Micro-optical Devices Research Articles

Direct laser writing for micro-optical devices using a negative photoresist.

Direct laser writing (DLW) via two-photon absorption (TPA) has attracted much attention as a new microfabrication technique because it can be applied to fabricate complex, three-dimensional (3D) microstructures. In this study, 3D microstructures and micro-optical devices of micro-lens array on the micrometer scale are fabricated using the negative photoresist SU-8 through TPA with a femtosecond laser pulse under a microscope. The effects of the irradiation conditions on linewidths, such as laser power, writing speed, and writing cycles (a number of times a line is overwritten), are investigated before the fabrication of the 3D microstructures. Various microstructures such as woodpiles, hemisphere and microstructures, 3D micro-lens and micro-lens array for micro-optical devices are fabricated. The shape of the micro-lens is evaluated using the shape analysis mode of a laser microscope to calculate the working distance of the fabricated micro-lenses. The calculated working distance corresponds well to the experimentally measured value. The focusing performance of the fabricated micro-lens is confirmed by the TPA fluorescence of an isopropyl thioxanthone (ITX) ethanol solution excited by a Ti:sapphire femtosecond laser at 800 nm. Micro-lens array (assembled 9 micro-lenses) are fabricated. Nine independent woodpile structures are simultaneously manufactured by DLW via TPA to confirm the multi-focusing ability using the fabricated micro-lens array.

Direct imaging of tunable photonic nanojets from a self-assembled liquid crystal microdroplet.

We report the direct experimental observation of electrically tunable photonic nanojets (PNJs) generated from self-assembled liquid crystal (LC) microdroplets formed by dispersing nematic LCs in polymer matrix. Optical measurements were performed with a home-built laser-scanning confocal microscope system. PNJs with subwavelength beam waists were successfully obtained from LC microdroplets of 5μm diameter, similar to those from SiO2 microspheres of the same size. By applying external voltage of a few volts, some of major properties of PNJs, such as beam length and brightness, are tuned. Electro-tunable PNJs from self-assembled LC microdroplets may open the way for the development of novel micro-optical devices.

Micro-Optics for Lab-on-a-Chip

Lab-on-a-chip (LOC) is nowadays one of the most interesting topics at the field of biomedicine and biosensing applications. Mainly devices that integrate a detection system right into a chip with microfluidic network are highly desirable. In this article we will present Mach-Zehnder interferometer created in IP-Dip photoresist by direct laser lithography system for laboratory on chip applications. Interferometer will be later used for biosensing application as integrated micro-optical device inside a microfluidic chip. Measurements of transmission characteristics of water solution with different sugar concentration show promising ability of the Mach-Zehnder interferometer to detect different liquids on principle of refractive index change.

Testing of Piezo-Actuated Glass Micro-Membranes by Optical Low-Coherence Reflectometry.

In this work, we have applied optical low-coherence reflectometry (OLCR), implemented with infra-red light propagating in fiberoptic paths, to perform static and dynamic analyses on piezo-actuated glass micro-membranes. The actuator was fabricated by means of thin-film piezoelectric MEMS technology and was employed for modifying the micro-membrane curvature, in view of its application in micro-optic devices, such as variable focus micro-lenses. We are here showing that OLCR incorporating a near-infrared superluminescent light emitting diode as the read-out source is suitable for measuring various parameters such as the micro-membrane optical path-length, the membrane displacement as a function of the applied voltage (yielding the piezo-actuator hysteresis) as well as the resonance curve of the fundamental vibration mode. The use of an optical source with short coherence-time allows performing interferometric measurements without spurious resonance effects due to multiple parallel interfaces of highly planar slabs, furthermore selecting the plane/layer to be monitored. We demonstrate that the same compact and flexible setup can be successfully employed to perform spot optical measurements for static and dynamic characterization of piezo-MEMS in real time.

Parametric Resonance in Electrostatically Actuated Micromirrors

We consider an electrostatically actuated torsional micromirror, a key element of recent optical microdevices. The mechanical response is analyzed with specific emphasis on its nonlinear features. We show that the mirror motion is an example of parametric resonance, activated when the drive frequency is twice the natural frequency of the system. The numerical model, solved with a continuation approach, is validated with very good accuracy through an extensive experimental campaign.

