Topological corner states in a silicon nitride photonic crystal membrane with a large bandgap.

The theory of band topology has inspired the discovery of various topologically protected states in the regime of photonics. It has led to the development of topological photonic devices with robust property and versatile functionalities, like unidirectional waveguides, compact power splitters, high-Q resonators, and robust lasers. These devices mainly rely on the on-chip photonic crystal (PhC) in Si or III-V compound materials with a fairly large bandgap. However, the topological designs have rarely been applied to the ultra-low-loss silicon nitride (SiN) platform which is widely used in silicon photonics for important devices and integrated photonic circuits. It is mainly hindered by the relatively low refractive index. In this work, we revealed that a rhombic PhC can open a large bandgap in the SiN slab, and thus support robust topological corner states stemming from the quantization of the dipole moments. Meanwhile, we propose the inclination angle of rhombic lattice, as a new degree of freedom, to manipulate the characteristics of topological states. Our work shows a possibility to further expand the topological protection and design flexibility to SiN photonic devices.

Impact of Cavity Geometry on Microlaser Dynamics.

We experimentally investigate spatiotemporal lasing dynamics in semiconductor microcavities with various geometries, featuring integrable or chaotic ray dynamics. The classical ray dynamics directly impacts the lasing dynamics, which is primarily determined by the local directionality of long-lived ray trajectories. The directionality of optical propagation dictates the characteristic length scales of intensity variations, which play a pivotal role in nonlinear light-matter interactions. While wavelength-scale intensity variations tend to stabilize lasing dynamics, modulation on much longer scales causes spatial filamentation and irregular pulsation. Our results will pave the way to control the lasing dynamics by engineering the cavity geometry and ray dynamical properties.

In-plane emission manipulation of random optical modes by using a zero-index material.

In this work, we have proposed to implement a zero-index material (ZIM) to control the in-plane emission of planar random optical modes while maintaining the intrinsic disordered features. Light propagating through a medium with near-zero effective refractive index accumulates little phase change and is guided to the direction determined by the conservation law of momentum. By enclosing a disordered structure with a ZIM based on all-dielectric photonic crystal (PhC), broadband emission directionality improvement can be obtained. We find the maximum output directionality enhancement factor reaches 30, around 6-fold increase compared to that of the random mode without ZIM. The minimum divergence angle is ∼6° for single random optical mode and can be further reduced to ∼3.5° for incoherent multimode superposition in the far field. Despite the significant directionality enhancement, the random properties are well preserved, and the Q factors are even slightly improved. The method is robust and can be effectively applied to the disordered medium with different structural parameters, e.g., the filling fraction of scatterers, and different disordered structure designs with extended or strongly localized modes. The output direction of random optical modes can also be altered by further tailoring the boundary of ZIM. This work provides a novel and universal method to manipulate the in-plane emission direction as well as the directionality of disordered medium like random lasers, which might enable its on-chip integration with other functional devices.

Electrically-pumped compact topological bulk lasers driven by band-inverted bound states in the continuum.

One of the most exciting breakthroughs in physics is the concept of topology that was recently introduced to photonics, achieving robust functionalities, as manifested in the recently demonstrated topological lasers. However, so far almost all attention was focused on lasing from topological edge states. Bulk bands that reflect the topological bulk-edge correspondence have been largely missed. Here, we demonstrate an electrically pumped topological bulk quantum cascade laser (QCL) operating in the terahertz (THz) frequency range. In addition to the band-inversion induced in-plane reflection due to topological nontrivial cavity surrounded by a trivial domain, we further illustrate the band edges of such topological bulk lasers are recognized as the bound states in the continuum (BICs) due to their nonradiative characteristics and robust topological polarization charges in the momentum space. Therefore, the lasing modes show both in-plane and out-of-plane tight confinements in a compact laser cavity (lateral size ~3λlaser). Experimentally, we realize a miniaturized THz QCL that shows single-mode lasing with a side-mode suppression ratio (SMSR) around 20 dB. We also observe a cylindrical vector beam for the far-field emission, which is evidence for topological bulk BIC lasers. Our demonstration on miniaturization of single-mode beam-engineered THz lasers is promising for many applications including imaging, sensing, and communications.

Photonic Majorana quantum cascade laser with polarization-winding emission.

Topological cavities, whose modes are protected against perturbations, are promising candidates for novel semiconductor laser devices. To date, there have been several demonstrations of topological lasers (TLs) exhibiting robust lasing modes. The possibility of achieving nontrivial beam profiles in TLs has recently been explored in the form of vortex wavefront emissions enabled by a structured optical pump or strong magnetic field, which are inconvenient for device applications. Electrically pumped TLs, by contrast, have attracted attention for their compact footprint and easy on-chip integration with photonic circuits. Here, we experimentally demonstrate an electrically pumped TL based on photonic analogue of a Majorana zero mode (MZM), implemented monolithically on a quantum cascade chip. We show that the MZM emits a cylindrical vector (CV) beam, with a topologically nontrivial polarization profile from a terahertz (THz) semiconductor laser.

