Incident Beam Spot Research Articles

Design and Study of a Two-Dimensional (2D) All-Optical Spatial Mapping Module.

Sequentially timed all-optical mapping photography is one of the main emerging ultra-fast detection technologies that can be widely applicable to ultra-fast detection at the picosecond level in fields such as materials and life sciences. We propose a new optical structure for an all-optical spatial mapping module that can control the optical field of two-dimensional imaging while improving spectral resolution and detector sensor utilization. The model of optical parameters based on geometrical optics theory for the given structure has been established, and the theoretical analysis of the inter-frame energy crosstalk caused by incident beam spot width, chromatic aberration, and main errors of the periscope array has been conducted. The optical design of the two-dimensional (2D) all-optical spatial mapping module was finally completed using ZEMAX OpticStudio 2018 software. The results show that our optical module can realize targets of 16 frames and 1.25 nm spectral resolution.

Imaging and reconstruction of high-energy proton beam spot before target based on secondary gamma rays.

The distribution and stability of the incident proton beam spot are critical for the stable operation of a high-power spallation target. This study proposes a method to capture images of the incident proton beam spot based on secondary gamma rays. The distribution of the backward secondary gamma rays irradiated by the primary proton beam on the incident surface of the target is close to the distribution of the protons and can be measured at a low-radiation position far from the target area. A relation of distributions between the incident protons and the secondary gammas is constructed by using the point response function of this pinhole imaging system. The proposed method of imaging is suitable for monitoring the distribution of the proton beam on the target in facilities that use a beam power of several megawatts or tens of megawatts, such as spallation neutron sources or accelerator-driven subcritical systems.

Investigation on the properties of the formation and coherence of intense fringe near nonlinear medium slab

Abstract Near medium intense (NMI) fringe is a kind of intense fringe which can be formed near Kerr medium in high-power laser beam propagation. The formation properties of NMI fringe and the relations between NMI fringe and related important parameters are systematically investigated. It is found that it is the co-existence of two wirelike phase-typed scatterers in the incident beam spot which is mainly responsible for the high intensity of NMI fringe. From the viewpoint of coherent superposition, the formation process of NMI fringe is analyzed, and the mechanism that NMI fringe is formed by the coherent superposition of the localized bright fringes in the exit field of Kerr medium slab is demonstrated. The fluctuations of NMI fringe properties with beam wavelength, scatterer spacing and object distance are studied, the coherence of NMI fringe are revealed, and the approximate periodicity of the appearance of remarkable NMI fringe for these parameters are obtained. Especially, it is found that the intensity of NMI fringe is very sensitive to scatterer spacing. Besides, the laws about how NMI fringe properties will be changed by the modulation properties of scatterers and the medium thickness are demonstrated.

A deflection optical sensor based on a Scotch tape waveguide with an integrated grating coupler

A deflection sensor based on a plastic waveguide cantilever is presented and demonstrated. The waveguide cantilever is made of conventional adhesive tape and integrates a metal grating coupler at its deflecting end. Light impinging the grating is coupled into the tape waveguide and guided to a fixed photodetector on which the opposite end of the waveguide is anchored. The photodiode acts as the cantilever support and converts guided optical power into a photocurrent (sensor response). Deflection optical monitoring relies on the variation of the overlap of the incident light beam spot with the grating coupler as a function of the cantilever deflection. This approach leads to a larger deflection sensitivity than that obtained by a method based just on the variation of the grating coupling efficiency with the incidence angle. A 14.85-mm-long cantilever sensor has been fabricated and exhibits a linear working range of 2 decades with a maximum deflection sensitivity of 0.2 μA/μm and a resolution of 1.7 μm, limited by the interrogation light source noise. Noise analysis indicates the feasibility of sub-nanometric deflection resolution.

Off-axis catadioptric fisheye wide field-of-view optical receiver for free space optical communications

We propose a wide field-of-view optical receiver design based on a fisheye lens and an off-axis catadioptric structure for free-space optical communications. The design utilizes a novel fisheye lens group to compress a wide field angle into a narrow field angle and produce the appropriately collimated light that can effectively be coupled into the following aperture of a catadioptric telescope. An off-axis catadioptric telescope with aspheric surface mirrors is designed to compress the incident beam spot size, compensate for the high order optical aberrations and eliminate light loss due to an obstruction. The parallel exit rays are reflected on a double-level tracking mechanism by feeding the position signal from a quadrant detector to correct the pointing error and optimize the coupling efficiency into an optical fiber. The final wide field-of-view optical receiver design is presented along with the evaluation of optical performance results and tracking characteristics. The proposed optical receiver not only can provide a 60-deg wide field-of-view to expand the tracking range, but also mitigates optical aberrations to improve the tracking accuracy for free space optical communication systems in a turbulent atmosphere.

