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Finding the solution to a large category of optimization problems, known as the NP-hard class, requires an exponentially increasing solution time using conventional computers. Lately, there has been intense efforts to develop alternative computational methods capable of addressing such tasks. In this regard, spin Hamiltonians, which originally arose in describing exchange interactions in magnetic materials, have recently been pursued as a powerful computational tool. Along these lines, it has been shown that solving NP-hard problems can be effectively mapped into finding the ground state of certain types of classical spin models. Here, we show that arrays of metallic nanolasers provide an ultra-compact, on-chip platform capable of implementing spin models, including the classical Ising and XY Hamiltonians. Various regimes of behavior including ferromagnetic, antiferromagnetic, as well as geometric frustration are observed in these structures. Our work paves the way towards nanoscale spin-emulators that enable efficient modeling of large-scale complex networks.

Microscale and nanoscale electrically pumped lasers are expected to play an indispensable role in future photonic integrated circuits. Small footprint, low threshold, high efficiency, and large side-mode suppression ratio (SMSR) are among the most important characteristics that such on-chip emitters should exhibit—allowing for large-scale integrability, low energy consumption, and stable output power. Microring resonators are one of the main contenders for the realization of such compact light sources. However, the use of microring laser cavities has been so far hindered by their tendency to operate in multiple modes. Quite recently, several studies have shown that notions from parity-time (PT)-symmetry and non-Hermitian physics can be utilized to effectively enforce single-mode operation in semiconductor microring laser arrangements. However, as of now, most works in this area were mainly devoted to the proof of concept demonstrations, while the feasibility of practical implementation has remained largely unexplored. In this article, we demonstrate the first regrowth free, electrically pumped microscale PT-symmetric laser. Our results can pave the way toward the utilization of exceptional points in more diverse applications ranging from on-chip light sources and modulators to laser beam steering systems and active sensors.

Spin models arise in the microscopic description of magnetic materials, where the macroscopic characteristics are governed by exchange interactions among the constituent magnetic moments. Recently, there has been a growing interest in complex systems with spin Hamiltonians, largely due to the rich behaviors exhibited by such interactions at the macroscale. Along these lines, it has been shown that certain classes of optimization problems involving large degrees of freedom can be effectively mapped into classical spin models. In this vein, the respective extremum can be found by identifying the ground state of the associated spin Hamiltonian. Here, we show both theoretically and experimentally, that the cooperative interplay among vectorial electromagnetic modes in coupled metallic nanolasers can be utilized as a means to emulate certain types of spin-like systems. The ensuing spin exchange interactions are in general anisotropic, in a way similar to that encountered in magnetic materials involving spin-orbit coupling. For some topologies, we find that these active nanophotonic structures are governed by a classical XY Hamiltonian that exhibits two phases akin to those associated with ferromagnetic (FM) and antiferromagnetic (AF) materials. In addition, we show that in certain configurations, the electromagnetic field distribution can undergo geometrical frustration, depending on the lattice shape and the transverse resonant modes supported by the individual cavity elements. Our results could pave the way towards a new scalable nanophotonic platform to study spin exchange interactions, that can in turn be potentially exploited to investigate more large-scale networks, emulate some magnetic materials, or to address a variety of optimization problems., Comment: 30 pages

Topological insulator lasers (TILs) are a recently introduced family of lasing arrays in which phase locking is achieved through synthetic gauge fields. These single frequency light source arrays operate in the spatially extended edge modes of topologically non-trivial optical lattices. Because of the inherent robustness of topological modes against perturbations and defects, such topological insulator lasers tend to demonstrate higher slope efficiencies as compared to their topologically trivial counterparts. So far, magnetic and non-magnetic optically pumped topological laser arrays as well as electrically pumped TILs that are operating at cryogenic temperatures have been demonstrated. Here we present the first room temperature and electrically pumped topological insulator laser. This laser array, using a structure that mimics the quantum spin Hall effect for photons, generates light at telecom wavelengths and exhibits single frequency emission. Our work is expected to lead to further developments in laser science and technology, while opening up new possibilities in topological photonics., Topological insulator lasers offer robustness and efficiency due to their unique properties but usually require cryogenic temperatures or optical pumping. Here the authors demonstrate an electrically pumped topological insulator laser operating at room temperature.

