Enhancement of quantum cascade laser intersubband transitions via coupling to resonant discrete photonic modes of subwavelength gratings.

We present an optical spectroscopic study of InGaAs/AlInAs active region of quantum cascade lasers grown by low pressure metal organic vapor phase epitaxy combined with subwavelength gratings fabricated by reactive ion etching. Fourier-transformed photoluminescence measurements were used to compare the emission properties of structures before and after processing the gratings. Our results demonstrate a significant increase of the photoluminescence intensity related to intersubband transitions in the mid-infrared, which is attributed to coupling with the grating modes via so called photonic Fano resonances. Our findings demonstrate a promising method for enhancing the emission in optoelectronic devices operating in a broad range of application-relevant infrared.

Threshold performance of pulse-operating quantum-cascade vertical-cavity surface-emitting lasers.

Quantum-cascade (QC) vertical-cavity surface-emitting lasers (VCSELs) could combine the single longitudinal mode operation, low threshold currents, circular output beam, and on-wafer testing associated with VCSEL configuration and the unprecedented flexibility of QCs in terms of wavelength emission tuning in the infrared spectral range. The key component of QC VCSEL is the monolithic high-contrast grating (MHCG) inducing light polarization, which is required for stimulated emission in unipolar quantum wells. In this paper, we demonstrate a numerical model of the threshold operation of a QC VCSEL under the pulse regime. We discuss the physical phenomena that determine the architecture of QC VCSELs. We also explore mechanisms that influence QC VCSEL operation, with particular emphasis on voltage-driven gain cumulation as the primary mechanism limiting QC VCSEL efficiency. By numerical simulations, we perform a thorough analysis of the threshold operation of QC VCSELs. We consider the influence of optical and electrical aperture dimensions and reveal the range of aperture values that enable single transversal mode operation as well as low threshold currents.

Experimental Demonstration of Light Focusing Enabled by Monolithic High-Contrast Grating Mirrors.

We present the first experimental demonstration of a planar focusing monolithic subwavelength grating mirror. The grating is formed on the surface of GaAs and focuses 980 nm light in one dimension on the high-refractive-index side of the mirror. According to our measurements, the focal length is 475 µm (300 µm of which is GaAs) and the numerical aperture is 0.52. The intensity of the light at the focal point is 23 times larger than that of the incident light. To the best of our knowledge, this is the highest value reported for a grating mirror. Moreover, the full width at half-maximum (FWHM) at the focal point is only 3.9 µm, which is the smallest reported value for a grating mirror. All of the measured parameters are close to or very close to the theoretically predicted values. Our realization of a sophisticated design of a focusing monolithic subwavelength grating opens a new avenue to technologically simple fabrication of the gratings for use in diverse optoelectronic materials and applications.

Planar focusing reflectors based on monolithic high contrast gratings: design procedure and comparison with parabolic mirrors.

Here, we describe in detail a procedure for the numerical design of planar focusing mirrors based on monolithic high contrast gratings. We put a special emphasis on the reconstruction of the hyperbolic phase of these mirrors and we conclude that the phase does not have to be perfectly mimicked to obtain a focusing reflector. We consider here the grating mirrors that focus light not in the air but in the GaAs substrate and we compare them with conventional parabolic reflectors of corresponding dimensions. The light intensity at the focal point of the focusing grating mirrors was found to be comparable to that of the parabolic reflector. Moreover, the reflectivity of the focusing grating mirrors is almost as high as that of parabolic mirrors covered with an additional reflecting structure, if the ratio of the reflector width to the focal length is less than 0.6. Planar focusing grating mirrors offer a good alternative to parabolic mirrors, especially considering the complexity of fabricating three-dimensional structures compared to planar structures.

Nearly perfect transmission of unpolarized infrared radiation through a one-dimensional metal grating embedded in a monolithic high-contrast grating.

We demonstrate a conceptually simple polarization-independent mechanism for nearly perfect infrared light transmission through a subwavelength one-dimensional metal grating implemented in the grooves of a deep-subwavelength monolithic high-contrast grating (metalMHCG). We provide theoretical background explaining the transmission mechanism, which eliminates Fresnel reflection as well as significantly reduces metal absorption and the reflection of transverse electric and transverse magnetic light polarizations. Careful design of a metalMHCG implemented at the interface between the regions of high refractive index contrast enables the coincidence of high transmission conditions for both light polarizations, enabling up to 97% transmission of polarization-independent infrared radiation. Our analysis shows excellent electrical properties of the metalMHCG as evidenced by sheet resistance of 2 ΩSq-1 facilitating straightforward horizontal electron transport and vertical injection of the current into the semiconductor substrate on which the electrode is implemented.

