Band 8, Heft 5

We discuss phonon-induced non-Markovian and Markovian features in QD-based quantum nanooptics. We cover lineshapes in linear absorption experiments, phonon-induced incoherence in the Heitler regime, and memory correlations in two-photon coherences. To qualitatively and quantitatively understand the underlying physics, we present several theoretical models that capture the non-Markovian properties of the electron–phonon interaction accurately in different regimes. Examples are the Heisenberg equation of motion approach, the polaron master equation, and Liouville propagator techniques in the independent boson limit and beyond via the path integral method. Phenomenological modeling overestimates typically the dephasing due to the finite memory kernel of phonons and we give instructive examples of phonon-mediated coherence such as phonon-dressed anticrossings in Mollow physics, robust quantum state preparation, cavity feeding, and the stabilization of the collapse and revival phenomenon in the strong coupling limit of cavity quantum electrodynamics.

Phononic crystals and acoustic metamaterials are periodic structures whose effective properties can be tailored at will to achieve extreme control on wave propagation. Their refractive index is obtained from the homogenization of the infinite periodic system, but it is possible to locally change the properties of a finite crystal in such a way that it results in an effective gradient of the refractive index. In such case the propagation of waves can be accurately described by means of ray theory, and different refractive devices can be designed in the framework of wave propagation in inhomogeneous media. In this paper we review the different devices that have been studied for the control of both bulk and guided acoustic waves based on graded phononic crystals.

Metamaterials have been characterized by effective medium parameters over the past decades due to the subwavelength nature of meta-atoms. Once the metamaterials are fabricated, their functions become fixed or tunable. Recently, the concept of digital metamaterials has been introduced, in which, for instance, the constitutive 1-bit meta-atom is digitalized as “0” or “1” corresponding to two opposite electromagnetic (EM) responses. The digital metamaterials set up a bridge between the physical world and the information world. More interestingly, when the digital meta-atom is programmable, a single metamaterial can be used to realize different functions when programmed with different coding sequences. Moreover, as the states of programmable meta-atoms can be quickly switched, it enables the wave-based information coding and processing on the physical level of metamaterials in real time. For these reasons, we prefer to call digital metamaterials with programmable meta-atoms as “information metamaterials.” In this review article, we introduce two basic principles for information metamaterials: Shannon entropy on metamaterials to measure the information capacity quantitatively and digital convolution on metamaterials to manipulate the beam steering. Afterwards, two proof-of-concept imaging systems based on information metamaterials, i.e. programmable hologram and programmable imager, are presented, showing more powerful abilities than the traditional counterparts. Furthermore, we discuss the time-modulated information metamaterial that enables efficient and accurate manipulations of spectral harmonic distributions and brings new physical phenomena such as frequency cloaking and velocity illusion. As a relevant application of time-modulated information metamaterials, we propose a novel architecture of wireless communication, which simplifies the modern wireless communication system. Finally, the future trends of information metamaterials are predicted.

Nonradiating sources of energy have traditionally been studied in quantum mechanics and astrophysics but have received very little attention in the photonics community. This situation has changed recently due to a number of pioneering theoretical studies and remarkable experimental demonstrations of the exotic states of light in dielectric resonant photonic structures and metasurfaces, with the possibility to localize efficiently the electromagnetic fields of high intensities within small volumes of matter. These recent advances underpin novel concepts in nanophotonics and provide a promising pathway to overcome the problem of losses usually associated with metals and plasmonic materials for the efficient control of light-matter interaction at the nanoscale. This review paper provides a general background and several snapshots of the recent results in this young yet prominent research field, focusing on two types of nonradiating states of light that both have been recently at the center of many studies in all-dielectric resonant meta-optics and metasurfaces: optical anapoles and photonic bound states in the continuum . We discuss a brief history of these states in optics, as well as their underlying physics and manifestations, and also emphasize their differences and similarities. We also review some applications of such novel photonic states in both linear and nonlinear optics for the nanoscale field enhancement, a design of novel dielectric structures with high- Q resonances, nonlinear wave mixing, and enhanced harmonic generation, as well as advanced concepts for lasing and optical neural networks.

Single-photon sources are one of the key components in quantum photonics applications. These sources ideally emit a single photon at a time, are highly efficient, and could be integrated in photonic circuits for complex quantum system designs. Various platforms to realize such sources have been actively studied, among which semiconductor quantum dots have been found to be particularly attractive. Furthermore, quantum dots embedded in bottom-up-grown III–V compound semiconductor nanowires have been found to exhibit relatively high performance as well as beneficial flexibility in fabrication and integration. Here, we review fabrication and performance of these nanowire-based quantum sources and compare them to quantum dots in top-down-fabricated designs. The state of the art in single-photon sources with quantum dots in nanowires is discussed. We also present current challenges and possible future research directions.