Low-Cost Fabrication of All-Polymer Components for Integrated Photonics

For various applications such as optical communication, sensor technology, and optical interconnects, microoptical polymer devices show great promise. Especially, straight and bent optical waveguides as well as beam splitters represent the building blocks of these devices. In this paper, we introduce a novel combination of simple fabrication techniques and cost-efficient polymer materials for the fabrication of planar polymer optical waveguides. We present a low-cost fabrication process through hot embossing and doctor blading techniques and investigate the use of UV curing printing ink and optical adhesive as waveguide core materials with an emphasis on economical fabrication and low optical losses in the near infrared and the visible range of the light spectrum. The refractive indices, the propagation losses and the bend losses of the fabricated waveguides are characterized. We demonstrate propagation losses as low as 0.09 and 0.74 dB/cm for wavelength of 850 and 633 nm, respectively. Furthermore, we investigate the crosstalk between adjacent waveguides as a function of the distance separating them and present an application of the presented fabrication technique in the form of beam splitters. We also investigate beam splitters having spliting ratios of 1:2, 1:4, and 1:8 and demonstrate excess losses per branching region of down to 0.11 dB and high output uniformity. All these experimental values represent important benchmarks, which demonstrate the capability of the presented method to be used for the design of more complex polymer photonic devices.

Phase gratings with tunable diffraction efficiency.

We present a new principle for tuning the diffraction efficiency of an optical grating and its implementation in a micro-optical device. The overlap of two phase gratings is used to vary the effective phase shift and hence the diffraction efficiency. We study the working principle using Fourier Optics to simulate the diffraction pattern in the far field and design and realize a device based on integrated piezo actuation. We find good agreement between simulation and experiment and observe a suppression of the first diffraction order intensity by more than 97% and response times of less than 3 ms.

Flexible Vesiculovasoscopy Using a Microoptical System in a Human Cadaver Model: An Experimental Approach for Atraumatic Endoscopy of the Seminal Tract.

The most common pathologies of the seminal tract are persistent hematospermia, seminal vesicle stones, and seminal duct obstruction. Endoscopic diagnostic work-up of the seminal tract is impeded by complex anatomy and lack of technical equipment. To date, there is no standardized endoscopic approach. The purpose of this study was to investigate the applicability and feasibility of a flexible microoptical device for atraumatic endoscopy of the seminal tract in a male human cadaver. The transurethral endoscopic examination was performed on a male cadaver. No premortal interventions or diseases of the genitourinary tract had been reported. The seminal orifice was identified via cystoscopy and accessed by the Seldinger technique using a hydrophilic guidewire and ureteral catheter. Retrograde endoscopic inspection of the distal seminal tract was performed using a miniaturized flexible endoscope. An antegrade endoscopic inspection of the seminal tract was carried out via high scrotal access to the vas deferens. Structures of the seminal tract, such as the ejaculatory duct, seminal vesicles, and distal portion of the ductus deferentes, were visualized using the miniaturized endoscope. Image quality allowed identification of anatomical structures and characterization of tissue properties. The technical limitations we observed involved the system's maneuverability. Initial results of this novel endoscopic approach to the seminal tract using a flexible microoptical system are encouraging. However, considerable anatomical limitations of the targeted organs necessitate further refinements of the technical equipment. This approach might improve diagnostics and treatment of genitourinary diseases. Future surgical techniques may include intraseminal laser therapy or endoocclusion to monitor fertility in men.

Flexible Vesiculo-Vasoscopy Using a Micro-Optical System in a Human Cadaver Model: An Experimental Approach for Atraumatic Endoscopy of the Seminal Tract

