Micro-optical Devices Research Articles

Controllable Preparation of Fused Silica Micro Lens Array through Femtosecond Laser Penetration-Induced Modification Assisted Wet Etching.

As an integrable micro-optical device, micro lens arrays (MLAs) have significant applications in modern optical imaging, new energy technology, and advanced displays. In order to reduce the impact of laser modification on wet etching, we propose a technique of femtosecond laser penetration-induced modification-assisted wet etching (FLIPM-WE), which avoids the influence of previous modification layers on subsequent laser pulses and effectively improves the controllability of lens array preparation. We conducted a detailed study on the effects of the laser single pulse energy, pulse number, and hydrofluoric acid etching duration on the morphology of micro lenses and obtained the optimal process parameters. Ultimately, two types of fused silica micro lens arrays with different focal lengths but the same numerical aperture (NA = 0.458) were fabricated using the FLPIM-WE technology. Both arrays exhibited excellent geometric consistency and surface quality (Ra~30 nm). Moreover, they achieved clear imaging at various magnifications with an adjustment range of 1.3×~3.0×. This provides potential technical support for special micro-optical systems.

Fabrication of liquid-filled zoom compound eyes with a tunable focal length

The conventional bionic compound eye system encounters challenges associated with a complex zoom structure, susceptibility to wear, and elevated costs. In this paper, a preparation method of liquid-filled zoom compound eyes is proposed. A polydimethylsiloxane (PDMS) film is integrated with a microfluidic chamber composed of polymethyl methacrylate (PMMA). The microlens array (MLA, hexagonal array, diameter 800 µm, center distance 850 µm) on the PDMS film is prepared by a wettability-guided dipping method. The sub-eye focal length on zoom compound eyes (ZCEs) is 1.42 mm. By controlling the volume of deionized (DI) water injected into the chamber from 2.41 to 2.71cm3, the focal length of the main lens is adjusted from infinity to 55.51 mm, and the FOV is adjusted from 32° to 58°. This large aperture ZCE combines the advantages of monocular and compound eyes. This method has great potential for advanced micro-optical devices with a wide field of view and tunable imaging capabilities.

Scanning Electrochemical Probe Lithography for Ultra-Precision Machining of Micro-Optical Elements with Freeform Curved Surface.

Two challenges should be overcome for the ultra-precision machining of micro-optical element with freeform curved surface: one is the intricate geometry, the other is the hard-to-machining optical materials due to their hardness, brittleness or flexibility. Here scanning electrochemical probe lithography (SECPL) is developed, not only to meet the machining need of intricate geometry by 3D direct writing, but also to overcome the above mentioned mechanical properties by an electrochemical material removal mode. Through the electrochemical probe a localized anodic voltage is applied to drive the localized corrosion of GaAs. The material removal rate is obtained as a function of applied voltage, motion rate, scan segment, etc. Based on the material removal function, an arbitrary geometry can be converted to a spatially distributed voltage. Thus, a series of micro-optical element are fabricated with a machining accuracy in the scale of 100 s of nanometers. Notably, the spiral phase plate shows an excellent performance to transfer parallel light to vortex beam. SECPL demonstrates its excellent controllability and accuracy for the ultra-precision machining of micro-optical devices with freeform curved surface, providing an alternative chemical approach besides the physical and mechanical techniques.

Micro-optical elements from optical-quality ZIF-62 hybrid glasses by hot imprinting

Hybrid glasses derived from meltable metal-organic frameworks (MOFs) promise to combine the intriguing properties of MOFs with the universal processing ability of glasses. However, the shaping of hybrid glasses in their liquid state – in analogy to conventional glass processing – has been elusive thus far. Here, we present optical-quality glasses derived from the zeolitic imidazole framework ZIF-62 in the form of cm-scale objects. These allow for in-depth studies of optical transparency and refraction across the ultraviolet to near-infrared spectral range. Fundamental viscosity data are reported using a ball penetration technique, and subsequently employed to demonstrate the fabrication of micro-optical devices by thermal imprinting. Using 3D-printed fused silica templates, we show that concave as well as convex lens structures can be obtained at high precision by remelting the glass without trading-off on material quality. This enables multifunctional micro-optical devices combining the gas uptake and permeation ability of MOFs with the optical functionality of glass. As an example, we demonstrate the reversible change of optical refraction upon the incorporation of volatile guest molecules.

