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Monolithic high contrast gratings (MHCGs) composed of a one-dimensional grating patterned in a monolithic layer provide up to 100% optical power reflectance and can be fabricated in almost any semiconductor and dielectric material used in modern optoelectronics. MHCGs enable monolithic integration, polarization selectivity, and versatile phase tuning. They can be from 10 to 20 times thinner than distributed Bragg reflectors. The subwavelength dimensions of MHCGs significantly reduce the possibility of ensuring the smoothness of the sidewalls of the MHCG stripes and make precise control of the shape of the MHCG stripe cross-section difficult during the etching process. The question is then whether it is more beneficial to improve the etching methods to obtain a perfect cross-section shape, as assumed by the design, or whether it is possible to find geometrical parameters that enable high optical power reflectance using the shape that a given etching method provides. Here, we present a numerical study supported by the experimental characterization of MHCGs fabricated in various materials using a variety of common surface nanometer-scale shaping methods. We demonstrate that MHCG stripes with an arbitrary cross-section shape can provide optical power reflectance of nearly 100%, which greatly relaxes their fabrication requirements. Moreover, we show that optical power reflectance exceeding 99% with a record spectral bandwidth of more than 20% can be achieved for quasi-Trapezoidal cross-sections of MHCGs. We also show that sidewall corrugations of the MHCG stripes have only a slight impact on MHCG optical power reflectance if the amplitude of the corrugation is less than 16% of the MHCG period. This level of stripe fabrication precision can be achieved using the most current surface etching methods. Our results are significant for the design and production of a variety of photonic devices employing MHCGs. The flexibility with regard to cross-section shape facilitates the reliable fabrication of highly reflective subwavelength grating mirrors. This in turn will enable the manufacture of monolithically integrated high-quality-factor optical micro-and nanocavity devices.

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.

We achieve 200 mW optical output power, 18 GHz bandwidth, and 35% wall plug efficiency with 19-element (novemdecuple) top-surface-emitting 980 nm oxide-confined vertical-cavity surface-emitting laser (VCSEL) arrays – a record combination of these three attributes. We grow wafers by metal-organic vapor phase epitaxy and process arrays into a unique two-dimensional honeycomb geometry with top mesa inter-VCSEL ridge connectors and p-metal anode interconnections. We study the impact on array performance of inter-VCSEL spacing and bottom n-doped mesa area. The top emitting VCSELs are optically uncoupled but electrically connected in parallel via anode ring-to-ring p-metal along the ridges and a common horseshoe ohmic cathode n-metal. We demonstrate the potential of our arrays as sources for optical wireless communication via 25 Gb/s data transmission tests.

Electrically parallel and optically uncoupled triple and septuple oxide confined vertical-cavity surface-emitting laser (VCSEL) arrays emitting at about 980 nm where each VCSEL has an oxide aperture diameter ( φ ) of ∼7.5 μm transmit data at 40 Gbps at room temperature (RT) in a back-to-back test configuration using standard two-level non-return-to-zero pulse amplitude modulation at bias current densities ( J ) of 17.4 and 10.3 kA/cm2, respectively. Reference single VCSELs processed with the VCSEL arrays on the same wafer pieces with φ ∼7.5, 13.5, and 19.5 μm transmit data at up to 45, 40, and 35 Gbps at RT at J of 20.4, 13.3, and 10.7 kA/cm2, respectively, where the latter two single VCSELs have total emitting areas nearly equal to the total additive emitting areas of the triple and septuple VCSEL arrays, respectively. The RT optical output power at J ∼10 and 15 kA/cm2 is 11.4 and 17.2 mW for the triple arrays and 31.1 and 45.9 mW for the septuple arrays, respectively. By increasing the number of emitters in an array for a fixed φ per VCSEL from 3 to 7 we increase the total optical output power and simultaneously we decrease the J required for error free digital data transmission at 40 Gbps.

