Dielectric metasurfaces are two‐dimensional structures composed of nano‐scatterers that manipulate the phase and polarization of optical waves with subwavelength spatial resolution, thus enabling ultra‐thin components for free‐space optics. While high performance devices with various functionalities, including some that are difficult to achieve using conventional optical setups have been shown, most demonstrated components have fixed parameters. Here, we demonstrate highly tunable dielectric metasurface devices based on subwavelength thick silicon nano‐posts encapsulated in a thin transparent elastic polymer. As proof of concept, we demonstrate a metasurface microlens operating at 915 nm, with focal distance tuning from 600 μm to 1400 μm (over 952 diopters change in optical power) through radial strain, while maintaining a diffraction limited focus and a focusing efficiency above 50%. The demonstrated tunable metasurface concept is highly versatile for developing ultra‐slim, multi‐functional and tunable optical devices with widespread applications ranging from consumer electronics to medical devices and optical communications.
Detecting the optical vortices of darkness hidden in an ultra‐weak background is a difficult task. Here we report an experiment demonstrating that the optical vortices can be directly visualized and identified with a smaller number of photons. Our method is based on the extension of the spiral phase contrast technique to incorporate vortex phase plates (VPP) of high‐order topological charges. In our experiment, we prepare optical vortex arrays of interesting structures such as Arabic numerals and the wings carrying various topological charges. By placing various VPP filters in the Fourier plane of a 4f imaging system, the embedded vortices of an incident ultra‐weak light field can be visualized, revealing both their positions and topological charges. It is found that a higher order vortex generally requires a smaller number of photons to be detected. Our method may find potential application in the fields of astronomical optics and biosensing in an ultra‐weak light environment.
Recently, metasurfaces have received increasing attention due to their ability to locally manipulate the amplitude, phase and polarization of light with high spatial resolution. Transmissive metasurfaces based on high‐index dielectric materials are particularly interesting due to the low intrinsic losses and compatibility with standard industrial processes. Here, it is demonstrated numerically and experimentally that a uniform array of silicon nanodisks can exhibit close‐to‐unity transmission at resonance in the visible spectrum. A single‐layer gradient metasurface utilizing this concept is shown to achieve around 45% transmission into the desired order. These values represent an improvement over existing state‐of‐the‐art, and are the result of simultaneous excitation and mutual interference of magnetic and electric‐dipole resonances in the nanodisks, which enables directional forward scattering with a broad bandwidth. Due to CMOS compatibility and the relative ease of fabrication, this approach is promising for creation of novel flat optical devices.
Traditional detour‐phase hologram is a powerful optical device for manipulating phase and amplitude of light, but it is usually not sensitive to the polarization of light. By introducing the light‐metasurface interaction mechanism to the traditional detour phase hologram, we design a novel plasmonic nano‐slits assisted polarization selective detour phase meta‐hologram, which has attractive advantages of polarization multiplexing ability, broadband response, and ultra‐compact size. The meta‐hologram relies on the dislocations of plasmonic slits to achieve arbitrary phase distributions, showing strong polarization selectivity to incident light due to the plasmonic response of deep‐subwavelength slits. To verify its polarization sensitive and broadband responses, we experimentally demonstrate two holographic patterns of an optical vortex and an Airy beam at p‐ and s‐polarized light with wavelengths of 532nm, 633nm and 780nm, respectively. Furthermore, we realize an application example of the meta‐hologram as a polarization multiplexed photonic device for multi‐channel optical angular momentum (OAM) generation and detection. Such meta‐holograms could find widespread applications in photonics, such as chip‐level beam shaping and high‐capacity OAM communication.
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.
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.
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.
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.
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.
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.
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.
We propose all‐dielectric metasurfaces that can be actively re‐configured using the phase‐change material Ge2Sb2Te5 (GST) alloy. With selectively controlled phase transitions on the composing GST elements, metasurfaces can be tailored to exhibit varied functionalities. Using phase‐change GST rod as the basic building block, we have modelled metamolecules with tunable optical response when phase change occurs on select constituent GST rods. Tunable gradient metasurfaces can be realized with variable supercell period consisting of different patterns of the GST rods in their amorphous and crystalline states. Simulation results indicate a range of functions can be delivered, including multilevel signal modulating, near‐field coupling of GST rods, and anomalous reflection angle controlling. This work opens up a new space in exploring active meta‐devices with broader applications that cannot be achieved in their passive counterparts with permanent properties once fabricated.
Optical metasurfaces are thin‐layer subwavelength‐patterned structures that interact strongly with light. Metasurfaces have become the subject of several rapidly growing areas of research, being a logical extension of the field of metamaterials towards their practical applications. Metasurfaces demonstrate many useful properties of metadevices with engineered resonant electric and magnetic optical responses combined with low losses of thin‐layer structures. Here we introduce the basic concepts of this rapidly growing research field that stem from earlier studies of frequency‐selective surfaces in radiophysics, being enriched by the recent development of metamaterials and subwavelength nanophotonics. We review the most interesting properties of photonic metasurfaces, demonstrating their useful functionalities such as frequency selectivity, wavefront shaping, polarization control, etc. We discuss the ways to achieve tunability of metasurfaces and also demonstrate that nonlinear effects can be enhanced with the help of metasurface engineering.
