Wide angle anapole excitation in stacked resonators.

In the search for resonances with high localized field strengths in all-dielectric nanophotonics, novel states such as anapoles, hybrid anapoles and bound states in the continuum have been realized. Of these, the anapoles are the most readily achievable. Interaction between vertically stacked disks supporting anapole resonances increases the field localization further. When fabricated from materials with high non-linear coefficients, such stacked disk pillars can be used as non-linear antennas. The excitation of such 3D pillars often includes off normal incidence when using focusing optics. Therefore, it is important to evaluate the angular and polarization response of such pillars. In the paper we fabricate pillars with three AlGaAs disks in a stack separated by stems of GaAs. The angular and polarization responses are evaluated experimentally with integrating sphere measurements and numerically through simulation, multipole decomposition and quasi-normal modes. We find that the stacked geometry shows hybridized anapole excitation for a broad span of incidence angles, with tunability of the individual multipolar response up to octupoles, including an electric octupole anapole, and we show how the average enhanced confined energy varies under angled excitation. The results show that the vertical stacked geometry can be used with highly focusing optics for efficient in-coupling to the hybridized anapole.

Analysis of Nanowire pn-Junction with Combined Current-Voltage, Electron-Beam-Induced Current, Cathodoluminescence, and Electron Holography Characterization.

We present the characterization of a pn-junction GaAs nanowire. For the characterization, current-voltage, electron-beam-induced current, cathodoluminescence, and electron holography measurements are used. We show that by combining information from these four methods, in combination with drift-diffusion modelling, we obtain a detailed picture of how the nanowire pn-junction is configured and how the recombination lifetime varies axially in the nanowire. We find (i) a constant doping concentration and 600 ps recombination lifetime in the n segment at the top part of the nanowire; (ii) a 200-300 nm long gradient in the p doping next to the pn-junction; and (iii) a strong gradient in the recombination lifetime on the p side, with 600 ps lifetime at the pn-junction, which drops to 10 ps at the bottom of the p segment closest to the substrate. We recommend such complementary characterization with multiple methods for nanowire-based optoelectronic devices.

Optimized absorption of light in perovskite nanowire solar cells.

Metal halide perovskite nanowires (PrvNWs) have recently emerged as an interesting path for nanostructured solar cells. Here, we model the absorption of light in PrvNW arrays for varying diameter and length of the PrvNWs and period for the array by solving the Maxwell equations. For long enough bare PrvNW arrays, we find that the optimum diameter is fixed to that which places the absorption peak from the HE11waveguide mode in the PrvNWs to the vicinity of the bandgap wavelength. In contrast, when we include a transparent conductive oxide (TCO) top contact layer, the optimum diameter shifts to a larger value by 100 nm. The origin of this shift is traced to a reduced reflection at the interface between the TCO layer and the PrvNW array when the PrvNW's diameter is larger. Overall, we find that 1500 nm long PrvNWs can reach 90% of the broadband absorption potential, making this system of high interest for photovoltaics.

Beyond ray optics absorption of light in CsPbBr3perovskite nanowire arrays studied experimentally and with wave optics modelling.

We study experimentally and with wave optics modelling the absorption of light in CsPbBr3perovskite nanowire arrays fabricated into periodic pores of an anodized aluminum oxide matrix, for nanowire diameters from 30 to 360 nm. First, we find that all the light that couples into the array can be absorbed by the nanowires at sufficient nanowire length. This behavior is in strong contrast to the expectation from a ray-optics description of light where, for normally incident light, only the rays that hit the cross-section of the nanowires can be absorbed. In that case, the absorption in the sample would be limited to the area fill factor of nanowires in the hexagonal array, which ranges from 13% to 58% for the samples that we study. Second, we find that the absorption saturates already at a nanowire length of 1000-2000 nm, making these perovskite nanowires promising for absorption-based applications such as solar cells and photodetectors. The absorption shows a strong diameter dependence, but for all diameters the transmission is less than 24% already at a nanowire length of 500 nm. For some diameters, the absorption exceeds that of a calculated thin film with 100% coverage. Our analysis indicates that the strong absorption in these nanowires originates from light-trapping induced by the out-of-plane disorder due to random axial position of each nanowire within its pore in the matrix.

