Improvement In Transverse Resolution Research Articles

In-vivo sub-diffraction adaptive optics imaging of photoreceptors in the human eye with annular pupil illumination and sub-Airy detection.

Adaptive optics scanning light ophthalmoscopy (AOSLO) allows non-invasive visualization of the living human eye at the microscopic scale; but even with correction of the ocular wavefront aberrations over a large pupil, the smallest cells in the photoreceptor mosaic cannot always be resolved. Here, we synergistically combine annular pupil illumination with sub-Airy disk confocal detection to demonstrate a 33% improvement in transverse resolution (from 2.36 to 1.58 μm) and a 13% axial resolution enhancement (from 37 to 32 μm), an important step towards the study of the complete photoreceptor mosaic in heath and disease. Interestingly, annular pupil illumination also enhanced the visualization of the photoreceptor mosaic in non-confocal detection schemes such as split detection AOSLO, providing a strategy for enhanced multimodal imaging of the cone and rod photoreceptor mosaic.

Image scanning microscopy with multiphoton excitation or Bessel beam illumination.

Image scanning microscopy is a technique of confocal microscopy in which the confocal pinhole is replaced by a detector array, and the image is reconstructed most straightforwardly by pixel reassignment. In the fluorescence mode, the detector array collects most of the fluorescent light, so the signal-to-noise ratio is much improved compared with confocal microscopy with a small pinhole, while the resolution is improved compared with conventional fluorescence microscopy. Here we consider two cases in which the illumination and detection point spread functions are dissimilar: illumination with a Bessel beam and multiphoton microscopy. It has been shown previously that for Bessel beam illumination in image scanning microscopy with a large array, the imaging performance is degraded. On the other hand, it is also known that the resolution of confocal microscopy is improved by Bessel beam illumination. Here we analyze image scanning microscopy with Bessel beam illumination together with a small array and show that an improvement in transverse resolution (width of the point spread function) by a factor of 1.78 compared with a conventional fluorescence microscope can be obtained. We also examine the behavior of image scanning microscopy in two- or three-photon fluorescence and for two-photon excitation also with Bessel beam illumination. The combination of the optical sectioning effect of image scanning microscopy with multiphoton microscopy reduces background from the sample surface, which can increase penetration depth. For a detector array size of two Airy units, the resolution of two-photon image scanning microscopy is a factor 1.85 better and the peak of the point spread function 2.84 times higher than in nonconfocal two-photon fluorescence. The resolution of three-photon image scanning microscopy is a factor 2.10 better, and the peak of the point spread function is 3.77 times higher than in nonconfocal three-photon fluorescence. The resolution of two-photon image scanning microscopy with Bessel beam illumination is a factor 2.13 better than in standard two-photon fluorescence. Axial resolution and optical sectioning in two-photon or three-photon fluorescence are also improved by using the image scanning modality.

High-resolution and extended-depth-of-field photoacoustic endomicroscopy by scanning-domain synthesis of optical beams.

Photoacoustic endomicroscopy (PAEM) is capable of imaging fine structures in digestive tract. However, conventional PAEM employs a tightly focused laser beam to irradiate the object, which results in a limited depth-of-field (DOF). Here, we propose a scanning-domain synthesis of optical beams (SDSOB) to optimize both transverse resolution and the DOF by synthetic effective focused beams in scanning domain for the PAEM. By utilizing the SDSOB technique, multiple defocused and scattered beams are refocused to synthesize virtual focuses covering a large range of depth. A transverse point spread function that is 5.7-time sharper, and a transverse spatial bandwidth that is 8.5-time broader than those of the conventional PAEM were simulatively obtained through SDSOB-PAEM at the defocus distance of 2.4 mm. We validated the transverse resolution improvement at both in- and out-focus positions via phantom experiments of carbon fibers. In addition, in vivo rabbit experiments were conducted to acquire vascular images over radial depth range of 900 µm. And further morphological analysis revealed that the SDSOB images were acquired with abundant vascular branches and nodes, large total-length and small average-length of blood vessels, which indicated that the SDSOB-PAEM achieved high-resolution imaging in distinct rectal layers. All these results suggest that the SDSOB-PAEM possesses high transverse resolution and extended DOF, which demonstrates the SDSOB-PAEM can provide more accurate information for clinical assessment.

