A diode‐pumped Yb:YAG MOPA‐System for the unprecedented generation of transform limited pulses with variable pulse duration in the range between 10 ps and 100 ps is presented. First applications relying on unique pulse parameters as modulation free spectrum, tunability and coherence length, namely the direct laser interference patterning (DLIP) and laser cooling of stored relativistic ion beams are highlighted. Pulses are generated by a mode‐locked fs‐oscillator while the spectral bandwidth is narrowed in the subsequent regenerative amplifier by an intra‐cavity grating monochromator. Two alternative booster amplifiers were added to increase the pulse energy to 100 μJ and 10 mJ, respectively.
Wide‐angle, polarization‐independent structural reflective colors from both directions based on a one‐dimensional photonic crystal are demonstrated. Our device produces a distinct and saturated color with high angular tolerant performance up to ±70° for any polarization state of an incident light wave, which is highly desirable for a broad range of research areas. Moreover, the purity of the color and luminous intensity of the proposed device are improved as compared to conventional colorant‐based color filters and colloidal glasses. The present approach may have the potential to replace existing color filters and pigments and pave the way for various applications, including color displays and image sensor technologies.
An ultrathin Huygens metasurface is proposed to manipulate orthogonally polarized transmitted waves independently. The Huygens metasurface consists of two layers of dielectric substrates and three layers of artificial metallic structures with ultrathin electric thickness. In physics, two orthogonal electric dipoles and two orthogonal magnetic dipoles are supported by each unit cell of the metasurface, which enable the complete control of phase distributions in both vertical and horizontal directions. Based on this feature, a polarization beam splitter with large splitting angle is designed and fabricated to demonstrate the capability and flexibility of the proposed Huygens metasurface. The numerical simulations and measurement results have a good match, indicating the good performance on independent controls of orthogonally polarized transmitted waves.
Polymers such as benzocyclobutene are commonly used as embedding materials for semiconductor nanostructures. During the curing process of the polymer up to 250 °C, a significant impact of strain can be induced on the embedded semiconductor material due to different thermal expansion coefficients. This strain has been revealed by X‐ray diffraction in free‐standing GaAs nanowires grown on a silicon substrate, embedded in a polymer matrix. It will be shown that this strain is released during the X‐ray irradiation if additionally an external static electric field is applied.
Leaky modes are below‐cutoff waveguide modes that lose part of their energy to the continuum of radiation modes during propagation. In photonic nanowire lasers, leaky modes have to compete with almost lossless above‐cutoff modes and are therefore usually prevented from crossing the lasing threshold. The situation is drastically different in plasmonic nanowire systems where the above‐cutoff plasmonic modes are very lossy because of their strong confinement to the metal surface. Due to gain guiding, the threshold gain of the hybrid electric leaky mode does not increase strongly with reduced wire diameter and stays below that of all other modes, making it possible to observe leaky‐mode lasing. Plasmonic ZnO nanowire lasers operating in the gain‐guided regime could be used as coherent sources of surface plasmon polaritons at the nanoscale or as surface plasmon emitting diodes with an emission angle that depends on the nanowire diameter and the color of the surface plasmon polariton.
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.
Optical waveguide theory is an established part of optical physics. Yet only recently have fundamental phenomena such as spatial eigenmodes and principal modes been demonstrated experimentally. This work leverages recently developed techniques enabling detailed spatiotemporal characterisation of multimode fibre to provide new insights into the fundamentals of fibre propagation. This paper presents detailed analysis of all 420 of a fibre's principal modes and spatial eigenmodes and compares the similarity and differences between these two phenomena. It was found that even over very short lengths, the principal modes can not only significantly suppress modal dispersion but are also a more physically meaningful basis than spatial eigenmodes.
