Fresnel Reflection Losses Research Articles

Nano-imprinting of refractive-index-matched indium tin oxide sol-gel in light-emitting diodes for eliminating total internal reflection.

Refractive-index (RI)-matched nanostructures are implemented in GaN-based light-emitting diodes (LEDs) for enhancing light output efficiency. The RI-matched indium tin oxide (ITO) nanostructures are successfully implemented in GaN-based lateral LEDs by using ITO sol–gel and nanoimprint lithography. The ITO sol–gel nanostructures annealed at 300 °C have RI of 1.95, showing high transparency of 90% and high diffused transmittance of 34%. Consequently, the light output power in LEDs with the RI-matched nanostructures increases by 8% in comparison with that in LEDs containing flat ITO. Ray tracing and finite-difference time-domain (FDTD) simulations show that the RI-matched nanostructures on the transparent current spreading layer dramatically reduce Fresnel reflection loss at the interface of the current spreading layer with the nanostructure and extract confined waveguide lights in LEDs.

High-power and highly efficient composite laser with an anti-reflection coated layer between a laser crystal and a diamond heat spreader fabricated by room-temperature bonding.

We have succeeded in bonding a Nd:YAG crystal and an anti-reflection coated diamond crystal to realize a composite laser which minimizes the Fresnel-reflection loss at the bonded interface for the first time. This newly developed composite laser has realized higher output power at higher pump power than a non-composite Nd:YAG laser while keeping nearly the same slope efficiency.

Ultrafast laser inscription in ZBLAN integrated optics chips for mid-IR beam combination in astronomical interferometry.

Astronomical interferometry is a unique technique that allows observation with angular resolutions on the milliarcsec scale by combining the light of several apertures hundreds of meters apart. The PIONIER and GRAVITY instruments at the Very Large Telescope Interferometer have demonstrated that silica-based integrated optics (IO) provide a small-scale and highly stable solution for the interferometric beam combination process. Yet, important science cases such as exoplanet hunting or the spectroscopic characterization of exoplanetary atmospheres are favorable for observation in the mid-IR, namely the atmospheric windows L and L' band (3-4 µm), a wavelength range that is not covered by conventional silica-based IO. Here, we propose laser-inscribed IO 2×2 couplers in ZBLAN and experimentally assess the critical properties of the component for broadband mid-IR interferometry. We measure the splitting ratio over the 2.5 to 5.0 µm range and find excellent broadband contrast over the L (3.1-3.6 µm) and L' (3.6 - 4.0 µm) bands. Furthermore, we quantify the dispersion properties of the coupler and find a phase variation as low as 0.02 rad across the L and L' band, respectively. By optimizing the NA of our injection beam, we measured a very high total throughput of 58% over the L band including Fresnel reflection and coupling losses. We also compare our findings to recent advances in mid-IR IO in GLS and discuss its advantages and disadvantages for the implementation in future mid-IR interferometers.

Drastic thermal effects reduction through distributed face cooling in a high power giant-pulse tiny laser

Shape design of sapphire/Nd3+:YAG based distributed face cooling (DFC) chip is reported with reduction of thermal effects as compared with those from a conventional Nd3+:YAG chip. The CW diode laser pumped round-trip cavity loss was 0.51% from a 9-disk bonded DFC chip, which was close to the theoretically calculated total Fresnel reflection loss of 0.2% from 8 Sapphire/Nd3+:YAG interfaces. The depolarization ratio from an 8-disk bonded DFC chip was 40 times lower than that from YAG/Nd3+:YAG chip. The DFC chip underwent no crack at pump power of 86 W while Nd3+:YAG single chip suffered crystal crack under pump power around 54 W. Over megawatt peak power from DFC tiny integrated laser is demonstrated at 1 kHz with 3-pulse burst modes. It is concluded that DFC structure could relieve thermal effects as expected.

