Actively variable-spectrum optoelectronics with black phosphorus.

Room-temperature optoelectronic devices that operate at short-wavelength and mid-wavelength infrared ranges (one to eight micrometres) can be used for numerous applications1-5. To achieve the range of operating wavelengths needed for a given application, a combination of materials with different bandgaps (for example, superlattices or heterostructures)6,7 or variations in the composition of semiconductor alloys during growth8,9 are used. However, these materials are complex to fabricate, and the operating range is fixed after fabrication. Although wide-range, active and reversible tunability of the operating wavelengths in optoelectronic devices after fabrication is a highly desirable feature, no such platform has been yet developed. Here we demonstrate high-performance room-temperature infrared optoelectronics with actively variable spectra by presenting black phosphorus as an ideal candidate. Enabled by the highly strain-sensitive nature of its bandgap, which varies from 0.22 to 0.53 electronvolts, we show a continuous and reversible tuning of the operating wavelengths in light-emitting diodes and photodetectors composed of black phosphorus. Furthermore, we leverage this platform to demonstrate multiplexed nondispersive infrared gas sensing, whereby multiple gases (for example, carbon dioxide, methane and water vapour) are detected using a single light source. With its active spectral tunability while also retaining high performance, our work bridges a technological gap, presenting a potential way of meeting different requirements for emission and detection spectra in optoelectronic applications.

Multicolor detour phase holograms based on an Al plasmonic color filter.

The remarkable advances in nanofabrication that have occurred over the last decade present opportunities for the realization of new types of holograms. In this work, for the first time to the best of our knowledge, a method for phase multicolor holograms based on nanohole arrays is described. The nanoholes are in an aluminum film that is interposed between the glass substrate and a silicon dioxide layer. The nanoholes serve as color filters for blue, green, and red wavelengths and provide the necessary phase distribution via the detour phase method. Our nanohole arrays are optimized to maximize the transmission efficiency of the red, green, and blue channels and to minimize the cross-talk between them. We design two multicolor holograms based on these filters and simulate their performance. The results show good fidelity to the desired holographic images. The proposed structure has the advantages of being very compact, of requiring only a simple fabrication method with one lithography step, and of employing materials (aluminum and silicon dioxide) that are compatible with standard CMOS technology.

Large-area epitaxial growth of InAs nanowires and thin films on hexagonal boron nitride by metal organic chemical vapor deposition.

Large-area epitaxial growth of III-V nanowires and thin films on van der Waals substrates is key to developing flexible optoelectronic devices. In our study, large-area InAs nanowires and planar structures are grown on hexagonal boron nitride templates using metal organic chemical vapor deposition method without any catalyst or pre-treatments. The effect of basic growth parameters on nanowire yield and thin film morphology is investigated. Under optimised growth conditions, a high nanowire density of 2.1×109cm-2is achieved. A novel growth strategy to achieve uniform InAs thin film on h-BN/SiO2/Si substrate is introduced. The approach involves controlling the growth process to suppress the nucleation and growth of InAs nanowires, while promoting the radial growth of nano-islands formed on the h-BN surface. A uniform polycrystalline InAs thin film is thus obtained over a large area with a dominant zinc-blende phase. The film exhibits near-band-edge emission at room temperature and a relatively high Hall mobility of 399 cm-2/(Vs). This work suggests a promising path for the direct growth of large-area, low-temperature III-V thin films on van der Waals substrates.

Bright Mid-Wave Infrared Resonant-Cavity Light-Emitting Diodes Based on Black Phosphorus.

The mid-wave infrared (MWIR) wavelength range plays a central role in a variety of applications, including optical gas sensing, industrial process control, spectroscopy, and infrared (IR) countermeasures. Among the MWIR light sources, light-emitting diodes (LEDs) have the advantages of simple design, room-temperature operation, and low cost. Owing to the low Auger recombination at high carrier densities and direct bandgap of black phosphorus (bP), it can serve as a high quantum efficiency emitting layer in LEDs. In this work, we demonstrate bP-LEDs exhibiting high external quantum efficiencies and wall-plug efficiencies of up to 4.43 and 1.78%, respectively. This is achieved by integrating the device with an Al2O3/Au optical cavity, which enhances the emission efficiency, and a thin transparent conducing oxide [indium tin oxide (ITO)] layer, which reduces the parasitic resistance, both resulting in order of magnitude improvements to performance.

Genetic optimization of mid-infrared filters for a machine learning chemical classifier.