Objectives: The most common pathologies of the seminal tract are persistent hematospermia, seminal vesicle stones, and seminal duct obstruction. Endoscopic diagnostic work-up of the seminal tract is impeded by complex anatomy and lack of technical equipment. To date, there is no standardized endoscopic approach. Purpose of this study was to investigate the applicability and feasibility of a flexible micro-optical device for atraumatic endoscopy of the seminal tract in a human male cadaver. Materials and Methods: The transurethral endoscopic examination was performed on a male cadaver. No premortal interventions or diseases of the genitourinary tract had been reported. The seminal orifice was identified through cystoscopy and accessed by Seldinger technique using a hydrophilic guidewire and ureteral catheter. Retrograde endoscopic inspection of the distal seminal tract was performed using a miniaturized flexible endoscope. An antegrade endoscopical inspection of the seminal tract was carried out through high scrotal access to the vas deferens. Results: Structures of the seminal tract such as the ejaculatory duct, seminal vesicles, and distal portion of the ductus deferentes were observed using the miniaturized endoscope. Image quality allowed identification of anatomical structures and characterization of tissue properties. The technical limitations we observed involved the system's maneuverability. Conclusions: Initial results of this novel endoscopic approach to the seminal tract using a flexible micro-optical system are encouraging. However, considerable anatomical limitations of the targeted organs necessitate further refinements of the technical equipment. This approach might improve diagnostics and treatment of genitourinary diseases. Future surgical techniques may include intraseminal laser therapy or endo-occlusion to monitor fertility in men. Acknowledgment: We would like to thank Schoelly Fiberoptics GmbH, Denzlingen, Germany, for providing the instruments and technical support for this study. The cadaver (body donor) was provided by the Institute for Anatomy, Medical Faculty, Justus-Liebig-University, Giessen. Institutional funding (Medical Faculty of the University of Freiburg) and nonfinancial industrial funding (providing the instruments and technical support for the study) were by Schoelly Fiberoptic GmbH, Denzlingen, Germany. D.S., M.M., S.H., F.A., and W.W. have no conflicts of interest or financial ties to disclose. M.S. consultant contract with Schoelly Fiberoptics GmbH, Denzlingen, Germany, and NeoTract, Inc., Pleasanton, USA. T.D. Lilly Deutschland GmbH (stocks/stock options, employee: through family member), Pfizer (speaker honorarium), Takeda Pharma GmbH (speaker honorarium), AMS Deutschland (speaker honorarium), Jenapharm GmbH (speaker honorarium), Glenwood (speaker honorarium), Cheplapharm Arzneimittel GmbH (consultant), Marpinion GmbH (Advisory Board member, consultant), Advance Medical S.A., Barcelona (ESP) (Consultatnt, Reviewer). A.M. consultant contract with Schoelly Fiberoptics GmbH, Denzlingen, Germany. Runtime of video: 6 mins 10 secs

Reconfigurable Liquid Whispering Gallery Mode Microlasers.

Engineering photonic devices from liquid has been emerging as a fascinating research avenue. Reconfigurably tuning liquid optical micro-devices are highly desirable but remain extremely challenging because of the fluidic nature. In this article we demonstrate an all-liquid tunable whispering gallery mode microlaser floating on a liquid surface fabricated by using inkjet print technique. We show that the cavity resonance of such liquid lasers could be reconfigurably manipulated by surface tension alteration originated from the tiny concentration change of the surfactant in the supporting liquid. As such, remarkable sensing of water-soluble organic compounds with a sensitivity of free spectral range as high as 19.85 THz / (mol · mL−1) and the detectivity limit around 5.56 × 10−3 mol · mL−1 is achieved. Our work provides not only a novel approach to effectively tuning a laser resonator but also new insight into potential applications in biological, chemical and environmental sensing.

High Efficiency Coupling of Optical Fibres with SU8 Micro-droplet Using Laser Welding Process

Apart from micro- structure fabrication, ablation, lithography etc., lasers find a lot of utility in various areas like precision joining, device fabrication, local heat delivery for surface texturing and local change of microstructure fabrication of standalone optical micro-devices (like microspheres, micro-prisms, micro-scale ring resonators, optical switches etc). There is a wide utility of such systems in chemical/ biochemical diagnostics and also communications where the standalone optical devices exist at a commercial scale but chip based devices with printed optics are necessary due to coupling issues between printed structures and external optics. This paper demonstrates a novel fabrication strategy used to join standalone optical fibres to microchip based printed optics using a simple SU8 drop. The fabrication process is deployed for fiber to fiber optical coupling and coupling between fiber and printed SU-8 waveguides. A CO2 laser is used to locally heat the coupling made up of SU8 material. Optimization of various dimensional parameters using design of experiments (DOE) on the bonded assembly has been performed as a function of laser power, speed, cycle control, spot size so on so forth. Exclusive optical [RF] modelling has been performed to estimate the transmissibility of the optical fibers bonded to each other on a surface with SU8. Our studies indicate the formation of a Whispering gallery mode (WGM) across the micro-droplet leading to high transmissibility of the signal. Through this work we have thus been able to develop a method of fabrication for optical coupling of standalone fibers or coupling of on-chip optics with off-chip illumination/detection.

