Quantum Cascade Surface Emitting Lasers

Abstract

A low-cost single frequency laser, emitting in the mid-infrared spectral region and dissipating minimal electrical power, is a key ingredient for the next generation of portable gas sensors for high-volume applications involving chemical sensing of important greenhouse and pollutant gases. Herein, a Quantum Cascade Surface Emitting Laser (QCSEL) is proposed, which is implemented as a short linear cavity with high reflectivity coated end-mirrors to suppress any edge emission and employs a buried semiconductor diffraction grating to extract the light from the surface. By wafer-level testing, the cavity length scaling is investigated, mirror reflectivities larger than 0.9 are extracted, and a pulsed threshold power dissipation of 237 mW for an emission wavelength near 7.5 µm is achieved. Finally, single-mode emission with a side-mode suppression ratio larger than 33 dB is demonstrated for a 248 µm short cavity, which is mounted with the epitaxial layer up and operated in continuous wave at 20 $^{\circ}{\rm C}$ .

1 Introduction

The mid-infrared (MIR) spectral region, spanning from 3 to 30 µm, is the molecular “fingerprint” region for many important organic and inorganic molecules, as they exhibit strong and narrow absorption lines within this region.^[^{1, 2}^] Miniaturized optical gas sensors based on MIR absorption spectroscopy are highly attractive^[³^] for many applications such as industrial process control, environmental monitoring, and medical diagnosis.^[⁴^] To enable low-cost and portable MIR gas sensors, compact, and low power dissipation single-mode light sources operating in the range of interest are of uttermost importance.

The Quantum Cascade Laser (QCL), relying on intersubband transitions, is an excellent candidate for a compact and coherent MIR light source because the emission wavelength can be tailored in wide ranges between 3 and 24 µm, as well as between 60 and 300 µm.^[⁵^] The QCL can be modulated up to tens of GHz^[⁶^] and exhibits narrow linewidths,^[⁷^] making it the source of choice for fast high-resolution gas spectroscopy in the MIR.