Miniaturized mid-infrared spectrometers present opportunities for applications that range from health monitoring to agriculture. One approach combines arrays of spectral filters with infrared photodetectors, called filter-array detector-array (FADA) microspectrometers. A paper recently reported a FADA microspectrometer in tandem with machine learning for chemical identification. In that work, a FADA microspectrometer with 20 filters was assembled and tested. The filters were band-pass, or band-stop designs that evenly spanned the microspectrometer's operating wavelength range. However, given that a machine learning classifier can be trained on an arbitrary filter basis, it is not apparent that evenly spaced filters are optimal. Here, through simulations with noise, we use a genetic algorithm to optimize six bandpass filters to best identify liquid and gaseous chemicals. We report that the classifiers trained with the optimized filter sets outperform those trained with evenly spaced filter sets and those handpicked to target the absorption bands of the chemicals investigated.

Mid-infrared spectral reconstruction with dielectric metasurfaces and dictionary learning.

Mid-infrared (MIR) spectroscopy has numerous industrial applications and is usually performed with Fourier-transform infrared (FTIR) spectrometers. While these work well for many purposes, there is currently much interest in alternative approaches that are smaller and lighter, i.e., MIR microspectrometers. Here we investigate all-dielectric metasurfaces as spectral filters for MIR microspectrometers. Two metasurface types are studied. For the first, we design, fabricate, and test a metasurface with a narrow and angularly tunable transmission stop band. We use it to reconstruct the transmission spectra of various materials. The second metasurface, investigated theoretically, possesses narrow passband features via symmetry-protected bound states in the continuum.

Vectorial Holograms with Spatially Continuous Polarization Distributions.

Metasurface-based holography presents opportunities for applications that include optical displays, data storage, and optical encryption. Holograms that control polarization are sometimes referred to as vectorial holograms. Most studies on this topic have concerned devices that display different images when illuminated with different polarization states. Fewer studies have demonstrated holographic images whose polarization varies spatially, i.e., as a function of the position within the image. Here, we experimentally demonstrate a vectorial hologram that produces an image with a spatially continuous distribution of polarization states, for the first time to our knowledge. An unlimited number of polarization states can be achieved within the image. Furthermore, the holographic image and its polarization map (polarization vs position in image) are independent. The same image can be thus encoded with different polarization maps. As far as we know, our approach is conceptually new. We anticipate that it could broaden the application scope of metasurface holography.

Detector-Only Spectrometer Based on Structurally Colored Silicon Nanowires and a Reconstruction Algorithm.

Spectroscopy is a cornerstone in the field of optics. Conventional spectrometers generally require two elements. The first provides wavelength selectivity, for example, diffraction grating or Michelson interferometer. The second is a detector (or detector array). Many applications would benefit from very small and lightweight spectrometers. This motivates us to investigate what may be regarded as an ultimate level of miniaturization for a spectrometer, in which it consists solely of a detector array. We demonstrate a chip containing 24 pixels, each comprising a silicon nanowire (Si NW) array photodetector formed above a planar photodetector. The NWs are structurally colored, enabling us to engineer the responsivity spectra of all photodetectors in the chip. Each pixel thus combines wavelength selectivity and photodetection functions. We demonstrate the use of our chip to reconstruct the spectrum of an unknown light source impinging upon it. This is achieved by an algorithm that takes as its inputs the measured photocurrents from the pixels and a library of their responsivity spectra.

Metasurface with metallic nanoantennas and graphene nanoslits for sensing of protein monolayers and sub-monolayers.

Biomolecule sensing plays an important role in both fundamental biological studies and medical diagnostic applications. Infrared (IR) spectroscopy presents opportunities for sensing biomolecules as it allows their fingerprints to be determined by directly measuring their absorption spectra. However, the detection of biomolecules at low concentrations is difficult with conventional IR spectroscopy due to signal-to-noise considerations. This has led to recent interest on the use of nanostructured surfaces to boost the signals from biomolecules in a method termed surface enhanced infrared spectroscopy. So far, efforts have largely involved the use of metallic nanoantennas (which produce large field enhancement) or graphene nanostructures (which produce strong field confinement and provide electrical tunability). Here, we propose a nanostructured surface that combines the large field enhancement of metallic nanoantennas with the strong field confinement and electrical tunability of graphene plasmons. Our device consists of an array of plasmonic nanoantennas and graphene nanoslits on a resonant substrate. We perform systematic electromagnetic simulations to quantify the sensing performance of the proposed device and show that it outperforms designs in which only plasmons from metallic nanoantennas or plasmons from graphene are utilized. These investigations consider the model system of a representative protein-goat anti-mouse immunoglobulin G (IgG) - in monolayer or sub-monolayer form. Our findings provide guidance for future biosensors for the sensitive quantification and identification of biomolecules.