Microresonator‐based Kerr frequency comb (microcomb) generation can potentially revolutionize a variety of applications ranging from telecommunications to optical frequency synthesis. However, phase‐locked microcombs have generally had low conversion efficiency limited to a few percent. Here we report experimental results that achieve conversion efficiency ( on‐chip comb power excluding the pump) in the fiber telecommunication band with broadband mode‐locked dark‐pulse combs. We present a general analysis on the efficiency which is applicable to any phase‐locked microcomb state. The effective coupling condition for the pump as well as the duty cycle of localized time‐domain structures play a key role in determining the conversion efficiency. Our observation of high efficiency comb states is relevant for applications such as optical communications which require high power per comb line.
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.
A major aim of researchers working in the field of optics and photonics is to mold the flow of light in optical structures and devices. In the regime of ballistic light propagation, transformation optics has given a certain boost, for which optical invisibility cloaking devices are striking examples. Our capability to mold the flow of light in the regime of diffuse light propagation in light‐scattering media has fallen behind—while diffuse light from clouds, white wallpaper, computer monitors, and light‐emitting diodes is literally all around us every day. In this review, we summarize progress in steering the flow of diffuse light in turbid media which was triggered by the mathematical analogy between electrostatics, magnetostatics, stationary heat conduction, and stationary light diffusion. We give an extensive tutorial introduction to the mathematics of the diffusion equation for light and its solutions, present an overview on the current experimental state‐of‐the‐art of simple core–shell invisibility cloaking, and compare these experiments with diffusion theory as well as with more advanced modelling based on Monte Carlo simulations. The latter approach enables spanning the bridge from diffusive to ballistic light propagation.
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.
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.
In this work, we report optomechanical coupling, resolved sidebands and phonon lasing in a solid‐core microbottle resonator fabricated on a single mode optical fiber. Mechanical modes with quality factors (Qm) as high as 1.57 × 104 and 1.45 × 104 were observed, respectively, at the mechanical frequencies and . The maximum Hz is close to the theoretical lower bound of 6 × 1012 Hz needed to overcome thermal decoherence for resolved‐sideband cooling of mechanical motion at room temperature, suggesting microbottle resonators as a possible platform for this endeavor. In addition to optomechanical effects, scatter‐induced mode splitting and ringing phenomena, which are typical for high‐quality optical resonances, were also observed in a microbottle resonator.
Periodic structures with a sub‐wavelength pitch have been known since Hertz conducted his first experiments on the polarization of electromagnetic waves. While the use of these structures in waveguide optics was proposed in the 1990s, it has been with the more recent developments of silicon photonics and high‐precision lithography techniques that sub‐wavelength structures have found widespread application in the field of photonics. This review first provides an introduction to the physics of sub‐wavelength structures. An overview of the applications of sub‐wavelength structures is then given including: anti‐reflective coatings, polarization rotators, high‐efficiency fiber–chip couplers, spectrometers, high‐reflectivity mirrors, athermal waveguides, multimode interference couplers, and dispersion engineered, ultra‐broadband waveguide couplers among others. Particular attention is paid to providing insight into the design strategies for these devices. The concluding remarks provide an outlook on the future development of sub‐wavelength structures and their impact in photonics.
Transport of subwavelength electromagnetic (EM) energy has been achieved through near‐field coupling of highly confined surface EM modes supported by plasmonic nanoparticles, in a configuration usually on a two‐dimensional (2D) substrate. Vertical transport of similar modes along the third dimension, on the other hand, can bring more flexibility in designs of functional photonic devices, but this phenomenon has not been observed in reality. In this paper, designer (or spoof) surface plasmon resonators (‘plasmonic meta‐atoms’) are stacked in the direction vertical to their individual planes in demonstrating vertical transport of subwavelength localized surface EM modes. The dispersion relation of this vertical transport is determined from coupled‐mode theory and is verified with a near‐field transmission spectrum and field mapping with a microwave near‐field scanning stage. This work extends the near‐field coupled resonator optical waveguide (CROW) theory into the vertical direction, and may find applications in novel three‐dimensional slow‐light structures, filters, and photonic circuits.