Nonlinear compression of mid-infrared supercontinuum generation in dispersion-engineered As2S5 chalcogenide ridge waveguide

We numerically demonstrated nonlinear compression of mid-infrared (mid-IR) supercontinuum (SC) generation in As2S5 chalcogenide glass (ChG) ridge waveguides, achieving a small dispersion value at the pump wavelength of 2.5 µm by adjusting the waveguide width (normal dispersions of -10.547 ps.nm−1.km−1 and the anomalous dispersions of +5.314 ps.nm−1.km−1). These waveguides were designed using the negative slope of the dispersions with the negative third-order dispersion, which are applied to generate the nonlinear compression of SC generation. Using a 50-fs pulse with the peak power of 2000 W, the two waveguides could compress the maximum pulse peak power of 6900 W (> 3.4 octaves) and 6360 W (> 3.1 octaves) and generate the widest SC spectra, spanning from 1.20 µm to 12.96 µm and from 1.25 µm to >13 µm with only short waveguides 0.85 mm and 1 mm long, respectively. The key process behind SC formation in such ChG waveguides is related to self-phase modulation, four-wave mixing, and nonlinear compression. This particular design is effective, and ChG waveguides can generate high peak power and the widest spectra of SC generation. Moreover, the waveguides are also relatively flexible in design, which is concerned with optical design and engineering, and micro-optical devices. As the ultra-wideband mid-IR SC source, high pulse peak power, very short waveguides, and low-energy pulses (<1 pJ) are important for on-chip mid-IR SC sources, the proposed work would offer the greatest benefits in practical application.

Research on the Processing of Image and Spectral Information in an Infrared Polarization Snapshot Spectral Imaging System

In order to solve the problems of a low target recognition rate and poor real-time performance brought about by conventional infrared imaging spectral detection technology under complex background conditions or in the detection of targets of weak radiation or long distance, a kind of infrared polarization snapshot spectral imaging system (PSIFTIS) and a spectrum information processing method based on micro-optical devices are proposed in this paper, where the synchronous acquisition of polarization spectrum information is realized through the spatial modulation of phase with a rooftop-shaped multi-stage micro-mirror and the modulation of the polarization state of light with a micro-nanowire array. For the polarization interference image information obtained, the infrared polarization spectrum decoupling is realized by image segmentation, optical path difference matching, and image registration methods, the infrared polarization spectrum reconstruction is realized by Fourier transform spectral demodulation, and the infrared polarization image fusion is realized by decomposing and reconstructing the high- and low-frequency components of the polarization image based on the Haar wavelet transform. The maximum spectral peak wavenumber error of the four polarization channels of the polarization spectrum reconstruction is less than 2 cm−1, and the polarization angle error is within 1°. Ultimately, compared with the unprocessed polarization image unit, the peak signal-to-noise ratio is improved by 45.67%, the average gradient is improved by 8.03%, and the information entropy is improved by 56.98%.

Imine-Linked 2D Conjugated Porous Organic Polymer Films for Tunable Acid Vapor Sensing.

Two-dimensional (2D) films of conjugated porous organic polymers (C-POPs) can translate the rich in-plane functionalities of conjugated frameworks into diverse optical and electronic applications while addressing the processability issues of their crystalline analogs for adaptable device architectures. However, the lack of facile single-step synthetic routes to obtain large-area high-quality films of 2D-C-POPs has limited their application possibilities so far. Here, we report the synthesis of four mechanically robust imine-linked 2D-C-POP free-standing films using a single-step fast condensation route that is scalable and tunable. The rigid covalently bonded 2D structures of the C-POP films offer high stability for volatile gas sensing in harsh environments while simultaneously enhancing site accessibility for gas molecules due to mesoporosity by structural design. Structurally, all films were composed of exfoliable layers of 2D polymeric nanosheets (NSs) that displayed anisotropy from disordered stacking, evinced by out-of-plane birefringent properties. The tunable in-plane conjugation, different nitrogen centers, and porous structures allow the films to act as ultraresponsive colorimetric sensors for acid sensing via reversible imine bond protonation. All the films could detect hydrogen chloride (HCl) gas down to 0.05 ppm, far exceeding the Occupational Safety and Health Administration's permissible exposure limit of 5 ppm with fast response time and good recyclability. Computational insights elucidated the effect of conjugation and tertiary nitrogen in the structures on the sensitivity and response time of the films. Furthermore, we exploited the exfoliated large 2D NSs and anisotropic optoelectronic properties of the films to adapt them into micro-optical and triboelectric devices to demonstrate their real-time sensing capabilities.

Biomimetic Compound Eyes with Gradient Ommatidium Arrays.

