Air-glass Interface Research Articles

Refractive index profilometry using the total internally reflected light field.

A full-field polarization-based technique is presented for quantitative evaluation of the spatial distribution of the refractive index in macro and micro samples. The sample is mounted on a glass-air interface of a prism, illuminated by a linearly polarized collimated light beam, and two intensity frames are digitally recorded with specific orientations of an analyzer. The pair of intensity data frames captured with this simple setup is combined through an algorithm specially developed for the purpose, to yield the phase difference between the transverse electric and transverse magnetic components of the total internally reflected light field. The phase difference is then related to the refractive index of the sample. Experimental results for refractive index variations in a laser-etched glass plate and red blood corpuscles are presented. One of the salient features of the proposed technique is that the depth of measurement is dependent on the penetration depth of the sample's evanescent field, which is typically of the order of a few hundred nanometers, thereby facilitating refractive index measurements along a thin section of the sample.

Geometric phase topology in weak measurement

The geometric phase visualization proposed by Bhandari (R Bhandari 1997 Phys. Rep. 281 1–64) in the ellipticity-ellipse orientation basis of the polarization ellipse of light is implemented to understand the geometric aspects of weak measurement. The weak interaction of a pre-selected state, acheived via spin-Hall effect of light (SHEL), results in a spread in the polarization ellipticity (η) or ellipse orientation (χ) depending on the resulting spatial or angular shift, respectively. The post-selection leads to the projection of the η spread in the complementary χ basis results in the appearance of a geometric phase with helical phase topology in the η − χ parameter space. By representing the weak measurement on the Poincaré sphere and using Jones calculus, the complex weak value and the geometric phase topology are obtained. This deeper understanding of the weak measurement process enabled us to explore the techniques’ capabilities maximally, as demonstrated via SHEL in two examples—external reflection at glass-air interface and transmission through a tilted half-wave plate.

Tailoring Nano-Textures for Optimized Light In-Coupling in Liquid Phase Crystallized Silicon Thin-Film Solar Cells

Thin-film solar cells based on liquid phase crystallized silicon (LPC Si) with 8–20 μm thick absorber layers demand for advanced light management to achieve high photocurrent densities. Open-circuit voltages (Voc) >600 mV underline the high silicon material quality of LPC silicon thin-films on nano-textured glass superstrates. We present a 500 nm-pitched sinusoidal nano-texture which outperforms larger pitched gratings with respect to light in-coupling at the buried glass-silicon interface. In the wavelength range of interest reflection of incident light is minimized to values close to 4%, which is the reflection at the sun-facing air-glass interface. Further, the electronic material quality of sinusoidally textured devices is analyzed on basis of a comparison of maximum achieved open-circuit voltages on different texture types. The Voc on sinusoidally textured glass superstrates could be raised to 630 mV by changing the interlayer deposition method from a PVD to a PECVD process. Thus, we are able to unify high optical and electronic properties of silicon absorber layers on sinusoidaly textured glass substrates. These results constitute a crucial step toward fully exploiting the optical potential of LPC silicon thin-film solar cells.

Second (1178 nm) and third (1242 nm) Stokes Raman fiber lasers without intermediate Stokes cavities

We report and propose a simple Raman fiber laser scheme that generates two or three order Raman Stokes components by using a single strong (unidirectional) cavity formed by a high-reflecting fiber Bragg grating and air-glass interface (fiber output); the intermediate cavities are non-grating, weak and bi-directional cavities that serve as ‘virtual links’ or energy reservoirs. Once the strong cavity reaches operation, it practically consumes (converts) all the energy from pump and intermediate components into a single and clamped (unidirectional) signal. For example, the use of second-Stokes fiber Bragg grating together with glass-air output operated and harvested practically all the energy. Analogously, third Stokes emission was obtained by changing the grating and hence relying on first and second non-grating formed intermediate cavities. The system uses commercial silica fiber and minimizes the use of lossy and costly fiber Bragg gratings. This proposal broadens the possibilities for covering the entire 1000–2000 nm window for applications that use silica fibers.

Light management in perovskite solar cells and organic LEDs with microlens arrays.

We demonstrate enhanced absorption in solar cells and enhanced light emission in OLEDs by light interaction with a periodically structured microlens array. We simulate n-i-p perovskite solar cells with a microlens at the air-glass interface, with rigorous scattering matrix simulations. The microlens focuses light in nanoscale regions within the absorber layer enhancing the solar cell. Optimal period of ~700 nm and microlens height of ~800-1000 nm, provides absorption (photocurrent) enhancement of 6% (6.3%). An external polymer microlens array on the air-glass side of the OLED generates experimental and theoretical enhancements >100%, by outcoupling trapped modes in the glass substrate.

