Far-Field and Non-Intrusive Optical Mapping of Nanoscale Structures.

Far-field high-density optics storage and readout involve the interaction of a sub-100 nm beam profile laser to store and retrieve data with nanostructure media. Hence, understanding the light-matter interaction responding in the far-field in such a small scale is essential for effective optical information processing. We present a theoretical analysis and an experimental study for far-field and non-intrusive optical mapping of nanostructures. By a comprehensive analytical derivation for interaction between the modulated light and the target in a confocal laser scanning microscopy (CLSM) configuration, it is found that the CLSM probes the local density of states (LDOSs) in the far field rather than the sample geometric morphology. With a radially polarized (RP) light for illumination, the far-field mapping of LDOS at the optical resolution down to 74 nm is obtained. In addition, it is experimentally verified that the target morphology is mapped only when the far-field mapping of LDOS coincides with the geometric morphology, while light may be blocked from entering the nanostructures medium with weak or missing LDOS, hence invalidating high-density optical information storage and retrieval. In this scenario, nanosphere gaps as small as 33 nm are clearly observed. We further discuss the characterization for far-field and non-intrusive interaction with nanostructures of different geometric morphology and compare them with those obtainable with the projection of near-field LDOS and scanning electronic microscopic results.

Non-invasive optical imaging using the extension of the Fourier-domain shower-curtain effect.

Optical imaging for non-self-luminous objects surrounded by complex scattering environments is scientifically challenging and technologically important. We propose a non-invasive imaging method by externally sending the illuminating light through the scattering medium and by detecting and analyzing the speckle patterns. The imaging of the object is recovered by extending the application scope of the Fourier-domain shower-curtain effect. It is found that the imaging depth is substantially extended and that faster imaging restoration is realized with the improved illumination scheme assisted with optical lenses, hence making it possible to apply the non-invasive optical imaging technique for practical applications.

Generation of arbitrary partially coherent Bessel beam array with a LED for confocal imaging.

As the most important class of self-imaging beams, Bessel Beams (BBs) have been extensively studied, and various applications in optical trapping, communication, imaging and quantum studies have been found. In this paper, we propose a new method to generate arbitrary (quasi-) BB arrays by using a single LED light source. The method is simpler, cheaper, and more compatible than other existing methods. The key idea of the proposed method is to form spatially controllable incoherent point sources used to generate BB array imaging. Detailed theoretical deduction, analysis of properties of the generated BB array and comparison with those generated by coherent light sources are depicted. Further application to confocal imaging shows that the BB array is promising for fast, super depth-of-field imaging and multi-particle optical manipulations.

Depth-resolved and auto-focus imaging through scattering layer with wavelength compensation.

Imaging techniques through turbid materials have been extensively studied in recent years. The challenge now is to recover objects in a large field of view with depth-resolving ability. We present a method to image through a thin scattering layer automatically with the depth of the object detectable. By revealing the wavelength-depth-matching relation based on the axial memory effect, this method can automatically search the optimal wavelength of the reference light and compute the depth of the object. The no-reference image quality assessment function and rule-based searching algorithm are used in the searching process. The proposed method is promising for dynamic object tracking.

Nanoscopic Spotlight in a Spindle Semiconductor Nanowire.

Theoretically, no matter how thin a nanowire is, it can transport light in the form of an evanescent field. However, in practice, the low propagation efficiency induced by complex dissipation makes light transport difficult to realize when the nanowire is distinctly thinner than ∼ λ/2. Accordingly, nanowire photonics research at such a scale is limited. Herein, light propagation was achieved in a very thin spindle nanowire (diameter below 70 nm), in which a nanoscopic spotlight formed. The nanowire output a maximum emission in the transverse dimension as small as ∼53 nm. The finite-difference time-domain (FDTD) simulation implied that the increased dimension gradient near the tip induced a maximum leakage of the propagating light at a transverse feature, precisely determined by the intrinsic feature of the nanowire. Moreover, a spectrum splitter phenomenon was observed and demonstrated based on the wavelength-dependent light propagation behavior in such a nanowire. These results contribute to the rational design of nanoscopic near-field illuminant, optoelectric, and photobiological probes with improved resolution largely superior to the so-called subwavelength level.

Ultrafast switching of optical singularity eigenstates with compact integrable liquid crystal structures.

By using the strong nonlinear effect and ultrafast electronic response of cholesteric liquid crystals (CLC), ultrafast all optical switching between polarization vortex and phase vortex is realized in a system combining CLC and q-plate. The experimental result shows that switching with high modulation depth can be accomplished in less than 1 picosecond. Furthermore, CLC and q-plates will enable compact integrated devices with sub-mm thicknesses.

Imaging of objects through a thin scattering layer using a spectrally and spatially separated reference.

