Focused Beam Intensity Research Articles

Radiation force of a special correlated beam with off-axis multiple vortices on Rayleigh particles

In order to explore the potential application of a superimposed Hermite-Gaussian correlated Schell-model beams with off-axis multi-vortices in optical trapping, based on the generalized Collins formula, the focused beam intensity and radiation force on particles are derived in detail. Numerical results show that the gradient force and capture stability range increase with the increase of topological charge. By changing the vortex off-axis distance, the precise position control of the particles can be made. Adjusting the number of vortices can control the existence of vortex singularity on the side lobe of the beam, and generate or close the optical potential well or optical cage in real time. By selecting the appropriate beam order, two kinds of particles with different refractive indexes can be captured. The beams are expected to be applied to optical trapping of multiple particles at different positions.

Camera for transmission He+ ion microscopy

The intensity of transmitted He+ ions and atoms from a focused He+ beam was measured using a direct impact onto a camera located 20 cm below the sample. The camera consisted of a 256 × 256 array of Si p-i-n diodes (pixels) each 55 × 55 μm2 in area. Static, focused beam intensity profiles show a uniform distribution, as expected from the small de Broglie wavelength (80 fm) and coherent source. From the size of the beam spot, the half-angle beam convergence angle can be directly measured and compared with theoretical predictions based on column geometries. The detector count rate was consistent with an efficiency of 75%, when compared to the current measured by electrostatic beam blanking. The intensity profile of a neutral atom component was measured to have a 17% peak or 1.3% integrated intensity at a beam energy of 25 keV, compared to the ion beam. The intensity of transmitted particles through 50 nm amorphous carbon was less than that of graphite flakes, likely affected by thickness and channeling. Transmission images as a function of time to form 20 nm holes in graphite were obtained.

Coherent x-ray diffraction imaging of ZnO nanostructures under confined illumination

Coherent x-ray diffraction imaging has been used to study a single ZnO nanorod in a confined illuminating condition. The focused beam size is smaller than the length of the nanorod, and the diffraction intensity is strongly dependent on the illumination position. The density maps show that the nanorod width in the radial direction is around 210 nm and has a length of 1.5 μm, in agreement with the scanning electron microscope measurement. Reconstructed phase maps show a maximum phase change of 0.8 radians. The reconstructed direct space structures reveal the exit wavefront profile, which includes that of the focused x-ray beam. The beam profile presents in reconstructions some ‘hill and valley’ surface features with a typical size of a few tens of nanometres and are attributed to the noise due to the slow variation of the focused beam intensity along the boundary. A single ZnO tetrapod has been investigated with the same method to recover the beam profile in the horizontal direction.

Amplification of subpico second UV pulses in the multistage GARPUN-MTW Ti:sapphire — KrF laser system

Terawatt 248-nm UV pulses with an energy of 0.62 J, a width of no more than 1ps, and a divergence of 20 μrad have been obtained in the first experiments on the GARPUN-MTW Ti:sapphire — KrF laser system at double-pass amplification of frequency-tripled pulses from the Ti:sapphire front end in wide-aperture electron-beam-pumped KrF amplifiers. The contrast of short pulses relative to the amplified spontaneous emission is found to be ∼106 for energy densities and ∼1011 for intensities. The specificity of short-pulse amplification in an active medium with fast population inversion recovery and advantages of amplification of trains of short pulses or short and long pulses are discussed. The peak power of single subpicosecond pulses in this laser system can be increased to 30TW, and the focused beam intensity can be as high as 1020 W cm-2.

Development of a Polymer Coating-Based Microlens Array on Isotropically Wet-Etched Quartz Substrates for Maskless Lithography Application

A polymer coating-based microlens array (MLA) was developed on an isotropically wet-etched quartz substrate for maskless photolithography application. The developed MLA showed excellent light focusing ability and uniformity, and a dense fill factor. The obtained focal length ranged from 32.2 to 45.4 µm depending on the curvature of quartz and the thickness of an ultraviolet (UV) adhesive. A small spot size of 1.55 µm and an uniformly focused beam intensity were obtained at the focal plane, confirming that the fabricated MLA has excellent uniformity and good focal ability. The fabricated MLA was applied to UV lithography. Beams were well focused onto a photoresist when UV passed through the MLA. Depending on the variable distance from the MLA, beam size on the photoresist was controlled. Variable micropatterns were realized on the photoresist. Even at high a temperature, the interface between quartz and the UV adhesive was thermally stable and lens performance characteristics remained unchanged.

