Intrinsic polarity inversion in III-nitride waveguides for efficient nonlinear interactions.

III-nitrides provide a versatile platform for nonlinear photonics. In this work, we explore a new promising configuration - composite waveguides containing GaN and AlN layers with inverted polarity, i.e., having opposite signs of the χ(2) nonlinear coefficient. This configuration allows us to address the limiting problem of the mode overlap for nonlinear interactions. Our modelling predicts a significant improvement in the conversion efficiency. We confirm our theoretical prediction with the experimental demonstration of second harmonic generation with an efficiency of 4%W-1cm-2 using a simple ridge waveguide. This efficiency is an order of magnitude higher compared to the previously reported results for III-nitride waveguides. Further improvement, reaching a theoretical efficiency of 30%W-1cm-2, can be achieved by reducing propagation losses.

Low-loss GaN-on-insulator platform for integrated photonics.

III-Nitride semiconductors are promising materials for on-chip integrated photonics. They provide a wide transparency window from the ultra-violet to the infrared that can be exploited for second-order nonlinear conversions. Here we demonstrate a photonics platform based on epitaxial GaN-on-insulator on silicon. The transfer of the epi-material on SiO2 is achieved through wafer bonding. We show that quality factors up to 230 000 can be achieved with this platform at telecommunication wavelengths. Resonant second harmonic generation is demonstrated with a continuous wave conversion efficiency of 0.24%/W.

Up to 300†K lasing with GeSn-On-Insulator microdisk resonators.

GeSn alloys are the most promising direct band gap semiconductors to demonstrate full CMOS-compatible laser integration with a manufacturing from Group-IV materials. Here, we show that room temperature lasing, up to 300 K, can be obtained with GeSn. This is achieved in microdisk resonators fabricated on a GeSn-On-Insulator platform by combining strain engineering with a thick layer of high Sn content GeSn.

III-nitride on silicon electrically injected microrings for nanophotonic circuits.

Nanophotonic circuits using group III-nitrides on silicon are still lacking one key component: efficient electrical injection. In this paper we demonstrate an electrical injection scheme using a metal microbridge contact in thin III-nitride on silicon mushroom-type microrings that is compatible with integrated nanophotonic circuits with the goal of achieving electrically injected lasing. Using a central buried n-contact to bypass the insulating buffer layers, we are able to underetch the microring, which is essential for maintaining vertical confinement in a thin disk. We demonstrate direct current room-temperature electroluminescence with 440 mW/cm2 output power density at 20 mA from such microrings with diameters of 30 to 50 µm. The first steps towards achieving an integrated photonic circuit are demonstrated.

Solving thermal issues in tensile-strained Ge microdisks.

We propose to use Ge-dielectric-metal stacking to allow one to address both thermal management with the metal as an efficient heat sink and tensile strain engineering with the buried dielectric as a stressor layer. This scheme is particularly useful for the development of Ge-based optical sources. We demonstrate experimentally the relevance of this approach by comparing the optical response of tensile-strained Ge microdisks with an Al heat sink or an oxide pedestal. Photoluminescence indicates a much reduced temperature rise in the microdisk (16 K with Al pedestal against 200 K with SiO2 pedestal under a 9 mW continuous wave optical pumping). An excellent agreement is found with finite element modeling of the temperature rise. This original stacking combining metal and dielectrics is promising for integrated photonics where thermal management is an issue.

Phase-matched second harmonic generation with on-chip GaN-on-Si microdisks.

We demonstrate phase-matched second harmonic generation in gallium nitride on silicon microdisks. The microdisks are integrated with side-coupling bus waveguides in a two-dimensional photonic circuit. The second harmonic generation is excited with a continuous wave laser in the telecom band. By fabricating a series of microdisks with diameters varying by steps of 8 nm, we obtain a tuning of the whispering gallery mode resonances for the fundamental and harmonic waves. Phase matching is obtained when both resonances are matched with modes satisfying the conservation of orbital momentum, which leads to a pronounced enhancement of frequency conversion.

