Broadband generation of quasi bound-state-in-continuum modes using subwavelength truncated cone resonators.

To allow a high quality factor (Q-factor) to a sub-wavelength dielectric resonator, quasi-bound states in the continuum (Q-BICs) have gained much interest. However, the Q-BIC resonance condition is too sensitive to the geometry of the resonator, and its practical broadband generation on a single-wafer platform has been limited. Here we present that, employing the base angle as a structural degree of freedom, the truncated nano-cone resonator supports the Q-BIC resonance with a high Q-factor of >150 over a wide wavelength range of >100 nm. We expect our approach will boost the utilization of the Q-BIC resonance for various applications requiring broadband spectral tuning.

Optical stimulation of cardiac cells with a polymer-supported silicon nanowire matrix.

Electronic pacemakers can treat electrical conduction disorders in hearts; however, they are invasive, bulky, and linked to increased incidence of infection at the tissue-device interface. Thus, researchers have looked to other more biocompatible methods for cardiac pacing or resynchronization, such as femtosecond infrared light pulsing, optogenetics, and polymer-based cardiac patches integrated with metal electrodes. Here we develop a biocompatible nongenetic approach for the optical modulation of cardiac cells and tissues. We demonstrate that a polymer-silicon nanowire composite mesh can be used to convert fast moving, low-radiance optical inputs into stimulatory signals in target cardiac cells. Our method allows for the stimulation of the cultured cardiomyocytes or ex vivo heart to beat at a higher target frequency.

Polarization Control of Deterministic Single-Photon Emitters in Monolayer WSe2.

Single-photon emitters, the basic building blocks of quantum communication and information, have been developed using atomically thin transition metal dichalcogenides (TMDCs). Although the bandgap of TMDCs was spatially engineered in artificially created defects for single-photon emitters, it remains a challenge to precisely align the emitter's dipole moment to optical cavities for the Purcell enhancement. Here, we demonstrate position- and polarization-controlled single-photon emitters in monolayer WSe2. A tensile strain of ∼0.2% was applied to monolayer WSe2 by placing it onto a dielectric rod structure with a nanosized gap. Excitons were localized in the nanogap sites, resulting in the generation of linearly polarized single-photon emission with a g(2) of ∼0.1 at 4 K. Additionally, we measured the abrupt change in polarization of single photons with respect to the nanogap size. Our robust spatial and polarization control of emission provides an efficient way to demonstrate deterministic and scalable single-photon sources by integrating with nanocavities.

Photon-Triggered Current Generation in Chemically-Synthesized Silicon Nanowires.

A porous Si segment in a Si nanowire (NW), when exposed to light, generates a current with a high on/off ratio. This unique feature has been recently used to demonstrate photon-triggered NW devices including transistors, logic gates, and photodetection systems. Here, we develop a reliable and simple procedure to fabricate porous Si segments in chemically synthesized Si NWs for photon-triggered current generation. To achieve this, we employ 100 nm-diameter chemical-vapor-deposition grown Si NWs that possess an n-type high doping level and extremely smooth surface. The NW regions uncovered by electron-beam resist become selectively porous through metal-assisted chemical etching, using Ag nanoparticles as a catalyst. The contact electrodes are then fabricated on both ends of such NWs, and the generated current is measured when the laser is focused on the porous Si segment. The current level is changed by controlling the power of the incident laser and bias voltage. The on/off ratio is measured up to 1.5 × 104 at a forward bias of 5 V. In addition, we investigate the porous-length-dependent responsivity of the NW device with the porous Si segment. The responsivity is observed to decrease for porous segment lengths beyond 360 nm. Furthermore, we fabricate nine porous Si segments in a single Si NW and measure the identical photon-triggered current from each porous segment; this single NW device can function as a high-resolution photodetection system. Therefore, our fabrication method to precisely control the position and length of the porous Si segments opens up new possibilities for the practical implementation of programmable logic gates and ultrasensitive photodetectors.

Switching of Photonic Crystal Lasers by Graphene.

Unique features of graphene have motivated the development of graphene-integrated photonic devices. In particular, the electrical tunability of graphene loss enables high-speed modulation of light and tuning of cavity resonances in graphene-integrated waveguides and cavities. However, efficient control of light emission such as lasing, using graphene, remains a challenge. In this work, we demonstrate on/off switching of single- and double-cavity photonic crystal lasers by electrical gating of a monolayer graphene sheet on top of photonic crystal cavities. The optical loss of graphene was controlled by varying the gate voltage Vg, with the ion gel atop the graphene sheet. First, the fundamental properties of graphene were investigated through the transmittance measurement and numerical simulations. Next, optically pumped lasing was demonstrated for a graphene-integrated single photonic crystal cavity at Vg below -0.6 V, exhibiting a low lasing threshold of ∼480 µW, whereas lasing was not observed at Vg above -0.6 V owing to the intrinsic optical loss of graphene. Changing quality factor of the graphene-integrated photonic crystal cavity enables or disables the lasing operation. Moreover, in the double-cavity photonic crystal lasers with graphene, switching of individual cavities with separate graphene sheets was achieved, and these two lasing actions were controlled independently despite the close distance of ∼2.2 µm between adjacent cavities. We believe that our simple and practical approach for switching in graphene-integrated active photonic devices will pave the way toward designing high-contrast and ultracompact photonic integrated circuits.

