Search

Lima\c{c}on-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 Lima\c{c}on-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 occurring on different spatiotemporal scales. The dominant process is an alternate oscillation between two output beams with a period as short as a few nanoseconds., Comment: 7 pages, 6 figures

Laser-based generation of random numbers The security of our digital networks is underpinned by the ability to generate streams of random numbers or bits. As networks expand in an ever-connected way, the challenge is to increase the generation rate of the random numbers to keep pace with demand. Kim et al. designed a chip-scale laser diode that generates random bits at an ultrahigh rate (see the Perspective by Fischer and Gauthier). By tailoring the geometry of the cavity, they were able to exploit the spatiotemporal interference of many lasing modes to generate picosecond-scale emission intensity fluctuations in space and time, producing ultrafast random bit streams in parallel. Such a device will find a wide range of applications requiring an ultrafast, compact, robust, and energy-efficient random bit generator. Science , this issue p. 948 ; see also p. 889

We employ spatio-temporal interference of many lasing modes for ultrafast, scalable random number generation. The laser diode with a specially-designed cavity produces hundreds of bitstreams at a total rate of two orders-of-magnitude faster than state-of-the-art.

We experimentally demonstrate that a slight modification of the Fabry-Perot cavity geometry has a profound impact on lasing performance. The near-planar cavity laser supports hundreds of transverse lasing modes with directional and stable output.

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 ultra-compactness. However, these criteria have been achieved only partially with conventional beam-steering devices based on waveguides, micro-electrical mechanical systems, spatial light modulators, and gratings, which would substantially limit the application of optical wireless broadcasting communication techniques. In this study, we designed and experimentally demonstrated an ultra-compact full-duplex meta-broadcasting communication system, 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 meta-device dimensions as small as 2 mm × 2 mm. In particular, the beam-steering meta-devices are mass-manufactured by a complementary metal-oxide-semiconductor processing platform, which shows their potential for large-scale commercial applications. The demonstrated meta-broadcasting 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. This article is protected by copyright. All rights reserved.

Spatiotemporal interference of numerous lasing modes in a specially designed cavity accelerates random bit generation in hundreds of spatial channels. The accumulative bit rate reaches 250 Terabit/s, two orders of magnitude higher than the state-of-the-art.

Fast and reliable random number generation (RNG) is a key requirement for, e.g., secure telecommunication or quantum simulations. This has driven the development of physical random number generators to replace deterministic, pseudo-random number generators. Semiconductor lasers exhibiting chaotic dynamics induced for example by optical feedback [1] can provide up to 1 Tbit/s, but their speed is ultimately limited by the time scales of the laser dynamics. Parallel RNG via spatial or spectral multiplexing can further increase the speed but previous demonstrations were limited by low intrinsic speed or a small number of channels.

We demonstrate ultrafast random bit generation using spatio-temporal interference of many modes in a specially-designed laser diode. Our scheme produces 127 bit streams in parallel at a total bit rate of 250 terabits per second.

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. Agency for Science, Technology and Research (A*STAR) National Research Foundation (NRF) Published version Agency for Science, Technology and Research (IAF-PP A1789a0024); National Research Foundation Singapore (NRF-CRP18-2017-02).

In order to address the non-Gaussian and dynamic features in the process, this paper combined the two- step principal component analysis (TS-PCA) and the preliminary-summation-based PCA (PS-PCA). By using summation and two-step PCA strategy, PS-TS-PCA can eliminate the influence of dynamic relationship among the process data and address the non-Gaussian feature at the same time. And hence PS-TS-PCA achieved good performance in both dynamic and non-Gaussian process monitoring. In addition, this paper also proposed new strategy to eliminate the correlation relationship between two data sample, and proved that least squares algorithm can estimate the dynamic structure in non-Gaussian process.

We present experimental and numerical studies of broad-area semiconductor lasers with chaotic ray dynamics. The emission intensity distributions at the cavity boundaries are measured and compared to ray tracing simulations and numerical calculations of the passive cavity modes. We study two different cavity geometries, a D-cavity and a stadium, both of which feature fully chaotic ray dynamics. While the far-field distributions exhibit fairly homogeneous emission in all directions, the emission intensity distributions at the cavity boundary are highly inhomogeneous, reflecting the non-uniform intensity distributions inside the cavities. The excellent agreement between experiments and simulations demonstrates that the intensity distributions of wave-chaotic semiconductor lasers are primarily determined by the cavity geometry. This is in contrast to conventional Fabry–Perot broad-area lasers for which the intensity distributions are to a large degree determined by the nonlinear interaction of the lasing modes with the semiconductor gain medium. Ministry of Education (MOE) National Research Foundation (NRF) Published version The authors thank A D Stone, M Constantin, O Hess and T Harayama for fruitful discussion. The work at Yale is supported partly by the Office of Naval Research (ONR) with MURI Grant N00014-13-1-0649, and the Air Force Office of Scientific Research (AFOSR) under Grant FA9550-16-1-0416. YZ and QJW acknowledge support from the Ministry of Education, Singapore (Grants MOE2016-T2-1-128 and MOE2016-T2-2-159) and the National Research Foundation, Competitive Research Program (NRF-CRP18-2017-02).

