Shima Gholam-Mirzaei

Laser waveform control of extreme ultraviolet high harmonics from solids

Solid-state high-harmonic sources offer the possibility of compact, high-repetition-rate attosecond light emitters. However, the time structure of high harmonics must be characterized at the sub-cycle level. We use strong two-cycle laser pulses to directly control the time-dependent nonlinear current in single-crystal MgO, leading to the generation of extreme ultraviolet harmonics. We find that harmonics are delayed with respect to each other, yielding an atto-chirp, the value of which depends on the laser field strength. Our results provide the foundation for attosecond pulse metrology based on solid-state harmonics and a new approach to studying sub-cycle dynamics in solids.

High-harmonic generation in amorphous solids

High-harmonic generation in isolated atoms and molecules has been widely utilized in extreme ultraviolet photonics and attosecond pulse metrology. Recently, high-harmonic generation has been observed in solids, which could lead to important applications such as all-optical methods to image valance charge density and reconstruct electronic band structures, as well as compact extreme ultraviolet light sources. So far these studies are confined to crystalline solids; therefore, decoupling the respective roles of long-range periodicity and high density has been challenging. Here we report the observation of high-harmonic generation from amorphous fused silica. We decouple the role of long-range periodicity by comparing harmonics generated from fused silica and crystalline quartz, which contain the same atomic constituents but differ in long-range periodicity. Our results advance current understanding of the strong-field processes leading to high-harmonic generation in solids with implications for the development of robust and compact extreme ultraviolet light sources.Although higher harmonic generation from solids has become of interest in many fields, its observation is typically limited to crystalline solids. Here, the authors demonstrate that higher harmonics can be generated from amorphous solids.

High harmonic generation in ZnO with a high-power mid-IR OPA

We generate high-order harmonics in a-cut (11–20) ZnO at a high repetition rate of 50 kHz, using the tunable mid-infrared pulses (3–4 μm wavelength) from a high-power optical parametric amplifier. For driving laser pulses with 3.8 μm central wavelength, we observe nonperturbative harmonic spectra that well exceed the material band gap. The harmonic spectra depend strongly on the orientation of the crystal with respect to the laser polarization, with odd harmonics exhibiting periodicities of π/2 for a polarization within the (11–20) crystal plane. Energy conversion efficiencies of ∼10−6 per harmonic are measured for the 9th–13th harmonics, yielding an average power of more than 0.2 μW for the 13th harmonic.

Spectral broadening and pulse compression of a 400 μJ, 20 W Yb:KGW laser using a multi-plate medium

We investigate the potential of a multi-plate medium consisting of thin fused silica plates to generate few-cycle pulses from a moderately high energy (400 μJ) and average power (20 W) Yb:KGW laser centered at 1025 nm. By optimizing the thicknesses and positions of the plates, we mitigate the losses associated with spatial and spectral distortions that can accompany self-focusing in bulk solids. Pulses with an initial duration of ∼280 fs were compressed using chirped mirrors, after spectral broadening in a multi-plate medium consisting of 6 mm of fused silica in total, by a factor of >5 to 50 fs. Further spectral broadening in a second stage also consisting of 6 mm of fused silica in total enabled compression to 18 fs with 40 μJ pulse energy, with the energy efficiency limited primarily by the geometry of the chirped mirror compressors.We investigate the potential of a multi-plate medium consisting of thin fused silica plates to generate few-cycle pulses from a moderately high energy (400 μJ) and average power (20 W) Yb:KGW laser centered at 1025 nm. By optimizing the thicknesses and positions of the plates, we mitigate the losses associated with spatial and spectral distortions that can accompany self-focusing in bulk solids. Pulses with an initial duration of ∼280 fs were compressed using chirped mirrors, after spectral broadening in a multi-plate medium consisting of 6 mm of fused silica in total, by a factor of >5 to 50 fs. Further spectral broadening in a second stage also consisting of 6 mm of fused silica in total enabled compression to 18 fs with 40 μJ pulse energy, with the energy efficiency limited primarily by the geometry of the chirped mirror compressors.

Solid-state high-order harmonics driven by long-wavelength lasers

High-order harmonic generation (HHG), resulting from the interaction of an intense laser field with an atomic or molecular gas, has been of great importance to the study of ultrafast dynamics for more than two decades. In the last several years, HHG has been observed in condensed matter systems driven by intense mid-infrared lasers. Investigations of HHG from solids can offer new capabilities for studying electronic structure and ultrafast carrier dynamics in photo-excited materials. However, HHG from solids is not yet well-understood, and even the generation mechanism cannot be uniquely determined in many systems. In this paper, we experimentally investigate HHG driven in solids by a high-power femtosecond optical parametric amplifier, producing mid-IR driving pulses with tunable central wavelength and >10 μJ pulse energy. We generate coherent high order harmonic radiation in ZnO and Si crystals, and characterize the dependence of the harmonic spectrum on the 3D crystal orientation. We further compress the driving pulse duration to below three optical cycles and investigate the resulting high-order harmonic spectrum. Moreover, we investigate the potential to generate harmonics in novel materials with the goal of probing the ultrafast dynamics arising from strong-field photo-excitation in such materials.

High Harmonic Generation in ZnO from a 50 kHz OPA

We generate high-order harmonics in ZnO using a mid-IR OPA with 50 kHz repetition rate. Nonperturbative harmonic spectra beyond the band gap exhibit strong dependence on the crystal orientation with respect to the laser polarization.