Quantitative analysis of fringe visibility in grating-based x-ray phase-contrast imaging.

The newly developed x-ray differential phase-contrast imaging technique has attracted increasing research interest. In this study, we quantitatively analyze the fringe visibility obtained in differential phase-contrast imaging. Numerical results of the visibility for polychromatic x rays with different structure heights of absorption gratings are shown and discussed. Furthermore, the fringe visibility of the nonabsorption grating approach is calculated, and based on the results, we conclude that this approach can potentially be applied for higher x-ray photon energies. These analytic results will be useful for designing a differential phase-contrast imaging system for different applications.

Statistical precision in super-resolution optical fluctuation imaging.

The super-resolution optical fluctuation imaging (SOFI) technique enhances image spatial resolution by calculating the spatiotemporal cross-cumulants of independent stochastic intensity fluctuations of emitters. Ideally, SOFI eliminates any noise that is not correlated over time, but in practice, due to limited data lengths, the statistical uncertainty of cumulants will affect the continuities and homogeneities of SOFI images. Since the variance and signal-to-noise ratio (SNR) characterize cumulant statistical uncertainty, we determined theoretical expressions for these based on a single dataset. From a simulation of temporal fluctuations of blinking fluorescent emitters, we calculated the quantitative relation between the SNR of cumulants and multiple parameters of the blinking signal, such as the on-time ratio, acquisition frame to average blinking rate ratio, sequence length, and photon amplitude, which not only provides a physical interpretation for SOFI phenomena but also theoretical guidance to achieve optimal practical outcomes.

Nanoscale three-dimensional single particle tracking by light-sheet-based double-helix point spread function microscopy.

The double-helix point spread function (DH-PSF) microscopy has become an essential tool for nanoscale three-dimensional (3D) localization and tracking of single molecules in living cells. However, its localization precision is limited by fluorescent contrast in thick samples because the signal-to-noise ratio of the system is low due to the inherent low transfer function efficiency and background fluorescence. Here we combine DH-PSF microscopy with light-sheet illumination to eliminate out-of-focus background fluorescence for high-precision 3D single particle tracking. To demonstrate the capability of the method, we obtain the single fluorescent bead image with light-sheet illumination, with three-dimensional localization accuracy better than that of epi-illumination. We also show that the single fluorescent beads in agarose solution can be tracked, which demonstrates the possibility of our method for the study of dynamic processes in complex biological specimens.

Early exposure of rotating magnetic fields promotes central nervous regeneration in planarian Girardia sinensis.

Magnetic field exposure is an accepted safe and effective modality for nerve injury. However, it is clinically used only as a supplement or salvage therapy at the later stage of treatment. Here, we used a planarian Girardia sinensis decapitated model to investigate beneficial effects of early rotary non-uniform magnetic fields (RMFs) exposure on central nervous regeneration. Our results clearly indicated that magnetic stimulation induced from early RMFs exposure significantly promoted neural regeneration of planarians. This stimulating effect is frequency and intensity dependent. Optimum effects were obtained when decapitated planarians were cultured at 20 °C, starved for 3 days before head-cutting, and treated with 6 Hz 0.02 T RMFs. At early regeneration stage, RMFs exposure eliminated edema around the wound and facilitated subsequent formation of blastema. It also accelerated cell proliferation and recovery of neuron functionality. Early RMFs exposure up-regulated expression of neural regeneration related proteins, EGR4 and Netrin 2, and mature nerve cell marker proteins, NSE and NPY. These results suggest that RMFs therapy produced early and significant benefit in central nervous regeneration, and should be clinically used at the early stage of neural regeneration, with appropriate optimal frequency and intensity.

A 75-ps Gated CMOS Image Sensor with Low Parasitic Light Sensitivity.