Fluorescence excitation enhancement by waveguiding nanowires.

The optical properties of vertical semiconductor nanowires can allow an enhancement of fluorescence from surface-bound fluorophores, a feature proven useful in biosensing. One of the contributing factors to the fluorescence enhancement is thought to be the local increase of the incident excitation light intensity in the vicinity of the nanowire surface, where fluorophores are located. However, this effect has not been experimentally studied in detail to date. Here, we quantify the excitation enhancement of fluorophores bound to a semiconductor nanowire surface by combining modelling with measurements of fluorescence photobleaching rate, indicative of the excitation light intensity, using epitaxially grown GaP nanowires. We study the excitation enhancement for nanowires with a diameter of 50-250 nm and show that excitation enhancement reaches a maximum for certain diameters, depending on the excitation wavelength. Furthermore, we find that the excitation enhancement decreases rapidly within tens of nanometers from the nanowire sidewall. The results can be used to design nanowire-based optical systems with exceptional sensitivities for bioanalytical applications.

Enhanced Optical Biosensing by Aerotaxy Ga(As)P Nanowire Platforms Suitable for Scalable Production.

Sensitive detection of low-abundance biomolecules is central for diagnostic applications. Semiconductor nanowires can be designed to enhance the fluorescence signal from surface-bound molecules, prospectively improving the limit of optical detection. However, to achieve the desired control of physical dimensions and material properties, one currently uses relatively expensive substrates and slow epitaxy techniques. An alternative approach is aerotaxy, a high-throughput and substrate-free production technique for high-quality semiconductor nanowires. Here, we compare the optical sensing performance of custom-grown aerotaxy-produced Ga(As)P nanowires vertically aligned on a polymer substrate to GaP nanowires batch-produced by epitaxy on GaP substrates. We find that signal enhancement by individual aerotaxy nanowires is comparable to that from epitaxy nanowires and present evidence of single-molecule detection. Platforms based on both types of nanowires show substantially higher normalized-to-blank signal intensity than planar glass surfaces, with the epitaxy platforms performing somewhat better, owing to a higher density of nanowires. With further optimization, aerotaxy nanowires thus offer a pathway to scalable, low-cost production of highly sensitive nanowire-based platforms for optical biosensing applications.

Wafer-Scale Synthesis and Optical Characterization of InP Nanowire Arrays for Solar Cells.

Nanowire solar cells have the potential to reach the same efficiencies as the world-record III-V solar cells while using a fraction of the material. For solar energy harvesting, large-area nanowire solar cells have to be processed. In this work, we demonstrate the synthesis of epitaxial InP nanowire arrays on a 2 inch wafer. We define five array areas with different nanowire diameters on the same wafer. We use a photoluminescence mapper to characterize the sample optically and compare it to a homogeneously exposed reference wafer. Both steady-state and time-resolved photoluminescence maps are used to study the material's quality. From a mapping of reflectance spectra, we simultaneously extract the diameter and length of the nanowires over the full wafer. The extracted knowledge of large-scale nanowire synthesis will be crucial for the upscaling of nanowire-based solar cells, and the demonstrated wafer-scale characterization methods will be central for quality control during manufacturing.

Nanowire Oligomer Waveguide Modes towards Reduced Lasing Threshold.