We present a cavity design for broad-area semiconductor lasers that stabilizes lasing dynamics and produces directional emission. By tuning the cavity geometry, we control nonlinear interactions of the lasing modes with the gain medium.

A terahertz (THz) quantum cascade laser (QCL) is an electrically pumped semiconductor laser based on the inter-subband electron transitions in a multiple-quantum-well heterostructure. Comparing with many other THz wave generation methods, THz QCL is mostly developed with compact fingerprint, high power, and high efficiency in the demanding frequency range of 0.8–5.4 THz with the aid of advanced electronic engineering technology of the active materials. This triggers various important applications including nonlinear optics, astronomy, imaging, sensing, and spectroscopy. However, the applications of THz QCL require good output characteristics in terms of the emission spectrum, beam quality, power efficiency, and polarization control, which are beyond the reach of the electronic engineering techniques. The main focus here is placed on photonic engineering of the THz QCLs with attention to the significant improvement of THz QCL output characteristics. Various photonic solutions to manipulate the laser output are thoroughly reviewed. Some innovative photonic designs with impressive achievements are highlighted. Nonconventional cavities with exotic physics and special functionalities are also discussed in the end, which may be exploited for potential applications in the future. National Research Foundation (NRF) Accepted version Funding support is acknowledged from the Singapore Ministry of Education Tier 2 Program (MOE 2016-T2-1-128 and MOE2016-T2-2-159); Singapore National Research Foundation Competitive Research Program (NRF-CRP18-2017-02).

Random lasers employing multiple scattering and interference processes in highly disordered media have been studied for several decades. However, it remains a challenge to achieve a broadband multimode random laser with high scattering efficiency, particularly at long wavelengths. Here, we develop a new class of strongly multimode random lasers in the terahertz (THz) frequency range in which optical feedback is provided by multiple scattering from metal pillars embedded in a quantum cascade (QC) gain medium. Compared with the dielectric pillars or air hole approaches used in previous random lasers, metal pillars provide high scattering efficiency over a broader range of frequencies and with low ohmic losses. Complex emission spectra are observed with over 25 emission peaks across a 0.4 THz frequency range, limited primarily by the gain bandwidth of the QC wafer employed. The experimental results are corroborated by numerical simulations that show the lasing modes are strongly localized.

The ability to control the polarization state of emission from semiconductor lasers is essential for many applications in spectroscopy, imaging, and communications, inter alia, with monolithic integration approaches being extremely beneficial. Although manipulating the output polarization of radiation from a laser can be achieved through a number of approaches, obtaining continuous dynamic control, e.g., from linear to circular, remains extremely challenging. In this paper, we demonstrate that the polarization of terahertz (THz) frequency radiation can be continuously tuned electronically from linear to circular polarization by monolithically integrating in-plane metasurfaces with two phase-locked semiconductor-based THz quantum cascade lasers. Moreover, the metasurfaces—metal antenna arrays in this case—also act as efficient beam collimators, yielding a collimated beam divergence of ∼10° × 10°. Our results, however, have broad applicability to a wide range of semiconductor lasers operating from the visible to THz regions of the electromagnetic spectrum.

Random lasers are a special class of laser in which light is confined through multiple scattering and interference process in a disordered medium, without a traditional optical cavity. They have been widely studied to investigate fundamental phenomena such as Anderson localization, and for applications such as speckle-free imaging, benefitting from multiple lasing modes. However, achieving controlled localized multi-mode random lasing at long wavelengths, such as in the terahertz (THz) frequency regime, remains a challenge. Here, we study devices consisting of randomly-distributed pillars fabricated from a quantum cascade gain medium, and show that such structures can achieve transversemagnetic polarized (TM) multi-mode random lasing, with strongly localized modes at THz frequencies. The weak short-range order induced by the pillar distribution is sufficient to ensure high quality-factor modes that have a large overlap with the active material. Furthermore, the emission spectrum can be easily tuned by tailoring the scatterer size and filling fraction. These “designer” random lasers, realized using standard photolithography 2 techniques, provide a promising platform for investigating disordered photonics with predesigned randomness in the THz frequency range, and may have potential applications such as speckle-free imaging.

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. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

Broad-area semiconductor lasers used for high-power applications such as material processing and pump sources feature significant spatio-temporal instabilities [1]. Most attempts to stabilize them aim at reducing the number of transverse lasing modes via injection or feedback but fail at high powers. We demonstrate a different approach for stabilization via complex interference in asymmetric cavities with chaotic ray dynamics. This approach maintains multi-mode operation and is hence compatible with high-power operation [2].

