Yefeng Guan

Image restoration through thin turbid layers by correlation with a known object

A method to recover the image of an object behind thin turbid layers is developed by wavefront shaping technique. The optimized wavefront is generated by modulating the scattering light of a known object with a spatial light modulator. A Pearson Correlation Coefficient is introduced as a cost function for the optimization. A beam scanning method based on optical memory effect is proposed to further enlarge the Field-of-View (FOV). The experimental results show good fidelity and large FOV of the recovered image.

Phase controlled beam combining with nonlinear frequency conversion

A phase controlled beam combining via nonlinear optical conversion is proposed and demonstrated. This process involves the combining of the fields at the second harmonic frequency generated by non-collinear input fields. The arrangement of the excitation configuration allows the generated second-harmonic light waves to propagate collinearly, with phases coherently correlated. The manipulation of the conversion efficiency is then possible with the phase control of the input fields. The combined second-harmonic fields are shown to be conveniently and robustly variable from zero to a maximum value that greatly exceeds the second-harmonic field generated by a single laser beam. By using a self-adaptive control algorithm, it is possible to optimize the output without prior knowledge on each beamlet property. Either the second-harmonic output beam profile or the total second-harmonic output power can be optimized with the control algorithm.

Coherent Beam Combining with Second-Harmonic Generation Optimized with Adaptive Phase Control

A theoretical and experimental study of phase-controlled beam combining based on noncollinear frequency doubling is described. Optimized second-harmonic power and improved output intensity distribution are simultaneously achieved by a closed-loop phase control system via an adaptive algorithm. It is shown that the profile of the combined beam at the second-harmonic with 12 noncollinear frequency inputs can be shaped to a near-fundamental Gaussian mode, with a power enhancement of more than two orders of magnitude, compared to the second-harmonic signal generated with a single beam.

Optimizing lightwave transmission through a nano-tip

Optical microscopy with spatial resolution below the diffraction limit is at present attracting extensive attentions. Further advancement of the near-field scanning optical microscopy (NSOM), a practical super-resolution microscopy, is mainly limited by the low transmission of optical power through the nano-meter apex. This work shows that lightwave can be efficiently delivered to a sub-100 nm apex inside a tapered metallic guiding structure. The enhanced light delivery, about 5-fold, is made possible with an adaptive optimization of the transmission via a spatial light phase-modulator. Numerical simulation shows the mechanism for the efficient light delivery to be the selective excitation of predominantly the lowest-order transverse component of standing wavevector with proper input wavefront modulation, hence favoring the transmission of lightwave in the longitudinal direction. The demonstration of such efficient focusing, to about full-width at half-maximum of a quarter wavelength, has a direct and immediate application in the improvement of the existing NSOMs.

Adaptive light field transmission through a hollow tapered metallic probe

Lightwave transmission through a sub-100 nm apex inside a tapered metallic guiding structure is enhanced substantially with controlled wavefront modulation via an adaptive algorithm. The efficient delivery is satisfactorily explained with numerical simulations.

Functional photonics with a resonantly absorbing waveguide array

A waveguide array consisting of resonantly absorbing molecules is designed, fabricated and characterized. Spatial, spectroscopic and temporal control of light field is demonstrated with the novel structure.