The upper limit of the in-plane spin splitting of Gaussian beam reflected from a glass-air interface

Optical spin splitting has a promising prospect in quantum information and precision metrology. Since it is typically small, many efforts have been devoted to its enhancement. However, the upper limit of optical spin splitting remains uninvestigated. Here, we investigate systematically the in-plane spin splitting of a Gaussian beam reflected from a glass-air interface and find that the spin splitting can be enhanced in three different incident angular ranges: around the Brewster angle, slightly smaller than and larger than the critical angle for total reflection. Within the first angular range, the reflected beam can undergo giant spin splitting but suffers from low energy reflectivity. In the second range, however, a large spin splitting and high energy reflectivity can be achieved simultaneously. The spin splitting becomes asymmetrical within the last angular range, and the displacement of one spin component can be up to half of incident beam waist w0/2. Of all the incident angles, the spin splitting reaches its maximum at Brewster angle. This maximum splitting increases with the refractive index of the “glass” prism, eventually approaching an upper limit of w0. These findings provide a deeper insight into the optical spin splitting phenomena and thereby facilitate the development of spin-based applications.

Thermally stable WLEDs with excellent luminous properties by screen-printing a patterned phosphor glass layer on a microstructured glass plate.

In order to improve the luminous properties and thermal reliability of white light-emitting diodes (WLEDs), we proposed a promising phosphor-in-glass (PiG) converter, which was prepared by screen-printing a patterned phosphor glass layer on a microstructured glass plate. The patterned layer achieves four-quadrant phosphor geometry based on separated yellow Y3Al5O12:Ce3+ (YAG:Ce3+) and red CaAlSiN3:Eu2+ (CASN:Eu2+) phosphor parts. Comparison experiments between the patterned PiG with and without a microstructure array (MSA) were conducted at different phosphor glass thicknesses. Consequently, for the phosphor glass thickness of 75μm, the luminous efficacy (LE) of the proposed PiG is increased by 12.5% owing to the reduction of total internal reflection at the glass-air interface by the MSA, and the corresponding correlated color temperature (CCT) and color rendering index are 3701K and 85.1, respectively. Furthermore, the CCT deviations of patterned PiG are reduced from 405 to 115K and 1004 to 548K by the MSA at the average CCTs of 3800K and 5500K, respectively. The results demonstrate that the proposed PiG converter can improve the LE and angular color uniformity of multi-component PiG-based WLEDs simultaneously.

Imaging Single Molecular Machines Attached with Two BODIPY Dyes at the Air–Solid Interface: High Probability of Single-Step-Like Photobleaching and Nonscaling Intensity

Single-molecule fluorescence microscopy (SMFM) is a powerful technique in monitoring single molecular machine actions at ambient conditions. To improve the fluorescence intensity and photostability, one strategy is to attach multiple dyes to the same single molecular machine. However, it is unclear how the fluorescence property of the dyes will change when multiple dyes are compacted into the same molecule within a distance between them of ∼2 nm. In this study, we investigated the photophysics of two types of single molecular machines that are each equipped with two BODIPY dyes with different distances. We found that at the air–glass interface, single molecules attached with two dyes have a high tendency to show a single-step-like photobleaching, making them to appear like a single dye. We propose that the product of the first photobleaching event, possibly a superoxide anion radical, is involved in the destruction of the neighboring dye. In addition, the fluorescence intensity of the two-dye system does ...

Effect of tilt on the performance of a quarter-wave prism retarder

The rhomb-type achromatic prism retarder utilizes the phase difference introduced at the glass–air interface, where the beam of light undergoes total internal reflection. With the proper choice of glass materials and the number of total internal reflection, it is possible to obtain a desired phase difference. The phase difference introduced is also dependent on the angle of incidence of the light beam at the glass–air interface. A small change in angle of incidence causes a considerable change in phase difference. The change in the phase difference introduced with the change in external angle of incidence of the light beam in an achromatic prism-type quarter-wave retarder is studied both theoretically and experimentally. The experimental results obtained are found to be in accordance with the theoretical results.

A Prism-Based Optical Readout Method for MEMS Bimaterial Infrared Sensors

This letter demonstrates a novel prism-based optical-readout, which uses a single prism to detect the incoming TM polarized wave just below the critical angle. The method is used with a 35- $\mu \text{m}$ -pixel pitch MEMS thermal sensor, whose inclination angle changes with the absorbed infrared (IR) radiation that results in an increase in the reflectivity at the prism’s glass–air interface. We compared this approach with the conventional knife-edge method. Noise equivalent temperature difference for a single sensor was measured as 200 mK for knife-edge method, and 154 mK for the proposed critical angle approach. Our approach shows a significant improvement for the sensitivity of the IR sensor. Both methods utilize an AC-coupled readout method for a single MEMS pixel using a photodetector, which responds only to changes in the scene. This method can be scaled to achieve smart pixel cameras for read sensor arrays with low-noise and high-dynamic range.

