Transparent electrodes are essential components of optoelectronic devices, however, increasing requirements with respect to transmission at a level approaching 100% and sheet resistance below 1 Sq-1 are still a challenge. In this talk, we show that monolithic deep-subwavelength grating integrated with metal enables to reach those requirements for broad spectrum of polarized light. It facilitates injection of very high current densities exceeding 20 kA cm-2 not causing noticeable heat generation that meets the requirements of the most demanding optoelectronic devices such as semiconductor lasers.
We design and process more than 100 different 980 nm MHCG mirror designs, to determine optimal parameters for the use of the MHCGs as mirrors for VCSELs. We present measured power reflectance spectra and compare the results to our with numerical simulations. We discuss the impact of the actual processed geometric shape of the MHCG stripes on the measured power reflectance of the MHCGs..
980 nm VCSELs with different numbers of top dielectric DBR periods added to a 5.5-period top semiconductor DBR and with various oxide aperture diameters are investigated to determine the impact of the added dielectric DBR’s impact on the static and dynamic properties of the VCSELs. For VCSELs with the same oxide aperture diameter we observe smaller small-signal modulation bandwidth and lower D-factor for the VCSELs with more pairs of dielectric DBRs. For the VCSELs with 4 μm oxide aperture diameters with 8 and 12 periods of added top dielectric DBRs we measured bandwidths of 29 and 26 GHz, respectively.
We show our latest results on electrically-driven VCSELs incorporating a monolithic high contrast grating (MHCG) mirror. Via optimized processing techniques we achieve a 3-fold improvement in threshold current and optical output power and a 2-fold improvement in the small-signal modulation bandwidth frequency with respect to the first generation of our MHCG VCSELs.
High contrast gratings (HCGs) are nowadays very popular in research due to small dimensions and their highly reflective or transmissive properties. By proper alignment of HCG bars they may become focusing reflectors or lenses. Here we present simulations of GaAs-based planar focusing reflectors realized by monolithic HCGs. We present how to design focusing reflectors and discuss how to tune their reflectivity.
High contrast gratings (HCGs) are an attractive alternative to distributed Bragg reflectors (DBRs) as highly reflective mirrors for VCSELs. In our previous work we proposed the use of monolithic HCGs (MHCGs) to reduce the vertical thickness and simplify the epitaxial structure of VCSELs. In this work we discuss the optimization and fabrication of MHCGs. We also analyze the impact of processing imperfections on the power reflectance of MHCGs.
Presently quantum-cascade (QC) lasers enable emission at the wavelengths ranging from infrared to terahertz making them ideal light source for the distant detection of harmful gases and free-space optical communication. In those applications, requirements for the lasers include: narrow, single-fundamental-mode operation, low-divergent emitted beam, low threshold current and high speed modulation. Those properties are inherently owned by vertical-cavity surface-emitting lasers (VCSELs). However, when a QC is embedded into conventional vertical cavity, stimulated emission is impossible, because of the absence of the vertical electromagnetic wave component, which makes fundamentally impossible fabrication of QC VCSELs in their conventional design.
We propose a design of QC VCSEL in which top DBR mirror is replaced with a monolithic high-refractive-index contrast grating (MHCG). QCs are embedded within the MHCG stripes where the vertical component of the electromagnetic field is induced, enabling stimulated emission from the QCs.
Using a three-dimensional, fully vectorial optical model combined with an electrical model and gain model we discuss the distribution of the optical field, threshold current, emitted optical power and wall-plug efficiency of a 9 micro m AlInAs/InGaAs/InP QC VCSEL. Our anticipation shows that threshold current of QC VCSELs can be as low as 0.09 mA and the wall-plug efficiency at the level of 4%. We consider methods of current injection to active regions as well as methods of current and optical confinement.
The fabrication possibility of QC VCSELs opens new perspectives for merging the advantages of VCSELs with those of QCLs.
Fabrication of approx. 3 THz Al0.15Ga0.85As/GaAs QCLs grown by Molecular Beam Epitaxy equipped with Ta/Cu or Ti/Cu waveguide claddings will be presented.
Our previous studies showed that copper layers as the waveguide claddings are most promising in THz QCLs technology. The theoretical predictions showed that lasers with Ti/Cu or Ta/Cu claddings (where Ti and Ta play the role of diffusion barriers and improve adhesion) show the smallest waveguide losses when compared with other metals. The main important issue of the presentation will be the wafer bonding of the QCL active region and GaAs receptor wafer. We will compare the results of ex-situ and in-situ bonding technology. The structures were tested by optical microscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS). Our studies show that it was necessary to apply at least 5 nm-thick diffusion-barrier layers, as well as to keep all of the process temperatures below 400C in order to ensure the barrier tightness. The next important issue was control of composition of metallic claddings, in order to provide the control of the refractive index profiles of the claddings.
The ridge structure lasers were fabricated with ridge width in the range 100 – 140 µm, formed by dry plasma etching in BCl3/Cl2/Ar mixture in ICP RIE system.
The lasers operated with threshold current densities of approx. 1.2 kA/cm2 at 77 K and the Tmax = 130 K, when fed by 100-300 ns current pulses supplied with 0.3-1 kHz repetition frequencies.
