Resilient Graphene Ultrathin Flat Lens in Aerospace, Chemical, and Biological Harsh Environments.

The development of ultrathin flat lenses has revolutionized the lens technologies and holds great promise for miniaturizing the conventional lens system in integrated photonic applications. In certain applications, the lenses are required to operate in harsh and/or extreme environments, for example aerospace, chemical, and biological environments. Under such circumstances, it is critical that the ultrathin flat lenses can be resilient and preserve their outstanding performance. However, the majority of the demonstrated ultrathin flat lenses are based on metal or semiconductor materials that have poor chemical, thermal, and UV stability, which limit their applications. Herein, we experimentally demonstrate a graphene ultrathin flat lens that can be applied in harsh environments for different applications, including a low Earth orbit space environment, strong corrosive chemical environments (pH = 0 and pH = 14), and biochemical environment. The graphene lenses have extraordinary environmental stability and can maintain a high level of structural integrity and outstanding focusing performance under different test conditions. Thus, it opens tremendous practical application opportunities for ultrathin flat lenses.

Optomagnetic plasmonic nanocircuits.

The coupling between solid-state quantum emitters and nanoplasmonic waveguides is essential for the realization of integrated circuits for various quantum information processing protocols, communication, and sensing. Such applications benefit from a feasible, scalable and low loss fabrication method as well as efficient coupling to nanoscale waveguides. Here, we demonstrate optomagnetic plasmonic nanocircuitry for guiding, routing and processing the readout of electron spins of nitrogen vacancy centres. This optimized method for the realization of highly efficient and ultracompact plasmonic circuitry is based on enhancing the plasmon propagation length and improving the coupling efficiency. Our results show 5 times enhancement in the plasmon propagation length using (3-mercaptopropyl)trimethoxysilane (MPTMS) and 5.2 times improvement in the coupling efficiency by introducing a grating coupler, and these enable the design of more complicated nanoplasmonic circuitries for quantum information processing. The integration of efficient plasmonic circuitry with the excellent spin properties of nitrogen vacancy centres can potentially be utilized to extend the applications of nanodiamonds and yield a great platform for the realization of on-chip quantum information networks.

Spin-manipulated nanoscopy for single nitrogen-vacancy center localizations in nanodiamonds.

Due to their exceptional optical and magnetic properties, negatively charged nitrogen-vacancy (NV-) centers in nanodiamonds (NDs) have been identified as an indispensable tool for imaging, sensing and quantum bit manipulation. The investigation of the emission behaviors of single NV- centers at the nanoscale is of paramount importance and underpins their use in applications ranging from quantum computation to super-resolution imaging. Here, we report on a spin-manipulated nanoscopy method for nanoscale resolutions of the collectively blinking NV- centers confined within the diffraction-limited region. Using wide-field localization microscopy combined with nanoscale spin manipulation and the assistance of a microwave source tuned to the optically detected magnetic resonance (ODMR) frequency, we discovered that two collectively blinking NV- centers can be resolved. Furthermore, when the collective emitters possess the same ground state spin transition frequency, the proposed method allows the resolving of each single NV- center via an external magnetic field used to split the resonant dips. In spin manipulation, the three-level blinking dynamics provide the means to resolve two NV- centers separated by distances of 23 nm. The method presented here offers a new platform for studying and imaging spin-related quantum interactions at the nanoscale with super-resolution techniques.

Optimization of plasmonic nanostructure for nanoparticle trapping.

We present a detailed analysis of nanoparticle trapping using plasmonic nanostructures, which predicts an improvement of two orders of magnitude in trapping force obtained by optimizing the plasmon resonance of the nanostructures. As the result, a total of four orders of magnitude enhancement in trapping force can be achieved comparing to the case without the nanostructures. In addition, it is illustrated that tuning the resonance wavelength is achievable by varying the diameter and/or the height of the nanorods.

Three dimensional nanoparticle trapping enhanced by surface plasmon resonance.

We demonstrate a three dimensional nanoparticle trapping approach aided by the surface plasmon resonance of metallic nanostructures. The localized surface plasmon resonance effect provides strong electromagnetic field enhancement, which enables confinement of nanoparticles in the three-dimensional space. Numerical simulations indicate that the plasmonic structure provides approximately two orders of magnitude stronger optical forces for trapping nanoparticles, compared with that without nanostructures. The study of thermal effect of the plasmonic structure shows that the impact of the thermal force is significant, which may determine the outcome of the nanoparticle trapping.

Modulation of evanescent focus by localized surface plasmons waveguide.

