Overview of the experimental apparatus, which consists of an amplified Ti:sapphire laser system, a capillary waveguide in which high-order harmonics are produced, and a vacuum system that houses a toroidal mirror, a sample gas cell in the interaction region, and a homebuilt soft x-ray spectrometer. BS: beamsplitter; FL: focal length; : zero-order half-waveplate; GV: gate valve; TMP: turbomolecular pump. The two arrows pointing toward the HHG capillary denote the positions of the gas inlets, whereas the remaining arrow pointing away from the HHG capillary denotes the connection to a scroll pump that is used to evacuate the front end of the capillary. The inset shows the layout for the soft x-ray optics. The first toroidal mirror (TM1) and second toroidal mirror (TM2, located in the soft x-ray spectrometer) are used to refocus the divergent high-order harmonics into the sample gas cell and onto the detector plane, respectively. Spectral dispersion onto the CCD camera is achieved by means of a uniform line spaced reflection grating (G).
Close-up view of the optical arrangement used to achieve spatial overlap between the optical pump and soft x-ray probe beams at the interaction region with a 2° crossing angle. The pickoff mirror consists of a dielectric-coated mirror with a diameter hole drilled through the optic to allow passage of the soft x-ray probe beam, while the optical pump beam is reflected by the front surface. The interaction region is defined by the position of the sample gas cell. A collinear pump-probe overlap geometry can also be implemented with the pickoff mirror, at the expense of decreasing the available pump pulse energy.
Schematic illustration of the sample gas cell. The gas cell consists of a PTFE cylindrical top and a stainless steel base. The PTFE top has 150 or diameter holes drilled through it along the beam propagation axis to allow the pump and probe beams to pass through the cell. A Swagelok tube fitting welded to the stainless steel base enables the gas cell to be connected to the external sample gas manifold via a fluid feedthrough.
Foci positions for various photon energies after spectral dispersion by the grating. The coordinates are referenced to the point of incidence on the grating by the soft x-ray beam. The line drawn through the points corresponds to the position of the CCD array plane. The inset shows the deviation of the foci position from the detector plane as a function of photon energy.
Typical calibration curve for the homebuilt soft x-ray spectrometer, along with the marked positions of the Al -edge cut-off in the first four diffraction orders (○) and the transition in the first two diffraction orders (◼).
(a) High-order harmonic spectra obtained with of Ar as the HHG medium. The spectrum in the solid line is obtained with an input pulse energy of and exhibits relatively narrow individual peaks, whereas the spectrum in the dashed line is obtained with an input pulse energy of and shows broadband high-order harmonics. (b) High-order harmonic spectra obtained with of Ne as the HHG medium and an input pulse energy of . The Al -edge cutoff at is apparent and substantiates the presence of a continuum underlying the discrete harmonic peaks.
Spectra of the NIR driving pulse after propagation through the capillary waveguide. The spectrum in gray is obtained with an evacuated capillary. The dashed line corresponds to the spectrum taken with an Ar inlet pressure of and an input energy of , whereas the solid black line is obtained with the same Ar inlet pressure, but with a higher input pulse energy of . The former yields narrow individual peaks in the high-order harmonic spectrum, whereas the latter leads to broadband harmonic spectra.
(a) Wavelength tuning of high-order harmonics by increasing the Ne inlet pressure. An increase in inlet pressure leads to an observed linear increase in photon energy at the peak of the individual harmonics, as would be expected from the relation delineated by Eq. (3). (b) The sensitivity of blueshifting with respect to inlet pressure shows a linear increase with harmonic order, i.e., . This trend verifies that blueshifting of the fundamental light in the capillary waveguide is the dominant mechanism that enables wavelength tuning of the high-order harmonics.
Knife-edge scan traces for (a) the 25th harmonic produced with of Ar and of input pulse energy and (b) the 45th harmonic produced with of Ne and of input pulse energy. The measured beam waists are and for the 25th and 45th harmonics, respectively.
Wavelength-integrated spatial beam profiles of the high-order harmonics produced in (a) Ar and (b) Ne, recorded by a soft x-ray CCD camera positioned after the sample focus. The area of the image is . Intensity variations along the horizontal and vertical sections are shown below and to the left of each image, respectively.
(a) High-order harmonic spectrum taken with an evacuated gas cell (dashed line) and with the gas cell filled with of Xe (solid line). (b) Nonresonant absorption spectrum of Xe obtained from calculating the transmission at the peak of each high-order harmonic. Absorption cross sections obtained from Ref. 57 are also shown for comparison. (c) Resonant absorption lineshape due to the Xe transition. (d) Resonant absorption lineshape due to the Xe transition.
(a) High-order harmonic spectra taken with an evacuated gas cell (dashed line) and with the gas cell filled with of (solid line). Note the use of a logarithmic scale. (b) Resonant absorption spectrum of showing two broad transitions separated by the Br spin-orbit splitting of .
Time evolution of the harmonic from , in steps of . The circle on each spectrum is located at . The decrease in spectral intensity on going from is indicative of the onset of transient absorption due to formation of by sequential strong-field ionization of Xe atoms with an pump pulse.
Transient absorption spectrum showing both and produced by optical strong-field ionization of Xe atoms. The peak intensity of the pump pulse is . The absorption features observed at and are collected by employing the weak continuum present in the high-order harmonic spectrum. The absorption at consists of two unresolved transitions at and .
Time trace showing the transient absorption at as a function of pump-probe time delay. The data are fit to a convolution of a step function with a Gaussian of FWHM. This value sets the upper bound for the duration of the soft x-ray pulse.
Photon flux at the source for the most intense high-order harmonics shown in Figs. 6(a) and 6(b), obtained by integrating the number of counts on the CCD camera over a bandwidth of centered about the harmonic.
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