X. Checoury

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.

Tensile-strained germanium microdisks

We show that a strong tensile strain can be applied to germanium microdisks using silicon nitride stressors. The transferred strain allows one to control the direct band gap emission that is shifted from 1550 nm up to 2000 nm, corresponding to a biaxial tensile strain around 1%. Both Fabry-Perot and whispering gallery modes are evidenced by room temperature photoluminescence measurements. Quality factors up to 1350 and limited by free carrier absorption of the doped layer are observed for the whispering gallery modes. We discuss the strain profile in the microdisks as a function of the disk geometry. These tensile-strained microdisks are promising candidates to achieve Ge laser emission in compact microresonators.

Recent advances in germanium emission [Invited]

The optical properties of germanium can be tailored by combining strain engineering and n-type doping. In this paper, we review the recent progress that has been reported in the study of germanium light emitters for silicon photonics. We discuss the different approaches that were implemented for strain engineering and the issues associated with n-type doping. We show that compact germanium emitters can be obtained by processing germanium into tensile-strained microdisks.

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.

Recent Advances Toward Optical Devices in Semiconductor-Based Photonic Crystals

Photonic crystals, artificial, wavelength-scale multidimensional periodic structures, have given birth to a number of realizations in semiconductors. Photonic integrated circuits, especially around new integrated lasers, are challenging directions of research for miniaturization and new functions in optical telecommunications. We review the basic physics behind such applications and underline the current status of this very active research field worldwide

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.

Unidirectional band gaps in uniformly magnetized two-dimensional magnetophotonic crystals

By exploiting the concepts of magnetic group theory we show how unidirectional behavior can be obtained in two-dimensional magnetophotonic crystals (MOPhC) with uniform magnetization. This group theory approach generalizes all previous investigations of one-way MOPhCs including those based on the use of antiparallel magnetic domains in the elementary crystal cell. Here, the theoretical approach is illustrated for one MOPhC example where unidirectional behavior is obtained by appropriately lowering the geometrical symmetry of the elementary motifs. One-way transmission is numerically demonstrated for a photonic crystal slice.

Probing photonic crystals on silicon-on-insulator with Ge∕Si self-assembled islands as an internal source

We report the study of two-dimensional photonic crystals fabricated on silicon-on-insulator substrates. Ge∕Si self-assembled islands are embedded as an active internal optical source inside the photonic crystals. We present a detailed analysis of photonic crystal microcavities and waveguides using the room-temperature Ge∕Si island photoluminescence. The tunability of the microcavity resonant emission is demonstrated between 1.2 and 1.5μm. We show that the microcavity photoluminescence is weakly dependent on the temperature. The polarized transmission properties of W1 single-line defect waveguides are investigated using the photoluminescence as an internal source. The transmission spectra are correlated to those given by two-dimensional finite-difference time-domain calculations.

Near-infrared gallium nitride two-dimensional photonic crystal platform on silicon

We demonstrate a two-dimensional free-standing gallium nitride photonic crystal platform operating around 1550 nm and fabricated on a silicon substrate. Width-modulated waveguide cavities are integrated and exhibit loaded quality factors up to 34 000 at 1575 nm. We show the resonance tunability by varying the ratio of air hole radius to periodicity, and cavity hole displacement. We deduce a ∼7.9 dB/cm linear absorption loss for the suspended nitride structure from the power dependence of the cavity in-plane transmission.

Stacking patterns in self-assembly opal photonic crystals

In this letter the authors present both experimental and numerical studies of the optical properties of four-layer artificial opals. The stacking of four layers of spheres may arise according to three different arrangements: face-centered cubic, hexagonal close packed, or double hexagonal close packed. The study shows that the transmission spectra features are characteristic of the type of stacking, and thus, each color region observed under the optical microscope can be unambiguously associated with one of the stacking types.