Four-beam Configuration Research Articles

Fabrication of two-dimensional special photonic crystals by symmetry-lost beam interference lithography

An optical interference holographic setup with a symmetry-lost five or four-beam configuration is used to obtain two-dimensional special photonic crystals. By altering beam configuration and polarization combinations, novel structures can be achieved, such as tai chi-like, tadpole-like, and flag-like patterns. The experimental results with SU8 photoresist are the same with simulation. Process of two-dimensional special photonic crystals could provide an effective guidance for experimental design and fabrication of other complex structures, and thereby, promote the development and applications of novel photonic crystals.

Beam configuration selection for robust intensity-modulated proton therapy in cervical cancer using Pareto front comparison

The Pareto front reflects the optimal trade-offs between conflicting objectives and can be used to quantify the effect of different beam configurations on plan robustness and dose-volume histogram parameters. Therefore, our aim was to develop and implement a method to automatically approach the Pareto front in robust intensity-modulated proton therapy (IMPT) planning. Additionally, clinically relevant Pareto fronts based on different beam configurations will be derived and compared to enable beam configuration selection in cervical cancer proton therapy.A method to iteratively approach the Pareto front by automatically generating robustly optimized IMPT plans was developed. To verify plan quality, IMPT plans were evaluated on robustness by simulating range and position errors and recalculating the dose. For five retrospectively selected cervical cancer patients, this method was applied for IMPT plans with three different beam configurations using two, three and four beams. 3D Pareto fronts were optimized on target coverage (CTV D99%) and OAR doses (rectum V30Gy; bladder V40Gy). Per patient, proportions of non-approved IMPT plans were determined and differences between patient-specific Pareto fronts were quantified in terms of CTV D99%, rectum V30Gy and bladder V40Gy to perform beam configuration selection.Per patient and beam configuration, Pareto fronts were successfully sampled based on 200 IMPT plans of which on average 29% were non-approved plans. In all patients, IMPT plans based on the 2-beam set-up were completely dominated by plans with the 3-beam and 4-beam configuration. Compared to the 3-beam set-up, the 4-beam set-up increased the median CTV D99% on average by 0.2 Gy and decreased the median rectum V30Gy and median bladder V40Gy on average by 3.6% and 1.3%, respectively.This study demonstrates a method to automatically derive Pareto fronts in robust IMPT planning. For all patients, the defined four-beam configuration was found optimal in terms of plan robustness, target coverage and OAR sparing.

Measured hot-electron intensity thresholds quantified by a two-plasmon-decay resonant common-wave gain in various experimental configurations

The fraction of laser energy converted into hot electrons by the two-plasmon-decay instability is found to have different overlapped intensity thresholds for various configurations on the Omega Laser Facility [T. R. Boehly et al., Opt. Commun. 133, 495 (1997); J. H. Kelly et al., J. Phys. IV 133, 75 (2006)]. A factor-of-2 difference in the overlapped intensity threshold is observed between two- and four-beam configurations. The overlapped intensity threshold increases by a factor of 2 between the 4- and 18-beam configurations and by a factor of 3 between the 4- and 60-beam configurations. This is explained by a linear common-wave model where multiple laser beams drive a common electron-plasma wave in a wavevector region that bisects the laser beams (resonant common-wave region in k-space). These experimental results indicate that the hot-electron threshold depends on the hydrodynamic parameters at the quarter-critical density surface, the configuration of the laser beams, and the sum of the intensity of the beams that share the same angle with the common-wave vector.

Angular measurements in azimuth and elevation with 77 GHz radar sensors

An antenna concept for direction of arrival estimation in azimuth and elevation is proposed for 77 GHz automotive radar sensors. This concept uses the amplitude information of the radar signal for the azimuth angle and the phase information for the elevation angle. The antenna consists of a combination of a series-fed-array structure with a cylindrical dielectric lens. This concept is implemented into a radar sensor based on SiGe MMICs for validation. A two- and a four-beam configuration are presented and discussed with respect to angular accuracy and ambiguities.

