Effective Numerical Aperture Research Articles

STUDY OF THE MAJOR CONDITIONS INFLUENCING STEP HEIGHT MEASUREMENT USING INTERFERENCE MICROSCOPE

Step height measurements in micro and nano scale are essential to the micro-technology. The interferometric measurement is carried out using two main techniques: white light scanning interferometry (WLI) and phase stepping interferometry (PSI). A step height standard sample of nominal step height value h of 90.1 nm has been studied using the PSI mode with an optical filter light of λ = 633.4 nm against three parameters: vertical distance (z) between objective and sample, intensity of the incident light and the inclination angles of the sample to the incident light. The effect of numerical aperture of objectives has been referenced to some literature. The total budget of studied parameters for the sample 90.1 nm leads to expanded uncertainty of 0.46 nm.

Field-based dynamic light scattering microscopy: theory and numerical analysis

We present a theoretical framework for field-based dynamic light scattering microscopy based on a spectral-domain optical coherence phase microscopy (SD-OCPM) platform. SD-OCPM is an interferometric microscope capable of quantitative measurement of amplitude and phase of scattered light with high phase stability. Field-based dynamic light scattering (F-DLS) analysis allows for direct evaluation of complex-valued field autocorrelation function and measurement of localized diffusive and directional dynamic properties of biological and material samples with high spatial resolution. In order to gain insight into the information provided by F-DLS microscopy, theoretical and numerical analyses are performed to evaluate the effect of numerical aperture of the imaging optics. We demonstrate that sharp focusing of fields affects the measured diffusive and transport velocity, which leads to smaller values for the dynamic properties in the sample. An approach for accurately determining the dynamic properties of the samples is discussed.

Three-dimensional imaging of targets buried in a cluttered semi-infinite medium

The present contribution deals with the detection and characterization of targets embedded in a highly cluttered environment using an electromagnetic wave probing. The targets are buried in a semi-infinite host medium with a flat interface and random permittivity. A two-dimensional antenna array is located in air above the random medium for illuminating and detecting the scene. The poor impedance matching between the antennas and the host medium limits significantly the effective numerical aperture of the imaging system. To reconstruct the targets, we propose to combine the DORT (french acronym for Décomposition de l'Opérateur de Retournement Temporel) method with an iterative inversion technique and a frequency hopping procedure. Numerical experiments show that this approach permits the reconstruction of the targets even when they are located in a highly cluttered medium.

Utilization of Light Microscopy and FFT for MFA Measurement from Unstained Sections of Red Pine (Pinus Densiflora)

This study demonstrates the utilization of light microscopy and Fast Fourier Transform-Peak Finding (FPF) method for microfibril angle (MFA) measurement from unstained sections of red pine (Pinus densiflora). To obtain an image with optimal contrast and resolution for MFA measurement, effects of numerical aperture (NA) of condenser lens and color filters were investigated. About 60% of NA of the maximum condenser NA produced an image with optimal contrast, but a color filter with short wavelength range (DAPI) created images with improved resolution. Manual angle measurement and the FPF method were applied to the image with optimal contrast for MFA measurement. The experimental results from the FPF method were considered to be more repeatable and less subjective than those from the manual angle measurement.

Planar elliptical solid immersion lens based on a Cartesian oval

We report on the design of a single element planar solid immersion lens which is theoretically capable of confining light to a focal spot with full width half maximum of 0.29 λ (154 nm at λ = 532 nm) and achieving an effective numerical aperture of 1.732. Devices are fabricated and initial optical characterization is performed using a far-field imaging technique providing a lower bound on the numerical aperture of 0.7. Experimental results compare well with two dimensional finite element method simulations. Devices can be mass produced using ultraviolet lithography and produce focal volumes lower than expensive microscope objectives.

