High order harmonic generation (HHG) is the most typical extreme nonlinear optical phenomenon, where integer multiples of driving laser photon energy are emitted, and gives us the fundamental insight of non-perturbative light-matter interaction in solids. So far, HHG have been mainly studied in semiconductors which can be described with single electron approximation. However, little has been elucidated about HHG in strongly correlated electron systems such as Mott-insulators, where electron-electron correlation plays a dominant role. Here, we firstly report HHG from the Mott-insulating phase of Ca2RuO4. By changing sample temperature, we controlled the gap energy of Ca2RuO4 and investigated the relation between HHG emission properties and gap energy. We found that yields of high harmonics are well scaled by the gap energy: exponential increase of HHG yields with an increase of gap energy for the harmonic order greater than third [1]. We also study HHG theoretically in Mott-insulators and conclude that this anomalous behavior should originate from a strong coupling between charge and other degree of freedom (spin, orbital, etc…) with the cooperation of the thermal ensemble, and the resulting strongly temperature-dependent coherence between charge carriers [2]. Our results demonstrate that correlations between different degrees of freedom, which are a characteristic feature of strongly correlated solids, have significant and nontrivial effects on nonlinear optical responses.
[1] K. Uchida et al., arXiv:2106.15478 (PRL accepted).
[2] Y. Murakami et al., arXiv:2203.01029.
Metallic antenna arrays fabricated on high resistivity silicon are used to localize and enhance the incident THz field resulting in high electric field pulses with peak electric field strength reaching several MV/cm on the silicon surface near the antenna tips. In such high electric field strengths high density of carriers are generated in silicon through impact ionization process. The high density of generated carriers induces a change of refractive index in silicon. By measuring the change of reflectivity of tightly focused 800 nm light, the local density of free carriers near the antenna tips is measured. Using the NIR probing technique, we observed that the density of carriers increases by over 8 orders of magnitude in a time duration of approximately 500 fs with an incident THz pulse of peak electric field strength 700 kV/cm. This shows that a single impact ionization process is happening in a time duration of less than 20 fs. The measurement is repeated by exciting the sample with an optical pump beam at a wavelength of 400 nm. The optical pump sets the initial free carrier density before the THz-induced impact ionization. The measurements show that the carrier generation mechanism depends on the initial free carrier density which confirms that the carrier generation mechanism is impact ionization, rather than the alternative carrier generation mechanism in high electric field, i.e. Zener tunneling. Finally this technique can be extended to investigate carrier dynamics in other semiconductors.
By means of THz pump and optical probe spectroscopy, we observed that the incident THz pulse induces a strong spectral modulation of the 1s heavy-hole exciton peak of GaAs quantum wells due to Rabi splitting. Our precise measurements in the time domain show that the Rabi splitting follows the instantaneous THz electric pulse at extremely strong fields but persists up to a negative delay time of ~1 ps at weak fields. This field dependent dynamics of the Rabi splitting indicates that the excitonic field ionization governs a nonperturbative nonlinear dynamics of excitons by causing a reduction of the dephasing time
Intense terahetz (THz) pulses induce a photoluminescence (PL) flashes from undoped GaAs/AlGaAs quantum wells under continuous wave laser excitation. The number of excitons increases 10000-fold from that of the steady state. The THz electric field dependence and the relaxation dynamics of the PL flash intensity suggest that the strong electric field of the THz pulse ionizes trap states during the one-picosecond period of the THz pulse and release carriers existing in a giant reservoir containing many trap states in the AlGaAs layers.
Using a tilted-pump-pulse-front scheme, we generate single-cycle terahertz (THz) pulses by optical rectification of
femtosecond laser pulses in LiNbO3. In our THz generation setup, to obtain optimal THz beam characteristics and pumpto-
THz conversion efficiency the condition that an image of a grating coincides with a tilted-optical-pulse front is
fulfilled. Generated THz pulses have spectra centered at around 1 THz. A designed focusing geometry enables tight
focus of the THz beam with a spot size close to the diffraction limit, and the maximum THz electric field of 1.2 MV/cm
is obtained. In addition, the nonlinear interactions of GaAs quantum wells with the intense THz pulses have been
studied. Here we show that the intense THz pulse, unlike a DC bias, can generate a substantial number of electron-hole
pairs forming excitons that emit near-infrared luminescence. The bright luminescence associated with carrier
multiplication suggests that the carriers coherently driven by a strong field can efficiently gain enough kinetic energy to
induce a series of impact ionizations, which we demonstrate can increase the number of carriers by about three orders of
magnitude on picosecond timescale.
Although the importance of water for bio-materials have been believed, even the amount of hydration water have not
been defined. Despite picosecond time scale of the collective dynamics of bulk water, past studies for the hydration state
of solute with using NMR or neutron scattering have measured water dynamics only in nanosecond time scale and have
defined only the strongly perturbed water as the hydration water. However, it is expected that much more slightly
perturbed water exist near the solute surfaces. To define precisely the hydration state of lipid bilayer, which is the basic
structure of biomembrane, including slightly perturbed water, we have used terahertz time-domain spectroscopy, with
which picosecond dynamics of water can be measured. By comparing the terahertz results with the structural information
of the lipid/water system obtained by X-ray scattering, it is concluded that there is a long-range hydration layer on the
surface of lipid membrane on up to 4-5 water layers (1 nm) [1], which is 5 time as much as that in the previous reports
by NMR or neutron scattering. Our results indicates that the hydration water is important for the self-assembly of
biomolecules because its length scale is comparable to that of some interactions such as van der Waals interaction.
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