Evaluation of DBS Wind Measurement Technique in Different Beam Configurations for a VHF Wind Profiler

Abstract Atmospheric winds in the troposphere have been observed routinely for many years with wind profiling (VHF and UHF) radars using the Doppler beam swinging (DBS) technique. Accuracy of wind estimates using wind profiling radars with different beam configurations has its limitations due to both the system of observation and atmospheric conditions. This paper presents a quantitative analysis and evaluation of horizontal wind estimation in different beam configurations up to an altitude of 18 km using the mesosphere–stratosphere–troposphere (MST) radar located in Gadanki, India. Horizontal wind velocities are derived in three different ways using two-, three-, and four-beam configurations. To know the performance of each configuration, radar-derived winds have been compared with the winds obtained by simultaneous GPS sonde balloon measurements, which are considered to be a standard reference by default. Results show that horizontal winds measured using three different beam configurations are comparable in general but discrepancy varies from one beam configuration to the other. It is observed that horizontal winds measured using four-beam configuration (east, west, north, and south) have better estimates than the other two-beam configurations. The standard deviation was found to be varying from 1.4 to 2.5 m s−1 and percentage error is about 9.68%–12.73% in four-beam configuration, whereas in other beam configurations the standard deviation is about 1.65–3.9 m s−1 and the percentage error is about 11.29%–15.16% with reference to GPS sonde balloon–measured winds.

Predication of multi-dimensional photonic crystal structures generated by multi-beaminterference in holographic lithography

Photonic crystals (PCs) are synthetic micro-structures which have periodic refractive indexvariations and produce photonic bandgaps similar to electronic bandgaps produced by thecrystal potentials of semiconductors. Different methods have been proposed anddemonstrated to fabricate two- or three-dimensional photonic crystal structures. Amongthem, the holographic lithography method, in which multi-beam interferenceis employed, offers a number of advantages. These include the ability to createa large volume of uniform periodic structures through an irradiation process,and more degrees of freedom to control the structures. In this study, a model ispresented for predicting the multi-dimensional photonic crystal structures formedusing multi-beam interference. Various parameters, including beam propagationand polarization directions, beam intensities, and phase shifts are considered.Calculations have been carried out to simulate two four-beam configurations whichhave been popularly used in the fabrication of photonic crystals. It has beendemonstrated that the contours of the interference pattern are related to thepolarization states, the intensity ratios among the four beams, and the phasedelays. Therefore, by controlling the beam intensities, polarization directions,and phase delays, different structures can be obtained. The results presented inthis study provide a useful guide for choosing various optical parameters andselecting suitable photoresists to fabricate 2D and 3D photonic crystal structures.

The Clinical Value of Non-Coplanar Photon Beams in Biologically Optimized Intensity Modulated Dose Delivery on Deep-Seated Tumours

The aim of the present study is to compare the merits of different radiobiologically optimized treatment techniques using few-field planar and non-coplanar dose delivery on an advanced cancer of the cervix, with rectum and bladder as principal organs at risk. Classically, the rational for using non-coplanar beams is to minimize the overlap of beam entrance and exit regions and to find new beam directions avoiding organs at risk, in order to reduce damage to sensitive normal tissues. Two four-beam configurations have been extensively studied. The first consists of three evenly spaced coplanar beams and a fourth non-coplanar beam. A second tetrahedral-like configuration, with two symmetric non-coplanar beams at the same gantry angle and two coplanar beams, with optimized beam directions, was also tested. The present study shows that when radiobiologically optimized intensity modulated beams are applied to such a geometry, only a marginal increase in the treatment outcome can be achieved by non-coplanar beams compared to the optimal coplanar treatment. The main reason for this result is that the high dose in the beam-overlap regions is already optimally reduced by biologically optimized intensity modulation in the plane. The large number of degrees of freedom already incorporated in the treatment by the use of intensity modulation and radiobiological optimization, leads to the saturation of the benefit acquired by a further increase in the degrees of freedom with non-coplanar beams. In conclusion, the use coplanar of radiobiologically optimized intensity modulation simplifies the dose delivery, reducing the need for non-coplanar beam portals.