Effect of various microlens parameters on enhancement of light outcoupling efficiency of organic light emitting diode

Since organic light emitting diode (OLED) is a multilayer device where each layer has different refractive index, total internal reflection (TIR) plays an important role in limiting the efficiency of an OLED. Due to the presence of TIR, a major portion of light is trapped within the device in various wave guiding modes. Of the total light trapped in an OLED, we address only the part that is lost due to wave guiding mode arising from refractive index mismatch at the glass-air interface. Microlens array, to improve luminance, is a method that can be externally applied to the OLEDs without altering its electrical characteristics and is easy to use. Microlens arrays ranging from 10 to 40 μm have been fabricated using an organic elastomeric material polydimethylsiloxane (PDMS) by mold transfer technique. Maximum improvement of 25% in outcoupling efficiency for blue OLED is reported upon using the microlens array with diameter 10 μm. For a given diameter of microlens, out-coupling efficiency of OLED increases as height to diameter (H/D) ratio of microlens array approaches 0.5 (perfect hemisphere). It is also observed that outcoupling efficiency increases with the diameter of microlens for a given H/D ratio. The best luminescence improvement was observed for blue OLED, which can be explained by the higher refractive index of PDMS at lower wavelengths.

High power compatible internally sensed optical phased array.

The technical embodiment of the Huygens-Fresnel principle, an optical phased array (OPA) is an arrangement of optical emitters with relative phases controlled to create a desired beam profile after propagation. One important application of an OPA is coherent beam combining (CBC), which can be used to create beams of higher power than is possible with a single laser source, especially for narrow linewidth sources. Here we present an all-fiber architecture that stabilizes the relative output phase by inferring the relative path length differences between lasers using the small fraction of light that is back-reflected into the fiber at the OPA's glass-air interface, without the need for any external sampling optics. This architecture is compatible with high power continuous wave laser sources (e.g., fiber amplifiers) up to 100 W per channel. The high-power compatible internally sensed OPA was implemented experimentally using commercial 15 W fiber amplifiers, demonstrating an output RMS phase stability of λ/194, and the ability to steer the beam at up to 10 kHz.

Nanoantenna-controlled radiation pattern of the third-harmonic emission

We present the third-harmonic emission pattern of single and multiple gold nanoantennas excited by few-cycle infrared laser pulses. The angular distribution of the nonlinear emission is measured by back focal plane imaging with a high-numerical-aperture objective lens. The third-harmonic emission of a single-rod antenna has a dipole-like radiation pattern modified at the air–glass interface. Simultaneous excitation of multiple antennas under the same laser focus results in interferences of the far-field third-harmonic radiation, which can be well explained using a dipole model.

Enhanced Luminescence Characteristics of Remote Yellow Silicate Phosphors Printed on Nanoscale Surface‐Roughened Glass Substrates for White Light‐Emitting Diodes

Nonconventional ways to modify phosphor structures have been reported to enhance the overall luminous efficacy of white light‐emitting diodes (LED). Here, a nanoscale texturing technique of a glass substrate with the simple printing process of yellow (Ba,Sr,Ca)2SiO4:Eu+2 silicate phosphor paste is combined to achieve enhanced white luminescence performance. It is demonstrated that the luminous efficacy of the resulting printed phosphor layer can be enhanced by ≈16% as a result of controlling surface roughness of the substrate up to 151 nm. The substantial improvement obtained by texturing both sides of the substrate is attributed to the reduction of total internal reflection of rays at the glass–air interface, combined with reduction of specular reflection at the phosphor–glass interface. For the surface‐textured configuration, 3D ray tracing simulations reveal that more rays can be extracted with a widely scattered radiation pattern on the surface. Far‐field luminance uniformity is also found to be significantly improved as a result of the texturing technique.

Unveiling the photonic spin Hall effect with asymmetric spin-dependent splitting.

The photonic spin Hall effect (SHE) manifests itself as the spin-dependent splitting of light beam. Usually, it shows a symmetric spin-dependent splitting, i.e., the left- and right-handed circularly polarized components are equally separated in position and intensity for linear polarization incidence. In this paper, we theoretically propose an asymmetric spin-dependent splitting at an air-glass interface under the illumination of elliptical polarization beam and experimentally demonstrate it with the weak measurement method. The left- and right-handed circularly polarized components show expectedly unequal intensity distributions and unexpectedly different spin-dependent shifts. Remarkably, the asymmetric spin-dependent splitting can be modulated by adjusting the handedness of incident polarization. The inherent physics behind this interesting phenomenon is attributed to the additional spatial Imbert-Fedorov shift. These findings offer us potential methods for developing new spin-based nanophotonic applications.