Long-wave infrared magnetic mirror based on Mie resonators on conductive substrate.

Metal films are often used in optoelectronic devices as mirrors and/or electrical contacts. In many such devices, however, the π-phase shift of the electric field that occurs upon reflection from a perfect electric conductor (for which a metal mirror is a reasonable approximation) is undesirable. This is because it results in the total electric field being zero at the mirror surface, which is unfavorable if one wishes for example to enhance absorption by a material placed there. This has motivated the development of structures that reflect light with zero phase shift, as these lead to the electric field having an anti-node (rather than node) at the surface. These structures have been denoted by a variety of terms, including magnetic mirrors, magnetic conductors, and high impedance surfaces. In this work, we experimentally demonstrate a long-wave infrared device that we term a magnetic mirror. It comprises an array of amorphous silicon cuboids on a gold film. Our measurements demonstrate a phase shift of zero and a high reflectance (of ∼90%) at a wavelength of 8.4 µm. We present the results of a multipole analysis that provides insight into the physical mechanism. Lastly, we investigate the use of our structure in a photodetector application by performing simulations of the optical absorption by monolayer graphene placed on the cuboids.

All-dielectric nanotweezers for trapping and observation of a single quantum dot.

We report the optical trapping of a single streptavidin-coated CdSe/ZnS quantum dot whose overall diameter is around 15-20 nm, in a microfluidic chamber by an all-dielectric (silicon) nanotweezer with negligible local heating. The use of fluorescence microscopy allows us to readily observe trapping events, tracking the fluorescence emission from, and the position of, each individual trapped quantum dot as a function of time. The blinking behavior of the quantum dots is observed during the trapping process, that is, in the near field region of the silicon nanoantenna. We furthermore show that the continuous wave infrared laser employed to trap the quantum dots can also excite photoluminescence from them via two-photon absorption. We present Maxwell stress tensor simulations of optical forces applied to a single quantum dot in the nanoantenna's vicinity. This work demonstrates that all-dielectric nanotweezers are a promising means to handle quantum dots in solution, enabling them to be localized for observations over extended periods of time.

Digital cameras with designs inspired by the arthropod eye.

In arthropods, evolution has created a remarkably sophisticated class of imaging systems, with a wide-angle field of view, low aberrations, high acuity to motion and an infinite depth of field. A challenge in building digital cameras with the hemispherical, compound apposition layouts of arthropod eyes is that essential design requirements cannot be met with existing planar sensor technologies or conventional optics. Here we present materials, mechanics and integration schemes that afford scalable pathways to working, arthropod-inspired cameras with nearly full hemispherical shapes (about 160 degrees). Their surfaces are densely populated by imaging elements (artificial ommatidia), which are comparable in number (180) to those of the eyes of fire ants (Solenopsis fugax) and bark beetles (Hylastes nigrinus). The devices combine elastomeric compound optical elements with deformable arrays of thin silicon photodetectors into integrated sheets that can be elastically transformed from the planar geometries in which they are fabricated to hemispherical shapes for integration into apposition cameras. Our imaging results and quantitative ray-tracing-based simulations illustrate key features of operation. These general strategies seem to be applicable to other compound eye devices, such as those inspired by moths and lacewings (refracting superposition eyes), lobster and shrimp (reflecting superposition eyes), and houseflies (neural superposition eyes).

Experimental demonstration of infrared spectral reconstruction using plasmonic metasurfaces.

We computationally reconstruct short- to long-wave infrared spectra using an array of plasmonic metasurface filters. We illuminate the filter array with an unknown spectrum and measure the optical power transmitted through each filter with an infrared microscope to emulate a filter-detector array system. We then use the recursive least squares method to determine the unknown spectrum. We demonstrate our method with light from a blackbody. We also demonstrate it with spectra generated by passing the light from the blackbody through various materials. Our approach is a step towards miniaturized spectrometers spanning the short- to long-wave infrared based on filter-detector arrays.

Elliptical silicon nanowire photodetectors for polarization-resolved imaging.

Polarization-resolved imaging offers many advantages over conventional imaging because it provides additional information on materials and scenes. In this study, we present an image sensor pixel for polarization-resolved imaging based on an all-silicon nanowire device. As the structure has an intrinsically polarization-dependent response, it is not necessary to employ a polarizer. We fabricate pixels consisting of etched vertical silicon nanowires with elliptical cross-sections that incorporate vertical p-i-n junctions. Our photocurrent measurement reveals that the spectral responsivities are dependent on the polarization state of incident light. Polarization-resolved imaging is performed with fabricated devices. This approach is different from conventional approaches using polarization filters because absorbed light in the elliptical nanowire is converted to photocurrent while light absorbed by a polarization filter is discarded.

