This talk will report on the use of two different photoswitch-containing polymers for metasurface resonance tuning. Each photoswitch can be independently coated on a metasurface, and induce either red- or blueshifts in the metasurface resonance. However, the disparate wavelength tuning of the two photoswitches means they can also be used in combination. We demonstrate that light-responsive photoswitches can be employed alone, or in combination with other stimuli, to add advanced functionality to metasurface resonance tuning and enable potential applications in logic processing or optical neural networks.
The excitation of quasi-bound states in the continuum (quasi-BIC) in symmetry broken all dielectric metasurfaces have been vastly explored in the last years. The high Q-factor of quasi-BIC resonances make them attractive in sensing, electromagnetic induced transparency or non-linear optics. The resonance wavelength is fixed by the geometry of the metasurface, which is a constraint for applications, which require the tuning of the resonances to different spectral regions. In this work, we demonstrate the use of temperature as a means to fine-tune the quasi-BIC resonance in hydrogenated amorphous silicon (a-Si:H) metasurfaces.
Shaping of short laser pulses offers versatile applications in laser processing, quantum state encoding, ultrafast bio-chemical reactions, and optical communication. Optical metasurfaces have emerged as highly influential and versatile tools for exerting precise control over the properties of incident light. Dielectric Huygens' metasurfaces, in particular, possess the capability to finely tune phase while confining electric and magnetic modes within the resonators, thereby presenting additional prospects, including the exploration of nonlinear effects. In our study, we demonstrate pulse shaping at femtosecond time scale using spatially variant silicon Huygens’ metasurfaces. We experimentally achieve control over pulse dynamics, demonstrating the transformation of a single Gaussian pulse into two pulses as well as into a temporally stretched pulse. The excellent agreement between the measured output pulses and our simulations demonstrates the capability of our metasurfaces to generate precise pulse shapes with femtosecond-level temporal resolution.
Bound states in the continuum (BICs) are a category of localized states that exist within the continuum of radiating modes. The high Q-factor exhibited by these states makes quasi-BICs interesting for enhancing the emission from quantum emitters. Quasi-BICs have been experimentally realized in silicon for applications in the infrared wavelength range. Instead of silicon, hydrogenated amorphous silicon (a-Si:H) has been used for achieving quasi-BIC resonance in parts of visible spectra. Titanium dioxide (TiO2) has emerged as an alternate material for fabricating dielectric metasurfaces with high Q-factor in the visible spectral range due to its lower absorptive losses and high refractive index. However, the fabrication process for TiO2 nanostructures presents challenges compared to the well-established fabrication processes in silicon. Our work focuses on the design and fabrication of TiO2 metasurfaces supporting a quasi-BIC mode around 795 nm, with a theoretical Q-factor of 353. Experimental results reveal a maximum Q-factor of 258 at 791 nm. We discuss encountered fabrication constraints and explore possibilities for improvement in both design and fabrication processes. This study contributes to the understanding of quasi-BIC resonance in TiO2 metasurfaces, and opens avenues for further exploration in the utilization of TiO2 for high-Q dielectric metasurfaces, offering insights into the design and optimization of these structures.
Highly reflective metal coatings are essential for numerous optical elements. Established mirror coatings made of silver (Ag) and gold (Au) offer high and broadband self-reflection in the infrared (IR) spectral range but are susceptible to environmental influences and mechanical stress without suitable protective layers. In the long-wavelength spectral range, in particular, the absorption bands of these protective layers partially reduce the high mirror reflectivity again. However, the noble metal iridium (Ir) is hard, extremely dense, and thermally, mechanically, and chemically stable. Iridium provides a similarly high reflectivity in the mid (MIR) and far-infrared (FIR) spectral range, as silver and gold, and high resistance to environmental influences - even without protective layers. In this paper, the different deposition processes, as well as the optical and structural properties of iridium mirror coatings fabricated by atomic layer deposition (ALD) and by magnetron sputtering (MS), are presented and compared with each other. The complex refractive indices for ALD and MS deposited iridium mirror coatings were determined for wavelengths from 200 nm to 20 μm, complementing the existing literature values. We demonstrate that iridium mirror coatings offer a high and broadband reflectivity from the mid to far-infrared spectral range. In contrast to established – protected – silver and gold mirror coatings, the iridium coatings are environmentally durable and thermally stable up to 600°C, even without protective layers. Therefore, as an interesting mirror coating material, iridium has the potential for special applications in infrared astronomy and probably also for industrial instruments.
Subwavelength size of nanophotonic devices in use with laser pulses at the few-cycle limit raises new questions about the spatial distribution of the carrier-envelope phase (CEP). It has been shown that the complexity of the CEP distribution for few-cycle laser pulses can go far beyond the axial phase flip, known as the Gouy phase. Moreover, the phase landscape is governed by various laser parameters, some of which can be deliberately changed in order to control the CEP distribution. To be able to fully grasp the control over CEP an accessible and reliable way to measure the distribution is needed. The measurement of CEP distribution of few-cycle laser pulses is challenging and the method so far relies on vacuum apparatus. Recently discovered light-driven CEP sensitive currents in dielectrics, which can be detected with microsized on-chip electrodes, offer a new perspective for the phase detection. In this work, we present a proof-of-principle method for measuring the CEP spatial distribution. With on-chip scanning the focal volume of tightly focused oscillator laser pulses we obtain the three-dimensional map of the phase with resolution down to 500 nm.
