Limits of High Harmonic Generation conversion efficiency

Thesis (Ph. D.)--Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 157-164).High Harmonic Generation (HHG) is a fascinating phenomenon from both fundamental and technological point of view. It enables the generation of attosecond pulses and can have applications in EUV lithography and bio-microscopy. HHG can be described by the Three Step Model (TSM), due to the three stages of the process: ionization, propagation and recombination. However, HHG suffers from low efficiencies and a study, which shows the efficiency scaling with laser and material parameters is essential. For a long time experimentalists were using only 800 nm driver pulses from Ti:sapphire lasers. With the advent of new light sources like optical parametric amplifiers, different driving wavelengths became available and thus the scaling of the single atom response versus drive wavelength has attracted a lot of attention. A detailed analysis shows that the efficiency scales with w50 at the cutoff and w60 at the plateau region for a fixed EUV frequency, where w0 is the carrier frequency of the driver pulse. To understand the limitations of such a light source, we have developed a semi-analytic model for the computation of the conversion efficiency into a single harmonic for the plateau and cutoff regions. This model is one-dimensional, uses the TSM for the calculation of the single atom response and takes laser, material parameters and macroscopic effects into account. Closed form expressions for the plateau and cutoff regions are derived and used to calculate efficiencies for 400 and 800 nm driver pulses. The results are compared with experimental ones showing very good agreement. In order to investigate long-wavelength driven HHG efficiency, the 1-D model is extended to three dimensions taking into account spatiotemporal propagation effects, such as plasma defocusing and losses due to electron-neutral inverse bremsstrahlung. These phenomena change the phase matching along propagation, resulting in non-coherent harmonic generation and consequently poor efficiencies. We further study ways to mitigate the effect of plasma defocusing like the use of Supergaussian pulses and the use of Gaussian pulses with larger beam waists. The work presented can help us develop tools for an optimization study of HHG efficiency, in order to make useful EUV sources.by Vasileios-Marios Gkortsas.Ph.D

High-order harmonic generation in Xe, Kr, and Ar driven by a 2.1-\mu m source: high-order harmonic spectroscopy under macroscopic effects

We experimentally and numerically study the atomic response and pulse propagation effects of high-order harmonics generated in Xe, Kr, and Ar driven by a 2.1-\mu m infrared femtosecond light source. The light source is an optical parametric chirped-pulse amplifier, and a modified strong-field approximation and 3-dimensional pulse propagation code are used for the numerical simulations. The extended cutoff in the long-wavelength driven high-harmonic generation has revealed the spectral shaping of high-order harmonics due to the atomic structure (or photo-recombination cross-section) and the macroscopic effects, which are the main factors of determining the conversion efficiency besides the driving wavelength. Using precise numerical simulations to determine the macroscopic electron wavepacket, we are able to extract the photo-recombination cross-sections from experimental high-order harmonic spectra in the presence of macroscopic effects. We have experimentally observed that the macroscopic effects shift the observed Cooper minimum of Kr from 80 eV to 60-70 eV and wash out the Cooper minimum of Ar. Measured high-harmonic conversion efficiencies per harmonic near the cutoff are ~10^{-9} for all three gases.Comment: 19 pages, 8 figure