*This research is supported by The National Centre for Research and Development (bilateral cooperation, project no. 1/POLTUR-1/2016) and TUBITAK (Scientific and Technical Research Council of Turkey) project number 215E113.
Since the very first demonstration of a vertical-cavity surface-emitting laser (VCSEL) incorporating subwavelength high refractive index contrast grating (HCG) membrane mirror in 2007 by the group of Prof. Chang-Hasnain, numerous research groups around the world have presented devices based on the same concept emitting at wavelengths from ~400 to 1550 nm manufactured in gallium nitride (GaN), gallium arsenide (GaAs) and indium phosphide (InP) material systems. On one hand, an open access to a VCSEL cavity through an air gap combined with a very low inertia of an HCG mirror opened a way for a large range of emission wavelengths in MEMS tunable VCSELs. On the other hand, an air gap in a cavity generally hinders heat and current flow, while the potentially rather fragile HCG membrane is prone to mechanical instability. We present electrically-injected VCSELs incorporating monolithic HCG (MHCG) mirrors. An MHCG mirror being a special case of an HCG mirror, keeps the extraordinary features of an HCG such as scalability with wavelength, ultra-low thickness and very large power reflectance, but doesn't have to be surrounded by a low refractive index material and hence can be monolithically integrated with an all-semiconductor VCSEL cavity. We present an extensive analysis of the impact of the MHCG parameters on the modal properties and thermal stability of single- and double-mode devices, with various oxide apertures. We additionally compare MHCG VCSELs and generic distributed Bragg reflector VCSELs in terms of modal properties and temperature stability based on measured data and the results of computer simulations.
A physical structure constructed from stripes of a material with high refractive index that are separated with a low refractive index medium is called a high contrast grating (HCG). Here we present the simulations of long focal-length GaAs-based planar focusing monolithic HCG reflectors designed for 980 nm. We discuss how the focal spot size depends on the reflector size and how it is possible to improve the maximum value of the electric field intensity distribution.
Monolithically grown, electrically-injected VCSELs of a generic design - a short cavity, sandwiched between two distributed Bragg reflectors (DBRs) - can only be realized easily in a gallium arsenide (GaAs) material system which restricts the emission wavelength to ~600 - 1100 nm range. The smartphones market and emerging applications such as LIDAR (light detection and ranging), free space communication and face recognition create a demand for VCSELs emitting outside of this range. We demonstrate electrically-injected VCSELs incorporating a monolithic high contrast grating (MHCG) - a special case of a subwavelength high contrast grating mirror (HCG). MHCG can be made of most of the common materials used in optoelectronics and provides reflectivity close to 100% at a wavelength of interest in range from ultraviolet to infrared. In contrast to the HCG, the MHCG doesn't have to be surrounded by a low refractive index material and hence, can be monolithically integrated with the rest of the VCSEL cavity. In our design the greater part of the top DBR is substituted by an MHCG mirror which reduces the amount of required material and growth time by about 20%. We show continuous wave emission around 980 nm up to 75 °C ambient temperature. Our devices are quasi-single- and double-mode from threshold to rollover for 13.5 μm and 16.5 μm oxide aperture diameters respectively. Our MHCG VCSEL concept can be produced using material systems where lattice-matched and high reflectivity DBRs are not available to create devices emitting at wavelengths from ultraviolet to infrared.
Quantum-cascade vertical-cavity surface-emitting lasers (QC VCSELs) [1] combine features
of VCSELs in respect of low threshold current, high quality of output beam, possible high speed modulation and fabrication of two dimensional phase-coupled arrays and quantum cascade lasers (QCLs) due to their emission in a broad range of infrared radiation up to about 100 m.
In those structures vertical resonance and stimulated emission of photons is possible due to embedding QCs in the stripes of a monolithic high-refractive-index contrast grating (MHCG). Unipolar QCs provide flexibility in the number of the active regions used in the structure, leading to designs with distributed active regions enabling efficient stimulated emission. The expected high performance of QC VCSELs relies on sophisticated designing of MHCG and active regions which takes into account distributions of the QC VCSEL modes. Spatial distributions of modes are highly unintuitive and anticipation of them requires the use of numerical methods solving fully vectorial Maxwell eigenvalue problem.
In this article, we present the principles of QC VCSELs designing illustrated by examples of optimization of a structure emitting at the wavelength of 9 µm. Particularly, we demonstrate optimization of the MHCGs, the resonant cavities and the numbers of active regions in QC VCSELs. In this contribution, optimal designs with respect to minimal threshold current and maximal output power are presented.
[1] T. Czyszanowski: Quantum Cascade Vertical Cavity Surface Emitting Laser, IEEE Photon. Technol. Lett. vol.29, pp. 351-354, 2018
A new structure of semiconductor lasers called the quantum-cascade vertical-cavity surface emitting laser (QC VCSEL) is proposed in the present paper. A structure of the QC VCSEL is a cross of the quantum-cascade laser (QCL) and the vertical-cavity surface-emitting laser (VCSEL). The QC VCSEL is expected to demonstrate important advantages of laser emission of both the QCL and the VCSEL without their drawbacks. In the QC VCSEL, the monolithic highcontrast grating (MHCG) structure is applied to cope with the fundamental requirement of the polarization direction of the electro-magnetic radiation perpendicular to the quantum cascade (QC) necessary to initiate within it the stimulated emission. The QC VCSEL structure recommended in the present paper is a result of the advanced modeling with the aid of our comprehensive self-consistent optical-electrical model.
