High harmonic generation in solids attracted great attentions as a new scheme for frequency conversion. We report observation of an extremely efficient terahertz (THz) third- and fifth-harmonic generation in thin films of Cd3As2, a three-dimensional Dirac semimetal with massless electron dispersion, as it is observable with tabletop THz source at room temperature. Our THz pump-THz probe study with subcycle time resolution elucidates that the intraband current of coherently accelerated Dirac electrons is the main source of the THz harmonics as expected theoretically. The results pave the way toward novel devices for ultrafast THz electronics and photonics based on topological semimetals.
High harmonics produced in aligned molecules contain the structural information of bound-state electronic states. We
have produced high harmonics from N2 molecules aligned to arbitrary directions with 5-degrees steps. From the set of
high harnionic spectra, we have successfully reconstructed tomographic images of the highest occupied molecular
orbital (HOMO) of N2.
High harmonics produced in aligned molecules contain the structural information of the outermost electron orbital that preferentially ionizes in intense laser fields. We show a method to reconstruct a 3-dimensional (3D) structure of the molecular orbital. The method is based on the technologies to align molecules and to produce attosecond XUV pulses, both of which utilize intense ultrashort laser pulses. We measured a set of high harmonic spectra produced in differently aligned N2 molecules, and successfully reconstructed the image of the highest occupied molecular orbital (HOMO) with sub-angstrom resolution.
When an atom is ionized by an x-ray pulse in the presence of a laser field,the drift velocity of photoelectrons shows the phase dependence on the dressing field.We show how to use this effect to characterize single attosecond x-ray pulses.(i)Attosecond streak camera - the distortion of the photoelectron spectra induced by the laser field is used to map the temporal shape of the x-ray pulse to the photoelectron spectra.(ii) Attosecond SPIDER (spectral phase interferometry for direct electric-field reconstruction) - the spectral shearing interferometry of photoelectrons is used to directly retrieve the spectral phase of the x-ray pulse from the photoelectron spectra.
We describe the phase-matched high-order-harmonic generation of femtosecond Ti:sapphire laser pulses in a self-guiding channel. Generation efficiency of the high-order harmonic was improved by phase-matched propagation in the guiding channel. The harmonics around the 49th harmonic (16 nm) were enhanced by two orders of magnitude compared to those in the plateau with a 7-mm-long, self-guided pulse in Ne. High- harmonic conversion efficiency of 10-6 was obtained, producing > nJ harmonics in the cutoff region around the 49th harmonic. The results are well explained by considering both the intrinsic phase based on the single-atom response and the macroscopic phase matching in the high-intensity interaction region.
We present time- and space-resolved XUV spectra of boron and carbon plasmas created by focusing 100-fs laser pulses on a solid target to an intensity of 1017 W/cm2. Emission lines originating from He-like and H-like excited states from n equals 2 to the ionization limit are observed with a spatial resolution of 100 micrometers in the direction normal to the target plane and with a temporal resolution of up to 4 ps. The position of the ionization limited is seen to depend very crucially on the plasma parameters of density and temperature, and is explained through continuum lowering effects. We observed the dynamics of the continuum lowering for plasma slices at different distances from the target, and record a maximum lowering of 40 eV in He-like carbon (10% of the ionization potential) from the disappearance of the 1snp - 1s2 line and from the position of the continuum edge.
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