Parametric Trough Solar Collector With Commercial Evacuated Receiver: Performance Comparison at Plant Level

A new anidolic parametric trough solar collector (PmTC) having 8.12 m net width aperture has been recently proposed for a commercial evacuated receiver tube with an absorber diameter of 70 mm. Since the collector was designed ignoring transmission, absorption, and reflection optical losses, calculations of the optical efficiency and the incidence angle modifier (IAM) by means of Monte Carlo spectral raytracing simulations using real slope errors distributions and taking into account Fresnel reflection losses were done. Comparison with an Eurotrough parabolic trough collector (PTC) shows an optical penalization of 5.1% due to the reflectivity and additional soiling of the secondary mirror, to an increase in the end losses and to the Fresnel reflection losses. The National Renewable Energy Laboratory (NREL) system advisor model (SAM) was used to perform annual simulations of two commercial 50 MWe oil power plants without thermal energy storage located in Seville. A PTC solar field consisting of 90 loops, each one having four Eurotrough solar collector assemblies (SCA) with 150 m length was first modeled resulting in a gross production of 386 kWh/(m2 yr). A PmTC solar field with the same module length and similar SCA net aperture area was also simulated. A final configuration of 94 loops and four SCAs with 100 m length per loop yields a gross production of 379 kWh/(m2 yr) showing no improvement compared to the reference PTC plant. The present study allows to advance in the understanding of the potential of the anidolic optic to produce optical geometries able to effectively improve the PTC technology in the short-term projecting results at a commercial plant level.

Effect of reflection losses on stationary dielectric-filled nonimaging concentrators

The effect of Fresnel reflection and total internal reflection (TIR) losses on the performance parameters in refractive solar concentrators has often been downplayed because most refractive solar concentrators are traditionally the imaging type, yielding a line or point image on the absorber surface when solely interacted with paraxial etendue ensured by solar tracking. Whereas, with refractive-type nonimaging solar concentrators that achieve two-dimensional (rectangular strip) focus or three-dimensional (circular or elliptical) focus through interaction with both paraxial and nonparaxial etendue within the acceptance angle, the Fresnel reflection and TIR losses are significant as they will affect the performance parameters and, thereby, energy collection. A raytracing analysis has been carried out to illustrate the effects of Fresnel reflection and TIR losses on four different types of stationary dielectric-filled nonimaging concentrators, namely V-trough, compound parabolic concentrator, compound elliptical concentrator, and compound hyperbolic concentrator. The refractive index (RI) of a dielectric fill material determines the acceptance angle of a solid nonimaging collector. Larger refractive indices yield larger acceptance angles and, thereby, larger energy collection. However, they also increase the Fresnel reflection losses. This paper also assesses the relative benefit of increasing RI from an energy collection standpoint.

Enhancement of the electrical response in high concentrating photovoltaic systems by antireflective coatings based on silica nanoparticles

Silicon dioxide (SiO2) nanoparticles, obtained by sol-gel technique, were used to create an antireflective coating (ARC) for the use in a high concentrating photovoltaic (HCPV) device. These ARCs have shown a transmission increase from 92% up to 99% when deposited on glass substrate.The introduction of these same ARCs, with different optical paths, in the HCPV system has shown an overall power increase with strong dependence of the HCPV module performance (based on triple junction solar cells) upon the different ARC transmission spectra.Optimal ARC spectral shape was thus determined. A net additional power generation of 4.3% of the HCPV was measured, indicating almost complete elimination of Fresnel reflection losses in the system.

Effect of radius of curvature of a convex interface on the light output power of light-emitting diode package with double-layered encapsulant

We investigate the effect of the radius of curvature, R, of a convex interface on the light output power of an light-emitting diode package. The encapsulation layers consist of two layers, with refractive indices of 1.41 and 1.53. By using a geometrical calculation and 3-D ray-tracing simulation, we quantify the effect of R on the light extraction efficiency (LEEec) by examining the escape cone and the Fresnel reflection loss. The increase in the LEEec is explained with respect to the change of the incident angle and the re-extraction of the reflected rays at the curved interface.