We report on the extraction of silver losses in the range 10 K-180 K by performing temperature-dependent micro-photoluminescence measurements in conjunction with numerical simulations on silver-coated nanolasers around near-infrared telecommunication wavelengths. By mapping changes in the quality factor of nanolasers into silver-loss variations, the imaginary part of silver permittivity is extracted at cryogenic temperatures. The latter is estimated to reach values an order of magnitude lower than room-temperature values. Temperature-dependent values for the thermo-optic coefficient of III-V semiconductors occupying the cavity are estimated as well. This data is missing from the literature and is crucial for precise device modeling. Our results can be useful for device designing, the theoretical validation of experimental observations as well as the evaluation of thermal effects in silver-coated nanophotonic structures.

We report on the first observation of a gain-induced topological response in a photonic lattice exhibiting asymmetric long-range interactions enabled by unidirectional microrings under pumping. This new platform is used to implement the Haldane lattice.

The dynamical behavior of broken symmetric coupled cavity lasers is theoretically investigated. The frequency response of this class of lasers is obtained using small signal analysis under direct modulation. Our model predicts a modulation bandwidth enhancement as a broken symmetric laser, operating in the parity-time (PT) symmetry and non-PT symmetry domains. This theoretical prediction is numerically examined in a laser system based on an InGaAs quantum dot platform. Our results clearly show that in these structures, in addition to the injection current, the gain-loss contrast can be used as a new degree of freedom in order to control the characteristic poles of the frequency response function.

We demonstrate both theoretically and experimentally that coupled metallic nanolasers can be used to emulate spin-Hamiltonians. Depending on the geometry of the array, we observe ferromagnetic and antiferromagnetic behaviors, as well as geometric frustration.

This Conference Presentation, Towards the experimental demonstration of topological Haldane lattice in microring laser arrays was recorded at Photonics West 2020 held in San Francisco, California, United States.

This Conference Presentation, Towards electrically pumped topological insulator lasers was recorded at Photonics West 2020 held in San Francisco, California, United States.

We report on the first demonstration of a topological Haldane lattice in a photonic setting using active microring structures.

We demonstrate how the presence of gain-loss contrast between two coupled identical resonators can be used as a new degree of freedom to enhance the modulation frequency response of laser diodes. An electrically pumped microring laser system with a bending radius of 50 μm is fabricated on an InAlGaAs/InP MQW p-i-n structure. The room temperature continuous wave (CW) laser threshold current of the device is 27 mA. By adjusting the ratio between the injection current levels in the two coupled microrings, our experimental results clearly show a bandwidth improvement by up to 1.63 times the fundamental resonant frequency of the individual device. This matches well with our rate equation simulation model.

Non-Hermitian exceptional points (EPs) represent a special type of degeneracy where not only the eigenvalues coalesce, but also the eigenstates tend to collapse on each other. Recent studies have shown that in the presence of an EP, light-matter interactions are profoundly modified, leading to a host of novel optical phenomena ranging from enhanced sensitivity to chiral light transport. As of now, however, in order to stabilize a system at the vicinity of an exceptional point, its related parameters must be carefully tuned and/or continuously controlled. To overcome this limitation, here we introduce a new family of broadband exceptional points based on unidirectional coupling, implemented by incorporating an Sshaped waveguide in a microring cavity. In active settings, the resulting unidirectionality exhibits unprecedented resilience to perturbations, thus providing a robust and tunable approach for directly generating beams with distinct orbital angular momentum (OAM). This work could open up new possibilities for manipulating OAM degrees of freedom in applications pertaining to telecommunications and quantum computing, while at the same time may expand the notions of non-Hermiticity in the orbital angular momentum space.

