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The ability to control the instantaneous state of light, from high-energy pulses down to the single-photon level, is an indispensable requirement in photonics. This has, for example, facilitated spatiotemporal probing and coherent control of ultrafast light-matter interactions, and enabled capabilities such as generation of exotic states of light with complexity, or at wavelengths, that are not easily accessible. Here, by leveraging the multifunctional control of light at the nanoscale offered by metasurfaces embedded in a Fourier transform setup, we present a versatile approach to synthesize ultrafast optical transients with arbitrary control over its complete spatiotemporal evolution. Our approach, supporting an ultrawide bandwidth with simultaneously high spectral and spatial resolution, enables ready synthesis of complex states of structured space-time wave packets. We expect our results to offer unique capabilities in coherent ultrafast light-matter interactions and facilitate applications in microscopy, communications, and nonlinear optics.

Modular integration of plasmonic nanoantennas with 3D micropillar electrode arrays offer hybrid optical-electrical functionality, crucial for advancing in situ spatiotemporal characterization of correlated bioelectrical and biochemical processes.

Surface-enhanced Raman spectroscopy (SERS) has become a powerful technique for ultrasensitive biochemical detection providing molecular fingerprint information. Due to the strong dependence of surf...

By supporting localized plasmon modes, metal-based plasmonic nanostructures can confine optical fields at deep-subwavelength scale in various applications, such as biological and chemical sensing, nanoscale light emission, and solar energy harvesting. While Cu is a low-cost complementary metal oxide semiconductor (CMOS) compatible material, its poor chemical stability limits the use of Cu plasmonic nanodevices in corrosive biochemical aqueous environments. In this paper, we demonstrate that sub-10 nm Al2O3/HfO2 nanolaminated coatings can significantly extend the lifetime of Cu nanodisk arrays from ∼5 h to ∼180 days in the physiological environment of 1× phosphate-buffered saline (PBS) at 37 °C. Cu nanodisk arrays are fabricated using freestanding Au nanohole array films as the physical vapor deposition masks and sub-10 nm nanolaminated coatings composed of alternating Al2O3 and HfO2 nanolayers are grown on Cu nanodisk arrays by atomic layer deposition (ALD). Time-dependent optical extinction measurements of Cu nanodisk arrays are conducted in 1× solutions at 37 °C to investigate the anticorrosion performance for different pure and nanolaminated ALD coatings. We observe a linear relationship between the lifetime of Cu nanodisk arrays in 1× PBS at 37 °C and the nanolaminated coating thickness, and ∼1.3 nm nanolaminated coatings of ∼10 ALD cycles can extend the lifetime of Cu plasmonics up to ∼20 days. Furthermore, we find that the anticorrosion performance of Al2O3/HfO2 nanolaminated ALD coatings strongly depends on the processing and the geometric parameters, such as the annealing temperature and the nanolaminated backbone unit size.

Arbitrary spatiotemporal shaping requires complete control of the electric-field vector at each space-time coordinate. Here, we demonstrate simultaneous and independent tailoring of spatial and temporal properties of femtosecond pulses using a single-layer dielectric metasurface.

We use inverse design to develop an optimized nanophotonic geometry for efficient, waveguide-coupled, path-entangled photon pair sources based on single embedded quan-tum dots.

We demonstrate a scalable, integrated photonics infrastructure for optical atomic clocks. We laser cool and trap 4 × 105 87Sr atoms in a magneto-optical trap, using metasurface optics and a compact, single-chamber vacuum system.

We fabricated guided mode resonance filters (GMRFs) with two-dimensional (2-D) gratings operating in the 8 to 12 µm long-wave infrared (LWIR) region by depositing amorphous germanium (Ge) film to form a zero-contrast (ZC) waveguide-grating (WGG) on polished zinc selenide (ZnSe) substrates with and without antireflection coating (ARC). We employed high-spatial resolution e-beam lithography and reactive-ion etching (RIE) nanofabrication techniques. We characterized the fabricated filters for their polarization independent spectral performance using a tunable quantum cascade laser (QCL) system and a modified Fourier transform infrared (FTIR) spectrometer. Here, we will present both theoretical and experimental results and their comparison.

Ultrathin gold films are attractive for plasmonic and metamaterial devices, thanks to their useful optical and optoelectronic properties. However, deposition of ultrathin continuous Au films of a f...

We report $8-12\ \mu \mathrm{m}$ longwave infrared tunable notch filters using the guided-mode resonance (GMR) effect in dielectric metasurfaces. This region corresponds to an atmospheric window that is widely used for sensing and imaging applications for day/night situational awareness.

