We have achieved very efficient high-harmonic generation in a two-color laser field using a long gas jet of He. With the optimization of laser parameters and target conditions, strong harmonics were produced at 2͑2n +1͒th orders in an orthogonally polarized two-color field. The strongest harmonic at the 38th order ͑21.6 nm͒ reached an energy of 0.6 J with a 6 mm gas jet, giving a conversion efficiency as high as 2 ϫ 10 −4 .

(Doc. ID 155208); published Month X, XXXXWe demonstrate sub-100-attosecond timing jitter optical pulse trains generated from free-running, 77.6-MHz repetition-rate, mode-locked Er-fiber lasers. At -0.002(±0.001) ps 2 net cavity dispersion, the rms timing jitter is 70 as (224 as) integrated from 10 kHz (1 kHz) to 38.8 MHz offset frequency, when measured by a 24-as-resolution balanced optical cross-correlator. To our knowledge, this result corresponds to the lowest rms timing jitter measured from any mode-locked fiber lasers so far. The measured result also agrees fairly well with the Namiki-Haus analytic model of quantum-limited timing jitter in stretched-pulse fiber lasers.OCIS Codes: 060.3510, 140.4050, 270.2500, 320.7100 Femtosecond mode-locked fiber lasers are finding more and more applications, ranging from materials processing to precision measurements, owing to their excellent characteristics such as high gain, low noise, good thermal property, compactness, and easiness in building and operation. In particular, the excellent noise properties of mode-locked fiber lasers enable various time-frequency applications. In the frequency domain, the extremely low optical phase noise of mode-locked fiber lasers has enabled sub-mHz linewidth level frequency combs [1]. In the time domain, the fluctuation of pulse positions from perfectly periodic positions, i.e., timing jitter, has been expected to be extremely low as well. The quantumlimited timing jitter of free-running mode-locked lasers has been expected to be well below a femtosecond [2][3][4][5]. Due to their inherently low level, only very recently, accurate measurement of timing jitter up to the Nyquist frequency has been possible by using the balanced optical cross-correlation (BOC) method [6][7][8]. The use of attosecond-resolution BOC method has enabled the demonstration of 20-as-level jitter from Ti:sapphire solidstate lasers at 800 nm [9] and 200-as-level jitter from Ybfiber lasers at 1 µm [10]. The availability of mode-locked Er-fiber lasers at telecomm wavelength (1.5-1.6 µm) with attosecond timing jitter enables wide range of new applications in optical communications and photonic signal processing such as analog-to-digital conversion and long-distance clock distribution via fiber optic links. So far the best timing jitter performance demonstrated for free-running Er-fiber lasers is 2.6 fs (integrated from 10 kHz to 40 MHz) when the laser is operated at the stretched-pulse condition with positive net cavity dispersion [7]. The optimization of timing jitter in Er-fiber lasers toward the attosecond regime has not yet been shown.In this Letter we demonstrate sub-100-as timing jitter optical pulse trains from 78-MHz free-running modelocked Er-fiber lasers. The high-frequency timing jitter is scaled down by a factor of ~40 from few-fs [7] to sub-100-as level by a simple cavity dispersion control. At -0.002 (±0.001) ps 2 net cavity dispersion at 1582 nm center wavelength, the rms timing jitter is measured to be 70 as (224 as) integrated from 10 kHz (1 k...

Hexagonal boron nitride (h-BN) has been previously manufactured using mechanical exfoliation and chemical vapor deposition methods, which make the large-scale synthesis of uniform h-BN very challenging. In this study, we produced highly uniform and scalable h-BN films by plasma-enhanced atomic layer deposition, which were characterized by various techniques including atomic force microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray diffraction. The film composition studied by X-ray photoelectron spectroscopy and Auger electron spectroscopy corresponded to a B:N stoichiometric ratio close to 1:1, and the band-gap value (5.65 eV) obtained by electron energy loss spectroscopy was consistent with the dielectric properties. The h-BN-containing capacitors were characterized by highly uniform properties, a reasonable dielectric constant (3), and low leakage current density, while graphene on h-BN substrates exhibited enhanced electrical performance such as the high carrier mobility and neutral Dirac voltage, which resulted from the low density of charged impurities on the h-BN surface.

The polarization of high-harmonics from aligned N(2) molecules was measured by observing the visibility of spatial interference between two high-harmonics generated separately. The minimum visibility was observed at an angle of 60 degrees between the polarization of the harmonic generation laser field and the molecular orientation. In this case, the angular shift of harmonic polarization is 15 degrees from the molecular orientation. Our measurement of the visibility variation matches the theoretical prediction based on the harmonic field calculation for aligned N(2) molecules.

Characteristics of high harmonics generated from aligned N 2 molecules were investigated by rotating the molecules with respect to the polarization of harmonic generation pulses. The high harmonics in the plateau and the cutoff regions showed distinctive behaviour, which could be explained from the calculation of transition dipole moments between recolliding electron and multi-orbitals of N 2 molecule consisting of the highest occupied molecular orbital (HOMO) and the lower-lying orbital below HOMO (HOMO-1). It was also observed that the angular dependence of harmonics from aligned molecules with respect to the polarization of harmonic generation pulses was closely related to the geometrical structure of molecular orbitals, and that the harmonic in the transition region from the plateau to the cutoff was influenced comparably by both HOMO and HOMO-1. These results confirmed that high harmonic generation and molecular structure were closely linked.