Here we report the first experimental demonstration of signal trapping by a refractive index front in a silicon waveguide with a periodic perturbation, the so-called optical push broom effect. The refractive index front generated by a fast pump pulse captures and confines the energy of a CW signal propagating with smaller group velocity near to the band gap of a Bragg grating introduced in the waveguide. The energy of the CW signal is shifted in frequency, accelerated to the front velocity and thus accumulated inside the front. In the experiment a 2 ps free carrier front was generated via two photon absorption of the pump in the silicon waveguide. It collects approximately a 30 ps long packet of the CW signal inside it. The presented effect can be utilized to compress signals in time and space.

Nanoporous gold (npAu) has emerged as a potential candidate for many optical applications, exploiting its large surface-to-volume ratio and high broadband absorption. However, the physical origin of its enhanced visible and near infrared absorption remained unclear and was rather explained by simplified phenomenological models. Here, we have employed leveled-wave approximants to simulate the optical response of realistic npAu structures. First, our simulations reproduced well the experimental absorption spectra. Second, we identify multiple resonances in the gaps between dangling ligaments that occur at the top and bottom surfaces of npAu films as the key contribution to the broadband absorption. These resonances at the surface of npAu cannot be captured by bulk effective medium models and should be considered separately as a surface effect. The additional absorption due to dangling ligaments contributes up to 80% to the overall absorption of npAu. Our results provide deeper insights into the absorption behavior of npAu, indicating promising avenues for photocatalysis and sensing applications.

In this study, we report the experimental demonstration of optical pulse compression with a gradual refractive index front comoving in a periodically modulated silicon waveguide. This spatiotemporal effect, also called optical push broom, captures and confines the input signal pulse inside a faster propagating refractive index front, driven by a pump pulse. The signal is accelerated by the approaching front until its velocity matches the front's velocity, effectively stopping the light relative to the front. In contrast with the previous experiment where we used a cw signal, we now utilized a pulsed signal and thus can vary the initial delay between the signal and the pump. We employ the slowly varying envelope approximation to model the effect and successfully reproduce the experimental frequency shift at the output corresponding to the temporal delay at the input.

The chemical interface damping (CID) effect increases the collision frequency of free electrons in metals by changes to the metal surface. We have now experimentally disentangled the two contributions to CID: induced roughness and direct charge transfer. The latter is an important area of research in photoelectrochemistry with potential applications in light-induced chemical reactions. We present a broadband investigation of the CID effect on Au(111) covered by a self-assembled monolayer of decanethiol. Spectroscopic ellipsometry measurements show a photon-energy-dependent increase of the collision frequency. We observe a constant, photon energy-independent contribution, which is attributed to induced roughness, and a contribution that linearly increases with photon energy from about 1 eV upward, which we attribute to direct charge transfer. The onset of the charge transfer mechanism corresponds to the occupied orbitals of thiols bound to the Au surface, as confirmed by density functional theory calculations.

We demonstrate, through numerical simulations, an on-chip time lens and pulse self-steepening effect based on the optical push broom mechanism. A fast Kerr-induced refractive index front, generated within an optical waveguide, collects, traps, and concentrates the energy of a signal pulse propagating with a smaller group velocity, tuned near the bandgap of the periodically modulated waveguide. The high energy of the initial low-peak-power signal pulse accumulates within the front. This concentrated energy itself modifies the front and further steepens the leading edge of the signal pulse. The demonstrated effect can be utilized to compress signals in time and generate highly steep refractive index fronts.

Reflective coatings based on photonic crystals and photonic glasses are usually produced by traditional colloidal self-assembly techniques characterised by limited control over the deposition surface and lengthy processing times. The emergence of Additive Manufacturing combined with Colloidal Assembly (AMCA) has enabled fast and precise deposition of homogeneous photonic structures, whilst circumventing issues such as the undesired coffee-ring effect. However, the application of this technique was limited to flat substrates. This study investigates the AMCA of ceramic-based colloidal structures onto metallic curved surfaces, relevant to the field of thermal barrier coatings (TBCs). Our results demonstrate the homogeneous ceramic-based photonic glass coatings can be AMCA-printed on different substrates only when a conscious surface charge matching between the colloidal particles and the substrates is made. It also demonstrates the importance of controlling the contact angle of the suspension on the substrates and the printing geometry strategy, differing from traditional direct writing. We further demonstrate the versatility of this method by printing highly porous three-dimensional gadolinium zirconate structures onto curved Inconel substrates. These coatings are engineered for their use as reflective "photonic-based" thermal barrier coatings (rTBCs), capable of suppressing both radiative and conductive heat transport. The resultant AMCA-printed Gd2Zr2O7 rTBCs outperform state-of-the-art TBCs in terms of their reflectance properties and provide a reliable thermal protection to the underlying Inconel alloy, lowering its temperature by about 150 °C in a torch experiment.