Internal focus coupling efficiency for optical signal acquisition by using specialty optical taper

A specialty optical taper is proposed for optical signal acquisition in fiber sensors, optical communication and laser radar etc. Based on the profile curve equation of specialty optical taper, the internal focus coupling efficiency has been analyzed by using the ray-tracing method. Considering the Fresnel reflection loss, the relationships among the internal focus depth, taper length, shape factor, large-end radius, small-end radius, radial displacement, and coupling efficiency are analyzed, respectively. The results show that the internal focus coupling efficiency can be close to 100% as a certain range of incidence angle varies in radial directions. Finally, the influencing factors of analysis conclusion are also given.

Soret fishnet metalens antenna.

At the expense of frequency narrowing, binary amplitude-only diffractive optical elements emulate refractive lenses without the need of large profiles. Unfortunately, they also present larger Fresnel reflection loss than conventional lenses. This is usually tackled by implementing unattractive cumbersome designs. Here we demonstrate that simplicity is not at odds with performance and we show how the fishnet metamaterial can improve the radiation pattern of a Soret lens. The building block of this advanced Soret lens is the fishnet metamaterial operating in the near-zero refractive index regime with one of the edge layers designed with alternating opaque and transparent concentric rings made of subwavelength holes. The hybrid Soret fishnet metalens retains all the merits of classical Soret lenses such as low profile, low cost and ease of manufacturing. It is designed for the W-band of the millimeter-waves range with a subwavelength focal length FL = 1.58 mm (0.5λ0) aiming at a compact antenna or radar systems. The focal properties of the lens along with its radiation characteristics in a lens antenna configuration have been studied numerically and confirmed experimentally, showing a gain improvement of ~2 dB with respect to a fishnet Soret lens without the fishnet metamaterial.

Achieving graded refractive index by use of ZnO nanorods/TiO2 layer to enhance omnidirectional photovoltaic performances of InGaP/GaAs/Ge triple-junction solar cells

In this study, we theoretically and experimentally demonstrate high performance of antireflection (AR) coating composed of the ZnO nanorods (NRs) and TiO2 layers applied on InGaP/GaAs/Ge triple-junction solar cells. The high performance of this AR coating is due to the realization of a smooth gradient profile of refractive index fabricated by only two physical layers. First, due to the inherent inhomogeneous-nanoporous geometry along the surface normal, the ZnO NRs are reasonable to be considered as discrete multiple optical layers with low scattering loss, leading to the index smoothly increasing from air ambient toward down to the solar cells. Second, to compensate the gap of index between ZnO and solar cells, an additional TiO2 layer with index in between is hence necessary to insert under ZnO NRs, significantly further reducing the Fresnel reflection loss of the entire device. The ZnO NRs/ TiO2 layer shows a low wavelength-averaged (solar spectrum weighted) reflectance of 6.00% (4.78%) over a wide spectral range of λ=380–1800nm, and exhibits a hydrophobic surface with a water contact angle of 128.2°. At device level, we compared the photovoltaic performance of solar cells with and without AR coating, the short-circuit density (JSC) is enhanced by 31.8% and 23.8% for solar cells integrated with the ZnO NRs/TiO2 layer and conventional SiO2/TiO2 double-layer AR (DLAR) coating, respectively. Under a very large incident angle of solar illumination (θ=80°), the ZnO NRs/TiO2 layer remains static with JSC enhancement of 35.2%, whereas the JSC enhancement of conventional DLAR coating drops down to 9.4%. In addition, the ZnO NRs/TiO2 layer barely affects the open-circuit voltage and fill factor of the solar device. Therefore, the proposed ZnO NRs/TiO2 layer with a smoother graded refractive index change is highly promising for the AR coating applications for the next-generation solar cells.

Efficiency enhancement of silicon solar cells through a downshifting and antireflective oxysulfide phosphor layer

A simple, rapid, and low-cost anti-reflection coating (ARC) for polycrystalline silicon (pc-Si) solar cells has been developed by spin coating a solution of submicron spherical Gd2O2S:Eu3+ phosphor impregnated in a polyvinylpyrrolidone (PVP) matrix on the textured surface of the solar cell. Under a simulated one-sun illumination, this predesigned device achieved an enhancement of 6.74mA/cm2 in short-circuit current density and 2.76% in power conversion efficiency relative to those for a pristine cell. Based on photoluminescence, reflectance, and optical spectral studies, the underlying mechanism for these improvements can be attributed to the luminescent down-shifting (LDS) capability in the ultraviolet range and additional promotion of light absorption within the measured spectral region. Our results indicate that a suitable inorganic phosphor impregnated in a polymer matrix may potentially serve as a feasible solution to compensate for the spectral mismatch and Fresnel reflection loss of silicon solar cells.

