Off-axis Field Research Articles

Single and double quantum transitions in spin-mixed states under photo-excitation

Electronic spins associated with the Nitrogen-Vacancy (NV) center in diamond offer an opportunity to study spin-related phenomena with extremely high sensitivity owing to their high degree of optical polarization. Here, we study both single- and double-quantum transitions (SQT and DQT) in NV centers between spin-mixed states, which arise from magnetic fields that are non-collinear to the NV axis. We demonstrate the amplification of the ESR signal from both these types of transition under laser illumination. We obtain hyperfine-resolved X-band ESR signal as a function of both excitation laser power and misalignment of static magnetic field with the NV axis. This, combined with our analysis using a seven-level model that incorporates thermal polarization and double quantum relaxation, allows us to comprehensively analyze the polarization of NV spins under off-axis fields. Such detailed understanding of spin-mixed states in NV centers under photo-excitation can help greatly in realizing NV-diamond platform’s potential in sensing correlated magnets and biological samples, as well as other emerging applications, such as masing and nuclear hyperpolarization.

Aberration aware feature detection and description.

Feature detection and description are crucial for image matching, as better performance at this stage leads to more accurate matching results, which is essential for subsequent vision-based tasks. However, images captured by different optical systems may suffer from various optical aberrations, especially in off-axis field and out-of-depth-of-field regions, making it challenging for models to extract consistent feature locations and descriptors. In this paper, we propose what we believe to be a novel method for training feature detection and description networks by incorporating optical system aberrations modeled by point spread function(PSF). We introduce PSF augmentation and homographic PSF adaptation, which customize the training of feature detection and description models for specific optical systems using general unlabeled image datasets. Experimental results demonstrate that our method significantly improves the performance of feature detection and description in images captured by given cameras.

Optimizing off-axis fields for two-axis magnetometry with point defects

Vector magnetometry is an essential tool for characterizing the distribution of currents and magnetization in a broad range of systems. Point defect sensors, like the nitrogen vacancy center in diamond, have demonstrated impressive sensitivity and spatial resolution for detecting these fields. Measuring the vector field at a single point in space using single defects, however, remains an outstanding challenge. We demonstrate that careful optimization of the static bias field can enable simultaneous measurement of multiple magnetic field components with enhanced sensitivity by leveraging the nonlinear Zeeman shift from transverse magnetic fields, realizing an improvement in transverse sensitivity from >200 μT/Hz (no bias field) to 30 μT/Hz. This work quantifies the trade-off between the increased frequency shift from second-order Zeeman effects with decreasing contrast as off-axis field components increase, demonstrating the measurement of multiple components of the magnetic field from an exemplar antiferromagnet with a complex magnetic texture.

Fast coherent control of nitrogen-14 spins associated with nitrogen-vacancy centers in diamonds using dynamical decoupling

Abstract A nitrogen-vacancy (NV) center in a diamond enables the access to an electron spin, which is expected to present highly sensitive quantum sensors. Although exploiting a nitrogen nuclear spin improves the sensitivity, manipulating it using a resonant pulse requires a long gate time owing to its small gyromagnetic ratio. Another technique to control nuclear spins is a conditional rotation gate based on dynamical decoupling, which is faster but unavailable for nitrogen spins owing to the lack of transverse hyperfine coupling with the electron spin. In this study, we generated effective transverse coupling by applying a weak off-axis magnetic field. An effective coupling depends on the off-axis field; the conditional rotation gate on the nitrogen-14 spins of an NV center was demonstrated within 4.2 μs under an 1.8% off-axis field and a longitudinal field of approximately 280 mT. We estimated that a population transfer from the electron to nitrogen spins can be implemented with 8.7 μs. Our method is applicable to an ensemble of NV centers, in addition to a single NV center.

Charged Domain Walls in BaTiO3 Crystals Emerging from Superdomain Boundaries (Adv. Electron. Mater. 6/2023)

Charged Domain Walls In article number 2300005, Petr S. Bednyakov and Jiří Hlinka observe transient domain structure with multiple superdomains containing bundles of ferroelastic subdomains during the paraelectric to the ferroelectric transformation in the crystal of barium titanate under the off-axis electric field. Operando optical microscopy reveals that after the retraction of the paraelectric phase, the superdomain boundaries convert into the desired charged domain wall species.