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 lasing5,6,7,8 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 regime5,6,7,8. Here we demonstrate an electrically pumped terahertz quantum cascade laser based on topologically protected valley edge states9,10,11. 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.

We design and fabricate an on-chip laser source that produces a directional beam with low spatial coherence. The lasing modes are based on the axial orbit in a stable cavity and have good directionality. To reduce the spatial coherence of emission, the number of transverse lasing modes is maximized by fine-tuning the cavity geometry. In a cavity with the size of hundreds of micrometers, 1000 transverse modes lase simultaneously and independently, reducing the speckle contrast to 0.03. Decoherence is reached in a few nanoseconds as a result of frequency detuning of lasing modes. Such rapid decoherence will facilitate applications in ultrafast speckle-free full-field imaging. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

We design and fabricate an electrically pumped semiconductor laser producing a directional beam with low spatial coherence. The device features decoherence time of a few nanoseconds, which will enable ultrafast speckle-free imaging and sensing.

We investigate experimentally and theoretically the lasing behavior of dielectric microcavity lasers with chaotic ray dynamics. Experiments show multimode lasing for both D-shaped and stadium-shaped wave-chaotic cavities. Theoretical calculations also find multimode lasing for different shapes, sizes and refractive indices. While there are quantitative differences between the theoretical lasing spectra of the stadium and D-cavity, due to the presence of scarred modes with anomalously high quality factors, these differences decrease as the system size increases, and are also substantially reduced when the effects of surface roughness are taken into account. Lasing spectra calculations are based on Steady-State Ab Initio Laser Theory, and indicate that gain competition is not sufficient to result in single-mode lasing in these systems., 15 pages, 9 figures

In this paper, a new multiperiod mean semivariance portfolio selection with the transaction costs, borrowing constraints, threshold constraints and cardinality constraints is proposed. In the model, the return and risk of assets are characterized by mean value and semivariance, respectively. Because the semivariance operator is not separable, the optimal solution of the model is not time-consistent. The time-consistent strategy for this model can be obtained by using game approach. The time-consistent strategy, which is a mix integer dynamic optimization problem with path dependence, is approximately turned into a dynamic programming problem by approximate dynamic programming method. A novel discrete approximate iteration method is designed to obtain the optimal time-consistent strategy, and is proved linearly convergent. Finally, the comparison analysis of trade-off parameters is given to illustrate the idea of our model and the effectiveness of the designed algorithm.

Quantum cascade laser (QCL) is a semiconductor based laser in a superlattice structure based on intersubband transitions. Although QCLs have been achieved with high performance such as Watt-level emission, it is always highly desired to further improve the device performance with multiple functions , for example, achieving high beam collimation, arbitrary polarization control, and high speed modulation. It is also desired that those performance could be achieved through an integrated approach for miniaturalization, easy alignment and reducing cost. In this presentation, we will use Terahertz (THz) QCL as a demonstration example to obtain high collimated THz QCLs through plasmonic collimation designs with a record beam divergence, electrically tunable THz polarizations by designing integrated THz metasurfaces on a hybrid dielectric-plasmonic waveguides, and broadband graphene-based integrated THz modulators with a fast modulating speed.

Taming laser instabilities Broad-area and high-power lasers often suffer from instabilities owing to the chaotic interference of multiple modes within the cavity. Such instabilities can ultimately limit the operation of the laser or damage the cavity. The usual approach to minimizing such instabilities is to limit the number of modes in the cavity. Bittner et al. designed a chaotic cavity that disrupts the formation of self-organized structures that lead to instabilities (see the Perspective by Yang). This approach of fighting chaos with chaos by using the boundary condition of the cavity shape may provide a robust route to stabilizing lasers at high operating powers. Science , this issue p. 1225 ; see also p. 1201