We report on our initial attempt to characterize the intrinsic frequency response of metal-clad nanolasers. The probed nanolaser is optically biased and modulated, allowing the emitted signal to be detected using a high-speed photodiode at each modulation frequency. Based on this technique, the prospect of high-speed operation of nanolasers is evaluated by measuring the D-factor, which is the ratio of the resonance frequency to the square root of its output power (fR/P_out^2). Our measurements show that for nanolasers, this factor is an order of magnitude greater than that of other state-of-the-art directly modulated semiconductor lasers. The theoretical analysis, based on the rate equation model and finite element method simulations of the cavity is in full agreement with the measurement results., Comment: Optics Express

The synergetic use of gain and loss in parity-time symmetric coupled resonators has been shown to lead to single-mode lasing operation. However, at the corresponding resonance frequency, an ideal ring resonator tends to support two degenerate eigenmodes, traveling along the cavity in opposite directions. Here, we show a unidirectional single-moded parity-time symmetric laser by incorporating active S-bend structures with opposite chirality in the respective ring resonators. Such chiral elements break the rotation symmetry of the ring cavities by providing an asymmetric coupling between the clockwise (CW) and the counterclockwise (CCW) traveling modes, hence creating a new type of exceptional point. This property, consequently, leads to the suppression of one of the counter-propagating modes. In this paper, we first measure the extinction ratio between the CW and CCW modes in a single ring resonator in the presence of an S-bend waveguide. We then experimentally investigate the unidirectional emission in PT-symmetric systems below and above the exceptional point. Finally, unidirectional emission will be shown in systems of two S-bend ring resonators coupled through a link structure.

The ongoing miniaturization of semiconductor lasers has enabled ultra‐low threshold devices and even provided a path to approach thresholdless lasing with linear input-output characteristics. Such nanoscale lasers have initiated a discourse on the origin of the physical mechanisms involved and their boundaries, such as the required photon number, the importance of optimized light confinement in a resonator, and mode‐density enhancement. Here, high‐β metal‐clad coaxial nanolasers, which facilitate thresholdless lasing are investigated. Both the conventional lasing characteristics, as well as the photon statistics of the emitted light at 10 K under continuous wave excitation are experimentally and theoretically investigated. While the former lacks adequate information to determine the threshold to coherent radiation, the latter reveals a finite threshold pump power. The work confirms an important aspect of high‐β lasers, namely that a thresholdless laser does have a finite threshold pump power and must not be confused with a hypothetical zero‐threshold laser. Moreover, the results reveal an excitation power dependent β‐factor which needs to be taken into account to correctly describe the experimental data.

Coupled microring arrangements with balanced gain and loss, also known as parity-time symmetric systems, are investigated both analytically and experimentally. In these configurations, stable single-mode lasing can be achieved at pump powers well above threshold. This self-adaptive mode management technique is broadband and robust to small fabrication imperfections. The results presented in this paper provide a new avenue in designing mode selective chip-scale in-plane semiconductor lasers by utilizing the complex dynamics of coupled gain/loss cavities.

Conventional techniques for transverse mode discrimination rely on introducing differential external losses to the different competing mode sets, enforcing single-mode operation at the expense of additional losses to the desirable mode. We show how a parity-time (PT) symmetric design approach can be employed to achieve single mode lasing in transversely multi-moded microring resonators. In this type of system, mode selectivity is attained by judiciously utilizing the exceptional point dynamics arising from a complex interplay of gain and loss. The proposed scheme is versatile, robust to deviations from PT symmetry such as caused by fabrication inaccuracies or pump inhomogeneities, and enables a stable operation considerably above threshold while maintaining spatial and spectral purity. The experimental results presented here were obtained in InP-based semiconductor microring arrangements and pave the way towards an entirely new class of chip-scale semiconductor lasers that harness gain/loss contrast as a primary mechanism of mode selectivity.