Semiconductor nanowires offer a promising route of realizing nanolasers for the next generation of chip-scale optoelectronics and photonics applications. Established fabrication methods can produce vertical semiconductor nanowires which can themselves act both as a gain medium and as a Fabry-Pérot cavity for feedback. The lasing threshold in such nanowire lasers is affected by the modal confinement factor and end facet reflectivities, of which the substrate end reflectivity tends to be limited due to small refractive index contrast between the nanowire and substrate. These modal properties, however, also depend strongly on the modal field profiles. In this work, we use numerical simulations to investigate waveguide modes in vertical nanowire oligomers (that is, arrangements of few vertical nanowires close to each other) and their modal properties compared to single nanowire monomers. We solve for the oligomer waveguide eigenmodes which are understood as arising from interaction of monomer modes and further compute the reflectivity of these modes at the end facets of the nanowires. We consider either the nanowires or an additional coating layer as the gain medium. We show that both types of oligomers can exhibit modes with modal properties leading to reduced lasing threshold and also give directions for further research on the topic.

Management of light and scattering in InP NWs by dielectric polymer shell.

Understanding and management of light is of great importance for nanoscale devices. This report demonstrates enhanced absorption, photoluminescence and scattering in InP nanowires when coated with dielectric polymer shell. The shells increase absorption and emission by a factor of ∼2 and photoluminescence by a factor of ∼4. A thorough optical characterization is provided, including reflectance, transmission, luminescence and scattering to incident and transmitted directions. From this characterization, we derive the distribution of absorbed light within the structure (InP nanowires, Au seed particles and the substrate). Additionally, reflectance, transmission and emission are shown to become increasingly diffuse with the dielectric shells. The results are thought to provide better understanding in light-matter interaction in nanostructures, as well as to provide valuable tools for light and scattering management in nanoscale optoelectronics.

Optical far-field extinction of a single GaAs nanowire towards in situ size control of aerotaxy nanowire growth.

A substrate-free approach of semiconductor nanowire growth has been achieved by the aerotaxy technique previously. In this work, we propose an in situ method to monitor the size of nanowires through non-destructive optical-extinction measurements. Our work aims to build a theoretical look-up database of extinction spectra for a single nanowire of varying dimensions. We describe the origin of possible peaks in the spectra, for example due to nanowire-length dependent Fabry-Perot resonances and nanowire-diameter dependent TM and TE mode resonances. Furthermore, we show that the Au catalyst on top of the nanowire can be ignored in the simulations when the volume of the nanowire is an order of magnitude larger than that of the Au catalyst and the diameter is small compared to the incident wavelength. For the calculation of the extinction spectra, we use the finite element method, the discrete dipole approximation and the Mie theory. To compare with experimental measurements of randomly oriented nanowires, we perform an averaging over nanowire orientation for the modeled results. However, in the experiments, nanowires are accumulating on the quartz window of the measurement setup, which leads to increasing uncertainty in the comparison with the experimental extinction spectra. This uncertainty can be eliminated by considering both a sparse and a dense collection of nanowires on the quartz window in the optical simulations. Finally, we create a database of extinction spectra for a GaAs nanowire of varying diameters and lengths. This database can be used to estimate the diameter and the length of the nanowires by comparing the position of a peak and the peak-to-shoulder difference in the extinction spectrum. Possible tapering of nanowires can be monitored through the appearance of an additional peak at a wavelength of 700-800 nm.

Modal analysis of resonant and non-resonant optical response in semiconductor nanowire arrays.

Nanowire array solar cells have reached efficiencies where it becomes feasible to talk about creating tandem solar cells in order to achieve even higher efficiencies. An example of such a tandem solar cell could be a nanowire array embedded in a membrane and integrated on top of a Si bottom cell. Such a system, however, requires understanding and control of its interaction with light, especially to make sure that the low energy photons are transmitted to the bottom cell. The dependence of the optical response of a nanowire array on the nanowire length, diameter, array pitch, materials surrounding the nanowires, and absorption coefficient of the nanowire material is very strong and possibly resonant, indicating the complexity of the optical response. In this work, we use an eigenmode-based analysis to reveal underlying physics that gives rise to observed resonant and non-resonant behavior. First, we show that an effective refractive index can be defined at long wavelengths, where only a single mode propagates. Second, we analyze the origin of the resonant reflection when the next optical mode becomes propagating and can be 'trapped' in the array and interact with the fundamental mode. Additionally, we define two simple boundaries for the wavelength range of the resonant response: the resonances can only occur if there is more than 1 propagating mode in the array, and they disappear if the 1st diffracted order is propagating in the top or bottom material. Such resonance effects could be detrimental for tandem solar cells. We thus provide recommendations for tuning the geometry of the array and the nanowire materials in order to push the resonant regime to the absorbing regime of the nanowire, where absorption in the nanowires dampens the resonances. Finally, this work demonstrates the strength of an eigenmode-based analysis of the optical response of periodic nanostructures in terms of simplifying the analysis of a complex system.