In this study, a 40 × 48 pixel global shutter complementary metal-oxide-semiconductor (CMOS) image sensor with an adjustable shutter time as low as 75 ps was implemented using a 0.5-µm mixed-signal CMOS process. The implementation consisted of a continuous contact ring around each p+/n-well photodiode in the pixel array in order to apply sufficient light shielding. The parasitic light sensitivity of the in-pixel storage node was measured to be 1/8.5 × 107 when illuminated by a 405-nm diode laser and 1/1.4 × 104 when illuminated by a 650-nm diode laser. The pixel pitch was 24 µm, the size of the square p+/n-well photodiode in each pixel was 7 µm per side, the measured random readout noise was 217 e(-) rms, and the measured dynamic range of the pixel of the designed chip was 5500:1. The type of gated CMOS image sensor (CIS) that is proposed here can be used in ultra-fast framing cameras to observe non-repeatable fast-evolving phenomena.

Three dimensional multi-molecule tracking in thick samples with extended depth-of-field.

We present a non-z-scanning multi-molecule tracking system with nano-resolution in all three dimensions and extended depth of field (DOF), which based on distorted grating (DG) and double-helix point spread function (DH-PSF) combination microscopy (DDCM). The critical component in DDCM is a custom designed composite phase mask (PM) combining the functions of DG and DH-PSF. The localization precision and the effective DOF of the home-built DDCM system based on the designed PM were tested. Our experimental results show that the three-dimensional (3D) localization precision for the three diffraction orders of the grating are σ(-1st)(x, y, z) = (6.5 nm, 9.2nm, 23.4 nm), σ(0th)(x, y, z) = (3.7 nm, 2.8nm, 10.3 nm), and σ(+1s)(x, y, z) = (5.8 nm, 6.9 nm, 18.4 nm), respectively. Furthermore, the total effective DOF of the DDCM system is extended to 14 µm. Tracking experiment demonstrated that beads separated over 12 µm along the axial direction at some instants can be localized and tracked successfully.

Characteristic analysis of broadband plasmonic emitting devices based on transformation optics.

The crescent nanostructure with gain medium inside is theoretically studied to analyze the characteristic of plasmonic emitting with wide bandwidth. An accurate analytical model is built based on the transformation optics. In this model, the poles of the electrostatic potential function are in the second and the fourth quadrant of the complex plane if the imaginary part of the relative permittivity of the gain medium is larger than the loss compensation threshold, and then the extinction cross section is to be negative by integrating the electrostatic potential over the half complex plane via an inverse Fourier transform. The positive extinction cross section corresponds to absorption, and the negative corresponds to emission. The proposed analytical model agrees well with the numerical simulation results based on the finite element method, to give a physical insight into the loss compensation property of the plasmonic nanostuctures. Results show that the negative extinction cross section is realizable by introducing the gain medium into a plasmonic crescent nanowire, which is equivalent to an emitting device with wide bandwidth.

Sampling grating approach for X-ray differential phase contrast imaging.

Grating-based X-ray differential phase contrast imaging (GDPCI) typically employs the phase-stepping technique to extract an object's phase information. This method requires heavy radiation dosage and is time consuming. Another potential approach is the reverse projection (RP) method, which, however, relies on a synchrotron radiation source to obtain highly sensitive differential phase contrast(DPC) signal. Here, we present an alternative approach that enables the RP method to be used with a conventional X-ray source and substantially improves the sensitivity of the DPC signal by replacing the analyzer grating of the GDPCI with a sampling grating. This development represents a significant step towards obtaining fast and low-dosage DPC images in medical, biological, and industrial applications.

3D localization of high particle density images using sparse recovery.

If particles are too close in space, their images may be overlapped when they are observed with microscopes because of diffraction limitation, which makes them difficult to be distinguished or localized. This limitation also affects the efficiency of localization of those single-particle-localization microcopies, such as stochastic optical reconstruction microscopy (STORM) and (fluorescence) photoactivated localization microscopy [(F)PALM]. In this work, we developed a 3D sparse recovery (3D-SR) method, with the aim of localizing particles with high density in three dimensions, which cannot be resolved using original STROM or (F)PALM. A cylindrical lens was introduced to a traditional wide-field microscope in order to form the 3D point spread function for 3D-SR. The performance of the 3D-SR method was evaluated using simulation. Simulated results demonstrated that, even for particle densities as high as 4 µm-2 on a transversal projection, particles could still be localized with high accuracy. The standard deviations were found to be 25.59 nm along the transverse direction and 50.42 nm along the axial direction. Compared with the existing 3D localization methods used in high particle density cases, such as 3D-DAOSTORM, 3D-SR allows a higher activated fluorophore density per frame.