Surface-sensitive diamond photonic crystals for high-performance gas detection.

Diamond slotted photonic crystal (PhC) cavities were fabricated and used for gas detection. They exhibit wavelength sensitivity reaching a 350 nm per unit change of the refractive index of the gaseous environment of the PhC. With a simple oxidized surface termination, diamond PhCs display an ultrahigh sensitivity to the surface adsorption of polar molecules. Gaseous concentrations as low as 80 parts per million (ppm) of hexanol vapor in nitrogen are probed, and a detection limit in the ppm range is inferred, demonstrating a high interest of such devices for trace sensing.

Near-infrared III-nitride-on-silicon nanophotonic platform with microdisk resonators.

We have developed a nanophotonic platform with microdisks using epitaxial III-nitride materials on silicon. The two-dimensional platform consists of suspended waveguides and mushroom-type microdisks as resonators side-coupled with a bus waveguide. Loaded quality factors up to 80000 have been obtained in the near-infrared spectral range for microdisk diameters between 8 and 15 µm. We analyze the dependence of the quality factors as a function of coupling efficiency. We have performed continuous-wave second harmonic generation experiments in resonance with the whispering gallery modes supported by the microdisks.

Deep-UV nitride-on-silicon microdisk lasers.

Deep ultra-violet semiconductor lasers have numerous applications for optical storage and biochemistry. Many strategies based on nitride heterostructures and adapted substrates have been investigated to develop efficient active layers in this spectral range, starting with AlGaN quantum wells on AlN substrates and more recently sapphire and SiC substrates. Here we report an efficient and simple solution relying on binary GaN/AlN quantum wells grown on a thin AlN buffer layer on a silicon substrate. This active region is embedded in microdisk photonic resonators of high quality factors and allows the demonstration of a deep ultra-violet microlaser operating at 275 nm at room temperature under optical pumping, with a spontaneous emission coupling factor ß = (4 ± 2) 10(-4). The ability of the active layer to be released from the silicon substrate and to be grown on silicon-on-insulator substrates opens the way to future developments of nitride nanophotonic platforms on silicon.

Tensile-strained germanium microdisk electroluminescence.

We report room temperature electroluminescence of tensile-strained germanium microdisks. The strain is transferred into the microdisks using silicon nitride stressors. Carrier injection is achieved with Schottky contacts on n-type doped germanium. We show that a biaxial tensile-strain up to 0.72% can be transferred by optimizing the carrier injection profile. The transferred strain is measured by the electroluminescence spectral red-shift and compared to finite element modeling. We discuss the impact of this strain level to achieve population inversion in germanium.

Tensile Ge microstructures for lasing fabricated by means of a silicon complementary metal-oxide-semiconductor process.

In this work we study, using experiments and theoretical modeling, the mechanical and optical properties of tensile strained Ge microstructures directly fabricated in a state-of-the art complementary metal-oxide-semiconductor fabrication line, using fully qualified materials and methods. We show that these microstructures can be used as active lasing materials in mm-long Fabry-Perot cavities, taking advantage of strain-enhanced direct band gap recombination. The results of our study can be realistically applied to the fabrication of a prototype platform for monolithic integration of near infrared laser sources for silicon photonics.

Effective thermal resistance of a photonic crystal microcavity.

We present a simple method to accurately measure the effective thermal resistance of a photonic crystal microcavity. The cavity is embedded between two Schottky contacts forming a metal-semiconductor-metal device. The photocarriers circulating in the device provide a local temperature rise that can be dominated by Joule effect under certain conditions. We show that the effective thermal resistance (R(th)) can be experimentally deduced from the spectral shift of the cavity resonance wavelength measured at different applied bias. We deduce a value of R(th)1.6×10(4) KW(-1) for a microcavity on silicon-on-insulator, which is in good agreement with 3D thermal modeling by finite elements.

Imaging of photonic modes in an AlN-based photonic crystal probed by an ultra-violet internal light source.

