W. Schrenk

Microcavity-Integrated Graphene Photodetector

There is an increasing interest in using graphene1,2 for optoelectronic applications.3−19 However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to be developed to increase the absorption and take full advantage of its unique optical properties. We demonstrate that by monolithically integrating graphene with a Fabry-Pérot microcavity, the optical absorption is 26-fold enhanced, reaching values >60%. We present a graphene-based microcavity photodetector with responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.

High-temperature performance of GaAs-based bound-to-continuum quantum-cascade lasers

GaAs-based quantum-cascade lasers based on a bound-to-continuum transition have been realized and characterized. This band structure design combines the advantages of the well known three-well and superlattice active regions. We observed lasing of Fabry–Perot lasers in pulsed mode up to a temperature of 100 °C. Multimode emission with a pulsed peak power of 340 mW is observed at room temperature and 42 mW at 80 °C. Further, from aging tests we expect a lifetime of over 60 years for these devices.

GaAs/AlGaAs superlattice quantum cascade lasers at λ≈13 μm

We report the realization of an injection laser based on intraband transitions in a finite AlGaAs/GaAs superlattice. The active material is a 30 period sequence of injectors/active regions made from AlGaAs/GaAs quantum wells. By an applied electric field, electrons are injected into the second miniband of a chirped superlattice and relax radiative to the lowest miniband. At a heat-sink temperature of 10 K, the laser emission wavelength is 12.9 μm with peak optical powers exceeding 100 mW and a threshold current density of 9.8 kA/cm2. The maximum operating temperature is 50 K. For this device, a waveguide consisting of heavily doped GaAs cladding and low doped core layers has been used as a plasma-enhanced confinement.

Continuous-wave operation of distributed feedback AlAs/GaAs superlattice quantum-cascade lasers

We report on continuous-wave operation of first-order distributed feedback quantum-cascade lasers at λ=11.8 μm, based on interminiband transitions in a chirped AlAs/GaAs superlattice. Short devices operate in continuous-wave up to ∼30 K. The single-mode emission wavelength is continuously tunable with the temperature. A metallized surface-relief grating is used for feedback to achieve single-mode emission.

Gate insulation and drain current saturation mechanism in InAlN/GaN metal-oxide-semiconductor high-electron-mobility transistors

The authors investigate 2μm gate-length InAlN∕GaN metal-oxide-semiconductor high-electron-mobility transistors (MOS HEMTs) with 12nm thick Al2O3 gate insulation. Compared to the Schottky barrier (SB) HEMT with similar design, the MOS HEMT exhibits a gate leakage reduction by six to ten orders of magnitude. A maximal drain current density (IDS=0.9A∕mm) and an extrinsic transconductance (gme=115mS∕mm) of the MOS HEMT also show improvements despite the threshold voltage shift. An analytical modeling shows that a higher mobility of electrons in the channel of the MOS HEMT and consequently a higher number of electrons attaining the velocity saturation may explain the observed increase in gme after the gate insulation.

Monolithically integrated mid-infrared lab-on-a-chip using plasmonics and quantum cascade structures

The increasing demand of rapid sensing and diagnosis in remote areas requires the development of compact and cost-effective mid-infrared sensing devices. So far, all miniaturization concepts have been demonstrated with discrete optical components. Here we present a monolithically integrated sensor based on mid-infrared absorption spectroscopy. A bi-functional quantum cascade laser/detector is used, where, by changing the applied bias, the device switches between laser and detector operation. The interaction with chemicals in a liquid is resolved via a dielectric-loaded surface plasmon polariton waveguide. The thin dielectric layer enhances the confinement and enables efficient end-fire coupling from and to the laser and detector. The unamplified detector signal shows a slope of 1.8–7 μV per p.p.m., which demonstrates the capability to reach p.p.m. accuracy over a wide range of concentrations (0–60%). Without any hybrid integration or subwavelength patterning, our approach allows a straightforward and cost-saving fabrication.

GaAs/AlGaAs distributed feedback quantum cascade lasers

We report on the realization of distributed feedback quantum cascade lasers in the GaAs/AlGaAs material system. A metallized surface relief grating is used for feedback. Both single mode and double mode emission is observed at λ≈10 μm. The coupling coefficient is measured from the mode spacing for double mode emission to be 24 cm−1. The emission wave number can be tuned with the temperature at a rate of dν/dT≈0.048 cm−1/K.

Analysis of TM-polarized DFB laser structures with metal surface gratings

An analysis of distributed feedback (DFB) laser structures with metallized surface grating structures in TM polarization is presented. The modal properties of these structures are described using coupled-mode theory where the coupling coefficients are derived from rigorously computed on-resonant Floquet-Bloch solutions of the waveguide grating problem. Based on this theory, first- and second-order DFB quantum cascade laser structures operating at a wavelength of 10 /spl mu/m are investigated numerically. We show that, utilizing a metal stripe grating structure, second-order laser structures are feasible showing efficient surface emission, whereas radiation into the substrate is strongly suppressed. The fraction of stimulated emission power being emitted via the surface can be as high as 17.5% whereas a low threshold gain of 20 cm/sup -1/ is maintained.

Single-mode surface-emitting quantum-cascade lasers

We present high-power surface-emitting second-order distributed feedback quantum-cascade lasers in GaAs and InP material systems. The GaAs device, grown by molecular-beam epitaxy, showed single-mode peak output powers of 3 W at 78 K in pulsed operation. With the InP-based devices, which are grown by metalorganic vapor phase epitaxy, we obtained single-mode peak output powers of 1 W at room temperature. These are the highest output powers for surface emission of quantum-cascade lasers reported so far. The InP-based distributed feedback lasers also have very low threshold current densities and are working well above room temperature.

Surface-emitting distributed feedback quantum-cascade lasers

We report on the realization of second-order distributed feedback quantum-cascade lasers at λ=9.35 μm, where the active region consists of GaAs, AlGaAs, and strained InGaAs grown on GaAs. A metal-stripe surface grating structure allows a high surface emission efficiency for the TM-polarized light. The emitted power via the surface is in the range of 100 mW and exceeds the emitted power from one facet. A double-lobed surface-emission far-field pattern is obtained for the lasing mode. The single-mode emission wavelength is continuously tunable by the heat sink temperature.