Scalable single-mode surface-emitting laser via open-Dirac singularities.

Single-aperture cavities are a key component of lasers that are instrumental for the amplification and emission of a single light mode. However, the appearance of high-order transverse modes as the size of the cavities increases has frustrated efforts to scale-up cavities while preserving single-mode operation since the invention of the laser six decades ago1-8. A suitable physical mechanism that allows single-mode lasing irrespective of the cavity size-a 'scale invariant' cavity or laser-has not been identified yet. Here we propose and demonstrate experimentally that open-Dirac electromagnetic cavities with linear dispersion-which in our devices are realized by a truncated photonic crystal arranged in a hexagonal pattern-exhibit unconventional scaling of losses in reciprocal space, leading to single-mode lasing that is maintained as the cavity is scaled up in size. The physical origin of this phenomenon lies in the convergence of the complex part of the free spectral range in open-Dirac cavities towards a constant governed by the loss rates of distinct Bloch bands, whereas for common cavities it converges to zero as the size grows, leading to inevitable multimode emission. An unconventional flat-envelope fundamental mode locks all unit cells in the cavity in phase, leading to single-mode lasing. We name such sources Berkeley surface-emitting lasers (BerkSELs) and demonstrate that their far-field corresponds to a topological singularity of charge two, in agreement with our theory. Open-Dirac cavities unlock avenues for light-matter interaction and cavity quantum electrodynamics.

Scalable manufacturing of quantum light emitters in silicon under rapid thermal annealing.

Quantum light sources play a fundamental role in quantum technologies ranging from quantum networking to quantum sensing and computation. The development of these technologies requires scalable platforms, and the recent discovery of quantum light sources in silicon represents an exciting and promising prospect for scalability. The usual process for creating color centers in silicon involves carbon implantation into silicon, followed by rapid thermal annealing. However, the dependence of critical optical properties, such as the inhomogeneous broadening, the density, and the signal-to-background ratio, on centers implantation steps is poorly understood. We investigate the role of rapid thermal annealing on the dynamic of the formation of single color centers in silicon. We find that the density and the inhomogeneous broadening greatly depend on the annealing time. We attribute the observations to nanoscale thermal processes occurring around single centers and leading to local strain fluctuations. Our experimental observation is supported by theoretical modeling based on first principles calculations. The results indicate that annealing is currently the main step limiting the scalable manufacturing of color centers in silicon.

Lasing action from photonic bound states in continuum.

In 1929, only three years after the advent of quantum mechanics, von Neumann and Wigner showed that Schrödinger's equation can have bound states above the continuum threshold. These peculiar states, called bound states in the continuum (BICs), manifest themselves as resonances that do not decay. For several decades afterwards the idea lay dormant, regarded primarily as a mathematical curiosity. In 1977, Herrick and Stillinger revived interest in BICs when they suggested that BICs could be observed in semiconductor superlattices. BICs arise naturally from Feshbach's quantum mechanical theory of resonances, as explained by Friedrich and Wintgen, and are thus more physical than initially realized. Recently, it was realized that BICs are intrinsically a wave phenomenon and are thus not restricted to the realm of quantum mechanics. They have since been shown to occur in many different fields of wave physics including acoustics, microwaves and nanophotonics. However, experimental observations of BICs have been limited to passive systems and the realization of BIC lasers has remained elusive. Here we report, at room temperature, lasing action from an optically pumped BIC cavity. Our results show that the lasing wavelength of the fabricated BIC cavities, each made of an array of cylindrical nanoresonators suspended in air, scales with the radii of the nanoresonators according to the theoretical prediction for the BIC mode. Moreover, lasing action from the designed BIC cavity persists even after scaling down the array to as few as 8-by-8 nanoresonators. BIC lasers open up new avenues in the study of light-matter interaction because they are intrinsically connected to topological charges and represent natural vector beam sources (that is, there are several possible beam shapes), which are highly sought after in the fields of optical trapping, biological sensing and quantum information.

Tunable and enhanced optical force with bound state in the continuum.

Light-actuated motors, vehicles, and even space sails have drawn tremendous attention for basic science and applications in space, biomedical, and sensing domains. Optical bound states in the continuum (BIC) are topological singularities of the scattering matrix, known for their unique light-trapping capability and enhanced light-matter interaction. We show that BIC modes enable the generation of enhanced and tunable optical forces and torques. A sharp and controllable line shape is observed in force and torque spectra when approaching high-Q resonance BIC modes. Wavelength and polarization tunability are presented as an effective method to control forces on BIC enclosed structures. Finally finite-size simulations are performed to evaluate the practical applications for a BIC-assisted metavehicle.

