Recording and Revealing 2.5D Nanopatterned Hidden Information on Silk Protein Bioresists.

Nanopatterning on biomaterials has attracted significant attention as it can lead to the development of biomedical devices capable of performing diagnostic and therapeutic functions while being biocompatible. Among various nanopatterning techniques, electron-beam lithography (EBL) enables precise and versatile nanopatterning in desired shapes. Various biomaterials are successfully nanopatterned as bioresists by using EBL. However, the use of high-energy electron beams (e-beams) for high-resolutive patterning has incorporated functional materials and has caused adverse effects on biomaterials. Moreover, the scattering of electrons not absorbed by the bioresist leads to proximity effects, thus deteriorating pattern quality. Herein, EBL-based nanopatterning is reported by inducing molecular degradation of amorphous silk fibroin, followed by selectively inducing secondary structures. High-resolution EBL nanopatterning is achievable, even at low-energy e-beam (5 keV) and low doses, as it minimizes the proximity effect and enables precise 2.5D nanopatterning via grayscale lithography. Additionally, integrating nanophotonic structures into fluorescent material-containing silk allows for fluorescence amplification. Furthermore, this post-exposure cross-linking way indicates that the silk bioresist can maintain nanopatterned information stored in silk molecules in the amorphous state, utilizing for the secure storage of nanopatterned information as a security patch. Based on the fabrication technique, versatile biomaterial-based nanodevices for biomedical applications can be envisioned.

Resonant cavity phosphor.

While phosphors play an immensely important role in solid-state lighting and full-colour displays, it has been noted lately that their performance can be largely improved via structural engineering. Here, phosphor material is synergistically merged with yet another structurally engineered platform, resonant cavity (RC). When a 40-nm-thick colloidal quantum dot (CQD) film is embedded in a tailored RC with a moderate cavity quality factor (Q ≈ 90), it gains the ability to absorb the majority (~87%) of excitation photons, resulting in significantly enhanced CQD fluorescence (~29×) across a reasonably broad linewidth (~13 nm). The colour gamut covered by red and green pixels implemented using the RC phosphor-along with a broad bandwidth (~20 nm) blue excitation source-exceeds that of the sRGB standard (~121%). The simple planar geometry facilitates design and implementation of the RC phosphor, making it promising for use in real applications.

Engineering Silk Protein to Modulate Polymorphic Transitions for Green Lithography Resists.

Silk protein is being increasingly introduced as a prospective material for biomedical devices. However, a limited locus to intervene in nature-oriented silk protein makes it challenging to implement on-demand functions to silk. Here, we report how polymorphic transitions are related with molecular structures of artificially synthesized silk protein and design principles to construct a green-lithographic and high-performative protein resist. The repetition number and ratio of two major building blocks in synthesized silk protein are essential to determine the size and content of ß-sheet crystallites, and radicals resulting from tyrosine cleavages by the 193 nm laser irradiation induce the ß-sheet to α-helix transition. Synthesized silk is designed to exclusively comprise homogeneous building blocks and exhibit high crystallization and tyrosine-richness, thus constituting an excellent basis for developing a high-performance deep-UV photoresist. Additionally, our findings can be conjugated to design an electron-beam resist governed by the different irradiation-protein interaction mechanisms. All synthesis and lithography processes are fully water-based, promising green lithography. Using the engineered silk, a nanopatterned planar color filter showing the reduced angle dependence can be obtained. Our study provides insights into the industrial scale production of silk protein with on-demand functions.

Structurally engineered colloidal quantum dot phosphor using TiO2 photonic crystal backbone.

Photonic crystal (PhC) phosphor, in which the phosphor material is periodically modulated for an enhancement in color-conversion efficiency via resonant absorption of excitation photons, is a paradigm-shifting structural phosphor platform. Two-dimensional (2D) square-lattice PhC phosphor is currently considered the most advanced platform because of not only its high efficiency, but also its immunity to excitation polarization. In the present study, two major modifications are made to further improve the performance of the 2D PhC phosphor: increasing the refractive index contrast and planarizing the surface. The index contrast is improved by replacing the PhC backbone material with TiO2 whereas the surface planarization is achieved by removing excessive colloidal quantum dots from the surface. In comparison with the reference phosphor, the upgraded PhC phosphor exhibits ~59 times enhanced absorption (in simulations) and ~7 times enhanced emission (in experiments), both of which are unprecedentedly high. Our results not only brighten the viability and applicability of the PhC phosphor but also spur the phosphor development through structural engineering of phosphor materials.

Lasing at topological edge states in a photonic crystal L3 nanocavity dimer array.

