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Optical coatings are integral components of virtually every optical instrument. However, despite being a century-old technology, there are only a handful of optical coating types. Here, we introduce a type of optical coatings that exhibit photonic Fano resonance, or a Fano-resonant optical coating (FROC). We expand the coupled mechanical oscillator description of Fano resonance to thin-film nanocavities. Using FROCs with thicknesses in the order of 300 nm, we experimentally obtained narrowband reflection akin to low-index-contrast dielectric Bragg mirrors and achieved control over the reflection iridescence. We observed that semi-transparent FROCs can transmit and reflect the same colour as a beam splitter filter, a property that cannot be realized through conventional optical coatings. Finally, FROCs can spectrally and spatially separate the thermal and photovoltaic bands of the solar spectrum, presenting a possible solution to the dispatchability problem in photovoltaics, that is, the inability to dispatch solar energy on demand. Our solar thermal device exhibited power generation of up to 50% and low photovoltaic cell temperatures (~30 °C), which could lead to a six-fold increase in the photovoltaic cell lifetime. A thin-film optical coating exhibits Fano resonance showing promising applications in structural colouring of transparent objects and hybrid thermal and photovoltaic power generation.

Solar-based water sanitation is an environmentally friendly process for obtaining clean water that requires efficient light-to-heat-to-vapour generation. Solar-driven interfacial evaporation has potential, but the inability to control interfacial evaporators for solar tracking limits efficiency at large solar zenith angles and when using optical concentration. Furthermore, clogging affects the efficiency of the device. Here, we create a super-wicking and super-light-absorbing (SWSA) aluminium surface for efficient solar-based water sanitation. The measured evaporation rate exceeds that of an ideal device operating at 100% efficiency, which we hypothesize resulted from a reduced enthalpy of vaporization within the microcapillaries. Limited solar absorber–water contact for water transport minimizes heat losses to bulk water and maximizes heat localization at the SWSA surface. The device can be mounted at any angle on a floating platform to optimize incident solar irradiance and can readily be integrated with commercial solar-thermal systems. With a design that is analogous to bifacial photovoltaic solar panels, we show a 150% increase in efficiency compared with a single-sided SWSA. Given the open capillary channels, the device surface can be easily cleaned and reused. Using the SWSA surface to purify contaminated water, we show a decrease in the level of contaminants to well below the WHO and EPA standards for drinkable water. Solar-driven interfacial evaporation can be a sustainable process to obtain clean water but device efficiency and simplicity need improving. This study presents a super-wicking and super-light-absorbing aluminium surface with increased efficiency, a simple structure and strong performance.

Direct femtosecond (fs) laser processing is a maskless fabrication technique that can effectively modify the optical, electrical, mechanical, and tribological properties of materials for a wide range of potential applications. However, the eventual implementation of fs-laser-treated surfaces in actual devices remains challenging because it is difficult to precisely control the surface properties. Previous studies of the morphological control of fs-laser-processed surfaces mostly focused on enhancing the uniformity of periodic microstructures. Here, guided by the plasmon hybridisation model, we control the morphology of surface nanostructures to obtain more control over spectral light absorption. We experimentally demonstrate spectral control of a variety of metals [copper (Cu), aluminium (Al), steel and tungsten (W)], resulting in the creation of broadband light absorbers and selective solar absorbers (SSAs). For the first time, we demonstrate that fs-laser-produced surfaces can be used as high-temperature SSAs. We show that a tungsten selective solar absorber (W-SSA) exhibits excellent performance as a high-temperature solar receiver. When integrated into a solar thermoelectric generation (TEG) device, W-SSA provides a 130% increase in solar TEG efficiency compared to untreated W, which is commonly used as an intrinsic selective light absorber., Materials processing: laser modified metals Femtosecond-laser processing of materials has many potential applications including high-efficiency incandescent lamps and solar-thermal receivers. A longstanding challenge is the ability to control the emerging properties of the treated surfaces. A research team at Chunlei Guo’s lab from the Institute of Optics at the University of Rochester showed that the spectral absorption properties of treated metals can be controlled systematically by controlling the laser processing parameters. They showed that the formed structures act as interacting nano-antennas which absorbs light based on the excitation of electromagnetic resonances. As a direct application, they converted a Tungsten surface to a selective solar-absorber and used it to operate a thermoelectric generator which resulted in a 130% increase in efficiency compared to an untreated tungsten receiver.