Near-field modification of femtosecond laser beam to enhance single-shot pulse filamentation in glass medium

In this paper, a study of the effect of a transparent dielectric medium (soda-lime glass placed after an objective lens) on femtosecond pulse filamentation in Corning Eagle2000 glass is presented. The dielectric medium placed after the objective lens increases the effective numerical aperture of the focusing lens and tightly focuses the beam to elevate the power density in a tiny focal volume. A single-shot narrow filamentary void of diameter 3.8 μm is observed to propagate as long as 700 μm in Corning Eagle2000 glass. Elevated optical intensity confined in the small focal volume and low ionization losses per unit length in the narrow plasma column collectively enhance the filamentary propagation of the ultra-short pulse in bulk glass. The applied technique also offers an approach to control the length and width of a filamentary void structure in a transparent dielectric medium.

Enhancing the Strength of an Optical Trap by Truncation

Optical traps (tweezers) are beginning to be used with increasing efficacy in diverse studies in the biological and biomedical sciences. We report here results of a systematic study aimed at enhancing the efficiency with which dielectric (transparent) materials can be optically trapped. Specifically, we investigate how truncation of the incident laser beam affects the strength of an optical trap in the presence of a circular aperture. Apertures of various sizes have been used by us to alter the beam radius, thereby changing the effective numerical aperture and intensity profile. We observe significant enhancement of the radial and axial trap stiffness when an aperture is used to truncate the beam compared to when no aperture was used, keeping incident laser power constant. Enhancement in trap stiffness persists even when the beam intensity profile is modulated. The possibility of applying truncation to multiple traps is explored; to this end a wire mesh is utilized to produce multiple trapping that also alters the effective numerical aperture. The use of a mesh leads to reduction in trap stiffness compared to the case when no wire mesh is used. Our findings lead to a simple-to-implement and inexpensive method of significantly enhancing optical trapping efficiency under a wide range of circumstances.

Tuning donut profile for spatial resolution in stimulated emission depletion microscopy

In stimulated emission depletion (STED)-based or up-conversion depletion-based super-resolution optical microscopy, the donut-shaped depletion beam profile is of critical importance to its resolution. In this study, we investigate the transformation of the donut-shaped depletion beam focused by a high numerical aperture (NA) microscope objective, and model STED point spread function (PSF) as a function of donut beam profile. We show experimentally that the intensity profile of the dark kernel of the donut can be approximated as a parabolic function, whose slope is determined by the donut beam size before the objective back aperture, or the effective NA. Based on this, we derive the mathematical expression for continuous wave (CW) STED PSF as a function of focal plane donut and excitation beam profiles, as well as dye properties. We find that the effective NA and the residual intensity at the center are critical factors for STED imaging quality and the resolution. The effective NA is critical for STED resolution in that it not only determines the donut shape but also the area the depletion laser power is dispersed. An improperly expanded depletion beam will have negligible improvement in resolution. The polarization of the depletion beam also plays an important role as it affects the residual intensity in the center of the donut. Finally, we construct a CW STED microscope operating at 488 nm excitation and 592 nm depletion with a resolution of 70 nm. Our study provides detailed insight to the property of donut beam, and parameters that are important for the optimal performance of STED microscopes. This paper will provide a useful guide for the construction and future development of STED microscopes.

A high resolution soft X-ray scintillation detector based on the Young-Weierstrass points of a lens shaped YAG crystal

Shaping a YAG scintillator crystal into a truncated-ball lens enables to image with a high numerical aperture its front surface, where the image converted from x-rays to visible is localized. Hence, both resolution and luminosity gains can be expected. Moreover if the plane surface is set at the Young-Weierstrass point of the spherical refractive surface, stigmatic imaging is achieved. On this principle, we have constructed an imaging detector from a 10 mm diameter YAG:Ce sphere and a long working distance plane-apochromatic microscope objective which does not limit the numerical aperture. The effective numerical aperture of the built system is 1.08, giving a Rayleigh resolution limit of 0.3 μm. Images of test objects (diatoms) have been recorded, in contact mode, with 103 eV. Periodic features of 0.4 μm pitch are visible on these images. The field of view is close to 200μm. The device is intended as an aid for X-ray optics fine-tuning and characterization.