Simulation of 12C+12C elastic scattering at high energy by using the Monte Carlo method

The Monte Carlo method is used to simulate the 12C+12C reaction process. Taking into account the size of the incident 12C beam spot and the thickness of the 12C target, the distributions of scattered 12C on the MWPC and the CsI detectors at a detective distance have been simulated. In order to separate elastic scattering from the inelastic scattering with 4.4 MeV excited energy, we set several variables: the kinetic energy of incident 12C, the thickness of the 12C target, the ratio of the excited state, the wire spacing of the MWPC, the energy resolution of the CsI detector and the time resolution of the plastic scintillator. From the simulation results, the preliminary establishment of the experiment system can be determined to be that the beam size of the incident 12C is ϕ5 mm, the incident kinetic energy is 200–400 A MeV, the target thickness is 2 mm, the ratio of the excited state is 20%, the flight distance of scattered 12C is 3 m, the energy resolution of the CsI detectors is 1%, the time resolution of the plastic scintillator is 0.5%, and the size of the CsI detectors is 7 cm×7 cm, and we need at least 16 CsI detectors to cover a 0° to 5° angular distribution.

Radiometer standard for absolute responsivity calibrations from 950 nm to 1650 nm with 0.05% (k = 2) uncertainty

A near-IR radiometer standard with similar performance to silicon trap detectors has been developed to calibrate detectors and radiometers for absolute spectral power, irradiance and radiance responsivities between 950 nm and 1650 nm. The new radiometer standard is utilized at the Spectral Irradiance and Radiance Responsivity Calibrations using Uniform Sources (SIRCUS) which is the reference calibration facility of NIST for absolute responsivity. The radiometer standard is a sphere detector with a unique geometrical arrangement and it can convert the radiant power responsivity scale of the primary-standard cryogenic radiometer into a reference irradiance responsivity scale. The 0.05% (k = 2) scale conversion uncertainty is dominated by the two largest uncertainty components of the radiometer: the spatial non-uniformity of responsivity of less than 0.05% in power mode and the 0.03% angular responsivity deviation from the cosine function in a 5° angular range in irradiance mode. These small uncertainty components are the results of a tilted input aperture (relative to the sphere axis) and four symmetrically positioned InGaAs detectors around the incident beam spot in the sphere. With the new radiometer standard, it is expected that a thermodynamic temperature uncertainty of 10 mK (k = 2) can be achieved at 157 °C, the freeze temperature of the In fixed-point blackbody.

Aperture effects and mismatch oscillations in an intense electron beam

When an electron beam is apertured, the transmitted beam current is the integral of the incident beam current density over the aperture area. Space charge forces generally cause an increase in incident beam current to result in an increase in incident beam spot size. Under certain circumstances, the spot size will increase faster than the current, resulting in a decrease in current extracted from the aperture. When using a gridded electron gun, this can give rise to negative transconductance. Here, this effect is explored in the case of an intense beam propagating in a uniform focusing channel. It is shown that proper placement of the aperture can decouple the current extracted from the aperture from fluctuations in the source current, thereby isolating systems downstream of the aperture from upstream current fluctuations. It is also shown that apertures can serve to alter longitudinal space charge wave propagation by changing the relative contribution of velocity and current modulation present in the beam.

Laser pulse impact characterization using ultrasonic transducers

Experiments were conducted with an excimer laser (wavelength 248 nm) to cause damage to gelatinous materials, which have optical properties similar to skin tissue. The laser pulses, when they impact the specimen, generate an elastic wave that propagates through the material. A 150-MHz broadband ultrasonic transducer was placed underneath the specimen to observe the material response and the pressure generated by the laser pulse. Different incident laser energies and laser beam spot sizes were utilized to obtain a range of input parameters, and the corresponding transducer responses were recorded. Fourier analysis of the signals was performed to identify the frequency response from the laser pulse. Additional tests were carried out to observe the effects due to a train of laser pulses. Earlier tests were performed with 200-kHz hydrophone-calibrated transducer but due to the shortness of the laser pulses (∼20 ns), a higher frequency transducer was used to accurately characterize the laser impact. The initial results indicate that there is a dominant frequency response around the 25–30-MHz regime.