Compound eyes are high-performing natural optical perception systems with compact configurations, generating extensive research interest. Existing compound eye systems are often combinations of simple uniform microlens arrays; there are still challenges in making more ommatidia on the compound eye surface to focus to the same plane. Here, a biomimetic gradient compound eye is presented by artificially mimicking dragonflies. The multiple replication process efficiently endows compound eyes with the gradient characteristics of dragonfly compound eyes. Experimental results show that the manufactured compound eye allows multifocus imaging by virtue of the gradient ommatidium array arranged closely in a honeycomb pattern while ensuring excellent optical properties and compact configurations. Thousands of ommatidia showing a gradient trend at the millimeter scale while remaining relatively uniform at the micron scale have gradient focal lengths ranging from 260 to 450 μm. This gradient compound eye allows more ommatidia to focus on the same plane than traditional uniform compound eyes, which have experimentally been shown to capture more than 1100 in-plane clear images simultaneously, promising potential applications in micro-optical devices, optical imaging, and biochemical sensing.

Microheater-Integrated Microlens Array for Robust Rapid Fog Removal.

In extreme environments, fog formation on a microlens array (MLA) surface results in a device failure. One reliable solution for fog removal is to heat the surface using a microheater. However, due to the surface interference, the combination of these two microdevices remains elusive. In this study, we introduce lift-off and electroless plating into femtosecond laser processing to fabricate a microheater integrated MLA (μH-MLA) on the same substrate with high light transmittance, durability, and fog removal efficiency. Laser-induced micro-nano grooves enable the microheater to be tightly coupled with the MLA and have high heating performance, thus maintaining a stable performance for over 24 h during continuous operation as well as under long time ultrasonic vibration and mechanical friction. With a rapid response time (τ0.5) of 17 s and a high working temperature of 188 °C, the μH-MLA removed fog that covers the entire face within 14 s. Finally, we prove the use of this fabrication method in large areas and curved surface environments. This study provides a flexible, stable, and economical method to integrate micro-optical and microelectrical devices.

The optimization method with segmenting iteration strategy for a long-wavelength infrared metalens

Over the past several years, there has been a substantial surge in the utilization of metasurfaces in micro-electronics and micro-optical devices. As one of their applications, metalenses have aroused significant research interest. In this paper, a new optimization method based on micro co-fluctuation model is proposed to optimize the initial metalens for better focusing performance by adjusting the structural dimensions of nanopillars. A segmenting iteration strategy is used to legitimately reduce the computation space. Meanwhile, an improved Harris Hawks Optimizer (IHHO) is proposed to obtain the optimal solutions, in which a new formula for escape energy is defined to promote the diversity of solution space. The optimization results indicate that the proposed optimization method effectively enhances the focusing capability of the initial metalens with a numerical aperture (NA) of about 0.7 and an operating wavelength of 10.6 μm. The maximum field intensity of the central hot spot and the absolute efficiency have increased by approximately 59.02% and 71.13%, respectively, and the full width at half maximum (FWHM) remains nearly the same value, which provides the potential for achieving high-contrast imaging.

Calcination-Enhanced Laser-Induced Damage Threshold of 3D Micro-Optics Made with Laser Multi-Photon Lithography

Laser Direct Writing (LDW), also known as 3D multi-photon laser lithography of resins, is a promising technique for fabricating complex free-form elements, including micro-optical functional components. Regular organic or hybrid (organic–inorganic) resins are often used, with the latter exhibiting better optical characteristics, as well as having the option to be heat-treated into inorganic glass-like structures particularly useful for resilient micro-optics. This work is a continuation of our SZ2080™ calcination development of micro-optics, specifically studying the Laser-Induced Damage Threshold (LIDT). Such sol–gel-derived glass 3D micro-structures, particularly those that undergo heat treatment, have not been well-characterized in this respect. In this pilot study, we investigated the LIDT using the Series-on-One (S-on-1) protocol of functional micro-lenses produced via LDW and subsequently calcinated. Our results demonstrate that the LIDT can be significantly increased, even multiple times, by this approach, thus enhancing the resilience and usefulness of these free-form micro-optics. This work represents the first investigation in terms of LIDT into the impact of calcination on LDW-produced, sol–gel-derived glass micro-structures and provides important insights for the development of robust micro-optical devices.