A very promising recent trend in applied quantum physics is to combine the advantageous features of different quantum systems into what is called “hybrid quantum technology”. One of the key elements in this new field will have to be a quantum memory enabling to store quanta over extended periods of time. Systems that may fulfill the demands of such applications are comb‐shaped spin ensembles coupled to a cavity. Due to the decoherence induced by the inhomogeneous ensemble broadening, the storage time of these quantum memories is, however, still rather limited. Here we demonstrate how to overcome this problem by burning well‐placed holes into the spectral spin density leading to spectacular performance in the multimode regime. Specifically, we show how an initial excitation of the ensemble leads to the emission of more than a hundred well‐separated photon pulses with a decay rate significantly below the fundamental limit of the recently proposed “cavity protection effect”.
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.
The newly engineered ternary CdZnS/ZnS colloidal quantum dots (CQDs) are found to exhibit remarkably high photoluminescence quantum yield and excellent optical gain properties. However, the underlying mechanisms, which could offer the guidelines for devising CQDs for optimized photonic devices, remain undisclosed. In this work, through comprehensive steady‐state and time‐resolved spectroscopy studies on a series of CdZnS‐based CQDs, we unambiguously clarify that CdZnS‐based CQDs are inherently superior optical gain media in the blue spectral range due to the slow Auger process and that the ultralow threshold stimulated emission is enabled by surface/interface engineering. Furthermore, external cavity‐free high‐Q quasitoroid microlasers were produced from self‐assembly of CdZnS/ZnS CQDs by facile inkjet printing technique. Detailed spectroscopy analysis confirms the whispering gallery mode lasing mechanism of the quasitoroid microlasers. This tempting microlaser fabrication method should be applicable to other solution‐processed gain materials, which could trigger broad research interests.
This work presents analytical, numerical and experimental demonstrations of light diffracted through a logarithmic spiral (LS) nanoslit, which forms a type of switchable and focus‐tunable structure. Owing to a strong dependence on the incident photon spin, the proposed LS‐nanoslit converges incoming light of opposite handedness (to that of the LS‐nanoslit) into a confined subwavelength spot, while it shapes light with similar chirality into a donut‐like intensity profile. Benefitting from the varying width of the LS‐nanoslit, different incident wavelengths interfere constructively at different positions, i.e., the focal length shifts from 7.5 μm (at λ = 632.8 nm) to 10 μm (at λ = 488 nm), which opens up new opportunities for tuning and spatially separating broadband light at the micrometer scale.
Rectangular arrays of pyramidal recesses coated by silver film are investigated by means of polarization‐resolved nonlinear microscopy at 900 nm fundamental wavelength, demonstrating strong dependence of the dipole‐allowed SHG upon the lattice parameters. The plasmonic band gap causes nearly complete SHG suppression in arrays of 650 nm periodicity, whereas a sharp resonance at 550 nm periodicity is observed due to excitation of band edge Bloch states at fundamental frequency, accompanied by symmetry‐constrained interactions with similar modes at the second‐harmonic frequency. Additionally, coupling with modes at the bottom side of the silver film may lead to extraordinary optical transmission, opening a channel for SHG from the highly nonlinear GaAs substrate. Changing the lattice geometry enables SHG intensity modulation over three orders of magnitude, while the effective nonlinear anisotropy can be continuously switched between the two lattice directions, reaching values as high as ±0.96.
Metasurfaces, which consist of resonant metamaterial elements in the form of two‐dimensional thin planar structures, retain great capabilities in manipulating electromagnetic wave and potential applications in modifying interaction with fluorescent molecules. The metasurfaces with magnetic responses are favorable to weakening fluorescence quenching while less investigated in controlling fluorescence. In this paper, we demonstrate control over fluorescence emission by engineering the magnetic and electric modes in plasmonic metasurfaces consisting of 45‐nm‐thick gold split‐ring‐resonators (SRRs). The fluorescence emission exhibits an enhancement factor of ∼18 and is predominantly x‐polarized with assistance of the magnetic mode excited by oblique incidence with an x‐polarized electric field. The magnetic and electric modes excited by oblique incidence with a y‐polarized electric field contribute to the rotation of emission polarization with respect to the incident polarization. The results demonstrate manipulating the interaction of fluorescent emitters with different resonant modes of the SRR‐based metasurface at the nanoscale by the polarization of incident light, providing potential applications of metasurfaces in a wide variety of areas, including optical nanosources, fluorescence spectroscopy and compact biosensors.
