In virtual reality (VR) and augmented reality (AR) display, the vergence-accommodation conflict (VAC) is a significant issue. Thus, true-3D display technologies has been proposed to solve the VAC problem. Integral imaging (II) display, one of the most critical true-3D display technologies, has received increasing research recently. Significantly, anachromatic metalens array has realized a broadband metalens-array-based II (meta-II). However, the past micro-scale metalens arrays were incompatible with commercial micro-displays. Additionally, the elemental image array(EIA)rendering is slow. These device and algorithm problems prevent meta-II from being used for practical video-rate near-eye displays (NEDs). This research demonstrates a II-based NED combining a commercial micro-display and a metalens array. We make efforts in the hardware and software to solve the bottlenecks of video-rate metalens array II-based NED. The large-area nanoimprint technology fabricates the metalens array, and a novel real-time rendering algorithm is proposed to generate the EIA. We also build a see-through prototype based on our meta-II NED, demonstrating the effect of depth of field in AR, and the 3D parallax effect on the real mode. This work verifies the feasibility of nanoimprint technology for mass preparation of metalens samples, explores the potential of video-rate meta-II displays, which we can be applied in the fields of VR/AR and 3D display.
Chromatic dispersion represents the wavelength-dependent behavior of optical devices and limits their operation bandwidth. Due to the material dispersion restriction of refractive elements, dispersion engineering remains a challenge to imaging technology and optical communication. Recently, metalens offers an attractive approach to engineer the dispersion by introducing the additional degree of freedom with only a single layer of nanostructures. Here, we propose a method to design the dual-wavelength metalenses with controllable dispersion characteristic in transmission mode in the visible region. Three kinds of polarization-independent metalenses are demonstrated, including those with zero dispersion, positive axial dispersion, and negative axial dispersion. All the metalenses show high resolution with nearly diffraction-limited focusing. Our findings may provide an alternative way to design dual-wavelength functional devices in the fields of optical information processing, imaging technologies and complex fluorescence techniques.
Periodic silver coated hydrogen silsesquioxane nanopost arrays (HSQ@Ag NPAs) with various diameters were fabricated as surface-enhanced Raman scattering (SERS) active substrates, and the SERS performance of the substrates were studied both experimentally and numerically. Raman signals of Rhodamine 6G molecules absorbed on the HSQ@Ag NPAs were measured and showed excellent SERS performance with significant enhancement and high uniformity. The enhancement factor under 514.5 nm excitation wavelength increased firstly and then decreased, but increased monotonically under 633 nm excitation wavelength. Finite-difference time-domain simulations of electric field distribution and far field absorption demonstrated that SERS enhancement is related to strong electric enhancements under both excitation and Stokes wavelengths, and the strongest enhancement occurred in HSQ@Ag NPAs with localized surface plasmon resonance (LSPR) wavelength located in the region between excitation wavelength and Stokes wavelength. Thus, by tuning the LSPR wavelength to the region between excitation wavelength and Stokes wavelength via reasonably designing the parameters of the nanostructure, SERS substrate with excellent performance could be obtained. Our work could be helpful in understanding the fundamental mechanism of SERS and provides a possible way to reasonably design excellent SERS substrates.
Noble metal nanostructured thin films are of great interest as competitive surface enhanced Raman scattering (SERS) substrates due to their remarkable plasmonic properties in the visible wavelength. In this work, large-area vertical Ag columnar thin films of different thickness have been prepared on the glass substrates by the simple, cost-effective glancing angle deposition technique. The Raman spectra of R6G from these substrates are measured and the experimental SERS enhancement factor is found to have the maximal value of 2.5E+8 at the optimized thickness of 680 nm. Finite difference time domain simulations have been utilized to study the near-field plasmonic properties upon these films and the simulated structure is a geometric copy obtained from the SEM image topology rather than simplified regular nanostructures. The areal electric field enhancement is sensitive to the gap size and areal column density. The wavelength and polarization dependence of localized electric enhancement in subwavelength gaps, "hot spots", are studied and the electric enhancement at different film depths is also analyzed. The simulation SERS enhancement factors are calculated and show good agreement with the experimental ones. When the thickness increases, the areal electric field enhancement decreases while the number of adsorded molecules increases, so there exists an optimized thickness to maximize the SERS performance. These results help to further our understanding of the plasmonic properties of noble metal nanostructured thin films.