Surface structures with nanoscale size and periodicity are important in controlling the flow of light as well as light-matter interaction. Fabrication of these periodic nanostructure requires sophisticated and expensive nanofabrication methods that limits large scale production of such structures. Laser induced periodic surface structures (LIPSS) is a scalable and cheap method to create periodic structures. However, they suffer from lacking a long-range order. Here, we present a maskless fabrication technique of creating highly uniform subwavelength structures on nickel surface, using two collinear femtosecond laser beams at various temporal delays. Our femtosecond laser induced periodic surface structures (FLIPSSs) show a high spatial uniformity with a grating period ranging between 320–350 nm. The high spatial uniformity is a consequence of using two pulsed beams with short inter-beam delay time due to reduction in the propagation length of excited surface plasmon polaritons (SPPs). The subwavelength period is due to grating splitting mechanism where the two beams sequentially interact with existing structures to create a superimposed grating. The demonstrated fabrication method enables large scale fabrication of regular sub-wavelength structures.

Optical activation of material properties illustrates the potentials held by tuning light-matter interactions with impacts ranging from basic science to technological applications. Here, we demonstrate for the first time that composite nanostructures providing nonlocal environments can be engineered to optically trigger photoinduced charge-transfer-dynamic modulations in the solid state. The nanostructures explored herein lead to out-of-phase behavior between charge separation and recombination dynamics, along with linear charge-transfer-dynamic variations with the optical-field intensity. Using transient absorption spectroscopy, up to 270% increase in charge separation rate is obtained in organic semiconductor thin films. We provide evidence that composite nanostructures allow for surface photovoltages to be created, which kinetics vary with the composite architecture and last beyond optical pulse temporal characteristics. Furthermore, by generalizing Marcus theory framework, we explain why charge-transfer-dynamic modulations can only be unveiled when optic-field effects are enhanced by nonlocal image-dipole interactions. Our demonstration, that composite nanostructures can be designed to take advantage of optical fields for tuneable charge-transfer-dynamic remote actuators, opens the path for their use in practical applications ranging from photochemistry to optoelectronics., Controlling and modulating charge transfer dynamics in composite nanostructures, though promising for optoelectronic applications, remains a challenge. Here, the authors report optical control of charge separation and recombination processes in organic semiconductor-based composite nanostructures.

Femtosecond (fs) laser processing can significantly alter the optical, thermal, mechanical, and electrical properties of materials. Here, we show that fs-laser processing transforms aluminum (Al) to a highly efficient and multipronged heat exchanger. By optimizing the formed surface nano- and microstructures, we increase the Al emissivity and surface area by 700% and 300%, respectively. Accordingly, we show that fs-laser treated Al (fs-Al) increases the radiative and convective cooling power of fs-Al by 2100% and 300%, respectively, at 200 °C. As a direct application, we use fs-Al as a heat sink for a thermoelectric generator (TEG) and demonstrate a 280% increase in the TEG output power compared to a TEG with an untreated Al heat exchanger at 200 °C. The multipronged enhancement in fs-Al heat exchange properties lead to an increase in the TEG output power over a wide temperature (T) range (T>50°C). Conversely, a simple radiative cooling heat exchanger increases the TEG output power within a limited temperature range (T>150°C). We investigate the laser processing parameters necessary to maximize the spectral emissivity and surface area of fs-Al. Fs-Al promises to be a widely used and compact heat exchanger for passive cooling of computers and data centers as well as to increase the efficiency of TEGs incorporated in sensors and handheld electronics., Graphical abstract Image 1, Highlights • We produced a novel heat-exchanger by creating nano/micro surface structures on Al via femtosecond laser processing. • We increased the Al emissivity, surface area, radiative and convective cooling by 700%, 300%, 2100% and 300% respectively. • We demonstrated a 280% increase in the TEG output power compared with an untreated Al heat exchanger at 200 °C.