The long-wave infrared (LWIR) spectroscopy has emerged as a promising technique for applications ranging from medical diagnosis to satellite imaging and terrestrial imaging. However, traditional optical elements are not realized well for the LWIR region. In this work, notch filters for LWIR spectral range (8 ~ 12 μm) based on the guided mode resonance (GMR) effect were designed, fabricated, and characterized with Germanium (Ge) thin film on Zinc Selenide (ZnSe) substrates. In contrast to the typical photolithography process, which faces challenge of resolution limit, we used e-beam lithography process to pattern the grating. By using a reactive ion etching process with a mixture of etching and passivation gases, we fabricated grating with well-defined vertical sidewalls. Finite-difference time-domain (FDTD) method was used to calculate the optical responses and model the geometry of the notch filters, and the optical transmittance of fabricated filters agrees well with the calculations. Moreover, we demonstrate the improved notch filtering response with reduced Fabry-Perot noise by introducing anti-reflection layer on the bottom of the ZnSe substrate. Therefore, findings of this work will be useful for various filter fabrications that prefer high spatial resolution in the LWIR spectral region.

We describe recent progress in the development of spectrally tunable micro-engineered notch filters operating in the longwave infrared (LWIR) region from 8 to 12 µm based on using the guided-mode resonance (GMR) effect. The device structure consists of a subwavelength dielectric grating on top of a homogeneous waveguide using high-index dielectric transparent materials, i.e., germanium (Ge) with a refractive index of 4.0 and zinc selenide (ZnSe) with a refractive index of 2.4. We design the filters to reflect the incident broadband light at one (or more) narrow spectral band while fully transmitting the rest of the light. Filters based on one-dimensional (1-D) gratings are polarization-dependent and those based on two-dimensional (2-D) gratings are less polarization-dependent. We designed and characterized both 1-D and 2- D filters. Anti-reflection coatings (ARCs) were applied on the backside of some of the filter substrates to improve transmission over the entire spectral region. We carried out transmission measurements of these filters using two separate experimental setups—an automated tunable room-temperature quantum cascade laser (QCL) system as well as a modified Fourier Transform Infrared (FTIR) spectrometer with normal incidence of light on the sample. We will present filter designs, theoretical simulation, characterization experiments and results.

Metallic nano-optoelectrode arrays can simultaneously serve as nanoelectrodes to increase the electrochemical surface-to-volume ratio for high-performance electrical recording and optical nanoantennas to achieve nanoscale light concentrations for ultrasensitive optical sensing. However, it remains a challenge to integrate nano-optoelectrodes with a miniaturized multifunctional probing system for combined electrical recording and optical biosensing in vivo. Here, we report that flexible nano-optoelectrode-integrated multifunctional fiber probes can have hybrid optical-electrical sensing multimodalities, including optical refractive index sensing, surface-enhanced Raman spectroscopy, and electrophysiological recording. By physical vapor deposition of thin metal films through free-standing masks of nanohole arrays, we exploit a scalable nanofabrication process to create nano-optoelectrode arrays on the tips of flexible multifunctional fiber probes. We envision that the development of flexible nano-optoelectrode-integrated multifunctional fiber probes can open significant opportunities by allowing for multimodal monitoring of brain activities with combined capabilities for simultaneous electrical neural recording and optical biochemical sensing at the single-cell level.

We present new, to the best of our knowledge, large-scale, high-quality spectral filters operating in the long-wave infrared (LWIR) spectral region. We employ high-spatial resolution nanofabrication techniques to achieve large-area ( 12 m m × 12 m m ) spectrally tunable notch filters. Filter operation is based on the guided-mode resonance effect. The device structure consists of a germanium waveguide grating on top of a zinc selenide substrate. The filters reflect the incident broadband light at one (or more) narrow spectral bands while fully transmitting the rest. We tune the reflected wavelength by tilting the filter. Filters based on one-dimensional gratings are polarization sensitive. We fabricate prototype filters and characterize their polarization dependence and spectral tuning performance using a tunable quantum cascade laser system that spans the ∼ 7 − 13 µ m spectral band. We obtain an excellent agreement between the theoretical and experimental results.

By supporting multiple hybridized plasmonic modes within individual nanocavities, metal-insulator-metal nanolaminate plasmonic nanostructures on nanopillar arrays can enable nanoscale multiband SERS with large enhancement factors (> 106) over a wide visible to near-infrared spectral range.

We demonstrate arbitrary control of femtosecond timescale complex electrical-field transients using dielectric metasurfaces, enabling temporal vectorial ultrafast pulses with rich instantaneous polarization states and a simultaneously tailored spatial wavefront.

