Tight Focal Spot Research Articles

Electroencephalographic Response of Brain Stimulation by Shock Waves from Laser Generated Carbon Nanotube Transducer.

Neuromodulation is used to treat neurological disorders. Focused ultrasound can deliver acoustic energy to local regions of the brain, including deep brain structures. In addition, it is possible to induce the activation or inhibition of nerves through parameter adjustments of focused ultrasound. Laser-generated focused ultrasound (LGFUS) has demonstrated a potential use in precise therapeutic ultrasound applications owing to the ability to produce high-pressure, broadband frequency of shock waves with a tight focal spot, resulting in confined acoustic exposure of a small area. However, there have been few studies of neurostimulation using shock waves with pulse durations of several nanoseconds. The purpose of this study is to investigate the possibility of neurostimulation by shock waves generated from a focused Carbon Nanotube (fCNT) transducer. We measured electroencephalographic (EEG) signals in three rat brains before and after shock wave stimulation and compared them in the time and frequency domains. In the time domain, the number of peaks of EEG signals was measured significantly higher after shock wave stimulation than before stimulation in all three rats. The three rats showed differences in three frequency bands: theta(4-7 Hz), alpha(8-12), and 1-30 Hz, before and after shock wave stimulation (p < 0.001). These differences in EEG signals after shock wave stimulation of three rats were confirmed mainly because of shock waves. The stimulation of a rat brain was feasible using shock waves generated by the fCNT transducer. This study provides a basis for the applications of shock waves to brain stimulation for precise targeting.

Near-infrared doublet zoom lens based on Ge2Sb2Te5 metasurface

Although numerous studies showcase the capability of metalenses to serve as crucial elements of miniaturized optical systems, tunable focusing remains an ongoing challenge. Here, we propose a doublet zoom lens based on a Ge2Sb2Te5 (GST) metasurface at a wavelength of 1.55 μm. Multistep zooming is realized by the joint control of incident circular polarization inversion and GST phase transition. Three tight focal spots, 0.91 λ, 1.09 λ and 0.89 λ, are achieved from the composite lens. The devised highly-integrated doublet lens, with advantages such as as being multi-foci, easily operated and responding quickly, can find applications in fields that require ultracompact zoom imaging and beam focusing.

Numerical simulation of a plasmonic lens for laser light focusing

In this paper, the design of a plasmonic lens in gold and silver thin films for focusing the light with radial polarization is presented. Using the finite difference time domain method the optimal parameters of the plasmonic lens design are found. It was shown that the silver plasmonic lens produces a tight focal spot with a full width at half maximum of 0.38 of the incident light wavelength.

Enhancement of second harmonic microscopy images in collagen-based thick samples using radially polarized laser beams

The visualization of collagen-based tissues imaged with second harmonic generation (SHG) microscopy strongly depends on the polarization state of the scanning laser beam. This has been often explored using spatially homogeneous states (mainly linear or circular). However, vector beams (i.e. laser beams with spatially non-uniform polarization states) have become a powerful tool in different research field and technological applications. In particular, radially polarized vector beams are known to provide a tight focal spot and lateral resolution enhancement. Here we propose the use of radially polarized beams to improve the quality of SHG microscopy images. The performance of this particular type of vector beam is compared to that obtained with a circularly polarized light beam for different non-stained specimens and depth locations. Results show that independently of the sample’s thickness, the efficiency of SHG processes is increased when using the radially polarized beam, also leading to a higher image quality. Our results open the route for using vector beams to get a better visualization of biological details within the imaged areas, especially at deeper locations.

Simulation of laser light focusing with two-layer dielectric microcyl-inders

Focusing of a linearly polarized laser beam of wavelength 633 nm with two-layer dielectric microcylinders of a circular cross-section and 2-um diameter was simulated using a finite-difference time-domain (FDTD) method, implemented using the FullWAVE software. It was shown that using a cladding whose refractive index (1.8 or 1.9) is higher than that of the core (1.45), it is possible to increase the depth of focus by a factor of 2.57 multiplied by the incident wavelength and shift the focal spot position along the optical axis away from the microcylinder boundary. It was also shown that parameters of the microcylinder could be chosen in such a way that a tighter focal spot was generated, with its full width at half maximum of intensity being 2.27 of the incident wavelength. The intensity at this focus was shown to be 1.4 times higher than that at the focus generated with a homogeneous microcylinder.