Plasmonic nanostructures with strong light-matter interactions have been intensively explored in the past decades. The plasmonic photothermal effect has garnered significant research interest and triggered plenty of applications, such as photothermal therapy, photothermal imaging, and photocatalysis. Recently, plasmonic nanostructures are emerging as one of the most exciting candidates for solar vapor generation, inspiring the revival of solar-thermal-based water purification technologies. Here we present a review of state-of-the-art plasmonic-enhanced solar evaporation, including the theoretical background, various designs of plasmonic materials and structures, and their potential applications. The current challenges and future perspective are outlined as well.

The crystal form of semiconductor materials is keenly correlated with the photosensitivity of optoelectronic devices. Thus, understanding the crystal form-dependent photosensitivity mechanism is critical. In this work, the microemulsion phase transfer method was adopted to prepare α- and β-titanylphthalocyanine (TiOPc NPs) with an average diameter of 35 nm. The photosensitivity ( E 1/2 ) of α-TiOPc NPs was 2.73 times better than that of β-TiOPc NPs, which was characterized by photoconductors under the same measurement conditions. DFT was performed to explain the relationship between crystal form and photosensitivity by systematically calculating the charge transfer integrals for all possible dimers in the two different crystal forms. The hole and electron reorganization energies of TiOPc were respectively calculated to be 53.5 and 271.5 meV, revealing TiOPc to be a typical p -type semiconductor. The calculated total hole transfer mobility ( μ + ) ratio (2.83) of α- to β-TiOPc was almost identical to the experimental E 1/2 ratio (2.73) and the calculated photogeneration quantum efficiency ( η e-h ) ratio (2.23). In addition, the optimum hole transfer routes in the crystal of α- and β-TiOPc were all along with the [1 0 0] crystal orientation, which was determined by the calculated μ + . A high charge transfer mobility leads to a high photosensitive TiOPc crystal. Consequently, these results indicate that the selected theoretical calculation method is reasonable for indirectly explaining the relationship between crystal form and photosensitivity. The TiOPc molecular solid-state arrangements, namely, the crystal forms of TiOPc, have a strong influence on the charge transport behavior, which in turn, affects its photosensitivity.

Photonic topological insulators (PTIs) are electromagnetic structures with highly robust unidirectional edge states, originating from their nontrivial bulk band topology. Here, we propose a plasmonic PTI that supports highly confined one-way edge states capable of transporting deep subwavelength optical frequency plasmons through arbitrary paths without back-scattering. The structure consists of a simple planar array of coupled plasmonic nanoparticles arranged in a perturbed honeycomb lattice that exhibits nontrivial band topology. The operation frequency of the emergent edge states is shown to be independent of the lattice constant, allowing for the miniaturization of the structure. As a high-frequency PTI with a simple and planar design, this structure is compatible with well-established integrated nanofabrication technologies and may find application in planar, compact, and topologically robust integrated nanophotonic devices.

In this paper we discuss the optical response of laser-written plasmonic colours on silver coated via the atomic layer deposition of alumina. These colours are due to nanoparticles distributed on a flat surface and on a surface with periodic topographical features (i.e. ripples). The colours are observed to shift with increasing alumina film thickness. The colours produced by surfaces with ripples recover their original vibrancy and hue after the deposition of film of thickness ~60 nm, while colours arising from flat surfaces gradually fade and never recover. Analysis of the surfaces identifies periodic topographical features to be responsible for this behaviour. Finite-difference time-domain simulations unravel the role played by the alumina thickness in colour formation and confirm the rotations and recovery of colours for increasing alumina thickness. The coloured surfaces were evaluated for applications in colourimetric and radiometric sensing showing large sensitivities of up to 3.06/nm and 3.19 nm/nm, respectively. The colourimetric and radiometric sensitivities are observed to be colour dependent.