Stray Light Analysis and Suppression of the Visible to Terahertz Integrated Cloud Detection Optical System.

The wide-spectrum integrated imaging method can simultaneously obtain the spectral information of different spectral bands of the same target, which is conducive to the realization of the high-precision detection of target characteristics, and can simultaneously obtain more comprehensive elements such as the structure, shape, and microphysical parameters of the cloud. However, for stray light, the same surface has different characteristics at different wavelengths, and a wider spectral band means more complex and diverse sources of stray light, which renders the analysis and suppression of stray light more difficult. In this work, according to the characteristics of the visible-to-terahertz integrated optical system design scheme, the influence of material surface treatment on stray light was studied; the stray light analysis and optimization of the whole link of light transmission were carried out. For the sources of stray light in different channels, targeted suppression measures such as front baffle, field stop, special structure baffle, and reflective inner baffle were adopted. The simulation results indicate that when the off-axis field of view was greater than 10°. The point source transmittance (PST) of the terahertz channel is on the order of 10-4, the visible and infrared channels are less than 10-5, and the final terahertz PST was on the order of 10-8, while visible and infrared channels were lower than 10-11. Here, we present a method for stray light suppression based on conventional surface treatments for broadband imaging systems.

Numerically Stable and Computationally Efficient Expression for the Magnetic Field of a Current Loop

In this work, it is demonstrated that straightforward implementations of the well-known textbook expressions of the off-axis magnetic field of a current loop are numerically unstable in a large region of interest. Specifically, close to the axis of symmetry and at large distances from the loop, complete loss of accuracy happens surprisingly fast. The origin of the instability is catastrophic numerical cancellation, which cannot be avoided with algebraic transformations. All exact expressions found in the literature exhibit similar instabilities. We propose a novel exact analytic expression, based on Bulirsch’s complete elliptic integral, which is numerically stable (15–16 significant figures in 64 bit floating point arithmetic) everywhere. Several field approximation methods (dipole, Taylor expansions, Binomial series) are studied in comparison with respect to accuracy, numerical stability and computation performance. In addition to its accuracy and global validity, the proposed method outperforms the classical solution, and even most approximation schemes in terms of computational efficiency.

Off-axis electric field due to cylindrical geometries of charge distribution

Off-axis electric field due to cylindrical distribution of charge is studied in various geometries, including solid cylinder, cylindrical shell, disk, and ring. The electric fields of the charged solid cylinder and cylindrical shell can be found by numerical calculations, but closed forms expressed in terms of elliptic integrals are derived for the charged disk and ring. By investigating the radial and axial components of the electric fields systematically we reveal many interesting and new features that are implicit or unseen in the ordinary approach based on the cylindrical symmetry.

Dosimetric evaluation of off-axis fields and angular transmission for the 1.5 T MR-linac

Objective. GPU-oriented Monte Carlo dose (GPUMCD) is a fast dose calculation algorithm used for treatment planning on the Unity MR-linac. Treatments for the MR-linac must be calculated quickly and accurately, and must account for two important MR-linac aspects: off-axis positions and angular transmission through the cryostat, couch and MR-coils. Therefore, the aim of this research is to quantify the system-related errors for GPUMCD calculations over the range of clinically-relevant field configurations and gantry angles. Approach. Dose profiles (crossline, inline and PDD) were measured and calculated for varying field sizes, off-axis positions and depths. Eleven different (off-axis) positions were included. The angular transmission was investigated by measuring and calculating the transmission for multiple angles, taking the cryostat, couch and coils into account. Main results. Differences between absolute point doses were found to be within 1.7% for field sizes 2 × 2 cm2 and larger. The relative dose profiles in the crossline, inline and PDD direction illustrated maximum mean dose differences of 0.9pp, 0.8pp and 0.7pp of D max in the central region for field sizes 2 × 2 cm2 and larger. The 1 × 1 cm2 field size showed larger dosimetric errors for absolute point doses and relative dose profiles. The maximum mean DTA in the penumbra was 0.7 mm. The mean difference in angular transmission ranged from −0.33% ± 0.60% to 0.27% ± 0.91% using three treatment machines. Additionally, 77.1%–93.7% of the datapoints remained within 1% transmission difference. The largest transmission differences were present at the edges of the table. Significance. This research showed that the GPUMCD algorithm provides reliable dose calculations with a low uncertainty for field sizes 2 × 2 cm2 and larger, focusing on off-axis fields and angular transmission.

