Nanophotonic structures can enhance light-mater interaction at nanoscales helping to improve the detection sensitivity of biosensors. In this work, we present a sandwich-type immunosensor by combining a nanophotonic resonant waveguide grating (RWG) structure and upconverting nanoparticles (UCNPs). UCNPs are used to label a target biomarker captured by capture antibody molecules immobilized on the surface of the RWG structure, which is used to enhance the upconversion fluorescence (UCF) of UCNPs through excitation resonance. The immunosensor has an extremely low limit of detection (LOD) in sub-fg/mL level and a detection range of six orders of magnitude and can be used to detect a variety of biomarkers such as cardiac troponin I, tau protein and phosphated-tau protein, etc. The LOD of the immunosensor is greatly reduced due to the increased UCF of UCNPs and the reduction of nonspecific adsorption of detection antibody-conjugated UCNPs on the RWG substrate surface.
By adding a transverse heater pulse with controlled intensity distribution into the axicon ignitor-heater scheme for optically producing a plasma waveguide, three-dimensionally structured plasma waveguide can be fabricated. The additional heater pulse generates further heating of the plasma filament produced by the axicon pulses in a spatially and temporally controlled manner. The succeeding evolution of the plasma leads to a properly structured plasma waveguide that suits for targeted application. With this technique, induction of electron injection in a plasma-waveguide-based laser wakefield accelerator was achieved and resulted in production of a quasi-monoenergetic electron beam with an electron energy reaching 280 MeV and an energy spread as low as 1% in a 4-mm-long gas jet by properly setting the transverse heater pulse delay with respect to the axicon pulses. Furthermore, strong hard X-ray beam was observed upon further increase of transverse heater delay so that the irradiated section in the plasma waveguide acts as a plasma kicker to enhance betatron oscillation.
Efficient soft x-ray lasing was achieved by using plasma waveguide to confine the pump beam. With a 9-mm-long pure
krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400
folds. An output level of 8×1010 photon/shot is reached at an energy conversion efficiency of 2×10-6. Seeding the laser
with high-harmonic generation yields small divergence, high coherence, and controlled polarization. Application in digital
holographic microscopy was demonstrated.
We experimentally demonstrate the amplification of optical-field-ionization soft x-ray lasers in an optically preformed plasma waveguide for pure xenon, krypton, and argon gases, respectively. The lasing photon number of Ni-like Kr laser at 32.8 nm generated in waveguide is dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in a photon number of 8×1010 and an energy conversion efficiency of 2×10-6 with a pump pulse of just 235 mJ. In addition to the 46.9 nm main lasing line for Ne-like argon, the 45.1 and 46.5 nm lasing lines are also observed, indicative of the strong enhancement effect and the large gas density in the plasma waveguide.
With this technique multispecies parallel x-ray lasing is also demonstrated in a Kr-Ar mixed-gas waveguide. By seeding
optical-field-ionization plasma with high harmonic signals, 32.8-nm Kr laser output can be further improved to produce brighter and better collimated x-ray laser beams. Comparing with the same laser seeded only with spontaneous emission, seeding with high harmonics yields much smaller divergence, enhanced spatial coherence, and controlled polarization. With the illumination of high-brightness 32.8-nm x-ray laser pulses, single-shot x-ray digital holographic
microscopy with an adjustable field of view and magnification is demonstrated successfully. The ultrashort x-ray pulse duration combined with single-shot capability offers great advantage for flash imaging of delicate samples.
Efficient optical-field-ionization x-ray lasers driven by femtosecond laser pulses have been demonstrated in clustered gas
jets. With a tight focusing configuration, near saturation outputs of Pd-like xenon laser at 41.8 nm and Ni-like krypton
laser at 32.8 nm are generated at low pump energy of 200 mJ. By using the axicon-ignitor-heater scheme to produce a
9-mm-long plasma waveguide in a pure krypton gas jet, the lasing photon number of Ni-like Kr laser at 32.8 nm is
dramatically enhanced by about three orders of magnitude in comparison to that without plasma waveguide, resulting in
a photon number of 8×1010 and an energy conversion efficiency of
2×10-6 with a pump pulse of just 235 mJ. Besides for
producing various OFI collisional-excitation x-ray lasers of sufficient photon number for practical applications, an
optically-preformed plasma waveguide may also be a favorable choice for achieving OFI recombination x-ray lasers and
inner-shell x-ray lasers.
An optical-field-ionization soft x-ray laser with prepulse-controlled nanoplasma expansion in a cluster gas jet was demonstrated. Pd-like xenon lasing at 41.8-nm with 95 nJ pulse energy and 5-mrad divergence was achieved, indicating near-saturation amplification. In addition, by using deflectometry of a longitudinal probe pulse to resolve the spatiotemporal distribution of the preformed plasma, we characterize and control the plasma density distribution near the target surface for the development of solid-target x-ray lasers. We show that the use of prepulses in an ignitor-heater scheme can increase the scale length of the preformed plasma and how the effect varies with target
materials.
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