Femtosecond lasers are a powerful tool for high-precision material processing and functionalization. In my lab, the laser processing led to a range of technological developments, including the so-called black and colored metals, superhydrophillic and superhydrophobic surfaces. In this talk, I will discuss our recent developments in femtosecond laser micro- and nano-scale surface patterning, including the formation dynamics, the drastically altered surface functionalities, and the various applications.
This talk highlights two parallel technologies developed in my lab for solar energy harvesting. First, femtosecond lasers are utilized for material processing and functionalization, which enabled the creation of the so-called black and colored metals. Here, I will discuss the black and colored metals in a range of photo-thermal applications. Second, I will discuss our recent push in introducing a pure physics-based approach to enhance perovskite performance by utilizing metamaterials. This physics approach rivals the most advanced chemical engineering and provides a new pathway for enhancing perovskites' performance in photo-voltaic applications.
We show that the recombination rate from excitons in Lead Halide Perovskite (LHP) polycrystalline thin films is suppressed, rather than enhanced, when they are placed on hyperbolic metamaterials and a plasmonic substrate by 30% and 50%, respectively. We reveal that these LHP films enjoy a dominant in-plane transition dipole, which is responsible for the decrease in the recombination rate. In triple cation LHPs we observe a 10-fold decrease in the recombination rate, comparable to recombination rate suppression obtained by Photonic Crystal cavities. By controlling the recombination rate of LHPs, we demonstrate a 250% increase in photoresponsivity of LHP-based photodetectors.
We analyze the impact of angular selectivity on the radiative cooling performance of thermal emitters. We show that combining angular and spectral selectivity enables realizing deep subfreezing temperatures. While angularly selective thermal emitters are robust to environmental effects, they require management of non-radiative heat transfer processes. We introduce a general scheme to realize angularly and spectrally selective absorption/emission using a thin film stack consisting with a total thickness of ~ 9 μm; an order of magnitude less than previously proposed angular selective thermal emitters. We finally discuss the prospects of angular selectivity based on conic mirrors.
Femtosecond lasers are a powerful tool for high-precision material processing. Femtosecond laser induced micro- and nano-scale surface patterning can also lead to surface functionalization. In my lab, the laser processing led to a range of technological developments, including the so-called black and colored metals. In this talk, I will discuss our recent developments in laser-induced structural coloring and its various applications. I will also discuss some of our recent work on non-laser-induced structural coloring and applications. Other associated surface functionalization induced by laser irradiation, such as superhydrophillic and superhydrophobic surfaces, will also be discussed.
The performance of all-optical (AO) logic AND and OR gates, which are realized for the first time using reflective semiconductor optical amplifiers (RSOAs) as nonlinear elements, is theoretically investigated at 120 Gb / s. The switching modules that incorporate the RSOAs and exploit their potential for AO signal processing are the Mach–Zehnder interferometer and the delayed interferometer for the AND and OR operations, respectively. A performance comparison between an RSOA and a conventional semiconductor optical amplifier (SOA) is made by examining and assessing the quality factor against the operational critical parameters, including the effect of the amplified spontaneous emission. The obtained results confirm that these Boolean functions based on RSOAs can be executed at the target data rate with better performance than if conventional SOAs were used instead.
Perfect light absorption (PLA) in nanophotonics has a wide range of applications from solar-thermal based applications to radiative cooling. However, most of the proposed platforms require intense lithography which makes them of minor practical relevance. On the other hand, thin-film light absorbers are lithographically free and can be deposited cheaply on large area based on matured technologies. However, thin-film light absorbers were thought to have major limitation and cannot be tailored compared to metamaterials. Here, we show how to design PLA using thin-films in terms of wavelength range, bandwidth, spatial profile of optical losses, directionality and iridescence. We also show that iridescent free, PLA can occur by simply heating metallic thin-films when the metal is of low reflectance and its oxide is of high refractive index. We theoretically and experimentally demonstrate Generalized Brewster angle effect in thin film light absorbers. In addition, we demonstrate hydrogen sensing using three different PLA strategies showing record sensitivity and figure of merit. Furthermore, we show various strategies to create ultra-pure structural colors. Finally, we demonstrate different solar-thermal applications for novel thin-film PLA designs.
