SignificanceOptical tweezers (OTs) have emerged as an essential technique for manipulating nanoscopic particulates and biological specimens with sub-micron precision and have revolutionized various fields, including biology and colloidal physics. However, traditional optical trapping techniques often rely on moderate- to high-power continuous wave (CW) lasers, which can introduce unwanted thermal effects and photodamage to delicate samples. An innovative alternative has emerged through the utilization of femtosecond (fs) lasers at ultra-low average powers on the order of tens of microwatt. Unexpectedly overlooked until now, this method enables the direct trapping and manipulation of cells without relying on functionalized spheres.AimWe aim to compare the trap stiffness of CW and fs lasers in an unexplored average power regime (sub-1 mW) on cells within the intermediate-size regime.ApproachA CW or fs laser is used to trap cells in an inverted microscope setup. We trap five different pathogenic bacteria with different morphologies to compare trap stiffness.ResultsWe find that fs laser-assisted selective holding with ultra-low power (FLASH-UP) exhibits five times greater trap stiffness than CW-based OTs and can trap at lower intensities. Furthermore, we demonstrate that FLASH-UP does not impact cell motility.ConclusionsFLASH-UP displays higher trap stiffness at average powers below 1 mW and does not impact cell functionality. These results pave the way for ultra-low-power trapping of cells for applications in sorting, bio-sensing, in vivo cell manipulation, and single-cell analysis.
Optical tweezers (OT) has proven to be an indispensable tool for elucidating phenomena in colloidal physics and for biomedical applications. Additionally, OT has been used to apply sub-piconewton forces on microscopic particles, for example in cells, as well as to measure displacements with nanometer resolution to extrapolate mechanical properties. Recently, an OT platform based on light sheet microscopy with a continuous wave laser has been developed to trap microscopic dielectric particles. However, the reduced gradient force resulting from the light sheet intensity distribution produces a trap stiffness an order of magnitude lower than its traditional circularly symmetric Gaussian counterpart. As a result, a high laser power, on the order of 50 mW is required, which risks phototoxicity for biological applications. In this work, we first compare the trap stiffnesses of continuous wave and femtosecond pulsed laser sources on dielectric particles in sub-1 mW scale. Next, we demonstrate the OT of dielectric spheres using a flat-top light sheet generated by a femtosecond pulsed laser source utilizing average powers as low as 1 mW. We propose leveraging flat-top light sheet OT to characterize the local and average mechanical properties of biological specimens.
: We present a unique route to realize a novel class of intrinsically chiral nano-helices that achieves the critical goal of demonstrating high-speed control of opto-chirality over 50,000 cycles via the optically active and tunable Ge2Sb2Te5 nanopatterned medium.
Chalcogenide phase change materials (PCMs) are uniquely suited for spectral tuning applications due to their contrasting dielectric material properties. Recent headway has been made towards realizing tunable photonic devices using twodimensional, sub-wavelength resonators by carefully designing geometries that optimize optical, electrical, and thermal performances using multi-physics analyses and machine learning. In this paper, we tackle two other essential aspects for creating application-specific, tunable PCM devices: (1) scalability of the device size and (2) high-throughput fabrication techniques. We employ a deep ultraviolet (DUV) stepper projection lithography to manufacture over 100 densely packed GST metasurfaces, each with a sample size of 5×7 mm2, all on a 4-inch Al2O3 wafer. These metasurface structures were discovered using artificial neural network (ANN) techniques and confirmed by finite-difference-time domain calculations. The primary structures under investigation were nanobar configurations enabling amplitude modulation at short-wave infrared wavelengths to realize efficient optical switches for free space optical multiplexing. The DUV fabrication technique can easily be extended to other metasurface geometries to demonstrate multi-functional, non-volatile photonic devices.
A magnetron co-sputtering system was used for producing nickel-doped Ge2Sb2Te5 (GST-Ni) thin films. The nickel content in the thin film was adjusted by the ratio of the plasma discharge power applied to the GST and nickel targets, as well as a physical shuttering technique to further control the nickel deposition rate. The doping concentration of the film was confirmed using Energy Dispersion Spectroscopy (EDS) technique. Results from a four-point probe measurement indicate that the nickel doping can reduce the resistivity of GST in the amorphous state by nearly three orders of magnitude. The dopant’s influence on crystallization behavior was studied by analyzing X-Ray Diffraction (XRD) patterns of the pure GST and GST-Ni at different annealing temperatures. To examine the structural changes due to the nickel dopant, the thin films were investigated with the aid of Raman scattering. Additionally, we extracted the optical constants for both the amorphous and crystalline states of undoped-GST and GST-Ni films by ellipsometry. The results indicate that at low doping concentrations nickel does not appreciably affect the optical constants, but dramatically improves the electrical conductivity. Therefore, nickel-doping of GST a viable method for designing optical devices for lower operating voltages at higher switching speeds.
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