Fast stimulated emission nanoscopy based on single molecule localization.

For super-resolution microscopy methods based on single molecule stochastic switching and localization, to simultaneously improve the spatial-temporal resolution, it is necessary to maximize the number of photons that can be collected from single molecules per unit time. Here, we describe a novel approach to enhance the signal intensity (collected photons per second) from fluorescence probes by introducing a stimulated emission (SE) optical process. This process is based on the following two properties: first, with reasonable parameters, the photon emission rate can be significantly increased with SE; and second, the SE photons, which are spatially coherent with the stimulation beam, are more favorable for collection than fluorescence. Theoretical results have shown that signal intensity from a single fluorescent molecule can be greatly improved with SE. We therefore showed, using SE in combination with single molecule localization methodology, that fast imaging at a rate of 0.05 s per reconstructed image with lateral resolutions of ∼30 nm can be obtained.

Approach for differential phase contrast imaging in x-ray microscopy.

We propose a differential phase contrast imaging method in x-ray microscopy by utilizing a biased derivative filter, which is structurally similar to that used in visible optics, except that phase changes by the filter cannot be ignored in the x-ray range. However, it is demonstrated that the filter's phase retardation does not disturb its function of phase contrast imaging, and even enhances the signals to some extent. Theoretical formulations and corresponding numerical simulations show that the approach is capable of performing characteristic differential microscopic phase imaging with nanometer-scale resolution. Manageable parameters are also examined in detail for pursuing a high image quality.

Fast flexible multiphoton fluorescence lifetime imaging using acousto-optic deflector.

We present a fast and flexible fluorescence lifetime imaging microscopy which uses a two-dimensional acousto-optic deflector to achieve fast beam scanning across the sample and provides random access to the regions of interests (ROI). Experimental results using standard fluorescent dye and biological samples show that this system can make addressable fluorescence lifetime measurements and perform fast and flexible fluorescence lifetime imaging particularly to the discontinuous ROI in the sample.

Approach to multiparticle parallel tracking in thick samples with three-dimensional nanoresolution.

This Letter proposes a method referred to as distorted grating (DG) and double-helix point spread function (DH-PSF) combination microscopy (DDCM), which is capable of multiparticle parallel localization and tracking in a transparent sample thicker than 10 µm, the thickness of cells. A special phase mask, combining the field depth extension capabilities of DG with the three-dimensional (3D) nanolocalization capabilities of the DH-PSF, is designed for multiparticle parallel localization. Time-lapse tracking of one particle moving along the z axis and parallel tracking of two particles are simulated. Results demonstrate that, with only a single snapshot, particles can be localized, tracking with 3D nanoresolution wherever they are. The theoretical localization precisions of DDCM, DH-PSF, and multifocus microscopy are compared. DDCM results in almost constant localization precisions in all three dimensions for a depth of field larger than 10 µm. DDCM is expected to become a tool in investigations of important dynamic events in living cells.

Wavelength-multiplexing phase-sensitive surface plasmon imaging sensor.

A wavelength-multiplexing phase-sensitive surface plasmon resonance (SPR) imaging sensor offering wide dynamic detection range and microarray capability is reported. Phase detection is accomplished by performing self-interference between the s- and p- polarizations within the signal beam. A liquid crystal tunable filter is used to sequentially select the SPR excitation wavelength from a white light source. This wavelength-multiplexing approach enables fast detection of the sensor's SPR phase response over a wide range of wavelengths, thereby covering literally any regions of interest within the SPR dip and thus maintaining the highest sensitivity point at all times. The phase-sensitive approach is particularly important for imaging SPR sensing applications because of its less stringent requirements for intensity signal-to-noise ratio, which also means the possibility of using uncooled modest resolution analog-to-digital conversion imaging devices. Experimental results demonstrate a resolution of 2.7×10(-7) RIU with a dynamic range of 0.0138 RIU.

Synthesis and nonlinear optical properties of single-crystalline KNb3O8 nanowires.

