Refractive Index Interface Research Articles

Reducing Optical Reflection Loss for Perovskite Solar Cells Via Printable Mesoporous SiO2 Antireflection Coatings

AbstractThe power conversion efficiency (PCE) of single‐junction perovskite solar cells (PSCs) is being rapidly promoted towards their theoretical limit, with a certified value of 25.7%. Reducing optical loss will further contribute to PCE improvement. Here, the optical loss including reflection loss, absorption loss, and transmission loss in printable mesoscopic perovskite solar cells (p‐MPSCs) is analyzed. A printable mesoporous SiO2 antireflection coating for improving the transmittance of the fluorine‐doped tin oxide (FTO) glass substrate by reducing optical reflection at the air/glass interface is reported. With modulated porosity and thickness, the mesoporous SiO2 film constructs a graded refractive index interface and increases the transmittance of FTO glass by ≈2%–4% in the spectral range of 350–800 nm at normal incident angle with the highest transmittance improved from 85% to 89%. The SiO2 coating also exhibits wide‐angle and broadband antireflection properties. The coatings successfully help p‐MPSCs obtain about an average 3% enhancement in the short‐circuit current density (JSC) and PCE. This study demonstrates the necessity of optical management for efficient solar cells and provides a cost‐effective and scalable antireflection coating for the future realistic application of PSCs.

Fringe field-tunable LC refractive index interface for in-plane beam steering applications

We report on the electrically tunable optical structure based on dual-domain nematic liquid crystal (LC) alignment for in-plane beam steering applications. The device operates due to the total internal reflection of an extraordinary beam at the LC refractive index interface that separates homeotropic and planar-aligned nematics. Patterned electrodes were used in order to switch on the refractive index interface in the bulk of a planar-aligned LC layer. An outstanding feature of the proposed device is the function of tuning the spatial position of the LC interface by means of a fringing electric field, which allowed one to implement wide range light beam microscanning, as well as to realize in-plane angular beam steering with a milliradian resolution.

Single-molecule orientation localization microscopy for resolving structural heterogeneities between amyloid fibrils.

Simultaneous measurements of single-molecule positions and orientations provide critical insight into a variety of biological and chemical processes. Various engineered point spread functions (PSFs) have been introduced for measuring the orientation and rotational diffusion of dipole-like emitters, but the widely used Cramér-Rao bound (CRB) only evaluates performance for one specific orientation at a time. Here, we report a performance metric, termed variance upper bound (VUB), that yields a global maximum CRB for all possible molecular orientations, thereby enabling the measurement performance of any PSF to be computed efficiently (~1000× faster than calculating average CRB). Our VUB reveals that the simple polarized standard PSF provides robust and precise orientation measurements if emitters are near a refractive index interface. Using this PSF, we measure the orientations and positions of Nile red (NR) molecules transiently bound to amyloid aggregates. Our super-resolved images reveal the main binding mode of NR on amyloid fiber surfaces, as well as structural heterogeneities along amyloid fibrillar networks, that cannot be resolved by single-molecule localization alone.

Solitonic waveguide reflection at an electric interface.

A refractive index interface is dynamically induced in a bulk photorefractive material by biasing two adjacent regions with different electric fields, thus building up an electric wall. Effects of this interface on reflection, refraction and breathing of bright photorefractive solitons and their associated waveguides are numerically and experimentally studied as a function of the induced purely electric field gradient. Reflection and refraction efficiency depends on the amplitude and sign of the applied voltages that affect both the self-confining beam and the signals propagating inside the waveguide. Experimental tests are performed in nominally undoped lithium niobate samples.