Numerically focused full-field swept-source optical coherence microscopy with structured illumination.

This paper presents an experimental investigation of the possibility of transverse resolution improvement combined with effective numerically focused 3D imaging in full-field swept-source optical coherence microscopy (OCM) by using structured illumination and specific numerical post-processing. The possibility of transverse resolution improvement of the OCM coherence signal combined with the possibility of numerical focusing is demonstrated by imaging a resolution test target in the optical focus and defocus regions. The possibility of numerically focused 3D imaging with high transverse resolution is further demonstrated by imaging a 3D phantom and a biological sample. The results obtained demonstrate the feasibility and prospects of the combination of structured illumination and numerical focusing in Fourier domain OCM.

Pixel-Reassigned Spectral-Domain Optical Coherence Tomography

A novel method of pixel reassignment (PRA) is proposed to achieve transverse superresolution in the confocal microscopy. Inspired by it, for the first time, we report a pixel-reassigned spectral-domain optical coherence tomography (OCT) with $\sim$ 1.5 times transverse resolution improvement both in numerical simulations and imaging experiments. PRA is implemented by transversely shifting the detection fiber from the on-axis position to five off-axis positions, capturing cross-sectional images at each shifting step of the detection fiber and digitally reassigning them together to give a sharper image with moderately extended depth of focus and enhanced intensity. So, we believe that the PRA technique will improve the imaging performance and be beneficial to the development of the OCT community.

Focusing of light beams by the phase apodization pupil

We investigated reduction of the size of the illuminated beam in the focal region produced by the optical systems of NA = 0.99 has been. The intensity distributions of polarized light field in the focal volume for the phase apodization pupil have been discussed. The circular pupil in different phase apodization situations can be employed to control the field components in the resultant intensity distribution. We show that both axial and transverse resolution improvement in the focal distribution is possible by applying proper phase engineering in the annulus of the pupil function.

Amplitude modulation technique for designing metalenses with apodized and enhanced resolution focal spots

In this paper we show that engineering both phase and amplitude of the scattered light can be employed in designing metalenses with either higher resolution or apodized focal spots. C-shaped split-ring micro-resonators (CSRRs) with different geometrical parameters are selected to have a full control of amplitude and phase. While phase engineering is necessary for light focusing, amplitude modulation of the scattered wave can be applied to characterize the focal point properties such as resolution gain and sidelobe level. We show that both axial and transverse resolution improvement or apodization is possible in the far-field region by applying proper amplitude function. Amplitude modulation technique, which is introduced in this paper, paves a new way to design efficient lenses which can be utilized in imaging and lithography applications in all frequency ranges.

Visible-super-resolution infrared microscopy using saturated transient fluorescence detected infrared spectroscopy

A scanning visible-super-resolution microscope based on the saturation behaviour of transient fluorescence detected infrared (TFD-IR) spectroscopy is proposed. A Gaussian IR beam, a Gaussian visible beam and a Laguerre–Gaussian (LG) visible beam are used to obtain two separate two-color excitation fluorescence (2CF) images of the sample. The final image is obtained as the difference between the two recorded images. If the peak intensity of the LG beam is high enough to induce saturation in the fluorescence signal, the image can, in principle, have unlimited spatial resolution. A ∼3-fold improvement in transverse resolution over the visible diffraction limit (and far exceeding the IR diffraction limit) is easily achievable in present experimental setups.

