Showing papers in "ACS Photonics in 2018"

Training Deep Neural Networks for the Inverse Design of Nanophotonic Structures

Abstract: Data inconsistency leads to a slow training process when deep neural networks are used for the inverse design of photonic devices, an issue that arises from the fundamental property of nonuniqueness in all inverse scattering problems. Here we show that by combining forward modeling and inverse design in a tandem architecture, one can overcome this fundamental issue, allowing deep neural networks to be effectively trained by data sets that contain nonunique electromagnetic scattering instances. This paves the way for using deep neural networks to design complex photonic structures that require large training data sets.

Radiative Heat Transfer

Abstract: Thermal radiation is one of the most universal physical phenomena, and its study has played a key role in the history of modern physics. Our understanding of this subject has been traditionally bas...

Novel Nanostructures and Materials for Strong Light–Matter Interactions

Abstract: Quantum mechanical interactions between electromagnetic radiation and matter underlie a broad spectrum of optical phenomena. Strong light-matter interactions result in the well-known vacuum Rabi splitting and emergence of new polaritonic eigenmodes of the coupled system. Thanks to recent progress in nanofabrication, observation of strong coupling has become possible in a great variety of optical nanostructures. Here, we review recently studied and emerging materials for realization of strong light–matter interactions. We present general theoretical formalism describing strong coupling and give an overview of various photonic structures and materials allowing for realization of this regime, including plasmonic and dielectric nanoantennas, novel two-dimensional materials, carbon nanotubes, and molecular vibrational transitions. In addition, we discuss practical applications that can benefit from these effects and give an outlook on unsettled questions that remain open for future research.

Polaritonic Chemistry with Organic Molecules

Abstract: We present an overview of the general concepts of polaritonic chemistry with organic molecules, i.e., the manipulation of chemical structure that can be achieved through strong coupling between confined light modes and organic molecules. Strong coupling and the associated formation of polaritons, hybrid light–matter excitations, lead to energy shifts in such systems that can amount to a large fraction of the uncoupled transition energy. This has recently been shown to significantly alter the chemical structure of the coupled molecules, which opens the possibility to manipulate and control reactions. We discuss the current state of theory for describing these changes and present several applications, with a particular focus on the collective effects observed when many molecules are involved in strong coupling.

Cs2AgInCl6 Double Perovskite Single Crystals: Parity Forbidden Transitions and Their Application For Sensitive and Fast UV Photodetectors

Abstract: Double perovskite Cs2AgInCl6 is newly reported as a stable and environmentally friendly alternative to lead halide perovskites. However, the fundamental properties of this material remain unexplored. Here, we first produced high-quality Cs2AgInCl6 single crystals (SCs) with a low trap density of 8.6 × 108 cm–3, even lower than the value reported in the best lead halide perovskite SCs. Through systematical optical and electronic characterization, we experimentally verified the existence of the proposed parity-forbidden transition in Cs2AgInCl6 and identified the role of oxygen in controlling its optical properties. Furthermore, sensitive (dectivity of ∼1012 Jones), fast (3 dB bandwidth of 1035 Hz), and stable UV photodetectors were fabricated based on our Cs2AgInCl6 SCs, showcasing their advantages for optoelectronic applications.

Dynamic Beam Switching by Liquid Crystal Tunable Dielectric Metasurfaces

Abstract: Dynamic steering of laser beams by ultrathin optical metasurfaces is a significant research advance for possible applications in remote ranging and sensing. A unique platform for such important functionalities is offered by dielectric metasurfaces that have the highest transmission efficiency. However, the realization of dynamically tunable metasurfaces still remains a challenge. Here we experimentally demonstrate the dynamic switching of beam deflection by a silicon-nanodisk dielectric metasurface infiltrated with liquid crystals. In particular, we show the switching of a laser beam from 0° to a 12° angle with an efficiency of 50% by heating the metasurface to modify the liquid crystal state from nematic to isotropic. Our results open important opportunities for tunable ultrathin beam steering metadevices.