The spatial distribution of photon modes confined in a 0D cavity and a 1D W1 waveguide is investigated on AlN-based photonic crystal (PC) membranes by spectrally resolved scanning confocal microscopy in the ultra-violet spectral range. The influence of the fabrication-induced disorder of the PC on the photon modes is analyzed. The cavity modes are shown to be robust with respect to disorder, whereas the 1D modes of the W1 waveguide are localized near its cut-off frequency. Those modifications of the lowest energy photonic modes are compared to simulations of weakly disordered photonic structures.

Optical gain in single tensile-strained germanium photonic wire.

We have investigated the optical properties of tensile-strained germanium photonic wires. The photonic wires patterned by electron beam lithography (50 µm long, 1 µm wide and 500 nm thick) are obtained by growing a n-doped germanium film on a GaAs substrate. Tensile strain is transferred in the germanium layer using a Si3N4 stressor. Tensile strain around 0.4% achieved by the technique corresponds to an optical recombination of tensile-strained germanium involving light hole band around 1690 nm at room temperature. We show that the waveguided emission associated with a single tensile-strained germanium wire increases superlinearly as a function of the illuminated length. A 20% decrease of the spectral broadening is observed as the pump intensity is increased. All these features are signatures of optical gain. A 80 cm⁻¹ modal optical gain is derived from the variable strip length method. This value is accounted for by the calculated gain material value using a 30 band k · p formalism. These germanium wires represent potential building blocks for integration of nanoscale optical sources on silicon.

High quality factor nitride-based optical cavities: microdisks with embedded GaN/Al(Ga)N quantum dots.

We compare the quality factor values of the whispering gallery modes of microdisks (µ-disks) incorporating GaN quantum dots (QDs) grown on AlN and AlGaN barriers by performing room temperature photoluminescence (PL) spectroscopy. The PL measurements show a large number of high Q factor resonant modes on the whole spectrum, which allows us to identify the different radial mode families and to compare them with simulations. We report a considerable improvement of the Q factor, which reflects the etching quality and the relatively low cavity loss by inserting QDs into the cavity. GaN/AlN QDs-based µ-disks show very high Q values (Q>7000) whereas the Q factor is only up to 2000 in µ-disks embedding QDs grown on the AlGaN barrier layer. We attribute this difference to the lower absorption below bandgap for AlN barrier layers at the energies of our experimental investigation.

Quality factor of Si-based photonic crystal L3 nanocavities probed with an internal source.

We have investigated the quality factors of silicon-based photonic crystal nanocavities using the photoluminescence of a single layer of Ge/Si self-assembled islands as an internal source. We focus on membrane-type L3 elongated cavities with or without their lateral edge air holes shifted in position. The photoluminescence measurements are performed at room temperature. We show that the quality factor of the fundamental mode observed at a normalized frequency u = a/lambda~_ 0.25 is strongly dependent on the incident pump power. This dependence is associated with the free-carrier absorption of the photogenerated carriers. The slope of the quality factor vs. incident pump power gives access to the carrier recombination dynamics in these Si-based nanocavities. The measurements indicate that the carrier dynamics is controlled by nonradiative recombination associated with surface recombinations. A surface recombination velocity of 4.8 x 10(4) cm/s is deduced from the experiments. The spectral red-shift of the cavity modes as a function of incident pump power is correlated to the temperature rise due to thermo-optic effects. The measured temperature rise, which can reach 190 K, is correlated to the value estimated by a thermal analysis.

Long polaron lifetime in InAs/GaAs self-assembled quantum dots.

We have investigated the polaron dynamics in n-doped InAs/GaAs self-assembled quantum dots by pump-probe midinfrared spectroscopy. A long T1 polaron decay time is measured at both low temperature and room temperature, with values around 70 and 37 ps, respectively. The decay time decreases for energies closer to the optical phonon energy. The relaxation is explained by the strong coupling for the electron-phonon interaction and by the finite lifetime of the optical phonons. We show that, even for a large detuning of 19 meV from the LO photon energy in GaAs, the carrier relaxation remains phonon assisted.