A dielectric-metasurface-enabled pulse shaper able to tailor the temporal instantaneous polarization states within a near-infrared femtosecond pulse is demonstrated. Simultaneous complex wavefront shaping has been implemented by exploiting metasurfaces’ multi-functionalities within a single-pixel.

Ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in quantitative analysis because of its susceptibility to local optical field variations at plasmonic hotspots in metallo-dielectric nanostructures. Current SERS calibration approaches using Raman tags have inherent limitations due to spatial occupation competition with analyte molecules, spectral interference with analyte Raman peaks, and photodegradation. Herein, we report that plasmon-enhanced electronic Raman scattering (ERS) signals from metal can serve as an internal standard for spatial and temporal calibration of molecular Raman scattering (MRS) signals from analyte molecules at the same hotspots, enabling rigorous quantitative SERS analysis. We observe a linear dependence between ERS and MRS signal intensities upon spatial and temporal variations of excitation optical fields, manifesting the |E|4 enhancements for both ERS and MRS processes at the same hotspots in agreement with our theoretical prediction. Furthermore, we find that the ERS calibration's performance limit can result from orientation variations of analyte molecules at hotspots.

The conventional methods of creating superhydrophobic surface-enhanced Raman spectroscopy (SERS) devices are by conformally coating a nanolayer of hydrophobic materials on micro-/nanostructured plasmonic substrates. However, the hydrophobic coating may partially block hot spots and therefore compromise Raman signals of analytes. In this paper, we report a partial Leidenfrost evaporation-assisted approach for ultrasensitive SERS detection of low-concentration analytes in water droplets on hierarchical plasmonic micro-/nanostructures, which are fabricated by integrating nanolaminated metal nanoantennas on carbon nanotube (CNT)-decorated Si micropillar arrays. In comparison with natural evaporation, partial Leidenfrost-assisted evaporation on the hierarchical surfaces can provide a levitating force to maintain the water-based analyte droplet in the Cassie-Wenzel hybrid state, i.e., a Janus droplet. By overcoming the diffusion limit in SERS measurements, the continuous shrinking circumferential rim of the droplet, which is in the Cassie state, toward the pinned central region of the droplet, which is in the Wenzel state, results in a fast concentration of dilute analyte molecules on a significantly reduced footprint within several minutes. Here, we demonstrate that a partial Leidenfrost droplet on the hierarchical plasmonic surfaces can reduce the final deposition footprint of analytes by 3-4 orders of magnitude and enable SERS detection of nanomolar analytes (10-9 M) in an aqueous solution. In particular, this type of hierarchical plasmonic surface has densely packed plasmonic hot spots with SERS enhancement factors (EFs) exceeding 107. Partial Leidenfrost evaporation-assisted SERS sensing on hierarchical plasmonic micro-/nanostructures provides a fast and ultrasensitive biochemical detection strategy without the need for additional surface modifications and chemical treatments.

We present novel polarization independent, high-quality monolithic spectral filters based on the guided-mode resonance (GMR) effect with orthogonal linear gratings on either side of the substrate operating in the longwave infrared (LWIR) spectral region. We employ high-spatial resolution e-beam lithography and reactive-ion etching (RIE) nanofabrication techniques to achieve large-area (10×10 mm2) notch filters with subwavelength features. We fabricated prototype filters and characterized their polarization independent spectral performance with both coherent and incoherent incident light using a tunable quantum cascade laser (QCL) system that spans the ∼8–12 µm spectral band as well as a Fourier transform infrared (FTIR) spectrometer with collimated incident beam.

Optical nanoantennas can concentrate light and enhance light-matter interactions in subwavelength domain, which is useful for photodetection, light emission, optical biosensing, and spectroscopy. However, conventional optical nanoantennas operating at a single wavelength band are not suitable for multiband applications. Here, we propose and exploit an out-of-plane plasmonic engineering strategy to design and create composite optical nanoantennas that can support multiple nanolocalized modes at different resonant wavelengths. These multiresonant composite nanoantennas are composed of vertically stacked building blocks of metal-insulator-metal loop nanoantennas. Studies of multiresonant composite nanoantennas demonstrate that the number of supported modes depends on the number of vertically stacked building blocks and the resonant wavelengths of individual modes are tunable by controlling the out-of-plane geometries of their building blocks. In addition, numerical studies show that the resonant wavelengths of individual modes in composite nanoantennas can deviate from the optical response of building blocks due to hybridization of magnetic modes in neighboring building blocks. Using Au nanohole arrays as deposition masks to fabricate arrays of multilayered composite nanoantennas, we experimentally demonstrate their multiresonant optical properties in good agreement with theory predictions. These studies show that out-of-plane engineered multiresonant composite nanoantennas can provide new opportunities for fundamental nanophotonics research and practical applications involving optical multiband operations, such as multiphoton process, broadband solar energy conversion, and wavelength-multiplexed optical system.