Switchable optics based on guided mode resonance in lithographically patterned vanadium dioxide with integrated heating layer

Vanadium dioxide (VO2) has promising applications in smart windows and active micro-optical devices due to its thermochromic properties. However, the successful fabrication and patterning of VO2 thin films with the correct stoichiometry and phase are challenging. In this study, we investigated lithographically patterned and non-patterned VO2 thin films fabricated by reactive ion beam deposition, using variable angle spectroscopic ellipsometry, Raman spectroscopy, and transmission and reflection measurements. The results show that the refractive index and extinction coefficient exhibit significant changes for near-infrared wavelengths when heated above 68 °C, confirming its thermochromic properties. The Raman spectroscopy results indicate the formation of the monoclinic phase VO2(M) after annealing, which was not changed by reactive ion etching. Lithographically structured VO2-layers were successfully realized demonstrating the potential of VO2 as a material for active micro-optical devices, such as guided mode resonance filters with switchable reflectance. The results suggest that VO2 has great potential as a promising material for actively switched optical elements and micro-optical devices.

Nanoscale Printing of Indium-Tin-Oxide by Femtosecond Laser Pulses.

For constructing optical and electrical micro-devices, the deposition/printing of materials with sub-1 μm precision and size (cross-section) is required. Crystalline c-ITO (indium tin oxide) nanostructures were patterned on glass with sufficient precision to form 20-50 nm gaps between individual disks or lines of ∼250 nm diameter or width. The absorbed energy density [J/cm3] followed a second-order dependence on pulse energy. This facilitated high-resolution and precise nanoscale laser-writing at a laser wavelength of 515 nm. Patterns for optical elements such as circular gratings and micro-disks were laser-printed using ITO as a resist. Unexposed amorphous a-ITO was chemically removed in aqueous 1% vol. HF solution. This use of a-ITO as a solid resist holds promise for metamaterial and micro-optical applications.

Stress-induced birefringence in 3D direct laser written micro-optics.

3D direct laser writing is a widely used technology to create different nano- and micro-optical devices for various purposes. However, one big issue is the shrinking of the structures during polymerization, which results in deviations from the design and in internal stress. While the deviations can be compensated by adapting the design, the internal stress remains and induces birefringence. In this Letter, we successfully demonstrate the quantitative analysis of stress-induced birefringence in 3D direct laser written structures. After presenting the measurement setup based on a rotating polarizer and an elliptical analyzer, we characterize the birefringence of different structures and writing modes. We further investigate different photoresists and the implications for 3D direct laser written optics.

Artificial Hyper Compound Eyes Enable Variable-Focus Imaging on both Curved and Flat Surfaces.

The artificial compound eye (ACE) with zoom imaging requires complex power sources. Meanwhile, its curved substrate makes it difficult for the ACE to realize the zoom imaging on flat surfaces. To realize a wide field of view and a zoom function on both curved and flat surfaces simultaneously, a novel ACE is proposed, which is a bionic design inspired by an ancient creature, trilobite. Compared with a dragonfly, photosensitive units of a trilobite's compound eye are composed of ommatidia with different focal lengths. By learning from this concept, an artificial hyper compound eye (AHCE) was fabricated. Its basic components are five microlenses with different curvatures, and they are capable of being treated as five ommatidia with different focal lengths. Five ommatidia form a photosensitive unit to realize a zoom function. AHCE is capable of variable-focus imaging on curved surfaces. With the information share function, we found that the AHCE not only images on curved surfaces but also has a zoom-imaging function on flat surfaces. The results confirm that the AHCE demonstrates an advanced imaging capability, a variable-focus imaging function on both curved and flat surfaces, which may open new opportunities in developing advanced micro-optical devices.

Technology of static oblique lithography used to improve the fidelity of lithography pattern based on DMD projection lithography

This paper proposes a static oblique lithography (SOL) strategy based on conventional digital micromirror device (DMD) projection lithography. This strategy combines the oblique lithography on oblique scanning lithography with static lithography to ensure that we can produce high fidelity microstructures with very smooth curved edges in all directions and more accurate relative line positions. Not only does the single-axis high-precision micromotion stage of this strategy enable a significant reduction in fabrication cost, but also enables the lithography of subpixel-width curves with smooth edges used our improved sub-pattern quantization strategy. The intensity distribution of the actual diffracted light field at the focal point of the helical zone plate fabricated by the SOL is highly matched with the intensity distribution of the designed simulated light field. This shows that the SOL can play an important role in the field of micro-optical device fabrication.

