Dan

DMD-based LED-illumination Super-resolution and optical sectioning microscopy

Super-resolution three-dimensional (3D) optical microscopy has incomparable advantages over other high-resolution microscopic technologies, such as electron microscopy and atomic force microscopy, in the study of biological molecules, pathways and events in live cells and tissues. We present a novel approach of structured illumination microscopy (SIM) by using a digital micromirror device (DMD) for fringe projection and a low-coherence LED light for illumination. The lateral resolution of 90 nm and the optical sectioning depth of 120 μm were achieved. The maximum acquisition speed for 3D imaging in the optical sectioning mode was 1.6×107 pixels/second, which was mainly limited by the sensitivity and speed of the CCD camera. In contrast to other SIM techniques, the DMD-based LED-illumination SIM is cost-effective, ease of multi-wavelength switchable and speckle-noise-free. The 2D super-resolution and 3D optical sectioning modalities can be easily switched and applied to either fluorescent or non-fluorescent specimens.

Autofocusing of digital holographic microscopy based on off-axis illuminations

An auto-focusing method for digital holographic microscopy has been proposed by employing two off-axis illumination beams. When specimens are illuminated by two plane waves in different directions, it is found that the farther the reconstruction plane is from the image plane, the wider the two reconstructed images are separated from each other. Thus, the image plane can be determinated by finding the minimum of the variation between the two reconstructed object waves on both the amplitude and phase distributions. The feasibility of the proposed method is demonstrated by the corresponding simulation and experiment.

Autofocusing based on wavelength dependence of diffraction in two-wavelength digital holographic microscopy.

An autofocusing method for two-wavelength digital holographic microscopy (TWDHM) based on the wavelength dependence of the diffraction process is proposed. Red and green lights are employed for the illumination of the TWDHM, and the generated holograms are recorded simultaneously by a color CCD camera. Due to the wavelength dependency of the diffraction process, the farther the reconstruction plane is from the image plane, the larger the difference is between the red and green light distributions. Thus, the image plane can be determined by finding the minimum of the variation between the red and green lights on their amplitude distributions. The feasibility of the proposed method is demonstrated by simulation and experiment.

Dual-wavelength slightly off-axis digital holographic microscopy

We propose dual-wavelength digital holographic microscopy with a slightly off-axis configuration. The axial measurement range without phase ambiguity is extended to the micrometer range by synthesizing a beat wavelength between the two wavelengths with separation of 157 nm. Real-time measurement of the specimen is made possible by virtue of the high wavelength selectivity of the Bayer mosaic filtered color CCD camera. The principle of the method is exposed, and the practicability of the proposed configuration is demonstrated by the experimental results on a vortex phase plate and a rectangular phase step.

Intrinsic optical torque of cylindrical vector beams on Rayleigh absorptive spherical particles

The intrinsic optical torque of a focused cylindrical vector beam on a Rayleigh absorptive spherical particle is calculated via the corrected dipole approximation. Numerical results show that, for the radially polarized input field, the torque is distributed in the focal plane strictly along the azimuthal direction anywhere except at the focus. This shows a completely different property from what is observed in the focusing of a circularly polarized beam, where a strong axial torque component arises. For other cylindrically polarized input fields, the torque tends to align itself along the radial direction, as the polarization angle (the angle between the electric vector and the radial direction) changes from 0° to 90°. When limited to considering the torque at the equilibrium position, we find that only for those input fields with polarization angles larger than 50°, the particle experiences a nonzero torque at its equilibrium position. This is verified by showing quantitatively the effects of the polarization angle on the magnitude and orientation of the torque at the equilibrium position.

Parallel on-axis phase-shifting holographic phase microscopy based on reflective point-diffraction interferometer with long-term stability

Parallel on-axis two-step phase-shifting reflective point-diffraction interferometry for holographic phase microscopy based on Michelson architecture is proposed. A cube beamsplitter splits the object wave into two copies within the two arms. The reference wave is rebuilt by low-pass filtering with a pinhole-masked mirror. Both object and reference waves are split into two beams by a grating in a 4f imaging system; thus, two interferograms with quadrature phase-shift can be acquired simultaneously with the aid of polarization elements. The approach has the merit of nanometers-scale phase stability over hours due to its quasi-common-path geometry. It can make full use of camera spatial bandwidth while its temporal resolution is as fast as the camera frame rate. Phase imaging on microscale specimen is implemented, and the experimental results demonstrate that the proposed approach is suitable for investigating dynamic processes.

LED-based digital holographic microscopy with slightly off-axis interferometry

An LED illuminated Linnik-type digital holographic microscope (DHM) for high-quality phase imaging is presented by the adoption of slightly off-axis two-step blind-phase-shifting interferometry (TB-PSI). Slightly off-axis interferometry lowers the requirement on the angle between the object and the reference waves as well as the requirement on the resolving power of the charge-coupled device (CCD) camera. In addition, the apparatus is cost-effective and offers ease of alignment. The phase-shifting DHM is simply implemented by mechanically moving the reference mirror while disposing of a precise phase modulator such as a piezoelectric transducer (PZT). The phase shift between the two interferograms is extracted by Fourier transformation analysis, and then the phase image is reconstructed. The performance of the TB-PSI used in the scheme is analyzed. The phase imaging for nanostructured specimens is conducted, and the results demonstrate the feasibility of the scheme. The phase noise is reduced by 73% when compared to the result obtained with coherent illumination.

Off-axis digital holographic microscopy with LED illumination based on polarization filtering

A reflection mode digital holographic microscope with light emitting diode (LED) illumination and off-axis interferometry is proposed. The setup is comprised of a Linnik interferometer and a grating-based 4f imaging unit. Both object and reference waves travel coaxially and are split into multiple diffraction orders in the Fourier plane by the grating. The zeroth and first orders are filtered by a polarizing array to select orthogonally polarized object waves and reference waves. Subsequently, the object and reference waves are combined again in the output plane of the 4f system, and then the hologram with uniform contrast over the entire field of view can be acquired with the aid of a polarizer. The one-shot nature in the off-axis configuration enables an interferometric recording time on a millisecond scale. The validity of the proposed setup is illustrated by imaging nanostructured substrates, and the experimental results demonstrate that the phase noise is reduced drastically by an order of 68% when compared to a He-Ne laser-based result.

Virtual source for an Airy beam

We identify a virtual source for generating an Airy wave. A spectral integral expression is derived to describe the Airy wave, which, in the paraxial limit, yields the freely accelerating, nondiffracting, and finite energy Airy beam. From the spectral representation of the Airy wave, the first two orders of nonparaxial corrections to the paraxial Airy beam are determined. Also, a connection between the obtained Airy wave and the well-known complex source point spherical wave is given.

Long-Distance Axial Trapping with Focused Annular Laser Beams

Focusing an annular laser beam can improve the axial trapping efficiency due to the reduction of the scattering force, which enables the use of a lower numerical aperture (NA) objective lens with a long working distance to trap particles in deeper aqueous medium. In this paper, we present an axicon-to-axicon scheme for producing parallel annular beams with the advantages of higher efficiency compared with the obstructed beam approach. The validity of the scheme is verified by the observation of a stable trapping of silica microspheres with relatively low NA microscope objective lenses (NA = 0.6 and 0.45), and the axial trapping depth of 5 mm is demonstrated in experiment.