PHYSICAL MECHANISM OF TERAHERTZ GENERATION IN TWO-COLOR PHOTOIONIZATION

Two-color photoionization has been widely used as a versatile tool for intense, broadband terahertz (THz) radiation generation. In this scheme, an ultrashort laser's fundamental and its second harmonic pulses are co-focused into a gas of atoms or molecules, transforming them into plasma by photoionization. This process produces an intense THz pulse emitted in the forward direction. The main focus of this dissertation is to provide a physical understanding of such THz generation and investigate its generation mechanism at both microscopic and macroscopic levels. First, we examine the generation process by measuring the relative phase between two-color (fundamental and second harmonic) laser fields and the resulting THz field simultaneously. We discover that a relative phase of π/2 yields maximal THz outputs, consistent with a semi-classical plasma current model. We find that this optimal relative phase is independent of laser intensities, gas species, and two-color laser amplitude ratios. We also measure concurrent near-field photocurrents. All these measurements verify laser-produced plasma currents as a microscopic source for THz generation. We also investigate THz radiation from an ensemble of aligned air molecules in two-color laser fields. Our experiments show that THz radiation is strongly affected by molecular (nitrogen and oxygen) alignment. We explain this phenomenon in the context of the plasma current model combined with alignment-dependent ionization. Phase-matching is essential to achieve high-efficiency nonlinear frequency conversion. We discover THz generation by two-color photoionization in elongated air plasmas (filamentation) is naturally phase-matched in the off-axis direction, resulting in donut-shaped radiation profiles in the far field. Because of this off-axis phase-matching, THz yields increase almost linearly with the filament length, scalable for further THz energy enhancement. Lastly, we study the polarization of emitted THz radiation. In the case of in-line focusing geometry, we observe the polarization evolves from linear to elliptical with increasing plasma length. This ellipticity arises from two combined effects--successive polarization rotation of local THz plasma sources, caused by laser phase and polarization modulations, and the velocity mismatch between laser and THz, which produces an elliptical THz pulse from a series of time-delayed, polarization-rotating local THz fields

Attosecond synchronization of extreme ultraviolet high harmonics from crystals

The interaction of strong near-infrared (NIR) laser pulses with wide-bandgap dielectrics produces high harmonics in the extreme ultraviolet (XUV) wavelength range. These observations have opened up the possibility of attosecond metrology in solids, which would benefit from a precise measurement of the emission times of individual harmonics with respect to the NIR laser field. Here we show that, when high-harmonics are detected from the input surface of a magnesium oxide crystal, a bichromatic probing of the XUV emission shows a clear synchronization largely consistent with a semiclassical model of electron-hole recollisions in bulk solids. On the other hand, the bichromatic spectrogram of harmonics originating from the exit surface of the 200 $\mu$m-thick crystal is strongly modified, indicating the influence of laser field distortions during propagation. Our tracking of sub-cycle electron and hole re-collisions at XUV energies is relevant to the development of solid-state sources of attosecond pulses

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

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Waveform Control Of High-Harmonic Generation In Solids

We report a strong carrier-envelope-phase dependence of high-harmonics in bulk solids subjected to strong few-cycle laser fields. We discover that harmonics are delayed with respect to each other at the sub-cycle level, yielding an atto-chirp

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

Waveform Control Of High-Harmonic Generation In Solids

We report a strong carrier-envelope-phase dependence of high-harmonics in bulk solids subjected to strong few-cycle laser fields. We discover that harmonics are delayed with respect to each other at the sub-cycle level, yielding an atto-chirp

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

Crystal orientation-dependent polarization state of high-order harmonics

We analyze the crystal orientation-dependent polarization state of extreme ultraviolet (XUV) high-order harmonics from bulk magnesium oxide crystals subjected to intense linearly polarized laser fields. We find that only along high-symmetry directions in crystals high-order harmonics follow the polarization direction of the laser field. In general, the polarization direction of high-order harmonics deviates from that of the laser field, and the deviation amplitude depends on the crystal orientation, harmonic order and the strength of the laser field. We use a real-space electron trajectory model to understand the crystal orientation-dependent polarization state of XUV harmonics. The polarization analysis allows us to track the motion of strong-field-driven electron in conduction bands in two dimensions. These results have implications in all-optical probing of atomic-scale structure in real-space, electronic band-structure in momentum space, and in the possibility of generating attosecond pulses with time-dependent polarization in a compact setup.Comment: 5 pages, 5 figure