3 research outputs found
High Harmonic Generation without Tunnel-Ionization
A new High Harmonic Generation (HHG) scheme, which doesn't rely on
Tunnel-Ionization as the ionization mechanism but rather on Single-Photon
Ionization, is theoretically proposed and numerically demonstrated. The scheme
uses two driver fields: an extreme-ultraviolet driver which induces the
ionization, and a circularly-polarized, co-rotating, two-color infrared driver
carried at a fundamental frequency and its second harmonic which induces the
recollision. Using Classical and time-dependent Schr\"odinger equation
simulations of a model Argon atom, we show that in this scheme ionization is
essentially decoupled from recollision. Releasing the process from being
Tunneling-dependent reduces its degree of nonlinearity, which offers new
capabilities in attosecond science, such as generation of High Harmonics from
highly-charged ions, or from specific deep core electronic levels. It is shown
that the emitted high harmonics involve the absorption of photons of one color
of the infrared driver, and the emission of photons of the second color. This
calls for future examination of the possible correlations between the emitted
high harmonics
Time-refraction optics with single cycle modulation
We present an experimental study of optical time-refraction caused by time-interfaces as short as a single optical cycle. Specifically, we study the propagation of a probe pulse through a sample undergoing a large refractive index change induced by an intense modulator pulse. In these systems, increasing the refractive index abruptly leads to time-refraction where the spectrum of all the waves propagating in the medium is red-shifted, and subsequently blue-shifted when the refractive index relaxes back to its original value. We observe these phenomena in the single-cycle regime. Moreover, by shortening the temporal width of the modulator to ∼5–6 fs, we observe that the rise time of the red-shift associated with time-refraction is proportionally shorter. The experiments are carried out in transparent conducting oxides acting as epsilon-near-zero materials. These observations raise multiple questions on the fundamental physics occurring within such ultrashort time frames, and open the way for experimenting with photonic time-crystals, generated by periodic ultrafast changes to the refractive index, in the near future