Volume 9, Issue 6

A single photon can be coupled to collective charge oscillations at the interfaces between metals and dielectrics forming a single surface plasmon. The electromagnetic near-fields induced by single surface plasmons offer new degrees of freedom to perform an exquisite control of complex quantum dynamics. Remarkably, the control of quantum systems represents one of the most significant challenges in the field of quantum photonics. Recently, there has been an enormous interest in using plasmonic systems to control multiphoton dynamics in complex photonic circuits. In this review, we discuss recent advances that unveil novel routes to control multiparticle quantum systems composed of multiple photons and plasmons. We describe important properties that characterize optical multiparticle systems such as their statistical quantum fluctuations and correlations. In this regard, we discuss the role that photon-plasmon interactions play in the manipulation of these fundamental properties for multiparticle systems. We also review recent works that show novel platforms to manipulate many-body light-matter interactions. In this spirit, the foundations that will allow nonexperts to understand new perspectives in multiparticle quantum plasmonics are described. First, we discuss the quantum statistical fluctuations of the electromagnetic field as well as the fundamentals of plasmonics and its quantum properties. This discussion is followed by a brief treatment of the dynamics that characterize complex multiparticle interactions. We apply these ideas to describe quantum interactions in photonic-plasmonic multiparticle quantum systems. We summarize the state-of-the-art in quantum devices that rely on plasmonic interactions. The review is concluded with our perspective on the future applications and challenges in this burgeoning field.

Photonic integrated circuits (PICs) have enabled numerous high performance, energy efficient, and compact technologies for optical communications, sensing, and metrology. One of the biggest challenges in scaling PICs comes from the parasitic reflections that feed light back into the laser source. These reflections increase noise and may cause laser destabilization. To avoid parasitic reflections, expensive and bulky optical isolators have been placed between the laser and the rest of the PIC leading to large increases in device footprint for on-chip integration schemes and significant increases in packaging complexity and cost for lasers co-packaged with passive PICs. This review article reports new findings on epitaxial quantum dot lasers on silicon and studies both theoretically and experimentally the connection between the material properties and the ultra-low reflection sensitivity that is achieved. Our results show that such quantum dot lasers on silicon exhibit much lower linewidth enhancement factors than any quantum well lasers. Together with the large damping factor, we show that the quantum dot gain medium is fundamentally dependent on dot uniformity, but through careful optimization, even epitaxial lasers on silicon can operate without an optical isolator, which is of paramount importance for the future high-speed silicon photonic systems.

Lithium niobate (LiNbO 3 ) on insulator (LNOI) is a promising material platform for integrated photonics due to single crystal LiNbO 3 film’s wide transparent window, high refractive index, and high second-order nonlinearity. Based on LNOI, the fast-developing ridge-waveguide fabrication techniques enabled various structures, devices, systems, and applications. We review the basic structures including waveguides, cavities, periodically poled LiNbO 3 , and couplers, along with their fabrication methods and optical properties. Treating those basic structures as building blocks, we review several integrated devices including electro-optic modulators, nonlinear optical devices, and optical frequency combs with each device’s operating mechanism, design principle and methodology, and performance metrics. Starting from these integrated devices, we review how integrated LNOI devices boost the performance of LiNbO 3 ’s traditional applications in optical communications and data center, integrated microwave photonics, and quantum optics. Beyond those traditional applications, we also review integrated LNOI devices’ novel applications in metrology including ranging system and frequency comb spectroscopy. Finally, we envision integrated LNOI photonics’ potential in revolutionizing nonlinear and quantum optics, optical computing and signal processing, and devices in ultraviolet, visible, and mid-infrared regimes. Beyond this outlook, we discuss the challenges in integrated LNOI photonics and the potential solutions.