Spatiotemporal lasing dynamics in a Limaçon-shaped microcavity.

Limaçon-shaped microdisk lasers are promising on-chip light sources with low lasing threshold and unidirectional output. We conduct an experimental study on the lasing dynamics of Limaçon-shaped semiconductor microcavities. The edge emission exhibits intensity fluctuations over a wide range of spatial and temporal scales. They result from multiple dynamic processes with different origins and occur on different spatiotemporal scales. The dominant process is an alternate oscillation between two output beams with a period as short as a few nanoseconds.

Mass-Manufactured Beam-Steering Metasurfaces for High-Speed Full-Duplex Optical Wireless-Broadcasting Communications.

Beam-steering devices, which are at the heart of optical wireless-broadcasting communication links, play an important role in data allocation and exchange. An ideal beam-steering device features large steering angles, arbitrary channel numbers, reconfigurability, and ultracompactness. However, these criteria have been achieved only partially with conventional beam-steering devices based on waveguides, micro-electricalmechanical systems, spatial light modulators, and gratings, which will substantially limit the application of optical wireless-broadcasting communication techniques. In this study, an ultracompact full-duplex metabroadcasting communication system is designed and experimentally demonstrated, which exhibits beam steering angles up to ±40°, 14 broadcasting channels with capacity for downstream and upstream links up to 100 and 10 Gbps for each user channel, three operating modes for flexible signal switching, and metadevice dimensions as small as 2 mm × 2 mm. In particular, the beam-steering metadevices are mass-manufactured by a complementary metal-oxide-semiconductor (CMOS) processing platform, which shows their potential for large-scale commercial applications. The demonstrated metabroadcasting communication system merges optical wireless-broadcasting communications and metasurfaces, which reduces the complexity of beam-steering devices while significantly increasing their performance, opening up a new avenue for high-quality optical wireless-broadcasting communications.

Tailorable infrared emission of microelectromechanical system-based thermal emitters with NiO films for gas sensing.

Infrared gas sensors hold great promise in the internet of things and artificial intelligence. Making infrared light sources with miniaturized size, reliable and tunable emission is essential but remains challenging. Herein, we present the tailorability of radiant power and the emergence of new emission wavelength of microelectromechanical system (MEMS)-based thermal emitters with nickel oxide (NiO) films. The coating of NiO on emitters increases top surface emissivity and induces the appearance of new wavelengths between 15 and 19 µm, all of which have been justified by spectroscopic methods. Furthermore, a sensor array is assembled for simultaneous monitoring of concentrations of carbon dioxide (CO2), methane (CH4), humidity, and temperature. The platform shows selective and sensitive detection at room temperature toward CO2 and CH4 with detection limits of around 50 and 1750 ppm, respectively, and also shows fast response/recovery and good recyclability. The demonstrated emission tailorability of MEMS emitters and their usage in sensor array provide novel insights for designing and fabricating optical sensors with good performance, which is promising for mass production and commercialization.

Massively parallel ultrafast random bit generation with a chip-scale laser.

Random numbers are widely used for information security, cryptography, stochastic modeling, and quantum simulations. Key technical challenges for physical random number generation are speed and scalability. We demonstrate a method for ultrafast generation of hundreds of random bit streams in parallel with a single laser diode. Spatiotemporal interference of many lasing modes in a specially designed cavity is introduced as a scheme for greatly accelerated random bit generation. Spontaneous emission, caused by quantum fluctuations, produces stochastic noise that makes the bit streams unpredictable. We achieve a total bit rate of 250 terabits per second with off-line postprocessing, which is more than two orders of magnitude higher than the current postprocessing record. Our approach is robust, compact, and energy-efficient, with potential applications in secure communication and high-performance computation.

Electrically pumped topological laser with valley edge modes.

Quantum cascade lasers are compact, electrically pumped light sources in the technologically important mid-infrared and terahertz region of the electromagnetic spectrum1,2. Recently, the concept of topology3 has been expanded from condensed matter physics into photonics4, giving rise to a new type of lasing5-8 using topologically protected photonic modes that can efficiently bypass corners and defects4. Previous demonstrations of topological lasers have required an external laser source for optical pumping and have operated in the conventional optical frequency regime5-8. Here we demonstrate an electrically pumped terahertz quantum cascade laser based on topologically protected valley edge states9-11. Unlike topological lasers that rely on large-scale features to impart topological protection, our compact design makes use of the valley degree of freedom in photonic crystals10,11, analogous to two-dimensional gapped valleytronic materials12. Lasing with regularly spaced emission peaks occurs in a sharp-cornered triangular cavity, even if perturbations are introduced into the underlying structure, owing to the existence of topologically protected valley edge states that circulate around the cavity without experiencing localization. We probe the properties of the topological lasing modes by adding different outcouplers to the topological cavity. The laser based on valley edge states may open routes to the practical use of topological protection in electrically driven laser sources.

