We report complete spatial shaping (both phase and amplitude) of the second‐harmonic beam generated in a nonlinear photonic crystal. Using a collinear second‐order process in a nonlinear computer generated hologram imprinted on the crystal, the desired beam is generated on‐axis and in the near field. This enables compact and efficient one‐dimensional beam shaping in comparison to previously demonstrated off‐axis Fourier holograms. We experimentally demonstrate the second‐harmonic generation of high‐order Hermite–Gauss, top hats and arbitrary skyline‐shaped beams.
We uncover that the breaking point of the ‐symmetry in optical waveguide arrays has a dramatic impact on light localization induced by the off‐diagonal disorder. Specifically, when the gain/loss control parameter approaches a critical value at which ‐symmetry breaking occurs, a fast growth of the coupling between neighboring waveguides causes diffraction to dominate to an extent that light localization is strongly suppressed and the statistically averaged width of the output pattern substantially increases. Beyond the symmetry‐breaking point localization is gradually restored, although in this regime the power of localized modes grows upon propagation. The strength of localization monotonically increases with disorder at both broken and unbroken ‐symmetry. Our findings are supported by statistical analysis of parameters of stationary eigenmodes of disordered‐symmetric waveguide arrays and by analysis of dynamical evolution of single‐site excitations in such structures.
We report on the realisation of ultra‐small‐mode‐volume tunable dye lasers based on hemispherical open microcavities. The cavity mode volume is of the order of cubic micrometers, such that self‐diffusion of the dye molecules allows continuous wave operation over several minutes without the need for driven circulation. Such micro lasers could be integrated into lab‐on‐a‐chip devices. A rate‐equation model that incorporates the diffusion mechanism is used to predict the effect of the microcavity parameters on the lasing threshold.
Two hundred years after Malus' discovery of optical anisotropy, the study of polarization‐driven optical effects is as active as ever, generating interest in new phenomena and potential applications. However, in ultrafast optics, the influence of polarization is frequently overlooked being considered as either detrimental or negligible. Here we demonstrate that spatio‐temporal couplings, which are inherent for ultrafast laser systems with chirped‐pulse amplification, accumulate in multi‐pulse irradiation and lead to a strongly anisotropic light‐matter interaction. Our results identify angular dispersion in the focus as the origin for the polarization dependence in modification, yielding an increase in modification strength. With tight focusing (NA ≥ ∼0.4), this non‐paraxial effect leads to a manifestation of spatio‐temporal couplings in photo‐induced modification. We devise a practical way to control the polarization dependence and exploit it as a new degree of freedom in tailoring laser‐induced modification in transparent material. A near‐focus, non‐paraxial field structure analysis of an optical beam provides insight on the origin of the polarization dependent modification. However, single pulse non‐paraxial corrected calculations are not sufficient to explain the phenomena confirming the experimental observations and exemplifying the need for multi‐pulse analysis.
About twenty years ago, in the autumn of 1996, the first white light‐emitting diodes (LEDs) were offered for sale. These then‐new devices ushered in a new era in lighting by displacing lower‐efficiency conventional light sources including Edison's venerable incandescent lamp as well as the Hg‐discharge‐based fluorescent lamp. We review the history of the conception, improvement, and commercialization of the white LED. Early models of white LEDs already exceeded the efficiency of low‐wattage incandescent lamps, and extraordinary progress has been made during the last 20 years. The review also includes a discussion of advances in blue LED chips, device architecture, light extraction, and phosphors. Finally, we offer a brief outlook on opportunities provided by smart LED technology.
Surface plasmon polaritons (SPPs) have sparked enormous interest on nanophotonics beyond the diffraction limit for their remarkable capabilities of subwavelength confinements and strong enhancements. Due to the inherent polarization sensitivity of the SPPs [transverse‐magnetic (TM) polarization], it is a great challenge to couple the s‐polarized free‐space light to the SPPs. Here, an ultrasmall defect aperture (<λ2/2) is designed to directionally couple both the p‐ and s‐polarized incident beams to the single SPP mode in a broad bandwidth, which is guided by a subwavelength plasmonic waveguide. Simulations show that hot spots emerge at the sharp corners of the defect aperture when the incident beams illuminate it from the back side. The strong radiative fields from the hot spots are directionally coupled to the SPP mode because of the symmetry breaking of the defect aperture. By adjusting the structural parameters, both the unidirectional and bidirectional SPP coupling from the two orthogonal linear‐polarization incident beams are experimentally demonstrated. The polarization‐free coupling of the SPPs is of importance in circuits for fully optical processing of information with a deep‐subwavelength footprint.
We investigate the fractional Schrödinger equation with a periodic ‐symmetric potential. In the inverse space, the problem transfers into a first‐order nonlocal frequency‐delay partial differential equation. We show that at a critical point, the band structure becomes linear and symmetric in the one‐dimensional case, which results in a nondiffracting propagation and conical diffraction of input beams. If only one channel in the periodic potential is excited, adjacent channels become uniformly excited along the propagation direction, which can be used to generate laser beams of high power and narrow width. In the two‐dimensional case, there appears conical diffraction that depends on the competition between the fractional Laplacian operator and the ‐symmetric potential. This investigation may find applications in novel on‐chip optical devices.