Crystallographic Approach to Regenerated and Pathological Hard Tissues

A dominant inorganic substance in hard tissue is known to be a biological apatite (BAp)^nano-crystal which basically crystallizes in an anisotropic hcp lattice, and the BAp c-axis is parallel to extended collagen fibrils. We applied the microbeam X-ray diffractometer system with an incident beam spot 100µm or 50µm in diameter to the original, regenerated and pathological hard tissues in order to analyze the preferential alignment of the BAp c-axis as a parameter of bone quality closely relating to the mechanical function. We conclude that the BAp orientational distribution in the hard tissues is a new measure to evaluate stress distribution in vivo, nano-scale microstructure and the related mechanical function, healing process of the regenerated bone and progress of the bone diseases.

Growth, Characterization and Application of Single-Crystal La2-xSrxCuO4 Having a Gradient in Sr Concentration

We succeeded in growing sizable single crystals of La2-xSrxCuO4 (LSCO) having a gradient in the Sr concentration along the axis of crystal growth. These crystals were completely characterized and the gradient of x (x ∼0.03/cm), extending from x = 0.00 to 0.30, was determined from the positional dependences of local lattice constants and by inductively coupled plasma analysis. In accordance with the gradient of x, a gradient of hole concentration was also confirmed from the positional dependences of resistivity and superconducting transition temperatures. A real space boundary between the superconducting and the insulating phases was first observed as the boundary of the vortex state by scanning magnetic fields along the gradient of x using a scanning superconducting quantum interference device (SQUID) microscope. Precise measurement of the diamagnetic susceptibility of LSCO as a function of doping yielded a phase diagram that is consistent with one obtained from samples with constant x. We used these crystals for the phonon measurement with a synchrotron X-ray beam and succeeded in determining the doping dependence of the dispersion relation by changing the position of the incident beam spot on the crystals with the same experimental setup.

Improved Automatic Alignment of Pinhole Integrated with Photodiode

The spatial filter of a Si micromachined pinhole integrated with photodiodes is fabricated. The photodiode cells placed around the center pinhole can detect the relative position between the incident beam spot and the pinhole. The automatic alignment of the pinhole was realized using a feedback control based on the position signal obtained from the photodiode cells. In our previous study, the alignment accuracy achieved was ±0.5 µm for a pinhole of 50 µm diameter. In this study, the design is improved by reducing the separation length between the surrounding photodiode cells. An alignment accuracy of ±0.03 µm was demonstrated.

Si Micromachined Pinhole and Surrounding Photodiode for Incident Beam Position Sensing and Automatic Alignment

The spatial filter of a Si micromachined pinhole integrated with photodiodes is fabricated. The photodiode cells placed around the center pinhole can detect the relative position between the incident beam spot and the pinhole. The position signal obtained from a two or four cell type photodiode and position sensitive detector show the sensitivity even when the beam spot is near the center pinhole. Combined with the feedback control of the shape memory alloy actuator based on the obtained position signal, the pinhole is aligned automatically.

Dark-Field and Marginal Imaging with a Thin-Annular Detector in STEM

The use of a thin annular detector in a scanning transmission electron microscope is shown, theoretically and experimentally, to allow several imaging modes that may be of value for the study of thin specimens. The diffraction pattern on the detector plane may be expanded or contracted by means of post-specimen lenses to vary the collection angle of the thin annular detector to form dark- or bright-field images. Dark-field images obtained from annuli of various radii in the diffraction pattern can selectively reveal different components of the sample, as illustrated in the case of a sample containing platinum crystallites, amorphous carbon, and carbon nanotubes. Amorphous materials of different composition can be distinguished by selecting the main maxima in their diffraction patterns. If the central beam is enlarged so that it just fills the inner aperture of the detector, the “marginal” imaging mode so achieved gives an image contrast that is proportional to the square of the differential of the projected potential distribution. Any deflection of the incident beam spot due to a change of the projected potential gives bright contrast in the image.

