Surface emitting terahertz quantum cascade laser with a double-metal waveguide

Jonathan A Fan,Mohamed Lachab,A Giles Davies,Mikhail A Belkin,Federico Capasso,Suraj Khanna,Edmund H Linfield

doi:10.1364/oe.14.011672

Abstract

We investigate the implementation of surface emission via a second order grating in terahertz quantum cascade lasers with double-metal waveguides. Absorbing edge structures are designed to enforce anti-reflecting boundary conditions, which ensure distributed feedback in the cavity. The grating duty cycle is chosen in order to maximize slope efficiency. Fabricated devices demonstrate surface emission output powers that are comparable to those measured from edge-emitting double metal waveguide structures without gratings. The slope efficiency of surface emitting lasers is twice that of double-metal edge emitting structures. Surface emitting lasers show single mode behavior, with a beam divergence of approximately six degrees.

Highlights

The terahertz (THz) spectral range (1-10 THz) has historically been devoid of a compact source of coherent radiation
The waveguide must provide for low lasing threshold, high slope efficiency, and high mode confinement within the quantum cascade lasers (QCLs) active region, which is typically an order of magnitude thinner than the wavelength of THz radiation in air
Double-metal waveguide THz QCLs were processed from a GaAs-AlGaAs heterostructure based on the bound-to-continuum design reported in Ref. [18]

Summary

Introduction

The terahertz (THz) spectral range (1-10 THz) has historically been devoid of a compact source of coherent radiation. A proper waveguide design is critical to achieve good temperature performance in THz QCLs. The waveguide must provide for low lasing threshold, high slope efficiency, and high mode confinement within the QCL active region, which is typically an order of magnitude thinner than the wavelength of THz radiation in air. One way to increase slope efficiency and avoid the strong beam divergence, while still preserving high mode confinement, is to configure the waveguide for surface emission via a second-order grating in double-metal waveguides. The paper is organized as follows: in the second section, the optimal second-order grating design is calculated for surface emission in a double-metal waveguide QCL operating at approximately 3THz. The third section covers the details of the device fabrication and presents the experimental results of this study

Theory

Fabrication and experimental results

Findings

Conclusions