This work proves the feasibility of a novel concept of differential absorption lidar based on the Scheimpflug principle. The range‐resolved atmospheric backscattering signal of a laser beam is retrieved by employing a tilted linear sensor with a Newtonian telescope, satisfying the Scheimpflug condition. Infinite focus depth is achieved despite employing a large optical aperture. The concept is demonstrated by measuring the range‐resolved atmospheric oxygen concentration with a tunable continuous‐wave narrow‐band laser diode emitting around 761 nm over a path of one kilometer during night time. Laser power requirements for daytime operation are also investigated and validated with single‐band atmospheric aerosol measurements by employing a broad‐band 3.2‐W laser diode. The results presented in this work show the potential of employing the continuous‐wave differential absorption lidar (CW‐DIAL) technique for remote profiling of atmospheric gases in daytime if high‐power narrow‐band continuous‐wave light sources were to be employed.
All‐optical signal processing on nonlinear photonic chips is a burgeoning field. These processes include light generation, optical regeneration and pulse metrology. Nonlinear photonic chips offer the benefits of small footprints, significantly larger nonlinear parameters and flexibility in generating dispersion. The nonlinear compression of optical pulses relies on a delicate balance of a material's nonlinearity and optical dispersion. Recent developments in dispersion engineering on a chip are proving to be key enablers of high‐efficiency integrated optical pulse compression. We review the recent advances made in optical pulse compression based on nonlinear photonic chips, as well as the future outlook and challenges that remain to be solved.
Due to the broad scattering spectral profiles, localized surface plasmon resonances (LSPRs) of Pd nanoparticles have low resolution and limited sensitivity for hydrogen detection. In this work, we use a simple light‐irradiation method to demonstrate that free‐space light can be efficiently coupled into and from the microfiber whispering‐gallery modes (WGMs) by the Pd nanoantennas. The nanoantenna–microfiber cavity system provides strong intermodal coupling between LSPRs and WGMs, and induces significant modulation of the scattering spectra. A measured full width at half‐maximum of 3.2 nm at 622.7 nm is obtained, which is the narrowest in Pd nanoparticle‐based LSPR structures reported up to now. The ultranarrow resonances offer enhanced sensitivity to hydrogen gas detection with a figure of merit reaching ∼2.22. Other advantages of the Pd nanoantenna–microfiber cavity system including independence of precise alignment of excitation light, large tunability of the resonant wavelengths, easy and low‐cost fabrication of the system, have also been demonstrated.
The dielectric metasurface hologram promises higher efficiencies due to lower absorption than its plasmonic counterpart. However, it has only been used, up to now, for controlling linear‐polarization photons to form single‐plane holographic images in the near‐infrared region. Here, we report a transmission‐type metahologram achieving images in three colors, free from high‐order diffraction and twin‐image issues, with 8‐level modulation of geometric phase by controlling photon spin via precisely patterned Si nanostructures with varying orientations. The resulting real and virtual holographic images with spin dependence of incident photons natively enable the spin degeneracy removal of light, leading to a metahologram‐enabled spin Hall effect of light. Low‐absorption dielectrics also enable us to create holograms for short‐wavelength light down to 480 nm, thus spanning the three primary colors. It possesses the potential for compact color‐display chips using mature semiconductor processes, and holds significant advantages over previous metaholograms operating at longer wavelengths.
The integration of microactuators within a silicon photonic chip gave rise to the field of optical micro‐electro‐mechanical systems (MEMS) that was originally driven by the telecommunication market. Following the latter's bubble collapse in the beginning of the third millennium, new directions of research with considerable momentum appeared focusing on the realization and applications of miniaturized instrumentation in biology, chemistry, physics and materials science. At the heart of these applications light interferometry is a key optical phenomenon, in which miniaturized scanning interferometers are the manipulating optical devices. Monolithic free‐space optical interferometers realized on a silicon chip take advantage of the recent progress in the microfabrication technology that is enabling accurate control of the etching depth, the aspect ratio, the verticality and the curvature of the etched surfaces. The fabrication technology, the library of micro‐optical and mechanical components, the realized architectures and their characterization are described in detail in this review, followed by a discussion of the foreseen challenges.
The terahertz (THz) radiation from InGaN/GaN dot‐in‐a‐wire nanostructures has been investigated. A submicrowatt THz signal is generated with just ten vertically stacked InGaN quantum dots (QDs) in each GaN nanowire. Based on the experimental results and analysis, a single quantum wire is expected to generate an output power as high as 10 pW, corresponding to 1 pW per dot. These structures are among the most efficient three‐dimensional quantum‐confined nanostructures for the THz emission. By applying a reverse bias along the wires in a light‐emitting device (LED) consisting of such nanostructures, the THz output power is increased more than fourfold. Based on THz and photoluminescence (PL) experiments, the mechanism for the THz emission is attributed to dipole radiation induced by internal electric fields and enhanced by external fields.