Incoherently illuminated or luminescent objects give rise to a low-contrast speckle-like pattern when observed through a thin diffusive medium, as such a medium effectively convolves their shape with a speckle-like point spread function (PSF). This point spread function can be extracted in the presence of a reference object of known shape. Here it is shown that reference objects that are both spatially and spectrally separated from the object of interest can be used to obtain an approximation of the point spread function. The crucial observation, corroborated by analytical calculations, is that the spectrally shifted point spread function is strongly correlated to a spatially scaled one. With the approximate point spread function thus obtained, the speckle-like pattern is deconvolved to produce a clear and sharp image of the object on a speckle-like background of low intensity.

Ultrahigh Numerical Aperture Metalens at Visible Wavelengths.

Subwavelength imaging requires the use of high numerical aperture (NA) lenses together with immersion liquids in order to achieve the highest possible resolution. Following exciting recent developments in metasurfaces that have achieved efficient focusing and novel beam-shaping, the race is on to demonstrate ultrahigh-NA metalenses. The highest NA that has been demonstrated so far is NA = 1.1, achieved with a TiO2 metalens and back-immersion. Here, we introduce and demonstrate a metalens with a high NA and high transmission in the visible range, based on crystalline silicon (c-Si). The higher refractive index of silicon compared to TiO2 allows us to push the NA further. The design uses the geometric phase approach also known as the Pancharatnam-Berry (P-B) phase, and we determine the arrangement of nanobricks using a hybrid optimization algorithm (HOA). We demonstrate a metalens with NA = 0.98 in air, a bandwidth (full width at half-maximum, fwhm) of 274 nm, and a focusing efficiency of 67% at 532 nm wavelength, which is close to the transmission performance of a TiO2 metalens. Moreover, and uniquely so, our metalens can be front-immersed into immersion oil and achieve an ultrahigh NA of 1.48 experimentally and 1.73 theoretically, thereby demonstrating the highest NA of any metalens in the visible regime reported to the best of our knowledge. The fabricating process is fully compatible with microelectronic technology and therefore scalable. We envision the front-immersion design to be beneficial for achieving ultrahigh-NA metalenses as well as immersion metalens doublets, thereby pushing metasurfaces into practical applications such as high resolution, low-cost confocal microscopy and achromatic lenses.

Extended depth-resolved imaging through a thin scattering medium with PSF manipulation.

Human ability to visualize an image is usually hindered by optical scattering. Recent extensive studies have promoted imaging technique through turbid materials to a reality where color image can be restored behind scattering media in real time. The big challenge now is to recover objects in a large field of view with depth resolving ability. Based on the existing research results, we systematically study the physical relationship between speckles generated from objects at different planes. By manipulating a given single point spread function, depth-resolved imaging through a thin scattering medium can be extended beyond the original depth of field (DOF). Experimental testing of standard scattering media shows that the DOF can be extended up to 5 times and the physical mechanism is depicted. This extended DOF is benefit to 3D imaging through scattering environment, and it is expected to have important applications in science, technology, bio-medical, security and defense.

Generalized vector wave theory for ultrahigh resolution confocal optical microscopy.

Polarization modulation of a tightly focused beam in a confocal imaging scheme is considered for incident and collected light fields. Rigorous vector wave theory of a confocal optical microscopy is developed, which provides clear physical pictures without the requirement for fragmentary calculations. Multiple spatial modulations on polarization, phase, or amplitude of the illuminating and the detected beams can be mathematically described by a uniform expression. Linear and nonlinear excitation schemes are derived with tailored excitation and detection fields within this generalized theory, whose results show that the ultimate resolution achieved with the linear excitation can reach one-fifth of the excitation wavelength (or λ/5), while the nonlinear excitation scheme gives rise to a resolution better than λ/12 for two-photon fluorescence excitation and λ/20 for three-photon fluorescence excitation. Hence the resolution of optical microscopy with a near-infrared excitation can routinely reach sub-60 nm. In addition, simulations for confocal laser scanning microscopy are carried out with the linear excitation scheme and the fluorescent one, respectively.

High speed color imaging through scattering media with a large field of view.

Optical imaging through complex media has many important applications. Although research progresses have been made to recover optical image through various turbid media, the widespread application of the technology is hampered by the recovery speed, requirement on specific illumination, poor image quality and limited field of view. Here we demonstrate that above-mentioned drawbacks can be essentially overcome. The realization of high speed color imaging through turbid media is successfully carried out by taking into account the media memory effect, the point spread function, the exit pupil of the optical system, and the optimized signal to noise ratio. By retrieving selected speckles with enlarged field of view, high quality image is recovered with a responding speed only determined by the frame rates of the image capturing devices. The immediate application of the technique is expected to register static and dynamic imaging under human skin to recover information with a wearable device.

Optical sharper focusing in an anisotropic crystal.