Limits of High Harmonic Generation conversion efficiency

High Harmonic Generation (HHG) is a fascinating phenomenon from both fundamental and technological point of view. It enables the generation of attosecond pulses and can have applications in EUV lithography and bio-microscopy. HHG can be described by the Three Step Model (TSM), due to the three stages of the process: ionization, propagation and recombination. However, HHG suffers from low efficiencies and a study, which shows the efficiency scaling with laser and material parameters is essential. For a long time experimentalists were using only 800 nm driver pulses from Ti:sapphire lasers. With the advent of new light sources like optical parametric amplifiers, different driving wavelengths became available and thus the scaling of the single atom response versus drive wavelength has attracted a lot of attention. A detailed analysis shows that the efficiency scales with w50 at the cutoff and w60 at the plateau region for a fixed EUV frequency, where w0 is the carrier frequency of the driver pulse. To understand the limitations of such a light source, we have developed a semi-analytic model for the computation of the conversion efficiency into a single harmonic for the plateau and cutoff regions. This model is one-dimensional, uses the TSM for the calculation of the single atom response and takes laser, material parameters and macroscopic effects into account. Closed form expressions for the plateau and cutoff regions are derived and used to calculate efficiencies for 400 and 800 nm driver pulses. The results are compared with experimental ones showing very good agreement. In order to investigate long-wavelength driven HHG efficiency, the 1-D model is extended to three dimensions taking into account spatiotemporal propagation effects, such as plasma defocusing and losses due to electron-neutral inverse bremsstrahlung. These phenomena change the phase matching along propagation, resulting in non-coherent harmonic generation and consequently poor efficiencies. We further study ways to mitigate the effect of plasma defocusing like the use of Supergaussian pulses and the use of Gaussian pulses with larger beam waists. The work presented can help us develop tools for an optimization study of HHG efficiency, in order to make useful EUV sources

Conversion efficiency, scaling and global optimization of high harmonic generation

Closed form expressions for the high harmonic generation (HHG) conversion efficiency in the plateau and cut-off region are derived showing agreement with previous observations. Application of these results to optimal HHG-based-XUV-sources is discussed

Spatial hole burning in actively mode-locked quantum cascade lasers

A theoretical study of active mode-locking in quantum cascade lasers including spatial hole-burning is presented. It is found that spatial hole-burning reduces the pulse duration at the expense of slight pulse instability and strongly structured pulse shapes

Ultrafast Rabi flopping and coherent pulse propagation in a quantum cascade laser

Pulse propagation phenomena are central to ultrashort pulse generation and amplification in lasers1–5. In the coherent regime, the phase relationship between the pulse and the material transition is preserved, allowing both optical fields and material states to be controlled6. The most prominent form of coherent manipulation is Rabi flopping7, a phenomenon well established in few-level absorbers, including atoms and single quantum dots8–19. However, Rabi flopping is generally much weaker in semiconductors because of strong dephasing in the electronic bands, in contrast to discrete-level systems. Although low-density induced coherent oscillations have been observed in semiconductor absorbers11,13–20, coherent pulse propagation phenomena in active semiconductor devices have not been observed. In this Letter, we explore coherent pulse propagation in an operating quantum cascade laser and directly observe Rabi flopping and coherent pulse reshaping. This work demonstrates the applicability of few-level models for quantum cascade lasers and may stimulate novel approaches to short pulse generationPhysic

Mode-locking via active gain modulation in quantum cascade lasers

A mode-locking mechanism by active gain modulation is studied numerically and experimentally. The parameter window for the emission of stable pulse trains was found. Pulses as short as 3 ps (~0.5 pJ) were characterized by second-order autocorrelation

Scalable High-Energy Sub-Cycle Waveform Synthesis for High-Field Physics

We demonstrate coherent pulse synthesis from two few-cycle phase-stable OPCPAs, enabling scalable, high-energy arbitrary optical waveform generation on sub-cycle time scales, suitable for attosecond control of high-field physics experiments

Interplay of mulitphoton and tunneling ionization in short-wavelength-driven high-order harmonic generation

High-order harmonic generation efficiency is theoretically modeled and compared with experiments using 400 and 800 nm driver pulses. It is shown that, for a short drive wavelength and a Keldysh parameter larger than 1, the Ammosov-Delone-Krainov (ADK) ionization model does not give a good agreement between theory and experiment. Since the ADK ionization model only accounts for tunnel ionization, it underestimates the yield of low-order harmonics from the wings of the driver pulse. In contrast, the Yudin-Ivanov ionization model [ Phys. Rev. A 64 013409 (2001)], which accounts for both tunnel and multiphoton ionization, gives much better agreement with the experimental results.United States. Air Force Office of Scientific Research (grant FA9550-09-1-0212)United States. Air Force Office of Scientific Research (grant FA9550-10-1-0063)Alexander S. Onassis Public Benefit FoundationConselho Nacional de Pesquisas (Brazil