MURI #9: Research Highlights
- Isolated attosecond X-ray pulses in the water window, Chang
- High harmonic generation from solids, Corkum
- Attosecond experimental endstation, Agostini and DiMauro
- Attosecond transient reflectivity, Leone
- Dynamics of hydrated electrons, Daniel Neumark
- Attosecond polarization spectroscopy, Krausz
- Theoretic investigations of sub-cycle strong field effects, Mark I. Stockman
1. Isolated attosecond X-ray pulses in the water window, Chang
Chang’s group implemented the polarization gating technique (PG) with a two-cycle, 1.7 μm driving field that demonstrated an attosecond supercontinuum extending to the
water window spectral region. The ellipticity dependence of the high harmonic yield over a photon energy range much broader than previous work is measured and compared
with a semi-classical model. When polarization gating is applied, the carrier-envelope phase (CEP) is swept to study its influence on the continuum generation. PG with
one-cycle (5.7 fs) and two-cycle (11.3 fs) delay are tested, and both give continuous spectra spanning from 50 to 450 eV under certain CEP values, strongly indicating
the generation of isolated attosecond pulses in the water window region.
To further extend the attosecond photon energy, Chang’s group proposed a novel approach for efficient generation of mid-infrared pulses at 3.2 µm, which is based on numerical
simulations of the broadband-pumped dual-chirped optical parametric amplification (DC-OPA) in LiNbO3. The broad DC-OPA phase-matching bandwidth-spanning from 2.4 µm to 4.0 µm
is achieved by chirping both the broadband Ti:Sapphire pump pulses and the seed pulses in such a way that the individual temporal slice of pump spectrum is able to phase match
that of seed spectrum. This phase matching scheme allows the use of longer crystals without gain narrowing or loss of conversion efficiency. Furthermore, the commercially
available acousto-optic programmable dispersive filter ensures compression of such a broad bandwidth down to 20 fs (two optical cycles at 3.2 µm).
2. High harmonic generation from solids, Corkum
Fig. 1 Half-cycle HHG driving laser.
When intense light irradiates solids they respond nonlinearly. Under the extreme conditions, very thin crystals can have an important influence on the pulse structure,
allowing us to monolithically synthesize electric field transients. Using a 1.8 μm driver and sub-100 micron thick crystalline quartz, Corkum’s group controllably produce
time-dependent optical fields as short as ½ cycle pulses, as shown in Fig. 1.
The very high intensities that dielectrics can withstand without damage are near the intensities used in gas phase attosecond pulse generation. Thus a hybrid technology is possible.
Corkum’s group shows that without any intervening elements, they can measure the carrier envelop phase of a pulse while shaping a 1.8 m beam to optimally generate isolated attosecond
pulses in a second solid or gaseous medium.
High harmonics are also generated in semiconductors. Corkum’s team has studied silicon and ZnO. In earlier work they had shown that the dominant harmonic generating mechanism
in ZnO was re-collision. This year they uploaded an archive paper showing the same for silicon. In addition, they have calculated and measured the nonlinear response of
graphene and graphite.
3. Attosecond experimental endstation, Agostini and DiMauro
OSU has been tasked with designing a new photon spectrometer capable of resolving energies up to 600 eV, as well as building a new beamline to perform preliminary attosecond
transient absorption spectroscopy measurements. To this end, they have designed, built and commissioned a photon spectrometer with a spectral range of 20 – 1,240 eV. Using a
pair of flat field variable line spaced gratings, it is capable of resolving both the energy and spatial profile of the beam.
The OSU group has designed an attosecond light source to be used for MIR-attosecond pump-probe experiments. Care was taken to make the optics achromatic to take full advantage
of the wavelength-tunable laser systems that are available at OSU. An ellipsoidal mirror is used to refocus and demagnify the XUV beam by a factor of three onto a solid or
gaseous sample. The pump arm of the interferometer propagates outside of vacuum, which allows for inherently non-vacuum compatible optical elements to be added to the interferometer.
An active stabilization feedback loop can be implemented to maintain sub-cycle stability. The design of the beamline is modular to accommodate different end-stations. They have
designed and constructed two end-stations specifically for this project: a gas/solid sample target chamber and the aforementioned photon spectrometer.
Using a phase mask, a Gaussian beam profile can be converted into a TEM01 beam profile. When focused in a gas jet, this produces two sources of harmonic emission which spatially
interfere in the far field. A scheme for calibrating the spectrometer using this two source generation technique has been developed. In the future, harmonic spectroscopy can be
performed by selectively exciting one of the generation sources and measuring the fringe shift in the far field.
Beamline construction has been completed and we have taken our first absorption spectra. A 1675 nm fundamental beam (pulse duration ~65 fs) and a free expansion argon gas jet were
used to generate harmonics past the silicon absorption L-edge (~100 eV). A 0.2 µm zirconium metal filter was used as a high-pass filter to block the fundamental beam and harmonics
below ~50 eV. A 200 nm thick freestanding silicon crystal was placed at the focus of the XUV beam and the ground state absorption was measured, as shown in Fig. 2.
Fig. 2 Absorption spectrum of Si.
4. Attosecond transient reflectivity, Leone
Fig. 3 Long and short time attosecond transient reflectivity of Ge. Note the sub-cycle features during 5 fs excitation and broadening of hole region in lower panel.
Attosecond transient reflectivity measurements in the extreme ultraviolet (XUV) have been achieved at the University of California, Berkeley (Fig. 3) under the guidance of S. Leone.
The experiments obtain absolute calibrated reflectivity and resolve both the real and imaginary parts of the complex dielectric function down to subfemtosecond timescales.
