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.
Nanostructures that feature nonreciprocal light transmission are highly desirable building blocks for realizing photonic integrated circuits. Here, a simple and ultracompact photonic‐crystal structure, where a waveguide is coupled to a single nanocavity, is proposed and experimentally demonstrated, showing very efficient optical diode functionality. The key novelty of the structure is the use of cavity‐enhanced material nonlinearities in combination with spatial symmetry breaking and a Fano resonance to realize nonreciprocal propagation effects at ultralow power and with good wavelength tunability. The nonlinearity of the device relies on ultrafast carrier dynamics, rather than the thermal effects usually considered, allowing the demonstration of nonreciprocal operation at a bit‐rate of 10 Gbit s−1 with a low energy consumption of 4.5 fJ bit−1.
Whispering gallery modes (WGMs) have been exploited for a broad range of sensing applications. However, the vast majority of WGM sensors consist of passive resonators, requiring complex interrogation systems to be employed, ultimately limiting their practicality. Active resonators containing a gain medium, allowing remote excitation and collection of the WGM‐modulated fluorescence spectra, have emerged as an alternative to passive resonators. Although research is still in its infancy, recent progress has reduced the performance gap between the two paradigms, fueled by the potential for new applications that could not previously be realized. Here, recent developments in sensors based on active WGM microresonators are reviewed, beginning with a discussion of the theory of fluorescence‐based and lasing WGMs, followed by a discussion of the variety of gain media, resonator architectures, and emerging sensing applications. We conclude with a discussion of the prospects and future directions for improving active WGM sensors.
Nanophotonic beamsplitters are fundamental building blocks in integrated optics, with applications ranging from high speed telecom receivers to biological sensors and quantum splitters. While high‐performance multiport beamsplitters have been demonstrated in several material platforms using multimode interference couplers, their operation bandwidth remains fundamentally limited. Here, we leverage the inherent anisotropy and dispersion of a sub‐wavelength structured photonic metamaterial to demonstrate ultra‐broadband integrated beamsplitting. Our device, which is three times more compact than its conventional counterpart, can achieve high‐performance operation over an unprecedented 500 nm design bandwidth exceeding all optical communication bands combined, and making it one of the most broadband silicon photonics components reported to date. Our demonstration paves the way toward nanophotonic waveguide components with ultra‐broadband operation for next generation integrated photonic systems.
We experimentally demonstrate an optically‐pumped III‐V/Si vertical‐cavity laser with lateral emission into a silicon waveguide. This on‐chip hybrid laser comprises a distributed Bragg reflector, a III‐V active layer, and a high‐contrast grating reflector, which simultaneously funnels light into the waveguide integrated with the laser. This laser has the advantages of long‐wavelength vertical‐cavity surface‐emitting lasers, such as low threshold and high side‐mode suppression ratio, while allowing integration with silicon photonic circuits, and is fabricated using CMOS compatible processes. It has the potential for ultrahigh‐speed operation beyond 100 Gbit/s and features a novel mechanism for transverse mode control.
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.
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.
We demonstrate a scheme incorporating dual‐coupled microresonators through which mode interactions are intentionally introduced and controlled for Kerr frequency comb (microcomb) generation in the normal‐dispersion region. Microcomb generation, repetition rate selection, and mode locking are achieved with coupled silicon nitride microrings controlled via an on‐chip microheater. The proposed scheme shows for the first time a reliable design strategy for normal‐dispersion microcombs and may make it possible to generate microcombs in an extended wavelength range (e.g. in the visible) where normal material dispersion is likely to dominate.
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.
Photonic structures offer unique opportunities for controlling light‐matter interaction, including the photonic spin Hall effect associated with the transverse spin‐dependent displacement of a light beam that propagates in specially designed optical media. However, due to small spin‐orbit coupling, the photonic spin Hall effect is usually weak at the nanoscale. Here we suggest theoretically and demonstrate experimentally, in both optics and microwave experiments, the photonic spin Hall effect enhanced by topologically protected edge states in subwavelength arrays of resonant dielectric particles. Based on direct near‐field measurements, we observe the selective excitation of the topological edge states controlled by the handedness of the incident light. Additionally, we reveal the main requirements to the symmetry of photonic structures to achieve the topology‐enhanced spin Hall effect, and also analyse the robustness of the photonic edge states against the long‐range coupling.
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.
Guiding light in the low index region of a high refractive index contrast waveguide can be beneficial for many applications including sensing, nonlinear optics and electro‐optics. Existing methods to achieve this goal suffer from fabrication complexity, large loss, or poor optical confinement. We propose a simple structure to achieve a significant enhancement of light confinement in the low index medium. We explain the guiding principle of this structure using geometrical optics, and suggest a number of applications where this guiding structure can provide significant performance benefits. The combination of simplicity, ease of fabrication, and good confinement makes this waveguide an attractive choice for a wide range of applications. To illustrate this, we consider the integration of a nonlinear polymer with a silicon photonic waveguide, and show that significantly better performance with an easier fabrication process can be achieved using this new scheme.
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.