A novel approach to facilitate excitation and readout processes of isolated negatively charged nitrogen‐vacancy (NV) centers is proposed. The approach is based on the concept of all‐dielectric nanoantennas. It is shown that the all‐dielectric nanoantenna can significantly enhance both the emission rate and emission extraction efficiency of a photoluminescence signal from a single NV center in a diamond nanoparticle on a dielectric substrate. The proposed approach provides high directivity, large Purcell factor, and efficient beam steering, thus allowing an efficient far‐field initialization and readout of several NV centers separated by subwavelength distances.
Entangled photon pairs must often be spatially separated for their subsequent manipulation in integrated quantum circuits. Separation that is both deterministic and universal can in principle be achieved through anti‐coalescent two‐photon quantum interference. However, such interference‐facilitated pair separation (IFPS) has not been extensively studied in the integrated setting, which has important implications on performance. This work provides a detailed review of IFPS and examines how integrated device dependencies such as dispersion impact separation fidelity and interference visibility. The analysis applies equally to both on‐chip and in‐fiber implementations. When coupler dispersion is present, the separation performance can depend on photon bandwidth, spectral entanglement and the dispersion. By design, reduction in the separation fidelity due to loss of non‐classical interference can be perfectly compensated for by classical wavelength demultiplexing effects. This work informs the design of devices for universal photon pair separation of states with tunable arbitrary properties.
Optical whispering‐gallery mode (WGM) microcavities featuring ultrahigh Q factors and small mode volumes enhance significantly the interaction between light and matter, becoming an excellent platform for achieving ultralow‐threshold microlasers. However, the emission of traditional WGMs is isotropic due to the rotational symmetry of cavity geometries, which hinders the potential photonics applications. In this review, the progress in WGM microcavities towards unidirectional laser emission is summarized. When a subwavelength scatterer is placed on the boundary of the microcavity, the unidirectional emission occurs due to the collimation effect of the microcavity‐enhanced scattering field. Furthermore, microcavities deformed from the circular shapes can not only produce the chaos‐assisted unidirectional emission, but also maintain high Q factors by special design and fabrication processes. Finally, gratings along the circumference of the WGM microdisk or microring can scatter the WGMs in the vertical direction. The review also lists several important applications of these types of microcavities, such as wide‐band laser illumination source, free‐space coupling, evanescent‐field enhancement, optical energy storage, and sensing.
The progress on multi‐wavelength quantum cascade laser arrays in the mid‐infrared is reviewed, which are a powerful, robust and versatile source for next‐generation spectroscopy and stand‐off detection systems. Various approaches for the array elements are discussed, from conventional distributed‐feedback lasers over master‐oscillator power‐amplifier devices to tapered oscillators, and the performances of the different array types are compared. The challenges associated with reliably achieving single‐mode operation at deterministic wavelengths for each laser element in combination with a uniform distribution of high output power across the array are discussed. An overview of the range of applications benefiting from the quantum cascade laser approach is given. The distinct and crucial advantages of arrays over external cavity quantum cascade lasers as tunable single‐mode sources in the mid‐infrared are discussed. Spectroscopy and hyperspectral imaging demonstrations by quantum cascade laser arrays are reviewed.
We reveal unusually strong polarization sensitivity of electric and magnetic dipole resonances of high‐index dielectric nanoparticles placed on a metallic film. By employing dark‐field spectroscopy, we observe the polarization‐controlled transformation from high‐Q magnetic‐dipole scattering to broadband suppression of scattering associated with the electric dipole mode, and show numerically that it is accompanied by a strong enhancement of the respective fields by the nanoparticle. Our experimental data for silicon nanospheres are in an excellent agreement with both analytical calculations based on Green's function approach and the full‐wave numerical simulations. Our findings further substantiate dielectric nanoparticles as strong candidates for many applications in enhanced sensing, spectroscopy and nonlinear processes at the nanoscale.