Semiconductor planar microcavities significantly enhance the interaction between light and matter and are thus crucial as a fundamental research platform for investigations of quantum information processing, quantum dynamics, and exciton-polariton observations. Microcavities also serve as a very agile basis for modern resonant-cavity light-emitting and detecting devices now in large-scale production for applications in sensing and communication. The fabrication of microcavity devices composed of both common materials now used in photonics and uncommon or arbitrary materials that are new to photonics offers great freedom in the exploration of the functionalities of novel microcavity device concepts. Here we propose and carefully investigate two unique microcavity designs. The first design uses a monolithic high-index-contrast grating (MHCG) and a distributed Bragg reflector (DBR) as the microcavity mirrors. The second design uses two MHCGs as the microcavity mirrors. We demonstrate by numerical analysis that MHCG-DBR and MHCG-MHCG microcavities, whose lateral radial dimension is 16 μm, reach very large quality factors at the level of 104 and nearly 106, as well as purposely designed wavelength tuning ranges of 8 and 60 nm in both configurations, respectively. Our MHCG-MHCG microcavities with a very small size of 600 nm in the vertical dimension show extremely large quality factors, which can be explained by treating the optical modes as quasi-bound states in a continuum (BICs). Moreover, we verify our theoretical analysis and calibrate our simulation parameters by comparing to the experimental characteristics of an electrically injected MHCG-DBR microcavity vertical-cavity surface-emitting laser (VCSEL) emitting at a peak wavelength of about 980 nm. We use the calibrated parameters to simulate the emission characteristics of electrically injected VCSELs in various MHCG-DBR and MHCG-MHCG microcavity configurations to illustrate the influence of microcavity designs and their quality factors on the predicted lasing properties of the devices.

Networks of semiconductor lasers are the foundation of numerous applications and fundamental investigations in nonlinear dynamics, material processing, lighting, and information processing. However, making the usually narrowband semiconductor lasers within the network interact requires both high spectral homogeneity and a fitting coupling concept. Here, we report how we use diffractive optics in an external cavity to experimentally couple vertical-cavity surface-emitting lasers (VCSELs) in a 5×5 array. Out of the 25 lasers, we succeed to spectrally align 22, all of which we lock simultaneously to an external drive laser. Furthermore, we show the considerable coupling interactions between the lasers of the array. This way, we present the largest network of optically coupled semiconductor lasers reported so far and the first detailed characterization of such a diffractively coupled system. Due to the high homogeneity of the lasers, the strong interaction between them, and the scalability of the coupling approach, our VCSEL network is a promising platform for experimental investigations of complex systems, and it has direct applications as a photonic neural network.

We implement a fully parallel photonic neural network based on the spatially distributed modes of a large-area vertical-cavity surface-emitting laser. All photonic connections are realized in hardware and the system is capable of autonomous operation.

Single-emitter vertical-cavity surface-emitting lasers (VCSELs) and multiple-emitter VCSEL arrays designed for emission at 980 nm, and with large oxide aperture diameters ( φ ) from ∼10.5 to 33.5 μm and with φ of ∼7.5 μm that are configured in three-emitter and seven-emitter electrically parallel triangular and hexagonal arrays, respectively, have record bandwidths and optical output powers for VCSELs with such large cumulative emitting areas. We demonstrate room temperature (RT) maximum small-signal modulation bandwidths ( f 3dBmax) of 26.6 to 18.6 GHz with corresponding continuous wave (CW) optical output powers ( L ) of 16.2 and 47 mW for VCSELs with φ ∼13.5 and 33.5 μm, respectively. For parallel 980 nm triple and septuple arrays with φ ∼7.5 μm for each VCSEL, we demonstrate RT f 3dBmax of 25.5 and 24.8 GHz with corresponding CW L of 22.7 and 50.1 mW, respectively. We perform a comparative analysis of the RT results, which consist of standard static light output power–current–voltage ( LIV ) characteristics to determine the typical VCSEL figures-of-merit including wall plug efficiency (WPE), LI slope efficiency, spectral emission versus current ( I ), and small-signal frequency response curves. We examine the tradeoffs in optical output power, small-signal modulation bandwidth, and WPE for the various VCSEL designs, and find that while the combination of f 3dBmax and L are record values for single VCSELs and VCSEL arrays, these parameters fall off as the cumulative VCSEL array area increases to emit higher overall optical power.

High-performance computing hardware is crucial for advanced neural network (NN) schemes. Photonics promises strong advantages in terms of parallelism, yet until now scalable and integrable concepts are scarce and partially rely on exotic substrates. The majority of large scale and parallel photonic NN demonstrations are neither standalone nor autonomous [1] , usually lacking fundamental NN constituents or requiring substantial interaction with a classical electronic computer. In this contribution, we implement a fully parallel photonic reservoir computer based on the spatially distributed modes of an efficient and fast semiconductor laser [2] . Crucially, all neural network connections are realized in hardware, and our laser-based and fully parallel NN comprising ~100 neurons produces results without pre-or post-processing.