III-N-based edge-emitting lasers suffer from low refractive index contrast between GaN, AlGaN and InGaN layers, conventionally used in their epitaxial structures. This issue becomes more severe with an increase in wavelength at which those devices operate when tuning from blue-violet to real blue and green light. To overcome this issue and to increase the refractive index contrast other materials must be employed within the epitaxial structures replacing the standard nitride layers with materials with lower refractive index. We demonstrate results of effective-index numerical calculations performed for the state-of-the-art semipolar real blue (471 nm) and green (518 nm) edge-emitting lasers with structural modifications that include ITO, AlInN, plasmonic GaN:Ge and nanoporous GaN layers. Such solutions are extensively investigated for III-N-based EELs operating in blue-violet region but only separately. Using combination of these solutions we managed to increase optical confinement factor over twice in blue- and over 3.5-times in green-EELs.
In this paper, we present a novel design of a nitride-based VCSEL emitting at 414 nm and perform numerical analysis of optical, electrical and thermal phenomena. The bottom mirror of the laser is a Al(In)N/GaN DBR (Distributed Bragg Reflector), whereas the top mirror is realized as a semiconductor-metal subwavelength-grating, etched in GaN with silver stripes deposited between the stripes of the semiconductor grating. In this monolithic structure simulations show a uniform active-region current density on the level of 5.5 kA/cm2 for the apertures as large as 10 μm. In the case of a broader apertures, e.g. 40 μm, we showed that, assuming a homogeneous current injection at the level of 5.5 kA/cm2 , the temperature inside the laser should not exceed 360 K, which gives promise to improve thermal management by uniformisation of the current injection.
This paper shows the possibility of stimulated emission in quantum cascades (QC) embedded in a vertical cavity and proposes a design for the first quantum-cascade vertical-cavity surface-emitting laser (QC VCSEL). In the proposed design, the top VCSEL mirror is a monolithic high-refractive-index contrast grating (MHCG), which serves as both an optical coupler and as the region in which the vertical component of the electrical field is induced, enabling stimulating emission from the quantum cascades. Using a three-dimensional, fully vectorial optical model, a stand-alone MHCG is analysed in terms of its possible use as a QC VCSEL mirror. The distribution of the optical field and threshold gain in VCSELs with QC embedded in MHCG are also simulated.
Here we investigate the influence of the p- and n-oxide-aperture radii in all-semiconductor GaAs-based verticalcavity surface-emitting lasers (VCSELs), designed for 980 nm, on the modulation time constant (τ). Our analysis shows that the minimum value of τ is obtained if the oxide layers on both sides of the junction have identical depths. The simulations of the number of oxide layers on both p- and n-type sides reveal that double p- and n-oxidations are the most effective in the reduction of the modulation time constant as compared to single oxide layers.
We reduce the epitaxial design complexity of our conventional single-cavity oxide-aperture vertical-cavity surfaceemitting lasers (VCSELs) to reduce manufacturing costs while still meeting our internal 980 nanometer VCSEL performance goals via simplicity-in-design principles. We achieve maximum static single-mode optical output powers exceeding 4 milliwatts with small-signal modulation bandwidths exceeding 30 gigahertz at an ambient temperature of about 25 degrees Celsius for VCSELs with an oxide-aperture diameter of about 4 micrometers. Neighbor VCSELs with oxide-aperture diameters above 15 micrometers have maximum room temperature multiple-mode optical output powers of about 20 milliwatts with small-signal modulation bandwidths exceeding 20 gigahertz. The performance of our conventional oxide-confined 980 nanometer simplicity VCSELs exceeds the performance of our previously-reported and more complex 980 nanometer VCSEL epitaxial designs where we previously achieved maximum small-signal modulation bandwidths of about 26 gigahertz with oxide-aperture diameters of about 4 to 6 micrometers.
We propose a novel optical sensing system based on one device that both emits and detects light consisting of a verticalcavity surface-emitting laser (VCSEL) incorporating an high contrast grating (HCG) as a top mirror. Since HCGs can be very sensitive to the optical properties of surrounding media, they can be used to detect gases and liquid. The presence of a gas or a liquid around an HCG mirror causes changes of the power reflectance of the mirror, which corresponds to changes of the VCSEL’s cavity quality factor and current-voltage characteristic. By observation of the current-voltage characteristic we can collect information about the medium around the HCG. In this paper we investigate how the properties of the HCG mirror depend on the refractive index of the HCG surroundings. We present results of a computer simulation performed with a three-dimensional fully vectorial model. We consider silicon HCGs on silica and designed for a 1300 nm VCSEL emission wavelength. We demonstrate that our approach can be applied to other wavelengths and material systems.
Conditions of fabrication of first-order distributed-feedback surface gratings designed for single-mode Al0.45Ga0.55As/GaAs quantum cascades lasers with the emission wavelength of about 10 μm are presented. The 1-μm-deep rectangular-shaped gratings with the period of about 1.55 μm and duty cycle in the range of 65% to 71% made by the standard photolithography are demonstrated. The wavenumber difference of about 7 cm−1 at 77 K is observed for the radiation emitted by lasers fabricated from the same epitaxial structure with ridge widths in the range of 15 to 25 μm. Moreover, the emission wavelength of the lasers could be tuned with temperature at a rate of 1 nm/K in the temperature range of 77 to 120 K. The full width at half maximum of the emitted spectra is ∼0.4 cm−1.