Single-crystalline KNb(3)O(8) nanowires with widths of 100-300 nm and lengths up to tens of microns were synthesized by calcining Nb(2)O(5) powders in molten KCl and K(2)SO(4). The phase of the products was determined by means of x-ray diffraction, and the morphology and structure were characterized by using scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscopy and selected area electron diffraction techniques. The growth direction of the KNb(3)O(8) obtained was determined to be the [001] crystallographic direction. Meanwhile, the polarization response of the second-harmonic generation (SHG) response was investigated. The as-synthesized nanowires clearly exhibited a SHG response, which means that the nanowires were an efficient nanoscale second-harmonic light source. The excellent nonlinear optical property of KNb(3)O(8) shows potential for application in nano-optical devices.

Ultrafast, large-field multiphoton microscopy based on an acousto-optic deflector and a spatial light modulator.

We present an ultrafast, large-field multiphoton excitation fluorescence microscope with high lateral and axial resolutions based on a two-dimensional (2-D) acousto-optical deflector (AOD) scanner and spatial light modulator (SLM). When a phase-only SLM is used to shape the near-infrared light from a mode-locked titanium:sapphire laser into a multifocus array including the 0-order beam, a 136 µm × 136 µm field of view is achieved with a 60× objective using a 2-D AOD scanner without any mechanical scan element. The two-photon fluorescence image of a neuronal network that was obtained using this system demonstrates that our microscopy permits observation of dynamic biological events in a large field with high-temporal and -spatial resolution.

Addressable discrete-line-scanning multiphoton microscopy based on a spatial light modulator.

We developed a novel addressable discrete-line-scanning multiphoton microscope with high lateral and axial resolutions based on a spatial light modulator. Our discrete-line-focus design eliminates the cross talk that occurs in conventional one-dimensional line-scanning multiphoton microscopies. Additionally, a phase-only spatial light modulator is able to scan only a sample's target area by generating a specific discrete line focus according to the shape and location of the target area. Compared with other multiphoton microscopies, this technique shortens scanning time and minimizes photodamage by concentrating scanning energy and dwell time on the area of interest.

Analysis of field of view limited by a multi-line X-ray source and its improvement for grating interferometry.

X-ray phase-contrast imaging based on grating interferometry is a technique with the potential to provide absorption, differential phase contrast, and dark-field signals simultaneously. The multi-line X-ray source used recently in grating interferometry has the advantage of high-energy X-rays for imaging of thick samples for most clinical and industrial investigations. However, it has a drawback of limited field of view (FOV), because of the axial extension of the X-ray emission area. In this paper, we analyze the effects of axial extension of the multi-line X-ray source on the FOV and its improvement in terms of Fresnel diffraction theory. Computer simulation results show that the FOV limitation can be overcome by use of an alternative X-ray tube with a specially designed multi-step anode. The FOV of this newly designed X-ray source can be approximately four times larger than that of the multi-line X-ray source in the same emission area. This might be beneficial for the applications of X-ray phase contrast imaging in materials science, biology, medicine, and industry.

Addressable, large-field second harmonic generation microscopy based on 2D acousto-optical deflector and spatial light modulator.

We present an addressable, large-field second harmonic generation microscope by combining a 2D acousto-optical deflector with a spatial light modulator. The SLM shapes an incoming mode-locked, near-infrared Ti:Sapphire laser beam into a multifocus array, which can be rapidly scanned by changing the incident angle of the laser beam using a 2D acousto-optical deflector. Compared to the single-beam-scan technique, the multifocus array scan can increase the scanning rate and the field-of-view size with the multi-region imaging ability.

[High sensitivity conherent anti-stokes Raman scattering for linearly probing low concentration materials].

The authors made a theoretical analysis and experiment research on the relation of time-resolved coherent anti-Stokes Raman scattering (T-CARS) intensity and the sample concentrations in this paper. It was proved experimentally that the T-CARS intensity is quadratic at the concentration higher than 35%, but is linear with the sample concentration at the concentration lower than 20%, which fits with theoretical analysis. And the research results correct inaccurate previous perceptions, which is conducive to better interpretation and application of the CARS process. The linear relation between the intensity of the CARS with the sample concentration at low concentrations indicates that the CARS is allowed for direct and precise concentration measurements, therefore it will be of great importance in biology and biochemistry.