Subwavelength structures such as subwavelength gratings (SWGs) and subwavelength metamaterials are capable of tailoring the optical properties of materials and controlling the flow of light at the nanoscale. The effective indices of the subwavelength structured strip and slab waveguides can be changed in a wide range by choosing an appropriate duty cycle or a filling factor of silicon, which provides an effective method to manipulate the optical field and achieve effective index matching for functional devices. Recent advances in nanofabrication techniques have made it possible to implement subwavelength structures in silicon strip and slab waveguides. Here we review various approaches used to design subwavelength structures and achieve exotic optical responses and discuss how these structures can be used to realize high-performance silicon photonic devices. Both one-dimensional SWG devices and two-dimensional subwavelength metamaterial devices are covered in this review, including subwavelength structure–based polarization handling devices, mode manipulation devices, and building blocks for integrated optical interconnects. Perspectives on subwavelength structured silicon photonic devices are also discussed.

In this review, we focus on the summary of nonlinear optical microscopies (NOMs), which are stimulated Raman scattering (SRS), coherent anti-Stokes Raman scattering (CARS), second harmonic generation (SHG), and two-photon excited fluorescence (TPEF). The introduction is divided into two parts: the principle of SRS, CARS, TPEF, and SHG and their application to biology and two-dimensional materials. We also introduce the connections and differences between them. We also discuss the principle of plasmon-enhanced NOM and its application in the above two aspects. This paper not only summarizes the research progress in the frontier but also deepens the readers’ understanding of the physical principles of these NOMs.

Lanthanide-based upconversion materials convert low energy infrared photons into high energy visible photons. These materials are of interest in a myriad of applications such as solar energy harvesting, color displays and photocatalysis. Upconversion nanoparticles (UCNPs) are also of interest in biological applications as bioimaging and therapeutic agents. However, the intrinsic conversion efficiency of UCNPs remains low for most applications. In this review, we survey the recent work done in increasing the upconversion emission by changing the local electric field experienced by the UCNPs using photonic nanostructures. We review both the underlying theory behind this photonic manipulation as well as experimental demonstrations of enhancement. We discuss the recent developments in the more common plasmonic designs as well as the emerging field of dielectric nanostructures. We find that improvements in design and fabrication of these nanostructures in the last few years have led to reported enhancements of over three orders of magnitude. This large enhancement has been achieved in not only nanostructures on films but also in nanostructures that can be dispersed into solution which is especially relevant for biological applications.

Three-dimensional structure fabrication using discrete building blocks provides a versatile pathway for the creation of complex nanophotonic devices. The processing of individual components can generally support high-resolution, multiple-material, and variegated structures that are not achievable in a single step using top-down or hybrid methods. In addition, these methods are additive in nature, using minimal reagent quantities and producing little to no material waste. In this article, we review the most promising technologies that build structures using the placement of discrete components, focusing on laser-induced transfer, light-directed assembly, and inkjet printing. We discuss the underlying principles and most recent advances for each technique, as well as existing and future applications. These methods serve as adaptable platforms for the next generation of functional three-dimensional nanophotonic structures.

Mechanically controllable break junctions are one suitable approach to generate atomic point contacts and ultrasmall and controllable gaps between two metal contacts. For constant bias voltages, the tunneling current can be used as a ruler to evaluate the distance between the contacts in the sub-1-nm regime and with sub-Å precision. This ruler can be used to measure the distance between two plasmonic nanostructures located at the designated breaking point of the break junction. In this work, an experimental setup together with suitable nanofabricated break junctions is developed that enables us to perform simultaneous gap-dependent optical and electrical characterization of coupled plasmonic particles, more specifically bowtie antennas in the highly interesting gap range from few nanometers down to zero gap width. The plasmonic break junction experiment is performed in the focus of a confocal microscope. Confocal scanning images and current measurements are simultaneously recorded and exhibit an increased current when the laser is focused in the proximity of the junction. This setup offers a flexible platform for further correlated optoelectronic investigations of coupled antennas or junctions bridged by nanomaterials.