We propose semiconductor-metal subwavelength grating (SMSG) which can be implemented as VCSEL mirror. Such new type of SMSG plays a double role of the electric contact and mirror simultaneously. It facilitates high optical power reflectance, perfectly vertical current injection. Such construction eliminates the inbuilt current confinement and allows scaling of emitted power by simple variation of SMSG spatial dimensions. To give the credibility to proposed design we perform numerical analysis of VCSEL with SMSG using fully vectorial optical model. We discuss properties of the proposed design realized in arsenide-based material configuration.
This paper proposes a design for the monolithic high-contrast mirror designed for infrared radiation. We use a fully vectorial model to search for the construction parameters of semiconductor monolithic high-contrast grating (MHCG) mirror providing maximal power reflectance. Such mirror can play a role of optical coupler, being alternative for distributed Bragg reflectors (DBRs). DBRs for mid- and long-wavelength infrared radiation are technologically highly demanding in terms of uniform quarter-wavelength layers control. Our results comprise a complete image of possible highly reflecting MHCG mirror constructions for potential use in optoelectronic infrared devices and systems.
Conditions of fabrication of first order distributed-feedback surface gratings designed for single-mode Al0.45Ga0.55As/GaAs quantum cascades lasers with the emission wavelength of about 10μm are presented. The 1 μm-deep rectangular-shaped gratings with the period of about 1.55 μm and duty cycle in the range of 65-71% made by the standard photolithography are demonstrated. The wavenumber difference of about 7 cm-1 at 77 K is observed for the radiation emitted by lasers fabricated from the same epitaxial structure with ridge widths in the range of 15-25 μm. Moreover, the emission wavelength of the lasers could be tuned with temperature at a rate of 1 nm/K in the temperature range of 77-120 K. The full width at half maximum of the emitted spectra is ~ 0.4 cm-1.
We present results of computer simulations of vertical cavity surface emitting lasers (VCSELs) using novel, highreflectivity monolithic high refractive-index contrast grating (MHCG) mirrors and their more advanced version, partially covered by a thin metal layer - metallic MHCG (mMHCG) mirrors. The first experimental realization of this new class of mirrors is presented and discussed. We show that the metal layer does not deteriorate the high reflectivity of an mMHCG mirror, but in contrary, is a crucial element which allows high reflectivity and additionally opens a way for a more efficient electrical pumping of a VCSEL. Comparison of results of thermal-electrical-carrier-gain self-consistent simulations of both MHCG- and mMHCG-based VCSELs is presented and discussed. It is shown that using mHCG mirror as a top mirror of a VCSEL improves electrical characteristics and greatly decreases the differential resistance of the device.
High Contrast Gratings (HCGs) become an attractive alternative for Distributed Bragg Reflectors (DBRs) used as high reflecting mirrors for VCSELs. In this paper we propose to implement HCG or monolithic HCG as a top mirror of the 1650nm InP-based VCSEL intended for use as a methane sensing device. Its unique feature is related to the fact that light taking part in the resonance can be accessed without opening the laser cavity due to the slow light phenomenon which occurs in HCG. Particular designs of HCGs allow to concentrate significant part of the mode between the HCG stripes. In such constructions the presence of the substance in the vicinity of the HCG which interacts with light resonating in the laser will change its emission properties. This enables sensing absorption or change to the refractive index in proximity of the laser based on the emission parameters of the laser. We present a numerical analysis of 1650nm MHCG and HCG mirrors based on fully vectorial optical model. We found optimal parameters of HCGs and MHCGs to detect absorption and refractive index variations in the vicinity of the gratings, based on changes in power reflectance of analysed mirrors. Additionally we consider HCG and MHCG constructions which allow for broad wavelength tuning by the change of the refractive index of substance surrounding mirror.
Monolithic High refractive index Contrast Grating (MHCG) allows several-fold size reduction of epitaxial structure of VCSEL and facilitates VCSEL fabrication in all photonic material systems. MHCGs can be fabricated of material which refractive index is higher than 1.75 without the need of the combination of low and high refractive index materials. MHCGs have a great application potential in optoelectronic devices, especially in phosphide- and nitride-based VCSELs, which suffer from the lack of efficient monolithically integrated DBR mirrors. MHCGs can simplify the construction of VCSELs, reducing their epitaxial design to monolithic wafer with carrier confinement and active region inside and etched stripes on both surfaces in post processing. In this paper we present results of numerical analysis of MHCGs as a high reflective mirrors for broad range of refractive indices that corresponds to plethora of materials typically used in optoelectronics. Our calculations base on a three-dimensional, fully vectorial optical model. We investigate the reflectance of the MHCG mirrors of different design as the function of the refractive index and we show the optimal geometrical parameters of MHCG enabling nearly 100% reflectance and broad reflection stop-band. We show that MHCG can be designed based on most of semiconductors materials and for any incident light wavelength from optical spectrum.
