Brilliant quantum dots' photoluminescence from a dual-resonance plasmonic grating.

Semiconductor quantum dots (QDs) have recently caused a stir as a promising and powerful lighting material applied in real-time fluorescence detection, display, and imaging. Photonic nanostructures are well suited for enhancing photoluminescence (PL) due to their ability to tailor the electromagnetic field, which raises both radiative and nonradiative decay rate of QDs nearby. However, several proposed structures with a complicated manufacturing process or low PL enhancement hinder their application and commercialization. Here, we present two kinds of dual-resonance gratings to effectively improve PL enhancement and propose a facile fabrication method based on holographic lithography. A maximum of 220-fold PL enhancement from CdSe/CdS/ZnS QDs are realized on 1D Al-coated photoresist (PR) gratings, where dual resonance bands are excited to simultaneously overlap the absorption and emission bands of QDs, much larger than those of some reported structures. Giant PL enhancement realized by cost-effective method further suggests the potential of better developing the nanostructure to QD-based optical and optoelectronic devices.

Crossed grating sensing refractive index change in the non-laboratory environment.

Surface plasmon polaritons (SPPs) have been widely applied to refractive index (RI) sensing for their extremely high sensitivity to the surrounding RI change. Many efforts have been devoted to narrowing the linewidth of the SPP mode and enhancing the sensitivity of SPP sensors. However, most reported SPP-based RI sensing platforms could only operate in a laboratory environment for their bulky volume or sophisticated measuring systems. In this context, we have developed a miniaturized and portable RI sensing platform based on a 2D crossed grating coupled SPP sensor that can work under a non-laboratory environment. The crossed grating is fabricated by the laser interference lithography (LIL) method, which is cost-effective and reproductive. A series of glucose solutions with different concentrations have been used as analytes to verify the sensing performance of the fabricated crossed grating.

Investigation of angle-insensitive grating color filters at periods much smaller than the wavelength of incidence.

We designed and simulated one-dimensional (1D) and two-dimensional (2D) reflective grating color filters inside the aluminized polyethylene (PE) film. The filters have several advantages: high angle insensitivity (up to 45° for the 1D filter, 40° for the 2D filter), high reflectance at non-resonant wavelengths, deep resonance dips, and a large color gamut. Both structures are characterized by with their grating periods being much smaller than the wavelength of incidence. A grating modal analysis was utilized to reveal the physical mechanism behind such structures that exhibit angle-insensitive spectral responses which are favored in the fields of color display and packaging.

Multiwavelength achromatic super-resolution focusing via a metasurface-empowered controlled generation of focused cylindrically polarized vortex beams.

Non-invasive imaging beyond the diffraction limit and free from fluorescent labels in the visible is highly desired for microscopy. It remains a challenge to obtain such super-resolution focusing along with multiwavelength achromatic performance in the far field using an integratable and easily designed system. In this work, we demonstrate a straightforward metasurface-based method to realize multiwavelength achromatic generation and focusing of cylindrically polarized vortex beams (CPVBs). Attributed to the extra degrees of freedom of CPVBs and multi-section design, we have realized multiwavelength achromatic super-resolution focusing in the air with focal size tighter than that of normally used schemes like immersion metalenses or focused radially polarized beams. It is expected that this metasurface-empowered ultra-compact design will benefit potential applications which call for high resolution, like optical microscopy, laser processing, etc.

Inverse design of soliton microcomb based on genetic algorithm and deep learning.

Soliton microcombs generated by the third-order nonlinearity of microresonators exhibit high coherence, low noise, and stable spectra envelopes, which can be designed for many applications. However, conventional dispersion engineering based design methods require iteratively solving Maxwell's equations through time-consuming electromagnetic field simulations until a local optimum is obtained. Moreover, the overall inverse design from soliton microcomb to the microcavity geometry has not been systematically investigated. In this paper, we propose a high accuracy microcomb-to-geometry inverse design method based on the genetic algorithm (GA) and deep neural network (DNN), which effectively optimizes dispersive wave position and power. The method uses the Lugiato-Lefever equation and GA (LLE-GA) to obtain second- and higher-order dispersions from a target microcomb, and it utilizes a pre-trained forward DNN combined with GA (FDNN-GA) to obtain microcavity geometry. The results show that the dispersive wave position deviations of the inverse designed MgF2 and Si3N4 microresonators are less than 0.5%, and the power deviations are less than 5 dB, which demonstrates good versatility and effectiveness of our method for various materials and structures.