(Invited) Advanced Photon Management in Printed High-Efficiency Multijunction Solar Cells

High efficiency photovoltaic (PV) cells represent a clean and efficient method to utilize the abundant solar energy in both space and terrestrial applications. Advances in solar cell technology and associated light management systems are the primary determinants of improvements in efficiency. Single junction (SJ) solar cells are already near theoretical efficiency limits defined by thermalization losses and sub-bandgap transparency. Devices that incorporate multiple junctions (i.e. sub-cells) in monolithic stacks, known as multijunction (MJ) cells, provide one of the most attractive routes to ultrahigh efficiency. Over the last decade, increases in the efficiency of MJ cells correspond to nearly 1% (absolute) per year, reaching values that are presently ~44%. Further improvements, however, will require solutions to daunting challenges in achieving lattice-matched or metamorphic epitaxial growth in complex stacks and in maintaining current matched outputs from each of the sub-cells. Here we present materials and strategies to make printed multijunction solar cell structures, to bypass the challenges in conventional multijunction cell design. In the first part, I will talk about printing-based assembly of microscale, quadruple junction, four-terminal solar cells with measured efficiencies of 43.9% at concentrations exceeding 1000 suns, and modules with efficiencies of 36.5%. Secondly, I will introduce an alternative device architecture, which uses low refractive index interface materials for enhanced photon recycling in printed multijunction solar cell stacks. These results establish routes to ultrahigh efficiency cells and modules, with potential to approach thermodynamic efficiency limits and realize large-scale photovoltaic energy production.

Refraction and instability of optical vortices at an interface in a liquid crystal

The refraction of an optical vortex at a nonlinear refractive index interface in a nematic liquid crystal is studied using modulation theory and numerical solutions. It is found that the refraction of an optical vortex differs in fundamental ways from the refraction of an optical solitary wave. This is due to an instability mode which can be triggered if it interacts too strongly with the interface across which the nonlinear refractive index change is above a low threshold, leading to the break-up of the vortex into solitary waves. Vortex refraction is studied using both an approximate modulation theory, based on an averaged Lagrangian formulation of the governing equations, and numerical solutions. Excellent agreement is found between the approximate analytical theory and numerical solutions, including the angle of incidence at which the vortex becomes unstable.

The vector reflector

A linearly polarized Bessel beam, whose spatial frequencies correspond to the Brewster angle, impinging at normal incidence on a higher refractive-index interface is shown to lead to a reflected field that can be used to produce an azimuthally polarized optical vector beam.

Fermat's principle and the formal equivalence of local light-ray rotation and refraction at the interface between homogeneous media with a complex refractive index ratio

We derive a formal description of local light-ray rotation in terms of complex refractive indices. We show that Fermat's principle holds, and we derive an extended Snell's law. The change in the angle of a light ray with respect to the normal of a refractive index interface is described by the modulus of the refractive index ratio; the rotation around the interface normal is described by the argument of the refractive index ratio.

Comparison of Terahertz Pulse Imaging and Near-Infrared Spectroscopy for Rapid, Non-Destructive Analysis of Tablet Coating Thickness and Uniformity

In this study, coating thickness and uniformity of production-scale pharmaceutical tablets were investigated using near-infrared (NIR) and terahertz pulse imaging (TPI) spectroscopy. Two coating formulations were considered; samples for each coating formulation were obtained at 0, 1, 2, 3, 4, and 5% coating weight. NIR spectra were collected, and regressed with respect to batch percent weight gain. While standard errors of calibration (SEC) less than 0.5% were observed for both formulations, the calibrations were not specifically sensitive to coating thickness. An upper limit for NIR coating thickness analysis was estimated to be ~4- 6% weight gain for this system. The NIR calibrations were used as filters to choose subsets of samples for TPI, and as a secondary method for validation of TPI results. The features in TPS time-domain spectra result when an incident THz plane wave meets a refractive index interface, which may be converted to an absolute distance. Therefore, assuming that a discernible difference in refractive index between coating material and core exists, coating thickness can be determined non-destructively. Coating thickness measurements from TPI and NIR spectroscopy were compared to estimate the lower limit for quantitative TPI coating analysis; a lower limit of ~35 mm was obtained for this system. Optical microscopy was employed on a subset of samples to validate absolute thickness values; reasonable correlations between the three methods were obtained. TPI was considered advantageous relative to the other methods, as similar results were obtained without the need for destructive sampling or empirical calibration development.

Optical channel waveguide switch and coupler using total internal reflection

Light beam switching and coupling in a four-port channel waveguide-horn structure has been accomplished using electrooptic modulation of the critical angle of a refractive index interface in a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Y</tex> -cut LiNbO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> substrate. The resulting double-pole double-throw switch/coupler is potentially capable of simultaneously providing a combination of desirable characteristics.