The application of surface-enhanced Raman spectroscopy (SERS) for everyday quantitative analysis is hindered by the point-to-point variability of SERS substrates that arises due to the heterogeneous distribution of localized electromagnetic fields across a suite of plasmonic nanostructures. Herein, we adopt surface-enhanced elastic scattering as a SERS internal standard. Both elastic and inelastic (i.e., Raman) scattering are simultaneously enhanced by a given “hot spot”, and thus, the surface-enhanced elastic scattering signal provides a localized intrinsic internal standard that scales across all of the plasmon-enhanced electromagnetic fields within a substrate. Elastically scattered light originates from the amplified spontaneous emission (ASE) of the commercial laser, leading to the formation of a low-wavenumber pseudo band that arises from the interaction of the ASE and the edge filter. A theoretical model was developed to illustrate the underlying mechanism supporting this normalization approach. The normalized Raman signals are independent of the incident laser intensity and the density of “hot spots” for numerous SERS substrates. Following “hot-spot” (HS) normalization, the coefficient of variation for the tested SERS substrates decreases from 10 to 60% to 2%–7%. This approach significantly improves SERS quantitation of four chloroanilines and enables collection of highly reproducible analyte adsorption results under both static and dynamic imaging conditions. Overall, this approach provides a simple means to improve SERS reproducibility without the need to use additional chemicals as internal standards.

Leidenfrost evaporation-assisted Surface-enhanced Raman spectroscopy on hierarchical plasmonic micro/nanostructures provides fast ultrasensitive biochemical detection methods beyond the diffusion limit.

We demonstrate a novel all-dielectric, ultraviolet metasurface platform based on Tantalum Pentoxide, enabling high-performance meta-devices operating across the mid- and nearultraviolet frequency regimes.

Dielectric metasurfaces enable control of the temporal profile of large bandwidth, near-infrared femtosecond pulses. Using this approach, we demonstrate shaping of the time-domain polarization state within a single pulse.

We demonstrate a dielectric-metasurface-enabled pulse shaper able to tailor the temporal polarization of near-infrared femtosecond pulses over an ultrawide bandwidth, allowing three-dimensional vector-shaped pulses exhibiting rich time-evolving instantaneous polarization states within a single pulse.

Dielectric metasurfaces enable control of the temporal profile of large bandwidth, near-infrared femtosecond pulses. Using this approach, we demonstrate shaping of the time-domain polarization state within a single pulse.

We present novel integrated photodetectors based on the radiation pressure of a plasmonic mode. Light absorbed in a plasmonic or hybrid plasmonic/optical waveguide builds a voltage along the length of the waveguide via the photon-drag effect. We implement this device concept for the first time and investigate its potential for fast, broadband, and inexpensive optical detection in an integrated platform.

By supporting multiple hybridized surface plasmon resonances with spatial mode overlap, multiresonant nanolaminate plasmonic metamaterials can achieve broadband deep- subwavelength light concentration in the same nano-localized volume at multiple different wavelength ranges.

Multiresonant plasmonic nanoantennas can enhance nanolocalized multiphoton processes or enable wavelength-multiplexed nano-optic operations by supporting multiple spatially overlapped plasmonic modes. Nevertheless, current multiresonant plasmonic nanoantenna designs do not consider engineering multiresonant spectral responses with strict size and footprint constraints. Developing a strategy to engineer fixed-size nanoantennas with tunable multiresonant responses is highly desirable for maintaining controlled cellular responses at the nano-bio interface and achieving seamless integration with other nanodevices with predefined footprints. Here, we report that fixed-size tapered nanolaminate nanoantennas (TNLNAs) can achieve a wide double-resonance spectral tunability by only changing the metal-to-insulator thickness ratio (t/h). Three separate TNLNAs' samples (8/38 , 20/20, and 28/8 nm) with a nominal total height of ∼100 nm are created from a high-throughput nanofabrication technique. Specifically, we fabricated TNLNAs' samples by exploiting a nanohole array membrane from soft interference lithography as a deposition mask for electron-beam evaporation of alternating Au and SiO2 layers. Transmission and dark field scattering measurements show that TNLNAs support two distinct resonant features with t/h-dependent tunable resonant wavelengths in the range of 730–850 and 840–1050 nm, respectively. Numerical simulations reveal that (i) a bianisotropy-induced magnetoelectric response in top and bottom nanogaps due to the asymmetric tapered shape can enhance light trapping and achieve optical near-field intensity enhancements up to 1000-fold and (ii) while TNLNAs consisting of thin Au nanodisks at low t/h primarily support spatial overlap between modes with enhanced electric polarizability, TNLNAs consisting of thick Au nanodisks at high t/h support spatial overlap between modes with enhanced magnetic polarizability, evoking higher-order multipolar behaviors.