Transverse resolution improvement using rotating-grating time-multiplexing approach

The ability to improve the limited resolving power of optical imaging systems while approaching the theoretical diffraction limit has been an attractive discipline with growing interest over the last years due to its benefits in many applied optics systems. This paper presents a new approach to achieve transverse superresolution in far-field imaging systems, with direct application in both digital microscopy and digital holographic microscopy. Theoretical analysis and computer simulations show the validity of the presented approach.

Depth sectioning with the aberration-corrected scanning transmission electron microscope

The ability to correct the aberrations of the probe-forming lens in the scanning transmission electron microscope provides not only a significant improvement in transverse resolution but in addition brings depth resolution at the nanometer scale. Aberration correction therefore opens up the possibility of 3D imaging by optical sectioning. Here we develop a definition for the depth resolution for scanning transmission electron microscope depth sectioning and present initial results from this method. Objects such as catalytic metal clusters and single atoms on various support materials are imaged in three dimensions with a resolution of several nanometers. Effective focal depth is determined by statistical analysis and the contributing factors are discussed. Finally, current challenges and future capabilities available through new instruments are discussed.

Image Formation in Femtosecond Confocal Interference Microscopy

: Three-dimensional image formation in an interference confocal scanning microscope under ultrashort pulsed beam illumination is investigated in this study. The novelty of this new image system is that it keeps advantages in femtosecond interferometry but also provides a femtosecond-resolved three-dimensional image without necessarily using an ultrafast detector. For a 5-fs pulsed beam illumination, spatial resolution in the axial and transverse directions in this system is improved by approximately 45% and 15%, respectively, compared with that in the case of continuous wave illumination. However, strong chromatic aberration caused by an ultrashort pulsed beam can result in a degradation of spatial and temporal resolution, whereas weak chromatic aberration may lead to an improvement in transverse resolution.

Effects of Fresnel diffraction on confocal imaging with an annular lens

yOptoelectronic Imaging Group, Department of Applied Physics, Victoria Universityof Technology, PO Box 14428, MCMC, VIC 8001, AustraliazDepartment of Physical Optics, University of Sydney, NSW 2006, AustraliaSubmitted 9 January 1997, accepted 26 June 1997Abstract. The effect of the Fresnel diffraction on the imaging properties of confocalmicroscopy with one circular aperture and one annular aperture is investigated. Theresults show that the Fresnel diffraction patterns alter the axial imaging properties moreseriously than the transverse imaging properties. The axial response of such a confocalmicroscope is sharpened, but severely distorted.Keywords: Fresnel diffraction, annular pupil, confocal microscopy1. IntroductionAnnular aperture is a widely used spatial filter in confocalmicroscopy. It gives some improvement in transverseresolution, but a degradation in axial resolution appears ina confocal system with an annular structure [1,2]. Recentresearch suggests that the use of annular apertures as spatialfilters can suppress the scattered photons [3]. For example,a reflection-mode confocal system with an annular aperturein the illumination path is reported to show a significantenhancement in signal-to-noise ratio for imaging through ahighly scattering medium [4].In practice, an annular aperture cannot be placed exactlyat the back focal plane of an objective. In this case,Fresnel diffraction by the annular aperture exists. Fresneldiffraction of a finite-sized aperture can produce bright anddark fringes because of the interference of the waveletscontributed from points over the aperture [5,6]. Theintensity modulation caused by the interference fringes canalter the imaging properties of confocal microscopy, sincethe effective pupil function of the objective illuminatedby the Fresnel patterns changes accordingly, resulting inthe alteration of the intensity in the focal region [7–10].Therefore, it is important to investigate the effect of Fresneldiffraction on the imaging performance of a confocalsystem with an annular aperture.This paper is organized as follows. Section 2 presentsthe Fresnel diffraction of an annular aperture. The effectof Fresnel diffraction patterns of an annular aperture on theimaging properties of confocal microscopy is discussed insection 3.2. Fresnel diffraction of an annular apertureIf an aperture is placed in the back focal plane of anobjective then focusing is described by Debye theory. TheFresnel number is infinity in this situation. However, ifthe aperture is placed away from the back focal plane by adistance