In recent years, sub-wavelength metamaterials-based light perfect absorbers have been the subject of many studies. The most frequently utilized absorber configuration is based on nanostructured plasmonic metals. However, two main drawbacks were raised for this design architecture. One is the fabrication complexity and large scale incompatibility of these nano units. The other one is the inherent limitation of these common metals which mostly operate in the visible frequency range. Recently, strong interference effects in lithography-free planar multilayer designs have been proposed as a solution for tackling these drawbacks. In this paper, we reveal the extraordinary potential of bismuth (Bi) metal in achieving light perfect absorption in a planar design through a broad wavelength regime. For this aim, we adopted a modeling approach based on the transfer matrix method (TMM) to find the ideal conditions for light perfect absorption. According to the findings of our modeling and numerical simulations, it was demonstrated that the use of Bi in the metal-insulator-metal-insulator (MIMI) configuration can simultaneously provide two distinct functionalities; a narrow near unity reflection response and an ultra-broadband near perfect absorption. The reflection behavior can be employed to realize additive color filters in the visible range, while the ultra-broadband absorption response of the design can fully harvest solar irradiation in the visible and near infrared (NIR) ranges. The findings of this paper demonstrate the extraordinary potential of Bi metal for the design of deep sub-wavelength optical devices.

Vortex field in photonic metamaterials has been revealed in recent years, which provides an extra degree of freedom to control the behavior of electromagnetic waves, such as the unidirectional propagation in topological metamaterials and orbital angular momentum multiplexing in optical fibers. Achieving topologically protected unidirectionality needs an external source either with a vortex index of m  = ±1 (i.e. 2 πm phase winding around phase singularity) or with a spin index of ±1 (i.e. right- or left-handed circular polarization). However, it is still an open question on the relationship between higher-order (| m  | > 1) vortex field and topological metamaterials. Here, we study the excitation of valley bulk states by a source with a higher-order vortex index in Kagome photonic crystals. The selection rule for the excitation of valley bulk states is derived and can be applied to predict the selective excitation of valley bulk states by a source with a higher-order vortex index. Interestingly, we find that the Fourier spectrum and the averaged Fourier spectrum of the E z component enable us to identify the vortex index of the source. Besides, it is shown that the unidirectional edge states can be excited by a source with a vortex index of 1 or 2 in a straight or 120° detour waveguide. Our work paves the way for the demonstration of the selection rule and may have potential applications in optical mode identification.

Microring resonators, as a fundamental building block of photonic integrated circuits, are well developed into numerous functional devices, whose performances are strongly determined by microring’s resonance lineshapes. We propose a compact structure to reliably realize Lorentzian, Fano, and electromagnetically induced transparency (EIT) resonance lineshapes in a microring. By simply inserting two air-holes in the side-coupled waveguide of a microring, a Fabry-Perot (FP) resonance is involved to couple with microring’s resonant modes, showing Lorentzian, Fano, and EIT lineshapes over one free spectral range of the FP resonance. The quality factors, extinction ratios (ERs), and slope rates (SRs) in different lineshapes are discussed. At microring’s specific resonant wavelength, the lineshape could be tuned among these three types by controlling the FP cavity’s length. Experiment results verify the theoretical analysis well and represent Fano lineshapes with ERs of about 20 dB and SRs over 280 dB/nm. The reliably and flexibly tunable lineshapes in the compact structure have potentials to improve microring-based devices and expand their application scopes.

Thin films containing small-sized quantum dots (QDs) and nanoparticles have shown strong optical nonlinearities caused by the confinement effect. Here, we report the study of third-order optical nonlinearities of silver sulfide (Ag 2 S) QD thin films using 800 and 400 nm, 30 fs pulses. The absorption spectrometry and transmission electron microscopy are used to characterize the synthesized 80 and 500 nm Ag 2 S QD films. The giant enhancement of nonlinearities is observed up to three to six orders of magnitude larger compared to those for the bulk and liquid Ag 2 S samples. We also demonstrate the efficient high-order harmonic generation in the plasmas produced during ablation of the Ag 2 S QD thin films. The analysis of the dynamics of the QD-containing plasma spreading allowed optimization of the delay between the heating and the driving pulses for an enhancement of harmonics conversion efficiency.

Monolithic waveguide laser devices are required to achieve on-chip lasing. In this work, a new design of a monolithic device with embedded Ag nanoparticles (NPs) plus the Nd:YAG ridge waveguide has been proposed and implemented. By using Ag + ion implantation, the embedded Ag NPs are synthesized on the near-surface region of the Nd:YAG crystal, resulting in the significant enhancement of the optical nonlinearity of Nd:YAG and offering saturable absorption properties of the crystal at a wide wavelength band. The subsequent processing of the O 5+ ion implantation and diamond saw dicing of crystal finally leads to the fabrication of monolithic waveguide with embedded Ag NPs. Under an optical pump, the Q-switched mode-locked waveguide lasers operating at 1 μm is realized with the pulse duration of 29.5 ps and fundamental repetition rate of 10.53 GHz, owing to the modulation of Ag NPs through evanescent field interaction with waveguide modes. This work introduces a new approach in the application of monolithic ultrafast laser devices by using embedded metallic NPs.

