Broadband miniaturized spectrometers with a van der Waals tunnel diode.

Miniaturized spectrometers are of immense interest for various on-chip and implantable photonic and optoelectronic applications. State-of-the-art conventional spectrometer designs rely heavily on bulky dispersive components (such as gratings, photodetector arrays, and interferometric optics) to capture different input spectral components that increase their integration complexity. Here, we report a high-performance broadband spectrometer based on a simple and compact van der Waals heterostructure diode, leveraging a careful selection of active van der Waals materials- molybdenum disulfide and black phosphorus, their electrically tunable photoresponse, and advanced computational algorithms for spectral reconstruction. We achieve remarkably high peak wavelength accuracy of ~2 nanometers, and broad operation bandwidth spanning from ~500 to 1600 nanometers in a device with a ~ 30×20 µm2 footprint. This diode-based spectrometer scheme with broadband operation offers an attractive pathway for various applications, such as sensing, surveillance and spectral imaging.

Facile layer-by-layer fabrication of semiconductor microdisk laser particles.

Semiconductor-based laser particles (LPs) with exceptionally narrowband spectral emission have been used in biological systems for cell tagging purposes. Fabrication of these LPs typically requires highly specialized lithography and etching equipment, and is typically done in a cleanroom environment, hindering the broad adoption of this exciting new technology. Here, using only easily accessible laboratory equipment, we demonstrate a simple layer-by-layer fabrication strategy that overcomes this obstacle. We start from an indium phosphide (InP) substrate with multiple epitaxial indium gallium arsenide phosphide (InGaAsP) layers which are sequentially processed to yield LPs of various compositions and spectral properties. The LPs isolated from each layer are characterized, exhibiting excellent optical properties with lasing emission full width at half maximum as narrow as < 0.3 nm and typical thresholds of approximately 6 pJ upon excitation using a 3 ns pulse duration 1064 nm pump laser. The high quality of these particles renders them suitable for large-scale biological experiments including those requiring spectral multiplexing.

Reconstructive spectrometers taper down in price.

The development of a low-cost compact reconstructive spectrometer paves the way towards portable pm-resolution spectroscopy.

Laser particle activated cell sorting in microfluidics.

Laser particles providing bright, spectrally narrowband emission renders them suitable for use as cellular barcodes. Here, we demonstrate a microfluidic platform integrated with a high-speed spectrometer, capable of reading the emission from laser particles in fluidic channels and routing cells based on their optical barcodes. The sub-nanometer spectral emission of each laser particle enables us to distinguish individual cells labeled with hundreds of different laser colors in the near infrared. Furthermore, cells tagged with laser particles are sorted based on their spectral barcodes at a kilohertz rate by using a real-time field programmable gate array and 2-way electric field switch. We demonstrate several different flavors of sorting, including isolation of barcoded cells, and cells tagged with a specific laser color. We term this novel sorting technique laser particle activated cell sorting (LACS). This flow reading and sorting technology adds to the arsenal of single-cell analysis tools using laser particles.

Laser particles with omnidirectional emission for cell tracking.

The ability to track individual cells in space over time is crucial to analyzing heterogeneous cell populations. Recently, microlaser particles have emerged as unique optical probes for massively multiplexed single-cell tagging. However, the microlaser far-field emission is inherently direction-dependent, which causes strong intensity fluctuations when the orientation of the particle varies randomly inside cells. Here, we demonstrate a general solution based on the incorporation of nanoscale light scatterers into microlasers. Two schemes are developed by introducing either boundary defects or a scattering layer into microdisk lasers. The resulting laser output is omnidirectional, with the minimum-to-maximum ratio of the angle-dependent intensity improving from 0.007 (-24 dB) to > 0.23 (-6 dB). After transfer into live cells in vitro, the omnidirectional laser particles within moving cells could be tracked continuously with high signal-to-noise ratios for 2 h, while conventional microlasers exhibited frequent signal loss causing tracking failure.