Transmissive-type metasurfaces represent an ultrathin alternative to traditional optical elements, e.g., lenses and waveplates. However, transmissive-type plasmonic metasurfaces (PMs) have significantly low efficiency compared to dielectric metasurfaces and reflective type PMs particularly in the visible range. For example, the state-of-the-art geometric PMs transmission efficiency is ≤10% with extinction ratios ~ 0 dB. The low transmission efficiency is mainly due to three loss channels (i) absorption losses in metals, (ii) diffraction to undesired high-orders, and most importantly (iii) symmetric forward-backward scattering which puts a 25% theoretical limit on cross-polarization conversion for ultrathin metasurfaces. We use tunable, multipole-interference-based meta-atoms to address all loss channels simultaneously. The experimentally demonstrated transmission efficiency and extinction ratio of our geometric PM are 42.3% and 7.8dB, respectively. As for dielectric metasurfaces, we demonstrate a new class of metasurfaces where the meta-atoms consist of a simple anti-reflective coating (ARC). ARCs enable the control over the entire 2 pi phase range by varying the dielectric films thicknesses while realizing ~ 99% transmission efficiency even in the visible range. The metasurface consists of patches of ARC meta-atoms with dielectric optical thicknesses much lower than that required in Fresnel optics to control the entire phase range.
Imaging transient dynamics of materials and light-matter interaction at the nanoscale is of great interest to the study of condensed phase dynamics and to the field of nano-photonics. However, optical interrogation of the ultrafast dynamics of nanostructures has not been demonstrated as they are diffraction limited. Furthermore, optical methods are limited due to the low scattering of nanostructures and the strong background reflection. Accordingly, The ultrafast imaging of laser induced nanostructure melting was demonstrated via femtosecond x-ray diffraction imaging which provided relatively high temporal (~10 ps) and high spatial (~10 nm) resolution. However, this technique suffers from the inherent difficulty of using a femtosecond X-ray laser source and the damaging nature of the femtosecond X-ray laser probe required for single-shot imaging. Consequently, X-ray pump-probe imaging was never used to image the re-solidification dynamics of surface structures. On the other hand, cryo-electron microscopy achieved sub-nanometer resolution for single particles, however, it requires experiments to be performed in vacuum with highly specialized and costly instrumentation. Here, we we employ a time-resolved variant of ultramicroscopy that we recently developed to study the ultrafast dynamics of laser ablated surfaces. The technique is non-destructive and allows us to compare the transient image with the initial/final image. Accordingly, we determine the characteristic times for melting and re-solidification of nanostructures using optical wavelengths. We also study the formation and melting of Si nanostructures and image, for the first time, the process of non-thermal melting which occurs on the sub-picosecond time scale.
KEYWORDS: Solar energy, Thin films, Thin film coatings, Optical coatings, Energy efficiency, Solar cells, Photovoltaics, Energy conversion efficiency, Beam splitters, Optical components
Increasing the efficiency and cost effectiveness of solar energy generation allowed them to compete with traditional carbon-based energy sources in many energy markets worldwide. However, a major problem facing the proliferation of solar energy generation is energy storage. Photovoltaic (PV) generators enjoy relatively high efficiency but suffer from high electric energy storage costs. On the other hand, solar-thermal energy conversion enables storing heat and dispatch electricity at lower storage costs but with less efficiency compared to photovoltaics. Hybridizing both solar energy conversion can address the energy storage problem. Furthermore, single junction PVs are unable to convert a large portion of the solar spectrum to energy which eventually lead to PV thermalization. Spectral beam splitting is a promising method to achieve high efficiency solar energy conversion while hybridizing electric and thermal solar energy generation.
Here, we use novel thin-film based optical coatings to develop single element selective light reflector/absorber that reflects within the wavelength range corresponding to a PV cell absorption band, while absorbing the remaining solar spectrum. We show that reflecting solar light on a PV cell using a silver mirror is less efficient and has higher temperature compared to using selective light reflector/absorber when using optical concentration exceeding 2 suns. We demonstrate hybrid PV generation and water sanitation/desalination using an Aluminum sheet with thin-film selective light reflector /absorber deposited on one side, while the other side is treated with femtosecond laser to become superwicking.
We present a new class of thin-film based metamaterials that exhibits Fano resonance with wide range of potential applications. We realize Fano resonance via thin-film interference between a broadband (continuum) and a narrowband (discrete) light absorbers. Fano resonant optical coatings (FROCs) exhibit selective light reflection, similar to distributed bragg reflectors, with narrower bandwidth and overall significantly less thickness for a given wavelength range. Accordingly, FROCs produce vibrant colors superior to structural coloring via selective light absorption that has been demonstrated using metamaterials and thin-film cavities. We control the iridescence of the produced colors and can produce iridescent free, ultra-pure colors spanning the entire visible spectrum. Furthermore, we show that FROCs can be used as narrowband beam splitters, as opposed to being simple color filter similar to metal-dielectric cavities. Finally, we utilize the absorption/reflection properties of FROCs in energy applications and show that by selectively reflecting light within the absorption band of Si photovoltaic cell, while absorbing the rest of the solar spectrum, we obtain higher power from PV cells as opposed to a normal silver mirror, while increasing the FROC temperature significantly. Accordingly, FROCs can play a crucial role in hybrid, solar-PV and solar-thermal power generation which is of major importance in recent years due to limitation on electric energy storage. By coating an Aluminum sheet with FROC, while processing its back side to be superwicking, we demonstrate single-element spectral splitting that generate electricity from a PV cell while using the generated heat for water desalination.