Theory of multibeam stagger-tuned gyroklystrons

An analytical theory describing the tradeoff in the bandwidth and gain in multibeam stagger-tuned gyroklystrons (GKLs) is developed. A two stage multibeam configuration (i.e., the device consisting only of input and output cavities) is considered in detail. The analysis of two-, three- and four-beam configurations allows one to evaluate the increase in bandwidth due to multibeam configuration and the tradeoff in the bandwidth and gain due to the stagger tuning.

Spatial effects in two- and four-beam interference of partially entangled biphotons

Fourth-order interference patterns of biphotons generated by type-I spontaneous parametric downconversion are examined for beams with partial angular and spectral entanglement, both for degenerate and nondegenerate constituent photons. Two-beam configurations using a single beam-splitter interferometer and a Mach-Zehnder interferometer are explicitly studied, as are four-beam configurations using pairs of beam splitters and MachZehnder interferometers. The interference pattern generally comprises a number of harmonic functions of the path-length difference with frequencies and visibilities that usually depend on direction, and may have a finite or infinite duration ~fourth-order coherence length!. The relation between the visibility and the spectral indistinguishability of the two beams is established. Certain components of the interference pattern are independent of direction so that they are not washed out by the use of apertures of finite size. @S1050-2947~98!09305-6#

Dynamical effects of the dipole-dipole interaction in three-dimensional optical lattices

We present a many-body theory of the dipole-dipole interaction in three-dimensional optical lattices generated by a four-beam configuration, specializing to the case of ${J}_{g}=\frac{1}{2}\ensuremath{\rightarrow}{J}_{e}=$$\frac{3}{2}$ transitions. We construct a many-body interaction Hamiltonian in coordinate representation for an antiferromagnetic fcc optical lattice, the Schr\odinger field operators being expanded on a basis of Wannier functions. We discuss the main characteristics of the dipole-dipole matrix elements giving rise to bound-bound and bound-free atom interactions in the lattice. Because of the anisotropy of the dipole-dipole interaction, specific directions can be favored for transport and scattering processes. Furthermore, since the dipole-dipole interaction depends on atomic magnetic quantum numbers, the dipole-dipole potential resembles a spin-dependent potential, and can give rise to atomic hopping with simultaneous change in the magnetic quantum number.

Crystallography of optical lattices.

We present a theoretical study of three- and four-beam configurations that generalize, in the two-dimensional (2D) and three-dimensional (3D) cases, the well-known 1D lin \ensuremath{\perp} lin (two counterpropagating beams having crossed linear polarizations) and magnetic-assisted Sisyphus effect cooling schemes. It is shown how the Bravais lattice is determined from the propagation directions of the laser beams while the basis is associated with the polarizations of the incident waves. The optical potentials are calculated for a ${\mathit{J}}_{\mathit{g}}$=1/2\ensuremath{\rightarrow}${\mathit{J}}_{\mathit{e}}$=3/2 transition for a variety of angular and polarization configurations and some of the characteristics of the atomic motion inside a single micrometer-sized potential well are predicted.

Trapping and levitation of a dielectric sphere with off-centred Gaussian beams. I. Experimental

The authors describe an experiment in which a dielectric sphere, a few mu m in size, is trapped and levitated by two vertical laser beams, whose separation can be varied continuously. Each beam is itself the superposition of two counterpropagating (up and down) Gaussian beams. This four-beam configuration has applications as a double optical levitation trap or as an all-optical dynamometer. The report is dedicated to the experimental study of forces exerted on a dielectric sphere in water in a non-symmetrical situation, i.e. with off-centred beams. The authors derive the shapes of the longitudinal and of the transverse forces as a function of the distance between the sphere centre and the beam axis in the linear regime, i.e. in conditions where these forces are found proportional to the beam intensity.