Substrate-emitting ring interband cascade lasers

Abstract

We demonstrate interband cascade lasers fabricated into ring-shaped cavities with vertical light emission through the substrate at a wavelength of λ ≈ 3.7 μm. The out-coupling mechanism is based on a metallized second-order distributed feedback grating. At room-temperature, a pulsed threshold current-density of 0.75 kA/cm2 and a temperature-tuning rate of 0.3 nm/°C are measured. In contrast to the azimuthal polarization of ring quantum cascade lasers, we observe a radial polarization of the projected nearfield of ring interband cascade lasers. These findings underline the fundamental physical difference between light generation in interband and intersubband cascade lasers, offering new perspectives for device integration.

The Interband Cascade Laser

“…Vertical emission can also be obtained using a second-order grating to diffract a beam propagating in the plane, an approach applied to both QCLs [147] and ICLs. In 2006, NRL demonstrated an optically-pumped type-II "W" photonic crystal distributed feedback laser that emitted from the top surface at λ = 3.7 µm [148], with cw operation at 81 K and pulsed operation above 200 K. More recently, TU Wien and U. Würzburg demonstrated ICLs fabricated into rings and patterned with metallized second-order DFB gratings to provide vertical emission via the substrate [149]. The pulsed threshold current density at room temperature was 750 A/cm 2 for a single-mode wavelength of 3.73 µm, although a second mode appeared at temperatures above 20 • C. The latest ring ICLs were mounted epitaxial-side-down and lased in cw mode to 38 • C [150].…”

Continuous-wave operation of vertically emitting ring interband cascade lasers at room temperature

“…Thus, they are especially attractive for mobile, miniaturized, and battery-powered sensing applications. While edge-emitting ICLs based on GaSb are operating in cw mode at room temperature, 2 vertical light emission has so far been limited to pulsed operation 11,12 or to lower temperatures. 13 A vertical-cavity surface-emitting laser (VCSEL) based on type-II quantum wells (QWs) in a single-stage active region was shown to operate in cw mode up to À7 C at 4 lm, 13 while devices relying on type-I QWs achieved cw operating temperatures of up to 5 C at 3 lm.…”

“…In contrast to VCSELs, where the maximum output power is limited due to their relatively small gain volume, the output power of ring ICLs can be scaled by adjusting the width and diameter of the waveguide. 12 In addition, surface emitters do not rely on a cleaved facet for light emission, as opposed to edge-emitting devices. No additional phase difference is introduced by the position of the cleave edge with regard to the grating as for ridge devices, making the wavelength selection more accurate.…”

“…In previous studies, epi-side up mounting in combination with a multiple mm-wide hole in the heat sink was used to facilitate the collection of light, limiting the devices to pulsed operation due to poor thermal dissipation. 12 In this work, we report on epi-side down mounted ring ICLs allowing cw operation at temperatures of up to 38 C. In addition, the width of the waveguide is reduced to 4 lm compared to 10 lm 12 for the first generation of devices, to suppress lasing of higher order lateral modes. 20 To compensate for the resulting reduction in optical output power due to the smaller gain area, an outer ring diameter of $799 lm was chosen to obtain an active volume corresponding to a device with a ridge waveguide of length 2.5 mm.…”

“…In the following, we point out the major differences and structure improvements compared to the previously published ring ICLs. 12 The waveguide and grating etch is performed in a Cl/Ar-chemistry using an inductively coupled plasma (ICP) reactive ion etching (RIE) process as opposed to the previously used SiCl 4 /Ar-process. This leads to more vertical slightly positively sloped sidewalls as compared to the observed negative slope using the former process.…”