Axial optical trap stiffness influenced by retro-reflected beam

We studied experimentally how the beam retro-reflected from a planar interface (microscope slide) influences the axial stiffness of a single beam trap. Since the incident and retro-reflected beams interfere, weak intensity maxima and minima form a standing wave superposed on the axial single focused beam intensity envelope. Therefore there exists competition between the single beam trap and the weak standing wave traps. It results in microsphere hops to a new stable position at certain distances of the beam focus from the retro-reflecting surface. We analysed the behaviour of two polystyrene spheres of different sensitivities to the weak standing wave (diameters 690 and 820 nm) placed close to surfaces with different reflectivities (common glass R = 0.4% and reflective coating R = 13%). We used quadrant photodiode placed in a back focal plane of the microscope objective to track the position of the particle trapped in the single beam trap. Analyses of the thermal motion of trapped bead provided trap stiffnesses at different distances of the beam focus from the retro-reflecting surface and it also revealed a non-harmonic shape of the axial potential profile at the distances where the bead hops. We compared the experimental results with theoretical simulations based on Lorentz–Mie scattering theory.

Fabrication of elliptically figured mirror for focusing hard x rays to size less than 50nm

In this study, we designed, fabricated, and evaluated a hard x-ray focusing mirror having an ideally focused beam with a full width at half maximum in the intensity profile of 36nm at an x-ray energy of 15keV. The designed elliptically curved shape was fabricated by a computer-controlled fabrication system using plasma chemical vaporization machining and elastic emission machining, on the basis of surface profiles accurately measured by combining microstitching interferometry with relative angle determinable stitching interferometry. A platinum-coated surface was employed for hard x-ray focusing with a large numerical aperture. Line-focusing tests on the fabricated elliptical mirror are carried out at the 1-km-long beamline of SPring-8. A full width at half maximum of 40nm was achieved in the focused beam intensity profile under the best focus conditions.

Structures and its dipole moments of half-sandwich type metal–benzene (1:1) complexes determined by 2-m long electrostatic hexapole

The transition metal–benzene (1:1) complexes are synthesized by laser evaporation method followed by the size and structure selection with 2-m long electrostatic hexapole field. Electric dipole moments of half-sandwich type binary complexes for a series of the transition metals, i.e. Ti, V, Co, and Ni, with benzene molecule were determined by measuring the focusing curves, namely the dependence of the focused beam intensity upon the hexapole field strength. It is found that the dipole moments of Ti–C6H6 and V–C6H6 are larger than the ones of Co–C6H6 and Ni–C6H6. Such large dipole moments of Ti–C6H6 and V–C6H6 can be ascribed to the strong orbital interaction due to charge transfer between the metal atom and benzene molecule.

Focusing a beam of ultracold spin-polarized hydrogen atoms with a helium-film-coated quasiparabolic mirror.

We formed the first ``atomic-optics'' beam of electron-spin-polarized hydrogen atoms using a quasiparabolic polished copper mirror coated with a hydrogen-atom-reflecting film of superfluid $^{4}\mathrm{He}$. The mirror was located in the gradient of an 8-T solenoidal magnetic field and mounted on an ultracold cell at 350 mK. After the focusing by the mirror surface, the beam was again focused with a sextupole magnet. The mirror, which was especially designed for operation in the magnetic field gradient of our solenoid, increased the focused beam intensity by a factor of about 7.5.

Quantum reflection: Focusing of hydrogen atoms with a concave mirror.

We use a concave spherical mirror to focus at 18-mm-diam beam of H atoms down to 0.5 mm. The mirror consists of a fused-quartz substrate polished to optical precision and coated with a liquid-{sup 4}He film to obtain high reflectivity. The temperature dependence of the focused beam intensity enables us to study the influence of the dynamic surface roughness on the reflection of the H atoms. Both zero-point fluctuations and thermal excitations turn out to be of importance. A monolayer of {sup 3}He does not significantly affect the results.

Focusing of synchcrotron radiation by a multilayer-cylindrical tungsten-carbon Bragg reflector

A curved Bragg reflector for focusing synchrotron radiation has been fabricated using a recently developed magnetron sputtering equipment. The Bragg reflector consists of alternate thin-film layers of W and C deposited on a cylindrical substrate of 46 mm in radius of curvature, covering a 50×50-mm2 area. The focusing properties of the Bragg reflector were examined at beamline 15B of the Photon Factory. The focused beam intensity per unit area was six times higher than the unfocused beam, and the observed variations in beam size and shape with optical distances showed qualitative agreement with ray-tracing calculations.

Performance of Radial Focus Applied-B Ion Diode on a Coaxial Pulse Power Machine

A high-power magnetically insulated ion diode with radial geometry was successfully operated on a Reiden IV-H (2 MV, 200 kA) coaxial pulse power machine with across-current feed converter. The generated proton beams were focused ballistically onto the axis. Data of the beam trajectory measurements showed a large aberration of beam focusing in the diode gap region. This may be due to the electron virtual cathode distortion. The reduction of beam focusability was improved simply by correcting the shape of the anode surface. The focused beam intensity of 3×10 10 W/ cm 2 , which was measured by an ion pinhole camera, was compared with predicted values from the trajectory measurements and the beam brightness. Focus reduction factors in the diode were improved from 0.15 to 0.41.