A hybrid broadband metalens operating at ultraviolet frequencies

The investigation on metalenses have been rapidly developing, aiming to bring compact optical devices with superior properties to the market. Realizing miniature optics at the UV frequency range in particular has been challenging as the available transparent materials have limited range of dielectric constants. In this work we introduce a low absorption loss and low refractive index dielectric material magnesium oxide, MgO, as an ideal candidate for metalenses operating at UV frequencies. We theoretically investigate metalens designs capable of efficient focusing over a broad UV frequency range (200–400 nm). The presented metalenses are composed of sub-wavelength MgO nanoblocks, and characterized according to the geometric Pancharatnam–Berry phase method using FDTD method. The presented broadband metalenses can focus the incident UV light on tight focal spots (182 nm) with high numerical aperture (hbox {NA}approx 0.8). The polarization conversion efficiency of the metalens unit cell and focusing efficiency of the total metalens are calculated to be as high as 94%, the best value reported in UV range so far. In addition, the metalens unit cell can be hybridized to enable lensing at multiple polarization states. The presented highly efficient MgO metalenses can play a vital role in the development of UV nanophotonic systems and could pave the way towards the world of miniaturization.

Experimental investigation of the energy backflow in the tight focal spot

Using two identical microobjectives with a numerical aperture NA = 0.95, we experimentally demonstrate that the on-axis intensity near the tight focal spot of an optical vortex with a topological charge 2 is zero for right-handed circular polarization and nonzero for left-handed circular polarization. This serves to confirm that in the latter case there is a reverse energy flow on the optical axis, as testified by a very weak local maximum (the Arago spot) detected at the center of the measured energy flow distribution, caused by diffraction of the direct energy flow by a 300 nm circle (the diameter of a reverse energy flow tube). The comparison of numerical and experimental intensity distributions shows that it is possible to determine the diameter of the reverse energy flow "tube", which is equal to the distance between the adjacent intensity nulls. For NA = 0.95 and a 532 nm incident wavelength, the diameter of the on-axis reverse energy flow "tube" is measured to be 300 nm. It is also experimentally shown that when an optical beam with second-order cylindrical polarization is focused with a lens with NA = 0.95, there is a circularly symmetric energy flow in the focus with a very weak maximum in the center (the Arago spot), whose distribution is determined by diffraction of the direct energy flow by a 300 nm circular region, where the energy flow is reverse. This also confirms that in this case, there is a reverse energy flow on the optical axis.

Versatile and scalable fabrication method for laser-generated focused ultrasound transducers.

A versatile and scalable fabrication method for laser-generated focused ultrasound transducers is proposed. The method is based on stamping a coated negative mold onto polydimethylsiloxane, and it can be adapted to include different optical absorbers that are directly transferred or synthesized in situ. Transducers with a range of sizes down to 3 mm in diameter are presented, incorporating two carbonaceous (multiwalled carbon nanoparticles and candle soot nanoparticles) and one plasmonic (gold nanoparticles) optically absorbing component. The fabricated transducers operate at central frequencies in the vicinity of 10 MHz with bandwidths in the range of 15–20 MHz. A transducer with a diameter of 5 mm was found to generate a positive peak pressure greater than 35 MPa in the focal zone with a tight focal spot of 150 μm in lateral width. Ultrasound cavitation on the tip of an optical fiber was demonstrated in water for a transducer with a diameter as small as 3 mm.

Sub-wavelength tight-focusing of terahertz waves by polarization-independent high-numerical-aperture dielectric metalens.

A focusing device is one of the key elements for terahertz applications, including homeland security, medicine, industrial inspection, and other fields. Sub-wavelength tight-focusing of terahertz waves is attractive for microscopy and spectroscopy. Flat optical lenses based on metasurfaces have shown potential in diffraction-limit focusing and advantages of ultrathin thickness and lightweight for large-aperture optics. However previously reported THz metalenses suffered from either polarization-dependency or small numerical aperture (NA), which greatly limits their focusing performance. In this paper, to achieve high-NA and polarization-free operation, we proposed a polarization-independent dielectric metasurface with a sub-wavelength period of 0.4λ. A planar terahertz lens based on such metasurface was designed for a wavelength of λ = 118.8 μm with a focal length of 100λ, a radius of 300λ, and a high NA of 0.95, which was fabricated with a silicon-on-insulator wafer. The experimental results demonstrate a tight focal spot with sub-wavelength full widths at half-maxima of 0.45λ and 0.61λ in the x and y directions, respectively, on the focal plane. In the x direction, the size of 0.45λ is even smaller than the diffraction limit 0.526λ (0.5λ/NA). Such a metalens is favorable for sub-wavelength tight-focusing terahertz waves with different polarizations, due to its polarization independence. The metalens has potential applications in THz imaging, spectroscopy, information processing, and communications, among others.