The development of all-dielectric metasurfaces vigorously prompts the applications of optical metasurfaces for the visible and near-IR light range. Compared to IR or longer wavelength light, visible and near-IR light have shorter wavelengths. As a result, surface roughness and imperfections of all-dielectric metasurfaces have larger scattering or absorption of visible and near-IR light, thereby directly affecting the performance of an all-dielectric metasurface. In this article, a volume-current method is adopted to study the effect of metasurface roughness. Numerical calculations based on the finite difference time domain (FDTD) method are also used to study the relationship between the effects of metasurface roughness and the optical resonant modes. Numerical predictions based on our theoretical studies fit the experimental data well. Further, the effect of different roughness levels on the all-dielectric metasurface performance is predicted. More importantly, a method utilizing resonant-mode engineering to enhance the metasurface performance (e.g. incident angle insensitivity) is also proposed and demonstrated. This work deepens our understanding of the working mechanism of all-dielectric metasurfaces and paves the way for their use in a broad spectrum of applications.

Micro-ring resonators made of titanium dioxide were decorated with local light sources comprising CdSe/CdS colloidal quantum dot aggregates. The active micro-resonators are operated to achieve efficient evanescent excitation of nearby co-planar integrated waveguides. Coupled-mode analysis and numerical simulations are used to capture the dynamic of the optical interaction between locally activated resonators and integrated waveguides. In this context, we exemplify the key role of resonator intrinsic loss. Next, we show that locally activated or bus-waveguide excited resonators are in optimum waveguide interaction for the same so-called critical coupling condition, although the physical origin of this property is different for each configuration. More importantly, we found that a locally activated resonator is a fabrication imperfection tolerant configuration for the coupling light of local sources into waveguides. This remarkable property originates from the opposite change of the power cycling into the resonator and the waveguide coupling efficiency as a function of the resonator-waveguide separation gap. By operating an 8-μm-radius ring resonator with loaded quality factors around Q  = 2100, we experimentally demonstrate a 5.5-dB enhancement of the power coupled into the output waveguide compared to a direct local source waveguide excitation.

High-precision placement of rare-earth ions in scalable silicon-based nanostructured materials exhibiting high photoluminescence (PL) emission, photostable and polarized emission, and near-radiative-limited excited state lifetimes can serve as critical building blocks toward the practical implementation of devices in the emerging fields of nanophotonics and quantum photonics. Introduced herein are optical nanostructures composed of arrays of ultrathin silicon carbide (SiC) nanowires (NWs) that constitute scalable one-dimensional NW-based photonic crystal (NW-PC) structures. The latter are based on a novel, fab-friendly, nanofabrication process. The NW arrays are grown in a self-aligned manner through chemical vapor deposition. They exhibit a reduction in defect density as determined by low-temperature time-resolved PL measurements. Additionally, the NW-PC structures enable the positioning of erbium (Er 3+ ) ions with an accuracy of 10 nm, an improvement on the current state-of-the-art ion implantation processes, and allow strong coupling of Er 3+ ions in NW-PC. The NW-PC structure is pivotal in engineering the Er 3+ -induced 1540-nm emission, which is the telecommunication wavelength used in optical fibers. An approximately 60-fold increase in the room-temperature Er 3+ PL emission is observed in NW-PC compared to its thin-film analog in the linear pumping regime. Furthermore, 22 times increase in the Er 3+ PL intensity per number of exited Er ions in NW-PC was observed at saturation while using 20 times lower pumping power. The NW-PC structures demonstrate broadband and efficient excitation characteristics for Er 3+ , with an absorption cross-section (~2 × 10 −18 cm 2 ) two-order larger than typical benchmark values for direct absorption in rare-earth-doped quantum materials. Experimental and simulation results show that the Er 3+ PL is photostable at high pumping power and polarized in NW-PC and is modulated with NW-PC lattice periodicity. The observed characteristics from these technologically friendly nanophotonic structures provide a promising route to the development of scalable nanophotonics and formation of single-photon emitters in the telecom optical wavelength band.