Spectrally selective emitters that endure extreme temperatures (exceeding 1,000 degrees C) are vital for thermophotovoltaic energy harvesting. Here, we report a 2D photonic crystal emitter composed of yttria-stabilized zirconia particles on a tungsten mirror, fabricated through a simple and scalable self-assembly process. The emitter demonstrates spectral stability for 2 h and structural stability for 14 h at 1,400 degrees C under high vacuum, with degradation attributed to zirconium nitridation, which is avoided under Ar or forming gas atmospheres. Long-term durability is demonstrated over 6 months and 200 thermal cycles at 1,050 degrees C, effectively mitigating tungsten oxidation. The emitter achieves 52% spectral efficiency for a 0.72 eV photovoltaic band gap. This work offers perspectives on designing and implementing spectrally selective emitters that remain stable at high temperatures and resilient in harsh environments, representing a significant step forward in developing robust thermophotovoltaic energy-harvesting systems.

Chip-integrated chirped or apodized Bragg gratings (CBGs) are key components for on-chip dispersion compensation. They are increasingly sought after for applications such as dispersion compensation for telecommunications, integrated chirped pulse amplifiers [1], mode-locked lasers [2], and ultrashort pulse soliton generation [3] on a silicon chip. However, maintaining flat group-delay dispersion (GDD) over a large bandwidth without strong ripples is challenging. These ripples typically arise from imperfect apodization profiles, and reflections at the grating boundaries, which can introduce phase inconsistencies and disrupt the desired dispersion profile.

In this study, we report a first experimental demonstration of pulse compression by a gradual refractive index front moving in a periodically modulated silicon waveguide, the so-called optical push broom effect. Optical push broom captures and confines the input signal pulse in a faster propagating refractive index front, driven by a pump pulse. This is a spatio-temporal analogue of light trapping in a tapered plasmonic waveguide where light is continuously changing its wavevector approaching zero group velocity and, thus, stopped without reflection. Here the signal is accelerated by the front until the signal velocity matches the front velocity, thus stopping the light in respect to the front. We employ the slowly varying envelope approximation to model this phenomenon. Notably, we well reproduced the experimental frequency shift at the output corresponding to the temporal delay at the input.

We demonstrate an on-chip time lens utilizing the optical pushbroom effect. Experimental validation is achieved within a silicon Bragg grating waveguide. By generating a fast front of free carriers, we capture and concentrate the energy of a signal wave with reduced group velocity. The presented effect can be utilized to compress signals in time and space. (C) 2024 The Authors

The precise mechanism governing the reversible semiconductor-to-metal transition (SMT) in V2O5 remains elusive, yet its investigation is of paramount importance due to the remarkable potential of V2O5 as a versatile "smart" material in advancing optoelectronics, plasmonics, and photonics. In this study, distinctive experimental insights into the SMT occurring in amorphous V2O5 through the application of highly sensitive, temperature-dependent, in situ analyses on a V2O5 thin film deposited on soda-lime glass are presented. The ellipsometry measurements reveal that the complete SMT occurs at approximate to 340 degrees C. Remarkably, the refractive index and extinction coefficients exhibit reversible characteristics across visible and near-infrared wavelengths, underscoring the switch-like behavior inherent to V2O5. The findings obtained from ellipsometry are substantiated by calorimetry and in situ secondary ion mass spectrometry analyses. In situ electron microscopy observations unveil a separation of oxidation states within V2O5 at 320 degrees C, despite the thin film retaining its amorphous state. The comprehensive experimental investigations effectively demonstrate that alterations in electronic state can trigger the SMT in amorphous V2O5. It is revealed for the first time that the SMT in V2O5 is solely contingent upon electronic state changes, independent of structural transitions, and importantly, it is a reversible transformation within the amorphous state itself. Unveiling the Mystery: The reversible semiconductor-to-metal Transition in amorphous V2O5. This study employs advanced, temperature-dependent in situ analyses, providing compelling evidence for the elusive phenomenon. Through meticulous investigation, the specific conditions triggering this transition are identified. The findings demonstrate that changes in the electronic state serve as the driving force behind the semiconductor-to-metal Transition, preserving its amorphous structure of V2O5 without necessitating any structural transformations. image