This conference presentation, “Structural and optical properties of atomically engineered Ir/Al2O3 nanocomposites” was presented at the Nonlinear Optics and its Applications 2022 conference at SPIE Photonics Europe 2022.
Optical surfaces are achieving increasingly complex shapes which brings out challenges to functionalize them conformally for improved performance. Besides, plastic optics pose a great challenge on coating technologies due to their large coefficient of thermal expansion and poor adhesion of functional coatings. Here, the potential of plasma enhanced atomic layer deposition (PEALD) technique to develop uniform and 3D-conformal films on polycarbonate (PC) (Makrolon) planar and dome substrates has been explored. It enables to grow conformal Al2O3, TiO2 and SiO2 films on steeply curved PC substrates. Moreover, we demonstrate an 11-layer antireflection (AR) coating reaching about 0.2% reflection at 905 nm wavelength on the entire outer surface of several PC domes along with a consistent optical performance on the inner surface. The adhesion and environmental stability tests according to ISO-9211-04 resulted in promising adhesive and environmentally durable films on PC dome optics. These results suggest a possible way to grow uniform, dense, conformal, and stable optical coatings on sensitive polymer PC substrates for desired optical applications.
Atomic layer deposition (ALD) enables coating complex shaped substrates with excellent uniformity along the surface of the optic. Recently developed nanoporous SiO2 layers have been applied as single layer antireflection coatings on fused silica substrates at both 1064 nm and 532 nm wavelengths. The LIDT in the nanosecond regime at both 1064 nm and 532 nm of these nanoporous SiO2 coatings as well as the bare substrates were investigated. The stability of the coatings with respect to LIDT has been evaluated under normal atmospheric conditions, dry air with relative humidity < 10% and nitrogen atmosphere. The multiple pulse damage characteristic for 5000 shots showed in all cases no significant pulse dependence. At 532 nm wavelength, the 0%-LIDT value is between 60 J/cm2 and 70 J/cm2, which is comparable to the values measured on uncoated substrates (80 J/cm2). In case of 1064 nm the 0%-LIDT is only between 40 J/cm2 and 50 J/cm2 (uncoated substrate: 100 J/cm2) which is attributed to generated defects during the fabrication process.
In this study, single layer antireflection (AR) coatings have been realized using nanoporous SiO2 thin films made by
atomic layer deposition (ALD) and wet chemical etching. The run-to-run and etch-to-etch reproducibility of nanoporous
coatings have been monitored nearly one year. Excellent reproducibility of the film thickness and refractive index, and
accordingly of the optical function are demonstrated. Furthermore, the stability of the coatings in vacuum as well as at
200°C has been analyzed.
Complex interference multilayer systems typically implemented in high-performance optics consists of several layers of low and high refractive index materials. Low mechanical stress of the coatings is desired to avoid cracking and delamination of the film or a deformation of the substrate. It is known that the ion energies in plasma-assisted deposition can be employed to control material properties and thereby mechanical stress. In this study, we evaluate the influence of substrate biasing on mechanical stress and optical properties of alumina (Al2O3) coatings deposited by plasma enhanced atomic layer deposition (PEALD). Substrate biasing up to -300 V was applied during O2 plasma exposure in the second step of a two-step PEALD process. To distinguish the physical effect of ion bombardment from the physico-chemical effect, a substrate bias of -100 V was applied separately and only during Ar plasma exposure that constituted the third step of a three-step PEALD process. Al2O3 films were characterized using spectroscopic ellipsometry, spectrophotometry, xray photoelectron spectroscopy (XPS), x-ray diffractometry (XRD), x-ray reflectometry (XRR), Fourier transform infrared spectroscopy (FT-IR), wafer-curvature measurement and atomic force microscopy (AFM).
High-reflective coatings are indispensable in order to manufacture mirrors with highest possible reflectivity. The maximum reflectivity can be achieved by all-dielectric coatings; however, the spectral bandwidth of these mirrors is limited. For astronomical applications metal based coatings (Al, Au, Ag) are commonly applied, as they allow high reflectivity and at the same time a broad spectral bandwidth.
Antireflective coatings are essential to improve transmittance of optical elements. Most research and development of AR coatings has been reported on a wide variety of plane optical surfaces; however, antireflection is also necessary on nonplanar optical surfaces. Physical vapor deposition (PVD), a common method for optical coatings, often results in thickness gradients on strongly curved surfaces, leading to a failure of the desired optical function. In this work, optical thin films of tantalum pentoxide, aluminum oxide and silicon dioxide were prepared by atomic layer deposition (ALD), which is based on self-limiting surface reactions. The results demonstrate that ALD optical layers can be deposited on both vertical and horizontal substrate surfaces with uniform thicknesses and the same optical properties. A Ta2O5/Al2O3/ SiO2 multilayer AR coating (400-700 nm) was successfully applied to a curved aspheric glass lens with a diameter of 50 mm and a center thickness of 25 mm.