An electrically parallel, optically uncoupled, 19-element 980 nm top-surface-emitting vertical-cavity surface-emitting laser (VCSEL) array with inter-VCSEL metal and top mesa ridge connectors, processed using a two-dimensional honeycomb lattice basis with oxide aperture diameters of ϕ ~10.5 m per VCSEL exhibits a record combination of continuous wave optical output power (150 mW), -3 dB small-signal modulation bandwidth (17 GHz), and wall plug efficiency (30%) at a conservative operating bias current density of 10 kA/cm2. To demonstrate the potential of the VCSEL array as an optical source for future fifth generation free-space optical communication systems we perform a data transmission test at 25 Gb/s across OM1 multiple-mode optical fiber using simple 2-level pulse amplitude modulation and pseudorandom binary patterns of word length 27-1. We achieve a bit rate of 25 Gb/s with a bit error ratio below 1 x 10-12.

Here we present how the monolithic high contrast grating (MHCG) mirror focuses light. The studied grating has a shape of a square with 300 micrometers side. The light focuses along one of the sides of the square. As a light source in our experiment we use a vertical-cavity surface-emitting laser that emits 980 nm. In our setup the light shines from above and the grating is on the bottom of the substrate. Based on numerous images taken by a camera attached to an optical microscope we generated a movie showing how the light intensity changes as a function of height above the grating. The FWHM at the focal point is around 5 micrometers and is observed around 200 micrometers above the top surface of the substrate. The measured focal length is in perfect agreement with the simulated data. Moreover, the light intensity at the focal point is more than 10 times larger as compared to the light intensity reflected by Au mirror reference. The project (POIR.04.04.00-00-4358/17) is financed by FNP

We experimentally demonstrate and elucidate by numerical simulations that breaking circular symmetry of large apertures of vertical-cavity surface-emitting lasers (VCSELs) significantly enhances their emission properties by increasing the optical density of states. Specifically, deformed shapes of circular oxide apertures of VCSELs enhance stimulated emission and suppress undesired non-radiative recombination contributing to an increase in output optical output power of more than 60% and in quantum efficiency of more than 10%. Our example deformed VCSEL structures demonstrate that the optical density of states appears to be of high importance for conventional optoelectronic devices in accordance to the predictions of quantum electrodynamics theory.

High contrast gratings (HCGs) are diffraction gratings whose period is less than the wavelength of light, made of a material with a high refractive index. Monolithic HCGs (MHCGs) are made of the same material as the cladding. They can be made of almost any material used in optoelectronics. We show experimentally and via simulations that shaping the cross-section of the MHCG stripes enables very broad high reflection spectrum.

To meet the performance goals of fifth generation (5G) and future sixth generation (6G) optical wireless communication (OWC) and sensing systems, we seek to develop low-cost, reliable, compact lasers capable of sourcing 5&ndash, 20 Gb/s (ideally up to 100 Gb/s by the 2030s) infrared beams across free-space line-of-sight distances of meters to kilometers. Toward this end, we develop small arrays of electrically parallel vertical cavity surface emitting lasers (VCSELs) for possible future use in short-distance (tens of meters) free-space optical communication and sensing applications in, for example, homes, data centers, manufacturing spaces, and backhaul (pole-to-pole or pole-to-building) optical links. As a starting point, we design, grow by metal&ndash, organic vapor phase epitaxy, fabricate, test, and analyze 980 nm top-emitting triple VCSEL arrays. Via on-wafer high-frequency probe testing, our arrays exhibit record bandwidths of 20&ndash, 25 GHz, optical output powers of 20&ndash, 50 mW, and error-free data transmission at up to 40 Gb/s&mdash, all extremely well suited for the intended 5G short-reach OWC and sensing applications. We employ novel p-metal and top mesa inter-VCSEL connectors to form electrically parallel but optically uncoupled (to reduce speckle) arrays with performance exceeding that of single VCSELs with equal total emitting areas.

We implement a fully parallel photonic neural network based on the spatially distributed modes of a large-area semiconductor laser. All photonic connections are realized in hardware and the system is capable of autonomous operation.

