Virginie Moreau

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.

Surface-emitting quantum cascade lasers with metallic photonic-crystal resonators

Surface emitting photonic-crystal quantum cascade lasers operating at λ≈7.3 μm are demonstrated. The photonic crystal resonator is written solely on the top metallization layer. The mismatch between the modes supported by metallized and nonmetallized regions yields enough optical feedback to achieve laser action. The devices exhibit single-mode emission with a side mode suppression ratio of ≈20 dB, the wavelength is lithographically tunable across a range of almost 70 cm−1, and the radiation is emitted from the surface. The maximum operating temperature is 220 K. The divergence of the output beam, which is doughnut-shaped, is approximately 9°.

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.

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.

Transient thermoreflectance imaging of active photonic crystals

Transient thermoreflectance imaging is used to study the dynamics of the temperature inside active two-dimensional photonic crystals (PhCs). We developed a pump-probe setup suited for optically pumped devices that presents submicrosecond time resolution and submicrometer spatial resolution. Characteristic thermal dissipation times of 429 ns in a PhC Bloch mode cavity and of 999 ns in a PhC membrane are measured. This technique gives also access to the diffusivity of the suspended PhC.

Proof-of-principle of surface detection with air-guided quantum cascade lasers

We report a proof-of-principle of surface detection with air-guided quantum cascade lasers. Laser ridges were designed to exhibit an evanescent electromagnetic field on their top surface that can interact with material or liquids deposited on the device. We employ photoresist and common solvents to provide a demonstration of the sensor setup. We observed spectral as well as threshold currents changes as a function of the deposited material absorption curve. A simple model, supplemented by 2D numerical finite element method simulations, allows one to explain and correctly predict the experimental results.

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

Quantum cascade photonic crystal lasers: Design, fabrication, and applications

The design and fabrication of quantum cascade photonic crystal surface emitting lasers in the mid infrared for intra-cavity spectroscopy and integration with microfluidic delivery is presented.

Near-field imaging of the evanescent electric field on the surface of a quantum cascade laser

We report the observation by scanning near-field optical microscopy of the evanescent electric field on the surface of a quantum cascade laser. The devices have been designed in order to let a consistent portion of its optical mode to leak out of the top surface. The laser wavelength in vacuum is ¿ 7.7 ¿m