The design of micro‐optical resonator arrays are introduced and tailored towards refractive index sensing applications, building on the previously unexplored benefits of open dielectric stacks. The resonant coupling of identical hollow cavities present strong and narrow spectral resonance bands beyond that available with a single Fabry Perot interferometer. Femtosecond laser irradiation with selective chemical etching is applied to precisely fabricate stacked and waveguide‐coupled open resonators into fused silica, taking advantage of small 12 nm rms surface roughness made available by the self‐alignment of nanograting planes. Refractive index sensing of methanol‐water solutions confirm a very attractive sensing resolution of 6.5 × 10−5 RIU. Such high finesse optical elements open a new realm of optofluidic sensing and integrated optical circuit concepts for detecting minute changes in sample properties against a control solution that may find importance in chemical and biological sensors, telecom sensing networks, biomedical probes, and low‐cost health care products.
As an emerging topic, photonic‐assisted microwave measurements with distinct features such as wide frequency coverage, large instantaneous bandwidth, low frequency‐dependent loss, and immunity to electromagnetic interference, have been extensively studied recently. In this article, we provide a comprehensive overview of the latest advances in photonic microwave measurements, including microwave spectrum analysis, instantaneous frequency measurement, microwave channelization, Doppler frequency‐shift measurement, angle‐of‐arrival detection, time–frequency analysis, compressive sensing, and phase‐noise measurement. A photonic microwave radar, as a functional measurement system, is also reviewed. The performance of the photonic measurement solutions is evaluated and compared with the electronic solutions. Future prospects using photonic integrated circuits and software‐defined architectures to further improve the measurement performance are also discussed.
High efficiency, broad bandwidth, and robust angular tolerance are key considerations in photonic device design. Here, a few‐layer, asymmetric light transmitting metasurface that simultaneously satisfies all the above requirements is reported. The metasurface consists of coupled metallic sheets. It has a measured transmission efficiency of 80%, extinction ratio of 13.8 dB around 1.5 μm, and a full width half maximum bandwidth of 1.7 μm. It is as thin as 290 nm, has good performance tolerance against the angle of incidence and constituent nano‐structure geometry variations. This work demonstrates a practical asymmetric light transmission device with optimal performance for large scale manufacturing.
Ultracompact directional optical nanoantennas are rapidly gaining popularity yet still challenging tasks to construct at the nanoscale. Here, we experimentally demonstrate unidirectional emission from a fluorescent nanodiamond coupled with a single gold nanorod. Different configurations of the assembled hybrid nanostructures were realized via step‐by‐step atomic force microscope nanomanipulation. The emission patterns can be controlled by adjusting the configurations, i.e., the gold nanorod orientation and separation with respect to the nanodiamond. Numerical simulation results reveal that the unidirectional emission can be ascribed to the interference between the electromagnetic fields produced by the dipole‐like source and the out‐of‐phase dipole induced in the gold nanorod. The proposed hybrid nanostructures remarkably exhibit highly unidirectional emission even when the emitter is positioned up to 50 nm away from the nanorod antenna and present a broad working spectral bandwidth of ∼200 nm. The distinct features of this hybrid structure suggest potential applications for novel nanoscale light sources, sensing, and quantum information.
High power single mode quantum cascade lasers with a narrow far field are important for several applications including surgery or military countermeasure. Existing technologies suffer from drawbacks such as operation temperature and scalability. In this paper we introduce a fabrication approach that potentially solves simultaneously these remaining limitations. We demonstrate and characterize deep etched, buried photonic crystal quantum cascade lasers emitting around a wavelength of 8.5 μm. The active region was dry etched before being regrown with semi‐insulating Fe:InP. This fabrication strategy results in a refractive index contrast of 10% allowing good photonic mode control, and simultaneously provides good thermal extraction during operation. Single mode emission with narrow far field pattern and peak powers up to 0.88 W at 263 K were recorded from the facet of the photonic crystal laser, and lasing operation was maintained up to room temperature. The lasing modes emitted from square photonic crystal mesas with a side length of 550μm, were identified as slow Bloch photonic crystal modes by means of three‐dimensional photonic simulations and measurements.
Photonic waveguide arrays provide an excellent platform for simulating conventional topological systems, and they can also be employed for the study of novel topological phases in photonics systems. However, a direct measurement of bulk topological invariants remains a great challenge. Here we study topological features of generalized commensurate Aubry‐André‐Harper (AAH) photonic waveguide arrays and construct a topological phase diagram by calculating all bulk Chern numbers, and then explore the bulk‐edge correspondence by analyzing the topological edge states and their winding numbers. In contrast to incommensurate AAH models, diagonal and off‐diagonal commensurate AAH models are not topologically equivalent. In particular, there appear nontrivial topological phases with large Chern numbers and topological phase transitions. By implementing Thouless pumping of light in photonic waveguide arrays, we propose a simple scheme to measure the bulk Chern numbers.
The feasibility of realizing a photonic Floquet topological insulator (PFTI) in an atomic ensemble is demonstrated. The interference of three coupling fields will split energy levels periodically, to form a periodic refractive index structure with honeycomb profile that can be adjusted by different frequency detunings and intensities of the coupling fields. This in turn will affect the appearance of Dirac cones in momentum space. When the honeycomb lattice sites are helically ordered along the propagation direction, gaps open at Dirac points, and one obtains a PFTI in an atomic vapor. An obliquely incident beam will be able to move along the zigzag edge of the lattice without scattering energy into the PFTI, due to the confinement of edge states. The appearance of Dirac cones and the formation of a photonic Floquet topological insulator can be shut down by the third‐order nonlinear susceptibility and opened up by the fifth‐order one.