Laser-generated broadband antireflection structures for freeform silicon lenses at terahertz frequencies.

We present a flexible technology to generate broadband antireflection (AR) structures for the terahertz (THz) frequency range on planar and curved surfaces of silicon optics. Ultrashort laser pulses are used to ablate the surface to form a pattern of conical pillars with a period of 30 μm. These subwavelength structures act as an effective medium with gradual transition of the refractive index from air to silicon, which reduces the Fresnel reflection losses. The characterization with the THz time-domain spectroscopy system shows an AR effect for a frequency range of 0.1-1.5 THz with a maximum enhancement of the spectral amplitude by ca. 32% at 0.4 THz for planar surfaces. In addition, we demonstrate laser-generated AR structures on convex silicon lenses of both photoconductive emitter and detector devices. Here, the THz pulse amplitude can be increased by about 28%, and single frequencies even show an improvement of the spectral amplitude up to 58%.

A Three-Step Procedure for the Design of Broadband Terahertz Antireflection Structures Based on a Subwavelength Pyramidal-Frustum Grating

Materials in the terahertz (THz) region have high refractive indices, thus leading to significant Fresnel reflection loss. We present a step-by-step 2-D THz antireflection grating design procedure using thin film theory and coupled-coefficient-matrix method. A thin film with desirable properties is first optimized by genetic algorithm, and a `translation' from thin film to multilevel grating is then implemented with the coupled-coefficient-matrix method. A final smoothing step leads to a square-pyramidal-frustum grating. Using rigorous coupled-wave analysis, we demonstrate a subwavelength square-pyramidal-frustum grating based on silicon producing broadband (0.5 to 5 THz) and efficient (transmittance>95%) polarization-independent antireflection effects. The grating parameters are evaluated, showing that our design is resistant to parameter deviations such as grating depth and incidence angle.

Terahertz epsilon-near-zero graded-index lens

An epsilon-near-zero graded-index converging lens with planar faces is proposed and analyzed. Each perfectly-electric conducting (PEC) waveguide comprising the lens operates slightly above its cut-off frequency and has the same length but different cross-sectional dimensions. This allows controlling individually the propagation constant and the normalized characteristic impedance of each waveguide for the desired phase front at the lens output while Fresnel reflection losses are minimized. A complete theoretical analysis based on the waveguide theory and Fermat's principle is provided. This is complemented with numerical simulation results of two-dimensional and three-dimensional lenses, made of PEC and aluminum, respectively, and working in the terahertz regime, which show good agreement with the analytical work.

Accurate determination of optical nonlinear absorption coefficients by the combination of reflection and transmission open-aperture z-scan measurements

In most z-scan measurements, the Fresnel reflection loss is generally negligible, which is reasonable for transparent samples or materials with small nonlinear coefficients. However, for semi-transparent samples with low transmission or samples with large nonlinear coefficients, the Fresnel reflection loss attributed to the nonlinear effect is large and must be considered in z-scan measurement and data fitting. In this study, we proposed a method and established the corresponding theoretical model and experimental configuration of combining the reflection open-aperture z-scan and the transmission open-aperture z-scan measurements to determine accurately the nonlinear absorption coefficients by considering the Fresnel reflection loss. The Sb2Te3 film was used as an example. The nonlinear absorption coefficient was experimentally measured and theoretically fitted by our model. The theoretical fitting curves are highly consistent with the experimental data, indicating that the theoretical model and the configuration of the experimental setup can reasonably and feasibly obtain an accurate nonlinear absorption coefficient in actual applications.