The study of femtosecond laser structural coloring has recently attracted a great amount of research interest. These studies, however, have only been carried out in air. At the same time, laser ablation has also been actively studied in liquids as they provide a unique environment for material processing. However, surprisingly, structural coloring has never been performed in liquids. In this work, we perform the first study of metal structural coloring in liquid and compare the results to metal structural coloring in air. Colors created in liquid are formed by nanoparticle-induced plasmonic absorption and result in a range of colors transitioning from purple to orange. Surface structures formed in liquid are less hierarchical and more uniform than those formed in air, producing a surface with a much higher reflectance due to reduced light trapping, resulting in a more vibrant color. However, colorization formed in water suffers from less uniform colorization due to turbulence at the air–water and water-target interfaces, resulting in slight changes to the laser beam's focus during processing. Finally, finite-difference-time-domain simulation based on the measured surface structures is used to understand the role of plasmonic resonance in colorization.

Summary Imaging photo-induced ultrafast dynamics of nanostructure phase transition is of great interest to the fields of laser-matter interactions and nanotechnology. However, conventional ultrafast far-field optical imaging methods cannot image nanostructures as their scattering scales as D6, with D being the diamater, leading to a vanishing signal-to-noise ratio. Here, we use ultrafast ultramicroscopy to capture the spatiotemporal evolution of surface nanostructures as they undergo melting, spallation, and re-solidification processes. Our experimental observations, combined with finite difference time domain (FDTD) simulations, show agreement with molecular dynamic simulations on ultrashort laser pulse-irradiated metallic nanoparticles and suggest the occurrence of melting of nanostructures followed by photomechanical spallation within a few picoseconds. At longer timescales, we image the re-solidification dynamics of the melted nanostructures occurring within nanoseconds. The re-solidification time for nanostructured surface occurs an order of magnitude faster than for an initially flat surface. Our study demonstrates a simple but powerful far-field optical approach for studying ultrafast dynamics of nanostructures.

The effect of laser fluence on the surface morphology and field emission properties of nickel (Ni) has been investigated. Circular shaped Ni targets are irradiated with Nd:YAG laser (1064 nm, 10 Hz, 10 ns) at various fluences ranging from 5.2 to 26 J/cm2 in air. For low fluence ranging from 5.2 to 10.4 J/cm2, SEM analysis reveals the growth of unorganized channels, grains, droplets, and ridges. Whereas, at moderate fluence of 15.6 J/cm2, the formation of ridges and cones along with few number of holes are observed. However, at high fluence regime ranging from 20 to 26 J/cm2, a sharp transition in morphology from ridges to holes has been observed. The laser structured Ni targets are also investigated for field emission properties by recording their I–V characteristics and Fowler–Nordheim (F–N) plots. The enhancement in field emission factor (β) and the reduction in turn on field are found to be dependent upon the laser fluence and morphology of the grown structures. For samples treated at low and moderate fluences, the growth of cones, channels and ridges is responsible for enhancement of β factor ranging from 121 to 178. Whereas, for samples treated at high fluence region, the formation of pores and holes is responsible for significant field convergence and consequently resulting in substantial enhancement in β factor to 276.

The fabrication of subwavelength two-dimensional (2D) structures on metals is of paramount importance to modern nanophotonics. Here we report a method to fabricate 2D conic structures on nickel surfaces using a single beam with three temporally delayed pulses. The 2D structures are fabricated over the entire irradiated region with relatively high uniformity. By controlling the delay between the three pulses, we control the effect of each pulse in creating laser-induced periodic surface structures which enables the control of the 2D structure features, namely, the period and structure dimensions. We explain the results based on the surface plasmon polariton-femtosecond laser interference model.

Wastewater recycling could be part of an environmentally feasible solution to an increasing global water crisis. In a recent work, we made a push toward that end, introducing a super-wicking, super-light-absorbing solar trackable panel for water sanitation.