One-step formation of a plasmonic grating with an ultranarrow resonance linewidth for sensing.

Nanograting-based plasmonic sensors are capable of real-time and label-free detection for biomedical applications. Simple and low-cost manufacturing methods of high-quality sensors are always demanding. In this study, we report on a one-step etch-free method achieved by directly patterning a photoresist on a copper substrate using laser interference lithography. Large area uniform gratings with a period of 600 nm were fabricated on the copper film, and its refractive index sensing performance was tested using glucose as analyte. By replacing the metallic grating ridges with photoresist ridges, the Ohmic absorption and radiative scattering losses of surface plasmons were greatly reduced. As a result, a much sharper resonance linewidth (∼ 10 nm) was experimentally obtained. Compared with pure metallic gratings, the reported structure is characterized by sharper resonance and a much easier fabrication process, making it a cost-effective plasmonic sensor with high quality.

Enhanced sensing performance from trapezoidal metallic gratings fabricated by laser interference lithography.

Nanograting-based plasmonic sensors are widely regarded as promising platforms due to their real-time label-free detection and ease of integration. However, many reported grating structures are too complicated to fabricate, which limits their application. We propose a 1D bilayer metallic grating with trapezoidal profile as a near-infrared plasmonic sensor in the spectral interrogation. Trapezoidal gratings tend to perform better than rectangular gratings as sensors, particularly as they can detect at oblique incidence to obtain higher performance. Furthermore, we have successfully fabricated such a grating with a period of 633 nm over a 2.25 cm2 area using a two-step approach based on laser interference lithography. Glucose detection has been conducted to experimentally validate the sensing performance of the as-prepared grating. The measured sensitivity and figure-of-merit can reach up to 786 nm/RIU and 30, respectively. Our research sheds new light on the development of cost-effective sensing devices.

Magnetometry based on the effect of laser-induced plasmas in a sodium-containing environment.

The magneto-optical resonance response of sodium atoms generated by a high-energy solid-state pulse Nd:YAG laser is studied in different external magnetic fields. We investigate the resonance fluorescence signal of sodium atoms in a simulated sea fog environment based on the laser-induced plasma (LIP) effect. By ionizing an NaCl solution spray to generate sodium atoms in an atmospheric environment, we build a Bell-Bloom magneto-optical resonance system under laboratory conditions. With the help of laser-induced breakdown spectroscopy (LIBS) and extinction spectrum, we obtain sodium atoms with a lifetime of 250 µs. A narrowband tunable continuous wave (CW) 589-nm laser tuned at the D2 line with a modulation frequency around the Larmor frequency is used as the pump beam to polarize sodium atoms in the test magnetic field. We find that the magneto-optical resonance signals vary with different external magnetic fields and the positions of the resonance signal are consistent with the theoretical values. An intrinsic magnetometric sensitivity of 620.4 pT in a 1-Hz bandwidth is achieved.

Polarization-independent long-wave infrared transmission bandpass filters based on guided mode resonance.

Subwavelength dielectric gratings based on guided mode resonances (GMRs) provide a compact, low-cost, and readily fabricable platform for narrowband spectral filters applied in numerous fields, such as sensors, spectrometers, and hyperspectral imaging. We numerically designed two long-wave infrared transmission bandpass filters based on Ge-ZnS-Ge thin film stacks and 2D zero-contrast gratings. The first filter is attributed to double GMR mode coupling, exhibiting a broadband high reflectance between 8 and 12 µm and a narrow transmission peak with a spectral linewidth of 53 nm and transmittance efficiency of 98% at the resonance. The other filter adopts a low-refractive-index material BaF2 as the substrate, which displays a lower-transmittance sideband from 8.6 to 9.4 µm, and a higher transmission efficiency up to 100% at the resonance. The physical mechanisms of two spectral responses are all relevant to guided mode coupling and Fabry-Perot resonance.

Photonic thermometer with a sub-millikelvin resolution and broad temperature range by waveguide-microring Fano resonance.