We present experimental results showing alternating lasing and non-lasing regions for the short-wavelength longitudinal mode in a GaAs-based 850 nm coupled-cavity vertical-cavity surface-emitting laser (CC-VCSEL). These regions are situated between the laser threshold and roll-off for this mode. The analyzed structure consists of two identical AlGaAs cavities with GaAs quantum wells, separated with 11.5 pairs of middle DBR. The current apertures are realized by ion-implantation for the top cavity and selective oxidation for the bottom cavity. We then perform fully-vectorial three-dimensional cold-cavity optical simulations to theoretically investigate optical density radial and on-optical-axis profiles of the first order transverse modes corresponding to the two longitudinal modes. We show that the short-wavelength fundamental mode λS-LP01 is subject to periodic changes of its optical field distribution when changing the oxide aperture radius, which can lead to weaker resonance of the short-wavelength LP01 mode within the coupled cavity structure.
Conventional High-index Contrast Gratings (HCG) consist of periodically distributed high refractive index stripes surrounded by low index media. Practically, such low/high index stack can be fabricated in several ways however low refractive index layers are electrical insulators of poor thermal conductivities. Monolithic High-index Contrast Gratings (MHCGs) overcome those limitations since they can be implemented in any material with a real refractive index larger than 1.75 without the need of the combination of low and high refractive index materials. The freedom of use of various materials allows to provide more efficient current injection and better heat flow through the mirror, in contrary to the conventional HCGs. MHCGs can simplify the construction of VCSELs, reducing their epitaxial design to monolithic wafer with carrier confinement and active region inside and etched stripes on both surfaces in post processing. We present numerical analysis of MHCGs using a three-dimensional, fully vectorial optical model. We investigate possible designs of MHCGs using multidimensional optimization of grating parameters for different refractive indices.
This paper reports on numerical analysis of longitudinal mode discrimination in coupled-cavity AlInAs/InGaAs/InP
quantum cascade lasers. Using a three dimensional, self-consistent model of physical phenomena in edge emitting laser
we performed exhaustive analysis of geometrical parameters of CC QCL on spectral characteristics. We discuss the
enhancement of the single mode operation in multi-section designs concerning variable dimensions of sections and air
gaps between sections and provide designing guidelines assuring single-mode operation. We also show impact of
independent current tuning of laser sections inducing Stark effect and heating as additional elements enhancing single
mode operation.
In this paper we present optical design and simulation results of vertical-cavity surface-emitting lasers (VCSELs) that
incorporate monolithic subwavelength high refractive-index-contrast grating (MHCG) mirrors - a new variety of HCG
mirror that is composed of high index material surrounded only on one side by low index material. We show the impact
of an MHCG mirror on the performance of 980 nm VCSELs designed for high bit rate and energy-efficient optical data
communications. In our design, all or part of the all-semiconductor top coupling distributed Bragg reflector mirror is
replaced by an undoped gallium-arsenide MHCG. We show how the optical field intensity distribution of the VCSEL’s
fundamental mode is controlled by the combination of the number of residual distributed Bragg reflector (DBR) mirror
periods and the physical design of the topmost gallium-arsenide MHCG. Additionally, we numerically investigate the
confinement factors of our VCSELs and show that this parameter for the MHCG DBR VCSELs may only be properly
determined in two or three dimensions due to the periodic nature of the grating mirror.
In the talk we show the process of modeling complete physical properties of VCSELs and we present a step-by-step development of its complete multi-physics model, gradually improving its accuracy. Then we introduce high contrast gratings to the VCSEL design, which strongly complicates its optical modeling, making the comprehensive multi-physics VCSEL simulation a challenging task. We show, however, that a proper choice of a self-consistent simulation algorithm can still make such a simulation a feasible one, which is necessary for an efficient optimization of the laser prior to its costly manufacturing.
In this paper we present the simulation results of an oxide-confined, InGaAs/GaAs based vertical-cavity surface-emitting laser with three different configurations of the oxide apertures. We analyze the impact of the number and position of oxide layers on the carrier distribution in the laser's active region, distribution of the optical modes, and modulation properties.
Distributed Bragg reflectors (DBRs) are typically used as the highly reflecting mirrors of vertical-cavity surface-emitting lasers (VCSELs). In order to provide optical field confinement, oxide apertures are often incorporated in the process of the selective wet oxidation of high aluminum-content DBR layers. This technology has some potential drawbacks such as difficulty in controlling the uniformity of the oxide aperture diameters across a large-diameter (≥ 6 inch) production wafers, high DBR series resistance especially for small diameters below about 5 μm despite elaborate grading and doping schemes, free carrier absorption at longer emission wavelengths in the p-doped DBRs, reduced reliability for oxide apertures placed close to the quantum wells, and low thermal conductivity for transporting heat away from the active region. A prospective alternative mirror is a high refractive index contrast grating (HCG) monolithically integrated with the VCSEL cavity. Two HCG mirrors potentially offer a very compact and simplified VCSEL design although the problems of resistance, heat dissipation, and reliability are not completely solved. We present an analysis of a double HCG 980 nm GaAs-based ultra-thin VCSEL. We analyze the optical confinement of such a structure with a total optical thickness is ~1.0λ including the optical cavity and the two opposing and parallel HCG mirrors.