Noble metal nano-structured thin films show great electromagnetic enhancement due to the surface plasmon resonance effect. It can be used for surface enhanced Raman scattering (SERS), which means markers of ultra low concentration can be detected, having great potential in biosensing applications. In this work, we introduce glancing angle deposition (GLAD), which is based on the traditional physical vapor deposition, to prepare vertical columnar thin films (CTFs). Anodic aluminum oxide (AAO) is used as template during vacuum deposition, and nanodots of ordered distribution are obtained. Subsequently, these nanodots are used as the pre-constructs in GLAD, and the fabricated columns have a much bigger separation. Rhodamine 6G (R6G) SERS spectra of the nanostructure thin films are measured to study their optical properties. Due to the separation of columns, CTFs on nanodots show stronger intensity than CTFs on blank substrates. What’s more, the uniformity is also improved by the separation, making the substrates more applicable for biosensing. CTFs on nanodots of different heights are prepared. SERS measurements show that as the height increases, SERS effect increases due to much more hot spots of electromagnetic enhancement and coupling. Our method proves to be a feasible, low-cost, large-area preparation method for ultra high sensitivity SERS substrate.
Hardware architecture of parallel computation is proposed for generating Fraunhofer computer-generated holograms (CGHs). A pipeline-based integrated circuit architecture is realized by employing the modified Fraunhofer analytical formulism, which is large scale and enables all components to be concurrently operated. The architecture of the CGH contains five modules to calculate initial parameters of amplitude, amplitude compensation, phases, and phase compensation, respectively. The precalculator of amplitude is fully adopted considering the “reusable design” concept. Each complex operation type (such as square arithmetic) is reused only once by means of a multichannel selector. The implemented hardware calculates an 800×600 pixels hologram in parallel using 39,319 logic elements, 21,074 registers, and 12,651 memory bits in an Altera field-programmable gate array environment with stable operation at 50 MHz. Experimental results demonstrate that the quality of the images reconstructed from the hardware-generated hologram can be comparable to that of a software implementation. Moreover, the calculation speed is approximately 100 times faster than that of a personal computer with an Intel i5-3230M 2.6 GHz CPU for a triangular object.
Silicon thin-films are fabricated on quartz substrates in the substrate temperature range 500~600 °C by electron-beam
evaporation (EBE). Scanning Electron Microscopy (SEM) is employed to characterize the surface morphology of the
films and measure the grain size. It is found that, there is an optimum temperature for getting large grain size. The X-ray
diffraction (XRD) results show that the preferred crystallographic orientation on quartz wafer is (111). Raman spectra
demonstrate the crystallization condition of the films and the crystalline volume fraction are calculated. The results
indicate that the growth characteristics of the films depend significantly on the substrate temperature. Silicon thin-films
are observed to be amorphous under the temperature of 560 °; above this temperature, they are polycrystalline. In other
words, 560 °C is the critical temperature of the films transforming from amorphous silicon to poly-Si. The optimum
temperature at about 580 ° for the growth of the film on quartz substrates is obtained. The films exhibit the largest grain
size, highest value of crystalline volume fraction and the best structural quality when the substrate is kept at this
temperature.
Based on the theory of optical fiber coupling and Fabry–Perot interference, a model of extrinsic Fabry–Perot interferometric (EFPI) optical fiber sensor for measuring strain is analyzed, and the theoretical model is demonstrated. A formula of white-light interfered EFPI optical fiber strain sensor is obtained. The system of the optical fiber sensor is designed using an LED source as light source to obtain reflected spectrum. The experimental results show that it coincides well with the computational simulation of theoretical model, which means the model is accurate.
Narrow frequency sharp angular filters with integral times of quarter wavelength layer structure, which can be fabricated with commonly used coating machines and work in visible and near infrared, are designed. These filters possess not only a narrow frequency pass band but also a sharp angular pass breadth in visible and near infrared. The experimental result agrees with the theoretical calculations well. These dual functional filters have many potential applications.
