Bow-tie Configuration Research Articles

Circular Bragg grating single-photon source design exploiting extreme dielectric confinement

Abstract We theoretically propose a single-photon source design based on a circular Bragg grating microcavity incorporating two air triangles in a bowtie configuration. Our proposed design allows for extreme light confinement with a Purcell factor of approximately 166 with a bandwidth of ~9 nm for a triangle separation of 5 nm combined with a collection efficiency above 80 % . The Purcell factor is enhanced by a factor of 8 relative to the reference cavity thanks to the reduction of the mode volume. Furthermore, we demonstrate the critical role of the dipole emitter polarization orientation and spatial position in achieving optimal Purcell enhancement.

Materials for Dielectric Nanotweezers in the Near-Visible Region

Nanotweezers made of dielectric nanoantennas have enabled optical trapping and manipulation of nanoscale objects. Commonly used dielectric materials with a high refractive index, notably silicon (Si), suffer from absorption at visible wavelengths and thus Joule heating of the structure and the trapped object. They are thus normally considered unsuitable for trapping applications in this wavelength range. In this work, dielectric materials with low absorption at 785 nm are studied to minimize Joule heating. We consider a bowtie configuration, which is simulated for four different dielectric materials and optimized for maximum electric field enhancement. The results show a significant electric field enhancement in the bowtie gap, which can act as a “hotspot” for optical trapping and enhancement of Raman scattering of a single nanosphere. We numerically investigate optical trapping of nanospheres and compare the results with an analytical expression. Three nanospheres with different refractive indices are used as examples. We further study the temperature increase and the thermally induced flow around the bowtie nanoantenna to understand if and how the trapping is perturbed by Joule heating. We find that nanoantennas made by Si on a Si substrate, although absorbing, can be used for trapping quantum dots (QDs) and polystyrene (PS) beads. The required input intensities for stable trapping are approximately 10 and 50 mW/μm2, for QDs and PS beads, respectively, inducing temperature increases of ∼1.4 K and ∼7 K, respectively. For transparent materials, the input intensity is the limiting factor, and gallium phosphide (GaP) requires the lowest input intensity due to its high refractive index. However, Si is a far more common and standard material and thus gives a significantly easier fabrication process.

Microsecond pulsed yellow emission by intracavity doubled optically pumped two-chip VECSEL

A quasi-continues-wave (QCW) 100 μs pulsed yellow laser at 589 nm, based on intracavity frequency doubling (ICFD) of identical two-chip vertical-external-cavity surface-emitting laser (VECSEL) with bow-tie configuration, is demonstrated. Each VECSEL chip is optically QCW-pumped by an 808 nm laser diode, emitting a fundamental wavelength around 1178 nm. A birefringence filter is inserted in the laser cavity to select the linear p-polarized oscillation and tune the operation wavelength. Using a lithium-triborate (LiB3O5) as the frequency doubled crystal, a total frequency doubled output power of 264 mW at 589 nm is achieved, operating at a pulse duration of 194 μs and a pulse repetition rate of 1 kHz. A good beam quality factor is measured to be M 2 = 2.15. These results represent a novel scheme for the QCW multiple gain chips VECSEL ICFD system, opening up a new perspective for the realization of impact long pulse 589 nm laser for sodium guide-star.

Multiple slot modules for high field magnetic resonance imaging array coils.

Mitigating coupling effects between coil elements represents a continuing challenge. Here, we present a 16-bowtie slot volume coil arranged in eight independent dual-slot modules without the use of any decoupling circuits. Two electrically short "bowtie" slot antennas were used to form a "module." A bowtie configuration was chosen because electromagnetic modeling results show that bowtie slots exhibit improved efficiency when compared to thin rectangular slots. An eight-module volume coil was evaluated through electromagnetic modeling, bench tests, and MRI experiments at 4.7T. Bench tests indicate that worst-case coupling between modules did not exceed -14.5dB. MR images demonstrate well-localized patterns about single excited modules confirming the low coupling between modules. Homogeneous MR images were acquired from a synthesized quadrature birdcage transmit mode. MRI experiments show that the RF power requirements for the proposed coil are 9.2 times more than a birdcage coil. Whereas from simulations performed to assess the proposed coil losses, the total power dissipated in the phantom was 1.1 times more for the birdcage. Simulation results at 7T reveal an equivalent B1 + homogeneity when compared with an eight-dipole coil. Although exhibiting higher RF power requirements, as a transmit coil when the power availability is not a restriction, the inherently low coupling between electrically short slots should enable the use of many slot elements around the imaging volume. The slot module described in this paper should be useful in the design of multi-channel transmit coils.

