Xiangfan Chen

Multi-level multi-thermal-electron FDTD simulation of plasmonic interaction with semiconducting gain media: applications to plasmonic amplifiers and nano-lasers

Interactions between a semiconducting gain medium and confined plasmon-polaritons are studied using a multilevel multi-thermal-electron finite-difference time-domain (MLMTE-FDTD) simulator. We investigated the amplification of wave propagating in a plasmonic metal-semiconductor-metal (MSM) waveguide filled with semiconductor gain medium and obtained the conditions required to achieve net optical gain. The MSM gain waveguide is used to form a plasmonic semiconductor nano-ring laser(PSNRL) with an effective mode volume of 0.0071 microm3, which is about an order of magnitude smaller than the smallest demonstrated integrated photonic crystal based laser cavities. The simulation shows a lasing threshold current density of 1kA/cm2 for a 300 nm outer diameter ring cavity with 80 nm-wide ring. This current density can be realistically achieved in typical III-V semiconductor, which shows the experimental feasibility of the proposed PSNRL structure.

Numerical and experimental investigation of light trapping effect of nanostructured diatom frustules

Recent advances in nanophotonic light-trapping technologies offer promising solutions in developing high-efficiency thin-film solar cells. However, the cost-effective scalable manufacturing of those rationally designed nanophotonic structures remains a critical challenge. In contrast, diatoms, the most common type of phytoplankton found in nature, may offer a very attractive solution. Diatoms exhibit high solar energy harvesting efficiency due to their frustules (i.e., hard porous cell wall made of silica) possessing remarkable hierarchical micro-/nano-scaled features optimized for the photosynthetic process through millions of years of evolution. Here we report numerical and experimental studies to investigate the light-trapping characteristic of diatom frustule. Rigorous coupled wave analysis (RCWA) and finite-difference time-domain (FDTD) methods are employed to investigate the light-trapping characteristics of the diatom frustules. In simulation, placing the diatom frustules on the surface of the light-absorption materials is found to strongly enhance the optical absorption over the visible spectrum. The absorption spectra are also measured experimentally and the results are in good agreement with numerical simulations.

Scalable nanofabrication of U-shaped nanowire resonators with tunable optical magnetism

Split ring resonators have been studied extensively in reconstituting the diminishing magnetism at high electromagnetic frequencies in nature. However, breakdown in the linear scaling of artificial magnetism is found to occur at the near-infrared frequency mainly due to the increasing contribution of self-inductance while reducing dimensions of the resonators. Although alternative designs have enabled artificial magnetism at optical frequencies, their sophisticated configurations and fabrication procedures do not lend themselves to easy implementation. Here, we report scalable nanofabrication of U-shaped nanowire resonators (UNWRs) using the high-throughput nanotransfer printing method. By providing ample area for conducting oscillating electric current, UNWRs overcome the saturation of the geometric scaling of the artificial magnetism. We experimentally demonstrated coarse and fine tuning of LC resonances over a wide wavelength range from 748 nm to 1600 nm. The added flexibility in transferring to other substrates makes UNWR a versatile building block for creating functional metamaterials in three dimensions.

High‐Speed 3D Printing of Millimeter‐Size Customized Aspheric Imaging Lenses with Sub 7 nm Surface Roughness

Advancements in three-dimensional (3D) printing technology have the potential to transform the manufacture of customized optical elements, which today relies heavily on time-consuming and costly polishing and grinding processes. However the inherent speed-accuracy trade-off seriously constrains the practical applications of 3D-printing technology in the optical realm. In addressing this issue, here, a new method featuring a significantly faster fabrication speed, at 24.54 mm3 h-1 , without compromising the fabrication accuracy required to 3D-print customized optical components is reported. A high-speed 3D-printing process with subvoxel-scale precision (sub 5 µm) and deep subwavelength (sub 7 nm) surface roughness by employing the projection micro-stereolithography process and the synergistic effects from grayscale photopolymerization and the meniscus equilibrium post-curing methods is demonstrated. Fabricating a customized aspheric lens 5 mm in height and 3 mm in diameter is accomplished in four hours. The 3D-printed singlet aspheric lens demonstrates a maximal imaging resolution of 373.2 lp mm-1 with low field distortion less than 0.13% across a 2 mm field of view. This lens is attached onto a cell phone camera and the colorful fine details of a sunset moths wing and the spot on a weevils elytra are captured. This work demonstrates the potential of this method to rapidly prototype optical components or systems based on 3D printing.

