Hybrid Microlens Research Articles

Hybrid microlens array/grating fabrication based on microfluidic system and its application in the enhancement of spectral resolution

Nowadays, micro-spectrometer has become an important research direction in the field of spectral analysis. The hybrid microlens array (MLA)/gratings element performs both focusing and diffraction duties, and its short focal length allows the optical system to be smaller. In this work, a unique and cost-effective fabrication method for hybrid MLA/grating based on the microfluidic system is described. By this method, the grating line-density, microlens curvature and aperture size of the hybrid MLA/grating can be effectively controlled, so as to bring different diffraction and focusing effects. Finite element simulation is used to describe the principle of the method. The relationship between bulge height and membrane thickness, pressure, hole size was investigated using a profile observation system. Wave optics simulation and diffraction measurements were used to demonstrate the optical properties of this hybrid element. It can be found that the diffracted light is converged at focus points after MLA/grating, and the full width at half maximum (FWHM) is significantly narrowed. The diffraction experimental results are consistent with the simulation results. The MLA/grating structure reduces FWHM of first-order diffracted light by a factor of 7 compared to same type of the planar grating. Therefore, the MLA/grating element can effectively improve the spectral resolution. Due to its small focal length, the MLA/grating has the potential to be used in micro-spectrometers.

Design and fabrication of hybrid MLAs/gratings for the enhancement of light extraction efficiency and distribution uniformity of OLEDs.

Extracting light from organic light-emitting diodes (OLEDs) and improving the angular distribution are essential for their commercial applications in illumination and displays. In this work, hybrid microlens arrays (MLAs) and gratings with periods and depths in the scale of submicron have been designed and incorporated on the lighting surface of OLEDs for simultaneous enhancement of light outcoupling efficiency and angular distribution improvement. It is found that the augmentation of light extraction efficiency is mainly attributed to the MLAs, while the gratings can improve the viewing angle by increasing the angular distribution uniformity. A novel approach was proposed by combining photoresist thermal reflow, soft-lithography and plasma treatments on polydimethylsiloxane (PDMS) surfaces synergistically to realize gratings on the wavy surface of MLAs. It has been proved that with the hybrid MLAs/gratings, the external quantum efficiency (EQE) of the OLED can reach up to 22.8%, which increased by 24% compared to that of bare OLED. Moreover, the OLED with the hybrid MLAs/gratings showed an obvious lateral enhancement at wider viewing angle.

Longitudinal optical imaging technique to visualize progressive axonal damage after brain injury in mice reveals responses to different minocycline treatments

A high-resolution, three-dimensional, optical imaging technique for the murine brain was developed to identify the effects of different therapeutic windows for preclinical brain research. This technique tracks the same cells over several weeks. We conducted a pilot study of a promising drug to treat diffuse axonal injury (DAI) caused by traumatic brain injury, using two different therapeutic windows, as a means to demonstrate the utility of this novel longitudinal imaging technique. DAI causes immediate, sporadic axon damage followed by progressive secondary axon damage. We administered minocycline for three days commencing one hour after injury in one treatment group and beginning 72 hours after injury in another group to demonstrate the method’s ability to show how and when the therapeutic drug exerts protective and/or healing effects. Fewer varicosities developed in acutely treated mice while more varicosities resolved in mice with delayed treatment. For both treatments, the drug arrested development of new axonal damage by 30 days. In addition to evaluation of therapeutics for traumatic brain injury, this hybrid microlens imaging method should be useful to study other types of brain injury and neurodegeneration and cellular responses to treatment.

Improvement of form accuracy and surface integrity of Si-HDPE hybrid micro-lens arrays in press molding

Press molding of silicon (Si)/high-density polyethylene (HDPE) composite is an important technology for producing thin hybrid infrared (IR) optics with microstructures. In this research, Si-HDPE hybrid micro-lens arrays were press molded under various conditions, and the form accuracy and surface integrity of the molded lenses were evaluated. Air trapping occurs inside the micro-lens cavities during molding in a non-vacuum environment, which leads to severe surface defects. To investigate the air trapping phenomenon, a new in-situ observation system was developed which enables real-time direct observation of the molding process. From the in-situ observations, it was found that air traps were formed among the HDPE pellets during melting, and an increase in the pressing force will increase the pressure of the trapped air, forming trenches on the lens surface. The trapped air also impacts the mold coating, causing trench formation on the coating surface. To minimize air trapping, the molding temperature, and pressing force must be strictly controlled. By performing press molding in a vacuum environment, trench formation was completely eliminated. Moreover, polymer shrinkage compensation was performed to improve the lens form accuracy.