Optical simulation of ultimate performance enhancement in ultrathin Si solar cells by semiconductor nanocrystal energy transfer sensitization.

Energy transfer (ET) from nanocrystals (NCs) has shown potential to enhance the optoelectronic performance of ultrathin semiconductor devices such as ultrathin Si solar cells, but the experimental identification of optimal device geometries for maximizing the performance enhancement is highly challenging due to a large parameter space. Here, we have demonstrated a general theoretical framework combining transfer matrix method (TMM) simulations and energy transfer (ET) calculations to reveal critical device design guidelines for developing an efficient, NC-based ET sensitization of ultrathin Si solar cells, which are otherwise infeasible to identify experimentally. The results uncover that the ET-driven NC sensitization is highly effective in enhancing the short circuit current (J SC) in sub-100 nm-thick Si layers, where, for example, the ET contribution can account for over 60% of the maximum achievable J SC in 10 nm-thick ultrathin Si. The study also reveals the limitation of the ET approach, which becomes ineffective for Si active layers thicker than 5 µm, being dominated by conventional optical coupling. The demonstrated simulation approach not only enables the development of efficient ultrathin Si solar cells but also should be applicable to precisely assessing and analyzing diverse experimental device geometries and configurations for developing new efficient ET-based ultrathin semiconductor optoelectronic devices.

Conformal Coating of Freestanding Particles by Vapor-Phase Infiltration.

A novel atomic layer method for encapsulating individual micro- and nano-particles with thin (sub-10-nm) dielectric films is presented. This method leverages the diffusion of vapor-phase precursors through an underlying inert polymer film to achieve growth of a metal oxide film on all sides of the particle simultaneously; even on the side that is in contact with the substrate. Crucially, the deposition is performed on stationary particles and does not require an agitation mechanism or a special reaction chamber. Here, conformal coatings of alumina are shown to improve stability in aqueous environments for two optically-relevant particles: compound semiconductor laser microparticles and lead halide perovskite nanocrystals.

Selective Equatorial Sclera Crosslinking in the Orbit Using a Metal-Coated Polymer Waveguide.

Purpose: Photochemical crosslinking of the sclera is an emerging technique that may prevent excessive eye elongation in pathologic myopia by stiffening the scleral tissue. To overcome the challenge of uniform light delivery in an anatomically restricted space, we previously introduced the use of flexible polymer waveguides. We presently demonstrate advanced waveguides that are optimized to deliver light selectively to equatorial sclera in the intact orbit. Methods: Our waveguides consist of a polydimethylsiloxane cladding and a polyurethane core, coupled to an optical fiber. A reflective silver coating deposited on the top and side surfaces of the waveguide prevents light leakage to nontarget, periorbital tissue. Postmortem rabbits were used to test the feasibility of in situ equatorial sclera crosslinking. Tensometry measurements were performed on ex vivo rabbit eyes to confirm a biomechanical stiffening effect. Results: Metal-coated waveguides enabled efficient light delivery to the entire circumference of the equatorial sclera with minimal light leakage to the periorbital tissues. Blue light was delivered to the intact orbit with a coefficient of variation in intensity of 22%, resulting in a 45 ± 11% bleaching of riboflavin fluorescence. A 2-fold increase in the Young's modulus at 5% strain (increase of 92% P < 0.05, at 25 J/cm2) was achieved for ex vivo crosslinked eyes. Conclusions: Flexible polymer waveguides with reflective, biocompatible surfaces are useful for sclera crosslinking to achieve targeted light delivery. We anticipate that our demonstrated procedure will be applicable to sclera crosslinking in live animal models and, potentially, humans in vivo.

Wavelength-encoded laser particles for massively multiplexed cell tagging.