The performance of surface-enhanced Raman spectroscopy (SERS) substrates is typically evaluated by calculating an enhancement factor (EF). However, it is challenging to accurately calculate EF values since the calculation often requires the use of model analytes and requires assumptions about the number of analyte molecules within the laser excitation volume. Furthermore, the measured EF values are target analyte dependent and thus it is challenging to compare substrates with EF values obtained using different analytes. In this study, we propose an alternative evaluation parameter for SERS substrate performance that is based on the intensity of the surface plasmon enhanced Rayleigh band (IRayleigh) that originates from the amplified spontaneous emission (ASE) of the laser. Compared to the EF, IRayleigh reflects the enhancing capability of the substrate itself, is easy to measure without the use of any analytes, and is universally applicable for the comparison of SERS substrates. Six SERS substrates with different states (solid, suspended in liquid, and hydrogel), different plasmonic nanoparticle identities (silver and gold), as well as different nanoparticle sizes and shapes were used to support our hypothesis. The results show that there are excellent correlations between the measured SERS intensities and IRayleigh as well as between the SERS homogeneity and the variation of IRayleigh acquired with the six SERS substrates. These results suggest that IRayleigh can be used as an evaluation parameter for both SERS substrate efficiency and reproducibility.

The photovoltage generated in metal films conflicts with the prevailing intuitive model of light-metal momentum exchange, establishing the need for a new microscopic model of radiation pressure, and newly revealing the distribution of optical forces.

Nanoparticle surface coatings dictate their fate, transport, and bioavailability. We used a gold nanoparticle-bacterial cellulose substrate and "hot spot"-normalized surface-enhanced Raman scattering (HSNSERS) to achieve in situ and real-time monitoring of ligand exchange reactions on the gold surface. This approach enables semiquantitative determination of citrate surface coverage. Following exposure of the citrate-coated nanoparticles to a suite of guest ligands (thiolates, amines, carboxylates, inorganic ions, and proteins), the guest ligand signal exhibited first-order growth kinetics, while the desorption mediated decay of the citrate signal followed a first-order model. Guest ligand functional group chemistry dictated the kinetics of citrate desorption, while the guest ligand concentration played only a minor role. Thiolates and BSA were more efficient at ligand exchange than amine-containing chemicals, carboxylate-containing chemicals, and inorganic salts due to their higher binding energies with the AuNP surface. Amine-containing molecules overcoated rather than displaced the citrate layer via electrostatic interaction. Citrate exhibited low resistance to replacement at high surface coverages, but higher resistance at lower coverage, thus suggesting a transformation of the citrate-binding mode during desorption. High resistance to replacement in streamwater suggests that the role of surface-adsorbed citrate in nanomaterial fate and transport must be better understood.

A partial removal of dielectric materials in nanogaps can activate “passive” hotspots in multigap nanoplasmonic systems for scalable high-performance Surface Enhanced Raman Spectroscopy (SERS) applications.

Integration of multilayered plasmonic nanostructures with hierarchical micro-nanopillar arrays can result in a superhydrophobic surface with high-performance Surface Enhanced Raman Spectroscopy (SERS) functionality.

Quasi-3D multilayer Metal-Insulator-Metal (MIM) nanostructures sitting on dielectric nanopillar arrays show unique multiresonant optical response both in far-field and near-field.

Plasmonic nanocavities can control light flows and enhance light-mater interactions at subwavelength scale, and thus can potentially be used as nanoscale components in integrated optics systems either for passive optical coupling, or for active optical modulation and emission. In this work, we investigated a new type of multilayered metal-insulator optical nanocavities that can support multiple localized plasmon resonances with ultra-small mode volumes. The total number of resonance peaks and their resonance wavelengths can be freely and accurately controlled by simple geometric design rules. Multi-resonance plasmonic nanocavities can serve as a nanoscale wavelength-multiplexed optical components in integrated optics systems, such as optical couplers, light emitters, nanolasers, optical sensors, and optical modulators.

We demonstrated and studied a new type of multi-resonant optical nanocavities with ultra-small mode volume. By geometric engineering of metal-dielectric multilayers, the total number of resonances and their spectral peaks can be tuned continuously.