In this work, by applying the femtosecond laser blackening technique directly to a tungsten incandescent lamp filament,
we dramatically brighten the tungsten lamp and enhance its emission efficiency by 60%. This process in fact leads the
tungsten incandescent light bulb to approach the 100% emission efficiency. A comparison study of emission and
absorption for the structured metal surface shows that Kirchhoff's law is applicable for the black metal. More
importantly, we demonstrate that we can even obtain partially polarized light as well as control the spectral range of the
optimal light emission from the laser-blackened tungsten lamp.
In this work, we show that the resonant angle of surface plasmons (SPs) excited on a unique type of
nanostructured metal can be significantly different from the calculated values. We also find that the excitation
of SPs can significantly enhance photoelectron emission on the structured metal surfaces.
Our study shows that a large variety of nano- and micro-scale structures can be controllably produced on metallic
biomaterials such as titanium, platinum, and gold using direct femtosecond laser processing techniques. This process
provides a way for controlling behaviors of both proteins and cells. Also, we find the laser treatment conditions for
producing femtosecond laser-induced periodic structures that can be suitable for surface-plasmon-based biosensors.
Recently, using a femtosecond laser surface structuring technique we turned highly reflective metals to highly
absorptive, creating the so-called "black metals". In this study, we made an even more significant advancement. Here,
we demonstrate that our femtosecond laser structuring technique not only allows us to create black metals but also gray
and even color metals. We show that our technique essentially provides a controllable modification of optical properties
of metals from the UV to THz spectral range via surface structuring on the nano-, micro-, and submillimeter-scales.
Based on experiments and a theoretical analysis, we raise questions on two fundamental mechanisms of femtosecond
laser desorption/ablation of solids, namely Coulomb explosion (CE) and plasma etching. The effects of laser-induced
ionization and surface charging are analyzed which can be responsible for ultrafast ions observed in time-of-flight mass-spectra
under ultrashort laser irradiation of solids. The importance of surface charging in formation of velocity
distributions of desorbed/ablated species has been revealed for conditions when the CE mechanism is inhibited. The
influence of ambient plasma formation on the dynamics of heating of metallic targets by femtosecond laser pulses is
studied based on 2D modeling of laser-induced target heating and dynamics of the ambient plasma. The calculations
show an intriguing picture of the laser-induced ambient gas motion. We propose a model of laser-induced breakdown of
an ambient gas in a region in front of the irradiated target and analyze plasma-chemical processes which can affect laser
processing of surfaces in the presence of air or highly reactive media.
We perform a comparison study of periodic structures on the surfaces of three different noble metals, Cu, Ag, and Au,
following femtosecond laser radiation. Under identical experimental conditions, laser-induced surface patterns show
distinctly different level of morphological clearness on the three different metals. Simply calculations based on metal
melting fail to explain the pattern differences. We show that our observation result from the competition of two
ultrafast processes, electron-phonon energy coupling and hot electron diffusion, following femtosecond laser heating of metals.
Surface relief topography and absorptivity enhancement due to surface structural modification
induced by multi-pulse femtosecond laser ablation of a bulk gold sample are studied. It is shown
that the enhancement of the absorptivity in multi-pulse ablation is caused by a combination of
nano-, micro-, macro-structural surface modifications. Femtosecond laser-induced surface
nanostructuring is found to be a significant factor enhancing the absorptivity. This absorptivity
enhancement of the nano-structured surface is explained as a result of both multiple light
reflections from produced ramified gold surface and resonant linear and nonlinear light
absorption by coupled nanostructures (spheres, wires, cones).
With recent advances in ultrafast laser technology, ablation using femtosecond laser pulses has found a wide range of applications. It is widely believed that ablation using femtosecond lasers has many unique advantages compared to longer-pulse ablation. One of the most significant features is believed to be that a negligible amount of
energy should remain in the sample following ablation because the deposited heat does not have enough time to travel into the bulk sample during the femtosecond laser pulse. In this paper, we discuss our new findings on thermal effects in femtosecond laser ablation of metals. In contrast to the previous common belief, we find that a significant amount of residual thermal energy is deposited in metal samples following multi-shot femtosecond laser ablation. We further
discuss how the absorptance of metals depends on the structural modifications. Lastly, we discuss the formation of periodic surface structures on metals induced by femtosecond laser pulses.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.