Femtosecond laser direct writing is widely exploited in surface periodic structures processing. However, this technique still faces challenges in obtaining high surface homogeneity and flexible morphology controllability. In this study, a flexible and efficient approach has been proposed to fabricate highly homogeneous and controllable nanogratings on silicon via chemical etching-assisted femtosecond laser modification. By precisely manipulating the laser-material interaction process, alternating amorphous-crystalline nanofringes are generated when employing femtosecond laser scanning over a Si sample, with almost no material removal. Following auxiliary chemical etching, highly homogeneous nanograting structures are obtained, and the morphology of the nanogratings can be flexibly managed through precisely controlling the duration of the etching process. Complex cross-scale patterns with remarkable structural colors that are visible under indoor light illumination are readily achieved on the sample surfaces exploiting our method. In addition, compared with traditional methods for laser-induced periodic surface structures, the fabrication efficiency is considerably improved. Our processing procedure offers potential applications in the fields of optics, nanoelectronics, and mechatronics.

Quantitative analysis of line edge roughness (LER) is very important for understanding the root causes of LER and thereby improving the pattern quality in near-field lithography (NFL), because LER has become the main limiter of critical dimension (CD) control as the feature size of nanostructures is scaled down. To address this challenge, the photoresist point-spread function of NFL with a contact plasmonic ridge nanoaperture can be employed to account for the physical and chemical effects involved in the LER-generation mechanism. Our theoretical and experimental results show that the sources of LER in NFL mainly come from the aerial image, material chemistry, and process. Importantly, the complicated decay characteristics of surface plasmon waves are demonstrated to be the main optical contributor. Because the evanescent mode of surface plasmon polaritons (SPPs) and quasi-spherical waves (QSWs) decay in the lateral direction, they can induce a small image log-slope and low photoresist contrast, leading to a large LER. We introduce an analytical model and demonstrate the relationship between LER and CD to estimate the pattern quality in NFL. We expect that these results can provide alternative approaches to further improve pattern uniformity and resolution, which can lead to advanced nanopatterning results in NFL.

The development of optical interconnect techniques greatly expands the communication bandwidth and decreases the power consumption at the same time. It provides a prospective solution for both intra-chip and inter-chip links. Herein reported is an integrated wavelength-division multiplexing (WDM)-compatible multimode optical switching system-on-chip (SoC) for large-capacity optical switching among processors. The interfaces for the input and output of the processor signals are electrical, and the on-chip data transmission and switching process are optical. It includes silicon-based microring optical modulator arrays, mode multiplexers/de-multiplexers, optical switches, microring wavelength de-multiplexers and germanium-silicon high-speed photodetectors. By introducing external multi-wavelength laser sources, the SoC achieved the function of on-chip WDM and mode-division multiplexing (MDM) hybrid-signal data transmission and switching on a standard silicon photonics platform. As a proof of concept, signals with a 25 Gbps data rate are implemented on each microring modulator of the fabricated SoC. We illustrated 25 × 3 × 2 Gbps on-chip data throughput with two-by-two multimode switching functionality through implementing three wavelength-channels and two mode-channel hybrid-multiplexed signals for each multimode transmission waveguide. The architecture of the SoC is flexible to scale, both for the number of supported processors and the data throughput. The demonstration paves the way to a large-capacity multimode optical switching SoC.

Sensitive solar-blind ultraviolet (UV) photodetectors are important to various military and civilian applications, such as flame sensors, missile interception, biological analysis, and UV radiation monitoring below the ozone hole. In this paper, a solar-blind UV photodetector based on a buried-gate graphene field-effect transistor (GFET) decorated with titanium dioxide (TiO 2 ) nanoparticles (NPs) was demonstrated. Under the illumination of a 325-nm laser (spot size ~2 μm) with a total power of 0.35 μW, a photoresponsivity as high as 118.3 A/W was obtained, at the conditions of zero gate bias and a source-drain bias voltage of 0.2 V. This photoresponsivity is over 600 times higher than that of a recently reported solar-blind UV photodetector based on graphene/vertical Ga 2 O 3 nanowire array heterojunction (0.185 A/W). Experiments showed that the photoresponsivity of the TiO 2 NPs decorated GFET photodetectors can be further enhanced by increasing the source-drain bias voltage or properly tuning the gate bias voltage. Furthermore, the photoresponse time of the TiO 2 NPs decorated GFET photodetectors can also be tuned by the source-drain bias and gate bias. This study paves a simple and feasible way to fabricate highly sensitive, cost-efficient, and integrable solar-blind UV photodetectors.