Topological photonics have provided new insights for the manipulation of light. Analogous to electrons in topological insulators, photons travelling through the surface of a topological photonic structure or the interface of two photonic structures with different topological phases are free from backscattering caused by structural imperfections or disorder. This exotic nature of the topological edge state (TES) is truly beneficial for nanophotonic devices that suffer from structural irregularities generated during device fabrication. Although various topological states and device concepts have been demonstrated in photonic systems, lasers based on a topological photonic crystal (PhC) cavity array with a wavelength-scale modal volume have not been explored. We investigated TESs in a PhC nanocavity array in the Su-Schrieffer-Heeger model. Upon optical excitation, the topological PhC cavity array realised using an InP-based multiple-quantum-well epilayer spontaneously exhibits lasing peaks at the topological edge and bulk states. TES characteristics, including the modal robustness caused by immunity to scattering, are confirmed from the emission spectra and near-field imaging and by theoretical simulations and calculations.

Tunable colloidal quantum dot distributed feedback lasers integrated on a continuously chirped surface grating.

We report a colloidal quantum dot (CQD) distributed feedback (DFB) laser structure containing a chirped grating. The device exhibits single-mode DFB lasing, of which the wavelength is spatially dispersed in a single chip. A period-chirped surface grating is fabricated using a modified Lloyd-type laser interference lithography setup, where a flat Lloyd's mirror is replaced with a concave one. A dense red-emitting CdSe/CdS/ZnS CQD film is prepared on a temporary substrate by spin-coating, which is subsequently released and wet-transferred onto a period-chirped quartz surface grating. Upon optical excitation, the fabricated DFB laser device lases in a single mode at a laser threshold of ∼400 µJ cm-2, with its lasing wavelength shifted linearly (in proportion to the grating pitch) along the chirp direction from 613.4 nm to 623.2 nm over a distance of ∼5.6 mm.

Electrical modulation of a photonic crystal band-edge laser with a graphene monolayer.

The electrical control of photonic crystal (PhC) lasers has been an attractive but challenging issue. Laser operation by electrical injection is of key importance for the viability and applicability of the PhC lasers. Another key factor is the electrical modulation of the laser output. The Fermi level of a graphene monolayer can be controlled by electrical gating, which adjusts its optical absorption. In this study, a graphene monolayer sheet is integrated on top of a two-dimensional PhC structure composed of InGaAsP multiple-quantum-wells (MQWs) in order to demonstrate the electrical modulation of a high-power (microwatt-scale) PhC band-edge laser. The introduced dielectric spacer layer presets the delicate balance between the optical gain from the MQWs and optical loss at the graphene monolayer. The proposed device is covered by an ion-gel film, which enables a low-voltage laser modulation at |Vg|≤1 V. The modulation is extensively investigated experimentally, and the obtained results are confirmed by performing numerical simulations.

High-Contrast Infrared Absorption Spectroscopy via Mass-Produced Coaxial Zero-Mode Resonators with Sub-10 nm Gaps.

We present a wafer-scale array of resonant coaxial nanoapertures as a practical platform for surface-enhanced infrared absorption spectroscopy (SEIRA). Coaxial nanoapertures with sub-10 nm gaps are fabricated via photolithography, atomic layer deposition of a sacrificial Al2O3 layer to define the nanogaps, and planarization via glancing-angle ion milling. At the zeroth-order Fabry-Pérot resonance condition, our coaxial apertures act as a "zero-mode resonator (ZMR)", efficiently funneling as much as 34% of incident infrared (IR) light along 10 nm annular gaps. After removing Al2O3 in the gaps and inserting silk protein, we can couple the intense optical fields of the annular nanogap into the vibrational modes of protein molecules. From 7 nm gap ZMR devices coated with a 5 nm thick silk protein film, we observe high-contrast IR absorbance signals drastically suppressing 58% of the transmitted light and infer a strong IR absorption enhancement factor of 104∼105. These single nanometer gap ZMR devices can be mass-produced via batch processing and offer promising routes for broad applications of SEIRA.

Anderson localizations and photonic band-tail states observed in compositionally disordered platform.

Anderson localization in random structures is an intriguing physical phenomenon, for which experimental verifications are far behind theoretical predictions. We report the first experimental confirmations of photonic band-tail states and a complete transition of Anderson localization. An optically activated photonic crystal alloy platform enables the acquisition of extensive experimental data exclusively on pure eigenstates, revealing direct evidence of band-tail states and Anderson localization transition within the band-tail states. Analyses of both experimental and simulated data lead to a comprehensive picture of photon localization that is highly consistent with theories by Anderson and others. We believe that our results provide a strong experimental foundation upon which both the fundamental understandings and application possibility of Anderson localization can be promoted significantly.