Theoretical and Experimental Study of Gyro-TWT Solenoids With Displacement and Tilt

Axis coincidence consideration is necessary in the design of gyrotron solenoids; besides, radial field component strongly affects the performance of gyrodevices owing to the transverse particle–wave interaction mechanism in it, especially for high-frequency operation. In this article, the theory of multiple eccentric coils is derived. It is capable of calculating the off-axis field inside the solenoid with specified displacement and tilt. Numerical calculations are validated by 3-D electromagnetic simulation methods. Based on the measured magnet field, the displacement and tilt of a three-coil magnet system are successfully predicted with a global optimization algorithm, and then, the magnet system is well calibrated using a pair of flanges.

Curving OLED microdisplays for mixed display optical design: Simulation and experimental validation of the display mechanical curving process

Freeform optical designs deal with off-axis field curvature, which can be corrected by using curved microdisplays, with a biconic shape. In this work, we first show using simulations how curved microdisplays permit increasing image quality, but may also improve the distortion, the system’s cumbersomeness and luminous flux in visual systems. Focusing on developing functional curved microdisplays, we therefore first designed a simulation process and then an experimental set-up to explore the mechanical limitations of silicon-based curved chips. The experimental results are compared to the simulation, allowing for more insight into realistic shapes for curved silicon dies. Tests were run for three different chip preparation processes, to evaluate their influence.

Off-axis propagation algorithm for partial reconstruction of wide-angle computer-generated holograms.

A method for reconstruction of partial off-axis areas of arbitrary size for wide-angle viewing computer generated holograms is presented. Proposed method employs paraxial spherical phase factors and modified propagation kernels. This significantly reduces the numerical space-bandwidth product needed for off-axis wave field calculations, which makes it an efficient propagation method. As a result, propagated wavefields of high-off axis and large size output windows can be obtained. To that end, a phase-space analysis for obtaining the proper condition for implementing spatial-frequency zero-padding for accurate wavefield propagation is carried out. Hence, suppression of aliased components and high spatial resolution is possible. Nevertheless, proposed algorithm faces a computer memory bottleneck when reconstructing very large off-axis areas due to too extensive zero-padding needed. To solve this problem, a memory optimized tiling implementation is introduced. Utility of the developed propagation tools are proven by partial reconstructions from a high-resolution hologram. The size of the reconstructions areas ranges from 100 × 100 mm2 up to 550 × 550 mm2.

Current Loop Off Axis Field Approximations With Excellent Accuracy and Low Computational Cost

The current loop is a fundamental building block of cylindrically symmetric magnetic calculations. However, the off-axis magnetic field involves the subtraction of elliptic integrals of the first and second kind, which is computationally expensive, hard to manipulate in equations, and difficult to visualize. By conducting a different binomial series expansion on the original loop integral, a series solution is created, which can be simplified to a set of approximation functions with useful characteristics: exactly correct along the axis and at distance, while in the current loop itself the relative error is limited, computationally simple, highly accurate, and easy to visualize for behavior or symmetry. For the radial field, the first and second orders fit where the parameters are optimized to minimize the relative peak error. Note the symmetry that occurs by expressing as function of the scaled expansion term <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> , which ranges from 0 to 1, allowing maximum relative errors of 0.025 for first order and 2.9E-4 for second order. For the axial field first order, due to the subtraction of terms, the accuracy is only modest but the second order has a 9E-4 maximum relative error, with zero error at the loop, on the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z =0$ </tex-math></inline-formula> loop plane. The axial field relative error stays low within the loop, but the outside stays low for any h until it falls to 1% of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z =0$ </tex-math></inline-formula> plane value, then it loses accuracy as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> is near where the field direction reverses sign due to slight differences in the zero crossing predicated coordinate, increasing again in accuracy as <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$z$ </tex-math></inline-formula> moves further from the reveal. Higher order approximations for both radial and axial add more <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$W$ </tex-math></inline-formula> terms with increasing accuracy—for radial, the third order is 1.8E-5, fourth order is 2.3E-6, and fifth order is 4.9E-7, while axial goes as third 4.6E-5, fourth 6E-6, and fifth 1.3E-6. Relative error plots in 3-D space are presented for all orders of approximations. The simplicity of these functions suggests new ways combining loops to optimize such things as field uniformity.