In this study, we present high efficiency embedded gratings produced by atomic layer deposition (ALD). The chosen embedding material is a nanolaminate, which consists of alternating arranged titanium dioxide (TiO2) and alumina (Al2O3) layers, where the TiO2 layers are by a factor of 25 thicker than the Al2O3 layers. Consequently, the refractive index nearly equal to the refractive index of pure TiO2 layers. Titanium dioxide has one of the highest refractive index among dielectrics and no absorption at the operating wavelength.
A pinhole free embedding of the grating is essential, since even tiny air pockets will reduce the efficiency of the diffraction optic. This has been successfully realized. However, the ALD coating produces indentations on the surface of the embedded grating. The method to remove the indentations in the excess layer on the embedded grating is discussed. The planarization is done by ion beam etching and the oxygen depletion of the top TiO2 component is fixed by thermal treatment in O2 atmosphere.
Finally, we developed an embedded grating with transmission efficiency higher than 97.0 % at 1030 nm wavelength. The experimentally measured efficiency is in excellent agreement with the theoretical value obtained by rigorous coupled wave analysis. In contrast, a conventional, binary grating with the same period reaches only a maximum theoretical efficiency of 92.3 % at the same wavelength in Littrow-configuration.
In this study, Al2O3:SiO2 composite films were grown using atomic layer deposition (ALD) with the thicknesses of Al2O3 and SiO2 being between 1 Å - 20 Å. The composition of the films was varied by changing the relative number of ALD cycles from 1 to 20. The optical properties of the layers were investigated with spectroscopic ellipsometry (SE). The experimental refractive indices of the composite films with Al2O3 and SiO2 ALD cycles of 1-10 were shown to be higher than the calculated values. This was attributed to the hampered growth of the SiO2 during the first ALD cycles. On the other hand, the experimental and calculated refractive indices of the mixture 20 cycles:20 cycles agreed very well indicating a nanolaminate behavior. Selective etching of the alloys 1:1 and 2:2 resulted in a nanoporous SiO2 films. The refractive index of the final porous SiO2 films was dependent on the thickness of the initial alloy layer.
This study focuses on the atomic layer deposition (ALD) of high quality SiO2 thin films for optical application. One of the challenges for the application of dielectric ALD layers in optical coatings is the realization of low absorption and scattering losses. Furthermore the layers have to be prepared with a precise controlled thickness and repeatable optical properties. SiO2 films were deposited using tris[dimethylamino]silane (3DMAS) and oxygen plasma on Si(100)substrates, quartz and BK7 glass substrates at temperatures between 100 °C and 300 °C. Film growth rate and refractive indices of SiO2 thin films were studied as function of deposition temperature. A linear growth behavior of SiO2 ALD films is confirmed, allowing a scalability of film thickness just by counting ALD cycles. The grown films are resistant to abrasion and possess good adhesion to glass substrates. The optical losses of the films are negligible in the investigated spectral range from 250 nm to 1100 nm. An antireflective (AR) coating was prepared by atomic layer deposition using SiO2 as low refractive index material and HfO2 as high refractive index material.
KEYWORDS: Hard x-rays, Atomic layer deposition, Ion beams, Zone plates, Electron beam lithography, X-rays, X-ray microscopy, Fabrication, X-ray optics, Microscopy
Developments and advances in the e-beam lithography (EBL) made it possible to reach resolutions in a single digit
nanometer range in the soft x-ray microscopy using Fresnel Zone Plates (FZP). However, it is very difficult to fabricate
efficient FZPs for hard x-rays via this conventional fabrication technique due to limitations in the achievable aspect
ratios. Here, we demonstrate the use of alternative fabrication techniques that depend on utilization of atomic layer
deposition and focused ion beam processing to deliver FZPs that are efficient for the hard X-ray range.
Advances in the deposition of metallic thin films are discussed. The ALD growth of ultrathin Ir films is analyzed by
transmission electron microscopy, energy dispersive X-ray spectroscopy, atomic force microscopy, and optical and
electrical measurements. The morphology of iridium metallic layers is assessed based on Ir/ Al2O3 nanolaminate films.
High resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy measurements show sharp
interfaces and pure Ir layers in the nanolaminates. The iridium films as polycrystalline. Excellent thickness control, high
uniformity and low roughness of ALD films are demonstrated. Four point probe measurements of the resistivity of Ir
coatings with various thicknesses have been performed and proved conductive layers with an Ir film thickness of ca. 10
nm. The optical properties of the Ir films deposited by ALD are similar to those of the bulk Ir. Thin iridum layers
deposited on high aspect ratio linear gratings have been successfully used as electrodes in the electrochemical deposition
of gold nanoparticles and gold layers. The gold deposition evolves through the formation of gold islands with ca. 40 nm
diameters that coalesce after ca. 60 seconds deposition. The density of the gold islands within the grating pattern is much
lower than on the flat region of the substrate. The combination of ALD with electrochemical deposition allows the
diversification of conductive layers on complex nanostructured surfaces.
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