This paper presents results of computer simulation of 1D monolithic high refractive index contrast grating (MHCG) reflector also called surface grating reflector (SGR). We analyzed optical properties of the GaAs reflector designed for 980 nm wavelength with respect to the grating parameters variation. We also determined the electric field patterns after reflection from the structure in several cases of parameters variation. We show that thanks to the scalability and design simplicity, proposed design is a promising candidate for simple, next generation vertical cavity surface emitting lasers emitting from ultra-violet to infrared.
In this paper we present results of computer optical simulations of VCSEL with modified high refractive index contrast grating (HCG) as a top mirror. We consider the HCG of two different designs which determine the lateral aperture. Such HCG mirror provides selective guiding effect. We show that proper design of aperture of HCG results in almost sixfold increase in cavity Q-factor for zero order mode and a discrimination of higher order modes.
We present the optimization of the carrier injection, heat flow and optical confinement aimed at single mode operation in anti-guiding long-wavelength VCSELs and VCSEL arrays. The analyzed structure incorporates InP/AlGaInAs quantum wells within an InP cavity. The cavity is bounded by GaAs/AlGaAs DBRs. The tunnel junction is responsible for carrier funneling into the active region. The air-gap etched at the interface between cavity and top DBR provides the confinement of the lateral modes. To rigorously simulate the physical phenomena taking place in the device we use a multi-physical model, which comprises three-dimensional models of optical (Plane Wave Admittance Method), thermal and electrical (Finite Element Method) phenomena. In the analysis we investigate the influence of the size of single and multiple emitters and the distance between the emitters in the case of the VCSEL arrays. As a result, we illustrate the complex competition of the modes and determine the geometrical parameters favoring specific array modes in the considered designs and compare the designs with respect to mode discrimination.
A self-consistent model of a GaAs-based 850 nm coupled-cavity vertical-cavity surface-emitting diode laser is presented. The analyzed laser consists of two identical AlGaAs cavities with GaAs quantum wells, separated with 10 pairs of middle DBR. The current apertures are realized by ion-implantation for the top cavity and selective oxidation for the bottom. To accurately simulate the physical phenomena present in the CW regime of the analyzed device, we use a multi-physical model, which comprises self-consistent Finite Element Method (FEM) thermo-electrical model. The numerical parameters have been found by the calibration based on experimental results. We have analyzed and shown the influence of the driving voltages on the temperature distribution within the analyzed structure and current densities in both cavities.
A numerical investigation of the current injection into the active region of electrically-pumped vertical-external-cavity surface-emitting lasers (E-VECSELs) is presented. To achieve high power of emission, a broad aperture is necessary, but such geometry favors multimode operation as the result of undesired current crowding. To reduce this effect, we propose a novel approach of selectively etched tunnel junctions in the form of coaxial rings. The paper presents the optimization of this novel geometry as an efficient approach for increasing the single mode output power of such laser.
Via experimental results supported by numerical modeling we report the energy-efficiency, bit rate, and modal properties of GaAs-based 980 nm vertical cavity surface emitting lasers (VCSELs). Using our newly established Principles for the design and operation of energy-efficient VCSELs as reported in the Invited paper by Moser et al. (SPIE 9001-02 ) [1] along with our high bit rate 980 nm VCSEL epitaxial designs that include a relatively large etalonto- quantum well gain-peak wavelength detuning of about 15 nm we demonstrate record error-free (bit error ratio below 10-12) data transmission performance of 38, 40, and 42 Gbit/s at 85, 75, and 25°C, respectively. At 38 Gbit/s in a back-toback test configuration from 45 to 85°C we demonstrate a record low and highly stable dissipated energy of only ~179 to 177 fJ per transmitted bit. We conclude that our 980 nm VCSELs are especially well suited for very-short-reach and ultra-short-reach optical interconnects where the data transmission distances are about 1 m or less, and about 10 mm or less, respectively.
We present the optimization of the carrier injection, heat flow and optical confinement aimed at single mode operation in anti-guiding long-wavelength VCSEL arrays. The analyzed structure incorporates InP/AlGaInAs quantum wells within an InP cavity. The cavity is bounded by GaAs/AlGaAs DBRs. The tunnel junction is responsible for carrier funneling into the active region. The air-gap etched at the interface between cavity and top DBR provides the confinement of the lateral modes. To rigorously simulate the physical phenomena taking place in the device we use a multi-physical model, which comprises three-dimensional models of optical (Plane Wave Admittance Method), thermal and electrical (Finite Element Method) phenomena. We perform an exhaustive modal analysis of a 1x3 VCSEL arrays. In the analysis we investigate the influence of the size and the distance between the emitters. As the result we illustrate the complex competition of the modes and determine the geometrical parameters favoring specific array modes in the considered array designs.
In this paper we present results of computer optical simulations of VCSEL with modified high refractive index contrast grating (HCG) as a top mirror. We consider the HCG of two different designs which determine the lateral aperture. Such HCG mirror provides selective guiding effect. We show that proper design of aperture of HCG results in almost sixfold increase in cavity Q-factor for zero order mode and a discrimination of higher order modes.