Ultrahigh Performance of Self-Powered β-Ga2O3 Thin Film Solar-Blind Photodetector Grown on Cost-Effective Si Substrate Using High-Temperature Seed Layer

Abstract: We demonstrated an ultrahigh-performance and self-powered β-Ga2O3 thin film solar-blind photodetector fabricated on a cost-effective Si substrate using a high-temperature seed layer (HSL). The polycrystalline β-Ga2O3 thin film deposited with HSL shows high performance in the solar-blind region in comparison to the amorphous Ga2O3 thin film deposited without HSL. The zero-bias digitizing sensor prototype with an HSL produces a digitized output bit with deep UV (DUV) light that exhibits a high on/off (I254 nm/Idark) ratio of >103, a record-low dark current of 1.43 pA, and high stability and reproducibility over 100 cycles even after >2100 h. The photodetector shows minimum persistent photoconductivity and fast response in milliseconds. The photodetector yields a responsivity of 96.13 A W–1 with an external quantum efficiency of 4.76 × 104 at 5 V for 250 nm monochromatic light. The photodetector shows a high response to even a rare weak signal of DUV (44 nW/cm2). These values are the highest reported to date...

Effective Radiative Cooling by Paint-Format Microsphere-Based Photonic Random Media

Abstract: We demonstrate that photonic media, when properly randomized to minimize the photon transport mean free path, can be used to coat a black substrate and reduce its temperature by radiative cooling Even under strong solar radiation, the substrate temperature could reach substantially below that of the ambient air Our random media that consist of silica microspheres considerably outperform commercially available solar-reflective white paint for daytime cooling We have achieved the outstanding cooling performance through a systematic study on light scattering, which reveals that the structural parameters of the random media for maximum scattering are significantly different from those of the commercial paint We have created the random media to maximize optical scattering in the solar spectrum and to enhance thermal emission in the atmospheric transparency window In contrast to previous studies, our random media do not require expensive processing steps or materials, such as silver, and can be applied to

Ultrasmall Bismuth Quantum Dots: Facile Liquid-Phase Exfoliation, Characterization, and Application in High-Performance UV–Vis Photodetector

Abstract: Two-dimensional (2D) monoelemental bismuth (Bi) crystal, one of the pnictogens (group VA), has recently attracted increasing interest because of its intriguing characteristics. Here, uniformly sized 2D Bi quantum dots (BiQDs) with an average diameter (thickness) of 4.9 ± 1.0 nm (2.6 ± 0.7 nm) were fabricated through a facile liquid-phase exfoliation (LPE) method, and the corresponding photoresponse was evaluated using photoelectrochemical (PEC) measurements. The as-fabricated BiQDs-based photodetector not only exhibits an appropriate capacity for self-driven broadband photoresponse but also shows high-performance photoresponse under low bias potentials ranging from UV to visible light in association with long-term stability of the ON/OFF switching behavior. In terms of these findings, it is further anticipated that the resultant BiQDs possess promising potential in UV–visible photodetection as well as in liquid optoelectronics. Our work may open a new avenue for delivering high-quality monoelemental pnict...

Highly Broadband Absorber Using Plasmonic Titanium Carbide (MXene)

Abstract: Control of light transmission and reflection through nanostructured materials has led to demonstration of metamaterial absorbers that have augmented the performance of energy harvesting applications of several optoelectronic and nanophotonic systems. Here, for the first time, a broadband plasmonic metamaterial absorber is fabricated using two-dimensional titanium carbide (Ti3C2Tx) MXene. Arrays of nanodisks made of Ti3C2Tx exhibit strong localized surface plasmon resonances at near-infrared frequencies. By exploiting the scattering enhancement at the resonances and the optical losses inherent to Ti3C2Tx MXene, high-efficiency absorption (∼90%) for a wide wavelength window of incident illumination (∼1.55 μm) has been achieved.

Color-Tunable Carbon Dots Possessing Solid-State Emission for Full-Color Light-Emitting Diodes Applications

Abstract: In this paper, a unique strategy is proposed to modulate fluorescence color of carbon dots (CDs) under both aqueous solution and solid state for bright multicolor light-emitting diodes (LEDs) applications. We report facile synthesis of dual-peak-emissive CDs with self-quenching-resistant character under solid state through hydrothermal method, and investigate the origins of dual-peak emission for the first time. In addition, based on the unique dual-peak-emissive phenomenon, acid-mediated PL behaviors were realized by changing the pH value of hydrothermal precursors. We realized the color-tunable fluorescence under aqueous solution from blue (B-CDs) to yellow-green (YG-CDs) color and solid state from yellow to orange-red color. The changed PL behaviors are attributed to the more conjugated structure inside CDs due to elevated carbonization degree. Furthermore, the red-shifted fluorescence from aqueous solution to solid state is ascribed to supramolecular cross-linking between adjacent particles. Thus, by ...

