Injectorless quantum cascade lasers for room-temperature short-wavelength emission by efficient second-harmonic generation

Semiconductor laser sources operating in the wavelength range from 2 µm to 15 µm are attractive components for numerous spectroscopy and free-space communication applications. However, wavelengths below 4 µm have been difficult to achieve due to limited conduction band offsets for the barrier materials and carrier scattering into indirect valleys of the well material [1]. A different approach to generate short wavelengths is to use intracavity frequency doubling. This has been successfully demonstrated in quantum cascade lasers [2]. However, their operation has been limited to cryogenic temperatures. Recently, a novel device design for nonlinear frequency mixing was proposed [3], which allows RT operation at wavelengths around 3.7 µm with peak output powers of around 150 nW at 14 kA/cm 2 [3]. For most applications, CW operation is often required, thus operating current densities have to be minimized, which is difficult to achieve without losing output performance. In this work we propose the use of strain-balanced injectorless QCL active regions for pumping the nonlinear section. Such injectorless QCLs have been shown to posses low threshold current densities and relatively high efficiencies [4, 5]. Here we demonstrate RT second-harmonic generation around 2.7 µm with peak output powers close to 10 µW at current densities of around 2 kA/cm 2 . We will also discuss possible device structures for wavelengths around 3.5 µm and a future development strategy for single-mode, two-color tunable laser sources.