Enhancing light harvesting of organic solar cells by using hybrid microlenses

Organic solar cells have drawn intense attentions in recent years due to their inherent advantages. But the relatively low power conversion efficiency is the main obstacle in the way of organic solar cell commercialization. One of the main reasons that limit the power conversion efficiency is the mismatch between electrical transmission properties and light absorption properties in an organic active layer. In this work, a highly efficient light trapping scheme with a hybrid microlens array is proposed to resolve this contradiction. This structure can achieve broadband absorption enhancement in the spectrum of interest by chromatic aberration correction and hole parameter adjustment. And the light trapping element can be separated from cells to avoid direct contact with an organic layer that may cause electrical defects. Moreover, it is also compatible with low cost manufacturing technologies.

Fabrication of SiO2 hybrid microlens structures using femtosecond laser nonlinear lithography

We fabricated three-dimensional SiO2 surfaces using femtoseond laser nonlinear optical lithography. By pattern transfers of polymerized resist patterns, which were written via nonlinear absorption, into underlying substrates, SiO2 hybrid microlens structures were created. Fresnel zone plate patterns were written on curved surfaces of microlenses whose diameters and curvature radii were 240 μm and 380 μm, respectively. The surface observation revealed that the surface roughness of the transferred SiO2 was improved with O2 concentration during plasma etching. When He-Ne laser light of 632.8 nm wavelength was coupled to the hybrid lenses, the focal lengths became shorter than those of the original refractive lenses. This shift amount of 216 μm was consistent with theoretical value of 213 μm, indicating nonlinear lithography was useful for the precise microfabrication of photonic devices with three-dimensional surfaces.

Silicon microlens structures fabricated by scanning-probe gray-scale oxidation

We report on the micromachining of silicon microlens structures by use of scanning-probe gray-scale anodic oxidation along with dry anisotropic etching. Convex, concave, and arbitrarily shaped silicon microlenses with diameters as small as 2 microm are demonstrated. We also confirm the high fidelity of pattern transfer between the probe-induced oxides and the etched silicon microlens structures. Besides the flexibility, the important features of scanning-probe gray-scale anodic oxidation are small pixel size and pitch (of the order of tens of nanometers), an unlimited number of gray-scale levels, and the possibility of creating arbitrarily designed microlens structures with exquisite precision and resolution. With this approach, refractive, diffractive, and hybrid microlens arrays can be developed to create innovative optical components.

Design of hybrid micro-diffractive-refractive optical element with wide field of view for free space optical interconnections

A new design is presented to achieve a hybrid micro-diffractiverefractive lens with wide field of view (WFOV) of 80 degrees integrated on backside of InGaAs / InP photodetector for free space optical interconnections. It has an apparent advantage of athermalization of optical system which working in large variation of ambient temperature ranging from -20 degrees C to 70 degrees C. The changing of focal length is only 0.504 m in the ambient temperature range with the hybrid microlens, which opto-thermal expansion coefficient matches with thermal expansion coefficient of AuSn solder bump used in corresponding flip-chip packaging system. The hybrid lens was designed via CODE-VTM professional software. The results show that the lens has good optical performance for the optical interconnection use.

Investigation of hybrid microlens integration with vertical-cavity surface-emitting lasers for free-space optical links

In a novel one-step process, a vertical-cavity surface-emitting laser (VCSEL, operation wavelength of 980 nm) is integrated with a hybrid microdiffractive lens by focused ion beam milling (FIBM) for use in freespace optical links. A hybrid microlens with a diameter of 100 m, numerical aperture of 0.56, and sag height of 4.196 m, combined with a diffractive lens with continuous relief and 6 annuli, was designed and fabricated in one step by FIBM on the back of a VCSEL with a GaAs substrate for beam collimation. A previous VCSEL integrated with a pure diffractive lens had a half-divergence angle of 0.6 degrees ; the half-divergence angle of the VCSEL with the hybrid microlens was improved to 0.3 degrees . Test results show that athermalization with respect to the variation in operating temperature can be realized with the hybrid microlens.

Hybrid microdiffractive–microrefractive lens with a continuous relief fabricated by use of focused-ion-beam milling for single-mode fiber coupling

The design, microfabrication, and testing of a hybrid microdiffractive-microrefractive lens with a continuous relief that is used with a laser diode for monomode fiber coupling is discussed. The hybrid microlens with a diameter as small as 65 mum and a numerical aperture of 0.25 is fabricated directly onto a spherical surface by use of focused-ion-beam milling technology. A focused Ga(+) ion beam is used to mill the continuous relief microstructure at an acceleration voltage of 50 kV. From the test results a coupling efficiency of 71% (-1.49 dB) was achieved at room temperature. A diffraction efficiency of the main diffractive order, -1, was measured to as high as 90.5% at a wavelength of 635 nm. This indicates that the hybrid microlens was suitably designed within the application requirements and satisfactorily manufactured with focused-ion-beam milling.