Design of Non-Deterministic Quasi-random Nanophotonic Structures Using Fourier Space Representations

Despite their seemingly random appearances in the real space, quasi-random nanophotonic structures exhibit distinct structural correlations and have been widely utilized for effective photon management. However, current design approaches mainly rely on the deterministic representations consisting two-dimensional (2D) discretized patterns in the real space. They fail to capture the inherent non-deterministic characteristic of the quasi-random structures and inevitably result in a large design dimensionality. Here, we report a new design approach that employs the one-dimensional (1D) spectral density function (SDF) as the unique representation of non-deterministic quasi-random structures in the Fourier space with greatly reduced design dimensionality. One 1D SDF representation can be used to generate infinite sets of real space structures in 2D with equally optimized performance, which was further validated experimentally using light-trapping structures in a thin film absorber as a model system. The optimized non-deterministic quasi-random nanostructures improve the broadband absorption by 225% over the unpatterned cell.

A novel piezoelectrically actuated 2-DoF compliant micro/nano-positioning stage with multi-level amplification

This article presents a novel two-degrees-of-freedom (2-DoF) piezo-actuated parallel-kinematic micro/nano-positioning stage with multi-level amplification. The mirror symmetric stage consists of four leverage mechanisms, two Scott-Russell mechanisms, and a Z-shaped flexure hinge (ZFH) mechanism. Taking advantage of the ZFH mechanism, 2-DoF motions with final-level flexural amplification and decoupled motion guidance are achieved. Analytical models of the stage are developed and validated through finite element analysis to characterize its working performance. Practical testing of a prototype stage is conducted to demonstrate the design process and to quantify its response characteristics. Due to the utilized multi-level amplification, a practical amplification ratio of 13.0 is realized by the prototype. The decoupled output motion guidance feature of the stage makes it amenable for implementation in raster scanning type of measurements.

Three-dimensional, polarization-sensitive, spectroscopic photon localization microscopy for parallel single-molecules imaging and tracking (Conference Presentation)

The spectroscopic information and the corresponding polarization states of a single-molecule emission possess wealth molecule-specific signatures that can be used to reveal the unique molecular electronic state, conformation, and its interactions with the host media. However, existing spectroscopic methods and advanced image analysis techniques, which can potentially provide quantitative analytical tools for the study of cellular dynamics, are yet limited by the diffraction limit. Therefore, developing a nanoscopic imaging platform for simultaneous acquisition of multiple molecular specific properties is highly desirable. Here we report a three-dimensional (3D), polarization-sensitive, spectroscopic photon localization microscopy (3D-Polar-SPLM) that simultaneously captures nanoscopic location of individual fluorescent emitters and their corresponding optical spectra and polarization states. To evaluate the capability of the imaging system, we imaged model system consisting quantum rods (QRs). Using 3D-Polar-SPLM, we spatially localized individual QRs with a lateral localization precision of 8 nm and an axial localization precision of 35 nm. In addition, we achieved a spectral resolution of 2 nm and a polarization angle measuring precision of 8 degrees. The spectral profile of the fluorescence emission provided a particle-specific signature for identifying individual QRs among the heterogeneous population, which significantly improved the fidelity in parallel 3D tracking of multiple QRs at a temporal resolution of 10 ms. Except its versatility, 3D-Polar-SPLM further provides advantageous in practical applications since it only employs a single light-path and therefore, is compatible with existing PALM/STORM, potentially bringing immediate impact to the broader research community, across physics, chemistry, material science and biology.