The ability of chiral media to differentiate circularly polarized light is conventionally characterized by circular dichroism (CD) which is based on the difference in the absorption of the incident light for different polarizations. Thus, CD probes the bulk properties of chiral media. Here, we introduce a new approach termed as circular phase-dichroism that is based on the surface properties and is defined as the difference of the reflection phase for different circularly polarized incident lights in characterizing chiral media. As a demonstration, we measure the reflection phase from planar chiral sawtooth metasurface for circularly polarized light in the visible range using a simple Fabry Perot interference technique. The measured circular phase-dichroism is also crosschecked by conventional CD measurement of the transmitted light and by full-wave simulations. Our results demonstrate the potential applications of circular phase-dichroism in sensing and metasurface characterizations.

Tip-based plasmonic nanofocusing, which delivers light into a nanoscale region and achieves localized electromagnetic (EM) field enhancement beyond the diffraction limit, is highly desired for light-matter interaction-based super-resolution imaging. Here, we present the plasmonic nanofocusing at the apex of a silver (Ag)-coated fiber tip with the internal illumination of a radial vector mode (RVM) generated directly in an optical fiber based on an acoustically-induced fiber grating (AIFG). As illustrated by theoretical calculation, a picture of the nanofocusing plasmonic tip given by analyzing the mode conversion process that the surface plasmon polariton (SPP) mode excited via the radial polarization optical mode can propagate to the apex of the plasmonic tip for nanofocusing because it is not cut off as the tip radius decreases; while the SPP mode which transited from the linear polarization optical mode cannot propagate to the tip apex for nanofocusing because it is cut off as the tip radius decreases. The electric field intensity enhancement factor |Eapex2|/|Einput2|$|{\rm{E}}_{{\rm{apex}}}^{\rm{2}}|/|{\rm{E}}_{{\rm{input}}}^{\rm{2}}|$ of a plasmonic tip with a tip radius of 20 nm was calculated to be ~2 × 10 3 . Furthermore, the electric field enhancement characteristic at the tip apex was also experimentally verified by using surface-enhanced Raman spectroscopy (SERS). The Raman scattering intensity was observed to be ~15 times as strong as that with internal illumination using the linear polarization mode (LPM), revealing their significantly different nanofocusing characteristics. A Raman sensitivity of 10 −14 m was achieved for the target analyte of malachite green (MG), denoting significant electric field enhancement and effective plasmonic nanofocusing. The energy conversion efficiency of the radial polarization optical mode to the corresponding SPP mode at the tip apex was measured to be ~17%. This light delivery technique can be potentially further exploited in near-field microscopy with improved resolution and conversion efficiency.

We realized a tunable Brillouin laser and a tunable Raman laser by using ultrahigh-quality-factor ( Q ) hybrid microbottle resonators. The whispering-gallery-mode microresonator-based Brillouin and Raman lasers possess unique advantages (e.g. low threshold, narrow line width, and flexible lasing wavelength region) and various potential applications. Efficient tuning of the Brillouin and Raman microresonator lasers is desirable in many cases. However, the corresponding lasing wavelength tunability is rarely reported. In this work, we realized the ultralow-threshold Brillouin and Raman lasers based on the proposed hybrid microbottle resonators with Q factors over 10 8 . Meanwhile, by feeding the control light through the axial direction of the hybrid microbottle resonators, a Brillouin lasing wavelength tuning range of 2.68 nm and a Raman lasing wavelength tuning range of 2.32 nm are realized, which are one order of magnitude and almost once larger than those reported in the previous works, respectively. Such tunable microlasers could find significant applications in light sources, microwave photonics, and optical sensing.

Surface-enhanced circular dichroism (SECD) of chiral molecules adsorbed on plasmonic nanostructures can substantially enhance chiroptical molecular signals by several orders, which is otherwise very weak to be directly measured. Several mechanisms were proposed to explain this extreme enhancement, but the exact mechanism is still controversial. We investigate strong higher-order multipole contribution to SECD near plasmonic nanostructures using the superposition T-matrix method and discuss how 3-dimensional full-field simulations implementing a homogeneous chiral medium have succeeded in the reconstruction of the extreme enhancement. We also discuss how theoretical studies modeling chiral molecules based on dipole approximation have failed to reconstruct the extreme enhancement and show that SECD enhancement of such chiral dipoles is directly governed by optical chirality enhancement. In addition, strong multipolar transitions in subwavelength chiral plasmonic nanoparticles are discussed based on the T-matrix. This work reviews theoretical frameworks describing chiral molecules, demonstrates significant contribution of a multipolar transition on the extreme SECD enhancement near plasmonic nanostructures, and emphasizes the importance of a multipolar transition in chiral nearfield interaction.