Practical problems with a proton probe

The Guelph proton microprobe has provided several thousand spot analyses of mineralogical, geochemical and metallurgical specimens, mainly through contract arrangements. User-friendly target chamber design with color TV viewing of the incident beam spot at magnification × 300 is crucial in this work. A new PC-based data accumulation system for both spot analysis and elemental mapping is described. Various enhancements to the GUPIX software for spectrum fitting and standardization are reviewed.

Wavelength dependence of pulsed laser ablation of calcified tissue

Pulsed laser ablation of calcified biological tissue was studied at several wavelengths in the near-ultraviolet, visible, near- and mid-infrared regions of the spectrum. The primary tissue model was bovine shank bone, while human arterial calcified plaque and normal human artery wall were also studied at selected wavelengths for comparison. Laser irradiances were on the order of MW/mm2, fluences ranged up to 1000 mJ/mm2, and repetition rates varied between 0.3-10 Hz. Spot sizes on the tissue surface ranged from 150 to 850 microns. Laser craters made with wavelengths between lambda = 295 nm and lambda = 375 nm and in the lambda = 3 microns region exhibited the highest quality ablation with clean, sharp cuts following closely the spatial contour of the incident beam. Craters drilled with visible wavelengths between lambda = 450 nm and lambda = 590 nm were generally larger than the incident laser beam spot, irregular in shape and often surrounded by large flakes of tissue debris. Ablation fluence thresholds increased with wavelength through the visible wavelengths and into the mid-infrared, but dropped to their lowest values near lambda = 3 microns. Fluence thresholds obtained with the tissue under a 1 mm depth of saline were approximately twice air thresholds. Ablation yields also varied with wavelength, probably due to increased scattering in the visible region, and were the same under saline as in air.

Diffraction Pattern Caused by X-Ray Thermal Diffuse Scattering in Absorbing Perfect Crystal

Excess diffraction lines observed as black lines across the 220 thermal diffuse spot and near the incident beam spot on X-ray film photographs of absorbing perfect germanium crystals were studied experimentally in detail by using monochromatized synchrotron radiations. The cause of the line was attributed to the anomalous transmission of the thermally scattered X-rays in the crystal. The experimental result was compared successfully with the theoretical calculation based on the dynamical diffraction theory of X-rays in the absorbing ideally perfect crystal.

On pulsed-laser heating of thin targets in air

A simple analytical model has been developed which describes the heating of a thin opaque target in air when subjected to irradiation from a high-power pulse laser. Effects due to the formation of a laser-supported absorption wave (LSAW) above the target surface are explicitly included in this model. The effective on-target flux distribution is assumed to be described by a radially expanding Gaussian profile in order to simulate the effects of enhanced coupling dominated by uv emission from the hot expanding LSAW plasma. Target heating inside as well as outside the incident beam-spot region is described. This model is an improvement over the previous approach [J.E. Robin, Appl. Phys. Lett. 30, 401 (1977)] which employed a radially expanding uniform flux distribution that was based on the assumption of a thermal-conduction-dominated enhanced-coupling mechanism.

Bit packing density of the BEAMOS target

BEAMOS, beam addressed metal oxide semiconductor, is an electron-beam-addressed semiconductor digital mass-memory device. Its operation is based on electron-beam writing of stored charge in an unstructured MOS layer and electron-beam reading of this stored charge through its effect on the surface-recombination velocity of the oxide silicon interface. The surface-recombination velocity controls the diffusion of electron-beam-generated carriers in the n layer of the np diode underlying the oxide layer. This control of carrier diffusion by surface-recombination-velocity patterns is the unique feature of the BEAMOS target concept. The data storage capacity of BEAMOS-type devices is limited by the electron-beam spot size and target construction. A model of the target resolution (bit packing density) is described which includes the effects of incident beam spot size, electron scattering in the target layers, and injected minority-carrier diffusion in the n layer of the target. Measurements of electron-beam scattering are summarized and applied to the writing and reading mechanisms of the target. The minority-carrier diffusion equation is solved by a relaxation method similar to the method of sources. This leads to a model for the target bit packing density, which includes both electron scattering and minority-carrier diffusion. For present BEAMOS targets, the dominant limitation is shown to be minority-carrier diffusion, which can be controlled by the n-layer thickness. Predictions of the model are compared with measurements on actual targets. The ultimate packing density of the concept is also discussed. Packing densities of more than 2×107 bits/cm2 have been demonstrated, and densities as high as 109 bits/cm2 may be possible.