We report on transverse mode discrimination in long-wavelength wafer-fused vertical-cavity surface-emitting lasers (VCSELs) incorporating ring-shaped air gap patterns at the fused interface between the active region and the top distributed Bragg reflector (DBR). These 60-nm deep patterns were implemented with the aim of favoring the fundamental mode while preserving high output power. The VCSELs under consideration emit in the 1310-nm band and incorporate an AlGaInAs-based quantum well active region, a regrown circular tunnel junction and undoped GaAs/AlGaAs DBRs. A large batch of devices with varying pattern dimensions was investigated by on-wafer mapping, allowing significant statistical analysis leading to conclusions on their typical behavior. We observe experimentally a dependence of the side-mode suppression ratio on the geometrical parameters of the patterns. In particular, we identified a design that statistically increases the maximal single-mode emitted power by more than 20%. Numerical simulations of the patterned-cavity VCSELs based on our fully three dimensional electrical, thermal and optical VCSEL computational model support these observations. They show that patterns with a large inner diameter actually confine the first-order transverse mode and enhance its modal gain. In smaller devices, this mode is pushed out of the optical aperture and suffers larger losses. Optimized parameters were found numerically for enhancing the single-mode properties of the devices with negligible penalty on emitted power and threshold current.
The introduction of a photonic-crystal to the VCSEL produces single mode emission in a very broad range of applied
currents. The mechanism responsible for the discrimination of high-order modes originates from two counter-acting
phenomena:
1) the PhC introduces lateral mode confinement through a strong waveguide effect and additionally by the Bragg
reflections from a regular net of PhC holes
2) the holes of the PhC destroy the vertical periodicity of the DBR and contribute to the selective reduction in reflectivity
of the mirror. As a result, the mode which overlaps the holes of the photonic crystal leaks through and becomes
discriminated.
We present numerical analysis of the influence of parameters of photonic crystal on the wavelength of emission, modal
gain, slope efficiency, emitted power and tuning range in single mode VCSELs. We recognise several mechanisms
determining high power emission in the single mode regime, which are: selective leakage, thermal focusing, waveguide
effect induced by the photonic-crystal, gain spectrum red shift and its maximum reduction with increase of driving
currents. We show that careful design of the photonic crystal allows for 10% increase in the emitted power of a singlemode
regime and it allows for broad range of the steering currents from 5 to 50 mA. Such attributes support tuning of the
single-mode emission over the 10 nm range of the spectrum.
We demonstrate the possibility of fabrication of InGaN laser diode with an extremely thin lower AlGaN cladding (200 nm) by using high electron concentration, plasmonic GaN substrate. The plasmonic substrates were fabricated by one of high-pressure methods – ammonothermal method or multi-feed-seed growth method and have an electron concentration from 5x1019 cm-3 up to 1x1020 cm-3. New plasmonic substrate devices, in spite of their extremely thin lower AlGaN cladding, showed identical properties to these manufactured with traditional, thick lower cladding design. They were characterized by identical threshold current density, slope efficiency and differential gain. Thin AlGaN devices are additionally characterized by low wafer bow and very low density of dislocations (<104 cm-2).
We present the optimization of the carrier injection, heat flow and optical confinement aimed for single mode operation.
The analyzed structure incorporates InAlGaAs quantum wells within InP cavity. The cavity is bounded by AlGaAs/GaAs
DBRs The tunnel junction is responsible for carrier funneling into the active region. The air-gap etched at the interface
between cavity and top DBR provides the confinement of the lateral modes. To rigorously simulate the physical
phenomena taking place in the device we used multi-physical model, which comprises three-dimensional models of
optical (Plane Wave Admittance Method), thermal and electrical (Finite Element Method) phenomena.
We perform the exhaustive modal analysis of the 1x3, 1x4 and 2x4 VCSEL arrays. In the analysis we investigate the
influence of the distance between emitters. The analysis is performed for broad range of injected currents from threshold
to the rollover. As the result we illustrate the complex competition of the modes, influence of the optical confinement on
structure of the modes and determine the geometrical parameters, which favor the array modes in the considered array
designs.
In the following paper a simulation of optically pumped vertical external cavity surface emitting lasers (VECSEL) with a
novel approach for the improvement of the heat management is presented. In recent VECSEL structures, it was common
to use one top diamond heat spreader in order to decrease the thermal resistance of the device by redistributing the heat
flow to the lateral regions and thus transporting heat down to the copper heat sink more efficiently. We present here
further improvement of the heat management by eliminating the bottom DBR from the heat flow path and substituting it
for a diamond with a High Contrast Grating (HCG). Hence the active region, which consists of 5 pairs of AlGaInAs
quaternary alloy quantum wells, is sandwiched between two diamond heat spreading layers.
The structure of Si HCG deposited on a diamond provides broad wavelength range in which reflectivity is close to 100%
for the emitted beam for perpendicular mode polarization with respect to the direction of the HCG trenches. The HCG
assures less than 20% reflection and near zero absorption of pumping light, hence it allows for on-axis bottom pumping
scheme and integration of the VECSEL with the pumping laser. According to the simulations 300 μm thick top diamond
heat spreader is enough to assure effective heat dissipation mechanism. Replacing the bottom DBR with the diamond
heat spreader will provide additional 10% reduction of the thermal impedance. The minimum of thermal impedance is
achieved for about 450 μm thick bottom diamond heat spreader.