Enhancement of Light Absorption in Silicon Nanowire Photovoltaic Devices with Dielectric and Metallic Grating Structures.

We report the enhancement of light absorption in Si nanowire photovoltaic devices with one-dimensional dielectric or metallic gratings that are fabricated by a damage-free, precisely aligning, polymer-assisted transfer method. Incorporation of a Si3N4 grating with a Si nanowire effectively enhances the photocurrents for transverse-electric polarized light. The wavelength at which a maximum photocurrent is generated is readily tuned by adjusting the grating pitch. Moreover, the electrical properties of the nanowire devices are preserved before and after transferring the Si3N4 gratings onto Si nanowires, ensuring that the quality of pristine nanowires is not degraded during the transfer. Furthermore, we demonstrate Si nanowire photovoltaic devices with Ag gratings using the same transfer method. Measurements on the fabricated devices reveal approximately 27.1% enhancement in light absorption compared to that of the same devices without the Ag gratings without any degradation of electrical properties. We believe that our polymer-assisted transfer method is not limited to the fabrication of grating-incorporated nanowire photovoltaic devices but can also be generically applied for the implementation of complex nanoscale structures toward the development of multifunctional optoelectronic devices.

Resonant magneto-optic Kerr effects of a single Ni nanorod in the Mie scattering regime.

We present a systematic, theoretical investigation of the polar magneto-optical (MO) Kerr effects of a single Ni nanorod in the Mie regime. The MO Kerr rotation, ellipticity, amplitude ratio, and phase shift are calculated as a function of the length and width of the nanorod. The electric field amplitude ratio of the MO Kerr effect is locally maximized when the nanorod supports a plasmonic resonance in the polarization state orthogonal to the incident light. The plasmonic resonances directly induced by the incident light do not enhance the amplitude ratio. In the Mie regime, multiple local maxima of the MO Kerr activity are supported by the resonant modes with different modal characteristics. From the viewpoint of first-order perturbation analysis, the spatial overlap between the incident-light-induced electric field and the Green function determines the local maxima.

Full three-dimensional power flow analysis of single-emitter-plasmonic-nanoantenna system.

We present a full three-dimensional (3D) power flow analysis of an emitter-nanoantenna system. A conventional analysis, based on the total Poynting vector, calculates only the coupling strength in terms of the Purcell enhancement. For a better understanding of the emitter-nanoantenna system, not only the Purcell enhancement but also complete information on the energy transfer channels is necessary. The separation of the pure scattering and emitter output Poynting vectors enables the quantification of the individual energy transfer channels. Employing the finite-difference time-domain method (FDTD), we examine a nanodisk antenna that supports the bright dipole and dark quadrupole resonance modes for which the power flow characteristics are completely distinct, and we analyze the power flow enhancements to the energy transfer channels with respect to the wavelength, polarization, and position of the emitter coupled to the antenna. The 3D power flow analysis reveals how the constructive or destructive interference between the emitter and the antenna resonance mode affects the power flow enhancements and the far-field radiation pattern. Our proposed power flow analysis should play a critical role in characterizing the emitter-antenna system and customizing its energy transfer properties for desired applications.

A double-strip plasmonic waveguide coupled to an electrically driven nanowire LED.

We demonstrate the efficient integration of an electrically driven nanowire (NW) light source with a double-strip plasmonic waveguide. A top-down-fabricated GaAs NW light-emitting diode (LED) is placed between two straight gold strip waveguides with the gap distance decreasing to 30 nm at the end of the waveguide and operated by current injection through the p-contact electrode acting as a plasmonic waveguide. Measurements of polarization-resolved images and spectra show that the light emission from the NW LED was coupled to a plasmonic waveguide mode, propagated through the waveguide, and was focused onto a subwavelength-sized spot of surface plasmon polaritons at the tapered end of the waveguide. Numerical simulation agreed well with these experimental results, confirming that a symmetric plasmonic waveguide mode was excited on the top surface of the waveguide. Our demonstration of a plasmonic waveguide coupled to an electrically driven NW LED represents important progress toward further miniaturization and practical implementation of ultracompact photonic integrated circuits.

Spin angular momentum-encoded single-photon emitters in a chiral nanoparticle-coupled WSe2 monolayer.

