A Broadband High-Diffraction-Efficiency Electro-Optic Bragg Deflector Based on Monolithic Dual-Grating Periodically-Poled Lithium Niobate

Electro-optic (EO) Bragg deflectors have been extensively used in a variety of applications. Recent developments show that bandwidths and deflection efficiencies, as well as angular bandwidths, would significantly limit the utilization of EO Bragg deflectors, especially for applications which need strong focusing, such as intra-cavity applications. In this paper, we introduce a broadband EO Bragg deflector based on periodically-poled lithium niobate with a monolithic dual-grating design. We analyzed the deflection properties of this device by using a modified coupled wave theory and showed that this device can be still efficient for a small beam radius under strong focusing, whereas a single-grating one becomes very inefficient. Using a 1064-nm laser beam with a 100-μm beam radius, we obtained a 74% deflection efficiency with a 190-V bias voltage with a 0.5-mm-thick and 7.5-mm-long dual-grating sample. The acceptance angle for the Bragg condition of this device is as large as a few tens of mrad. The potential bandwidth of this device exceeds 500 nm if the proper operation region is chosen.

Anomalous Beam Transport through Gabor (Plasma) Lens Prototype

An electron plasma lens is a cost-effective, compact, strong-focusing element that can ensure efficient capture of low-energy proton and ion beams from laser-driven sources. A Gabor lens prototype was built for high electron density operation at Imperial College London. The parameters of the stable operation regime of the lens and its performance during a beam test with 1.4 MeV protons are reported here. Narrow pencil beams were imaged on a scintillator screen 67 cm downstream of the lens. The lens converted the pencil beams into rings that show position-dependent shape and intensity modulation that are dependent on the settings of the lens. Characterisation of the focusing effect suggests that the plasma column exhibited an off-axis rotation similar to the m=1 diocotron instability. The association of the instability with the cause of the rings was investigated using particle tracking simulations.

Ultrahigh numerical aperture meta-fibre for flexible optical trapping

Strong focusing on diffraction-limited spots is essential for many photonic applications and is particularly relevant for optical trapping; however, all currently used approaches fail to simultaneously provide flexible transportation of light, straightforward implementation, compatibility with waveguide circuitry, and strong focusing. Here, we demonstrate the design and 3D nanoprinting of an ultrahigh numerical aperture meta-fibre for highly flexible optical trapping. Taking into account the peculiarities of the fibre environment, we implemented an ultrathin meta-lens on the facet of a modified single-mode optical fibre via direct laser writing, leading to a diffraction-limited focal spot with a record-high numerical aperture of up to NA ≈ 0.9. The unique capabilities of this flexible, cost-effective, bio- and fibre-circuitry-compatible meta-fibre device were demonstrated by optically trapping microbeads and bacteria for the first time with only one single-mode fibre in combination with diffractive optics. Our study highlights the relevance of the unexplored but exciting field of meta-fibre optics to a multitude of fields, such as bioanalytics, quantum technology and life sciences.

Constraints on small-scale heterogeneity in Earth's inner core determined from transmitted P waves

<div> <p>Scattered waves composing the coda of the PKiKP wave, reflected by the boundary of Earth's inner core at pre-critical range, reveal the existence of small-scale heterogeneity in the uppermost inner core. Since the shape this coda envelope is relatively insensitive to intrinsic viscoelastic attenuation, seismograms synthesized using the Axisem code (Nissen-Meyer, 2014) are exploited to determine whether heterogeneity spectra consistent with the coda envelope of PKiKP can contribute to the attenuation observed in long range PKIKP waves transmitted through the deeper inner core.  Peng et al. (2008) have shown that a range of possible parameters describing an exponential autocorrelation of small-scale heterogeneity can fit observed PKiKP coda envelopes, with the rms P velocity fluctuation trading off against the corner scale length parameter "a" of the heterogeneity spectrum. Testing the effects of a series of "a's" and velocity fluctuations that fit PKIKP coda envelopes we determined upper bounds to “a” and the rms P velocity fluctuation below 300 km depth in the inner core.  Parameter combinations of  “a” > 2 km and rms dVp/Vp > 2% can be eliminated from consideration because they produce too strong a coda following PKIKP. In the antipodal range (178<sup>o</sup> to 180<sup>o</sup>) we found that there exists a strong focusing of multiply scattered waves affecting the pulse width and coda of PKIKP.  The parameter combination "a"= 2 km and rms=1.2% produces a strong PKIKP coda, which is not observed in antipodal data.  This, coupled with the fact that Axisem ignores out-of-plane scattering, suggests that the attenuation of PKIKP observed beyond 160<sup>o</sup> is dominated by viscoelastic rather than scattering attenuation and that the rms P velocity fluctuation must decrease by at least a factor of 2 below 300 km to be consistent with the coda of antipodal PKIKP waves.</p> </div><div> <p>Peng, Z., Koper, K.D., Leyton, J.E., Shearer, P., J<em>. Geophys. Res.</em>, 113(B9), B09312, doi:10.1029/2007JB/005412, 2008.</p> </div><p>Nissen-Meyer, T., van Driel, M., Stähler, S. C., Hosseini, K., Hempel, S., Auer, L., Colombi, A., and Fournier, A. <em>Solid Earth</em>, 5, 425-446, https://doi.org/10.5194/se-5-425-2024, 2014.</p><p> </p>