Doping-induced plateau of strong electromagnetic confinement in the momentum space.

In this Letter, we present a design strategy for the realization of electrically powered bound states in the continuum (BIC) lasers. Despite growing attention of the optics community for BICs, practical uses of BICs in an active device are still unestablished. A large index contrast and out-of-plane symmetries that aid the formation of BICs are not trivial to achieve using conventional approaches for semiconductor laser design. Here, we propose a doping scheme to circumvent this issue. We also show that the introduction of material absorption due to carriers deteriorates the quality factor of BIC modes and show that a suitable compromise between electrical conductivity and optical loss can be achieved.

Experimental demonstration of single-mode topological valley-Hall lasing at telecommunication wavelength controlled by the degree of asymmetry.

Topology plays a fundamental role in contemporary physics and enables new information processing schemes and wave device physics with built-in robustness. However, the creation of photonic topological phases usually requires complex geometries that limit the prospect for miniaturization and integration and dispossess designers of additional degrees of freedom needed to control topological modes on-chip. By controlling the degree of asymmetry (DoA) in a photonic crystal with broken inversion symmetry, we report single-mode lasing of valley-Hall ring cavities at telecommunication wavelength. The DoA governs four photon confinement regimes at the interface of topologically distinct valley-Hall domains and evidences an interplay between the width of the topological bandgap and the quality factor of ring-like modes for single-mode operation. Our results open the door to novel optoelectronic devices and systems based on compact topological integrated circuits.

Broadband and linear polarization metasurface carpet cloak in the visible.

In the past few years, carpet cloaking has attracted interest because of its feasibility at optical frequencies and potential in stealth technologies. Metasurfaces have been proposed as a method to engineer ultra-thin carpet cloaking surfaces due to their abilities to manipulate wavefronts, polarization, and phase at subwavelength scale. However, achieving broadband carpet cloaking with a significant bandwidth is one of the key remaining challenges for metasurface designs. To date, broadband carpet cloaking based on metasurfaces has not been achieved, and operation has been limited to discrete wavelengths. Here, we propose and numerically demonstrate a novel metasurface design for broadband carpet cloaking with linear polarization at visible wavelengths from 650 nm to 800 nm. Our proposed method is a promising approach for broadband structured interfaces.

Lasing action in low-resistance nanolasers based on tunnel junctions.

We experimentally demonstrate the lasing action of a new nanolaser design with a tunnel junction. By using a heavily doped tunnel junction for hole injection, we can replace the p-type contact material of a conventional nanolaser diode with a low-resistance n-type contact layer. This leads to a significant reduction of the device resistance and lowers the threshold voltage from 5 V to around 0.95 V at 77 K. The lasing behavior is verified by the light output versus the injection current (L-I) characterization and second-order coherence function measurements. Because of less Joule heating during current injection, the nanolaser can be operated at temperatures as high as 180 K under CW pumping. The incorporation of heavily doped tunnel junctions may pave the way for other nanoscale cavity design for improved heat management.

Planar dielectric cylindrical lens at 800 nm and the role of fabrication imperfections.

Conventional optical components have been proposed to realize high-quality line focusing with uniform intensity distribution such as cylindrical lenses, segmented wedge-arrays, or a combination of prisms and spherical mirrors. Numerous factors such as the manufacturing tolerances or the need for precise alignment of conventional lenses cause wave front aberrations that impact the performance of optical systems. These aforementioned limitations affect the uniformity of the intensity distribution and the intercept factor of lenses. Here, we experimentally demonstrate an integrable planar dielectric cylindrical lens made of titanium dioxide for uniform line focusing and discuss the sensitivity of its performance to fabrication imperfections originating from non-ideal geometrical parameters. The lens has a numerical aperture of 0.247, an intercept factor of 0.85, and an efficiency of 79% at 800 nm.

Efficient design of random metasurfaces.

Random media introduce large degrees of freedom in device design and can thus address challenges in manipulating optical waves. Wave shaping with metasurfaces has mainly utilized periodic or quasi-periodic grids, and the potential of random arrangement of particles for devices has only come under investigation recently. The main difficulty in pursuing random metasurfaces is the identification of the degrees of freedom that optimize their efficiencies and functions. They can also encode information using the statistics of particle distribution. We propose a phase-map that accounts for the statistical nature of random media. The method takes into account effects of random near-field couplings that introduce phase errors by affecting the phase shift of elements. The proposed approach increases the efficiency of our random metasurface devices by up to ∼20%. This work paves the way toward the efficient design of random metasurfaces with potential applications in highly secure optical cryptography and information encoding.