We demonstrate a microring laser generating vortex beams with topological charge. By implementing a chiral S-bend element inside the active ring and a second-order grating structure around the sidewall, this system could effectively down-convert the large order whispering gallery mode to on-demand OAM values.

A multimode, chaotic, and broadband emission is an undesired characteristic of laser arrays which is an adverse effect of supermodes in coupled waveguides. Employing optical supersymmetry, we experimentally demonstrate single mode lasing in arrays of supersymmetric lasers where a superpatner array eliminates undesired lasing modes.

We present our progress towards the realization of room temperature current-injected coaxial nanolasers. Lasing action is demonstrated in a metallic coaxial resonator up to a temperature of 140 K.

We present an electrically pumped parity-time-symmetric coupled microring laser. Using the interplay between gain and loss, single mode operation is demonstrated with no penalty in terms of output power or threshold pump intensity.

In recent years, the concept of parity-time (PT) symmetry has received considerable attention in the field of optics and photonics. In PT-symmetric arrangements, the interaction between gain/loss-contrast and coupling leads to the formation of exceptional points in parameter space. At these junctures, not only the eigenvalues but also the eigenvectors tend to merge, resulting in a sudden reduction of the dimensionality of the eigen-space. Consequently, in the vicinity of such points, the eigenfrequencies are strongly affected by external perturbationsas the system regains its original dimensionality. This unique behavior can be utilized to fundamentally enhance the sensitivity of micro-resonators. Here, we experimentally investigate this effect in integrated semiconductor PT-symmetric microring lasers that are biased at exceptional points. Using this arrangement, we demonstrate >10- fold enhancement in sensitivity. Our results also show that unlike standard microcavities, the parity-time symmetric system responds to the square-root of the perturbation. Our work provides a new avenue for enhancing the sensitivity of optical integrated sensors.

Heptamer arrays of coupled metallic nanolasers are demonstrated. The lasers operate in a single transverse and longitudinal mode, have low thresholds, and are capable of generating high output powers.

We provide an overview of our recent work on PT-symmetric microring lasers and present our newest theoretical findings and experimental results on mode control and lasing dynamics in such configurations. In contrast to conventional approaches, the exceptional-point dynamics arising from the complex interplay of gain and loss allow for the suppression of parasitic resonances with virtually no detrimental effects on the overall system efficiency. Finally, we show how PT-symmetrically structured active materials can serve as a powerful tool to tailor the modal content of lasing emissions from larger-area transversely multimoded architectures.

The last two decades have witnessed tremendous advancements in the area of nanophotonics and plasmonics. Undoubtedly, the introduction of metallic structures has opened a path towards light confinement and manipulation at the subwavelength scale { a regime that was previously thought to be out of reach in optics. Of central importance is to devise efficient light sources to power up the future nanoscale optical circuits. Coaxial resonators can provide a platform to implement such subwavelength sources. They support ultrasmall cavity modes and offer large mode-emitter overlap as well as multifold scalability. Given their large modulation bandwidth, they hold promise for high speed optical interconnects { where they can be used for light generation and modulation simultaneously. In addition, the possibility of thresholdless operation in such devices may have implications in developing the next generation of efficient lighting systems. In this review article, the physics and applications of coaxial nanolasers will be discussed.

We present a new class of ultra-sensitive PT-symmetric photonic molecules based on higher-order exceptional points. Methods to realize such sensitive structures are explored in InGaAsP arrangements by appropriately controlling the perturbations in the system.

The second-order coherence function is measured for a metallic coaxial nanolaser using a modified Hanbury Brown-Twiss technique. The results indicate that such nanoscale lasers can indeed generate coherent radiation.

