material system which operate up to 333 K (in pulsed regime) at λ = 17.8 μm. They employ metal-metal optical waveguides and the threshold current density is 1.6 kA/cm2 at 78 K. We also report distributed-feedback devices obtained using the same laser material via a 1st-order Bragg grating inscribed in the sole top metallic contact. Spectral single mode operation with more than 20 dB side mode suppression ratio is achieved at a temperature of 300 K. Large wavelength tuning rates, of the order of 1.5 nm/K, are demonstrated. A wavelength coverage of 0.38 μm is achieved in single-mode regime over a temperature range of 255 K.

Long-infrared InAs-based quantum cascade lasers operating at 291 K (λ=19 μm) with metal-metal resonators

We demonstrate quantum cascade lasers in the InAs/AlSb material system emitting at wavelengths of λ = 19 μm and λ = 21 μm. The maximum operating temperatures are 291 K and 250 K, and the threshold current densities at 78 K are as low as 0.6 kA/cm2 and 1.3 kA/cm2 for the lasers emitting at λ = 19 μm and λ = 21 μm, respectively. These values represent the best performance to date for quantum cascade lasers operating above λ = 16 μm. Although the devices employ metal-metal waveguide geometries, the diffraction effects which typically hinder the output beam of THz devices are not observed.

Optical Mode Control of Surface-Plasmon Quantum Cascade Lasers

Surface-plasmon waveguides based on metallic strips can provide a two-dimensional optical confinement. This concept has been successfully applied to midinfrared quantum cascade lasers, processed as ridge waveguides, to demonstrate that the lateral extension of the optical mode can be influenced solely by the width of the device top contact. In this configuration, the waveguide mode has a reduced interaction with the top metal and the ridge sidewalls. This results in lower propagation losses and higher performances. For devices operating at a wavelength of lambdaap7.5 mum, the room-temperature threshold current density was reduced from 6.3 to 4.4 kA/cm2 with respect to larger devices with full top metallization

A micromechanical resonator to reach the quantum regime

We present a new micromechanical resonator designed for the observation of its quantum ground state (QGS). To reach QGS, a high frequency resonator with the lowest possible mass and the highest possible quality factor, coupled with an extremely sensitive measurement technique, has to be implemented. Using a high-finesse Fabry-Perot cavity with a mirror coated on the resonator, we expect benefits from the unique sensitivity of optical interferometry (10−38 m2/Hz) and from the optomechanical coupling between the light and the micro-resonator both to laser cool the resonator down to its ground state and to observe its residual quantum position fluctuations. We present the resonator we have developed for that purpose, which takes advantage from the high intrinsic quality factor of single crystal quartz and is designed to obtain a high resonance frequency (a few MHz) as well as a low mass (a few tens of µg). A length extension mode is used in order to avoid any deformation of the mirror surface and so to preserve the intrinsic quality factor of the resonator. A dedicated crystallographic orientation and a beam equilateral cross-section have been defined with respect to the quartz trigonal symmetry, allowing the micromachining of the resonator by wet etching. A beam cross-section area of 10−2 mm2 has been chosen to ease the deposit of the multilayered mirror. First mechanical characterizations of the resonator give a resonance frequency of 3.6 MHz, with a 25 µg mass and a quality factor of 390 000. Next steps will be the coating of the low-loss mirror on the resonator and its implementation in the Fabry-Perot cavity.

A micro-resonator for fundamental physics experiments and its possible interest for time and frequency applications

A dedicated micro-resonator in a length-extension mode (LEM) has been developing for fundamental physics experiments, aiming to detect the quantum ground state of a resonator. The experiment is based on a high frequency, high quality factor and low mass micro-resonator, implemented in a high finesse Fabry-Perot cavity at cryogenic temperature. Very interesting preliminary measurements have been obtained on a 3.9 MHz, 25 µg resonator, given a 1.95 million quality factor, at room temperature and under a 5 10−2 mbar vacuum. By its planar configuration compatible with collective etching process, this resonator should also find interesting applications in the field of ultra stable oscillator (USO) and future quartz-MEMS USO.