Breaking the spatial symmetry in optical systems has become a key approach to the study of nonlinear dynamics, wave chaos, and non-Hermitian physics. Moreover, it enables tailoring of the spatiotemporal properties of such systems. Breaking the circular symmetry of lasers yields a more uniform light intensity profile within the optical aperture and makes uniform the spectral distribution of the optical states (modes). Those effects are known to enhance spontaneous as well as stimulated emission and consequently suppress undesired nonradiative recombination in the active region, but their importance for laser emission is not fully understood so far. In this paper, using the example of vertical-cavity surface-emitting lasers, we show that intentionally deformed optical apertures induce a more uniform light intensity distribution within the optical aperture, related to wave chaos, and a higher density of optical states, enhancing stimulated emission as predicted by quantum electrodynamics theory. These two phenomena contribute to increasing the optical output power by more than 60% and quantum efficiency by more than 10%. The results of this study are of significant importance for a variety of lasers, showing a clear link between the fundamentals of their operation and quantum electrodynamics and providing a general, robust method of enhancing emitted power for high-power broad-area lasers., Narodowe Centrum Nauki (2015/18/E/ST7/00572); German Research Foundation (Collaborative Research Center 787).

We achieve record concurrent combinations of bandwidth (18 GHz), optical output power (150 mW), and wall plug efficiency (30%) with a unique arrangement of 19-element, electrically parallel 980 nm vertical cavity surface emitting laser (VCSEL) arrays. We use a new two-dimensional, quasi honeycomb geometry with inter-VCSEL ridge connectors—made nonconducting by selective thermal oxidation—to improve heat dissipation and facilitate a single top surface anode contact. Via on-wafer probing we perform static and dynamic measurements over the wide temperature range of 23 °C to 85 °C and extract, report, and discuss key array figures-of-merit.

Novemdecuple (19-element) electrically parallel, optically uncoupled vertical cavity surface emitting laser arrays exhibit 18 GHz bandwidth, transmit pseudorandom digital data at 25 Gb/s with a bit error ratio of 1 x 10-13, and emit up to 200 mW at 980 nm.

Compact groups of seven electrically-parallel, optically uncoupled, top-surface-emitting 980 nm vertical-single-cavity surface-emitting lasers arranged in a unique interconnected honeycomb pattern exhibit record concurrent combinations of large optical output power (70 to 100 mW), wall plug efficiency, and small-signal modulation bandwidth (15 to 21 GHz).

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 design, produce, characterize, and compare 850 nm vertical cavity surface emitting lasers (VCSELs) with one and two oxide aperture layers, and with cavity optical thicknesses of 0.5λ and 1.5λ. We process five VCSEL wafers side by side with varying oxide aperture diameters from about 4 to 16 m and perform on-wafer static and dynamic testing. From optical output power-current-voltage characteristics we extract and compare threshold currents, differential series resistances, and wall plug efficiencies. We measure the dynamic 2-port scattering parameters (S11 and S21) to determine the small signal modulation frequency response of the VCSEL and the combined VCSEL and photodetector optical link. By extracting and comparing the D-factor, modulation current efficiency factor, -3 dB bandwidth, and resistanceinductance- capacitance (RLC) circuit elements we find only a small difference in the static and dynamic performance characteristics of the five VCSEL designs, with slightly higher bandwidth for the half-lambda cavity VCSELs with two top oxide apertures.

We investigate triple and septuple electrically parallel, optically uncoupled 980 nm vertical-cavity surface-emitting laser (VCSEL) arrays with varying intra-array VCSEL-to-VCSEL spacings and oxide aperture diameters. We demonstrate 20 to 40 gigabit-per-second back-to-back data transmission with a bit error ratio of 1x10-13. Our VCSELs enable optical wireless communication for a plethora of sensors, smartphones, and other Internet of Things gadgets.

Vertical-cavity surface-emitting lasers with simplified epitaxial structures for integration exhibit small-signal modulation bandwidths (f3dB) exceeding 35 gigahertz. Devices for free-space communications and sensing exhibit f3dB exceeding 20 gigahertz and operating powers exceeding 25 milliwatts.

