Yan-Hao Yu

Focal varying microlens array

We report a novel microlens array with different curvature unit lenses (MLADC) fabricated with femtosecond laser direct writing technology. The MLADC consisted of hexagonal hyperboloid unit microlenses, which have different heights and curvatures from others. The unique optical performance of imaging and focusing capability were demonstrated. An object was imaged at different positions from the MLADC by unit lenses, as the ability of adjusting the curvature of the image plane for overall MLADC. In addition, the experiment had a good agreement with simulation results, which was based on the analysis of the finite element method. The novel MLADC will have important applications in improving the performance of optical systems, especially in field curvature correction and real-time three-dimensional imaging.

Sapphire-Based Fresnel Zone Plate Fabricated by Femtosecond Laser Direct Writing and Wet Etching

Here, we report a sapphire-based Fresnel zone plate (FZP), which is fabricated by femtosecond laser direct writing assisted with subsequent wet etching. With this method, we solved the problem of high surface roughness caused by ultrafast femtosecond laser processing. We have obtained ~12-nm average surface roughness smaller than 1/25 of the optical working wavelength. As-formed sapphire FZP also exhibited a well-defined geometry. More importantly, ultraviolet (UV) light focusing and imaging can be easily achieved. Due to the high material hardness, thermal and chemical stabilities of sapphire, such sapphire FZP, may have great potential in UV imaging and focusing under some harsh environments.

Fabrication of an anti-reflective microstructure on sapphire by femtosecond laser direct writing

Herein, we report a facile approach for the maskless production of subwavelength-structured antireflective surfaces on sapphire with high and broadband transmittance in the mid-IR: femtosecond laser direct writing assist with wet etching. With this method, inverted pyramid and cone arrays with a pitch of about 2 μm and a total height of near 900 nm on the sapphire were produced. The resulting subwavelength structures greatly suppress specular reflection at normal incidence. The transmission measurements between 3 and 5 μm are in agreement with the simulations performed using VirtualLab, and the transmittance reached a maximum value of 92.5% at 4 μm. The sapphire with subwavelength structures also exhibits angle-independent transmittance characteristics up to a high θ=60°. Therefore, these subwavelength structures on sapphire are of great technological importance in mid-IR optics, especially for the harsh-condition-applicable windows of military mid-IR devices.

Electro-optical detection based on large Kerr effect in polymer-stabilized liquid crystals.

In this Letter, polymer-stabilized liquid crystals with experimentally observed large electro-optic effect are introduced to the electro-optical detection to improve the voltage sensitivity. The Kerr constant of materials prepared in this study reached as high as 7.2×10(-9) m/V(2), increasing by 1000 times the sensitivity of the conventional electro-optical materials. The noncontact detection configuration, using a laser beam as a probe, enables quick two-dimensional scanning measurements.

Ultrahigh sensitivity electric field detection with a liquid electro-optical film

In this Letter, an electro-optical probe configuration with polar molecule liquids as the sensing film is proposed to improve the voltage sensitivity. This method exhibited increases in intrinsic sensitivities better than 0.1 mV/√Hz, 2 orders of magnitude larger than the normal method using a GaAs probe in the same measurement system. Based on the mechanism of orientation polarization, the electro-optic coefficient was measured to be 250 pm/V by the Teng-Man method at a modulation field of 100 Hz. This technology will be promising in applications of low-frequency field detection.

Biomimetic construction of hierarchical structures via laser processing

Biomimetic hierarchical structures are an important branch among bio-inspired materials. These structures are responsible for rich physical and chemical phenomena. In this paper, we briefly reviewed facile methods to fabricate large-area biomimetic hierarchical structures via laser processing. One is based on femtosecond laser direct writing, and the other is based on two-beam interference lithography. In details, the first method is laser ablation on ZnS thin films via femtosecond direct writing combined with a phase mask. The second method is a multi-exposure lithography by two-beam interference on a photoresist with varied exposure angles to acquire structural hierarchies with periodicity. Owing to the outstanding patterning ability of the two laser-processing approaches, a diversity of large-area hierarchical structures can be obtained and show rich biomimetic functions such as superhydrophobicity.

Tapered and Tip-Grounded Waveguide Electrooptical Microsensors

A tip-grounded waveguide microsensor was proposed to overcome the difficulty of quantitative voltage calibration in electrooptical detection for integrated circuit (IC) test. On this basis, we optimized the thickness of the electrooptical material of the sensor to eliminate the influence of the circuit layout on the measured signals. The improved sensor in return made it possible to calibrate the voltage with known reference electric signals quantitatively. This method circumvented the uncertainty of the probe conditions of each measurement point. Finally, a calibration accuracy of better than 6% was obtained, which satisfied broad applications in the IC industry.

Micro-buried spiral zone plate in a lithium niobate crystal

We present a micro-buried spiral zone plate (MBSZP) in the lithium niobate crystal fabricated with femtosecond laser direct writing technology. The microstructures of the MBSZP are buried under the surface of the crystal, which ensures the stability of the optical performance in various refractive index environments. The optical performances of imaging and focusing capabilities were demonstrated. In addition, the experiment showed good agreement with simulation results based on the optical wave propagation method. This novel optical element will have important applications in multistate information encoding, optical manipulation, quantum communication, and computation, especially in high integration, contact coupling, and variable refractive index environments.

Hybrid Refractive–Diffractive Optical Vortex Microlens

We report a novel hybrid refractive-diffractive microlens combined with spiral phase for the generation of optical vortex, which is fabricated via femtosecond laser direct writing technology. The unique optical performance of focusing capability is demonstrated. At the focus position, the hollow focus with different integer topological charges is investigated. Moreover, experimental results are supported by finite-element calculation. The novel microlens generating an optical vortex will fulfill important applications in optical manipulation, multistate information encoding, quantum communication, and computation, particularly in the compaction, integration, and simplification of optical vortex generation system.

Anomalous Electro-Optic Effect in Polar Liquid Films

We present an anomalous electro-optic effect in polar liquid films: liquids, usually considered to be isotropic, possess the linear electro-optic effect that occurs only in materials lacking inversion symmetry. Due to the observed large effect in the low-frequency range and slow response speed, this strange effect was thought to come from the field-induced orientation of large mass. Therefore, we brought forward a physical model that contributed to the interpretation of this phenomenon: field-induced pre-oriented, short-range orderly dipole clusters in liquid films break the macroscopic symmetry and results in this asymmetric effect. Finally, combined with spectral analysis, the formation of clusters induced by an electric pulse was proved.