Control of spontaneously emitted light lies at the heart of quantum optics. It is essential for diverse applications ranging from miniature lasers and light-emitting diodes, to single-photon sources for quantum information, and to solar energy harvesting. To explore such new quantum optics applications, a suitably tailored dielectric environment is required in which the vacuum fluctuations that control spontaneous emission can be manipulated. Photonic crystals provide such an environment: they strongly modify the vacuum fluctuations, causing the decay of emitted light to be accelerated or slowed down, to reveal unusual statistics, or to be completely inhibited in the ideal case of a photonic bandgap. Here we study spontaneous emission from semiconductor quantum dots embedded in inverse opal photonic crystals. We show that the spectral distribution and time-dependent decay of light emitted from excitons confined in the quantum dots are controlled by the host photonic crystal. Modified emission is observed over large frequency bandwidths of 10%, orders of magnitude larger than reported for resonant optical microcavities. Both inhibited and enhanced decay rates are observed depending on the optical emission frequency, and they are controlled by the crystals’ lattice parameter. Our experimental results provide a basis for all-solid-state dynamic control
of optical quantum systems.

Controlling the dynamics of spontaneous emission from quantum dots by photonic crystals

Thirty years ago, Coullet et al. proposed that a special optical field exists in laser cavities bearing some analogy with the superfluid vortex. Since then, optical vortices have been widely studied, inspired by the hydrodynamics sharing similar mathematics. Akin to a fluid vortex with a central flow singularity, an optical vortex beam has a phase singularity with a certain topological charge, giving rise to a hollow intensity distribution. Such a beam with helical phase fronts and orbital angular momentum reveals a subtle connection between macroscopic physical optics and microscopic quantum optics. These amazing properties provide a new understanding of a wide range of optical and physical phenomena, including twisting photons, spin-orbital interactions, Bose-Einstein condensates, etc., while the associated technologies for manipulating optical vortices have become increasingly tunable and flexible. Hitherto, owing to these salient properties and optical manipulation technologies, tunable vortex beams have engendered tremendous advanced applications such as optical tweezers, high-order quantum entanglement, and nonlinear optics. This article reviews the recent progress in tunable vortex technologies along with their advanced applications.

/pdf/optical-vortices-30-years-on-oam-manipulation-from-2yknluuuga.pdf

Optical vortices 30 years on: OAM manipulation from topological charge to multiple singularities

Graphene plasmons were predicted to possess ultra-strong field confinement and very low damping at the same time, enabling new classes of devices for deep subwavelength metamaterials, single-photon nonlinearities, extraordinarily strong light-matter interactions and nano-optoelectronic switches. While all of these great prospects require low damping, thus far strong plasmon damping was observed, with both impurity scattering and many-body effects in graphene proposed as possible explanations. With the advent of van der Waals heterostructures, new methods have been developed to integrate graphene with other atomically flat materials. In this letter we exploit near-field microscopy to image propagating plasmons in high quality graphene encapsulated between two films of hexagonal boron nitride (h-BN). We determine dispersion and particularly plasmon damping in real space. We find unprecedented low plasmon damping combined with strong field confinement, and identify the main damping channels as intrinsic thermal phonons in the graphene and dielectric losses in the h-BN. The observation and in-depth understanding of low plasmon damping is the key for the development of graphene nano-photonic and nano-optoelectronic devices.

Highly confined low-loss plasmons in graphene-boron nitride heterostructures

During the past few years, metasurfaces have been used to demonstrate optical elements and systems with capabilities that surpass those of conventional diffractive optics. Here, we review some of these recent developments, with a focus on dielectric structures for shaping optical wavefronts. We discuss the mechanisms for achieving steep phase gradients with high efficiency, simultaneous polarization and phase control, controlling the chromatic dispersion, and controlling the angular response. Then, we review applications in imaging, conformal optics, tunable devices, and optical systems. We conclude with an outlook on future potentials and challenges that need to be overcome.

https://www.degruyter.com/document/doi/10.1515/nanoph-2017-0129/pdf

A review of dielectric optical metasurfaces for wavefront control

Rapidly growing demands for fast information processing have launched a race for creating compact and highly efficient optical devices that can reliably transmit signals without losses. Recently discovered topological phases of light provide novel opportunities for photonic devices robust against scattering losses and disorder. Combining these topological photonic structures with nonlinear effects will unlock advanced functionalities such as magnet-free nonreciprocity and active tunability. Here, we introduce the emerging field of nonlinear topological photonics and highlight the recent developments in bridging the physics of topological phases with nonlinear optics. This includes the design of novel photonic platforms which combine topological phases of light with appreciable nonlinear response, self-interaction effects leading to edge solitons in topological photonic lattices, frequency conversion, active photonic structures exhibiting lasing from topologically protected modes, and many-body quantum topological phases of light. We also chart future research directions discussing device applications such as mode stabilization in lasers, parametric amplifiers protected against feedback, and ultrafast optical switches employing topological waveguides.