Absorption of light in a single vertical nanowire and a nanowire array.

Both a single III-V semiconductor nanowire and an array of such nanowires have shown promise for solar cell applications. However, the correspondence between the optical properties of the single nanowire and the nanowire array has not been studied. Here, we perform electromagnetic modeling of InP nanowires to study this relationship. We find that a single nanowire can show at an absorption peak, a remarkably high absorption cross-section that is more than 50 times the geometrical cross-section. With optimization of the diameter of the single nanowire, the short-circuit current density is 30 times higher than in a bulk solar cell. With such a strong absorption, we predict an apparent efficiency >500% for the single nanowire solar cell. In contrast, we show that an efficient nanowire array solar cell cannot rely on strong absorption just through the absorption peak. Instead, the nanowires need to be packed rather closely to enhance the absorption of the full solar spectrum. At the optimum diameter for the nanowire array, neighboring nanowires compete strongly for absorption of incident photons at the absorption peak, which limits the absorption per nanowire by a factor of 18. As a result, the single InP nanowire is optimized at a diameter of 110 nm while the nanowires in the array are optimized at a considerably larger diameter of 180 nm. Importantly, we show analytically the coupling efficiency of incident light into the fundamental HE11 guided mode and consecutive absorption of the mode in the nanowires. With that analysis, we explain that a single nanowire shows two different absorption pathways-one through coupling into the guided mode and another by coupling into the nanowire through the sidewall. This analytical analysis also shows at which period the neighboring nanowires in an array start to compete for absorption of incident photons.

Physics and design for 20% and 25% efficiency nanowire array solar cells.

Bottom-up fabricated single-junction III-V nanowire array solar cells have shown efficiency up to 15.3%, which is approximately half of the conventional Shockley-Queisser detailed balance efficiency limit of 33.6%. Here, based on numerical and analytical opto-electronics modeling and analysis, we give guidelines for (i) geometry that gives strong absorption as well as (ii) the design of efficient p-n junction and electrical contacts in the nanowires to reach 20% and 25% efficiency. We exemplify the impact of eight different optical and electrical loss mechanisms in a 15% and a 25% design. We also provide an analytical equation for estimating the efficiency drop due to resistive losses in the top contact layer for varying cell size.

Nanowires for Biosensing: Lightguiding of Fluorescence as a Function of Diameter and Wavelength.

Semiconductor nanowires can act as nanoscaled optical fibers, enabling them to guide and concentrate light emitted by surface-bound fluorophores, potentially enhancing the sensitivity of optical biosensing. While parameters such as the nanowire geometry and the fluorophore wavelength can be expected to strongly influence this lightguiding effect, no detailed description of their effect on in-coupling of fluorescent emission is available to date. Here, we use confocal imaging to quantify the lightguiding effect in GaP nanowires as a function of nanowire geometry and light wavelength. Using a combination of finite-difference time-domain simulations and analytical approaches, we identify the role of multiple waveguide modes for the observed lightguiding. The normalized frequency parameter, based on the step-index approximation, predicts the lightguiding ability of the nanowires as a function of diameter and fluorophore wavelength, providing a useful guide for the design of optical biosensors based on nanowires.