In this paper, we present the modulation of a tightly focused evanescent field by a nano-plasmonic waveguide, which consists of two silver nanorods lying on the interface of two dielectric media. Linearly polarized and radially polarized illuminating beams are investigated under the influence of localized surface plasmons effect. It is demonstrated that different polarization components of the tightly focused evanescent field can be modulated accordingly. The results obtained from the finite difference time domain simulation show that super-resolved focal spot can be achieved using the nano-plasmonic waveguide structure.

Strong tangential force within a small trapping volume under near-field Laguerre-Gaussian beam illumination.

Near-field rotation of a trapped particle under focused evanescent Laguerre-Gaussian beam illumination is theoretically investigated by mapping the two-dimensional transverse trapping efficiency exerting on the particle. It is revealed that the severe focal field deformation associated with a focused evanescent Laguerre-Gaussian beam causes a significant impact on the transverse trapping performance of the microparticle. Compared with the far-field trapping force, strong tangential force components have been observed in the transverse efficiency mapping, which potentially lead to rotational motions to the particle within a small trapping volume in the optical near-field.

A single beam near-field laser trap for optical stretching, folding and rotation of erythrocytes.

To understand the fundamental mechanical and viscoelastic properties of RBCs, one needs laser tweezers in which cells can not only be trapped, but also be stretched, folded, and rotated. Stretching, folding and rotating an RBC is particularly important in order to reveal the shear elasticity of the RBC membrane. Here we show a single beam near-field laser trapping technique under focused evanescent wave illumination for optical stretching, folding and rotation of a single RBC. This multifunctional manipulation method will provide a new platform for measuring cell properties such as the membrane elasticity, viscoelasticity and deformability.

Comment on: Computation of the optical trapping force using an FDTD based technique.

In this comment, problems associated with an oversimplified FDTD based model used for trapping force calculation in recent papers "Computation of the optical trapping force using an FDTD based technique" [Opt. Express 13, 3707 (2005)], and "Rigorous time domain simulation of momentum transfer between light and microscopic particles in optical trapping" [Opt. Express 12, 2220 (2004)] are discussed. A more rigorous model using in Poynting vector is also presented.

Characterization of gradient-index lens-fiber spacing toward applications in two-photon fluorescence endoscopy.

We report on the experimental investigation into the characterization of two-photon fluorescence microscopy based on the separation distance of a single-mode optical fiber coupler and a gradient-index (GRIN) rod lens. The collected two-photon fluorescence signal exhibits a maximum intensity at a defined separation distance (gap length) where the increasing effective excitation numerical aperture is balanced by the decreasing confocal emission collection. A maximum signal is found at gap lengths of approximately 2, 1.25, and 1.75 mm for GRIN lenses with pitches of 0.23, 0.25, and 0.29 wavelength at 830 nm. The maximum two-photon fluorescence signal collected corresponds to a threefold reduction of axial resolution (38.5 microm at 1.25 mm), compared with the maximum resolution (11.6 microm at 5.5 mm), as shown by the three-dimensional imaging of 10 microm beads. These results demonstrate an intrinsic trade-off between signal collection and axial resolution.

Use of a single-mode fiber coupler for second-harmonic-generation microscopy.

We present a compact second-harmonic-generation (SHG) microscope based on a three-port single-mode fiber coupler. The fiber coupler is used to deliver a near-infrared ultrashort-pulsed laser beam as well as to collect the SHG signal in the visible wavelength range. The SHG microscope exhibits an axial resolution of 1.8 microm, which shows a slight enhancement of the optical sectioning effect compared with that for two-photon excitation at the same excitation wavelength. It is also demonstrated that SHG and two-photon fluorescence images under parallel and perpendicular laser excitation polarization can be simultaneously obtained.

Optical trapping force with annular and doughnut laser beams based on vectorial diffraction.

The inadequacy of the optical trapping model based on ray optics in the case of describing the optical trapping performance of annular and doughnut laser beams is discussed. The inadequacy originates from neglecting the complex focused field distributions of such beams, such as polarization and phase, and thus leads to erroneous predictions of trapping force. Instead, the optical trapping model based on the vectorial diffraction theory, which considers the exact field distributions of a beam in the focal region, needs to be employed for the determination of the trapping force exerted on small particles. The theoretical predictions of such a trapping model agree with the experimentally measured results.

Nonlinear optical microscopy based on double-clad photonic crystal fibers.

We report on a nonlinear optical microscope that adopts double-clad photonic crystal fibers for single-mode illumination delivery and multimode signal collection. It is demonstrated that two-photon fluorescence and second harmonic generation signals can be simultaneously collected in such a microscope with axial resolution of 2.8 microm and 2.5 mum, respectively. The delivery and detection efficiencies of the photonic-crystal-fiber- based microscope are significantly improved by approximately 3 times and 40 times compared with those in the single-mode fiber-optic microscope. The high resolution three-dimensional second harmonic generation images from rat tail tendon demonstrate the effectiveness of the system.