Discovery of active off-axis hydrothermal vents at 9° 54′N East Pacific Rise

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50'N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50'N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.

Blanco DECam Bulge Survey (BDBS) IV: Metallicity distributions and bulge structure from 2.6 million red clump stars

ABSTRACT We present photometric metallicity measurements for a sample of 2.6 million bulge red clump stars extracted from the Blanco DECam Bulge Survey (BDBS). Similar to previous studies, we find that the bulge exhibits a strong vertical metallicity gradient, and that at least two peaks in the metallicity distribution functions appear at b &amp;lt; −5°. We can discern a metal-poor ([Fe/H] ∼ −0.3) and metal-rich ([Fe/H] ∼ +0.2) abundance distribution that each show clear systematic trends with latitude, and may be best understood by changes in the bulge’s star formation/enrichment processes. Both groups exhibit asymmetric tails, and as a result we argue that the proximity of a star to either peak in [Fe/H] space is not necessarily an affirmation of group membership. The metal-poor peak shifts to lower [Fe/H] values at larger distances from the plane while the metal-rich tail truncates. Close to the plane, the metal-rich tail appears broader along the minor axis than in off-axis fields. We also posit that the bulge has two metal-poor populations – one that belongs to the metal-poor tail of the low latitude and predominantly metal-rich group, and another belonging to the metal-poor group that dominates in the outer bulge. We detect the X-shape structure in fields with |Z| &amp;gt; 0.7 kpc and for stars with [Fe/H] &amp;gt; −0.5. Stars with [Fe/H] &amp;lt; −0.5 may form a spheroidal or ‘thick bar’ distribution while those with [Fe/H] $\gtrsim$ −0.1 are strongly concentrated near the plane.

TR12 centers in diamond as a room temperature atomic scale vector magnetometer

The family of room temperature atomic scale magnetometers is currently limited to nitrogen-vacancy centers in diamond. However, nitrogen-vacancy centers are insensitive to strong off-axis magnetic fields. In this work, we show that the well-known TR12 radiative defect in diamond, exhibits strong optically detected magnetic resonance (ODMR) signal under optical saturation. We also demonstrate that the spin system responsible for the magnetic resonance is an excited triplet state that can be coherently controlled at room temperature on a single defect level. The high optically detected magnetic resonance contrast, which is maintained even for strong off-axis magnetic fields, suggests that TR12 centers can be used for vector magnetometry even at high field.

Collimation of an anastigmat objective with three off-axis aspherical mirrors

Subject of study. Objectives for a wide variety of wavelengths from the visible to the mid-infrared are currently under development for use in small spacecraft. Because of advantages such as the ability to operate over a broad spectral range, the use of low-weight mirror fabrication technologies, and their smaller size, reflective objectives are preferred for space-based applications. This paper discusses the collimation of reflective objectives with off-axis aspheric mirrors (objectives with an off-axis field of view). Collimation of reflective objectives has traditionally been performed using interferometers operating in autocollimation mode, with collimation being verified using compensatory designs. In the case of compact multimirror objectives, it is not always possible to find space for the collimation equipment (either an interferometer with an auxiliary mirror or a holographic compensator). Method. An alternative technique was developed for sequential element-by-element collimation of a prototype multimirror objective, based on monitoring focal-spot shapes and comparing them against a calculated focal-spot shape. We discuss collimation of a four-mirror anastigmat objective, in which the first mirror is an off-axis concave ellipsoid, the second mirror is an off-axis convex hyperboloid, the third mirror is a flat, and the fourth mirror is an off-axis concave ellipsoid. The objective was collimated using a He-Ne laser (wavelength 0.63 µm), with the distribution of light in the focal spot being recorded with a television camera; collimation was verified using a continuously tunable CO laser (emission wavelength of 5.415 µm) and an HF laser (emission wavelength of 2.8–3.2 µm), and the distribution of light in the focal spots was measured with an InSb photodetector array. The focal spots were inspected using a microscope whose object plane coincided with the focal plane of the element being collimated. The entrance aperture of the objective was illuminated by a collimated laser beam parallel to the collimation axis. The quality of collimation was determined by comparing the shapes of the observed focal spots against those theoretically calculated for an ideally collimated objective. Main results. Our research revealed that a test collimation technique based on the shape of the focal spots is insufficiently sensitive to miscollimation of individual mirrors and scattering within a multiple-mirror system. In addition, the technique fails to provide the required collimation accuracy. Practical significance. The proposed technique will be used to develop specialized optoelectronic instrumentation for continuous, step-by-step verification of all optical elements in a four-mirror anastigmat to sufficient accuracy for collimation.

