We report a selective injection design for GaAs-based Photonic-Crystal Surface-Emitting Lasers (PCSELs). COMSOL and FDTD simulations are carried out to design the injection electrode size to achieve largest gain overlapping factors with optical mode and lowest gain threshold. The PCSEL devices are fabricated with GaAs-based Multiple Quantum Well (MQW) wafer. Devices with surface area of 250×250 μm2 are fabricated with different injection electrode sizes. Testing results show that the best beam properties and an output power of 750 mW were achieved with a 150 μm p-electrode design, demonstrating selective injection impact to PCSEL beam profile.
Adjoint optimization is used to construct libraries of varying numbers of arbitrarily shaped elements. These libraries are then used to create and compare four metalens designs. Each of the 0.5 mm diameter metalens are designed to have a NA of 0.5 and is optimized for wavelengths from 4 to 5 µm. Based on libraries consisting of 4, 8, 16, and 32 elements, the different silicon MWIR metalenses show little to no degradation in performance with increasing library size. Across the different library sizes, the transmission increases by 2%, the focal effieciency increases by 1% and the FWHM decreases by less than a percent.
We investigate the effect of fabrication tolerances on photonic multimode waveguides operating in the vicinity of a third-order exceptional point degeneracy (EPD), known as a stationary inflection point (SIP). An EPD is a point in the parameter space where two or more Bloch eigenmodes coalesce in an infinite periodic waveguide, and at an SIP three modes coalesce to form the frozen mode. Waveguides operating near an SIP exhibit slow-light behavior in finite-length waveguides with anomalous cubic scaling of the group delay with waveguide length. The frozen mode facilitates stronger light-matter interactions in active media, resulting in a significant increase in the effective gain within the cavity. However, systems operating near an EPD are also exceptionally sensitive to fabrication deviations. In this work, we explore wave propagation and the impact of various fabrication imperfections in analytic models and in fabricated photonic chips for three mirrorless devices operating near an SIP. To advance the concept of the SIP laser, we also analyze how the addition of gain and loss affects the SIP performance. Our results show that while minor deviations from the ideal parameters can prevent perfect mode coalescence at the EPD, the frozen mode remains resilient to small perturbations and a significant degree of mode degeneracy prevails. These findings provide critical insights into the design and fabrication of passive and active photonic devices operating near high-order EPDs, paving the way for their practical implementation in a wide range of applications.
A mode-locked VECSEL is reported using a novel hybrid semiconductor saturable absorber mirror (SESAM) consisting of a semiconductor absorber region bonded to a curved dielectric partial reflector. The hybrid SESAM is realized by direct bonding of the saturable absorber to a commercially available ultrafast output coupler, nominally 99.4% reflectivity and GDD of ≤ |20 fs2| with a radius of curvature of 10 cm. In a linear cavity where the curved output coupler is the hybrid SESAM, a pulse-width of 410 femtoseconds is achieved at a repetition rate of 4.2 GHz for a VECSEL operating at a wavelength of 1030 nm.
A single transverse mode high pulse-energy GaSb VECSEL emitting at 2030 nm was studied. The peak power exceeds 500 W while maintaining good beam quality throughout the operation range. The cavity employs a Pockels cell combined with a low-loss thin film polarizer to selectively dump the intracavity energy in a 10 ns pulse. Thermal mitigation of the gain chip is achieved by both gain-switching and utilizing a long wavelength pump laser at 1470 nm compared to the traditional 980 nm pump for GaSb VECSELs. The laser has promise for incoherent LIDAR, materials processing, gas sensing, and nonlinear optics.
Mid infrared frequency combs allow for high resolution absorption spectroscopy of molecular species, which have strong signatures in this spectral region. Dual comb spectroscopy can provide broadband and high-resolution capability, but requires two fully stabilized frequency combs which adds complexity to the system.