Large-scale single-cell analyses have become increasingly important given the role of cellular heterogeneity in complex biological systems. However, no current techniques enable optical imaging of uniquely-tagged individual cells. Fluorescence-based approaches can only distinguish a small number of distinct cells or cell groups at a time because of spectral crosstalk between conventional fluorophores. Here we investigate large-scale cell tracking using intracellular laser particles as imaging probes that emit coherent laser light with a characteristic wavelength. Made of silica-coated semiconductor microcavities, these laser particles have single-mode emission over a broad range from 1170 to 1580 nm with sub-nm linewidths, enabling massive spectral multiplexing. We explore the stability and biocompatibility of these probes in vitro and their utility for wavelength-multiplexed cell tagging and imaging. We demonstrate real-time tracking of thousands of individual cells in a 3D tumour model over several days showing different behavioural phenotypes.

Quantum Hilbert Hotel.

In 1924 David Hilbert conceived a paradoxical tale involving a hotel with an infinite number of rooms to illustrate some aspects of the mathematical notion of "infinity." In continuous-variable quantum mechanics we routinely make use of infinite state spaces: here we show that such a theoretical apparatus can accommodate an analog of Hilbert's hotel paradox. We devise a protocol that, mimicking what happens to the guests of the hotel, maps the amplitudes of an infinite eigenbasis to twice their original quantum number in a coherent and deterministic manner, producing infinitely many unoccupied levels in the process. We demonstrate the feasibility of the protocol by experimentally realizing it on the orbital angular momentum of a paraxial field. This new non-Gaussian operation may be exploited, for example, for enhancing the sensitivity of NOON states, for increasing the capacity of a channel, or for multiplexing multiple channels into a single one.

Measurement of the complex nonlinear optical response of a surface plasmon-polariton.

We observe experimentally the self-phase modulation of a surface plasmon-polariton (SPP) propagating along a gold film bounded by air in a Kretschmann-Raether configuration. Through analyzing the power dependence of the reflectance curve as a function of the incidence angle, we characterize the complex-valued nonlinear propagation coefficient of the SPP. Moreover, we present a procedure that can further extract the complex value of the third-order nonlinear susceptibility of gold from our experimental data. Our work provides direct insights into nonlinear control of SPPs utilizing the nonlinearity of metals, and serves as a practical method to measure the complex-valued third-order nonlinear susceptibility of metallic materials.

Time-domain measurements of reflection delay in frustrated total internal reflection.

We present experimental evidence that the contribution of the Goos-Hänchen shift to tunneling delay is suppressed in frustrated total internal reflection. We use a Hong-Ou-Mandel interferometer to perform direct time measurements of reflection delays with femtosecond resolution at optical frequencies, and take advantage of a liquid-crystal-filled double-prism structure to dynamically change the refractive index of the barrier region.

Optimizing photonic crystal waveguides for on-chip spectroscopic applications.

We investigate the applicability of photonic crystal waveguides to high-resolution on-chip spectrometers. We argue that the figure of merit by which their performance should be gauged is not the group index bandwidth product, which photonic crystal waveguides are usually optimized for, but the working finesse, which relates to the maximum number of spectral lines resolvable by a slow-light spectrometer. Through numerical simulation, we show that a properly-optimized photonic crystal waveguide could form the basis of a spectrometer with a spectral resolution of 0.04 nm over a 12.5 nm bandwidth near 1550 nm and with a footprint six times smaller than a conventional spectrometer.

Surface-plasmon polaritons on metal-dielectric nanocomposite films.

We observe experimentally that the reflectances of metal-dielectric nanocomposite films in the Kretschmann configuration show different characteristics, depending on the metal fill fraction f, that fall into one of three distinct regimes. In the "metallic" regime, in which f is large, the film supports conventional surface-plasmon polaritons (SPPs), and one can tailor the properties of the SPPs by controlling the value of f. In the "dielectric" regime, in which f is small, the film does not support any surface modes. In the intermediate "lossy" regime, the nanocomposite film supports a SPP mode that is different from that of a "metallic" film. These results are explained by using an anisotropic effective medium model and mode analysis.