Nanosecond pulsed laser ablation phenomena of single crystal Ge (100) has been investigated by employing photoacoustic deflection as well as SEM analysis techniques. Nd: YAG laser (1064 nm, 10 ns, 1–10 Hz) at various laser fluences ranging from 0.2 to 11 J cm −2 is employed as pump beam to ablate Ge targets. In order to evaluate in-situe ablation threshold fluence of Ge by photoacoustic deflection technique, Continuous Wave (CW) He–Ne laser (632 nm, power 10 mW) is employed as a probe beam. It travels parallel to the target surface at a distance of 3 mm and after passing through Ge plasma it causes deflection due to density gradient of acoustic waves. The deflected signal is detected by photodiode and is recorded by oscilloscope. The threshold fluence of Ge, the velocity of ablated species and the amplitude of the deflected signal are evaluated. The threshold fluence of Ge comes out to be 0.5 J cm −2 and is comparable with the analytical value. In order to compare the estimated value of threshold with ex-situe measurements, the quantitative analysis of laser irradiated Ge is performed by using SEM analysis. For this purpose Ge is exposed to single and multiple shots of 5, 10, 50 and 100 at various laser fluences ranging from 0.2 to 11 J cm −2 . The threshold fluence for single and multiple shots as well as incubation coefficients are evaluated. It is observed that the value of incubation co-efficient decreases with increasing number of pulses and is therefore responsible for lowering the threshold fluence of Ge. SEM analysis also reveals the growth of various features such as porous structures, non-uniform ripples and blisters on the laser irradiated Ge. It is observed that both the fluence as well as number of laser shots plays a significant role for the growth of these structures.

The spectroscopic and morphological studies of silver has been investigated after irradiation of Nd:YAG laser by employing Laser Induced Breakdown Spectroscopy (LIBS) and Scanning Electron Microscope (SEM) analyses. Silver targets were exposed to various laser fluences ranging from 5.2 J cm−2 to 41.7 J cm−2 under ambient environment of argon at a pressure of 100 Torr. LIBS analysis revealed that when the laser fluence was increased from 5.2 J cm−2 to 10.4 J cm−2, an increasing trend for both electron temperature and electron number density of plasma was observed due to enhanced energy deposition. Afterwards, with increasing fluence from 10.4 J cm−2 to 15.6 J cm−2, a decreasing trend was achieved which was attributed to the shielding effect. With further increase in fluence from 15.6 J cm−2 to a maximum value of 41.7 J cm−2, a saturation region was achieved and insignificant changes in both plasma parameters were observed. This saturation is explainable on the basis of the formation of a self-regulating regime. SEM analysis revealed the formation of flakes and ridges on the surface of silver at the lowest fluence of 5.2 J cm−2. For increased fluences ranging from 10.4 J cm−2 to 41.7 J cm−2, distinct and unorganized melted channels were the dominant features. However, at a maximum fluence of 41.7 J cm−2, micro and nano crystallites of lamellar (tabular) shape were also formed. At peripheral ablated areas, flakes, cavities, cones and craters with multiple ablated layers were formed. The modifications in surface morphology of laser-ablated silver have been correlated with plasma parameters.

Fabrication of smooth noble metal ultrathin films is crucial to many optical and electronic devices. However, the metals’ adatom-adatom cohesive force is usually stronger than the adatom-substrate adhesive force leading to a Vollmer-Weber, island-like, growth mode. This phenomenon imposes limitations on the surface smoothness and minimum (percolation) thickness necessary to obtain a smooth film. Here, we demonstrate a facile method to fabricate ultra-smooth ultrathin silver (Ag) films on silicon via physical vapor deposition. By removing the oxide layer on silicon substrates using a hydrofluoric acid treatment, Ag atoms bind strongly to the energetically favorable silicon atoms leading to smooth Ag films. We compare the results for Ag deposited on HF treated and untreated Si substrates for different Ag thicknesses. Our results show that HF acid treatment and annealing lead to a significant reduction in the surface roughness (∼0.5 nm), narrower peak-to-valley height distribution, and higher Kurtosis. Continuous Ag films were obtained down to Ag thickness of 5 nm. We expect our results to play a crucial role in minimizing electronic and optical losses for optoelectronic, plasmonic and optical metamaterial devices.