The method was applied to germanium single crystal semiconductor material via the Ge 3d core level transition at 30 eV. Core level probing accesses information about both
the valence band holes and conduction band electrons after excitation. The real part of the dielectric function reveals sub-femtosecond to few femtosecond timing of the
buildup of screened Coulomb interaction. The imaginary part obtains the creation and loss rates of the electron and hole carrier densities and hot carrier thermalization.
In the figure, subcycle oscillations are observed during the build up of the signal when field-induced excitation at higher intensities is applied. In addition, broadening
of the spectral features during the first 10 fs (not shown) is indicative of the timescale for charge carrier screening to build up. Hot carriers are scattered initially
into other regions of the band structure and also excite phonons on longer timescales, leading to all-assignable features in the band structure and bandgap renormalization,
accessed by the XUV transitions. This capability provides an important new window onto quantum electronic materials, with potential future applications to topological
spin-protected surface states and emerging electronic phenomena.
5. Dynamics of hydrated electrons, Daniel Neumark
Neumark’s group have carried out time-resolved photoelectron spectroscopy (TRPES) on liquid jets with femtosecond lasers with two goals in mind: elucidating the dynamics of
hydrated electrons, and investigating the energetics and dynamics of DNA subunits in water. Their recently published work on hydrated electrons showed that the lifetime of
the photoexcited p-state of the electron exhibits a strong isotope effect; the lifetime is about 70 fs in H2O and 100 fs in D2O. This isotope effect is consistent with the
non-adiabatic picture of hydrated electron relaxation dynamics. They have also been carrying out two types of experiments on DNA subunits (nucleobases, nucleosides, and nucleotides).
First, they generate hydrated electrons in liquid jets in which DNA subunits are involved, with the goal of measuring the binding energies of these electrons to the subunits.
Secondly, they use time-resolved photoelectron spectroscopy to study the relaxation dynamics of the DNA subunits subsequent to electronic excitation.
Neumark’s group is currently upgrading their experiment significantly, with the goal of generating femtosecond XUV pulses out to 100 eV and using these in TRPES experiments.
They are also learning how to use flat rather than cylindrical water jets. Flat jets may have advantages in TRPES experiments and will be critical in planned attosecond
transient absorption experiments at and above the carbon K-edge.
6. Attosecond polarization spectroscopy, Krausz
The research at the Ludwig Maximilians University and the Max Planck Institute of Quantum Optics focused on the use of precisely-resolved laser fields to observe the nonlinear
polarization of materials, and the associated light-matter energy exchange, on a sub-cycle time-resolved basis with attosecond time resolution. This approach, described in
Sommer et al., Nature 534, 86 (2016), describe a new route towards attosecond experiments based on coherent field measurements, as shown in Fig. 4. Combined with the recent
work at MPQ utilizing electro-optic sampling in the near-infrared spectral range (Keiber et al., Nature Photonics 10, 129), and now in the visible, will allow for attosecond
observations of energy dynamics in a compact system with no need for a single vacuum chamber or XUV optic
Fig. 4 Energy dynamics recorded in fused silica using the attosecond polarization spectroscopy technique [from Nature 534, 86 (2016)].
(a) Reversible and irreversible energy transfer as a function of time for varying electric field strength. At low field strengths, the laser
induces a significant modification of the optical properties of the solid, but the energy returns to the optical field - higher intensities
result in the generation of electron-hole pairs and the long-lived transfer of energy from the field to the solid. (b) Transmitted pulse
intensity profile at low and high intensity, showing the energy redistribution within the optical field. (c) Comparison of laser-field driven
switching with state of the art electronic devices.
7. Theoretic investigations of sub-cycle strong field effects, Mark I. Stockman
Fig. 5 Residual CB population in graphene after a two-oscillation pulse (fs duration) where the first optical cycle is left circularly-polarized with amplitude F0 = 0.5 V/Å,
and the second cycle is right circularly-polarized with amplitude 0.75F0 (this waveform is shown in the inset). The separatrices (i.e., the lines separating trajectories that
contain and do not contain the Dirac points) are indicated by the blue lines superimposed on the distributions.
During this grant period, the main focus was on attosecond processes in novel two-dimensional materials and semiconductors. Among these is graphene where the wide bandwidth of
the valence and conduction bands allows for changes of electronic population at subcycle time intervals. One of the most interesting results have been a prediction of attosecond
reciprocal-space interferometry in graphene subjected to a few-cycle circularly polarized pulses of a moderately high amplitude. In the reciprocal space, such a pulse moves
electrons in circular orbits. Passing close to the Dirac points, the electrons undergo excitation from the valence band (VB) to the conduction band (CB). Such excitation
events originating from different optical cycles of the ultrashort pulse interfere, as illustrated in Fig. 5, forming interferograms. These interferograms are highly chiral;
they carry information about such fundamental topological properties of the reciprocal space as Berry curvature, Berry fluxes, and the band dispersions. They can be measured
using time-resolved angle-resolved photoemission spectroscopy (TR-ARPES).
Stockman’s group has also predicted a possibility of attosecond optical field control of electron population and symmetry in a two dimensional material silicene, which is similar
to graphene but with Si atoms replacing the C atoms with a resulting double-sheet structure. The strong optical field shifts electron population toward one of these two sheets
effectively reducing the symmetry from honeycomb to triangular lattice, allowing for wealth of non-isotropic nonlinear effects. In another development, they have predicted
theoretically and have shown experimentally that the adiabatic semi-metallization of dielectrics observed earlier for silica and quartz is a universal phenomenon, which is
observed also in alumina (sapphire) and calcium fluoride. The strong-field coherent attosecond phenomena associated with intraband motion of electrons have been shown to take place.