Fano resonance theoretically is an effective approach for sensitivity enhancement in photonic sensing applications, but the reported methods suffer from complicated structure and fabrication, narrow dynamic range, etc. In this article, we propose a photonic thermometer with sub-millikelvin resolution and broad temperature measurement range implemented by a simple waveguide-microring Fano structure. An air hole is introduced at the center of the coupling region of the waveguide of an all-pass microring resonator. The effective refractive index theory is used to design its equivalent phase shift and therefore the lineshape of the Fano resonance. Experimental results showed that the quality factor and the Fano parameter of the structure were invariant in a broad temperature range. The wavelength-temperature sensitivity was 75.3 pm/℃, the intensity-temperature sensitivity at the Fano asymmetric edge was 7.49 dB/℃, and the temperature resolution was 0.25 mK within 10℃ to 90℃.

Giant polarization anisotropic optical response from anodic aluminum oxide templates embedded with plasmonic metamaterials.

Plasmonic metamaterials enable extraordinary manipulation of key constitutive properties of light at a subwavelength scale and thus have attracted significant interest. Here, we report a simple and convenient nanofabrication method for a novel meta-device by glancing deposition of gold into anodic aluminum oxide templates on glass substrates. A methodology with the assistance of ellipsometric measurements to examine the anisotropy and optical activity properties is presented. A tunable polarization conversion in both transmission and reflection is demonstrated. Specifically, giant broadband circular dichroism for reflection at visible wavelengths is experimentally realized by oblique incidence, due to the extrinsic chirality resulting from the mutual orientation of the metamaterials and the incident beam. This work paves the way for practical applications for large-area, low-cost polarization modulators, polarization imaging, displays, and bio-sensing.

Large-angle beaming from asymmetric nanoslit-corrugation structures.

The beaming effect in single apertures surrounded by periodic corrugations and the manipulation of beaming directions from such structures has gained considerable attention since discovery. Different materials and structural profiles have been studied in this context but directional beaming at angles larger than 45° has not been achieved. We design and demonstrate nanoslits in a gold film flanked by corrugations, which give rise to beaming angles ranging from 45° to 60°. While the previous designs are based on achieving constructive interference at the aimed beaming angle, our approach complements such constructive interference with destructive interference at 0° and, as a result, enhances the directional beaming effect at angles larger than 45°. The structures are fabricated by electron beam lithography with two consecutive lift-off processes. The experimental far-field intensity distributions agree well with the designs.

Volume holographic recording in Al nanoparticles dispersed phenanthrenequinone-doped poly(methyl methacrylate) photopolymer.

High-performance material plays a crucial role in holographic data storage, which is a noteworthy technology with potential applications in the field of high capacity data storage. We report on a new kind of holographic storage material based on aluminum nanoparticles (Al NPs) dispersed phenanthrenequinone-doped poly(methyl methacrylate) (PQ/PMMA) photopolymer. Al NPs are efficiently synthesized in a monomer solvent using laser ablation in liquids without chemical precursors. It is shown that an increase in diffraction efficiency and recording sensitivity is achieved in both traditional holography and polarization holography by doping with Al NPs. After 4 h of ablation, the new material exhibited an improvement in the diffraction efficiency for both traditional holography and polarization holography from 2.85% to 57.15% and from 0.6% to 4.07%, respectively. We also investigated the image recording and reconstruction performance for both traditional and polarization holography and the results indicate that the proposed material has noticeable potential as a holographic storage material. Additionally, it also possesses excellent potential for holographic position multiplexing recording. We conclude that laser ablation in a liquid is a promising option for processing low-cost nano-doped holographic storage material.

Improving the polarization-holography performance of PQ/PMMA photopolymer by doping with THMFA.

Increasing photosensitizer concentration has been considered as an effective approach to improve the performance of holographic material. In this paper, we report on new method for increasing the saturated dissolvability of photosensitizer PQ within polymeric media by introducing copolymerization monomer into the PQ/PMMA. The photosensitizer concentration of PQ was increased from 0.7wt% to 1.3wt%, compared with the typical PQ/PMMA sample. Besides, we investigated performance of polarization holographic recordings in typical PQ/PMMA and copolymerization monomer-containing PQ/PMMA with the orthogonally polarized signal and reference waves. And the doping of THFMA component resulted in a significant improvement of diffraction intensity and photosensitivity. In addition, high-quality holographic image reconstruction was realized in our home-made material.

Highly directive plasmonic structure with double resonance at excitation and emission for molecule-enhanced fluorescence.

