Volume 9, Issue 3

Topological photonics emerged as a novel route to engineer the flow of light. Topologically protected photonic edge modes, which are supported at the perimeters of topologically nontrivial insulating bulk structures, are of particular interest as they may enable low-loss optical waveguides immune to structural disorder. Very recently, there has been a sharp rise of interest in introducing gain materials into such topological photonic structures, primarily aiming at revolutionizing semiconductor lasers with the aid of physical mechanisms existing in topological physics. Examples of remarkable realizations are topological lasers with unidirectional light output under time-reversal symmetry breaking and topologically protected polariton and micro/nanocavity lasers. Moreover, the introduction of gain and loss provides a fascinating playground to explore novel topological phases, which are in close relevance to non-Hermitian and parity-time symmetric quantum physics and are, in general, difficult to access using fermionic condensed matter systems. Here, we review the cutting-edge research on active topological photonics, in which optical gain plays a pivotal role. We discuss recent realizations of topological lasers of various kinds, together with the underlying physics explaining the emergence of topological edge modes. In such demonstrations, the optical modes of the topological lasers are determined by the dielectric structures and support lasing oscillation with the help of optical gain. We also address recent research on topological photonic systems in which gain and loss, themselves, essentially influence topological properties of the bulk systems. We believe that active topological photonics provides powerful means to advance micro/nanophotonics systems for diverse applications and topological physics, itself, as well.

Owing to their wide direct bandgap tunability, III-nitride (III-N) compound semiconductors have been proven instrumental in the development of blue light-emitting diodes that led to the so-called solid-state lighting revolution and blue laser diodes that are used for optical data storage. Beyond such conventional optoelectronic devices, in this review, we explore the progress made in the past 15 years with this low refractive index material family for the realization of microdisks as well as 2D and 1D photonic crystal (PhC) membrane cavities. Critical aspects related to their design and fabrication are first highlighted. Then, the optical properties of passive PhC structures designed for near-infrared such as their quality factor and their mode volume are addressed. Additional challenges dealing with fabrication pertaining to structures designed for shorter wavelengths, namely the visible to ultraviolet spectral range, are also critically reviewed and analyzed. Various applications ranging from second and third harmonic generation to microlasers and nanolasers are then discussed. Finally, forthcoming challenges and novel fields of application of III-N photonic cavities are commented.

Last decade, halide perovskites demonstrate high potential for efficient, tunable, and cheap photonic sources. Recently, single-particle perovskite lasers of various compositions and shapes with all dimensions close or smaller than the emitted wavelengths were demonstrated experimentally in a broad range of temperatures. In this review, we aim to cover not only the recent progress in the single-particle perovskite lasers but also provide a comprehensive analysis on strategies to achieve the most compact perovskite lasers with the best working parameters.

Targeted drug delivery and real-time detection both play an important role for studying the specificity of a single cancer cell and the development of anticancer drugs. However, a method that simultaneously enables safe and efficient targeted drug delivery and noninvasive, free-label cell detection is highly desirable but challenging. Here, we report an all-optical method that combines fiber optical tweezers with laser Raman microspectroscopy, which can achieve targeted drug delivery to a single cancer cell using optical manipulation in vitro quickly and accurately by a tapered fiber probe, and simultaneously record the corresponding active characteristics of the targeted cancer cell under the contact of delivered drug through a Raman spectrometer. Using the method, drug delivery and release can be flexibly controlled by turning on/off the trapping laser beam propagating in the fiber, which can avoid the complex systems and is highly autonomous and controllable. Moreover, the detection of cell activity does not require any dye calibration and processing, and it is noninvasive. In addition, for a single suspension cell, optical trapping of the cell using another fiber tip can overcome the low efficiency of targeted drug delivery and the poor stability of the Raman spectrum caused by Brownian motion of the cell. This all-optical method provides a promising approach to conduct pharmacologic studies with the reaction of cancer cell and drugs at the level of a single cell.

We revisit plasmon modes in nanoparticle dimers with conductive or insulating junction resulting in conductive or capacitive coupling. In our study, which combines electron energy loss spectroscopy, optical spectroscopy, and numerical simulations, we show the coexistence of strongly and weakly hybridised modes. While the properties of the former ones strongly depend on the nature of the junction, the properties of the latter ones are nearly unaffected. This opens up a prospect for independent engineering of individual plasmon modes in a single plasmonic antenna. In addition, we show that Babinet’s principle allows to engineer the near field of plasmon modes independent of their energy. Finally, we demonstrate that combined electron energy loss imaging of a plasmonic antenna and its Babinet-complementary counterpart allows to reconstruct the distribution of both electric and magnetic near fields of localised plasmon resonances supported by the antenna, as well as charge and current antinodes of related charge oscillations.