Brain-inspired computing concepts like artificial neural networks have become promising alternatives to classical von Neumann computer architectures. Photonic neural networks target the realizations of neurons, network connections and potentially learning in photonic substrates. Here, we report the development of a nanophotonic hardware platform of fast and energy-efficient photonic neurons via arrays of high-quality vertical cavity surface emitting lasers (VCSELs). The developed 5 × 5 VCSEL arrays provide high optical injection locking efficiency through homogeneous fabrication combined with individual control over the laser wavelengths. Injection locking is crucial for the reliable processing of information in VCSEL-based photonic neurons, and we demonstrate the suitability of the VCSEL arrays by injection locking measurements and current-induced spectral fine-tuning. We find that our investigated array can readily be tuned to the required spectral homogeneity, and as such show that VCSEL arrays based on our technology can act as highly energy efficient and ultra-fast photonic neurons for next generation photonic neural networks. Combined with fully parallel photonic networks our substrates are promising for ultra-fast operation reaching 10 s of GHz bandwidths, and we show that a single non-linear transformation based on our lasers will consume only about 100 fJ per VCSEL, which is highly competitive, compared to other platforms., The research leading to these results received funding from the Volkswagen Foundation via the project 'NeuroQNet' and the German Research Foundation supported this work via the Collaborative Research Center CRC 787. Additionally, this work was supported by the Spanish State Research Agency (AEI) through the Severo Ochoa and María de Maeztu Program for Centers and Units of Excellence in R&D (MDM-2017-0711).

Neural networks are one of the disruptive computing concepts of our time. However, they fundamentally differ from classical, algorithmic computing in a number of fundamental aspects. These differences result in equally fundamental, severe and relevant challenges for neural network computing using current computing substrates. Neural networks urge for parallelism across the entire processor and for a co-location of memory and arithmetic, i.e. beyond von Neumann architectures. Parallelism in particular made photonics a highly promising platform, yet until now scalable and integratable concepts are scarce. Here, we demonstrate for the first time how a fully parallel and fully implemented photonic neural network can be realized using spatially distributed modes of an efficient and fast semiconductor laser. Importantly, all neural network connections are realized in hardware, and our processor produces results without pre- or post-processing. 130+ nodes are implemented in a large-area vertical cavity surface emitting laser, input and output weights are realized via the complex transmission matrix of a multimode fiber and a digital micro-mirror array, respectively. We train the readout weights to perform 2-bit header recognition, a 2-bit XOR and 2-bit digital analog conversion, and obtain < 0.9 10^-3 and 2.9 10^-2 error rates for digit recognition and XOR, respectively. Finally, the digital analog conversion can be realized with a standard deviation of only 5.4 10^-2. Our system is scalable to much larger sizes and to bandwidths in excess of 20 GHz., 16 pages, 4 figures

We present an experimental and theoretical analysis of chromatic aberration in a monolithic metasurface focusing mirror. The planar focusing mirror is based on a monolithic high contrast grating made from GaAs, designed for a wavelength of 980 nm. Light is focused on the high refractive index side of the mirror. Our measurements, performed between 890 and 1050 nm, indicate a shift of the focal point position that is inversely proportional to the wavelength. The experimental results are in very good agreement with our simulations, in terms of both the position of the focal point and the spectral dependence. Based on our numerical simulations, we show that simply modifying the grating height does not lead to significant alteration of the focal length or to any noticeable reduction in chromatic aberration. Using numerical simulations, we analyze how the height of the stripes, the refractive index of the grating material, and its dispersion combine to influence the chromatic aberration of the mirror.

Directly current-modulated 980-nm vertical-cavity surface-emitting lasers (VCSELs) with oxide-aperture-diameters of $1.5~\mu \text{m}$ exhibit a small-signal −3 dB modulation bandwidth of 31 GHz and a single-mode light-output-power of 3 mW with a side-mode-suppression-ratio (SMSR) of 49 dB at 25 °C at a forward bias current of 3 mA. At 85 °C, the maximum bandwidth is about 25 GHz, the peak power is about 2 mW, and the SMSR remains 49 dB. The bandwidth for a 5.5- $\mu \text{m}$ oxide-aperture-diameter VCSEL reaches 26 and 21 GHz at 25 °C and 85 °C, respectively, with corresponding maximum multi-mode optical output power of about 10 and 6 mW.

A low-cost method to adjust the mirror reflectivity of vertical-cavity surface-emitting lasers (VCSELs) helps to optimize the cavity photon lifetime. The joint variation of photon lifetime and the oxide aperture diameter of our 980-nm VCSELs allows optimization procedures between the laser parameters. A new indicator helps to judge the optimum mirror reflectivity to enable increased bit rates in concert with reduced energy consumption, as well as temperature stability, and a smaller bias current density. Error-free back-to-back data transmission at 50 Gb/s over a wide temperature range from 25 °C to 75 °C for completely unchanged driving conditions is presented amongst other progress, including a 400-fJ/b data transmission energy efficiency at 50 Gb/s.