Optimized efficiency in InP nanowire solar cells with accurate 1D analysis.

Semiconductor nanowire arrays are a promising candidate for next generation solar cells due to enhanced absorption and reduced material consumption. However, to optimize their performance, time consuming three-dimensional (3D) opto-electronics modeling is usually performed. Here, we develop an accurate one-dimensional (1D) modeling method for the analysis. The 1D modeling is about 400 times faster than 3D modeling and allows direct application of concepts from planar pn-junctions on the analysis of nanowire solar cells. We show that the superposition principle can break down in InP nanowires due to strong surface recombination in the depletion region, giving rise to an IV-behavior similar to that with low shunt resistance. Importantly, we find that the open-circuit voltage of nanowire solar cells is typically limited by contact leakage. Therefore, to increase the efficiency, we have investigated the effect of high-bandgap GaP carrier-selective contact segments at the top and bottom of the InP nanowire and we find that GaP contact segments improve the solar cell efficiency. Next, we discuss the merit of p-i-n and p-n junction concepts in nanowire solar cells. With GaP carrier selective top and bottom contact segments in the InP nanowire array, we find that a p-n junction design is superior to a p-i-n junction design. We predict a best efficiency of 25% for a surface recombination velocity of 4500 cm s-1, corresponding to a non-radiative lifetime of 1 ns in p-n junction cells. The developed 1D model can be used for general modeling of axial p-n and p-i-n junctions in semiconductor nanowires. This includes also LED applications and we expect faster progress in device modeling using our method.

Measurement of Nanowire Optical Modes Using Cross-Polarization Microscopy.

A method to detect optical modes from vertical InGaAs nanowires (NWs) using cross-polarization microscopy is presented. Light scattered from the optical modes in the NWs is detected by filtering out the polarized direct reflection with a crossed polarizer. A spectral peak and a valley were seen to red-shift with increasing NW diameter in the measured spectra. The peak was assigned to scattering from the TE01 optical mode and the valley was an indication of the HE11 mode, based on finite-element and scattering matrix method simulations. The cross-polarization method can be used to experimentally determine the spectral positions of the TE01 and HE11 optical modes. The modes are significantly more visible in comparison to conventional reflectance measurements. The method can be beneficial in the characterization of NW solar cells, light-emitting diodes and lasers where precise mode control is required.

Time-resolved photoluminescence characterization of GaAs nanowire arrays on native substrate.

Time-resolved photoluminescence (TRPL) measurements of nanowires (NWs) are often carried out on broken-off NWs in order to avoid the ensemble effects as well as substrate contribution. However, the development of NW-array solar cells could benefit from non-destructive optical characterization to allow faster feedback and further device processing. With this work, we show that different NW array and substrate spectral behaviors with delay time and excitation power can be used to determine which part of the sample dominates the detected spectrum. Here, we evaluate TRPL characterization of dense periodic as-grown GaAs NW arrays on a p-type GaAs substrate, including a sample with uncapped GaAs NWs and several samples passivated with AlGaAs radial shell of varied composition and thickness. We observe a strong spectral overlap of substrate and NW signals and find that the NWs can absorb part of the substrate luminescence signal, thus resulting in a modified substrate signal. The level of absorption depends on the NW-array geometry, making a deconvolution of the NW signal very difficult. By studying TRPL of substrate-only and as-grown NWs at 770 and 400 nm excitation wavelengths, we find a difference in spectral behavior with delay time and excitation power that can be used to assess whether the signal is dominated by the NWs. We find that the NW signal dominates with 400 nm excitation wavelength, where we observe two different types of excitation power dependence for the NWs capped with high and low Al composition shells. Finally, from the excitation power dependence of the peak TRPL signal, we extract an estimate of background carrier concentration in the NWs.

Optical analysis of a III-V-nanowire-array-on-Si dual junction solar cell.