Self-similar fractals provide a degree of freedom for varying the resonance frequency due to the multiscale geometric features involved and are an ideal candidate for ultrabroadband absorbing devices – especially in the terahertz (THz) band where there is a lack of natural absorbing materials. Metasurface-based THz absorbers often suffer from poor broadband performance, whereas strongly absorbing broadband devices are typically complex multilayer structures. Here, we numerically demonstrate an ultrabroadband, ultrathin, polarization-insensitive, wide-angle, single-layer planar metasurface THz absorber by integrating different Cayley tree fractal resonators into one supercell based on the frequency shifting and multiresonance bands of different fractal orders. In terms of physics, we have exploited the self-similar nature of fractal geometry to engineer the multimodal spectrum of this system. With increasing fractal order N, an increasing number of modes can be excited with certain degeneracies where each mode corresponds to plasmon oscillations at different geometric scales inside fractal. As a result, broad, multipeaked spectra with large degeneracy numbers can be achieved with larger N. Finally, by placing fractals of different order N into one supercell, the coupling and superposition of the neighboring resonances exhibit the desired ultrabroadband response. The proposed absorber provides a wide incident wave angle with a full-width half-maximum absorption bandwidth of more than one octave, i.e. 3.88 THz. Greater than 80% absorption is achieved over a frequency range of 3 THz. Owing to its performance, this work is a step forward in realizing perfect blackbody absorbers that can be easily integrated with bolometric sensing technology to make high-efficient THz-sensing devices.

A strategy is proposed to achieve wideband tunable perfect plasmonic absorption in graphene nanoribbons by employing attenuated total refraction (ATR) in Otto prism configuration. In this configuration, the Otto prism with a deep-subwavelength dielectric spacer is used to generate tunneling evanescent waves to excite localized plasmons in graphene nanoribbons. The influence of the configuration parameters on the absorption spectra of graphene plasmons is studied systematically, and the key finding is that perfect absorption can be achieved by actively controlling the incident angle of light under ATR conditions, which provides an effective degree of freedom to tune the absorption properties of graphene plasmons. Based on this result, it is further demonstrated that by simultaneously tuning the incident angle and the graphene Fermi energy, the tunable absorption waveband can be significantly enlarged, which is about 3 times wider than the conventional cavity-enhanced configuration. Our proposed strategy to achieve wideband, tunable graphene plasmons could be useful in various infrared plasmonic devices.

Coupled light-matter modes supported by plasmonic metasurfaces can be combined with topological principles to yield subwavelength topological valley states of light. This study gives a systematic presentation of the topological valley states available for lattices of metallic nanoparticles (NPs): all possible lattices with hexagonal symmetry are considered as well as valley states emerging on a square lattice. Several unique effects that have yet to be explored in plasmonics are identified, such as robust guiding, filtering, and splitting of modes, as well as dual-band effects. These are demonstrated by means of scattering computations based on the coupled dipole method that encompass full electromagnetic interactions between NPs.

The effect of V-pits in the upper waveguide (UWG) on device performance of GaN-based laser diodes (LDs) has been studied. Experimental results demonstrate that in comparison with the LDs with u-In 0.017 Ga 0.983 N/u-GaN multiple UWG or u-In 0.017 Ga 0.983 N one, the LDs with a single u-GaN UWG has the best device performance. They have a smaller threshold current density, and a larger and more stable output optical power. The lowest threshold current density is as low as 1.3 kA/cm 2 , and the optical power reaches to 2.77 W. Furthermore, atomic force microscopy suggests that the deterioration of device performance of former kinds of devices may be attributed to the increase of V-pits’ size and quantity in the undoped-In 0.017 Ga 0.983 N UWG layer, and these V-pits could introduce more nonradiative recombination centers and exacerbate the inhomogeneity of injection current. Moreover, theoretical calculation results indicate that the increase of leakage current and optical loss are additional reasons for the device performance deterioration, which may be caused by a reduction of the potential barrier height for electrons in the quantum wells and by an increased background electron concentration in UWG.

Adaptive metasurfaces (MSs) provide immense control over the phase, amplitude and propagation direction of electromagnetic waves. Adopting phase-change materials (PCMs) as an adaptive medium allows us to tune functionality of MSs at the meta-atom length scale providing full control over MS (re-)programmability. Recent experimental progress in the local switching of PCM-based MSs promises to revolutionize adaptive photonics. Novel possibilities open new challenges, one of which is a necessity to understand and be able to predict the phase transition behavior at the sub-micrometer scale. A meta-atom can be switched by a local deposition of heat using optical or electrical pulses. The deposited energy is strongly inhomogeneous and the resulting phase transition is spatially non-uniform. The drastic change of the material properties during the phase transition leads to time-dependent changes in the absorption rate and heat conduction near the meta-atom. These necessitate a self-consistent treatment of electromagnetic, thermal and phase transition processes. Here, a self-consistent multiphysics description of an optically induced phase transition in MSs is reported. The developed model is used to analyze local tuning of a perfect absorber. A detailed understanding of the phase transition at the meta-atom length scale will enable a purposeful design of programmable adaptive MSs.