Surface Photochemical Corrosion as a Mechanism for Fast Degradation of InGaN UV Laser Diodes.

We studied degradation mechanisms of ultraviolet InGaN laser diodes emitting in the UVA range. Short wavelength nitride devices are subjected to much faster degradation, under the same packaging and testing conditions, than their longer wavelength counterparts. Transmission electron microscopy analysis of the degraded laser diodes showed pronounced damage to facets in the area of the active layer (waveguide, quantum wells, and electron blocking layer). Energy-dispersive X-ray spectroscopy showed that the active layers were heavily oxidized, forming a compound close in composition to Ga2O3 with proportional addition of Al in the respective area. The oxidation depth was roughly proportional to the intensity of the optical field. We propose UV-light-induced water splitting on a semiconductor surface as a mechanism of the oxidation and degradation of these devices.

Transparent electrode employing deep-subwavelength monolithic high-contrast grating integrated with metal.

This paper demonstrates designs of transparent electrodes for polarized light based on semiconductor deep-subwavelength monolithic high-contrast gratings integrated with metal (metalMHCG). We provide theoretical background explaining the phenomena of high transmittance in the gratings and investigate their optimal parameters, which enable above 95% transmittance for sheet resistance of 2 ΩSq-1 and over 90% transmittance for extremely small sheet resistance of 0.04 ΩSq-1 in a broad spectral range below the semiconductor band-gap. The analysis is based on our fully vectorial optical model, which has been verified previously via comparison with the experimental characteristics of similar structures. The transparent electrodes can be realized in any high refractive index material used in optoelectronics and designed for light in spectral ranges starting from ultra-violet with no upper limit for the wavelength of the electromagnetic waves. They not only enable lateral transport of electrons but can also be used as an electric contact for injecting current into a semiconductor.

Tuning of reflection spectrum of a monolithic high-contrast grating by variation of its spatial dimensions.

We report the first experimental parametric analysis of subwavelength monolithic high-contrast grating (MHCG) mirrors. To date, subwavelength grating mirrors have been fabricated by suspending a thin grating membrane in the air or placing it on a low refractive index material - a scheme that requires sophisticated processing and makes the gratings sensitive to mechanical stress, impeding current injection, and heat dissipation if used in active devices. Inherently MHCGs are well suited for optoelectronic devices because they can be fabricated in all possible material systems. Here we demonstrate above 90% optical power reflectance, strong polarization discrimination. Based on experimental analysis aided by numerical simulations, we demonstrate the possibility of tuning the spectral characteristics of MHCGs reflectance for more than 200 nm via modification of the duty cycle of the MHCG stripes. We show our MHCG tuning method is convenient to define the properties of MHCG devices during the device processing.

Numerical Investigation of the Impact of ITO, AlInN, Plasmonic GaN and Top Gold Metalization on Semipolar Green EELs.

In this paper, we present the results of a computational analysis of continuous-wave (CW) room-temperature (RT) semipolar InGaN/GaN edge-emitting lasers (EELs) operating in the green spectral region. In our calculations, we focused on the most promising materials and design solutions for the cladding layers, in terms of enhancing optical mode confinement. The structural modifications included optimization of top gold metalization, partial replacement of p-type GaN cladding layers with ITO and introducing low refractive index lattice-matched AlInN or plasmonic GaN regions. Based on our numerical findings, we show that by employing new material modifications to green EELs operating at around 540 nm it is possible to decrease their CW RT threshold current densities from over 11 kA/cm2 to less than 7 kA/cm2.

Influence of Various Bottom DBR Designs on the Thermal Properties of Blue Semiconductor-Metal Subwavelength-Grating VCSELs.

In this paper, we consider several designs for nitride-based vertical-cavity surface-emitting lasers (VCSELs) with a top semiconductor-metal subwavelength grating (SMSG) as the facet mirror. The constructions of the bottom distributed Bragg reflectors (DBRs) used in the VCSEL designs were inspired by devices demonstrated recently by several research groups. A multiparameter numerical analysis was performed, based on self-consistent thermal and electrical simulations. The results show that, in the case of small aperture VCSEL designs, dielectric-based DBRs with metallic or GaN channels enable equally efficient heat dissipation to designs with monolithically integrated DBRs. In the case of broad aperture designs enabled by SMSGs, monolithically integrated DBRs provide much more efficient heat dissipation in comparison to all other considered designs.

Subwavelength grating as both emission mirror and electrical contact for VCSELs in any material system.