Nonlinear topological photonics

We propose an effective approach to generate tunable orbital angular momentum (OAM) beam based on all-fiber fused coupler consisting of a single mode fiber (SMF) and a two-mode fiber. The fundamental mode in the SMF is directly coupled to the LP11 mode group (TE0,1, $\text {HE}^{\text {even,odd}}_{2,1}$ , TM0,1) by appropriately phase matching the modes in the fiber coupler. The experimental results demonstrate that the OAM beam is produced by combining $\text {HE}^{\text {even}}_{2,1}$ and TE0,1 (or $\text{HE}^{\text {odd}}_{2,1}$ and TM0,1) with a $\pi $ /2 phase shift and the OAM topological charge could be tuned from $l = -1$ to $l =+1$ by adjusting a polarizer set at the output end of fiber.

Tunable Orbital Angular Momentum Generation Using All-Fiber Fused Coupler

In this paper we propose a graphene plasmons isolator based on non-reciprocal coupling within double graphene layer waveguide structure on a magneto-optical substrate. The difference in modal indices of graphene plasmons in opposite directions enables non-reciprocal coupling, which is theoretically investigated via coupled mode theory (CMT) and shows good agreement with numerical finite elements methods (FEM). The non-reciprocal coupling endows such system functionalities as magnetically controlled “plasmons circulator” or “plasmons isolator”. For the latter case, systematical investigations of the insertion losses and isolation ratios with respect to the structural parameters, material properties, environmental parameters, fabrication errors as well as dielectric damping are presented. Theoretical investigation has shown an isolation ratio as 42 dB. The proposed plasmons circulator and isolator may serve as potential building blocks in graphene plasmons circuits.

Graphene plasmons isolator based on non-reciprocal coupling

We investigate the field enhancement and gradient force in the graphene-coated nanowire pairs in this paper. The real part of modal index, normalized propagation lengths, surface charge distributions and field distributions of the six lowest orders’ plasmon modes in the graphene-coated nanowire pairs is presented. Studies have shown that the six lowest orders’ modes can be divided into two groups due to the monopole-monopole hybridizations or the dipole-dipole interactions. The field enhancement in the slot region of the graphene-coated nanowire pairs can be as large as 107 times, which is six orders’ magnitude larger than the counterpart in silver nanowire pairs. Meanwhile, gradient force between the two nanowires can be as high as 20 nN·μm−1·mW−1, which is more than one order of magnitude (∼50 times) larger than the silver nanowire pairs and the previous results from other slot waveguides or coupled waveguides. The field enhancement or gradient force in the graphene-coated nanowire pairs may have applications in single biomolecule manipulation or detection.

Field Enhancement and Gradient Force in the Graphene-Coated Nanowire Pairs

A semianalytical method based on the perturbation theory is developed to calculate the bending losses of individual modes of few-mode fibers (FMFs); it is applicable for optical fibers with arbitrary circularly symmetric index profile, especially for trench-assisted fibers. The bending performance of trench-assisted step-index FMFs and parabolic-index FMFs are investigated with three key parameters (i.e., the refractive index difference of trench-cladding, the width of the trench, and the distance of the core-trench). Then, a performance index is defined to estimate the bending performance for FMFs. It is shown that changing the distance of the trench-core, for each order of mode, there is a minimum bending loss, which can be used for fiber optimization. This optimization position (core-trench distance) decreases as the mode order increases. It is found that the bending performance of parabolic-index FMFs is better than that of step-index FMFs with fixed core radius and cutoff wavelength. The conclusions are helpful for understanding the mechanism of bending loss for FMFs, and make contributions to designing and manufacturing of few-mode bend-insensitive fibers.

Bending losses of trench-assisted few-mode optical fibers.

High absorption efficiency is particularly desirable for various microtechnological applications. In this paper, a nearly perfect terahertz absorber for transverse magnetic (TM) polarization based on T-shaped InSb array is proposed and numerically investigated. Incident wave at the Fabry-Perot resonant frequency can be totally absorbed into the narrow grooves between the two adjacent T-shaped InSb arms. The absorption mechanism is theoretically and numerically studied by using the Fabry-Perot model and the finite element method (FEM), respectively. It is found that the proposed absorber has large angle tolerance. Moreover, the absorption peak can be controlled by varying the temperature. Furthermore, a new absorption peak will emerge while breaking the symmetry of the T-shaped InSb array. This tunable and angle-independent THz perfect absorber may find important applications in THz devices such as microbolometers, coherent thermal emitters, solar cells, photo detectors, and sensors.

Bofeng Zhu

Papers

Tunable Orbital Angular Momentum Generation Using All-Fiber Fused Coupler

Graphene plasmons isolator based on non-reciprocal coupling

Field Enhancement and Gradient Force in the Graphene-Coated Nanowire Pairs

Bending losses of trench-assisted few-mode optical fibers.

Tunable Terahertz Plasmonic Perfect Absorber Based on T-Shaped InSb Array