The behavior of a parity-time (PT) symmetric coupled microring system is studied when operating in the vicinity of an exceptional point. Using the abrupt phase transition around this point, stable single-mode lasing is demonstrated in spectrally multi-moded micro-ring arrangements., 5 pages, 6 figures

Wavelength tuning in a single mode parity-time (PT) symmetric semiconductor microring laser is demonstrated. Stable continuous tuning over a spectral range of 4 nm has been obtained at telecom wavelengths by adjusting the ambient temperature.

Single-transverse mode lasing is demonstrated in a broad-area PT-symmetric semiconductor microring arrangement. The proposed scheme is versatile, robust to fabrication errors, and allows for high brightness operation while maintaining spectral purity.

The sensitivity of microcavities used for particle detection is at best proportional to the perturbation (ϵ). Here we propose a novel microresonator sensor using exceptional points in PT-symmetric arrangements showing an enhanced sensitivity (ϵ1/2).

Second-order coherence measurements can unambiguously determine whether a light source is generating coherent radiation, that is, whether it is a laser. In a systematic study reported by Kreinberg et al., this measurement is used to additionally reveal intriguing physics associated with ultra-small resonators, such as the identification of sub- or super-radiance emission.

Upconversion processes have found widespread applications in drug delivery, bio-imaging and solar-cells. In this paper we present a theoretical model that analyzes the impact of a plasmonic shield structure on the quantum yield of upconversion nanoparticles. We use this model to assess the efficiency of NaYF 4 : Tm 3+ Yb 3+ /NaYF 4 core-shell nanoparticles when embedded in a polymer matrix and covered by a metallic can-like structure. We find that as a result of this specific plasmonic structure, the upconversion luminescence from NIR to UV can be increased by a factor of 30.

Low noise mode-locked lasers and stabilized optical frequency combs are receiving considerable attention due to their broad spectrum of applications which ranges from signal processing to communications to metrology. Progress has been made in the realization of ultralow noise pulse trains by using ultralow expansion (ULE) quartz etalons for filtering the axial mode groups. An important step towards miniaturization of these systems is the integration of a high finesse on-chip optical filter that would serve to replace the ULE etalon. In this paper, we report our experimental results towards the realization of such a high finesse cavity based on a silicon microring resonator.

By exploiting the inherent characteristics of dark-state resonators, we experimentally realize a single-frequency integrated microring laser system. This semiconductor laser can remain single-mode, even at high pump power levels, while allowing tunability over a wide spectral range. Our results demonstrate the potential of exceptional points as a versatile tool for mode selection in micro-cavity laser configurations.

We show that the upconversion emission spectra of Tm³⁺ and Yb³⁺ codoped β-NaYF₄-NaYF₄ core-shell nanoparticles can be judiciously modified by means of plasmonic nanocavities. Our analysis indicates that more than a 30-fold increase in conversion efficiency to the UV spectral band can be expected by engineering the NIR absorption and the local density of states. The effect of the nanocavity on the resulting radiation patterns is discussed. Our results are exemplified in cylindrical cavity geometries.

Single-mode lasing is demonstrated in an electrically injected coupled microring arrangement at telecommunication wavelengths by exploiting the unique physics associated with parity-time-symmetry.

The interplay between array geometry and the whispering galley-like modes of individual nanolasers can lead to the direct generation of structured light in nanoscale. Such nanolaser lattices emitting OAM beams are experimentally demonstrated.

A semiconductor microring laser based on a dark state configuration is demonstrated. Single mode lasing is achieved by judiciously exploiting non-Hermitian exceptional points.

We report coherent emission behavior of high-β metallic coaxial nanolasers under pulsed illumination conditions. Time-resolved photon statistical measurements show a transition from thermal to coherent emission within the envelope of the excitation pulse.

We report on the first demonstration of electrically pumped topological insulator laser arrays. When pumping is applied to the elements along the perimeter, a sharp single-mode lasing peak can be detected across the array.

We report on the temporal dynamics of second-order correlation function of high-β nanolasers excited with Gaussian-shaped pulses. We show that the nature of output radiation from the nanolaser depends on the excitaiton pulse envelope.