Miniature Schwarzschild objective as a micro-optical component free of main aberrations: concept, design, and first realization with silicon-glass micromachining.

This paper presents the conception of a new micro-optical component fabricated within the wafer-level approach: a micromachined reflective objective, the so-called micro-Schwarzschild objective, characterized by superior optical performances than widespread microlenses. The system, made of two vertically integrated mirrors, works in transmission similarly as microlenses. While the specific geometric configuration of the two-mirrors allows elimination of most common optical aberrations, the reflective architecture provides inherent achromaticity. This paper presents in detail the optical design and analyzes fabrication tolerances. It also describes a fabrication flow chart based on silicon micromachining done at the wafer level that could allow production of thousands of such micro-optical devices within a single fabrication run. The realized prototype employs the two-step KOH etching process to generate the micromirror pairs followed by glass reflow for the secondary mirror generation and selective metallic deposition. Despite an insufficient mirror quality attributed to this specific silicon etching technique and highlighted by the reflective configuration, the objective fabrication in terms of alignment, bonding, and coating is shown as feasible.

Synthesis of AlN hexagonal bipyramids by carbothermal reduction nitridation

Wurtzite-structured aluminum nitride (AlN) hexagonal bipyramids were prepared via a carbothermal reduction nitridation method using alumina and carbon as the raw materials. The growth of the AlN hexagonal bipyramids is dominated by the vapor-solid (VS) mechanism with Fe2O3, SiO2 and Na2O acting as mineralizers under alternate gas pressures. The preferred growth direction of the AlN hexagonal bipyramids is [0001]. Photoluminescence spectra of the AlN hexagonal bipyramids showed near-band edge emission at 512nm at room temperature. The AlN hexagonal bipyramids might be useful for optical micro-devices.

Tracking micro-optical resonances for identifying and sensing novel procaspase-3 protein marker released from cell cultures in response to toxins

The response of cells to toxins is commonly investigated by detecting intracellular markers for cell death, such as caspase proteins. This requires the introduction of labels by the permeabilization or complete lysis of cells. Here we introduce a non-invasive tool for monitoring a caspase protein in the extracellular medium. The tool is based on highly sensitive optical micro-devices, referred to as whispering-gallery mode biosensors (WGMBs). WGMBs are functionalized with antibodies for the specific and label-free detection of procaspase-3 released from human embryonic kidney HEK293 and neuroglioma H4 cells after introducing staurosporine and rotenone toxins, respectively. Additional tests show that the extracellular accumulation of procaspase-3 is concomitant with a decrease in cell viability. The hitherto unknown release of procaspase-3 from cells in response to toxins and its accumulation in the medium is further investigated by Western blot, showing that the extracellular detection of procaspase-3 is interrelated with cytotoxicity of alpha-synuclein protein (aSyn) overexpressed in H4 cells. These studies provide evidence for procaspase-3 as a novel extracellular biomarker for cell death, with applications in cytotoxicity tests. Such WGMBs could be applied to further identify as-yet unknown extracellular biomarkers using established antibodies against intracellular antigens.

Implantable micro-optical semiconductor devices for optical theranostics in deep tissue

Optical therapy and diagnostics using photoactivatable molecular tools are promising approaches in medical applications; however, a method for the delivery of light deep inside biological tissues remains a challenge. Here, we present a method of illumination and detection of light using implantable micro-optical semiconductor devices. Unlike in conventional transdermal light delivery methods using low-energy light (>620 nm or near-infrared light), in our method, high-energy light (470 nm) can also be used for illumination. Implanted submillimeter-sized light-emitting diodes were found to provide sufficient illumination (0.6–4.1 mW/cm2), and a complementary metal–oxide–semiconductor image sensor enabled the detection of fluorescence signals.