Diamond magnetometer enhanced by ferrite flux concentrators.

Magnetometers based on nitrogen-vacancy (NV) centers in diamond are promising room-temperature, solid-state sensors. However, their reported sensitivity to magnetic fields at low frequencies (≾1 kHz) is presently ≿10 pT s1/2, precluding potential applications in medical imaging, geoscience, and navigation. Here we show that high-permeability magnetic flux concentrators, which collect magnetic flux from a larger area and concentrate it into the diamond sensor, can be used to improve the sensitivity of diamond magnetometers. By inserting an NV-doped diamond membrane between two ferrite cones in a bowtie configuration, we realize a ~250-fold increase of the magnetic field amplitude within the diamond. We demonstrate a sensitivity of ~0.9 pT s1/2 to magnetic fields in the frequency range between 10 and 1000 Hz. This is accomplished using a dual-resonance modulation technique to suppress the effect of thermal shifts of the NV spin levels. The magnetometer uses 200 mW of laser power and 20 mW of microwave power. This work introduces a new degree of freedom for the design of diamond sensors by using structured magnetic materials to manipulate magnetic fields.

Multicomponent gas detection based on concise CW-cavity ring-down spectroscopy with a bow-tie design.

Concise open-path continuous-wave cavity ring-down spectroscopy (CW-CRDS) with a bow-tie cavity structure is demonstrated in the single- and dual-optical-path experiments for multicomponent gas detection, e.g., greenhouse gas concentration evaluation in ambient air. Owing to its features of optical feedback suppression and small free spectral range (FSR), the bow-tie configuration shows its special advantages in the realization of both a compact arrangement and two counter-propagating non-interference optical paths. The minimum of the Allan deviation reaches 1.6×10-10 cm-1 for an integration time of 100s, corresponding to the noise equivalent absorption coefficient of 1.6×10-9 cm-1 Hz-1/2. The detection sensitivity of methane is deduced to be 0.9 ppbv with its absorption cross section of 1.48×10-20 cm2/molecule in the 512 decays averaging mode. A wavelength-correction method is proposed to reduce by about 30% the uncertainty in the measurements caused by the deviation in the wavelength resonance between incident laser and ring-down cavity. The concentrations of greenhouse gases in ambient air are measured by the open-path CW-CRDS with the uncertainties of 0.02, 100, and 10 ppmv for CH4, H2O, and CO2, respectively.

Injection-seeded single frequency 2.05 μm output by ring cavity optical parametric oscillator

An injection-seeded single-resonant optical parametric oscillator (SROPO) with single frequency nanosecond pulsed 2.05 μm wavelength output is presented. Based on two potassium titanyl phosphate crystals and pumped by a 1064 nm single frequency laser pulse, injection seeding is performed successfully by using the ramp-hold-fire technique in a ring cavity with a bow-tie configuration. The SROPO provides 2.65 mJ single frequency signal pulse output with a 17.6 ns pulse duration at a 20 Hz repetition rate. A near-diffraction-limited beam is achieved with a beam quality factor M2 of about 1.2. The spectrum linewidth of the signal pulse is around 26.4 MHz, which is almost the Fourier-transform-limited value.