An improved bull's eye nanostructure is proposed as a substrate for surface-enhanced fluorescence. Optimized by finite-difference time-domain, annular corrugation on both upper and lower surfaces is placed around a nanobow tie in the middle. The whole structure is designed for the excitation and emission process of a certain fluorescence simultaneously, which can enhance the fluorescence intensity enormously (35 times for an excitation field and 120 times for radiation, independently). Meanwhile, it is able to confine the far-field emission energy within a divergence angle of ±3.5°. The dual-resonant enhancement and directivity of such a substrate allows higher sensitivity during fluorescence detection. Furthermore, the designed resonance and directional light control at emission may effectively reduce the required energy at excitation, which is ideal for those molecules with low laser damage threshold.

Volume holographic recording in Irgacure 784-doped PMMA photopolymer.

Photoinitiator plays a crucial role on photopolymer. Unlike photoinitiator phenanthrenequinone (PQ), the solubility of Irgacure 784 dissolved in MMA is very high. In this paper, we use Irgacure 784 as photoinitiator doped in poly(methyl methacrylate)(PMMA)to make a bulk photopolymer with high photoinitiator concentration for holographic data storage. The effect of concentration of photoinitiator and record intensity are experimentally investigated. The results reveal the material has an optimum condition in holographic recording. A comparison between our material and the material of PQ doped PMMA is carried out by experiment. It is found that our material has better performance on diffraction efficiency, refractive index modulation and recording sensitivity. Besides, this material also has polarization sensitivity, which can be applied to polarization holography. With the capacity of recording polarization holography and conventional intensity holography simultaneously, the Irgacure 784 doped PMMA material is expected to be applied in holographic data storage.

Dual-channel recording based on the null reconstruction effect of orthogonal linear polarization holography.

We report on dual-channel recording within polarization holography written by orthogonal linear polarization waves. The null reconstruction effect (NRE) of linear polarization holography was experimentally achieved at a large cross-angle of π/2 inside the polarization-sensitive media. Based on the NRE, two polarization encoded holograms were recorded in a dual-channel recording system with negligible inter-channel crosstalk. The two polarization multiplexed holograms could then be sequentially or simultaneously readout by shifting the polarization state of reference wave with the best signal-to-noise of 18:1 obtained within the experiment.

Inverse polarizing effect of an elliptical-polarization recorded hologram at a large cross angle.

We report on the inverse polarizing effect (IPE) of an elliptical-polarization recorded hologram at a large recording angle. The IPE is a polarizing phenomenon in which the reconstructed signal switches the major and minor axes and keeps the original polarization, direction compared, to that of the signal wave. In reviewing the case of a linear-polarization and circular-polarization recorded hologram, we found that the IPE is a unique phenomenon for elliptical polarization. The IPE was observed at the cross angle of 38° experimentally, and was theoretically explained using tensor theory to remove paraxial limitation.

Null reconstruction of orthogonal circular polarization hologram with large recording angle.

We report on the null reconstruction of polarization volume hologram recorded by orthogonal circularly polarized waves with a large cross angle. Based on the recently developed tensor theory for polarization holography, the disappearance of the reconstruction was analytically verified, where a nice agreement was found between the experimental and theoretical results. When the polarization and intensity hologram attain a balance, not only the null reconstruction but also the faithful reconstruction can be realized by the illumination of the orthogonal reference wave and original reference wave. As a consequence of the hologram recorded without paraxial approximation, the null reconstruction may lead to important applications, such as a potential enhancement in optical storage capacity for volume holograms.

Enhanced dual-band infrared absorption in a Fabry-Perot cavity with subwavelength metallic grating.

The performance of infrared (IR) dual-band detector can be substantially improved by simultaneously increasing IR absorptions for both sensor bands. Currently available methods only provide absorption enhancement for single spectral band, but not for the dual-band. The Fabry-Perot (FP) cavity generates a series of resonances in multispectral bands. With this flexibility, we introduced a novel type of dual-band detector structure containing a multilayer FP cavity with two absorbing layers and a subwavelength-period grating mirror, which is capable of simultaneously enhancing the middle wave infrared (MWIR) and the long wave infrared (LWIR) detection. Compared with the bare-absorption-layer detector (common dual-band detector), the optimized FP cavity can provide about 13 times and 17 times absorption enhancement in LWIR and MWIR bands respectively.