Yanghui Li

Microsphere based microscope with optical super-resolution capability

We experimentally demonstrated that the microsphere can discern the details of the object whose sizes are below the conventional diffractive limit and such super-resolution capability can be reinforced if semi-immersing the corresponding microspheres in liquid droplet, producing a sharper contrast and a comparatively smaller magnification factor. The microsphere is considered as a channel that connects the near-field evanescent wave and the transmission one in far field. A conjecture based on this is proposed to explain the mechanism of super-resolution and the corresponding phenomenon.

Far-field super-resolution imaging using near-field illumination by micro-fiber

We propose an approach that uses a conventional optical microscope to achieve optical super-resolution. This approach will provide a direct, non-invasive, and far-field access in the observation of both metallic and non-metallic objects with a spatial resolution of tens-of-nanometres, achieved in a single snap shot. This method, combines near-field illumination by the micro-fiber and a passive spatial frequency shift by the sample itself, offering a promising approach to surface tomography imaging without localized field enhancement by surface plasmon polaritons. Our results present the potential of breaking the diffractive barrier using a conventional wide-field optical microscope within the visible spectrum.

Manipulation of doughnut focal spot by image inverting interferometry

Based on image inverting interference combined with phase modulation, we theoretically demonstrate that the doughnut focal spot can readily be manipulated, and either shrinkage or expansion of size of the central dark spot is possible in a large scale (peak-to-peak value: 0.555λ-0.830λ, or 93.3%-140.8% compared with the standard one). As the interference and phase modulation can both be achieved by a double Porro prism, it is feasible to introduce this approach into optical tweezers to improve their performance. As much as 33.9% intensity of stimulated emission depletion (STED) beam can be reduced if the further optimized configuration is utilized in STED microscopy.

Continuous manipulation of doughnut focal spot in a large scale.

We theoretically demonstrate that the doughnut focal spot can continuously be manipulated by synthetically using various beam modulation techniques. Comparatively, a more evident effect can be expected by different orders of phase modulation, while accurate manipulation stems from changing the phase diversity between two arms in an image inverting interferometer (III). The size of central dark spot can thus be continuously adjusted in a theoretically infinite scale, although it may actually be limited by resolution of Spatial Light Modulator (SLM). This approach brings additional flexibility to many applications, such as optical tweezers.

Reduction of coating induced polarization aberrations by controlling the polarization state variation

The mechanism of coating induced polarization state variation is analysed by the Jones matrix. Pauli spin matrices are used to establish the relationship between coating induced polarization state variation and polarization aberrations. To reduce coating induced polarization aberrations, we propose that δ = 0 and Ts = Tp at arbitrary incident angle should be appended as two additional optimization goals of optical coating design when the requirements of transmittance are met. Two typical anti-reflection (AR) coatings are designed and the polarization state variation induced by them is simulated. The MTF (modulation transfer function) calculated by polarization ray tracing is applied to evaluate the polarization aberrations of the practical lithography objective system with the two AR coatings. All the obtained results show that the coating induced polarization aberrations can be reduced by optimizing the angle dependent properties of the optical coating without additional optical elements.

Atomic layer deposition of Al2O3 and TiO2 multilayers for application as broadband antireflection coating

A thick Al2O3 film is inserted as a pre-deposited layer to overlay the transition region on bare BK7 in atomic layer deposition. The average reflectance is 0.535% (400~680nm) in practice, compared with the designed 0.408%.

Anti-reflection coating at 550 nm fabricated by atomic layer deposition

Trimethylamine (TMA), TiCl4, and water are applied as the precursors to deposit Al2O3 and TiO2. With different substrate temperatures, the optical properties and surface morphologies of the two oxides TiO2 and SiO2 are studied, respectively. With substrate temperature of 120 ◦C, amorphous TiO2 can be obtained, and the surface roughness (RMS) is only 0.928 nm. Applying Al2O3 and TiO2 deposited in 120 ◦C as low and high refractive index materials, anti-reflection (AR) coating at single point (550 nm) is designed. Furthermore, with the calibrated growth rates, this AR coating is fabricated, and its ultimate reflectance for the AR coating at 550 nm is less than 0.2%, which can meet the requirement for most applications. OCIS codes: 310.1210, 310.1860, 310.6860. doi: 10.3788/COL201311.S10205.