On the basis of the optical thin film interference theory, a novel thin film multi-frequency filtering technology applied to CWDM filter, a comb-like filter, which is not applied to select single signal channel from the multi-channel, but to separate the multi-channel into two different sets of channels, is proposed in the article. According to the central wavelength and the width of pass-band of 8-channel CWDM system that were defined by the ITU-T, two kinds of thin-film filters for 8-channel CWDM system have been designed by the technology. A specific expression for these thin film structures has been described and the parameters of these structures have been analyzed and optimized. The transmission curves of the designed structures show that the design results are agreement with that of the theoretical calculation well. We can implement the function of the 8-channel CWDM using two above-mentioned thin-film filters, which make the CWDM system low-cost, simpleness and easy collimation. These thin film structures have the advantages of good stability, arbitrary wavelength spacing between the adjacent channels, perfect rectangular shape of the pass-band, less layers, low cost and easy encapsulation, which would make it more attractive to use in CWDM system and other fields.
A microphotonic switch with three input waveguides and two output waveguides and integrated with an optical power splitter has been proposed. It is fabricated on the multimode interference principle in Si-based SiGe material system and configured for a 3x2 symmetrical structure of the three input waveguides and the two output waveguides of the device. The central waveguide section is based on a multimode interference and incorporated with an activated carrier injection element. The operating wavelengths of the device are specially designed for 1545, 1550, and 1555 nm conventional-band wavelengths. The measured crosstalk is at around -17 dB and the average insertion loss is about 2.3 dB. At switch-ON state, the measured injection current is 370 mA corresponding to an injection current density of 950 A/cm2.
Fiber optical Fabry-Perot (F-P) interferometric sensors have been used for the measurement of various physical parameters. A novel miniature asymmetric F-P interferometric cavity applied to fiber optical sensors is designed in this work; and its structure consists of fiber, complex metallic thin films, air or dielectric tunable layer, in which the reflectance of the cavity in response to phase thickness of the tunable layer can be approximated to saw wave function so it can improve the degree of linearity and the dynamic range of the sensors. The measurand to sense, which modulates the optical path of the tunable layer to change the reflectance of the structure, can be detected and demodulated. The response of the reflectance of the structure to the phase thickness of the tunable layer has been calculated and analyzed with optical thin film characteristic matrix method, and an expression for the response of this structure has been described. The design method to obtain optimal parameters of the F-P interferometric cavity has been concluded in this work. The result indicates that this miniature structure possess a high degree of linearity, sensitivity and dynamic range.
In this paper, a kind of absorptive thin film, for the first time, was used in laser welding of SiO2, Si and LiNbO3. This absorptive thin film of three-layer metal-dielectric-metal structure is designed for further reducing the high reflectance of the Nd:YAG laser beam in the surface of the tin layer that is utilized as solder between the transparent parent materials. The actual absorption exceeds 99%. This combination of absorber and solder transformed the laser energy into heat effectively and decreased the minimum necessary incident laser power transmitting through the transparent parent materials. As a result, the damage of the parent materials, which is suffered from laser transmission, is avoided; on the other hand, the laser power is of full utilization and saved.
Based on the optical thin film interference theory, a novel thin film multi-half-wave multi-frequency filtering technology applied to DWDM interleaver, a comb-like filter, is proposed in this report. A flexible on-demand design of arbitrary frequency spacing can be easily obtained by this thchnology. The parameters of this thin film structure have been analyzed and optimized, and a specific expression for these thin film structure has been described. 50G-spaced and 100G-spaced DWDM interleavers with flat-top passband consisting of less than 30 layers have been designed and fabricated. The process of fabrication is also discussed in this paper. These thin film interleavers have the advantages of good stability, arbitrary wavelength spacing between the adjacent channels, rectangular shape of the pass-band, less layers, low cost, and easy encapsulation, which would make it more attractive to use in DWDM system. The above-mentioned technology exhibits the flexibility in design and the advantages of thin film coating, which will have more applications besides interleaver.
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