Over the last two decades, integrated whispering-gallery-mode resonators have been increasingly used as the basic building blocks for selective filters, high-sensitivity sensors, and as nonlinear converters. In the latter two cases, optimum performance is achieved when the intra-cavity power or the resonance feature contrast are maximum. For devices with transversely singlemode resonator and access waveguides, the above-mentioned conditions are obtained when the system is critically coupled i.e. when the coupler power transfer rate corresponds to the single-pass intra-cavity loss. Designing coupled resonators for which critical-coupling is maintained over a large spectral range is therefore attractive to facilitate sensing or nonlinear frequency conversion.
In this paper, we theoretically show, using a generic model based on the universal description of the device spectral characteristics and a coupled-mode theory treatment of the coupling section, that access-waveguide-coupled resonators can exhibit a wideband critical-coupling bandwidth when their constitutive resonator and access waveguides are different i.e. when they are phase-mismatched. To illustrate this, we have calculated the spectral response of Si3N4/SiO2 racetrack resonators and have found that, when the coupler beat-length becomes achromatic, the device critical-coupling bandwidth is expanded by more one order of magnitude compared to their phase-matched counterpart.
In this article, we apply the coupled-mode theory to vertically-coupled micro-disk resonators presenting an asymmetric distribution of refractive index and a multilayer separation region between the two waveguide cores, resulting in an effective propagation constant phase-mismatch in the coupling region. We introduce a criterion which, given the coupler overall permittivity distribution, clarifies how to best choose the individual decomposition index profiles among the various possible solutions. Following our recent experimental demonstration we subsequently exploit the derived decomposition to evaluate the theoretical transmission characteristics of an AlGaAs/AlOx-based structure as function of wavelength and as function of the position of the resonator relative to the access waveguide.We show that the resonant dips of the intensity transmission, spaced by the cavity FSR, are modulated by an envelop which governs the coupling regime of the resonator-waveguide system.
Integrated whispering-gallery mode resonators are attractive devices which have found applications as selective filters, low-threshold lasers, high-speed modulators, high-sensitivity sensors and even as nonlinear converters. Their performance is governed by the level of detrimental (scattering, bulk, bending) loss incurred and the usable loss represented by the coupling rate between the resonator and its access waveguide. Practically, the latter parameter can be more accurately controlled when the resonator lies above the access waveguide, in other words, when the device uses a vertical integration scheme. So far, when using such an integration technique, the process involved a rather technically challenging step being either a planarization or a substrate transfer step. In this presentation, we propose and demonstrate an alternative method to fabricate vertically-coupled whispering-gallery mode resonators on III-V semiconductor epitaxial structures which has the benefit of being planarization-free and performed as single-side top-down process. The approach relies on a selective lateral thermal oxidation of aluminum-rich AlGaAs layers to define the buried access waveguide and enhance the vertical confinement of the whispering-gallery mode into the resonator. As a first experimental proof-of-principle of this approach, 75 µm-diameter micro-disk devices exhibiting quality factor reaching ~4500 have been successfully made.
This work is devoted to the design of high contrast grating mirrors taking into account the technological constraints and tolerance of fabrication. First, a global optimization algorithm has been combined to a numerical analysis of grating structures (RCWA) to automatically design HCG mirrors. Then, the tolerances of the grating dimensions have been precisely studied to develop a robust optimization algorithm with which high contrast gratings, exhibiting not only a high efficiency but also large tolerance values, could be designed. Finally, several structures integrating previously designed HCGs has been simulated to validate and illustrate the interest of such gratings.