Enhanced Second-Harmonic Generation Using Broken Symmetry III-V Semiconductor Fano Metasurfaces

Abstract: All-dielectric metasurfaces, two-dimensional arrays of subwavelength low loss dielectric inclusions, can be used not only to control the amplitude and phase of optical beams, but also to generate new wavelengths through enhanced nonlinear optical processes that are free from some of the constraints dictated by the use of bulk materials. Recently, high quality factor (Q) resonances in these metasurfaces have been revealed and utilized for applications such as sensing and lasing. The origin of these resonances stems from the interference of two nanoresonator modes with vastly different Q. Here we show that nonlinear optical processes can be further enhanced by utilizing these high-Q resonances in broken symmetry all-dielectric metasurfaces. We study second harmonic generation from broken symmetry metasurfaces made from III–V semiconductors and observe nontrivial spectral shaping of second-harmonic and multifold efficiency enhancement induced by high field localization and enhancement inside the nanoresonators.

Adjoint Method and Inverse Design for Nonlinear Nanophotonic Devices

Abstract: The development of inverse design, where computational optimization techniques are used to design devices based on certain specifications, has led to the discovery of many compact, nonintuitive structures with superior performance. Among various methods, large-scale, gradient-based optimization techniques have been one of the most important ways to design a structure containing a vast number of degrees of freedom. These techniques are made possible by the adjoint method, in which the gradient of an objective function with respect to all design degrees of freedom can be computed using only two full-field simulations. However, this approach has so far mostly been applied to linear photonic devices. Here, we present an extension of this method to modeling nonlinear devices in the frequency domain, with the nonlinear response directly included in the gradient computation. As illustrations, we use the method to devise compact photonic switches in a Kerr nonlinear material, in which low-power and high-power pul...

High Responsivity β-Ga2O3 Metal–Semiconductor–Metal Solar-Blind Photodetectors with Ultraviolet Transparent Graphene Electrodes

Abstract: We demonstrated high responsivity metal–semiconductor–metal (MSM) solar-blind photodetectors by integrating exfoliated β-Ga2O3 microlayers with graphene, which is a deep ultraviolet (UV) transparent and conductive electrode. Photodetectors with MSM structures commonly suffer from low responsivity, although they feature a facile fabrication process, low dark current, and fast response speed. The β-Ga2O3 MSM solar-blind photodetectors with graphene electrodes exhibited excellent operating characteristics including higher responsivity (∼29.8 A/W), photo-to-dark current ratio (∼1 × 106%), rejection ratio (R254nm/R365nm, ∼9.4 × 103), detectivity (∼1 × 1012 Jones), and operating speed to UV-C wavelengths, compared with MSM photodetectors with conventional metal electrodes. Absence of shading by the integration of graphene with β-Ga2O3 allows maximum exposure to the incident photons, suggesting a great potential for deep UV optoelectronic applications.

Strong Light–Matter Interaction in Quantum Emitter/Metal Hybrid Nanostructures

Abstract: Surface plasmon polaritons (SPPs) are spatially confined electromagnetic field modes at a metal-dielectric interface capable of generating intense near-field optical forces on ultrafast time scales. Within the field of photonics, SPPs carry significant potential for guiding and manipulating light on the nanoscale. The intense SPP fields substantially enhance light–matter interactions with quantum emitters (QEs). Thus, hybrid systems comprised of SPP resonators and various types of QEs constitute key components of the modern photonics applications. Recent advances in nanotechnology have enabled fabrication of high quality QE/metal hybrid nanostructures, in which several aspects of light–matter interactions, including those in the quantum regime have been demonstrated and extensively studied. The present Perspective explores the central phenomenon in light–matter interaction in emitter/metal hybrid nanostructures, namely, the strong dipole coupling between QEs and SPPs, particularly between excitons (Xs) an...