Silicon waveguide grating antennas (SWGAs) have been widely employed to interface the guided and radiation modes in various integrated photonic systems. However, ultrasmall feature sizes or heteromaterial integrations are usually required to obtain long propagation length and small far-field divergence. Moreover, for conventional SWGAs, the diffraction strength is wavelength sensitive, so the output power and far-field divergence will deviate in the beam steering process. In this paper, we propose and demonstrate a novel approach to engineer the diffraction in SWGA by harnessing the bound state in the continuum (BIC). A new degree of freedom is attained in diffraction engineering by introducing the “modified” diffraction formula. The side-wall emission can be dramatically depressed by building the quasi-BIC at critical waveguide width, leading to ultrauniform diffraction. The extremely weak diffraction strength (~3.3 × 10 −3 dB/μm) has been experimentally realized for the fabricated device with a large feature size (~60 nm). From the measurement results, one can predict a centimeter-scale propagation length and an ultrasmall divergence (~0.027°). Moreover, the diffraction strength dispersion can be flattened for SWGA with critical waveguide width. Such effect has also been experimentally verified. Our proposed design is the first one that introduces the BIC effect into SWGA optimization, paving the way for precise diffraction engineering and high-performance integrated optical antennas.

Mode division multiplexing (MDM) technology has been well known to researchers for its ability to increase the link capacity of photonic network. While various mode processing devices were demonstrated in recent years, the reconfigurability of multi-mode processing devices, which is vital for large-scale multi-functional networks, is rarely developed. In this paper, we first propose and experimentally demonstrate a scalable mode-selective converter using asymmetrical micro-racetrack resonators (MRRs) for optical network-on-chip. The proposed device, composed of cascaded MRRs, is able to convert the input monochromatic light to an arbitrary supported mode in the output waveguide as required. Thermo-optical effect of silicon waveguides is adopted to tune the working states of the device. To test the utility, a device for proof-of-concept is fabricated and experimentally demonstrated based on silicon-on-insulator substrate. The measured spectra of the device show that the extinction ratios of MRRs are larger than 18 dB, and modal crosstalk for selected modes are all less than −16.5 dB. The switching time of the fabricated device is in the level of about 40 μs. The proposed device is believed to have potential applications in multi-functional and intelligent network-on-chip, especially in reconfigurable MDM networks.

The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified. Here, by comparing conventional contact- and newly developed tapping-mode PTIR, we show that the PTIR sensitivity to those weakly-scattering, high-Q (up to ≈280) modes is, contrary to a previous hypothesis, unrelated to the probe operation (contact or tapping) and is instead linked to PTIR ability to detect tip-launched dark, volumetrically-confined polaritons, rather than nanostructure-launched HPhPs modes observed by other techniques. Furthermore, we show that in contrast with plasmons and surface phonon-polaritons, whose Q -factors and optical cross-sections are typically degraded by the proximity of other nanostructures, the high- Q HPhP resonances are preserved even in high-density hBN frustum arrays, which is useful in sensing and quantum emission applications.

The advent of planar metalenses, including the super-oscillatory lens (SOL) and the supercritical lens (SCL) with distinctive interference properties, has profoundly impacted on the long-lasting perception of the far-field optical diffraction limit. In spite of its conspicuous success in achieving marvelously small focal spots, the planar metalens still faces tough design and fabrication challenges to realize high focusing efficiency. In this work, we demonstrated a dual-mode laser fabrication technique based on two-photon polymerization for realizing the multilevel phase SCL with focusing efficiency spiking. Synergistically controlling two types of movement trajectory, which is implemented with the piezo stage and the scanning galvo mirror, enables the fabrication of complicated structures with sub-diffraction-limit feature size. By utilizing such advantage, SCLs with discretized multilevel phase configurations are explicitly patterned. The experimental characterization results have shown that a four-level phase SCL can focus light into a sub-diffraction-limit spot with the lateral size of 0.41 λ/NA (NA is the numerical aperture), while achieve the focal spot intensity and the energy concentration ratio in the focal region 7.2 times and 3 times that of the traditional binary amplitude-type SCL with the same optimization conditions, respectively. Our results may release the application obstacles for the sub-diffraction-limit planar metalens and enable major advances in the fields from label-free optical super-resolution imaging to high precision laser fabrication.