Fast (non-)diffusive Quasi-Geostrophic Magneto-Coriolis Modes in the Earth's core

<p> Fast changes of Earth's magnetic field could be explained by inviscid and diffusion-less quasi-geostrophic (QG) Magneto-Coriolis modes. We present a hybrid QG model with columnar flows and three-dimensional magnetic fields and find modes with periods of a few years at parameters relevant to Earth's core. These fast Magneto-Coriolis modes show strong focusing of their kinetic and magnetic energy in the equatorial region, while maintaining a relatively large spatial structure along the azimuthal direction. Their properties agree with some of the observations and inferred core flows. We find additionally, in contrast to what has been assumed previously, that these modes are not affected significantly by magnetic diffusion. The model opens a new way of inverting geomagnetic observations to the flow and magnetic field deep within the Earth's outer core.</p>

Capillary-force-induced collapse lithography for controlled plasmonic nanogap structures

The capillary force effect is one of the most important fabrication parameters that must be considered at the micro/nanoscale because it is strong enough to deform micro/nanostructures. However, the deformation of micro/nanostructures due to such capillary forces (e.g., stiction and collapse) has been regarded as an undesirable and uncontrollable obstacle to be avoided during fabrication. Here, we present a capillary-force-induced collapse lithography (CCL) technique, which exploits the capillary force to precisely control the collapse of micro/nanostructures. CCL uses electron-beam lithography, so nanopillars with various shapes can be fabricated by precisely controlling the capillary-force-dominant cohesion process and the nanopillar-geometry-dominant collapse process by adjusting the fabrication parameters such as the development time, electron dose, and shape of the nanopillars. CCL aims to achieve sub-10-nm plasmonic nanogap structures that promote extremely strong focusing of light. CCL is a simple and straightforward method to realize such nanogap structures that are needed for further research such as on plasmonic nanosensors.

Polarization Insensitive, Broadband, Near Diffraction-Limited Metalens in Ultraviolet Region

Metasurfaces in the ultraviolet spectrum have stirred up prevalent research interest due to the increasing demand for ultra-compact and wearable UV optical systems. The limitations of conventional plasmonic metasurfaces operating in transmission mode can be overcome by using a suitable dielectric material. A metalens holds promising wavefront engineering for various applications. Metalenses have developed a breakthrough technology in the advancement of integrated and miniaturized optical devices. However, metalenses utilizing the Pancharatnam–Berry (PB) phase or resonance tuning methodology are restricted to polarization dependence and for various applications, polarization-insensitive metalenses are highly desirable. We propose the design of a high-efficiency dielectric polarization-insensitive UV metalens utilizing cylindrical nanopillars with strong focusing ability, providing full phase delay in a broadband range of Ultraviolet light (270–380 nm). The designed metalens comprises Silicon nitride cylindrical nanopillars with spatially varying radii and offers outstanding polarization-insensitive operation in the broadband UV spectrum. It will significantly promote and boost the integration and miniaturization of the UV photonic devices by overcoming the use of Plasmonics structures that are vulnerable to the absorption and ohmic losses of the metals. The focusing efficiency of the designed metalens is as high as 40%.

Strongly Focused Circularly Polarized Optical Vortices Regulated by a Fractal Conical Lens

In this paper, a recently-proposed pure-phase optical element, the fractal conical lens (FCL), is introduced for the regulation of strongly-focused circularly-polarized optical vortices in a high numerical aperture (NA) optical system. Strong focusing characteristics of circularly polarized optical vortices through a high NA system in cases with and without a FCL are investigated comparatively. Moreover, the conversion between spin angular momentum (SAM) and orbital angular momentum (OAM) of the focused optical vortex in the focal vicinity is also analyzed. Results revealed that a FCL of different stage S could significantly regulate the distributions of tight focusing intensity and angular momentum of the circularly polarized optical vortex. The interesting results obtained here may be advantageous when using a FCL to shape vortex beams or utilizing circularly polarized vortex beams to exploit new-type optical tweezers.