Mid-infrared Vertical cavity surface emitting lasers (MIR-VCSEL) are very attractive compact sources for spectroscopic measurements above 2μm, relevant for molecules sensing in various application domains. A long-standing issue for long wavelength VCSEL is the large structure thickness affecting the laser properties, added for the MIR to the tricky technological implementation of the antimonide alloys system. In this paper, we propose a new geometry for MIR-VCSEL including both a lateral confinement by an oxide aperture, and a high-contrast sub-wavelength grating mirror (HCG mirror) formed by the high contrast combination AIOx/GaAs in place of GaSb/A│AsSb top Bragg reflector. In addition to drastically simplifying the vertical stack, HCG mirror allows to control through its design the beam properties. The robust design of the HCG has been ensured by an original method of optimization based on particle swarm optimization algorithm combined with an anti-optimization one, thus allowing large error tolerance for the nano-fabrication. Oxide-based electro-optical confinement has been adapted to mid-infrared lasers, byusing a metamorphic approach with (Al) GaAs layer directly epitaxially grown on the GaSb-based VCSEL bottom structure. This approach combines the advantages of the will-controlled oxidation of AlAs layer and the efficient gain media of Sb-based for mid-infrared emission. We finally present the results obtained on electrically pumped mid-IR-VCSELs structures, for which we included oxide aperturing for lateral confinement and HCG as high reflectivity output mirrors, both based on AlxOy/GaAs heterostructures.
Development of a reliable, selective, sensitive, technique for atmospheric trace gas concentrations monitoring is a critical challenge in science and engineering. Tunable single-frequency laser in the 2.3 to 3.3µm wavelength range, working in a continuous regime at room temperature can be used for absorption spectroscopy to identify and quantify several gases such as methane (greenhouse gases) and ethylene (food-processing) which are studied in the IES. We report here on the design and fabrication of 1st to 4th order distributed-feedback (DFB) antimonide-lasers diodes in the 2.3 to 3.3µm wavelength range. This process is applied to all studied structures grown by molecular beam epitaxy (MBE) on GaSb substrate.
Electromagnetic modeling helps us to determine the Bragg grating period as well as the global geometry of the structure in order to optimize both modal discrimination and optical power of the lasing mode. The grating is defined by holographic lithography.
Two DFB laser diode designs are proposed and investigated in parallel:
-Side wall corrugation DFB: A corrugation on the lateral sides of the ridge waveguide is transferred by both wet and dry on a hard mask followed by a Cl2/N2 dry etching in the III-V heterostructure.
-Buried DFB: The MBE growth is stopped at the top of the active region. Then the Bragg grating is etched by Ar sputtering . A MBE regrowth process is performed allowing the growth of the upper cladding layer. Next chemical etching of the mesa is done with fluoro-chromic acid.
Si3N4 isolation and evaporation of ohmic contacts ends those processes.
Finally we will show the results on the fabrication and characterization of the devices.
This work is supported by the ANR NexCILAS international project, ANR MIDAS project, NUMEV labex and RENATECH national Network.
Mesoscopic photonic crystal based mirrors are exploited to theoretically conceive and analyse a novel high-Q factor Fabry-Perot-like cavity. Thanks to a beam focussing effect in reflection these mirrors efficiently confine and stabilise a mode inside a planar cavity, even for non-paraxial small beam sizes, mimicking the behaviour of curved mirrors. Furthermore, these mirrors show a higher reflectivity with respect to their standard distributed Bragg reflector counterparts, which allow these cavities to reach a maximum Q factor higher than 104 when 5-period-long mirrors are considered. Moreover, these cavities show other intriguing features as a beamforming behaviour and transverse translational invariance offered by the planar geometry. The latter opens interesting possibilities for lasing and biodetection. The optimization of the cavity size and the performances in terms of Q factor, energy storage and confinement are detailed.
We demonstrate random lasing emission in an active planar slab of AlGaAs/GaAs randomly perforated with subwavelength circular holes. Spectrally-resolved imaging of both the diffusion in the passive regime together with the lasing emission allows identifying exteneded lasing modes in the weakly diffusive regime.
Planar 2D photonic crystals are of tremendous interest for integrated optics applications. In the last years, several
possible laser cavities have been proposed in that prospect. In this paper, we review our work on 2D photonic
crystal second order DFB lasers. We will show that an affine deformation of the photonic crystal allows the
fabrication of closely spaced arrays of high Q DFB lasers. Combining waveguide and photonic crystal affine
deformations, we demonstrate arrays of optically pumped single mode DFB lasers with controlled wavelength
spacing. Potential extension of this scheme will be discussed, altogether with potential for electrical pumping
Laser diodes emitting in the mid-infrared 2-2.7 μm range are of particular interest for spectroscopic applications and
especially for trace gas detection due to the presence of strong absorption bands of several species  of pollutants.