Terahertz resonant nanoantennas have recently become a key tool to investigate otherwise inaccessible interactions of such long-wavelength radiation with nano-matter. Because of their high-aspect-ratio rod-shaped geometry, resonant nanoantennas suffer from severe loss, which ultimately limits their field localization performance. Here we show, via a quasi-analytical model, numerical simulations, and experimental evidence, that a proper tapering of such nanostructures relaxes their overall loss, leading to an augmented local field enhancement and a significantly reduced resonator mode volume. Our findings, which can also be extended to more complex geometries and higher frequencies, have profound implications for enhanced sensing and spectroscopy of nano-objects, as well as for designing more effective platforms for nanoscale long-wavelength cavity quantum electrodynamics.

Gold nanoparticles (Au NPs) have a wide range of applications because of their localized surface plasmon resonance properties. Femtosecond laser is considered to be an effective method for preparing Au NPs because of its characteristics of ultrashort irradiation periods and ultrahigh intensities. In this study, a novel method is proposed to produce an Au NP-attached substrate using the spatially and temporally shaped femtosecond laser. Laser-induced periodic surface structures (LIPSS) are designed to obtain the localized optical field enhancement, which leads to the femtosecond laser spatially reshaping, enabling the deposition of Au NPs by photoreduction on silicon substrates. The Au NPs prepared by this method exhibit morphological controllability and chemical stability, especially excellent spatial selectivity and uniformity, resulting in the tunable and stable surface-enhanced Raman scattering (SERS) applications. Also, the temporally shaped femtosecond pulses are introduced to further increase the enhancement factors of the SERS. This method successfully achieves the controllable morphology synthesis and selective deposition of Au NPs on the substrate simultaneously, which provides a promising candidate for SERS substrates fabrication, and holds potential applications in optoelectronics, such as molecular detection and biosensors.

In this paper, a dual-band reflective meta-hologram is designed providing two distinct information channels whose field intensity distributions can be independently manipulated at the same time. The proposed pure-phase meta-hologram is composed of several frequency-dispersive coding meta-atoms possessing each of 2-bit digital statuses of “00”, “01”, “10”, and “11” at either the lower (X-band) or the higher (Ku-band) frequency band. Relying on the weighted Gerchberg-Saxton phase retrieval algorithm, different illustrative examples have been provided to theoretically inspect the dual-band performance of our coding meta-hologram. Numerical simulations validate the proposed frequency multiplexing meta-holography with the ability to project two different high-quality images with low cross-talk on two X-band and Ku-band near-field channels located at distinct pre-determined distances from the metasurface plane. As proof of concept, two meta-hologram samples are fabricated, and the experimental results corroborate well the numerical simulations and theoretical predictions. The designed meta-hologram features all fascinating advantages of the coding metasurfaces while its performance overcomes that of previous studies due to providing two information channels rather than the conventional single-channel holography. The frequency multiplexing acquired by the proposed bi-spectral coding meta-hologram may provide great opportunities in a variety of applications, such as data storage and information processing.

Topological optics is an emerging research area in which various topological and geometrical ideas are being proposed to design and manipulate the behaviors of photons. Here, the photonic spin Hall effect on the surfaces of topological Weyl semimetal (WSM) films was studied. Our results show that the spin-dependent splitting (i.e. photonic spin Hall shifts) induced by the spin-orbit interaction is little sensitive to the tilt α t of Weyl nodes and the chemical potential μ in type-I WSM film. In contrast, photonic spin Hall shifts in both the in-plane and transverse directions present versatile dependent behaviors on the α t and μ in type-II WSM film. In particular, the largest in-plane and transverse spin Hall shifts appear at the tilts between −2 and −3, which are ~40 and ~10 times of the incident wavelength, respectively. Nevertheless, the largest spin Hall shifts for type-II WSM film with positive α t are only several times of incident wavelength. Moreover, the photonic spin Hall shifts also exhibit different variation trends with decreasing the chemical potential for different signs of α t in type-II WSM films. This dependence of photonic spin Hall shifts on tilt orientation in type-II WSM films has been explained by time-reversal-symmetry-breaking Hall conductivities in WSMs.