Optically-transparent micro-ring resonator enables longitudinal cortical imaging by photoacoustic microscopy (Conference Presentation)

High-resolution optical longitudinal cortical imaging usually uses cranial window, which involves removing a skull portion and sealing the exposed brain area with a transparent cover glass, allowing ballistic photons to reach the cortex with minimal disturbance of the brain function. It enables obtaining high-resolution brain images in extended periods of time for long-term neuronal activity studies using confocal and two-photon microscopies. Photoacoustic microscopy (PAM), as the only imaging method that directly measure absorption contrast, is a complementary functional imaging method to provide absorption related brain information, such as total concentration of hemoglobin and oxygen saturation of hemoglobin. However, the use of traditional piezoelectric transducers (PZT) to collect ultrasound signal greatly limits the versatility of PAM. Though highly sensitive, PZT transducers are usually bulky and optically opaque. It blocks the light and is hard to be inserted into the limited distance between the optical objective and imaging sample, which are normally less than one millimeter when using a high- numerical aperture (NA) objective to achieve submicron resolution. Here, we developed a simple and cost-efficient soft nanoimprint lithography (NIL) process to fabricate fully embedded micro-ring resonator ultrasound detectors on optically transparent substrates, and integrated the detector onto a cranial window, making cranial window itself an ultrasonic detector. We implanted this functional cranial window on mouse head and achieved longitudinal monitoring of cortex vasculature using PAM. Our low-cost, disposable, and optically transparent detector may potentially reshape the longitudinal functional brain imaging using PAM in small animals.

High-throughput 3D printing of customized imaging lens

Recently, 3D printing has gone beyond being an industrial prototyping process and has gradually evolved as the tool to manufacture production-quality parts that are otherwise challenging by using traditional methods. Especially, translating 3D printing technique into the optical realm would dramatically improve the time- and cost-efficiency of customized optical elements, while conventional methods such as multiaxial lathes polishing, magnetorheological finishing, molding techniques are relatively expensive and time consuming. However, 3D printing also suffers from the inherent drawback: the reduced surface quality associated with the stair-stepping effect as a direct result of the layered deposition of the material. In this paper, we have demonstrated a time- and cost-effective single photon micro-stereolithography based 3D printing method to eliminate the layer stair-stepping effect. This method supports not only sub-voxel accuracy (~ 2 μm) of the surface in the range of 2 mm diameter, but also features deep sub-wavelength roughness (< 10 nm) of the surfaces and extremely good reproducibility. Furthermore, we designed and rapidly prototyped the aspherical lenses which not only feature low distortion, but also show remarkable optical quality in a broadband wavelength range from 400 nm to 800 nm.

Understanding the nanophotonic light-trapping structure of diatom frustule for enhanced solar energy conversion: a theoretical and experimental study

Recent designs in nanophotonic light-trapping technologies offer promising potential to develop high-efficiency thin-film solar cell at dramatically reduced cost. However, the lack of a cost effective scalable nanomanufacturing technique remains the main road-block. In nature, diatoms exhibit high solar energy harvesting efficiency due to their frustules (i.e., hard porous cell wall made of silica) possessing remarkable hierarchical nano-features optimized for the photosynthetic process through millions of years evolution. To explore this unique light trapping effect, different species of diatoms (Coscinodiscus sp. and Coscinodiscus wailesii) are cultured and characterized by Scanning electron microscope (SEM). Rigorous Coupled Wave Analysis (RCWA) and Finite-difference time-domain (FDTD) method are employed to numerically study the nanophotonic light-trapping effect. The absorption efficiency is significantly enhanced over the spectrum region centered on 450nm and 700nm where the electric fields are found strongly confined within the active layer. The transmission and reflection spectra are also measured by optical spectroscopy and the experimental results are in good agreement with numerical simulations.