Controlling the surface morphology at the subwavelength scale is one of the cornerstones of modern nanophotonics. Femtosecond laser-induced surface structuring is a viable technique for the large-scale formation of nano- and microscale structures. A typical example is the formation of one-dimensional laser-induced periodic surface structures (LIPSSs), which can lead to strong modifications of optical and wetting properties of the material surface. Creating two-dimensional (2D) patterned structures, however, is a more challenging and rewarding task. Here, we demonstrate a single step method for fabricating various subwavelength structures on the cobalt (Co) surface using different laser fluences (0.12–0.24 J/cm2) and time delay (0–30 ps) between double pulses. More importantly, we can control the geometry and organization of the formed structures demonstrating spherical, triangular, rhombic, and high spatial frequency LIPSSs using two temporally delayed orthogonally polarized femtosecond laser beams. We show that the laser fluence and delay time between the two beams are the controlling parameters for realizing the different surface morphologies. We provide a numerically supported, phenomenological model to explain the formed 2D structures. Our model employs elements from both the scattered surface-wave interference and the self-organization theories that are commonly used to explain uniform surface structures.

Direct femtosecond laser ablation enables the maskless fabrication of nano- and micro-scale structures on variety of materials. A typical example is the formation of femtosecond Laser Induced Periodic Surface Structures (fs-LIPSSs), which can lead to strong modification in electrical, optical, wetting, and field emission properties of materials. Here, we study the field emission properties of fs-LIPSSs. We created fs-LIPSSs and fs-LIPSSs covered with nano- and micro-scale structures at different laser fluences on Tungsten (W) and showed that these structures offer significant enhancement in electron field emission properties. We provide a phenomenological model to explain the enhancement of electron emission parameters. The enhancement in the field emission properties of laser irradiated W is explained based on the convergence of electric field lines at the ridges of the fs-LIPSSs and fs-LIPSSs covered with nanoscale structures, which in turn, enhances the local field intensity and the electron emission parameters. The direct fabrication of 1D subwavelength structures is an important step towards the creation of low-cost cathodes for various potential applications.

Flat optics suffer meager efficiency due to plasmonic losses at visible wavelengths. This issue has been addressed in this study by the substitution of dielectric materials. For this purpose, optical parameters (real and imaginary parts of refractive index) and surface roughness of TiO2 films were optimized, which are pre-requisite for the development of highly efficient dielectric metasurface devices. Electron beam evaporator was employed to deposit various ultrathin TiO2 films with subwavelength thickness. These deposited films were further processed via annealing at various temperatures to achieve the appropriate optical parameters. SEM analysis confirmed the absence of craters, cracks and rugged type features, whereas, AFM analysis revealed the smoothness of deposited films with least roughness. High refractive index above 2.2 with minimum absorption coefficient in the visible region was studied through ellipsometry analysis. FTIR spectroscopy showed transmission over 90% from the deposited thin film on glass substrate. The results of this study would have significant implications for material processing at nanoscale and dielectric metasurface fabrication which would in turn eliminate the requirement of costly and sophisticated setups for such fabrications.

Ti3O5 films are deposited with the help of an electron beam evaporator for their applications in metasurfaces. The film of subwavelength (632 nm) thickness is deposited on a silicon substrate and annealed at 400°C. The ellipsometry result shows a high refractive index above 2.5 with the minimum absorption coefficient in the visible region, which is necessary for high efficiency of transparent metasurfaces. Atomic force microscopy analysis is employed to measure the roughness of the as-deposited films. It is seen from micrographs that the deposited films are very smooth with the minimum roughness to prevent scattering and absorption losses for metasurface devices. The absence of grains and cracks can be seen by scanning electron microscope analysis, which is favorable for electron beam lithography. Fourier transform infrared spectroscopy reveals the transmission and reflection obtained from the film deposited on glass substrates. The as-deposited film shows high transmission above 60%, which is in good agreement with metasurfaces.