Previous work has demonstrated that frequency combs coupled with a high resolution spectrometer, consisting of a virtually imaged phased array (VIPA) along with a grating, can perform time-resolved, broadband and high- resolution absorption spectroscopy with a single frequency comb. The VIPA spectrometer disperses the spectrum in two dimensions and images it onto a focal plane detector array. If the comb teeth can be resolved, the VIPA is easily calibrated and provides comb-tooth resolved resolution and accuracy. However, in previous work, the repetition rate of the laser sources used was too low to be resolved directly, and additional passive "filter cavities" had to be employed to increase the effective repetition rate of the frequency comb. In this work we use a fully stabilized mid infrared frequency comb based on a 1.6 GHz repetition rate modelocked vertical external cavity surface emitting laser (VECSEL) and difference frequency generation to produce an off set free comb in the 3- 4 micron wavelength range. The source is directly coupled to the VIPA spectrometer to provide comb-tooth resolved absorption spectroscopy. We discuss the system's performance in gas absorption spectroscopy and its time resolving capabilities, which are limited only by the speed of the detector system.
In recent years, diffractive, discrete scatterer based optics such as metasurfaces have shown considerable promise in the realization of arbitrary optical functions. However, these optical elements are systems large numbers of tunable degrees of freedom that are impractical to tune using forward design methods. In parallel, there has been great progress in using computational inverse design methods to produce high quality nanophotonic elements. We show that this inverse design method is capable of handling the large scale of the three-dimensional electromagnetic scattering problem, and leads to a realistic path towards the computational design and optimization of these discrete scatterer based optics capable of performing arbitrary optical functions in the far field. Then, we present an experimental demonstration of an optical element at 1.55 μm that focuses light into a discrete helical pattern that is designed using an inverse method based on generalized Lorenz-Mie scattering theory. This optical function is realized by specifying a suitable figure of merit that encapsulates the performance of the optical element. The fabrication of these optical elements with such small length scales is done using the Nanoscribe GT two-photon lithography system.
We report on the simulation of cavity-dumped semiconductor disk lasers utilizing an intracavity Pockels cell. This technique is used to generate high peak power pulses with pulse lengths of nominally one cavity round-trip. These results are compared to experiments demonstrated using InGaAs quantum-well gain region operating at approximately 1 μm to generate micro-Joule level nanosecond pulses.
The modal characteristics of nonresonant five-element phase-locked arrays of 4.7-μm emitting quantum cascade lasers (QCLs) have been studied using spectrally resolved near- and far-field measurements and correlated with results of device simulation. Devices are fabricated by a two-step metal-organic chemical vapor deposition process and operate predominantly in an in-phase array mode near threshold, although become multimode at higher drive levels. The wide spectral bandwidth of the QCL’s core region is found to be a factor in promoting multispatial-mode operation at high drive levels above threshold. An optimized resonant-array design is identified to allow sole in-phase array-mode operation to high drive levels above threshold, and indicates that for phase-locked laser arrays full spatial coherence to high output powers does not require full temporal coherence.
In recent years, infrared plasmonics has turned towards materials that are wavelength and application tailorable, and which are geared towards CMOS processing. The transparent conductive oxides are very favorable towards infrared plasmonic applications for a number of reasons, one of which being the natural visible transparency due to their relatively large bandgap. Fluorine-doped tin oxide (FTO) is one such transparent and doping-tunable material that in addition is low cost due to spray deposition techniques that result in perfectly conformal coatings. In this work, a deposition recipe that gives high free carrier concentration was used to fabricate structures for demonstration of surface plasmon excitation. 1D gratings with a range of structural parameters were etched in silicon. Then the gratings were conformally coated with FTO by aqueous spray deposition. Excitation of surface plasmon polaritons (SPP) at mid- and long- wave infrared wavelengths on these gratings was demonstrated. The observed (SPP) excitation resonances agree will with analytical excitation calculations and numerical simulations. We show that grating heights of ~10-15% of the wavelength are optimum for achieving the strongest sharpest coupling to plasmonic resonances in the mid- and longwave infrared. The presented results are compared with similar etched silicon gratings coated with Ga-doped ZnO (GZO). The dominant difference between our FTO and GZO measurements is the free carrier concentration. The useful wavelength range is predicted for FTO based plasmonics and compared with other plasmonic host materials. The work presented here could play a key role in novel decreased-cost detectors, filters, and on-chip optoelectronics.
The history of semiconductor quantum optics group in the College of Optical Sciences will be discussed. The work from planar microcavities including VCSELs, photonic crystal cavities leading to the observation of strong-coupling between an L3 cavity and a quantum dot, and now metallic cavities coupled to quantum wells and quantum dots will be described.
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