Near infrared step-zoom doublet lens based on dielectric metasurfaces

Optical metasurfaces are ultrathin planar nanostructures that manipulate wavefronts and polarization of light. Here, we propose a step-zoom doublet lens of dual focal lengths composed of two dielectric metasurfaces at 1550 nm wavelength. The highly integrated doublet lens can operate with long and short focal length modes by easily converting the handedness of the incident circular polarized light, meanwhile the focal plane remains unchanged. Two tight focal spots 0.57λ and 0.79λ are achieved with corresponding numerical aperture of 0.95 and 0.66, respectively. Compared with a conventional zoom system, this step-zoom doublet lens is ultra-thin, compact and flexibility. The ultra-compact doublet lens can be applied in zoom imaging, infrared detection and on-chip photonics components.

Sharp focusing of vector optical vortices using a metalens

Using an FDTD-simulation, we numerically show that a spiral metalens with topological charge m = 1, numerical aperture 1, and a focal length equal to the incident wavelength produces a tight near-surface focal spot shaped as a circle for incident TE-waves or a doughnut for TM-waves. It is also demonstrated numerically and theoretically that in the intensity pattern of the resulting focal spot there are local (symmetric and asymmetric) regions, which can be located both at its center or on the periphery, where the energy flux has the opposite direction. For example, the reverse energy flux is found to occur at m = 1 and left-handed circular polarization or at m = 2 and right-handed circular polarization. This effect can be used for moving a microparticle in the opposite direction to the beam propagation.

Development of Focused Transcranial Magnetic Stimulation for Rodents by Copper-Array Shields.

Transcranial magnetic stimulation (TMS) is one of the most widely used noninvasive brain stimulation method. It has been utilized for both treatment and diagnosis of many neural diseases, such as neuropathic pain and loss of function caused by stroke. Existing TMS tools cannot deliver focused electric field to targeted penetration depth even though many important neurological disorders are originated from there. A breakthrough is needed to achieve noninvasive, focused brain stimulation. We demonstrated using magnetic shield to achieve magnetic focusing without sacrificing significant amount of throughput. The shield is composed of multiple layers of copper ring arrays, which utilize induced current to generate counter magnetic fields. We experimentally set up a two-pole stimulator system to verify device simulation. A transient magnetic field probe was used for field measurements. The focusing effect highly depends on the geometric design of shield. A tight focal spot with a diameter of smaller than 5mm (plotted in Matlab contour map) can be achieved by using copper ring arrays. With properly designed array structures and rings locations, the combined original and induced counter fields can produce a tightly focused field distribution with enhanced field strength at a depth 7.5mm beyond the shield plane, which is sufficient to reach many deep and critical parts of a mouse brain.

Improvement of focusing characteristics of a surface plasmonic lens for UV wavelength

We study a surface plasmonic lens (SP lens) made as multicircular slit apertures of subwavelength widths milled in a thin metal film for generating a subwavelength focal spot in the far-field region. We previously developed the SP lens with a size of less than 4 µm for a wavelength of 375 nm, consisting of three concentric circular slit apertures in aluminum (Al) thin film. We numerically calculated the electric field distribution of the SP lens and verified a tight focal spot of subwavelength size in the far-field region. However, the experimentally measured spot size at the focal position (1.3 µm height above the SP lens surface) was more than the wavelength. The large focused spot size compared with the calculated value is attributed to the incomplete fabrication of the slit aperture delineation. In this study, we improved the design parameters and the fabrication of the SP lens for the same wavelength. We simulated the electric field distribution and confirmed that a focal spot with subwavelength size (270 nm) and enlarged focal length (2.2 µm) was experimentally verified.

Spatiotemporal drug delivery using laser-generated-focused ultrasound system

Laser-generated-focused ultrasound (LGFU) holds promise for the high-precision ultrasound therapy owing to its tight focal spot, broad frequency band, and stable excitation with minimal ultrasound-induced heating. We here report the development of the LGFU as a stimulus for promoted drug release from microgels integrated with drug-loaded polymeric nanoparticles. The pulsed waves of ultrasound, generated by a carbon black/polydimethylsiloxane (PDMS)-photoacoustic lens, were introduced to trigger the drug release from alginate microgels encapsulated with drug-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles. We demonstrated the antibacterial capability of this drug delivery system against Escherichia coli by the disk diffusion method, and antitumor efficacy toward the HeLa cell-derived tumor spheroids in vitro. This novel LGFU-responsive drug delivery system provides a simple and remote approach to precisely control the release of therapeutics in a spatiotemporal manner and potentially suppress detrimental effects to the surrounding tissue, such as thermal ablation.

