Yuhang Chen

High-efficiency fabrication of aspheric microlens arrays by holographic femtosecond laser-induced photopolymerization

Manufacture of aspheric microlens has always been technically challenging for conventional approaches due to their complex curved profile and tiny sizes. Two-photon polymerization is capable of producing arbitrary shape with high spatial resolution, apart from the disadvantage of ultra-low rate of yield resulting from point-by-point writing strategy. Here, we report parallel fabrication of aspheric microlens arrays (AMLAs) by taking advantage of holographic femtosecond laser direct-writing. The inherent constraints of the spatial light modulator are taken into consideration for achieving improved intensity uniformity and enhanced diffraction efficiency. Closely-packed AMLAs with designable optical parameters are readily fabricated with excellent optical performance.

Two-photon-induced polarization-multiplexed and multilevel storage in photoisomeric copolymer film

We present a two-photon-induced polarization-multiplexed and multilevel data storage method with a bisazobenzene copolymer film. A polarization-adjustable femtosecond pulsed laser is used as writing beam to induce anisotropy, and the recorded information is retrieved by a CCD sensor from the film with corresponding polarized illumination. It is found that the optical axis of bisazobenzene molecules can be reoriented under two-photon excitation by the polarized femtosecond laser via a photoisomerization process. Polarization-multiplexed and multilevel storage is demonstrated by using this method. The capability to combine both advantages of these distinct techniques makes it a novel approach to obtain higher optical data density.

Femtosecond laser induced surface deformation in multi-dimensional data storage

We investigate the surface deformation in two-photon induced multi-dimensional data storage. Both experimental evidence and theoretical analysis are presented to demonstrate the surface characteristics and formation mechanism in azo-containing material. The deformation reveals strong polarization dependence and has a topographic effect on multi-dimensional encoding. Different stages of data storage process are finally discussed taking into consideration the surface deformation formation.

Comparative study on the nonperiodic and periodic gratings for scanning probe microscopy drift measurements

Performances of a special nonperiodic grating (NPG) and a conventional periodic grating (PG) for scanning probe microscopy (SPM) drift measurements are compared. Experimental results verify that the NPG has the advantages of robust measurements and large measurable range. The correlation peak contrast of the SPM image of the fabricated NPG reaches 17.8, which enables enough stable drift measurement by correlation analysis of sequentially scanned images. The measurable drift range relies on the grating size of the NPG, while it is limited by the grating pitch of the PG. For SPM drift characterization, the NPG prevails against the ordinary PG since both the peak contrast and the measurable range can be easily improved by more than one order of magnitude.

Detection of subsurface cavity structures using contact-resonance atomic force microscopy

To meet the surging demands for quantitative and nondestructive testing at the nanoscale in various fields, ultrasonic-based scanning probe microscopy techniques, such as contact-resonance atomic force microscopy (CR-AFM), have attracted increased attention. Despite considerable success in subsurface nanostructure or defect imaging, the detecting capabilities of CR-AFM have not been fully explored yet. In this paper, we present an analytical model of CR-AFM for detecting subsurface cavities by adopting a circular freestanding membrane structure as an equivalent cavity. The parameters describing the detection limits of CR-AFM for such structures include the detecting depth and the detectable area. These parameters are systematically studied for different cantilever eigenmodes for structures of different sizes and depths. The results show that the detecting depth depends on the structure size. The higher eigenmodes generally provide better detecting capabilities than the lower ones. For an experimental veri...

Atomic force microscopy force-distance curves with small amplitude ultrasonic modulation.

Force-distance curves were acquired on a highly oriented pyrolytic graphite (HOPG) specimen and a gold film specimen under ultrasonic modulation in atomic force microscopy (AFM). Measurements demonstrated that small amplitude ultrasonic oscillation of either the cantilever or the sample has significant impacts on the characteristics of force-distance curves. With the increase of excitation amplitude, the apparent pull-off force decreased gradually and the hysteresis between the approach and retraction curves reduced significantly. Furthermore, the decrease of the pull-off force was determined to be also relevant to the excitation frequency. With the assistance of contact resonance spectra, the pull-off force was verified to have a near-linear relationship with the cantilever contact oscillation amplitude. Theoretical analysis and subsequent numerical simulations well interpreted the experimental results. The emergence of large oscillating contact forces under ultrasonic modulation altered the force-distance curves, and such a mechanism was ascertained by further ultrasonic AFM imaging.

Fast Bits Recording in Photoisomeric Polymers by Phase-Modulated Femtosecond Laser

High storage capacity and recording-access rate are two critical factors that the current optical storage technology pursues. Efforts on shortening the laser wavelength, increasing the numerical aperture of objective, and increasing the optical drive speed have been made to reach this end. However, these methods have run their full course because of many existing difficulties on the physical and manufacturing aspect. People have to explore innovative ways to satisfy the requirement of both mass data and high rate. Multidimensional optical storage and holographic recording are two promising technologies to increase the capacity of memory and recording rate by orders of magnitude. We report on the fast bits recording in bisazobenzene functional polymer film by femtosecond laser holographic recording. The mechanism of the photo-induced anisotropy of the material is reviewed and discussed. The demonstration of multidimensional holographic recording is presented. Some influence factors in the holographic processing are discussed.

Mechanism and analysis of structural color in two typical butterfly scales

In this paper, the microstructures of two kinds of butterfly scales, Morpho type and Urania type, are obtained using scanning electron microscope (SEM) and transmission electron microscope (TEM). Three structure models of the first type are constructed and their corresponding reflectivity spectra are simulated using finite-difference time-domain (FDTD) method. The reflectivity spectrum of 3-D model and 2-D tower model are closer in agreement with the measurement result than the multi-layer model with equal branch and stem. For the second type scale, the structural colors mainly come from the sculpted multi-layer structure, and the 2-D planar multi-layer model can well explain the mechanism of structural colors.

Measuring stiffness and residual stress of thin films by contact resonance atomic force microscopy

A method based on contact resonance atomic force microscopy (AFM) was proposed to determine the mechanical properties of thin films. By analyzing the contact resonance frequencies of an AFM probe while the tip was in contact with the sample, the stiffness and residual stress of a freestanding circular SiN x membrane were evaluated quantitatively. The obtained magnitude of residual stress was in reasonable agreement with that determined by wafer curvature measurement. The method was verified to have much better mechanical sensitivity than the popular AFM bending test method. Its promising application to fast, nondestructive mechanical mapping of thin-film-type structures at the nanoscale was also demonstrated.

Visualizing subsurface defects in graphite by acoustic atomic force microscopy

We describe a versatile platform, which combines atomic force acoustic microscopy, ultrasonic atomic force microscopy and heterodyne force microscopy. The AFM system can enable in‐situ switching among these operation modes flexibly and thus benefit the discrimination of differences in mechanical properties and buried subsurface nanostructures. We demonstrate the potential of this platform for visualizing the subsurface defects of graphite. Our results show that tiny topographic edges are enhanced in acoustic oscillation signals whilst embedded defects and inhomogeneous in mechanical properties are made clearly distinguishable. The possibility of detecting subsurface defects in few‐layer graphene is further discussed with first‐principles calculations. Microsc. Res. Tech. 80:66–74, 2017.