For quasi-phase matching of high-harmonic generation at short wavelengths, a beat-wave pulse train with 66-fs pulse separation is generated from a two-color Ti:sapphire amplifier system. By using such pulse train to collide with the main driving pulse for harmonic generation in the interacting media, quasi-phase matching can be achieved. The 66-fs pulse separation matches to a quasi-phase-matching zone length of 4.9 μm, which corresponds to a dephasing length for 3-nm harmonic generation under 1.0×1018 cm−3 plasma density. The pulse train energy is 100 mJ, sufficient to support more than 1000 quasi-phase-matching zones.
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.
The band structures of dispersive photonic crystals are numerically investigated. Based on the fmite-difference time-domain (FDTD) method with some auxiliary variables, time-stepping formulas are derived to compute the evolution of electromagnetic fields. The photomc crystals made of dispersive materials can be described either by the Lorentzian model or by the Drude model. In this paper, we describe how to use the time domain technique to calculate the dispersion relation of photonic crystals and then introduce the recursive convolution scheme to treat the photonic crystals with frequency-dependent materials. Band structures of photomc crystals without dispersion are calculated with the conventional FDTD time-stepping formulas and those with Lorentzian type dispersion and Drude type dispersion are also calculated with the recursive convolution scheme.
Gain saturation and the induced refractive index variation in semiconductor optical amplifiers (SOAs) have been widely used for many optoelectronics operations, including frequency conversion, phase conjugation, switching, modulation, and laser mode locking. In this paper, we report the experimental and numerical results of using gain saturation in SOAs for all-optical switching and novel laser mode locking configurations. For all-optical switching, we fabricated an all-SOA nonlinear optical loop mirror and implemented efficient power-dependent switching in both cw and pulse modes. Also, numerical modeling was also conducted to show consistent trends with experimental data. For novel laser mode-locking configurations, we demonstrated numerically efficient operation of mode-locked semiconductor laser with multi-mode interference SOA structures. With the nonlinear coupling process, it was found that efficient pulse compression could be achieved, implying that stable mode locking is feasible. Also, with a ring cavity colliding-pulse mode locking can be implemented.
Giant unilamellar lipsomes (diameter $GTR 10 (mu) m) are important for cell-membrane research and controlled drug delivery. Mechanical properties of unilamellar lipsomes in different physiological conditions are crucial for their applications. For example, liquid-gel phase transition of the bilayer membrane under different temperatures determines the stability and activity of liposomes. Bending rigidity is the most closely related mechanical property to phase transition. Owing to the flexible nature of bilayer membranes, accurate measurements of the bending rigidity of membranes are difficult. Here we report an all-optical technique to directly measure the bending modulus of unilamellar lipsomes. We use differential confocal microscopy, a far-field optical profilmetry with 2-nm depth resolution to monitor the thermal fluctuations and the deformation of unilamellar lipsomes. From the amplitude changes of thermal fluctuations along with temperature we can directly determine the phase-transition temperature of the membrane structure. We then employ optical force to induce sub-micrometer deformation of the unilamellar lipsomes. From the deformation we can obtain their bending rigidity with simple calculation. We find the bending modulus decreases from 8-11 pico-erg to 0.5 to 0.9 pico-erg as the liposomes are heated across the phase-transition temperature. All measurements are done without contacting the samples and the shapes of the liposomes remain the same after the experiments.
KEYWORDS: Dielectrics, Near field, Optical properties, Radio propagation, Near field scanning optical microscopy, Diffraction, Magnetism, Optical character recognition, Optical microscopy, Light wave propagation
Recent application of tapered fiber tips in near field optical microscopy has demonstrated a resolution far exceed the diffraction limit. We investigate the propagation and tunneling of monochromatic light through a tapered fiber tip near a dielectric surface. The transmission rate through the tip is given for various circumstances and found to increase significantly with the dielectric constant of the sample.
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