Experimental study on high efficiency of Ti:sapphire laser to single-mode f iber coupling

We experimentally investigate an optimum scheme of coupling a collimated light from a Ti:sapphire laser source into a standard single-mode fiber (SMF). By adjusting the effective numerical aperture (NA) of coupling lens and eliminating the chromatic aberration, a coupling efficiency of around 70% is finally obtained. This result is close to the maximum value predicted by theoretical simulation. It is well demonstrated that high coupling efficiency between Ti:sapphire laser and SMF can also be obtained by optimizing certain parameters of a coupling lens, without employing any special optical components, or the specific fiber with complex structure.

Near-field optical recording with nanocomposite cover-layer for numerical aperture of 1.85

Near-field optical recording media with a nanocomposite cover-layer were prepared and tested using a gap servo, near-field recording system with an effective numerical aperture of 1.85. The refractive index of the nanocomposite cover-layer, which has a smooth surface, is 1.86, yielding a successful gap servo operation. However, the recording density cannot be maximized due to the refractive index of the cover-layer being lower than the required value for the effective numerical aperture. A recording density of 95 GB per disc can be achieved with a nanocomposite cover-layer having a refractive index of 1.86. The readout signal from 110 GB is not clear due to the reduced eventual numerical aperture in conjunction with the limited refractive index of the cover-layer.

Guiding and thermal properties of a hybrid polymer-infused photonic crystal fiber

In this work, we investigate the guiding properties of a hybrid polymer (poly-dimethylsiloxane)/silica photonic crystal fiber (PCF). In particular, we demonstrate how the basic guiding properties of a conventional PCF are changed due to the infusion of poly-dimethylsiloxane (PDMS) in its air-holes. We show that PDMS infiltration allows tuning of single mode operation, confinement loss, effective modal area (EMA) and numerical aperture (NA) with wavelength and/or temperature. This is primarily due to the enhancement of evanescent field interaction, lending some important characteristics for designing tunable fiber devices. Numerical calculations were performed for different relative hole sizes, d/Λ (0.35-0.75), of PCF for a 500-1700nm wavelength and 0-100°C temperature range, whereas direct comparison with a conventional air-filled PCF is also shown.

An annealing algorithm to correct positioning errors in ptychography

Ptychography offers the possibility of improving the resolution of atomic-scale (electron and X-ray) transmission microscopy without any of the demands of high quality lenses: its resolution is in theory only limited by the effective synthetic numerical aperture determined by the angular size of the detector. However, it has been realised experimentally that a major weakness of the approach is that the obtainable resolution is only as good as the accuracy to which the illuminating beam can be moved relative to the specimen. This can be catastrophic in the electron case because of thermal drift and hysteresis in the probe scan coils. We present here a computationally efficient extension of the 'ePIE' ptychographic reconstruction algorithm for correcting these errors retrospectively. We demonstrate its effectiveness using simulations and results from visible light and electron beam experiments that show it can correct positioning errors tens of times larger than the pixel size in the resulting image.

Aperture effect correction in spectroscopic full-field optical coherence tomography

Spatially resolved spectroscopic optical coherence tomography (OCT) has been demonstrated to be a convenient tool for spectral analysis in turbid media. For a full-field OCT configuration using a Mirau objective in the visible range, we found that the effective numerical aperture varies over the field of view, leading to field-dependent spectral shifts in the reconstructed spectra. Interferograms recorded with quasi-monochromatic lights are theoretically fitted with a general Mirau interference formula, and we propose a numerical correction method for white-light spectroscopy. The method is then tested successfully for the measure of the reflectivity of a plane gold sample.