Filter-free image sensor pixels comprising silicon nanowires with selective color absorption.

The organic dye filters of conventional color image sensors achieve the red/green/blue response needed for color imaging, but have disadvantages related to durability, low absorption coefficient, and fabrication complexity. Here, we report a new paradigm for color imaging based on all-silicon nanowire devices and no filters. We fabricate pixels consisting of vertical silicon nanowires with integrated photodetectors, demonstrate that their spectral sensitivities are governed by nanowire radius, and perform color imaging. Our approach is conceptually different from filter-based methods, as absorbed light is converted to photocurrent, ultimately presenting the opportunity for very high photon efficiency.

High throughput multichannel fluorescence microscopy with microlens arrays.

We present a multichannel fluorescence microscopy technique for high throughput imaging applications. A microlens array with over 140,000 elements is used to image centimeter-scale samples at up to 18.1 megapixels per second. Large field-of-view multichannel fluorescent imaging is demonstrated in both sequential and parallel geometries. The extended dynamic range of this approach is also discussed.

Surface-enhanced Raman scattering with Ag nanoparticles optically trapped by a photonic crystal cavity.

We demonstrate a reusable and reconfigurable surface enhanced Raman scattering (SERS) platform by optically trapping Ag nanoparticles with a photonic crystal cavity integrated with a microfluidic chip. High-performance SERS is performed in a very reproducible manner, owing to the fact that Ag aggregates are produced by optical trapping in a controllable process that is monitored in real-time by the cavity resonance shift that occurs with the trapping of each additional nanoparticle.

Directional Raman scattering from single molecules in the feed gaps of optical antennas.

Controlling light from single emitters is an overarching theme of nano-optics. Antennas are routinely used to modify the angular emission patterns of radio wave sources. "Optical antennas" translate these principles to visible and infrared wavelengths and have been recently used to modify fluorescence from single quantum dots and single molecules. Understanding the properties of single molecules, however, would be advanced were one able to observe their vibrational spectra through Raman scattering in a very reproducible manner but it is a hugely challenging task, as Raman scattering cross sections are very weak. Here we measure for the first time the highly directional emission patterns of Raman scattering from single molecules in the feed gaps of optical antennas fabricated on a chip. More than a thousand single molecule events are observed, revealing that an unprecedented near-unity fraction of optical antennas have single molecule sensitivity.

Smart mid-infrared metasurface microspectrometer gas sensing system.

Smart, low-cost and portable gas sensors are highly desired due to the importance of air quality monitoring for environmental and defense-related applications. Traditionally, electrochemical and nondispersive infrared (IR) gas sensors are designed to detect a single specific analyte. Although IR spectroscopy-based sensors provide superior performance, their deployment is limited due to their large size and high cost. In this study, a smart, low-cost, multigas sensing system is demonstrated consisting of a mid-infrared microspectrometer and a machine learning algorithm. The microspectrometer is a metasurface filter array integrated with a commercial IR camera that is consumable-free, compact ( ~ 1 cm3) and lightweight ( ~ 1 g). The machine learning algorithm is trained to analyze the data from the microspectrometer and predict the gases present. The system detects the greenhouse gases carbon dioxide and methane at concentrations ranging from 10 to 100% with 100% accuracy. It also detects hazardous gases at low concentrations with an accuracy of 98.4%. Ammonia can be detected at a concentration of 100 ppm. Additionally, methyl-ethyl-ketone can be detected at its permissible exposure limit (200 ppm); this concentration is considered low and nonhazardous. This study demonstrates the viability of using machine learning with IR spectroscopy to provide a smart and low-cost multigas sensing platform.

Scalable Fabrication of Black Phosphorous Films for Infrared Photodetector Arrays.

Bulk black phosphorous (bP) exhibits excellent infrared (IR) optoelectronic properties, but most reported bP IR photodetectors are fabricated from single exfoliated flakes with lateral sizes of < 100 µm. Here, scalable thin films of bP suitable for IR photodetector arrays are realized through a tailored solution-deposition method. The properties of the bP film and their protective capping layers are optimized to fabricate bP IR photoconductors exhibiting specific detectivities up to 4.0 × 108 cm Hz1/2 W-1 with fast 30/60 µs rise/fall times under λ = 2.2 µm illumination. The scalability of the bP thin film fabrication is demonstrated by fabricating a linear array of 25 bP photodetectors and obtaining 25 × 25 pixel IR images at ≈203 ppi with good spatial fidelity. This research demonstrates a commercially viable method of fabricating scalable bP thin films for optoelectronic devices including room temperature-operable IR photodetector arrays.