Ivan P. Christov

Fourth-order dispersion-limited solitary pulses

We analyze the performance of a Ti:sapphire self-mode-locked laser with near-zero second- and third-order dispersion. Our simulations show that, in the presence of fourth-order dispersion, solitary laser pulses can be supported within a wide parameter range, close to the experimental values of these parameters. We also conclude that it is possible to generate pulses much shorter than 10 fs if fourth-order dispersion is further reduced.

Quasi-phase matching of high-harmonics and attosecond pulses in modulated waveguides

We describe theoretically a new technique for quasi-phase-matched generation of high harmonics and attosecond pulses in a gas medium, in a high ionization limit. A corrugated hollow-core fiber modulates the intensity of the fundamental pulse along the direction of propagation, resulting in a periodic modulation of the harmonic emission at wavelengths close to the cutoff. This leads to an increase of the harmonic yield of up to three orders of magnitude. At the same time the highest harmonics merge in a broad band that corresponds to a single attosecond pulse, using 15-fs driving pulses.

Space-time focusing of femtosecond pulses in a Ti:sapphire laser

We present what is to our knowledge the first three-dimensional model of a femtosecond pulse propagating in a Ti:sapphire laser crystal, which includes dispersion, self-focusing, and finite response time of the medium and does not assume the slowly varying envelope approximation. The combined action of material dispersion and phase modulation leads to a dramatic space-time focusing on the pulse. Dispersion prevents catastrophic selffocusing and self-steepening of the pulse, even though the peak power of the pulse is much greater than the critical power for self-focusing filamentation. Extrapolation of these results to shorter-pulse durations shows that this space-time focusing mechanism can operate even for pulses with durations close to the response time of the Kerr nonlinearity.

Attosecond Time-Scale Intra-atomic Phase Matching of High Harmonic Generation

Using a model of high-harmonic generation that couples a fully quantum calculation with a semiclassical electron trajectory picture, we show that a new type of phase matching is possible when an atom is driven by an optimal optical waveform. For an optimized laser pulse shape, strong constructive interference is obtained in the frequency domain between emissions from different electron trajectories, thereby selectively enhancing a particular harmonic order. This work demonstrates that coherent control in the strong-field regime is possible by adjusting the peaks of a laser field on an attosecond time scale.

Attosecond time-scale feedback control of coherent X-ray generation

Abstract High-harmonic generation is an extreme, high-order, nonlinear process that converts intense, ultrafast, visible and infrared laser light pulses coherently into the soft X-ray region of the spectrum. We demonstrate that by optimizing the shape of an ultrafast laser pulse, we can selectively enhance this process by promoting strong constructive interference between X-ray bursts emitted from adjacent optical cycles. This work demonstrates that coherent control of highly nonlinear processes in the strong-field regime is possible by adjusting the relative timing of the crests of an electromagnetic wave on a sub-optical cycle, attosecond time scale.

Highly coherent light at 13 nm generated by use of quasi-phase-matched high-harmonic generation

By measuring the fringe visibility in a Youngs double pinhole experiment, we demonstrate that quasi-phase-matched high-harmonic generation produces beams with very high spatial coherence at wavelengths around 13 nm. To our knowledge these are the highest spatial coherence values ever measured at such short wavelengths from any source without spatial filtering. This results in a practical, small-scale, coherent, extreme-ultraviolet source that is useful for applications in metrology, imaging, and microscopy.

Generation of single-cycle attosecond pulses in the vacuum ultraviolet

Abstract A model for the interaction of a focused electron beam and a terawatt femtosecond optical pulse has been developed which demonstrates that single-cycle VUV pulses, with durations below 0.5 fs, can be generated. The ultrashort VUV pulses result from the nonlinear motion of the electrons in a ponderomotive force induced by a tightly focused femtosecond pump pulse. Our results demonstrate that the nonlinear properties of relativistic free electrons can be exploited to generate useful sources of light in the VUV.

Isolated EUV Pulses via CEP-insensitive Nonlinear Stabilization in a Waveguide

We discuss a new regime of harmonic generation, where bright isolated, suboptical cycle, EUV pulse can be generated via a mechanism relatively insensitive to carrier-envelope phase. Interplay between the transient plasma generated by the laser and the instantaneous laser intensity in the waveguide gives rise to a new non-linear stabilization mechanism where phase-matched EUV emission is localized within a sub-optical cycle.

Quantum Control of High-Order Harmonic Generation

We discuss the physical processes involved in the generation and optimization of extreme ultraviolet and soft x-ray light though the process of high-order harmonic generation. We show that by manipulating the sub-optical-cycle attosecond dynamics of this process using optimized waveguide structures and pulse shapes, we can control the energy of the emitted photons, the phase matching of the conversion process, and the spatial and temporal coherence of the light. High-order harmonic generation is a useful source of short wavelength light with ultrashort time duration. Thus, optimization and manipulation of high-order harmonic generation demonstrates control of electron dynamics on attosecond time scales.

Multiphoton EUV photonics

We demonstrate high-order quasi-phase-matched conversion of laser light into the water-window region of the spectrum around 300 eV for the first time. We also demonstrate phase matching in a fully ionized gas for the first time