Direct measurement of a radially polarized focused evanescent field facilitated by a single LCD.

In this paper, a tightly focused evanescent field produced by a total internal reflection objective lens under the illumination of a radially polarized beam generated using a single liquid crystal phase modulator is investigated. The field distributions have been directly mapped by a scanning near-field optical microscope. It is demonstrated both theoretically and experimentally that the introduction of radially polarized beam illumination combining with an annular beam illumination exhibits advantages in two aspects. On one hand, it corrects the focus elongation and splitting in a focused evanescent field associated with a linearly polarized beam. On the other hand, it significantly improves the lateral localization to approximately a quarter of the illumination wavelength, which is less than half of the size that is achievable under linearly polarized illumination.

Anomalous phenomenon of a focused evanescent Laguerre-Gaussian beam.

We demonstrate theoretically and experimentally an anomaly in the intensity distribution at the focal region of a Laguerre-Gaussian beam, when such a beam is focused by a high numerical aperture objective lens through an index-mismatched interface satisfying the total internal reflection condition. An asymmetric rotation of the focal field arising from the interplay of the phase shift induced by the total internal reflection and the helical phase of the Laguerre-Gaussian beam has been experimentally observed by a scanning near-field optical microscope. A cross-section analysis shows that the experimental results match well with the theoretical predictions.

Effective Mie scattering of a spherical fractal aggregate and its application in turbid media.

An effective Mie-scattering model is developed to deal with the scattering property of a spherical fractal aggregate consisting of scattering particles. In this model the scattered field of a scattering particle is given by the classical Mie-scattering theory. On the basis of the Monte Carlo simulation method, we determine the physical parameters of a scattering aggregate, the scattering efficiency Q, and the anisotropy value g, as well as their dependence on the size and the effective mean-free-path length of a scattering aggregate. Accordingly, photon migration through a microscope objective focused into a turbid medium including scattering aggregates is simulated to understand the effect of complex tissue on image quality.

Monte Carlo modeling of optical coherence tomography imaging through turbid media.

We combine a Monte Carlo technique with Mie theory to develop a method for simulating optical coherence tomography (OCT) imaging through homogeneous turbid media. In our model the propagating light is represented by a plane wavelet; its line propagation direction and path length in the turbid medium are determined by the Monte Carlo technique, and the process of scattering by small particles is computed according to Mie theory. Incorporated into the model is the numerical phase function obtained with Mie theory. The effect of phase function on simulation is also illustrated. Based on this improved Monte Carlo technique, OCT imaging is directly simulated and phase information is recorded. Speckles, resolution, and coherence gating are discussed. The simulation results show that axial and transversal resolutions decrease as probing depth increases. Adapting a light source with a low coherence improves the resolution. The selection of an appropriate coherence length involves a trade-off between intensity and resolution.

Exact radiation trapping force calculation based on vectorial diffraction theory.

There has been an interest to understand the trapping performance produced by a laser beam with a complex wavefront structure because the current methods for calculating trapping force ignore the effect of diffraction by a vectorial electromagnetic wave. In this letter, we present a method for determining radiation trapping force on a micro-particle, based on the vectorial diffraction theory and the Maxwell stress tensor approach. This exact method enables one to deal with not only complex apodization, phase, and polarization structures of trapping laser beams but also the effect of spherical aberration present in the trapping system.

Morphology-dependent resonance induced by two-photon excitation in a micro-sphere trapped by a femtosecond pulsed laser.

We report on the measurement of morphology-dependent resonance within a laser-trapped micro-sphere excited under two-photon absorption. Both trapping and two-photon excitation are simultaneously achieved by a single femtosecond pulsed laser beam. The effect of the laser power as well as the pulse width on the transverse trapping force is first investigated. The dependence of two-photon-induced morphology-dependent resonance on the scanning velocity of a trapped particle is then experimentally determined.

Near-field imaging by a micro-particle: a model for conversion of evanescent photons into propagating photons.

In this letter we present a physical model, both theoretically and experimentally, which describes the mechanism for the conversion of evanescent photons into propagating photons detectable by an imaging system. The conversion mechanism consists of two physical processes, near-field Mie scattering enhanced by morphology dependant resonance and vectorial diffraction. For dielectric probe particles, these two processes lead to the formation of an interference-like pattern in the far-field of a collecting objective. The detailed knowledge of the far-field structure of converted evanescent photons is extremely important for designing novel detection systems. This model should find broad applications in near-field imaging, optical nanometry and near-field metrology.