The ability to control the optical field in the vicinity of an individual nano‐object is an obvious stepping‐stone in the tailoring of light‐matter interactions at the nanoscale. Earlier reports on tailoring light fields in the vicinity of a nano‐object have been restricted by their dependence on cumbersome optical or fabrication techniques, have relied mostly on in‐plane electric field polarizations, and have been demonstrated only for bulk materials and structures with strong in‐plane anisotropies. In addition, traditional methods for manipulating the longitudinal electric fields are significantly hindered by the lack of appropriate probes that can be used to unambiguously measure or calibrate the light coupling efficiency to nano‐objects. Here, we demonstrate such a possibility for the specific case of optical second‐harmonic generation (SHG). Our technique relies on spatial phase‐shaping of a high‐order laser beam to tailor the longitudinal fields at the beam focus and allows SHG from an individual and well‐defined vertically‐aligned GaAs nanowire to be manipulated on demand. Our technique is applicable to tailoring the efficiency of nonlinear emission on the nanoscale and to arbitrary polarization control at the beam focus in general.
The future generation of modern illumination should not only be cheap and highly efficient, but also demonstrate high quality of light, light which allows better color differentiation and fidelity. Here we are presenting a novel approach to create a white solid‐state light source providing ultimate color rendition necessary for a number of applications. The proposed semi‐hybrid device combines a monolithic blue‐cyan light emitting diode (MBC LED) with a green‐red phosphor mixture. It has shown a superior color rendering index (CRI), 98.6, at correlated color temperature of around 3400 K. The MBC LED epi‐structure did not suffer from the efficiency reduction typical for monolithic multi‐color emitters and was implemented in the two most popular chip designs: “epi‐up” and “flip‐chip”. Redistribution of the blue and cyan band amplitudes in the white‐light emission spectrum, using the operating current, is found to be an effective tool for fine tuning the color characteristics.
Open‐access microcavities are emerging as a new approach to confine and engineer light at mode volumes down to the λ3 regime. They offer direct access to a highly confined electromagnetic field while maintaining tunability of the system and flexibility for coupling to a range of matter systems. This article presents a study of coupled cavities, for which the substrates are produced using Focused Ion Beam milling. Based on experimental and theoretical investigation the engineering of the coupling between two microcavities with radius of curvature of 6 m is demonstrated. Details are provided by studying the evolution of spectral, spatial and polarisation properties through the transition from isolated to coupled cavities. Normal mode splittings up to 20 meV are observed for total mode volumes around . This work is of importance for future development of lab‐on‐a‐chip sensors and photonic open‐access devices ranging from polariton systems to quantum simulators.
One of the challenges of the modern photonics is to develop all‐optical devices enabling increased speed and energy efficiency for transmitting and processing information on an optical chip. It is believed that the recently suggested Parity‐Time (PT) symmetric photonic systems with alternating regions of gain and loss can bring novel functionalities. In such systems, losses are as important as gain and, depending on the structural parameters, gain compensates losses. Generally, PT systems demonstrate nontrivial non‐conservative wave interactions and phase transitions, which can be employed for signal filtering and switching, opening new prospects for active control of light. In this review, we discuss a broad range of problems involving nonlinear PT‐symmetric photonic systems with an intensity‐dependent refractive index. Nonlinearity in such PT symmetric systems provides a basis for many effects such as the formation of localized modes, nonlinearly‐induced PT‐symmetry breaking, and all‐optical switching. Nonlinear PT‐symmetric systems can serve as powerful building blocks for the development of novel photonic devices targeting an active light control.
This article presents a novel III‐V on silicon laser. This work exploits the phenomenon that a passive silicon cavity, side‐coupled to a III‐V waveguide, will provide high and narrow‐band reflectivity into the III‐V waveguide: the resonant mirror. This results in an electrically pumped laser with a threshold current of 4 mA and a side‐mode suppression ratio up to 48 dB.
Surface‐plasmon‐polariton waves are two‐dimensional electromagnetic surface waves that propagate at the interface between a metal and a dielectric. These waves exhibit unusual and attractive properties, such as high spatial confinement and enhancement of the optical field, and are widely used in a variety of applications, such as sensing and subwavelength optics. The ability to precisely control the spatial and spectral properties of the surface‐plasmon wave is required in order to support the growing interest in both research and applications of plasmonic waves, and to bring it to the next level. Here, we review the challenges and methods for shaping the wavefront and spectrum of plasmonic waves. In particular, we present the recent advances in plasmonic spatial and spectral shaping, which are based on the realization of plasmonic holograms for the optical nearfield.
Since the surface plasmon polariton (SPP) has received a great deal of attention because of its capability of guiding light within the subwavelength scale, finding methods for arbitrary SPP field generation has been a significant issue in the area of integrated optics. To achieve such a goal, it will be necessary to generate a plasmonic complex field. In this paper, we propose a novel method for generating a plasmonic complex field propagating with arbitrary curvatures by using double‐lined distributed nanoslits. As a unit cell, two facing nanoslits are used for tuning both the amplitude and the phase of excited SPPs as a function of their tilted angles. For verification of the proposed design rule, the authors experimentally demonstrate some plasmonic caustic curves and Airy plasmons.