Highly n-doped GaN is a material of a reduced refractive index which may substitute AlGaN as a cladding layer in
InGaN laser diodes. In this study we focus on the determination of the optical absorption and the refractive index of
GaN:O having the electron concentration between 1·1018 - 8·1019 cm-3. Though the measured absorption coefficient for
the highest doped GaN are rather high (200 cm-1) we show, using an optical mode simulation, that you can design a
InGaN laser diode operating in blue/green region with decent properties and low optical losses. We propose to use
relatively thin AlGaN interlayer to separate plasmonic GaN from the waveguide and thus to dramatically reduce the
optical losses.
1300-nm, 1550-nm and 1480-nm wavelength, optically-pumped VECSELs based on wafer-fused InAlGaAs/InPAlGaAs/
GaAs gain mirrors with intra-cavity diamond heat-spreaders demonstrate very low thermal impedance of 4
K/W. Maximum CW output of devices with5 groups of quantum wells show CW output power of 2.7 W from 180μm
apertures in both 1300-nm and 1550-nm bands. Devices with 3 groups of quantum wells emitting at 1480 nm and with
the same aperture size show CW output of 4.8 W. These devices emit a high quality beam with M² beam parameter
below 1.6 allowing reaching a coupling efficiency into a single mode fiber as high as 70 %. Maximum value of output
power of 6.6 W was reached for 1300nm wavelength devices with 290μm aperture size.
A self-consistent pulse-operation model of an InP-based 1300-nm AlInGaAs vertical-cavity surface-emitting diode laser
with filled-photonic-crystal is presented. It is shown that low threshold characteristics and strong transverse-mode
discrimination can be simultaneously achieved for optimized photonic crystal structure for broad optical apertures.
We present a new three dimensional, fully vectorial optical modeling of oxide confined as well as shallow relief vertical-cavity surface-emitting laser. Our model is based on the combination of the plane wave expansion method with the method of lines resulting in a fast and accurate computational technique. We carry out hereby a comparison between the Plane Wave Admittance Method (PWAM) and other numerical approaches for VCSEL optical modeling and show very good agreement. Furthermore, this procedure makes it possible to find optimal basic computational parameters for the PWAM in the case of
VCSELs.
In this paper we present the application of a novel fully vectorial and three-dimensional computational method for planar devices to simulation of electromagnetic modes in classical and photonic-crystal-based VCSELs. We show the mathematical basis of the method and present results of computations of a resonant wavelength, optical losses, and a threshold gain of a classical arsenide VCSEL with oxide confinement and also of a purely photonic-crystal confined one. Furthermore we analyze the analytical reduction of computational domain to two dimensions in axisymmetric geometries with cylindrical-wave expansion, discuss the mathematical problems which occurs in such coordinates and suggest a method to overcome them.
Modern communication in absolute secrecy requires creation of new intrinsically secure quantum communication channels. It is particularly necessary during the first connection between two parties establishing then in assumed unconditional security the secret cryptographic key which is supposed to be used afterwards during normal information exchanging. This new emerging field of quantum information technology is based on a new type of light sources, in which numbers of emitted photons can be carefully controlled. Especially advantageous are sources of single photons emitted at strictly predetermined moments, so called single-photon devices. Then any possible eavesdropper activity will be followed by some unavoidable disturbance which alerts both communication parties to an event. In the present paper, the Purcell effect associated with enhancement of spontaneous emission coupled to a resonator is explained, methods used to produce streams of antibunched photons are given, mechanisms applied to control carrier injection into quantum dots are shown and some possible designs of single-photon devices are presented and described. These devices are based on taking advantage of both the Purcell effect and the atom-like energy spectrum of quantum dots.
The advanced three-dimensional fully self-consistent optical-electrical-thermal-gain model of the 1.3-μm (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser (VCSEL) has been developed to simulate its room-temperature (RT) continuous-wave (CW) performance characteristics and to enable its structure optimisation. The standard GaInNAs VCSEL structure with an intracavity-contacted configuration exhibits very nonuniform current injection into its active region, whereas a uniform current injection is important in long-wavelength VCSELs for low threshold, high-efficiency and stable-mode operation. Therefore we decided to insert an additional tunnel junction within the active-region neighbourhood. The tunnel junction is shown to enhance effectively hole injection via a lateral electron current, with only a modest increase (a small penalty) in voltage drop and series resistance compared to standard devices.
Performance of various possible designs of 400-nm nitride vertical-cavity surface-emitting lasers (VCSELs) has been analysed with the aid of the advanced three-dimensional (3D) thermal-electrical-optical-gain self-consistent threshold simulation. It has been demonstrated that it is practically impossible to reach the fundamental-mode operation in nitride VCSELs of the traditional design with two ring contacts. To enhance this desired operation, uniformity of current injection into VCSEL active regions should be dramatically improved. Therefore, we focused our research on designs with tunnel junctions and/or a semitransparent contact. In particular, it has been proved that the design with two cascading active regions, two tunnel junctions and a semitransparent contact may offer the most promising room-temperature performance characteristics for both pulse and continuous-wave operation. In particular, this design offers high mode selectivity with distinct fundamental transverse mode domination. Our simulations reveal, that the thickness and localization of a semitransparent contact as well as localization of active regions and tunnel junctions are crucial for a successful construction designing.
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