Optically and rheologically tailored polymers for applications in integrated optics

We demonstrate an epoxy acrylate based system on which various micro-optical applications can be realized. With this innovative and flexible material concept, it is possible to adjust viscosity (3mPas<η<46Pas) as required for different microstructuring processes e.g. inkjet printing, spin coating, and stereolithography and simultaneously precisely tailor refractive indices (1.518<nD,20<1.569) which is essential to implement optical sensor concepts. The practicability of this material system is demonstrated for optical waveguide production and selected applications. The fast and low cost modification of optical and rheological material properties is advantageous for the fabrication of micro-optical polymer devices including planar-optical polymer sensor networks.

3D Microfabrication Using Emulsion Mask Grayscale Photolithography Technique

Recently, the rapid development of technology such as biochips, microfluidic, micro-optical devices and micro-electromechanical-systems (MEMS) demands the capability to create complex design of three-dimensional (3D) microstructures. In order to create 3D microstructures, the traditional photolithography process often requires multiple photomasks to form 3D pattern from several stacked photoresist layers. This fabrication method is extremely time consuming, low throughput, costly and complicated to conduct for high volume manufacturing scale. On the other hand, next generation lithography such as electron beam lithography (EBL), focused ion beam lithography (FIB) and extreme ultraviolet lithography (EUV) are however too costly and the machines require expertise to setup. Therefore, the purpose of this study is to develop a simplified method in producing 3D microstructures using single grayscale emulsion mask technique. By using this grayscale fabrication method, microstructures of thickness as high as 500μm and as low as 20μm are obtained in a single photolithography exposure. Finally, the fabrication of 3D microfluidic channel has been demonstrated by using this grayscale photolithographic technique.

Near-IR Emission Enhancement of K2LuF5: Yb3+/Er3+ via the Cross-Relaxation Processes with the Assistance of Ce3+ Ions

Yb3+/Er3+ co-doped K2LuF5 micro-crystals have been synthesized via a facile and environmentally-friendly hydrothermal route. The effects of Ce3+ on the upconversion (UC) and downconversion (DC) luminescence properties of K2LuF5: 16%Yb3+, 2%Er3+ under the excitation of 980 nm laser diode (LD) are systematically investigated. The emission intensity located at 1.54 μm of 15 mol% Ce3+ doped sample is enhanced about 6 times, in comparison with that of Ce3+-free sample. It is proposed that the efficient cross-relaxation processes between Er3+ and Ce3+ ions contribute to the competition between UC and DC transitions, and the success in varying the UC and DC radiation rate. In addition, with an increasing of excitation power density, the cross-relaxation process of Ce3+ ions doped samples is promoted, which leads to a multicolor output. This study could be expected to promote the development of Ce3+ doped K2LuF5: 16%Yb3+, 2%Er3+ as micro-optical device.

Four-Wave Mixing (FWM) Effects within a Micro-Optical Device

Four-Wave Mixing (FWM) Effects within a Micro-Optical Device

自己組織化による3次元微細周期構造形成のための微小パーツの作製

We prepared 10 μm-sized poly(dimethylsiloxane) (PDMS)-based cubic microparts to form 3D microperiodic structures by self-assembly of the microparts. Self-assembly is a promising candidate for fabricating the 3D periodic structures such as optical microdevices. The PDMS-based microparts, which have high transparency in ultraviolet (UV) to near infrared (NIR) light, were fabricated by a combined process of photolithographic techniques and UV-ozone treatment. First, cubic molds of thick photoresist were formed by the lithography. Then, PDMS filled into the molds, following that it was cured by heating at 80°C. After that, the microparts were separated using UV-ozone treatment and ultrasonic microwaves. The microparts were dispersed into the water an ethanol mixed solution with surfactant F-127 to prepare the suspension. The surfactant was useful to prevent aggregation of microparts. Finally, we demonstrated the self-assembling of the PDMS microparts by putting a droplet of the suspension on a glass substrate, and drying at room temperature with turning substrate upside down. The microparts were partly aligned with the cubic closed packing by self-assembly. This result indicates that PDMS-based cubic microparts have possibility of forming three-dimensional periodic structures.