Full Stokes imaging polarimetry using dielectric metasurfaces

Abstract: Polarization is a degree of freedom of light carrying important information that is usually absent in intensity and spectral content. Imaging polarimetry is the process of determining the polarization state of light, either partially or fully, over an extended scene. It has found several applications in various fields, from remote sensing to biology. Among different devices for imaging polarimetry, division of focal plane polarization cameras (DoFP-PCs) are more compact, less complicated, and less expensive. In general, DoFP-PCs are based on an array of polarization filters in the focal plane. Here we demonstrate a new principle and design for DoFP-PCs based on dielectric metasurfaces with the ability to control polarization and phase. Instead of polarization filtering, the method is based on splitting and focusing light in three different polarization bases. Therefore, it enables full Stokes characterization of the state of polarization and overcomes the 50% theoretical efficiency limit of the polarizati...

Plasmonic Cavity Coupling

Abstract: The large losses of plasmonic nanocavities, orders of magnitude beyond those of photonic dielectric cavities, places them, perhaps surprisingly, as exceptional enhancers of single emitter light–matter interactions. The ultraconfined, sub-diffraction-limited mode volumes of plasmonic systems offer huge coupling strengths (in the 1–100 meV range) to single quantum emitters. Such strengths far outshine the coupling strengths of dielectric microcavities, which nonetheless easily achieve single emitter “strong coupling” due to the low loss rates of dielectric cavities. In fact, it is the much higher loss rate of plasmonic cavities that make them desirable for applications requiring bright, fast-emitting photon sources. Here we provide a simple method to reformulate lifetime measurements of single emitters in terms of coupling strengths to allow a useful comparison of the literature of plasmonic cavities with that of cavity-QED, typically more closely associated with dielectric cavities. Using this approach, we...

Electrically tunable slow light using graphene metamaterials

Abstract: Metamaterials with classical analogues of electromagnetically induced transparency open new avenues in photonics for realizing smaller, more efficient slow light devices without quantum approaches. However, most of the metamaterial-based slow light devices are passive, which limits their practical applications. Here, by combining diatomic metamaterials with a gated single-layer graphene, we demonstrate that the group delay of terahertz light can be dynamically controlled under a small gate voltage. Using a two coupled harmonic oscillators model, we show that this active control of group delay is made possible by an effective control of the dissipative loss of the radiative dark resonator by varying the graphene’s optical conductivity. Our work may provide opportunities in the design of various applications such as compact slow light devices and ultrasensitive sensors and switches.

DNA Origami Route for Nanophotonics

Abstract: The specificity and simplicity of the Watson–Crick base pair interactions make DNA one of the most versatile construction materials for creating nanoscale structures and devices. Among several DNA-based approaches, the DNA origami technique excels in programmable self-assembly of complex, arbitrary shaped structures with dimensions of hundreds of nanometers. Importantly, DNA origami can be used as templates for assembly of functional nanoscale components into three-dimensional structures with high precision and controlled stoichiometry. This is often beyond the reach of other nanofabrication techniques. In this Perspective, we highlight the capability of the DNA origami technique for realization of novel nanophotonic systems. First, we introduce the basic principles of designing and fabrication of DNA origami structures. Subsequently, we review recent advances of the DNA origami applications in nanoplasmonics, single-molecule and super-resolution fluorescent imaging, as well as hybrid photonic systems. We...

Color Centers in Hexagonal Boron Nitride Monolayers: A Group Theory and Ab Initio Analysis

Abstract: We theoretically study physical properties of the most promising color center candidates for the recently observed single-photon emissions in hexagonal boron nitride (h-BN) monolayers. Through our group theory analysis combined with density functional theory (DFT) calculations we provide several pieces of evidence that the electronic properties of the color centers match the characters of the experimentally observed emitters. We calculate the symmetry-adapted multielectron wave functions of the defects using group theory methods and analyze the spin–orbit and spin–spin interactions in detail. We also identify the radiative and nonradiative transition channels for each color center. An advanced ab initio DFT method is then used to compute energy levels of the color centers and their zero-phonon-line (ZPL) emissions. The computed ZPLs, the profile of excitation and emission dipole polarizations, and the competing relaxation processes are discussed and matched with the observed emission lines. By providing e...