Lensless digital holography with diffuse illumination through a pseudo-random phase mask

Microscopic imaging with a setup consisting of a pseudo-random phase mask, and an open CMOS camera, without an imaging objective, is demonstrated. The pseudo random phase mask acts as a diffuser for an incoming laser beam, scattering a speckle pattern to a CMOS chip, which is recorded once as a reference. A sample which is afterwards inserted somewhere in the optical beam path changes the speckle pattern. A single (non-iterative) image processing step, comparing the modified speckle pattern with the previously recorded one, generates a sharp image of the sample. After a first calibration the method works in real-time and allows quantitative imaging of complex (amplitude and phase) samples in an extended three-dimensional volume. Since no lenses are used, the method is free from lens abberations. Compared to standard inline holography the diffuse sample illumination improves the axial sectioning capability by increasing the effective numerical aperture in the illumination path, and it suppresses the undesired so-called twin images. For demonstration, a high resolution spatial light modulator (SLM) is programmed to act as the pseudo-random phase mask. We show experimental results, imaging microscopic biological samples, e.g. insects, within an extended volume at a distance of 15 cm with a transverse and longitudinal resolution of about 60 μm and 400 μm, respectively.

One-Head Near-Field Writing/Erasing on a Rewritable Dual-Layer Optical Disk Having High-Index Cover and Separation Layers

Near-field writing and erasing on a rewritable dual-layer optical disk with a single solid immersion lens (SIL) optical head is demonstrated for the first time. A novel beam expander enabled access to both information layers of the disk without exchanging two individual optical heads. A rewritable dual-layer disk having high-index (n=1.8) cover and separation layers was newly developed. By using the SIL optical system and the disk, an effective numerical aperture (NA) of 1.42 is achieved and a recording capacity of 180 Gbyte for the dual-layer rewritable disk is expected.

Ultrasmall spot size scanning laser ophthalmoscopy

An ultrasmall spot size scanning laser ophthalmoscope has been developed that employs an annular aberration-corrected incident beam to increase the effective numerical aperture of the eye thereby reducing the width of the probing light spot. Parafovea and foveal cone photoreceptor visibility determined from small area retinal image scans are discussed from the perspective of mode matching between the focused incident beam and the waveguide modes of individual cones. The cone visibility near the fovea centralis can be increased with the annular illumination scheme whereas the visibility of larger parafovea cones drops significantly as a consequence of poorer mode match. With further improvements of the implemented wavefront correction technology it holds promise for individual cone-photoreceptor imaging at the fovea centralis and for optical targeting of the retina with increased resolution.

Designing high performance Er +3-doped fiber amplifier in triangular-lattice photonic crystal fiber host towards higher gain, low splice loss

In order to improve the over-all performance of all-fiber amplifier using Er +3-doped triangular-lattice photonic crystal fiber (PCF), we performed a detailed study on the amplifying characteristics of the PCF host by varying all associated geometrical parameters towards utilizing controllable effective numerical aperture and tight modal confinement. A finite difference (FD) mode convergence analysis is used to determine the modal propagation characteristics of the structure, which is then used to solve a standard propagation and population rate equation. Our results show that a spectral gain of the amplifier as high as 51 dB and that too over a short length ∼2.5 m of the fiber is achievable by optimizing the transverse geometry of the fiber. Aimed at field-deployment of the amplifier as inline component, we calculate the important all-fiber characteristics, namely, beam divergence, bending loss and nonlinearity and then optimize the splicing/coupling loss (resulting from the fundamental mode mismatch only) of this active fiber device. Notably, the splice loss with standard telecom-grade SMF-28 fiber is considerably minimized through an improved mode-matching of the design. These results record a marked improvement in fiber amplifier research in realizing high-performance EDFA-PCF amplifiers.

Enhancing collection efficiency in large field of view multiphoton microscopy

Many multiphoton imaging applications would benefit from a larger field of view; however, large field of views (>mm) require low magnification objectives which have low light collection efficiencies. We demonstrate a light collection system mounted on a low magnification objective that increases fluorescence collection by as much as 20-fold in scattering tissues. This peripheral detector results in an effective numerical aperture of collection >0.8 with a 3-4 mm field of view.

217301 光造形法を用いた担体作製におけるNAの造形精度に及ぼす影響評価(OS07 細胞・生体組織3,オーガナイズド・セッション)

217301 光造形法を用いた担体作製におけるNAの造形精度に及ぼす影響評価(OS07 細胞・生体組織3,オーガナイズド・セッション)