Photonic Crystal Phosphors Integrated on a Blue LED Chip for Efficient White Light Generation.

Following the proof-of-concept experiment in the unit structure level, photonic crystal (PhC) phosphors-structurally engineered phosphor materials based on the nanophotonics principles-are integrated with a blue light-emitting diode (LED) chip to demonstrate a compact and efficient white light source. Red- or green-emitting CdSe-based colloidal quantum dots (CQDs) are coated on a Si3 N4 thin-film grating to fabricate PhC phosphors. The underlying PhC structure is designed such that the photonic band-edge modes at the zone center (k∣∣ = 0) are tuned to the energy of the blue excitation photons. By progressively stacking the PhC phosphor plates on a blue LED chip, the blue, green, and red emission intensities can be tightly controlled to obtain white light with the desired properties. The chromaticity coordinates, (0.332, 0.341), and correlated color temperature, 5500 K, are obtained from a stack of 3 red and 11 green PhC phosphor plates; in contrast, a stack of 5 red and 16 green reference phosphor plates are required to generate a similar white light. Overall, the PhC phosphors produce 8% higher total emission intensity out of 33% less amount of CQDs than the reference phosphors.

A colloidal quantum dot photonic crystal phosphor: nanostructural engineering of the phosphor for enhanced color conversion.

Phosphors, long-known color-converting photonic agents, are gaining increasing attention owing to the interest in white LEDs and related applications. Conventional material-based approaches to phosphors focus on obtaining the desired absorption/emission wavelengths and/or improving quantum efficiency. Here, we report a novel approach for enhancing the performance of phosphors: structural modification of phosphors. We incorporated inorganic colloidal quantum dots (CQDs) into a lateral one-dimensional (1D) photonic crystal (PhC) thin-film structure, with its photonic band-edge (PBE) modes matching the energy of 'excitation photons' (rather than 'emitted photons', as in most other PBE application devices). At resonance, we observed an approximately 4-fold enhancement of fluorescence over the reference bulk phosphor, which reflects an improved absorption of the excitation photons. This nano-structural engineering approach is a paradigm shift in the phosphor research area and may help to develop next-generation higher efficiency phosphors with novel characteristics.

Colloidal quantum dot lasers built on a passive two-dimensional photonic crystal backbone.

We report the room-temperature lasing action from two-dimensional photonic crystal (PC) structures composed of a passive Si3N4 backbone with an over-coat of CdSe/CdS/ZnS colloidal quantum dots (CQDs) for optical gain. When optically excited, devices lased in dual PC band-edge modes, with the modal dominance governed by the thickness of the CQD over-layer. The demonstrated laser platform should have an impact on future photonic integrated circuits as the on-chip coupling between active and passive components is readily achievable.

Period-chirped gratings fabricated by laser interference lithography with a concave Lloyd's mirror.

We developed a laser interference lithography (LIL) system for fabrication of period-chirped gratings, which would be useful for sophisticated optical components. Despite its simplicity, the developed LIL system, based on a Lloyd's mirror interferometer with a cylindrically concave mirror, can generate chirped gratings, yet over a large area at high throughput owing to the nature of LIL. We have derived exact theoretical equations needed for system design, built the LIL system, and subsequently realized period-chirped gratings. A fabricated sample whose center period is Λ≈600 nm exhibits a continuous period variation of ΔΛ=92 nm across 17 mm width.

A highly tunable and fully biocompatible silk nanoplasmonic optical sensor.

Novel concepts for manipulating plasmonic resonances and the biocompatibility of plasmonic devices offer great potential in versatile applications involving real-time and in vivo monitoring of analytes with high sensitivity in biomedical and biological research. Here we report a biocompatible and highly tunable plasmonic bio/chemical sensor consisting of a natural silk protein and a gold nanostructure. Our silk plasmonic absorber sensor (SPAS) takes advantage of the strong local field enhancement in the metal-insulator-metal resonator in which silk protein is used as an insulating spacer and substrate. The silk insulating spacer has hydrogel properties and therefore exhibits a controllable swelling when exposed to water-alcohol mixtures. We experimentally and numerically show that drastic spectral shifts in reflectance minima arise from the changing physical volume and refractive index of the silk spacer during swelling. Furthermore, we apply this SPAS device as a glucose sensor with a very high sensitivity of 1200 nm/RIU (refractive index units) and high relative intensity change.

Surface passivation of a photonic crystal band-edge laser by atomic layer deposition of SiO2 and its application for biosensing.