Spin angular momentum (SAM)-encoded single-photon emitters, also known as circularly polarized single photons, are basic building blocks for the advancement of chiral quantum optics and cryptography. Despite substantial efforts such as coupling quantum emitters to grating-like optical metasurfaces and applying intense magnetic fields, it remains challenging to generate circularly polarized single photons from a subwavelength-scale nanostructure in the absence of a magnetic field. Here, we demonstrate single-photon emitters encoded with SAM in a strained WSe2 monolayer coupled with chiral plasmonic gold nanoparticles. Single-photon emissions were observed at the nanoparticle position, exhibiting photon antibunching behavior with a g(2)(0) value of ~0.3 and circular polarization properties with a slight preference for left-circular polarization. Specifically, the measured Stokes parameters confirmed strong circular polarization characteristics, in contrast to emitters coupled with achiral gold nanocubes. Therefore, this work provides potential insights to make SAM-encoded single-photon emitters and understand the interaction of plasmonic dipoles and single photons, facilitating the development of chiral quantum optics.

Design of plasmonic nano-antenna for total internal reflection fluorescence microscopy.

We propose a gold modified bow-tie plasmonic nano-antenna, which can be suitably used in combination with total internal reflection fluorescence microscopy. The plasmonic nano-antenna, supporting well-separated multiple resonances, not only concentrates the total internal reflection evanescent field at the deep subwavelength scale, but also enhances fluorescence emission by the Purcell effect. Finite-difference time-domain computations show that the enhancement of the excitation light strongly correlates with the far-field radiation pattern radiated from the antenna. Depending on the antenna geometry, the resonant modes are widely tuned and their wavelengths can be easily matched to the diverse emission or excitation wavelengths of fluorophores.

Strain Engineering of Low-Dimensional Materials for Emerging Quantum Phenomena and Functionalities.

Recent discoveries of exotic physical phenomena, such as unconventional superconductivity in magic-angle twisted bilayer graphene, dissipationless Dirac fermions in topological insulators, and quantum spin liquids, have triggered tremendous interest in quantum materials. The macroscopic revelation of quantum mechanical effects in quantum materials is associated with strong electron-electron correlations in the lattice, particularly where materials have reduced dimensionality. Owing to the strong correlations and confined geometry, altering atomic spacing and crystal symmetry via strain has emerged as an effective and versatile pathway for perturbing the subtle equilibrium of quantum states. This review highlights recent advances in strain-tunable quantum phenomena and functionalities, with particular focus on low-dimensional quantum materials. Experimental strategies for strain engineering are first discussed in terms of heterogeneity and elastic reconfigurability of strain distribution. The nontrivial quantum properties of several strain-quantum coupled platforms, including 2D van der Waals materials and heterostructures, topological insulators, superconducting oxides, and metal halide perovskites, are next outlined, with current challenges and future opportunities in quantum straintronics followed. Overall, strain engineering of quantum phenomena and functionalities is a rich field for fundamental research of many-body interactions and holds substantial promise for next-generation electronics capable of ultrafast, dissipationless, and secure information processing and communications.

Nanobeam photonic bandedge lasers.

We demonstrate one-dimensional nanobeam photonic bandedge lasers with InGaAsP quantum wells at room temperature from the lowest dielectric band of photonic crystal nanobeam waveguides. The incident optical power at threshold is 0.6 mW (effectively ~18 µW). To confirm the lasing from the dielectric bandedge, the polarization and the photoluminescent spectra are taken from nanobeams of varying lattice constants. The observed shift of the lasing wavelength agrees well with the computational prediction.

Ultralow-threshold laser using super-bound states in the continuum.

Wavelength-scale lasers provide promising applications through low power consumption requiring for optical cavities with increased quality factors. Cavity radiative losses can be suppressed strongly in the regime of optical bound states in the continuum; however, a finite size of the resonator limits the performance of bound states in the continuum as cavity modes for active nanophotonic devices. Here, we employ the concept of a supercavity mode created by merging symmetry-protected and accidental bound states in the continuum in the momentum space, and realize an efficient laser based on a finite-size cavity with a small footprint. We trace the evolution of lasing properties before and after the merging point by varying the lattice spacing, and we reveal this laser demonstrates the significantly reduced threshold, substantially increased quality factor, and shrunken far-field images. Our results provide a route for nanolasers with reduced out-of-plane losses in finite-size active nanodevices and improved lasing characteristics.

Electrically driven strain-induced deterministic single-photon emitters in a van der Waals heterostructure.

