U. Oesterle

Low-loss channel waveguides with two-dimensional photonic crystal boundaries

We have used transmission measurements to estimate the propagation loss of submicron channels defined in two-dimensional photonic crystals patterned into a Ga(Al)As waveguide. The measured propagation loss of the fundamental mode is indistinguishable from the material absorption, setting an upper limit of 50 cm−1 (2 dB per 100 μm). We also find that, provided the etching is deep enough, propagation losses of these photonic crystal waveguides are lower than those of ridge waveguides etched in the same run.

Low-loss Al-free waveguides for unipolar semiconductor lasers

A promising waveguide design for midinfrared (λ=5–20u200aμm) unipolar semiconductor lasers is proposed and demonstrated in (Al)GaAs quantum cascade structures. In the latter, the active region is embedded between two GaAs layers, with an appropriate doping profile which allows optical confinement, with low waveguide losses and optimal heat dissipation. Low internal cavity losses of 20 cm−1 have been measured using different techniques for lasers with emission wavelength at ∼9 μm. At 77 K, these devices have peak output power in excess of 550 mW and threshold current of 4.7 kA/cm2.

Coupled semiconductor microcavities

The coupled semiconductor microcavity is a system in which there are three oscillators, two photonic and one electronic (quantum well excitons). It develops three strongly coupled modes which allow a wide design range for a variety of optoelectronic applications. The MBE grown structure is comprised of two λ sized GaAs cavities containing InxGa1−xAs quantum wells, separated by a common mirror. Reflectivity measurements show both two coupled photon mode behavior and three coupled mode behavior, i.e., two photon and one exciton, depending on the relative position of the exciton resonance.

Omnidirectional and compact guided light extraction from Archimedean photonic lattices

We address the issue of extracting light from a waveguide towards air in a compact way for randomly oriented guided waves. The goal is to enhance the extraction efficiency of light-emitting diodes while retaining planar processing. For incidence-angle-independent extraction, preferred lattice designs appear to possess a ring-shaped Fourier transform. We demonstrate this property for an Archimedean lattice. This system is the outer part of a resonant-cavity light-emitting diode. Data suggest that ∼40% extraction efficiency is at hand in a planar top-emitting device retaining its substrate.

DUAL-WAVELENGTH LASER EMISSION FROM A COUPLED SEMICONDUCTOR MICROCAVITY

We report photopumped operation of a monolithic coupled semiconductor microcavity laser. The structure consists of two λ-sized GaAs vertical cavities, one on top of the other, coupled together through a common mirror. Due to a wedge induced into each cavity, the detuning between the cavities can be continuously varied when moving across the sample. Depending on the detuning, laser action is simultaneously achieved at two different wavelengths or occurs only at one wavelength. At resonance, we observe coupled dual-wavelength laser emission at two widely spaced wavelengths (13 nm) with the same threshold and same dependence on pump power.We report photopumped operation of a monolithic coupled semiconductor microcavity laser. The structure consists of two λ-sized GaAs vertical cavities, one on top of the other, coupled together through a common mirror. Due to a wedge induced into each cavity, the detuning between the cavities can be continuously varied when moving across the sample. Depending on the detuning, laser action is simultaneously achieved at two different wavelengths or occurs only at one wavelength. At resonance, we observe coupled dual-wavelength laser emission at two widely spaced wavelengths (13 nm) with the same threshold and same dependence on pump power.

Coupled guide and cavity in a two-dimensional photonic crystal

We demonstrate, in a planar two-dimensional (2D) configuration, in the optical regime a clear association of two photonic crystal elements and the ability to produce a low-loss coupled system. A channel waveguide is brought to between two and five crystal rows (450 to 1126 nm) from a 2D microcavity fabricated in a GaAs/AlGaAs waveguide. We probe these two elements individually and explore their interaction.

Ultrahigh finesse microcavity with distributed Bragg reflectors

We have grown a very high finesse microcavity using distributed Bragg reflectors of AlxGa1−xAs and AlAs. The measured Fabry–Perot mode has a linewidth of 0.84 A at 930 nm. This implies a finesse in excess of 5500 and an effective (mirror corrected) finesse greater than 1450. Comparison with theoretical calculations for such a structure shows that (i) the growth rates are stable to 0.25% over 14 h and (ii) the internal losses are less than 1 cm−1.

Tuning InAs/GaAs quantum dot properties under Stranski-Krastanov growth mode for 1.3 μm applications

In this paper, we present a systematic study of the effect of growth parameters on the structural and optical properties of InAs quantum dot (QD) grown under Stranski-Krastanov mode by molecular beam epitaxy. The dot density is significantly reduced from 1.9x10(10) to 0.6x10(10) cm(-2) as the growth rate decreases from 0.075 to 0.019 ML/s, while the island size becomes larger. Correspondingly, the emission wavelength shifts to the longer side. By increasing the indium fraction in the InGaAs capping layer, the emission wavelength increases further. At indium fraction of 0.3, a ground state transition wavelength as long as 1.4 mum with the excited state transition wavelength of around 1.3 mum has been achieved in our dots. The optical properties of QDs with a ground state transition wavelength of 1.3 mum but with different growth techniques were compared. The QDs grown with higher rate and embedded by InGaAs have a higher intensity saturation level from excitation dependent photoluminescence measurements and a smaller intensity decrease from temperature dependent measurements. Finally, single mirror light emitting diodes with a QD embedded in InGaAs have been fabricated. The quantum efficiency at room temperature is 1.3%, corresponding to a radiative efficiency of 21.5%

Use of guided spontaneous emission of a semiconductor to probe the optical properties of two-dimensional photonic crystals

We describe an experimental setup, which allows assessing the optical properties of two-dimensional photonic crystals combined with a waveguide geometry, and etched into a light-emitting (GaAs/InGaAs) semiconductor. By means of a guiding layer, the spontaneous emission of the material is used as a built-in source to probe the properties of the etched microstructure, conveniently compared to the usual measurement schemes. We show polarized transmission and coefficients largely depending on the photonic crystal orientation.

High-finesse disk microcavity based on a circular Bragg reflector

We made disk-shaped microcavities of approximately 10u2009μm2 in area in a GaAs/AlGaAs waveguide structure by etching deep vertical concentric trenches. The trenches form a circular Bragg-like reflector that confines light in the remaining two lateral dimensions. We demonstrate from photoluminescence excited in the waveguide the confinement of discrete disk modes whose wave vector is mainly radial, in contrast with whispering gallery modes. Their quality factors up to Q=650 indicate in-plane reflectivities approaching 90%. In the near infrared, this represents a demonstration of wavelength-scale light confinement based on photonic crystal effects in two dimensions.