Optical super-resolution technique through tight focusing is a widely used technique to image material samples with anisotropic optical properties. The knowledge of the field distribution of a tightly focused beam in anisotropic media is both scientifically interesting and technologically important. In this paper, the optical properties of a uniaxial crystal with the optic axis perpendicular to the interface under a tight focusing configuration are studied with rigorous theoretical and numerical analysis. The significant effect of the Poynting vector on the focal position introduces an obvious displacement of the focal spot formed by the extraordinary waves (e-ray). Moreover, a sharper focus with a lateral size of 0.22λ is obtained as a result of the effective separation of the ordinary waves (o-ray) and the e-ray. It provides a new tool to fabricate optical structures with higher resolutions than that in an isotropic medium through the far-field method.

Controlled light field concentration through turbid biological membrane for phototherapy.

Laser propagation through a turbid rat dura mater membrane is shown to be controllable with a wavefront modulation technique. The scattered light field can be refocused into a target area behind the rat dura mater membrane with a 110 times intensity enhancement using a spatial light modulator. The efficient laser intensity concentration system is demonstrated to imitate the phototherapy for human brain tumors. The power density in the target area is enhanced more than 200 times compared with the input power density on the dura mater membrane, thus allowing continued irradiation concentration to the deep lesion without damage to the dura mater. Multibeam inputs along different directions, or at different positions, can be guided to focus to the same spot behind the membrane, hence providing a similar gamma knife function in optical spectral range. Moreover, both the polarization and the phase of the input field can be recovered in the target area, allowing coherent field superposition in comparison with the linear intensity superposition for the gamma knife.

Shaping self-imaging bottle beams with modified quasi-Bessel beams.

Coherent generated self-imaging bottle beams, typically formed by interfering two coherent quasi-Bessel beams, possess a periodic array of intensity maxima and minima along their axial direction. In practice, the overall quality of the self-repeating intensity patterns is prone to unresolved large intensity variations. In this Letter, we increased consistency of intensity of self-imaging bottle beams through a spatial frequency optimization routine. By doing so, we increased the effective length of self-imaging bottle beams by 74%. Further, we showed that this approach is applicable to higher-order self-imaging beams that display complex intensity structures. The enhancement in these modified self-imaging beams could play a significant role in optical trapping, imaging, and lithography.

Harnessing the point-spread function for high-resolution far-field optical microscopy.

The resolution limit of far-field optical microscopy is reexamined with a full vectorial theoretical analysis. A highly symmetric excitation optical field and optimized detection scheme are proposed to harness the total point-spread function for a microscopic system. Spatial resolution of better than 1/6λ is shown to be obtainable, giving rise to a resolution better than 100 nm with visible light excitation. The experimental measurement is applied to examine nonfluorescent samples. A lateral resolution of 1/5λ is obtained in truly far-field optical microscopy with a working distance greater than ∼500λ. Comparison is made for the far-field microscopic measurement with that of a nearfield scanning optical microscopy, showing that the proposed scheme provides a better image quality.

Effect of polarization purity of cylindrical vector beam on tightly focused spot.

The tightly focused spots of cylindrical vectors (CVs) are dependent on polarization composition. We experimentally demonstrate the effect of polarization purity (PP) of the CV beam on the tightly focused spot quantitatively, which should be strictly controlled for the effective applications of the CV beam. The focal spots measured by a knife-edge scanning method showed that the azimuthally polarized (AP) component increases the transverse field and the size of the focal spots, while the radially polarized component results in a nonzero intensity distribution at the center of the focus even in a high PP AP beam.

Minimized spot of annular radially polarized focusing beam.

We have experimentally demonstrated the measurement of a tighter focal spot generated by a radially polarized narrow-width annular beam with the double-knife-edge method. The reconstructed spot profiles indicate that sharper focus cannot be achieved by shrinking the annular aperture further. The smallest focal spot (0.0711λ(2)) is obtained in experiment with an annular factor of 0.91. An apodization function has been introduced with the consideration of the diffraction effect, which achieves good agreement with the experimental data. Our result shows that the diffraction effect should be considered with small topography structures of the incident beam.

Light propagation in a resonantly absorbing waveguide array.

Light propagation behavior in a resonantly absorbing waveguide array is analyzed. Both a Lorentzian line shape and an inhomogeneous broadened absorbing line shape are considered, with their imaginary and real part of the refractive index determined by a Kramers-Kronig relationship. The diffracted wave is shown to have the frequency spectra determined by the material absorption, dispersion as well as the waveguide structure. An interesting phenomenon is that a spectral hole is produced and becomes deeper in the diffraction spectrum as the thickness of the resonantly absorbing waveguide array increases. The experimental measurements conducted in a waveguide array are found to be in good agreement with the numerical results.

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.

Fabrication of photonic crystals with functional defects by one-step holographic lithography.

A one-step introduction of functional defects into a photonic crystal is demonstrated. By using a multi-beam phase-controlled holographic lithography, line-defects in a Bragg structure and embedded waveguides in a two-dimensional photonic crystal are fabricated. Intrinsic defect introduction into a 3-dimensional photonic crystal is also proposed. This technique gives rise to a substantial reduction of the fabrication complexity and a significant improvement on the accuracy of the functional defects in photonic crystals.