Quantum confinement in transition metal dichalcogenides (TMDCs) enables the realization of deterministic single-photon emitters. The position and polarization control of single photons have been achieved via local strain engineering using nanostructures. However, most existing TMDC-based emitters are operated by optical pumping, while the emission sites in electrically pumped emitters are uncontrolled. Here, we demonstrate electrically driven single-photon emitters located at the positions where strains are induced by atomic force microscope indentation on a van der Waals heterostructure consisting of graphene, hexagonal boron nitride, and tungsten diselenide. The optical, electrical, and mechanical properties induced by the local strain gradient were systematically analyzed. The emission at the indentation sites exhibits photon antibunching behavior with a g(2)(0) value of ~0.3, intensity saturation, and a linearly cross-polarized doublet, at 4 kelvin. This robust spatial control of electrically driven single-photon emitters will pave the way for the practical implementation of integrated quantum light sources.

Recent advances in nanocavities and their applications.

High quality factor and small mode volume in nanocavities enable the demonstration of efficient nanophotonic devices with low power consumption, strong nonlinearity, and high modulation speed, due to the strong light-matter interaction. In this review, we focus on recent state-of-the-art nanocavities and their applications. We introduce single nanocavities including semiconductor nanowires, plasmonic cavities, and nanostructures based on quasi-bound states in the continuum (quasi-BIC), for laser, photovoltaic, and nonlinear applications. In addition, nanocavity arrays with unique feedback mechanisms, including BIC cavities, parity-time symmetry coupled cavities, and photonic topological cavities, are introduced for laser applications. These various cavity designs and underlying physics in single and array nanocavities are useful for the practical implementation of promising nanophotonic devices.

Integration of Single-Photon Emitters in 2D Materials with Plasmonic Waveguides at Room Temperature.

Efficient integration of a single-photon emitter with an optical waveguide is essential for quantum integrated circuits. In this study, we integrated a single-photon emitter in a hexagonal boron nitride (h-BN) flake with a Ag plasmonic waveguide and measured its optical properties at room temperature. First, we performed numerical simulations to calculate the efficiency of light coupling from the emitter to the Ag plasmonic waveguide, depending on the position and polarization of the emitter. In the experiment, we placed a Ag nanowire, which acted as the plasmonic waveguide, near the defect of the h-BN, which acted as the single-photon emitter. The position and direction of the nanowire were precisely controlled using a stamping method. Our time-resolved photoluminescence measurement showed that the single-photon emission from the h-BN flake was enhanced to almost twice the intensity as a result of the coupling with the Ag nanowire. We expect these results to pave the way for the practical implementation of on-chip nanoscale quantum plasmonic integrated circuits.

Recent Progress in Nanolaser Technology.

Nanolasers are key elements in the implementation of optical integrated circuits owing to their low lasing thresholds, high energy efficiencies, and high modulation speeds. With the development of semiconductor wafer growth and nanofabrication techniques, various types of wavelength-scale and subwavelength-scale nanolasers have been proposed. For example, photonic crystal lasers and plasmonic lasers based on the feedback mechanisms of the photonic bandgap and surface plasmon polaritons, respectively, have been successfully demonstrated. More recently, nanolasers employing new mechanisms of light confinement, including parity-time symmetry lasers, photonic topological insulator lasers, and bound states in the continuum lasers, have been developed. Here, the operational mechanisms, optical characterizations, and practical applications of these nanolasers based on recent research results are outlined. Their scientific and engineering challenges are also discussed.

Multipolar lasing modes from topological corner states.

Topological photonics provides a fundamental framework for robust manipulation of light, including directional transport and localization with built-in immunity to disorder. Combined with an optical gain, active topological cavities hold special promise for a design of light-emitting devices. Most studies to date have focused on lasing at topological edges of finite systems or domain walls. Recently discovered higher-order topological phases enable strong high-quality confinement of light at the corners. Here, we demonstrate lasing action of corner states in nanophotonic topological structures. We identify several multipole corner modes with distinct emission profiles via hyperspectral imaging and discern signatures of non-Hermitian radiative coupling of leaky topological states. In addition, depending on the pump position in a large-size cavity, we generate selectively lasing from either edge or corner states within the topological bandgap. Our studies provide the direct observation of multipolar lasing and engineered collective resonances in active topological nanostructures.

Near-field sub-diffraction photolithography with an elastomeric photomask.

Photolithography is the prevalent microfabrication technology. It needs to meet resolution and yield demands at a cost that makes it economically viable. However, conventional far-field photolithography has reached the diffraction limit, which imposes complex optics and short-wavelength beam source to achieve high resolution at the expense of cost efficiency. Here, we present a cost-effective near-field optical printing approach that uses metal patterns embedded in a flexible elastomer photomask with mechanical robustness. This technique generates sub-diffraction patterns that are smaller than 1/10th of the wavelength of the incoming light. It can be integrated into existing hardware and standard mercury lamp, and used for a variety of surfaces, such as curved, rough and defect surfaces. This method offers a higher resolution than common light-based printing systems, while enabling parallel-writing. We anticipate that it will be widely used in academic and industrial productions.