Local phase method for designing and optimizing metasurface devices.

Metasurfaces have attracted significant attention due to their novel designs for flat optics. However, the approach usually used to engineer metasurface devices assumes that neighboring elements are identical, by extracting the phase information from simulations with periodic boundaries, or that near-field coupling between particles is negligible, by extracting the phase from single particle simulations. This is not the case most of the time and the approach thus prevents the optimization of devices that operate away from their optimum. Here, we propose a versatile numerical method to obtain the phase of each element within the metasurface (meta-atoms) while accounting for near-field coupling. Quantifying the phase error of each element of the metasurfaces with the proposed local phase method paves the way to the design of highly efficient metasurface devices including, but not limited to, deflectors, high numerical aperture metasurface concentrators, lenses, cloaks, and modulators.

Hybridized metamaterial platform for nano-scale sensing.

Plasmonic/metamaterial sensors are being investigated for their high sensitivity, fast response time, and high accuracy. We propose, characterize and experimentally realize subwavelength bilayer metamaterial sensors operating in the near-infrared domain. We measure the figure-of-merit (FOM) and the bulk sensitivity (S) of the two fundamental hybridized modes and demonstrate both numerically and experimentally that the magnetic dipolar mode, degenerate with the electric quadrupolar mode, has higher sensitivity to a variation of the refractive index compared to the electric dipolar mode. In addition, the hybridized system exhibits a four fold increase in the FOM compared to a standard dipolar plasmonic system.

From parabolic-trough to metasurface-concentrator: assessing focusing in the wave-optics limit.

Metasurfaces are promising tools toward novel designs for flat optics applications. As such, their quality and tolerance to fabrication imperfections need to be evaluated with specific tools. However, most such tools rely on the geometrical optics approximation and are not straightforwardly applicable to metasurfaces. In this Letter, we introduce and evaluate for metasurfaces parameters such as intercept factor and slope error usually defined for solar concentrators in the realm of ray-optics. After proposing definitions valid in physical optics, we put forward an approach to calculate them. As examples, we design three different concentrators based on three specific unit cells and assess them numerically. The concept allows for comparison of the efficiency of the metasurfaces and their sensitivities to fabrication imperfections and will be critical for practical systems implementation.

Swimming Microrobot Optical Nanoscopy.

Optical imaging plays a fundamental role in science and technology but is limited by the ability of lenses to resolve small features below the fundamental diffraction limit. A variety of nanophotonic devices, such as metamaterial superlenses and hyperlenses, as well as microsphere lenses, have been proposed recently for subdiffraction imaging. The implementation of these micro/nanostructured lenses as practical and efficient imaging approaches requires locomotive capabilities to probe specific sites and scan large areas. However, directed motion of nanoscale objects in liquids must overcome low Reynolds number viscous flow and Brownian fluctuations, which impede stable and controllable scanning. Here we introduce a new imaging method, named swimming microrobot optical nanoscopy, based on untethered chemically powered microrobots as autonomous probes for subdiffraction optical scanning and imaging. The microrobots are made of high-refractive-index microsphere lenses and powered by local catalytic reactions to swim and scan over the sample surface. Autonomous motion and magnetic guidance of microrobots enable large-area, parallel and nondestructive scanning with subdiffraction resolution, as illustrated using soft biological samples such as neuron axons, protein microtubulin, and DNA nanotubes. Incorporating such imaging capacities in emerging nanorobotics technology represents a major step toward ubiquitous nanoscopy and smart nanorobots for spectroscopy and imaging.

Predicting nonlinear properties of metamaterials from the linear response.

The discovery of optical second harmonic generation in 1961 started modern nonlinear optics. Soon after, R. C. Miller found empirically that the nonlinear susceptibility could be predicted from the linear susceptibilities. This important relation, known as Miller's Rule, allows a rapid determination of nonlinear susceptibilities from linear properties. In recent years, metamaterials, artificial materials that exhibit intriguing linear optical properties not found in natural materials, have shown novel nonlinear properties such as phase-mismatch-free nonlinear generation, new quasi-phase matching capabilities and large nonlinear susceptibilities. However, the understanding of nonlinear metamaterials is still in its infancy, with no general conclusion on the relationship between linear and nonlinear properties. The key question is then whether one can determine the nonlinear behaviour of these artificial materials from their exotic linear behaviour. Here, we show that the nonlinear oscillator model does not apply in general to nonlinear metamaterials. We show, instead, that it is possible to predict the relative nonlinear susceptibility of large classes of metamaterials using a more comprehensive nonlinear scattering theory, which allows efficient design of metamaterials with strong nonlinearity for important applications such as coherent Raman sensing, entangled photon generation and frequency conversion.