Radiation Enhancement by Graphene Oxide on Microelectromechanical System Emitters for Highly Selective Gas Sensing.

Infrared gas sensors have been proven promising for broad applications in Internet of Things and Industrial Internet of Things. However, the lack of miniaturized light sources with good compatibility and tunable spectral features hinders their widespread utilization. Herein, a strategy is proposed to increase the radiated power from microelectromechanical-based thermal emitters by coating with graphene oxide (GO). The radiation can be substantially enhanced, which partially stems from the high emissivity of GO coating demonstrated by spectroscopic methods. Moreover, the sp2 structure within GO may induce plasmons and thus couple with photons to produce blackbody radiation and/or new thermal emission sources. As a proof-of-concept demonstration, the GO-coated emitter is integrated into a multifunctional monitoring platform and evaluated for gas detection. The platform exhibits sensitive and highly selective detection toward CO2 at room temperature with a detection limit of 50 ppm and short response/recovery time, outperforming the state-of-the-art gas sensors. This study demonstrates the emission tailorability of thermal emitters and the feasibility of improving the associated gas sensing property, offering perspectives for designing and fabricating high-end optical sensors with cost-effectiveness and superior performance.

Suppressing spatiotemporal lasing instabilities with wave-chaotic microcavities.

Spatiotemporal instabilities are widespread phenomena resulting from complexity and nonlinearity. In broad-area edge-emitting semiconductor lasers, the nonlinear interactions of multiple spatial modes with the active medium can result in filamentation and spatiotemporal chaos. These instabilities degrade the laser performance and are extremely challenging to control. We demonstrate a powerful approach to suppress spatiotemporal instabilities using wave-chaotic or disordered cavities. The interference of many propagating waves with random phases in such cavities disrupts the formation of self-organized structures such as filaments, resulting in stable lasing dynamics. Our method provides a general and robust scheme to prevent the formation and growth of nonlinear instabilities for a large variety of high-power lasers.

A high performance, visible to mid-infrared photodetector based on graphene nanoribbons passivated with HfO2.

Graphene has drawn tremendous attention as a promising candidate for electronic and optoelectronic applications owing to its extraordinary properties, such as broadband absorption and ultrahigh mobility. Nevertheless, the absence of a bandgap makes graphene unfavorable for digital electronic or photonic applications. Although patterning graphene into nanostructures with the quantum confinement effect is able to open a bandgap, devices based on these graphene nanostructures generally suffer from low carrier mobility and scattering losses. In this paper, we demonstrated that encapsulation of an atomic layer deposited high-quality HfO2 film will greatly enhance the carrier mobility and decrease the scattering losses of graphene nanoribbons, because this high-k dielectric layer weakens carrier coulombic interactions. In addition, a photodetector based on HfO2 layer capped graphene nanoribbons can cover broadband wavelengths from visible to mid-infrared at room temperature, exhibiting ∼10 times higher responsivity than the one without a HfO2 layer in the visible regime and ∼8 times higher responsivity in the mid-infrared regime. The method employed here could be potentially used as a general approach to improve the performance of graphene nanostructures for electronic and optoelectronic applications.

Controllable synthesis of wurtzite Cu(2)ZnSnS(4) nanocrystals by hot-injection approach and growth mechanism studies.

Wurtzite Cu2 ZnSnS4 (WZ-CZTS) has been controllably synthesized through a hot-injection route. The crystal-growth mechanism of WZ-CZTS has been investigated by using time-dependent XRD patterns, TEM images, and absorption spectra analysis, and revealed that WZ-CZTS nucleated and grew from monoclinic Cu7 S4 nanocrystals through phase transformation of Cu7 S4 to WZ-CZTS within 5 min. The synthesis processes are dependent on precursor concentration, reaction temperature, reaction time, organic ligand, and sulfur source and have been studied in detail. It was revealed that the width of the WZ-CZTS nanocrystals was mainly controlled by the precursor concentration, which determines the diameter of Cu7 S4 . The length could be regulated by the ratio of dodecanethiol (DDT) to oleylamine (OLA) and reaction time.

Rational design of the nanowall photoelectrode for efficient solar water splitting.

The alternate dielectric component is introduced into a nanowall skeleton for the first time. As a photoelectrode, this novel model can optimize the process of photon absorption, charge separation/migration and surface reaction, resulting in superior photoelectrochemical performance.