WE‐EF‐207‐05: Monte Carlo Dosimetry for a Dedicated Cone‐Beam CT Head Scanner

Purpose:Cone‐Beam CT (CBCT) is an attractive platform for point‐of‐care imaging of traumatic brain injury and intracranial hemorrhage. This work implements and evaluates a fast Monte‐Carlo (MC) dose estimation engine for development of a dedicated head CBCT scanner, optimization of acquisition protocols, geometry, bowtie filter designs, and patient‐specific dosimetry.Methods:Dose scoring with a GPU‐based MC CBCT simulator was validated on an imaging bench using a modified 16 cm CTDI phantom with 7 ion chamber shafts along the central ray for 80–100 kVp (+2 mm Al, +0.2 mm Cu). Dose distributions were computed in a segmented CBCT reconstruction of an anthropomorphic head phantom with 4×10⁵ tracked photons per scan (5 min runtime). Circular orbits with angular span ranging from short scan (180° + fan angle) to full rotation (360°) were considered for fixed total mAs per scan. Two aluminum filters were investigated: aggressive bowtie, and moderate bowtie (matched to 16 cm and 32 cm water cylinder, respectively).Results:MC dose estimates showed strong agreement with measurements (RMSE<0.001 mGy/mAs). A moderate (aggressive) bowtie reduced the dose, per total mAs, by 20% (30%) at the center of the head, by 40% (50%) at the eye lens, and by 70% (80%) at the posterior skin entrance. For the no bowtie configuration, a short scan reduced the eye lens dose by 62% (from 0.08 mGy/mAs to 0.03 mGy/mAs) compared to full scan, although the dose to spinal bone marrow increased by 40%. For both bowties, the short scan resulted in a similar 40% increase in bone marrow dose, but the reduction in the eye lens was more pronounced: 70% (90%) for the moderate (aggressive) bowtie.Conclusions:Dose maps obtained with validated MC simulation demonstrated dose reduction in sensitive structures (eye lens and bone marrow) through combination of short‐scan trajectories and bowtie filters.Xiaohui Wang and David Foos are employees of Carestream Health

Bow-Tie Hybrid Plasmonic Waveguides

We propose a hybrid plasmonic waveguide that incorporates a semiconductor—insulator–metal bow-tie configuration within a low-index gap guarded by semiconductor and metallic nanostructures. Ultratight field confinement ( $A_{{\rm eff}} \sim \lambda ^2 /3100 - \lambda ^2 \!/\!120$ ) in conjunction with reasonable propagation distance ( $L \sim$ 56-138 μm) can be achieved simultaneously at a telecommunication wavelength. Compared to a conventional hybrid waveguide, the effective mode area and propagation loss of a typical bow-tie hybrid configuration are reduced by 2–89% and 34–62%, respectively, whereas the figure of merit is enhanced by 53–700%, along with increased power ratio inside the gap region for small gap cases. Studies on fabrication tolerance, waveguide crosstalk, and loss compensation reveal its robust property for practical implementations and remarkable feasibility to realize ultracompact passive and active components. Moreover, we also reveal the broadband feature of the waveguide and show that its concept can be applicable to various other metallic configurations as well, which may open up possibilities for the inventions of numerous high-performance plasmon waveguides and components.

Modeling of a Dielectric Elastomer Bender Actuator

The current smallest self-contained modular robot uses a shape memory alloy, which is inherently inefficient, slow and difficult to control. We present the design, fabrication and demonstration of a module based on dielectric elastomer actuation. The module uses a pair of bowtie dielectric elastomer actuators in an agonist-antagonist configuration and is seven times smaller than previously demonstrated. In addition, we present an intuitive model for the bowtie configuration that predicts the performance with experimental verification. Based on this model and the experimental analysis, we address the theoretical limitations and advantages of this antagonistic bender design relative to other dielectric elastomer actuators.

Contents: (Small 6/2014)

The self-assembly of silver nanoparticles into a bow-tie antenna configuration is achieved with the DNA origami method. Instead of complicated particle geometries, spherical silver nanoparticles are used. Formation of the structures in high yields is verified with transmission electron microscopy and agarose gel electrophoresis. According to finite-difference time-domain simulations, the antenna configuration could be used as a DNA sensor.

Self‐Assembled Silver Nanoparticles in a Bow‐Tie Antenna Configuration

The self-assembly of silver nanoparticles into a bow-tie antenna configuration is achieved with the DNA origami method. Instead of complicated particle geometries, spherical silver nanoparticles are used. Formation of the structures in high yields is verified with transmission electron microscopy and agarose gel electrophoresis. According to finite-difference time-domain simulations, the antenna configuration could be used as a DNA sensor.

High Conversion Efficiency and Power Stability of 532 nm Generation from an External Frequency Doubling Cavity

We present a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation (SHG) with a periodically poled KTP crystal (PPKTP). The cavity is a bow-tie ring configuration with a unitized structure. When the impedance matching is optimized, the coupling efficiency of the fundamental is as high as 95%. Taking into account both the high power output of the second harmonic and the stability of the system, we obtain over 500 mW green passing through the output cavity mirror, corresponding to a net conversion efficiency higher than 75.2%. Under these operating conditions, the power stability is better than ±0.25% during 5 h. It is the highest conversion efficiency and power stability ever produced in the bow-tie ring cavity with PPKTP for 532 nm generation.