Inverse design and demonstration of a compact on-chip narrowband three-channel wavelength demultiplexer

Abstract: In wavelength division multiplexing schemes, splitters must be used to combine and separate different wavelengths. Conventional splitters are fairly large with footprints in hundreds to thousands of square microns, and experimentally demonstrated multimode-interference-based and inverse-designed ultracompact splitters operate with only two channels and large channel spacing (>100 nm). Here we inverse design and experimentally demonstrate a three-channel wavelength demultiplexer with 40 nm spacing (1500, 1540, and 1580 nm) with a footprint of 24.75 μm2. The splitter has a simulated peak insertion loss of −1.55 dB with under −15 dB crosstalk and a measured peak insertion loss of −2.29 dB with under −10.7 dB crosstalk.

Si-Based GeSn Lasers with Wavelength Coverage of 2–3 μm and Operating Temperatures up to 180 K

Abstract: A Si-based monolithic laser is strongly desired for the full integration of Si-photonics. Lasing from the direct bandgap group-IV GeSn alloy has opened a new avenue, different from the hybrid III–V-on-Si integration approach. We demonstrated optically pumped GeSn lasers on Si with broad wavelength coverage from 2 to 3 μm. The GeSn alloys were grown using newly developed approaches with an industry standard chemical vapor deposition reactor and low-cost commercially available precursors. The achieved maximum Sn composition of 17.5% exceeded the generally acknowledged Sn incorporation limits found with similar deposition chemistries. The highest lasing temperature was measured as 180 K with the active layer thickness as thin as 260 nm. The unprecedented lasing performance is mainly due to the unique growth approaches, which offer high-quality epitaxial materials. The results reported in this work show a major advance toward Si-based mid-infrared laser sources for integrated photonics.

Suppressed Quenching and Strong-Coupling of Purcell-Enhanced Single-Molecule Emission in Plasmonic Nanocavities

Abstract: An emitter in the vicinity of a metal nanostructure is quenched by its decay through nonradiative channels, leading to the belief in a zone of inactivity for emitters placed within <10 nm of a plasmonic nanostructure. Here we demonstrate and explain why in tightly coupled plasmonic resonators forming nanocavities “quenching is quenched” due to plasmon mixing. Unlike isolated nanoparticles, such plasmonic nanocavities show mode hybridization, which can massively enhance emitter excitation and decay via radiative channels, here experimentally confirmed by laterally dependent emitter placement through DNA-origami. We explain why this enhancement of excitation and radiative decay can be strong enough to facilitate single-molecule strong coupling, as evident in dynamic Rabi-oscillations.

Deep Learning Enhanced Mobile-Phone Microscopy

Abstract: Mobile phones have facilitated the creation of field-portable, cost-effective imaging and sensing technologies that approach laboratory-grade instrument performance. However, the optical imaging interfaces of mobile phones are not designed for microscopy and produce distortions in imaging microscopic specimens. Here, we report on the use of deep learning to correct such distortions introduced by mobile-phone-based microscopes, facilitating the production of high-resolution, denoised, and color-corrected images, matching the performance of benchtop microscopes with high-end objective lenses, also extending their limited depth of field. After training a convolutional neural network, we successfully imaged various samples, including human tissue sections and Papanicolaou and blood smears, where the recorded images were highly compressed to ease storage and transmission. This method is applicable to other low-cost, aberrated imaging systems and could offer alternatives for costly and bulky microscopes, while ...

100 GHz Plasmonic Photodetector

Abstract: Photodetectors compatible with CMOS technology have shown great potential in implementing active silicon photonics circuits, yet current technologies are facing fundamental bandwidth limitations. Here, we propose and experimentally demonstrate for the first time a plasmonic photodetector achieving simultaneously record-high bandwidth beyond 100 GHz, an internal quantum efficiency of 36% and low footprint. High-speed data reception at 72 Gbit/s is demonstrated. Such superior performance is attributed to the subwavelength confinement of the optical energy in a photoconductive based plasmonic-germanium waveguide detector that enables shortest drift paths for photogenerated carriers and a very small resistance-capacitance product. In addition, the combination of plasmonic structures with absorbing semiconductors enables efficient and highest-speed photodetection. The proposed scheme may pave the way for a cost-efficient CMOS compatible and low temperature fabricated photodetector solution for photodetection b...