Chemical interface damping is a change in the effective collision frequency of conduction band electrons in metal originating from a chemical change of the metal interface. In this work, we present in-situ ellipsometric measurements that reveal the chemical interface damping effect from electrochemical oxidation of single crystal and polycrystalline gold films. We observe an increase in collision frequency of up to 21 meV for single-crystalline gold. To compare to results obtained with thiols and metal-oxides on gold nanoparticles, we normalize the collision frequency by the electron mean free path to the surface of the structure. We show that electrochemical gold oxidation provides a stronger effect on collision frequency than these coatings. Similar ellipsometric experiments have previously been conducted to investigate the optical properties of gold oxide, but without taking chemical interface damping into account. The change in reflection from oxidation of gold was solely attributed to the oxide coating. We also show that the chemical interface damping effect saturates at a larger effective oxide thickness, which is attributed to the stabilization of the gold-oxide interface.

The generation and utilization of hot charge carriers in plasmonic materials have emerged as a topic of significant importance with profound implications across multiple disciplines, including optoelectronics, photovoltaics, photocatalysis, and sensing. In this study, we investigate the hot electron transfer from nanoporous gold (npAu) dependent on the structure size, utilizing both the nanoscale feature size and the interconnected nature of this material. We employed photoelectron injection from nanoporous gold into the electrolyte under UV illumination as a test electron transfer process. Nanoporous gold thin films with sub-10 nm initial ligament diameter are stepwise coarsened by potential cycles in a photoelectrochemical setup, thereby allowing us to precisely probe the influence of the ligament diameter on the photocurrent response. The resulting ligament diameter variations are confirmed by scanning electron microscopy (SEM) analysis. As the ligament diameter increased from 8 to 16 nm, there was a corresponding decrease in quantum efficiency proportional to the inverse ligament diameter squared. Such a dependency is expected for electrons excited by surface collisions. For the small ligament diameter of 10 nm, we estimate an emission efficiency of excited 6sp electrons as 3.14%, reaching 23% for the surface-excited electrons.

The optical properties of dielectric materials with subwavelength cylindrical pores are commonly described by effective medium models. We compare the Maxwell Garnett and the Bruggeman effective medium models for porous silicon with simulations and experiments for the case of polarization orthogonal to the pore axis. The Maxwell Garnett model matches the results of the simulations even up to very high porosities. An experimental study of the effective permittivity of macroporous and mesoporous silicon is conducted by analyzing the Fabry-Pérot oscillations in the long-wavelength limit. These experimental results are also consistent with the Maxwell Garnett model. We advocate using this model for media with cylindrical pores in the future.

Photonic crystals (PhCs) are optical structures characterized by the spatial modulation of the dielectric function, which results in the formation of a photonic band gap (PBG) in the frequency spectrum. This PBG blocks the propagation of light, enabling filtering, confinement, and manipulation of light. Most of the research in this field has concentrated on static PhCs, which have fixed structural and material parameters, leading to a constant PBG. However, the growing demand for adaptive photonic devices has led to an increased interest in switchable PhCs, where the PBG can be reversibly activated or shifted. Vanadium dioxide (VO2) is particularly notable for its near-room-temperature insulator-to-metal transition (IMT), which is accompanied by significant changes in its optical properties. Here, we demonstrate a fabrication strategy for switchable three-dimensional (3D) PhCs, involving sacrificial templates and a VO2 atomic layer deposition (ALD) process in combination with an accurately controlled annealing procedure. The resulting VO2 inverse opal (IO) PhC achieves substantial control over PBG in the near-infrared (NIR) region. Specifically, the synthesized VO2 IO PhC exhibits PBGs near 1.49 and 1.03 μm in the dielectric and metallic states of the VO2 material, respectively, which can be reversibly switched by adjusting the external temperature. Furthermore, a temperature-dependent switch from a narrow-band NIR reflector to a broad-band absorber is revealed. This work highlights the potential of integrating VO2 into 3D templates in the development of switchable photonics with complex 3D structures, offering a promising avenue for the advancement of photonic devices with adaptable functionalities.