Beam commissioning of the first clinical biology‐guided radiotherapy system

This study reports the beam commissioning results for the first clinical RefleXion Linac. Methods: The X1 produces a 6 MV photon beam and the maximum clinical field size is 40 × 2 cm2 at source‐to‐axis distance of 85 cm. Treatment fields are collimated by a binary multileaf collimator (MLC) system with 64 leaves with width of 0.625 cm and y‐jaw pairs to provide either a 1 or 2 cm opening. The mechanical alignment of the radiation source, the y‐jaw, and MLC were checked with film and ion chambers. The beam parameters were characterized using a diode detector in a compact water tank. In‐air lateral profiles and in‐water percentage depth dose (PDD) were measured for beam modeling of the treatment planning system (TPS). The lateral profiles, PDDs, and output factors were acquired for field sizes from 1.25 × 1 to 40 × 2 cm2 field to verify the beam modeling. The rotational output variation and synchronicity were tested to check the gantry angle, couch motion, and gantry rotation. Results: The source misalignments were 0.049 mm in y‐direction, 0.66% out‐of‐focus in x‐direction. The divergence of the beam axis was 0.36 mm with a y‐jaw twist of 0.03°. Clinical off‐axis treatment fields shared a common center in y‐direction were within 0.03 mm. The MLC misalignment and twist were 0.57 mm and 0.15°. For all measured fields ranging from the size from 1.25 × 1 to 40 × 2 cm2, the mean difference between measured and TPS modeled PDD at 10 cm depth was −0.3%. The mean transverse profile difference in the field core was −0.3% ± 1.1%. The full‐width half maximum (FWHM) modeling was within 0.5 mm. The measured output factors agreed with TPS within 0.8%. Conclusions: This study summarizes our specific experience commissioning the first novel RefleXion linac, which may assist future users of this technology when implementing it into their own clinics.

Characterizing magnetically focused contamination electrons by off-axis irradiation on an inline MRI-Linac.

PurposeThe aim of this study is to investigate off‐axis irradiation on the Australian MRI‐Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field.MethodsDosimetric measurements were performed using a microDiamond detector, Gafchromic® EBT3 film, and MOSkin TM. Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2. Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source‐to‐surface distance (SSD) of 1.8 m for fields collimated centrally and off‐axis. PDD measurements were also acquired at isocenter for each off‐axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination.ResultsOff‐axis irradiation separates the majority of electron contamination from an x‐ray beam and was found to significantly reduce in‐field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm2 field, surface dose was reduced from 120.9% to 24.9%, 229.7% to 39.2%, and 355.3% to 47.3%, respectively. Monte Carlo simulations generally were within experimental error to MOSkin TM and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero.ConclusionsExperimental and simulation data were acquired for a range of field sizes to investigate off‐axis irradiation on an inline MRI‐Linac. The skin sparing effect was observed with off‐axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

Decoherence-protected quantum register of nuclear spins in diamond

Solid-state quantum registers are exceptional for storing quantum information at room temperature with long coherence time. Nevertheless, practical applications toward quantum supremacy require even longer coherence time to allow for more complex algorithms. In this work we propose a quantum register that lies in a decoherence-protected subspace to be implemented with nuclear spins nearby a nitrogen-vacancy center in diamond. The quantum information is encoded in two logical states composed of two carbon-13 nuclear spins, while an electron spin is used as ancilla for initialization and control. Moreover, by tuning an off-axis magnetic field we enable non-nuclear-spin-preserving transitions that we use for preparing and manipulating the register through stimulating Raman adiabatic passage. Furthermore, we consider more elaborated sequences to improve simultaneous control over the system yielding decreased gate time.