Nicklas Anttu

Energy transfer and light emission behavior at subwavelength scales are governed by near-field interactions between an emitter and its surrounding refractive-index environment, which can modify emission at the nanoscale and influence the far-field response in devices such as solar cells and light-emitting diodes (LEDs). Here, we analyze light emission in a spherical structure using a classical point dipole model with emphasis on how near-field interactions affect the spontaneous emission rate, as quantified by the Purcell factor. The system is studied under two refractive-index configurations: (1) an abrupt transition from an inner sphere to the surrounding medium and (2) a gradual transition with a graded refractive index, considered for both nonabsorbing and absorbing surrounding media. Our findings show that, within the fixed-amplitude dipole considered here, even a strong increase in near-field absorption does not lead to a comparable reduction in the far-field emission. In this model, the source supplies the additional power required to compensate for such near-field losses. This distinction between near-field enhancement and radiated power to the far field is important, for example, in studies of photon recycling in semiconductors and other optoelectronic devices, where both near-field absorption and radiative emission are present. An abrupt transition in the refractive index produces repeated peaks in the Purcell factor as a function of the inner sphere radius, whereas a graded refractive index smooths these peaks. In a graded system, with increasing grading thickness, the Purcell factor converges to the refractive index of the inner sphere when the grading thickness reaches the order of the wavelength. This behavior indicates that, even when the inner sphere has a higher refractive index than the surrounding medium, the emission is not limited by the surrounding refractive index. Thus, a graded refractive-index profile provides a viable strategy for mitigating reflection losses in emission. These insights are valuable for designing photon out-coupling and photon-recycling schemes and offer a pathway for improving the performance of optoelectronic devices, including solar cells and LEDs.

Light-assisted metal oxide-based chemiresistive gas sensors are widely explored for operation at relatively low temperatures, yet the investigation of the role of irradiance, as opposed to wavelength, remains underrepresented. Here, we systematically quantify the irradiance-dependent behavior of ordered mesoporous In₂O₃ under visible light illumination. Photoconductivity measurements reveal two distinct irradiance regimes consistent with trap-limited transport at low power and recombination- or saturation-limited transport at high power. Gas sensing experiments towards CO and H₂ show a pronounced non-monotonic response, reaching maximum responses of 0.74 for 135 ppm CO at 67 mW cm⁻² and 0.64 for 90 ppm H₂ at 11 mW cm⁻², followed by strong suppression at higher irradiance. Illumination also accelerated the response kinetics. At 60 ppm, t₉₀ decreases from 96 to 12 s for CO and 141 to 27 s for H₂, corresponding to an 8- and 5-fold faster response time, respectively. Near-ambient pressure-XPS under controlled atmosphere and density functional theory calculations indicate defect-mediated excitation. Oxygen vacancy states and illumination-induced modification of surface oxygen species govern this behavior. The results establish irradiance as a critical mechanistic parameter that determines whether In₂O₃ operates in a surface-controlled or bulk photoconductive regime. These findings highlight the need to explicitly optimize and report irradiance in illuminated gas sensor studies, and not only the power consumption of the light source.

Nanowires have an ideal shape for high-resolution imaging, with a small cross-section for high resolution and a long length for strong absorption. However, since the absorption in nanowires is dominated by nanophotonic effects, it is unclear what resolution and efficiency such devices could offer. Here, we investigate the limits of spatial resolution and efficiency of nanowire-based direct detectors using experiments and optical modeling. We demonstrate a direct detection scheme using a single pixel detector based on an InP 80 nm diameter nanowire diode by 3D imaging with a laser focus. Our detector has an apparent peak responsivity of 2.9 AW^-1 and a dynamic range of approximately 10⁶ in intensity. Optical modeling shows a clear optimum for the spatial resolution at around 100 nm nanowire diameter, while even smaller diameters lead to a loss of resolution. Additionally, we find that the nanowire diameter can be optimized for resolution and absorption simultaneously.

Semiconductor nanowires are an efficient platform for fluorescence-based biosensors. Here, we model how a GaP nanowire (i) enhances the excitation intensity at the position of the fluorophore attached to the nanowire sidewall, (ii) enhances the probability to collect photons emitted from the fluorophore, and (iii) through the Purcell effect increases the quantum yield of the fluorophore. With optimized design, we can reach a larger than 10² enhancement in signal. We also model imaging-based detection. There, waveguiding in the nanowire beats the limitations set by the depth of view in conventional microscopy, enabling the use of a long nanowire to enhance the binding-area for fluorophores. As an example, we can focus to the top of a 4000 nm long nanowire and reach a 25 times sharper image from a fluorophore at the bottom of the nanowire, as compared to such a 4000 nm defocusing in a conventional planar biosensor platform.

Semiconductor nanowires can enhance the signal of fluorescent molecules, thus significantly improving the limits of fluorescence detection in optical biosensing. In this work, we explore how the sensitivity can further be enhanced through “digital” detection of adequately spaced vertically aligned nanowires, employing single‐emitter localization methods, and bright‐field microscopy. Additionally, we introduce a systematic analysis pipeline aimed at harnessing this digital detection capability and evaluate its impact on detection sensitivity. Using a streptavidin‐biotin assay, we demonstrate that single‐emitter localization expands the dynamic range to encompass five orders of magnitude, enabling detections of concentrations ranging from 10 fM to 10 nM. This represents two to three orders of magnitude improvement in detection compared to methods that do not utilize single‐emitter localization. We validate our analysis framework by simulating an artificial dataset based on numerical solutions of Maxwell’s equations. Furthermore, we benchmark our results against total internal reflection fluorescence microscopy and find, in time‐resolved titration experiments, that nanowires offer higher sensitivity at the lowest concentrations, attributed to a combination of higher protein capture rate and higher intensity per single protein binding event. These findings suggest promising applications of nanowires in both endpoint and time‐resolved biosensing.

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 HE₁₁ waveguide 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.

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.

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.

We study experimentally and with wave optics modelling the absorption of light in CsPbBr₃ perovskite 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.

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 HE$_{11}$ waveguide 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.