Strain-tunable optical microlens arrays with deformable wrinkles for spatially coordinated image projection on a security substrate

As a new concept in materials design, a variety of strategies have been developed to fabricate optical microlens arrays (MLAs) that enable the miniaturization of optical systems on the micro/nanoscale to improve their characteristic performance with unique optical functionality. In this paper, we introduce a cost-effective and facile fabrication process on a large scale up to ~15 inches via sequential lithographic methods to produce thin and deformable hexagonally arranged MLAs consisting of polydimethylsiloxane (PDMS). Simple employment of oxygen plasma treatment on the prestrained MLAs effectively harnessed the spontaneous formation of highly uniform nanowrinkled structures all over the surface of the elastomeric microlenses. With strain-controlled tunability, unexpected optical diffraction patterns were characterized by the interference combination effect of the microlens and deformable nanowrinkles. Consequently, the hierarchically structured MLAs presented here have the potential to produce desirable spatial arrangements, which may provide easily accessible opportunities to realize microlens-based technology by tunable focal lengths for more advanced micro-optical devices and imaging projection elements on unconventional security substrates.

ENHANCEMENT OF WATER RESISTANCE AND MECHANICAL PROPERTIES OF MICROFIBRILLATED CELLULOSE BY GLYCEROL AND HOT-PRESSING TREATMENT

In this work, glycerol and hot-pressing treatment were utilized to enhance the water-resistant and mechanical properties of microfibrillated cellulose (MFC) film. Recently, MFC films have been suggested as promising materials for various applications such as flexible electronics, micro-optical devices, and biosensors. However, their poor water resistance and brittleness prevent the widespread use of MFC films. In this study, the combination of glycerol and hot-pressing treatment was used to overcome these limitations. While the plasticizing effect by glycerol greatly enhances the stretchability and flexibility of the MFC film, hot-pressing treatment improves its water resistance and mechanical strength. The wet strength of treated MFC film using a glycerol solution with a concentration of 2.5% (v/v) (H-MFC/2.5%G) was 41.3 MPa compared to 3.3 MPa of untreated MFC film. Moreover, the strain at break and tensile strength of this film was significantly improved. The present MFC film can be used as an eco-friendly, robust, and flexible platform for advanced applications.

Method for improving the speed and pattern quality of a DMD maskless lithography system using a pulse exposure method.

Maskless lithography based on a digital micromirror device (DMD) has the advantages of high process flexibility and a low production cost. However, due to the trade-off relationship between the pixel size and exposure area, it is challenging to achieve high resolutions and high patterning speeds at the same time, which hinders the wider application of this technology in micro- and nano-fabrication processes. In addition, micromirrors in DMDs create pixelated edges that limit the pattern quality. In this paper, we propose a novel DMD maskless lithography method to improve the pattern quality during high-speed continuous patterning by means of pulse exposure and oblique scanning processes. A unique criterion, the pixel occupancy, was devised to determine the parameters related to the pulse exposure and oblique scanning optimally. We also studied how the duty cycle of the pulse exposure affects the pattern quality. As a result, we were able to increase the scanning speed up to the speed limit considering the damage threshold of the DMD and improve the pattern quality by resolving the pixelation problem. We anticipate that this method can be used in various microfabrication fields with short product life cycles or in those that require custom designs, such as the manufacturing of PCBs, MEMS devices, and micro-optics devices, among others.

Polymer Crystallization with Configurable Birefringence in Double Emulsion Droplets

Biodegradable semicrystalline polymers are broadly used in microfluidic techniques to create microdroplets for various applications. However, it remains a fundamental challenge to modulate polymer crystallization and optical properties in microdroplets which have complicated spatial geometries. Here we investigate the crystallization of double emulsion droplets formed by poly(1,4-butylene adipate) (PBA), polycaprolactone (PCL), and their blends. Crystallization of these polymers drives the spherical liquid droplets to anisotropic solid microcapsules. Crystal growth shows a linear relationship with time, irrespective of flat-on, edge-on, or continuous twisting lamellar arrangement in the shell. We further reveal that the anisotropic shape and lamellar arrangement impart the crystallized microcapsule with orientation-sensitive birefringence. A representative feature is that all anisotropic microcapsules show two types of Maltese cross, depending on the microcapsule orientation. The PBA shell with flat-on lamellar arrangement always shows positive birefringence, while the PCL shell with edge-on lamellar growth may present negative or positive birefringence at specific orientation angles. Microcapsules of PCL/PBA blends are regulated by the composition of two polymers (fPCL), realizing the transition from PBA-dominated shell to PCL-dominated shell across fPCL = 0.5. We emphasize the peculiarities of crystallization behavior in double emulsion droplets by comparing with crystallization on plane substrate. This work may serve as a fundamental reference for understanding polymer crystallization and further expanding potential applications of semicrystalline polymers for optical microdevices.