These applications require single-frequency, wavelength tunable lasers. In this perspective, we have studied designs
made of two coupled cavities (C2), coupled by an intracavity Photonic Crystal (PhC) mirror as proposed by Happ et al.
. The first devices of this type have recently been proposed on GaSb, emitting at 1.9 μm .
In this paper, we demonstrate the first coupled cavity (C2) PhC devices operating above 2.3 μm.
Edge emitting photonic crystal (PhC) laser are the keystone opening the way to highly integrated planar photonic circuits. Most of the geometries investigated to date rely on the use of the hexagonal lattice which offers a fully opened photonic band gap. However, it was recently demonstrated that the square lattice based W1 waveguide geometry can provide single mode lasing across the gain whole spectral window (over 150 nm demonstrated under optical pumping). This rather unique property is of high interest for designing high yield, integrated, single mode lasers arrays. In this paper, we show that lasing occurs due to 2nd order DFB effect. Based on 2 and 3 dimensional FDTD computations, we show that the spectral selectivity of the square lattice arises from in plane and out of plane diffraction. We study the tunability options provided by this geometry using FDTD models. We show that whilst C-WDM is compatible with simple design schemes using lattice period variations, WDM and DWDM specifications require the use of a rectangular deformation of the lattice.
VCSELs (Vertical Cavity Surface Emitting Lasers) are nowadays more and more exploited in optoelectronic applications, monitoring their lasing power in a compact and low cost manner becomes crucial. To collect and control the output light, an external photodetector associated with an optical microlens array can be used. Integrated solutions based on the use of a bulk or QW photodetection section added in single-or double-cavity structures have also been proposed. Here, we have investigated a simpler solution based on a standard VCSEL array. Light emitted by a VCSEL has been electrically detected by adjacent VCSELs located in the same array, using in plane optical waveguiding of spontaneous emission in the intrinsic central zone of the devices. We show that the detected photocurrent can be related to the power of the emitting VCSEL. Signal intensity has been studied as a function of VCSELs distance. This method could lead to a more efficient way to monitor VCSEL emission.
High-power unipolar GaAs/AlGaAs lasers emitting in the 14-15 micrometers wavelength range under optical pumping by a pulsed CO2 laser are investigated. Operation of edge lasers with side-facet pumping gas well as broad-area lasers with normal-incidence pumping is demonstrated. We show that record high optical powers can be obtained from these quantum fountain unipolar lasers. Optical powers per facet as high as 6.6 W for edge lasers and 7.8 W for broad-area lasers are achieved with TM00 mode emission. Extended tunability of the lasing wavelength, (Delta) (lambda) /(lambda) approximately equals 2.5 percent, is observed by varying the pump wavelength. Operating temperatures as high as 137 K are presently achieved. Application of quantum fountain unipolar lasers to CO2 gas detection is demonstrated.
A new type of semiconductor unipolar laser operating in the mid-infrared spectral region, the Quantum Fountain intersubband laser, is demonstrated. It is based on optical pumping of a three-bound-state coupled quantum well structure in the GaAs/AlGaAs material system. The lasing transition occurs between the two excited states. Population inversion can be achieved by benefitting from LO-phonon resonance between the two lower subbands. The optical pumping scheme enables a simpler design of the active region than electrically pumped intersubband lasers. Moreover, because doped layers and metallic contacts are not necessary for the operation of the Quantum Fountain laser, free-carrier and plasmon absorptions can be minimised, thus allowing long- wavelength operation. Large optical gains are measured using pump-probe experiments with a free-electron laser. Lasing action under optical pumping by a pulsed CO2 laser has been achieved at a record long wavelength of 15.5 micrometer with an output peak power of the order of 0.6 W at low temperatures.