Effects of the refractive index of the encapsulant on the light-extraction efficiency of light-emitting diodes

We investigate the effects of the refractive index of the encapsulant on the light-extraction efficiency (LEE) of light-emitting diodes (LEDs) for GaN LEDs (n ≈ 2.5) and AlGaInP LEDs (n ≈ 3.0). For non-absorbing rectangular parallelepiped LED chips, as the refractive index of the encapsulant increases, the LEE first increases quasi-linearly, then increases sub-linearly, and finally a saturation is reached. Furthermore, LEDs with a dual-layer graded-refractive-index (GRIN) encapsulant (n(encapsulant 1) = 1.57 and n(encapsulant 2) = 1.41) is fabricated through a two-step curing process. We demonstrate that such an LED further enhances the LEE by reducing Fresnel reflection loss at the encapsulant/air interface by 35% compared with an LED encapsulated with a single-layer encapsulant (n(encapsulant) = 1.57).

Ultra-compact multilayer Si/SiO_2 GRIN lens mode-size converter for coupling single-mode fiber to Si-wire waveguide

We report on the fabrication and experimental demonstration of optical mode size transformation between standard single-mode fiber and 0.26 μm-thick Si-waveguide by 12 μm-thick Si/SiO(2) multilayer on-chip GRIN lens of lengths 16 μm or 24 μm butt-joint to 10 μm-wide terminated Si-waveguide. The overall coupling loss of the coupler was measured to be 3.45 dB in which the Fresnel reflection loss is estimated to be 2dB at the GRIN-lens/air interface. The on-chip integrated GRIN lens opens up the feasibility of a low cost passive aligned fiber-pigtailed electronic-photonics integrated circuits platform.

Optimization of photonics for corrugated thin-film solar cells

The amount of solar energy reaching the active (photovoltaic) layer in a thin-film solar cell can be increased by reducing the Fresnel reflection losses at the interfaces. By using corrugated interfaces (at the wavelength scale), adiabatic propagation of the electromagnetic radiation is achieved over a broad wavelength range throughout the structure, which leads to an increase in the light that is absorbed in the active layer and, ultimately, to the improvement of the photovoltaic conversion efficiency. In this article, we have considered the case of corrugated thin-film solar cell structures and we have studied theoretically the optimization of such structures from the point of view of photonics. The focus was put on periodic pyramidal interface corrugations because they were similar to those existing at the surface of corrugated transparent electrodes on which active layers can be deposited. Because of their technological importance, we chose to work with fluorine-doped tin oxide as front electrode material and with amorphous silicon as active material. Using an original three dimensional transfer matrix method, we solved the electromagnetic wave propagation problem in the general case of laterally periodic stratified media and we compared this solution with effective medium approximated solution. On the basis of typical pyramid sizes, we demonstrated, through numerical simulations, the optimization of the global light energy intake by means of corrugations of increasing complexity. The best structures were found to be based on pyramid arrays having subwavelength periods and aspect ratio values close to one. Typically, a pyramidal structure with base and height both equal to 300 nm led to a global energy intake equal to I=0.98 (integrated over the spectral range 400–710 nm), which represented a 24% improvement in comparison with the global energy intake of a planar structure (I=0.79).

Passively Q-Switched Quasi-Continuous-Wave Diode-Pumped Intracavity Optical Parametric Oscillator at 1.57 μm

We report on a passively Q-switched quasi-cw diode-pumped Nd:YAG including an intracavity optical parametric oscillator. The dynamics of this system is described by solving the coupled equations. The effect of the initial transmission of Cr4+:YAG saturable absorber on the signal wave operation is studied. Under optimum conditions, we achieve 2.3 m J energy at 1.57μm wavelength for 40 Hz repetition rate. The peak power of the pulses amounts to 0.88MW with the pulse width of 2.6ns. When the Fresnel reflection losses of the filters are taken into account, the pulse energy would be higher than 2.3 m J. To the best of our knowledge, this is the highest pulse energy and peak power for such a type of single resonant quasi-cw diode pumped Nd:YAG/Cr4+ :YAG IOPO laser.