VO2 Thermochromic Metamaterial-Based Smart Optical Solar Reflector

Abstract: Optical solar reflector smart radiators are able to control the temperature of spacecraft. This work demonstrates a novel smart optical solar reflector using a patterned thermo-chromic VO2 plasmonic meta-surface design. The VO2 meta-surface combines the temperature induced phase transition of VO2 with plasmonic resonances resulting in a significant enhancement of the infrared absorption. The enhanced absorption obtained at a reduced VO2 coverage results in superior emittance tunability Δe and lower solar absorptance α compared to a corresponding thin-film reflector. An emittance tunability of 0.48 is obtained for the meta-reflector design, representing a 30% improvement compared to the unstructured film. Meta-surface based smart optical solar reflectors offer a new route toward energy-efficient and cost-effective passive thermal control systems of spacecraft and other surfaces.

Ultrathin 2D Transition Metal Carbides for Ultrafast Pulsed Fiber Lasers

Abstract: Two-dimensional (2D) materials, such as graphene, transition metal dichalcogenides, and black phosphorus, have attracted intense interest for applications in ultrafast pulsed laser generation, owing to their strong light–matter interactions and large optical nonlinearities. However, due to the mismatch of the bandgap, many of these 2D materials are not suitable for applications at near-infrared (NIR) waveband. Here, we report nonlinear optical properties of 2D α-Mo2C crystals and the usage of 2D α-Mo2C as a new broadband saturable absorber for pulsed laser generation. It was found that 2D α-Mo2C crystals have excellent saturable absorption properties in terms of largely tunable modulation depth and very low saturation intensity. In addition, ultrafast carrier dynamic results of 2D α-Mo2C reveal an ultrashort intraband carrier recovery time of 0.48 ps at 1.55 μm. By incorporating 2D α-Mo2C saturable absorber into either an Er-doped or Yb-doped fiber laser, we are able to generate ultrashort pulses with ver...

Room Temperature Chiral Coupling of Valley Excitons with Spin-Momentum Locked Surface Plasmons

Abstract: We demonstrate room temperature chiral coupling of valley excitons in a transition metal dichalcogenide monolayer with spin-momentum locked surface plasmons At the onset of the strong coupling regime, we measure spin-selective excitation of directional flows of polaritons Operating under such conditions, our platform yields surprisingly robust intervalley contrasts (ca 40%) and coherence (ca 5–8%) as opposed to their total absence for the uncoupled valley excitons at room temperature These results open rich possibilities, easy to implement, in the context of chiral optical networks

Design of 2D Layered PtSe2 Heterojunction for the High-Performance, Room-Temperature, Broadband, Infrared Photodetector

Abstract: The rich variety and attractive properties of two-dimensional (2D) layered nanomaterials provide an ideal platform for fabricating next generation of advanced optoelectronic devices Recently, a newly discovered 2D layered PtSe2 thin film has exhibited outstanding broadband sensitivity and optoelectronic properties In our work, a large-area 2D layered PtSe2 thin film was used to construct the PtSe2/CdTe heterojunction infrared photodetector (PD) This PD exhibited a broad detection range coverage from 200 to 2000 nm with a high responsivity of 5065 mA/W, a high specific detectivity of 42 × 1011 Jones, a high current on/off ratio of 7 × 106, and a fast response speed of 81/436 μs at room temperature Additionally, the PtSe2/CdTe heterojunction PD exhibits excellent repeatability and stability in air The high-performance of the PtSe2/CdTe heterojunction PD demonstrated in this work reveals that it has great potential to be used for broadband infrared detection

Strong Exciton-Photon Coupling and lasing behavior in All-Inorganic CsPbBr3 Micro/nanowire Fabry-Pérot cavity

Abstract: All-inorganic perovskite micro/nanowire materials hold great promises as nanoscale coherent light source due to their superior optical and electronic properties. The coupling strength between exciton and photon in this system is important for their optical application, however, is rarely studied. In this work, we demonstrated the strong coupling of exciton-photon and polariton lasing in high quality CsPbBr3 micro/nanowires synthesized by a CVD method. By exploring spatial resolved PL spectra of CsPbBr3 cavity, we observed mode volume dependent coupling strength with a vacuum Rabi splitting up to 656 meV, as well as significant increase in group index. Moreover, low threshold polariton lasing was achieved at room temperature within strong coupling regime; the polariton characteristic is confirmed by comparing lasing spectra with waveguided output spectra and the dramatically reduced lasing threshold. Our present results provide new avenues to achieve high coupling strengths potentially enabling application...