Scattering, which is created by dust, clouds, white paints or imperfections of artificial material such as photonic crystal, is normally undesirable in the field of photonics. Unequivocally, a deep understanding of disorder can help to overcome the issues arising from scattering. On the other hand, it is also possible to utilize the disorder to create useful photonic devices. A most well-known example is the random laser, in which lasing action is sustained by multiple scattering of light within the disordered media. Disordered structures are also proposed for the applications including wavelength filters, optical switches, sensors and cavity quantum electrodynamics. Most of the research work on disordered structures has been focused on short wavelength range. Several years ago, this tide spreads to mid-infrared range with the experimental demonstration and theoretical optimization of mid-infrared random laser at ~10 μm. However, in the technically important terahertz (THz) frequency range, the disordered photonics are least explored. This thesis investigates disordered photonic systems based on the double-metal (DM) waveguide of THz quantum cascade laser (QCL) and explores the emission properties with the assistance of gain. As the first demonstration, I design and fabricate one-dimensional disordered grating structures on top of the DM waveguide of the THz QCLs. Disorder is introduced into the waveguide by dislocating the position of each aperture from a periodic configuration. Numerical calculation indicates that the introduced disorder creates multiple microcavities with small optical losses near the bandgap frequency window of the underlying long-range order. With electrical pumping, 8 emission peaks at around 3.2 THz are observed, which are located near the bandgap region of the periodic counterparts. Simulation work and experimental measurements provide strong evidence for spatial localization of light. Instead of utilizing long-range order for creating microcavities, I also investigate two-dimensional disordered structures consisting of randomly-distributed pillar scatterers fabricated from a quantum cascade gain medium, in which long-range order does not exist. Due to the strong scattering efficiency of a high-index dielectric pillar for transverse-magnetic (TM) polarized wave, it is shown that such structures can achieve multimode lasing, with strongly localized modes at THz frequencies. This is a typical feature of random lasers. The weak short-range order induced by the pillar distribution is sufficient to ensure high quality-factor modes that have a large overlap with the active material. It is also found that the emission spectrum can be easily tuned by tailoring the size and filling fraction of the scatterers. While the dielectric pillar scatterer is beneficial for better performance of multimode lasing (lower threshold, more lasing modes etc.) due to high scattering efficiency, it also limits the lasing to the Mie resonance frequency range, making the broadband random lasing applications difficult. The non-resonant scatterers feature a weaker scattering strength nonetheless a relatively frequency-insensitive scattering efficiency. Inspired by this, I further develop metallic pillars, a class of non-resonant scatterers with high scattering efficiency in the THz frequency range, enabling possible a broadband random laser. It is demonstrated that such random lasers, with metallic pillars embedded in THz QCL gain medium, outperform their dielectric-counterparts in terms of lasing spectral range, the number of lasing mode and laser threshold. Doctor of Philosophy (EEE)

The suppression of spatio-temporal instabilities of broad-area semiconductor lasers is demonstrated for microcavities with chaotic ray dynamics. We attribute the stabilization to the disruption of coherent instabilities by complex wave interference.

Multimode terahertz (THz) laser emission around 3.2 THz in one-dimensional (1-D) disordered systems with 20% disorder. Simulation and experimental work provide evidence for spatial localization of light of defect modes within the bandgap frequency window.

Random lasers were demonstrated in the mid-infrared and terahertz regimes in quantum cascade semiconductor laser platforms with electrical pumping. Air-holes, dielectric pillars and metallic pillars were employed as efficient scatterers to achieve multi-mode lasing.

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.

Multi-mode random terahertz lasing with strongly localized modes has been realized for the first time, by fabricating two dimensional randomly distributed quantum cascade pillars with a small short-range order.

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.

We demonstrate terahertz (THz) frequency laser emission around 3.2 THz from localized modes in one-dimensional disordered grating systems. The disordered structures are patterned on top of the double-metal waveguide of a THz quantum cascade laser. Multiple emission peaks are observed within a frequency range corresponding to the bandgap of a periodic counterpart with no disorder, indicating the presence of mode localization aided by Bragg scattering. Simulations and experimental measurements provide strong evidence for the spatial localization of the THz laser modes. National Research Foundation (NRF) Published version Ministry of Education—Singapore (MOE) (MOE 2016-T2-1-128); Agency for Science, Technology and Research (A*STAR) (1426500050); National Research Foundation Singapore (NRF) (NRF-CRP18-2017-02); Engineering and Physical Sciences Research Council (EPSRC) (EP/P021859/1); Royal Society and Wolfson Foundation.

We report our progress in the development of broadly tunable single-mode slot waveguide quantum cascade lasers based on a continuum-to-continuum active region design. The electroluminescence spectrum of the continuum-to-continuum active region design has a full width at half maximum of 440 cm−1 at center wavelength ~10 μm at room temperature (300 K). Devices using the optimized slot waveguide structure and the continuum-to-continuum design can be tuned continuously with a lasing emission over 42 cm−1, from 9.74 to 10.16 μm, at room temperature by using only current tuning scheme, together with a side mode suppression ratio of above 15 dB within the whole tuning range. Published version

Broadly tunable single-mode quantum cascade laser at ~10 μm with a monolithically integrated two-section slot structure is presented. The device shows ~80 cm−1 tuning range and ~20 dB side mode suppression ratio by adjusting the currents and temperature.

Electrically-pumped disordered photonic-crystal lasers in the mid-infrared regime at λ~ 10 μm have been demonstrated for the first time based on quantum cascade lasers. Both the extended and highly-localized lasing modes are generated by adjusting the degree of disorder.