A recent study [PRB 100, 075427 (2019)], finally, demonstrated the plasmon-analog of refractive index enhancement in metal nanostructures (MNSs), which has already been studied in atomic clouds for several decades. Here, we simply utilize this phenomenon for achieving continuously-tunable enhanced Cherenkov radiation (CR) in MNSs. Beyond enabling CR from slow-moving particles, or increasing its intensity, the phenomenon can be used in continuous-tuning of the velocity cutoff of particles contributing to CR. More influentially, this allows a continuously-tunable analysis of the contributing particles as if the data is collected from many different detectors, which enables data correction. The phenomenon can also be integrated into lattice MNSs, for continuous medium tuning, where a high density of photonic states is present and the threshold for the CR can even be lifted. Additionally, vanishing absorption can heal radiation angle distortion effects caused by the metallic absorption.

Cherenkov radiation (CR) is the electromagnetic shockwaves generated by the uniform motion of charged particles at a velocity exceeding the phase velocity of light in a given medium. In the Reststrahlen bands of hexagonal boron nitride (hBN), hyperbolic phonon polaritons (HPPs) are generated owing to the coupling between mid-infrared electromagnetic waves and strong anisotropic lattice vibrations. This study theoretically and numerically investigates the generation of volume CR based on HPPs in hBN with super-large wavevectors. Results reveal that CR can be generated using free electrons with an extremely low kinetic energy of 1 eV—two orders of magnitude lower than that reported in extant studies. The findings of this investigation provide new insights into significantly reducing the electron energy required for CR generation and potentially open new research avenues in the fields of CR and HPP.

The spin Hall effect of light, associated with spin-orbit interactions, describes a transport phenomenon with optical spin-dependent splitting, leading to a plethora of applications such as sensing, imaging, and spin-controlled nanophotonics. Although geometric meatsurfaces can mimic photonic spin Hall effect by spatially splitting left-hand circularly polarized and right-hand circularly polarized states of electromagnetic waves with anomalous refraction or reflection angles, the geometric phase generated by metasurfaces hinders metalenses to realize simultaneous focusing of different spin states, limiting further applications. Here, we propose and experimentally demonstrate an approach to realizing a spin Hall metalens that can focus terahertz waves with different spin states and flexibly manipulate spin-dependent focal points in multiple spatial dimensions based on a pure geometric phase. A dielectric metasurface consisting of micropillars with identical shape and different in-plane orientations is designed to realize the multidimensional manipulation of photonics spin Hall effect in terahertz region. Furthermore, helicity-dependent imaging is demonstrated by the terahertz spin Hall metalens. The uniqueness and robust approach for manipulating spin photons may have a significant impact on designing ultra-compact and multifunctional devices and spin photonics devices.

Blue-emissive nanocrystals (NCs) with high photoluminescence quantum yields (PL QYs) and excellent stability are essential for lighting and displays. Here, a facile top-down approach (including two steps: thermal annealing and ultrasonic treatment) by using aluminum acetylacetonate (Al(acac) 3 ) as a precursor is adopted to fabricate blue-emissive Al(acac) 3 NCs with high PL QY reaching 81.8%, the highest reported value for the aluminum compound-based NCs so far. Additionally, the as-fabricated Al(acac) 3 NC solution (in toluene) exhibits high stability under air atmosphere condition, maintaining 61.2% of initial PL QY after 1 year. Furthermore, solution-processed Al(acac) 3 NCs/poly(methyl methacrylate) (PMMA) composite film with blue emission is demonstrated. Finally, combinations of the blue-emitting Al(acac) 3 NCs/PMMA composite film with red-emitting and green-emitting CuInS 2 composite films are realized, resulting in remote ultraviolet-pumped white light-emitting diodes with a high color rendering index of 91. These findings inform new blue-emissive NCs and composite films, potentially paving the way to design new structures of lighting and display devices.