Rotational energy effect upon the branching fraction for reactive decay of the CsF+K collision complex

The influence of the rotational energy of the CsF molecule upon its reactivity with K has been measured by the crossed molecular beam technique over a range of relative translational energy Ētr from 3–6 kcal mol−1. The experiments involve the endoergic (1.8 kcal mol−1) reaction CsF+K⇄[CsFK]→Cs+KF, for which the reactive branching fraction FR had been previously measured in this laboratory using velocity-selected but otherwise ’’thermal’’ (Boltzmann) beams. In the present investigation, beams of low rotational states of CsF have been prepared by means of a quadrupole state selector focusing field placed beyond the usual slotted-disk velocity selector. This arrangement provides so-called ’’low-J’’ beams of CsF, with known speed distributions, which are then crossed with a well-characterized K beam. The measurements consist of angular distributions of reactive (i.e., Cs) and nonreactive (i.e., CsF) scattering for the ’’low-J’’ beam compared to the ’’thermal’’ reactant beam at essentially the same Ētr. The reactive yield and thus the reactive branching fraction FR is found to be significantly smaller for the ’’low-J’’ beam relative to the ’’thermal’’ beam of CsF. An analysis of the state selection and focusing properties of the quadrupole lens sytem has been carried out which accounts satisfactorily for the observed velocity dependence of the CsF focused beam intensity. This analysis yields a calculated rotational state distribution P (J), from which the average rotational energy Ērot of the reactant CsF molecules is obtained at each of the Ētr values studied. Ērot ranges from 0.13 to 0.04 kcal mol−1 for Ētr in the range 3–6 kcal mol−1, whereas Ērot for the ’’thermal’’ CsF beam is 2.52 kcal mol−1. (Thus the effect of applying the rod voltage to the quadrupoles is to ’’switch off’’ some 2.4 kcal mol−1 of CsF rotational energy.) This analysis also predicts the observed enhancement of the total scattered signal due to the focusing, which implies that the cross section for complex formation is the same (within ≈±10%) for the low-J and the thermal reagent molecules. By combining the data on the Ētr-dependence of FR for the rotationally cold CsF beams with the previous results for thermal beams it is possible to compare the relative importance of rotational versus translational energy at a given total energy. It is found that rotational energy of the CsF is significantly less effective than relative translational energy in promoting reactive decay of the complex. The theoretical implications of this result are discussed.

Aberration effects in the interaction of focused laser beams with matter

An analysis of the role of primary spherical aberration ϕ in governing the intensity distribution of a focused laser beam has been confirmed by recording the fluorescence caused by the passage through a dye solution of radiation from both an Nd3+ laser and an He-Ne laser.The agreement between these recorded distributions and those computed using the Fresnel-Kirchhoff integral leads to the specification of the limit ϕ less-than-or-eq, slant 1λ for the reliable use of the geometric-optics approximations d=fθ and l=2(radical 2-1)f2θ/D for the focused beam diameter d and effective length l of the focal volume in terms of the beam divergence θ, unfocused beam diameter D and lens focal length f. Use of these relations under conditions where ϕ>1λ can lead to grossly inaccurate evaluation of the focal dimensions and of the focused beam intensity.The role of spherical aberration in laser-beam-induced gas breakdown has been investigated experimentally using single 20 ps Nd3+ laser pulses and combinations of beam diameters and lenses which changed ϕ from 0·41λ to 164λ. The results confirm the importance of aberration in controlling the focused beam intensity. The breakdown threshold intensity for argon at atmospheric pressure was found to be 3 × 1013 W cm−2.

A Graphical Method for Calculating Intensities of Space-Focused Atomic and Molecular Beams

A simple graphical method for calculating focused beam intensities in an atomic or a molecular beam resonance apparatus using space focusing is described. The method takes into account the finite sizes of the source and detector and the distribution of velocities in the beam. It can be used to determine the effect of stops and other obstacles placed in the beam. In addition to serving as a calculational tool, the method provides physical insight into the focusing process. Using the method, the dependence of focused beam intensity on the length of the C field is investigated for a symmetric apparatus. It is found that as the C field is lengthened, the focused beam intensity first increases, then slowly decreases. The results of numerical calculations are presented for a molecular beam apparatus using quadrupole A and B fields, and a beam of carbon monoxide molecules. The implications for high resolution atomic and molecular beam spectroscopy are discussed.

Measurements of Focused Beam Intensity by a Double-Beam Maser

The efficiency of an electrostatic focuser for the production of population inversion as a function of focuser voltage was experimentally studied and compared with theory. The measurement of the effective beam intensity below the threshold of maser oscillation was made possible by the use of another beam which supplied the additional beam of active molecules entering the cavity resonator. The threshold conditions and the amplitude characteristics in a double-beam ammonia maser were observed and compared with theory. It is shown that the effective number of active molecules in the beam varies in proportion to V4 when the focuser voltage V is low, and to V2 when the voltage is high, in good agreement with the theoretical result. The theoretical evaluation of the number of focused active molecules in a wide range of the focuser voltage is confirmed so that the amplitude characteristics, the threshold condition, and the beam intensity below threshold can be calculated with sufficient accuracies.