We report on the conformal surface passivation of photonic crystal (PC) laser devices with an ultrathin dielectric layer. Air-bridge-type Γ-point band-edge lasers (BELs) are fabricated by forming a honeycomb lattice two-dimensional PC structure into an InGaAsP multiple-quantum-well epilayer. Atomic layer deposition (ALD) is employed for conformal deposition of a few-nanometer-thick SiO2 layer over the entire device surface, not only on the top and bottom surfaces of the air-bridge membrane but also on the air-hole sidewalls. Despite its extreme thinness, the ALD passivation layer is found to protect the InGaAsP BEL devices from harsh chemicals. In addition, the ALD-SiO2 is compatible with the silane-based surface chemistry, which allows us to use ALD-passivated BEL devices as label-free biosensors. The standard streptavidin-biotin interaction shifts the BEL lasing wavelength by ∼1 nm for the dipole-like Γ-point band-edge mode. A sharp lasing line (<0.2 nm, full width at half-maximum) and a large refractive index sensitivity (∼163 nm per RIU) produce a figure of merit as high as ∼800 for our BEL biosensor, which is at least an order of magnitude higher than those of more common biosensors that rely on a broad resonance peak, showing that our nanolaser structures are suitable for highly sensitive biosensor applications.

Novel and simple route to fabricate fully biocompatible plasmonic mushroom arrays adhered on silk biopolymer.

A fully biocompatible plasmonic quasi-3D nanostructure is demonstrated by a simple and reliable fabrication method using strong adhesion between gold and silk fibroin. The quasi-3D nature gives rise to complex photonic responses in reflectance that are prospectively useful in bio/chemical sensing applications. Laser interference lithography is utilized to fabricate large-area plasmonic nanostructures.

A fully biocompatible single-mode distributed feedback laser.

A fully biocompatible laser would be attractive in many aspects of biomedical research. Here we report a single-mode biocompatible distributed feedback laser consisting of silk, riboflavin and silver in the form of a freestanding film. The distributed feedback structure has a large surface area and flexibility. The fabricated laser exhibited single-mode lasing at a wavelength of 495 nm.

Enhanced fluorescence from CdSe/ZnS quantum dot nanophosphors embedded in a one-dimensional photonic crystal backbone structure.

A nano-engineered phosphor structure that produces enhanced fluorescence is reported. Two kinds of polymer materials with different refractive indices are spin-coated alternately to realize a one-dimensional (1D) photonic crystal (PC) phosphor platform, in which CdSe/ZnS core-shell quantum dots (QDs) were embedded as a fluorescence agent. The 1D PC phosphor structure is designed to match the pump photon energy with one of the photonic band-edges (PBEs), where the photon group velocity becomes zero, and thus the interaction between pump photons and fluorescent centres strengthened. A reference phosphor structure is also designed and fabricated; however, it has no PBE and exhibited bulk-like photonic properties. The fluorescence intensity from the 1D PC phosphors is examined during the pump photon energy scanning across the PBE. It is found that fluorescence from the 1D PC phosphor reaches its maximum when the pump photon energy coincides with the PBE, which is consistent with the theoretical prediction. In comparison with the reference phosphor, the fluorescence from the 1D PC phosphor is measured to be enhanced by a factor of 1.36.

Model calculations for enhanced fluorescence in photonic crystal phosphor.

We propose a novel photonic structure, based on the photonic crystal (PC) effect, which simulations show results in an improved fluorescence efficiency from embedded phosphor. To be specific, the phosphor pumping efficiency can be significantly improved by tuning the pump photon energy to a photonic band-edge (PBE) of the PC phosphor. We have confirmed this theoretically by calculating optical properties of one-dimensional PC phosphor structures using the transfer-matrix method and plane-wave expansion method. For a particular model structure based on a quantum dot phosphor, the fluorescence enhancement factor was estimated to be as high as 6.9 for a monochromatic pump source and 2.2 for a broad bandwidth (20 nm) pump source.

Combined laser interference and photolithography patterning of a hybrid mask mold for nanoimprint lithography.

A lithography technique that combines laser interference lithography (LIL) and photolithography, which can be a valuable technique for the low cost production of microscale and nanoscale hybrid mask molds, is proposed. LIL is a maskless process which allows the production of periodic nanoscale structures quickly, uniformly, and over large areas. A 257 nm wavelength Ar-Ion laser is utilized for the LIL process incorporating a Lloyd's mirror one beam inteferometer. By combining LIL with photolithography, the non-selective patterning limitation of LIL are explored and the design and development of a hybrid mask mold for nanoimprint lithography process, with uniform two-dimensional nanoscale patterns are presented. Polydimethylsiloxane is applied on the mold to fabricate a replica of the stamp. Through nanoimprint lithography using the manufactured replica, successful transfer of the patterns is achieved, and selective nanoscale patterning is confirmed with pattern sizes of around 180 nm and pattern aspect ratio of around 1.44:1.