A tandem solar cell consisting of a III-V nanowire subcell on top of a planar Si subcell is a promising candidate for next generation photovoltaics due to the potential for high efficiency. However, for success with such applications, the geometry of the system must be optimized for absorption of sunlight. Here, we consider this absorption through optics modeling. Similarly, as for a bulk dual-junction tandem system on a silicon bottom cell, a bandgap of approximately 1.7 eV is optimum for the nanowire top cell. First, we consider a simplified system of bare, uncoated III-V nanowires on the silicon substrate and optimize the absorption in the nanowires. We find that an optimum absorption in 2000 nm long nanowires is reached for a dense array of approximately 15 nanowires per square micrometer. However, when we coat such an array with a conformal indium tin oxide (ITO) top contact layer, a substantial absorption loss occurs in the ITO. This ITO could absorb 37% of the low energy photons intended for the silicon subcell. By moving to a design with a 50 nm thick, planarized ITO top layer, we can reduce this ITO absorption to 5%. However, such a planarized design introduces additional reflection losses. We show that these reflection losses can be reduced with a 100 nm thick SiO2 anti-reflection coating on top of the ITO layer. When we at the same time include a Si3N4 layer with a thickness of 90 nm on the silicon surface between the nanowires, we can reduce the average reflection loss of the silicon cell from 17% to 4%. Finally, we show that different approximate models for the absorption in the silicon substrate can lead to a 15% variation in the estimated photocurrent density in the silicon subcell.

Single-nanowire, low-bandgap hot carrier solar cells with tunable open-circuit voltage.

Compared to traditional pn-junction photovoltaics, hot carrier solar cells offer potentially higher efficiency by extracting work from the kinetic energy of photogenerated 'hot carriers' before they cool to the lattice temperature. Hot carrier solar cells have been demonstrated in high-bandgap ferroelectric insulators and GaAs/AlGaAs heterostructures, but so far not in low-bandgap materials, where the potential efficiency gain is highest. Recently, a high open-circuit voltage was demonstrated in an illuminated wurtzite InAs nanowire with a low bandgap of 0.39 eV, and was interpreted in terms of a photothermoelectric effect. Here, we point out that this device is a hot carrier solar cell and discuss its performance in those terms. In the demonstrated devices, InP heterostructures are used as energy filters in order to thermoelectrically harvest the energy of hot electrons photogenerated in InAs absorber segments. The obtained photovoltage depends on the heterostructure design of the energy filter and is therefore tunable. By using a high-resistance, thermionic barrier, an open-circuit voltage is obtained that is in excess of the Shockley-Queisser limit. These results provide generalizable insight into how to realize high voltage hot carrier solar cells in low-bandgap materials, and therefore are a step towards the demonstration of higher efficiency hot carrier solar cells.

Bipolar Photothermoelectric Effect Across Energy Filters in Single Nanowires.

The photothermoelectric (PTE) effect uses nonuniform absorption of light to produce a voltage via the Seebeck effect and is of interest for optical sensing and solar-to-electric energy conversion. However, the utility of PTE devices reported to date has been limited by the need to use a tightly focused laser spot to achieve the required, nonuniform illumination and by their dependence upon the Seebeck coefficients of the constituent materials, which exhibit limited tunability and, generally, low values. Here, we use InAs/InP heterostructure nanowires to overcome these limitations: first, we use naturally occurring absorption "hot spots" at wave mode maxima within the nanowire to achieve sharp boundaries between heated and unheated subwavelength regions of high and low absorption, allowing us to use global illumination; second, we employ carrier energy-filtering heterostructures to achieve a high Seebeck coefficient that is tunable by heterostructure design. Using these methods, we demonstrate PTE voltages of hundreds of millivolts at room temperature from a globally illuminated nanowire device. Furthermore, we find PTE currents and voltages that change polarity as a function of the wavelength of illumination due to spatial shifting of subwavelength absorption hot spots. These results indicate the feasibility of designing new types of PTE-based photodetectors, photothermoelectrics, and hot-carrier solar cells using nanowires.