We propose a quasiperiodic leveled -wave structure whose reciprocal space is represented by spherical belt sections, designed to achieve scattering only for the defined wavelength and direction of incident light. At the same time light is scattered only towards directions of k -vectors for which the waves are trapped by total internal reflection in the structured slab. The trapped light is only weakly scattered and thus spends a long time inside the slab and is attenuated by weak absorption in the slab. The incident light of other direction and/or wavelength is transmitted through the slab almost undisturbed. We quantitatively estimated the scattering mean free paths for incident and trapped light from the first -order Born approximation and develop an analytical model which predicts the absorption for a given slab thickness, refractive index contrast and spherical belt parameters. Reducing the refractive index contrast and thickness of the belt, the selectivity can be increased and the absorption contrast for incident light of different wavelengths can reach 70%. We present numerical simulations with absorption contrast of 63% for refractive index contrast of 0.1, which is in good agreement with our analytical model. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Significant suppression of radiation in 3D structured media with small refractive index 1.4-1.6, such as of glass or polymers, is a desirable feature yet to be obtained. For periodical structures this is realised at frequencies of the complete photonic band gap (CPBG), which up to now was demonstrated to open for materials with refractive index of at least 1.9. We present here a quasiperiodic 3D structure consisting of multiple overlapping gratings with a homogeneous distribution of Bragg peaks on a sphere in reciprocal space, which allows efficient suppression of emission. Recently we have presented the theoretical model, considering interaction with the neighbouring gratings only, that estimates a finite CPBG for arbitrarily small refractive indices and thus complete emission suppression in infinite structures. However, numerical simulations demonstrate a finite leakage of power from emitter not predicted by the model. Still the simulations show -10 dB suppression in 3D structures with optimised number of gratings. Astonishingly, as we show here, this limit is almost independent of the refractive index contrast. Also, the structures with a defined number of gratings show maximal suppression at certain refractive indices, losing the suppression even at higher refractive indices. The -10 dB suppression is demonstrated for refractive index contrast as low as 1.30.

The long‐term operation of refractory‐metal‐based metamaterials is crucial for applications such as thermophotovoltaics. The metamaterials based on refractory metals like W, Mo, Ta, Nb, and Re fail primarily by oxidation. Here, the use of the noble metal Ir is proposed, which is stable to oxidation and has optical properties comparable to gold. The thermal endurance of Ir in a 3‐layer‐system, consisting of HfO 2 /Ir/HfO 2 , by performing annealing experiments up to 1240 °C in a pressure range from 2 × 10 −6 mbar to 1 bar, is demonstrated. The Ir layer shows no oxidation in a vacuum and inert gas atmosphere. At temperatures above 1100 °C, the Ir layer starts to agglomerate due to the degradation of the confining HfO 2 layers. An in situ X‐ray diffraction experimental comparison between 1D multilayered Ir/HfO 2 and W/HfO 2 selective emitters annealed at 1000 °C, 2 × 10 −6 mbar, over 100 h, confirms oxidation stability of Ir while W multilayers gradually disappear. The results of this work show that W‐based metamaterials are not long‐term stable even at 1000 °C. However, the oxidation resistance of Ir can be leveraged for refractory plasmonic metamaterials, such as selective emitters in thermophotovoltaic systems with strong suppression of long wavelength radiation.

Reciprocal space engineering allows tailoring the scattering response of media with a low refractive‐index contrast. Here it is shown that a quasiperiodic leveled‐wave structure with well‐defined reciprocal space and random real space distribution can be engineered to open a complete photonic bandgap (CPBG) for any refractive‐index contrast. For these structures, an analytical estimation is derived, which predicts that there is an optimal number of Bragg peaks for any refractive‐index contrast. A finite 2D or 3D CPBG is expected at this optimal number even for an arbitrarily small refractive‐index contrast. Results of numerical simulations of dipole emission in 2D and 3D structures support the estimations. In 3D simulations, an emission suppression of almost 10 dB is demonstrated with a refractive index down to 1.38. The 3D structures are realized by additive manufacturing on millimeter scale for a material with a refractive index of n ≈ 1.59. Measurements confirm a strong suppression of microwave transmission in the expected frequency range.

Refractory tungsten (W) plays an important role in high temperature photonic/plasmonic applications. Previ-ously room temperature bulk single/poly-crystalline optical constants were extensively used to calculate the optical properties of W nanostructures at high temperatures. This might lead to a significant deviation between the predicted and measured optical properties due to the exclusion of electron-phonon interaction, as well as grain-boundary and surface-scattering.Herein, we show experimentally, how film thickness and temperature affects the optical losses in W. A drastic increase in the effective electron collision frequency is observed with decreasing the film thickness down to 5 nm, due to the grain-boundary and surface-scattering mechanisms. At sufficiently high temperatures (greater than 200 ?degrees C for W), the electron-phonon interaction eventually becomes the dominant mechanism, linearly increasing collision frequency with temperature, and it is independent of the geometry of the thin film structure. The impact of thickness and temperature-dependent optical properties of W is showcased with a hyperbolic 1D metamaterial structure acting as a thermophotovoltaic emitter. This work opens new directions in accurate prediction of the optical properties of nanostructures and design of efficient devices in various applications, such as aerospace, energy-efficient lighting, radiative cooling and energy harvesting, by incorporating thickness and temperature-dependent optical constants.