Semiconductor-metal subwavelength grating (SMSG) can serve a dual purpose in vertical-cavity surface-emitting lasers (VCSELs), as both optical coupler and current injector. SMSGs provide optical as well as lateral current confinement, eliminating the need for ring contacts and lateral build-in optical and current confinement, allowing their implementation on arbitrarily large surfaces. Using an SMSG as the top mirror enables fabrication of monolithic VCSELs from any type of semiconductor crystal. The construction of VCSELs with SMSGs requires significantly less p-type material, in comparison to conventional VCSELs. In this paper, using a three-dimensional, fully vectorial optical model, we analyse the properties of the stand-alone SMSG in a number of semiconductor materials for a broad range of wavelengths. Integrating the optical model with thermal and electrical numerical models, we then simulate the threshold operation of an exemplary SMSG VCSEL.

Optimal parameters of monolithic high-contrast grating mirrors.

In this Letter a fully vectorial numerical model is used to search for the construction parameters of monolithic high-contrast grating (MHCG) mirrors providing maximal power reflectance. We determine the design parameters of highly reflecting MHCG mirrors where the etching depth of the stripes is less than two wavelengths in free space. We analyze MHCGs in a broad range of real refractive index values corresponding to most of the common optoelectronic materials in use today. Our results comprise a complete image of possible highly reflecting MHCG mirror constructions for potential use in optoelectronic devices and systems. We support the numerical analysis by experimental verification of the high reflectance via a GaAs MHCG designed for a wavelength of 980 nm.

Optimization of electrically tunable VCSEL with intracavity nematic liquid crystal.

We optimize the wavelength tuning range of a Vertical-Cavity Surface-Emitting Laser with an intracavity layer of nematic Liquid Crystal (LC-VCSEL) lasing around 1.3 µm. The tunability is obtained by applying voltage to the liquid crystal layer, which esentially is to vary the refractive index from the extraordinary to the ordinary. We achieve 71.6 nm continuous tuning (without mode hopping) with liquid crystal thickness of about 3.2 µm. We investigate the impact of ambient temperature on the LC-VCSEL tuning range and show that mode-hop tuning can be achieved in the temperature range from -10°C to 50°C where the LC is in nematic phase.

Transverse mode control in high-contrast grating VCSELs.

This paper presents an extensive numerical analysis of a high-contrast grating VCSEL emitting at 0.98 µm. Using a three-dimensional, fully vectorial optical model, we investigate the influence of a non-uniform grating with a broad range of geometrical parameters on the modal behavior of the VCSEL. Properly designed and optimized, the high-contrast grating confines the fundamental mode selectively in all three dimensions and discriminates all higher order modes by expelling them from its central region. This mechanism makes single mode operation possible under a broad range of currents and could potentially enhance the single-mode output power of such devices. The high-contrast grating design proposed here is the only design for a VCSEL with three-dimensional, selective, optical confinement that requires relatively simple fabrication.

Transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers by intra-cavity patterning.

Transverse mode discrimination is demonstrated in long-wavelength wafer-fused vertical-cavity surface-emitting lasers using ring-shaped air gap patterns at the fused interface between the cavity and the top distributed Bragg reflector. A significant number of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing in particular the identification of a design that reproducibly increases the maximal single-mode emitted power by about 30 %. Numerical simulations support these observations and allow specifying optimized ring dimensions for which higher-order transverse modes are localized out of the optical aperture. These simulations predict further enhancement of the single-mode properties of the devices with negligible penalty on threshold current and emitted power.

Optimal radii of photonic crystal holes within DBR mirrors in long wavelength VCSEL.

The modal characteristics of a Photonic-Crystal Vertical-Cavity Surface-Emitting diode Laser (PC-VCSEL) have been investigated. Photonic crystal structure, realized by a regular net of air holes within the layers, has been etched in the upper DBR mirror. An advanced three-dimensional, vectorial electromagnetic model has been applied to a phosphide - based device design featuring InGaAlAs active region, AlGaAs/GaAs mirrors and a tunnel junction to confine the current flow. For the structure under consideration a single mode operation has been found for the hole diameter over photonic crystal lattice constant ratio between 0.1 - 0.3.

Single mode condition and modes discrimination in photonic-crystal 1.3 mum AlInGaAs/InP VCSEL.

We determine the single mode condition and analyze the modes discrimination of 1.3 mum InP based photonic-crystal vertical-cavity surfaceemitting diode laser. To this aim we apply the fully vectorial, three dimensional Plane Wave Admittance Method and analyze a broad range of photonic-crystal parameters such as hole etching depth, distance between the holes and their diameters.