Extreme optical nonlinearities in chalcogenide glass fibers embedded with metallic and semiconductor nanowires

A nanostructured chalcogenide-metal optical fiber is proposed. This hybrid nanofiber is embedded with a periodic array of triangular-shaped deep-subwavelength metallic (or semiconductor) nanowires set up in a bowtie configuration. Calculations show that the proposed nanostructured fiber supports a guided collective plasmonic mode enabling both subwavelength field confinement and extreme nonlinear light-matter interactions. A scheme is also proposed whereby the large linear absorption losses are compensated via nonlinear optical gain when semiconducting single-wall carbon nanotubes are used in place of metal.

Extreme nonlinear optical enhancement in chalcogenide glass fibers with deep-subwavelength metallic nanowires

A nanostructured chalcogenide-metal optical fiber is proposed. This hybrid nanofiber is embedded with a periodic array of triangular-shaped deep-subwavelength metallic nanowires set up in a bowtie configuration. Our simulations show that the proposed nanostructured fiber supports a guided plasmonic mode enabling both subwavelength field confinement and extreme nonlinear light-matter interactions, much larger than a bare chalcogenide nanowire of comparable diameter. This is all achieved with less than 3% by volume of metal content.

Isolated bilateral lateral geniculate infarction producing bow-tie visual field defects

Case report: We examined a unique case of stroke in a 29-year-old woman. Magnetic resonance imaging revealed isolated bilateral acute infarctions of the lateral geniculate bodies (LGB). Visual field testing demonstrated incongruous but homonymous bilateral visual field loss in a bow-tie configuration. Comments: Isolated bilateral damage to the LGB is a rare event. Literature review has revealed only 4 previous incidents of bilateral damage to both LGB. We present a literature review and a case of bilateral LGB infarction producing visual field defects suggestive of bilateral lateral choroidal artery involvement in an individual with classic migraine.

Design optimisation for high-power high-brightness parabolic bow-tie laser diodes

Optical sources that combine high power with high brightness are in great demand for various applications. Although sophisticated device designs have been demonstrated to have such desirable characteristics, to contain the device costs tapered geometry devices seem to provide attractive alternatives. Further, particularly for high-power operation, the bow-tie configuration is effective for further reducing the risk of catastrophic optical damage and for obtaining a good quality output beam. The authors present the detailed analysis that has been necessary to optimise both the material layer structure and the tapered laser geometry to achieve simultaneously high power and high brightness. Following the design guidelines, several 980 nm parabolic bow-tie lasers have been fabricated and characterised in-house, obtaining output powers in excess of 700 mW per facet in a diffraction-limited (2.7°) beam measured without the use of external lenses.

Prevalence of abnormal findings in magnetic resonance images of asymptomatic knees

One hundred fifteen asymptomatic Japanese volunteers aged from 13 to 76 years were examined by magnetic resonance imaging (MRI) to evaluate age-related meniscal degeneration and to determine the prevalence of discoid menisci. Each meniscus was graded in the anterior and posterior portions according to intrameniscal MRI signals. Discoid meniscus was diagnosed if a bow-tie configuration was noted on three or more contiguous sagittal sections. Meniscal abnormalities on MRI became more prevalent with age in both men and women. The posterior horn of the medial meniscus showed a significantly higher prevalence of degeneration than other parts of the meniscus. Discoid menisci were noted in 15 subjects (13% prevalence), always representing the lateral meniscus. Subchondral changes were observed in 13 subjects more than 40 years old, mostly women, and were located in the medial compartment. These abnormalities were not correlated with severity of degeneration in the posterior portion of the medial meniscus. This study demonstrates considerable prevalence of meniscal abnormalities in asymptomatic Japanese subjects.

Mixing in TlCaBaCuO superconducting films at 61 GHz

The results of mixing at 61 GHz using weak-link, grain-boundary junctions in Tl/sub 2/CaBa/sub 2/Cu/sub 2/O/sub 8/ thin films are discussed. Films were prepared using laser deposition of precursor material on MgO substrates followed by high-temperature postprocessing. Mixing experiments were conducted at 75 K, at an IF frequency of 1 GHz, using a bow-tie antenna configuration. The signal-to-noise ratio for the system at the IF frequency was approximately 10/sup 4/.