Lanthanide up-conversion features stepwise multi-photon processes, where the difference in photon number that is required for specific up-conversion process usually leads to significant variance in pumping-related processes/properties. In this work, a pumping-controlled dual-mode anti-counterfeiting strategy is conceived by taking advantage of the combination of up-conversion processes with different photon numbers. The combination of Er 3+ and Tm 3+ , which are spatially separated within a designed core/triple-shell nano-architecture, is taken as an example to illustrate such idea. Upon infrared excitation, the emission color of a designed pattern can be switched from red to purple by increasing the excitation power density from 5 to 11 W/cm 2 , while a bright luminescent trajectory including red, white and blue-green color with different length is observed when rotating the pattern above 600 rpm. In addition, the relative up-conversion emission intensities of the Er 3+ and Tm 3+ ions can be manipulated through tailoring interfacial or inner defects in the core/triple-shell nano-crystals, which enable an ultrahigh sensitivity for the pumping-controlled emission color variation to be observed under excitation power well below 11 W/cm 2 .

Graphene, a two-dimensional nanomaterial, possess unique photoelectric properties that have potential application in designing optoelectronic devices. The tunable optical absorption is one of the most exciting properties that can be used to improve the performance of silicon modulators. However, the weak light–matter interaction caused by the size mismatch between the optical mode fields and graphene makes the graphene-on-silicon modulator (GOSM) has large footprint and high energy consumption, limiting the enhancement of modulation efficiency. Here, we propose a broadband GOSM with orthogonal hybrid plasmonic waveguides (HPWs) at near-infrared wavelengths. The orthogonal HPWs are designed to compress the interaction region of optical fields and enhance the light-graphene interaction. The results show that the GOSM has a modulation depth of 26.20 dB/μm, a footprint of 0.33 μm 2 , a 3 dB modulation bandwidth of 462.77 GHz, and energy consumption of 2.82 fJ/bit at 1.55 μm. Even working at a broad wavelength band ranging from 1.3 to 2 μm, the GOSM also has a modulation depth of over 8.58 dB/μm and energy consumption of below 4.97 fJ/bit. It is anticipated that with the excellent modulation performance, this GOSM may have great potential in broadband integrated modulators, on-chip optical communications and interconnects, etc.

In recent years, noble metal-based solar absorbers have been extensively studied as their pronounced plasmonic resonances and high solar-to-thermal conversion efficiency. However, the high cost of noble metals is the unavoidable roadblock restricting the way towards scalability. In this work, we report a nickel-based photothermal absorbers, which is capable of realizing an average solar absorption of ∼97% in the range of 400–2500 nm originating from relatively weaker collective plasmonic resonances but more pronounced single electron excitation. Importantly, it is easily fabricated via the straightforward physical deposition and cost-effective with a raw material price of ∼0.3% gold and ∼20% of silver. We used it for interfacial solar vapor generation and realized an evaporation rate of ∼0.9 kg m −2  h −1 under one sun, almost comparable to the counterparts made from noble metals. The excellent performance combined with the cost effective and scalable fabrication process makes it be a promising candidate for mass off-grid solar desalination.

Photonic and plasmonic refractive index sensors are able to detect increasingly smaller refractive index changes and concentrations of clinically relevant substances. They typically exploit optical resonances and aim to maximise the field overlap with the analyte in order to achieve high sensitivity. Correspondingly, they operate on the basis of maximizing the bulk sensitivity, which favours spatially extended modes. We note that this strategy, counter-intuitively, is not necessarily suitable for detecting biomolecules and one should focus on the surface sensitivity instead. Here, we show that by confining light tightly in metal-insulator-metal (MIM) nanoresonators, the surface sensitivity is significantly increased despite a clear decrease in bulk sensitivity. In particular, we experimentally show the operation of third order MIM resonators which support both extended surface plasmon polariton (SPP) modes and localized MIM modes. We are able to demonstrate that the MIM mode has a sensitivity of 55 nm/RIU to a 10 nm layer, which is approximately twice as high as that of the SPP mode. Overall, our work emphasizes the importance of the surface sensitivity over the more commonly used bulk sensitivity and it shows a novel approach for improving it. These insights are highly relevant for the design of next generation optical biosensors.