Integrated metaphotonics: symmetries and confined excitation of LSP resonances in a single metallic nanoparticle.

Using numerical simulations, we demonstrate that the dipolar plasmonic resonance of a single metallic nanoparticle inserted in the core of a dielectric waveguide can be excited with higher order photonic modes of the waveguide only if their symmetry is compatible with the charge distribution of the plasmonic mode. For the case of a symmetric waveguide, we demonstrate that this condition is only achieved if the particle is shifted from the center of the core. The simple and comprehensive analysis presented in this contribution will serve as basis for applications in integrated nanophotonic/metamaterials devices, such as optical filters, modulators and mode converters.

Cramér-Rao bounds for determination of electric and magnetic susceptibilities in metasurfaces.

Accurate and robust characterization of metasurfaces and metamaterials in terms of effective parameters is critical to the design of novel metadevices. We compute the Cramér-Rao lower bounds on the variance of any estimator for both the electric and magnetic surface susceptibilities of metasurfaces. We show that retrieval of such effective properties is inherently difficult around resonances, most notably for low-loss metasurfaces. We also put forth a least-squares estimator to mitigate this difficulty for the normal components of susceptibility tensors, which are observed to be the most ill-behaved. The present work is relevant to the development of loss-compensated metasurfaces for which noise has to be closely considered for accurate and robust device characterization.

Modal amplification in active waveguides with hyperbolic dispersion at telecommunication frequencies.

We present a method for studying amplification of electromagnetic modes in active, circularly symmetric waveguides with hyperbolic dispersion. Using this method, we obtain a closed-form expression for the modal threshold condition. We find that modal amplification is possible in a region of the radius-wavelength phase-space with small enough radius so that propagation of the mode is permitted while modal energy and phase counter-propagate. At telecommunication frequencies, such a situation is achievable only when the absolute value of the real metal permittivity exceeds that of the active dielectric. We validate our theoretical conclusions with numerical simulations that explain the threshold condition in terms of an energy balance between the longitudinal and radial components of the electric field.

Creating pairs of exceptional points for arbitrary polarization control: asymmetric vectorial wavefront modulation.

Exceptional points (EPs) can achieve intriguing asymmetric control in non-Hermitian systems due to the degeneracy of eigenstates. Here, we present a general method that extends this specific asymmetric response of EP photonic systems to address any arbitrary fully-polarized light. By rotating the meta-structures at EP, Pancharatnam-Berry (PB) phase can be exclusively encoded on one of the circular polarization-conversion channels. To address any arbitrary wavefront, we superpose the optical signals originating from two orthogonally polarized -yet degenerate- EP eigenmodes. The construction of such orthogonal EP eigenstates pairs is achieved by applying mirror-symmetry to the nanostructure geometry flipping thereby the EP eigenmode handedness from left to right circular polarization. Non-Hermitian reflective PB metasurfaces designed using such EP superposition enable arbitrary, yet unidirectional, vectorial wavefront shaping devices. Our results open new avenues for topological wave control and illustrate the capabilities of topological photonics to distinctively operate on arbitrary polarization-state with enhanced performances.

Heterojunction silicon microwire solar cells.

We report radial heterojunction solar cells of amorphous silicon on crystalline silicon microwires with high surface passivation. While the shortened collection path is exploited to increase the photocurrent, proper choice of the wire radius and the highly passivated surface prevent drastic decrease in the voltage due to high surface-to-volume ratio. The heterojunction is formed by depositing a ∼12-16 nm of amorphous silicon on crystalline silicon wires of radius approximately equal to minority carrier diffusion length (∼10 µm). In spite of very short carrier lifetime (<1 µs), the microwire array devices generate photocurrent of ∼30 mA/cm(2), and the same time, voltages close to 600 mV are achieved, leading to efficiency in excess of 12% in extremely short carrier lifetime silicon. We also find that formation of nanocrystallites of silicon in the deposited film results in loss of the expected passivation.