Directional exciton-polariton photoluminescence emission from terminals of a microsphere-coupled organic waveguide

We report on angle-resolved, exciton-polariton photoluminescence measurements from asymmetric terminals of a microsphere-coupled organic waveguide (MOW). The MOW architecture consisted of a SiO2 microsphere coupled with a diaminoanthroquinone mesowire, self-assembled on a glass substrate. The angle-resolved emission measurements were performed using spatially filtered Fourier-plane optical imaging method. The light emanating from the sphere-terminus had two regions of angular emission in the Fourier-plane, of which one had azimuthal angular spread as small as 10°. The emission from wire terminus was uni-directional in nature, with some light emitted beyond the critical angle of glass-air interface. Our results highlight unique directional emission characteristics of a hybrid organic waveguide geometry and may have implications on single-element, exciton-polariton based light-emitting devices and lasers.

Nanoimprint-textured Glass Superstrates for Light Trapping in Crystalline Silicon thin-film Solar Cells

Recent progress in Liquid Phase Crystallization (LPC) of silicon enabled reaching a material quality of thin-film silicon solar cells on glass comparable to multicrystalline silicon wafers. However, decreasing the absorber thickness requires taking measures for efficient coupling and trapping light into the solar cell device. Here, we successfully integrated different periodic sub-micrometer sized structures at the sun-facing air-glass superstrate interface of 5 x 5 cm2 LPC silicon thin-film solar cells by nanoimprint lithography. These structured superstrates were modularly attached to the device and found to provide both, anti-reflective and scattering properties. In this study, the influence of the structure type and geometry is examined. With the structures used an enhancement in short-circuit current density of + 5% (relative) compared to the planar device could be achieved.

Luminescence imaging analysis of light harvesting from inactive areas in crystalline silicon PV modules

In a crystalline silicon (Si) wafer based photovoltaic (PV) module, a significant fraction of incoming photons falls onto the inactive areas of the module that do not generate electrical carriers (e.g. metal fingers, interconnection ribbons, cell spacing etc.). However, some of those photons can still be harvested via optical scattering at the inactive area and subsequent internal reflection at the glass–air interface, thereby contributing to the module’s output power. We propose a method using luminescence imaging to characterize the light harvesting from the inactive area in crystalline Si PV modules. By exploiting the reciprocity theorem relating luminescence emission to external quantum efficiency (EQE), a relative EQE map is extracted from a luminescence image of a PV module. The light harvesting efficiency at the different inactive areas can thus be directly quantified. For example in this paper, we apply the method to extract the light harvesting efficiency of two different backsheets (i.e. a white backsheet and a scattering tape) and of metal fingers (by screen and stencil printing) in one-cell mini-modules. We show that photons impinging on the white and the scattering tape backsheet close to the solar cell edges have a 20% and a 45% chance, respectively, to be harvested. Similarly, about 60% of the photons impinging on the metal fingers can be harvested. Furthermore, we estimate the enhancement of the effective active area in a typical multicrystalline Si PV module (2.1% due to the metal fingers, 0.2% due to the ribbons, 1.5% due to the white backsheet and 3.2% due to the scattering tape respectively) as well as the corresponding short-circuit current enhancement (0.20A, 0.02A, 0.13A and 0.29A, respectively).

Direct observation of a resolvable spin separation in the spin Hall effect of light at an air-glass interface

We theoretically and experimentally demonstrate that it is possible to directly observe the resolvable spin separation in the spin Hall effect of light at an air-glass interface by choosing optimal parameters. When a P-polarized light with a beam waist of 10 μm is incident around Brewster's angle, the two spin components of the reflected beam can be completely separated by eliminating the influence of the in-plane wavevector spread. This not only obviously reveals the strong impacts of the polarization state, the incident angle, the beam waist, and the in-plane wavevector spread, but also intuitively visualizes the observation of the spin Hall effect of light.

Control of optical spin Hall shift in phase-discontinuity metasurface by weak value measurement post-selection.

Spin Hall effect of light is a spin-dependent transverse shift of optical beam propagating along a curved trajectory, where the refractive index gradient plays a role of the electric field in spin Hall effect of solid-state systems. In order to observe optical spin Hall shift in a refraction taking place at air-glass interface, an amplification technique was necessary such as quantum weak measurement. In phase-discontinuity metasurface (PMS) a rapid phase-change along metasurface takes place over subwavelength distance, which leads to a large refractive index gradient for refraction beam enabling a direct detection of optical spin Hall shift without amplification. Here, we identify that the relative optical spin Hall shift depends on incidence angle at PMS, and demonstrate a control of optical spin Hall shift by constructing weak value measurement with a variable phase retardance in the post-selection. Capability of optical spin Hall shift control permits a tunable precision metrology applicable to nanoscale photonics such as angular momentum transfer and sensing.