Optical trapping Rayleigh dielectric particles with focused partially coherent dark hollow beams

The focusing properties of coherent and partially coherent dark hollow beams (DHBs) through a paraxial ABCD optical system are theoretically investigated. It is found that the evolution behavior of the intensity distribution of focused partially coherent DHBs is closely related to their spatial coherence. The radiation forces (RFs) of focused coherent and partially coherent DHBs acting on a Rayleigh dielectric particle are also theoretically investigated. Numerical results show that the coherent and partially coherent DHBs can be focused into a tight focal spot, which can be used to stably trap a Rayleigh dielectric particle with high refractive index at the focus point. The influences of different beam parameters, including the spatial coherence, beam waist width, beam order, and hollow parameter of partially coherent DHBs, on the RFs and the trap stiffness are analyzed in detail. Finally, the stability conditions for effective trapping particles are also discussed.

Micro-ultrasonic cleaving of cell clusters by laser-generated focused ultrasound and its mechanisms

Laser-generated focused ultrasound (LGFU) is a unique modality that can produce single-pulsed cavitation and strong local disturbances on a tight focal spot (<100 μm). We utilize LGFU as a non-contact, non-thermal, high-precision tool to fractionate and cleave cell clusters cultured on glass substrates. Fractionation processes are investigated in detail, which confirms distinct cell behaviors in the focal center and the periphery of LGFU spot. For better understanding of local disturbances under LGFU, we use a high-speed laser-flash shadowgraphy technique and then fully visualize instantaneous microscopic processes from the ultrasound wave focusing to the micro-bubble collapse. Based on these visual evidences, we discuss possible mechanisms responsible for the focal and peripheral disruptions, such as a liquid jet-induced wall shear stress and shock emissions due to bubble collapse. The ultrasonic micro-fractionation is readily available for in vitro cell patterning and harvesting. Moreover, it is significant as a preliminary step towards high-precision surgery applications in future.

Carbon-Nanotube Optoacoustic Lens for Focused Ultrasound Generation and High-Precision Targeted Therapy

We demonstrate a new optical approach to generate high-frequency (>15 MHz) and high-amplitude focused ultrasound, which can be used for non-invasive ultrasound therapy. A nano-composite film of carbon nanotubes (CNTs) and elastomeric polymer is formed on concave lenses, and used as an efficient optoacoustic source due to the high optical absorption of the CNTs and rapid heat transfer to the polymer upon excitation by pulsed laser irradiation. The CNT-coated lenses can generate unprecedented optoacoustic pressures of >50 MPa in peak positive on a tight focal spot of 75 μm in lateral and 400 μm in axial widths. This pressure amplitude is remarkably high in this frequency regime, producing pronounced shock effects and non-thermal pulsed cavitation at the focal zone. We demonstrate that the optoacoustic lens can be used for micro-scale ultrasonic fragmentation of solid materials and a single-cell surgery in terms of removing the cells from substrates and neighboring cells.

Sub-wavelength focal spot with long depth of focus generated by radially polarized, narrow-width annular beam

We demonstrate the formation of a sub-wavelength focal spot with a long depth of focus using a radially polarized, narrow-width annular beam. Theoretical analysis predicts that a tighter focal spot (approximately 0.4 lambda) and longer depth of focus (more than 4 lambda) can be formed by a longitudinal electric field when the width of the annular part of the beam is decreased. Experimental measurements using a radially polarized beam from a photonic crystal laser agree well with these predictions. Tight focal spots with long depths of focus have great potential for use in high-tolerance, high-resolution applications in optical systems.

Nanofocusing by phase delayed plasmonic nanostructures illuminated with a linearly polarized light

We report the near-field nanofocusing through two types of plasmonic nanostructures consisting of annular grooves and annular slit, respectively. The radius of one half of each circle is designed to be bigger than the other half by half wavelength of the surface plasmon polaritons. By illuminating the structures with linearly polarized light, one single tight focal spot was detected by the scanning near-field optical microscope. Compared with the method of using radially polarized light, this method does not need to keep the center of the illuminating light right on the center of the structure, hence is much easier to use. Furthermore, theoretical simulations based on the finite-difference time-domain method testified the experimental results.

Near-field focusing with optical phase antennas

We investigate the near-field focusing properties of three-dimensional phase antennas consisting of concentric rings designed to have source and image spots separated by several microns from the lens. Tight focal spots are obtained for silicon or gold rings patterned in a silica matrix. We analyze in detail the dependence of the performance of these lenses on geometrical parameters such as the number of rings, the ring thickness, and the focal distance. Subwavelength focal spots are found to form at distances of tens of wavelengths from the lens, thus suggesting applications to remote sensing and penlight microscopy and lithography.