Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances Muhammed Sayrac Phys-689 Texas A&M University 4/30/2015 J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

What is High Harmonic generation (HHG) When a sample is illuminated by an intense laser pulse, the sample will emit the high harmonics of the generation beam. HHG is an attractive method for generating coherent radiation in the extreme ultraviolet spectral region and is, nowadays, widely employed in atomic, molecular, plasma, and solid state physics. In addition, HHG allows producing extremely short attosecond pulses, which enable to analyze electron dynamics However, HHG suffers from its inherently low conversion efficiency, which hinders applications that are especially dependent on a high photon flux.

Mechanism of HHG Laser field K. Midorikawa, “Ultrafast dynamic imaging,” Nature photonics 5, (2011) P. B. Corkum, F. Krausz, “Attosecond Science,” Nature Physics, 3, (2007) XUV

Typical Shape of the HH Spectrum Plateau Cutoff Intensity ω1ω1 3ω 1 … … 31ω 1 Increasing harmonic order ω M. Murakami “High Harmonic Generation by Short Laser Pulses: Time-Frequency Behavior and Applications to Attophysics”, (2006)

Fano Resonance The Fano resonance is due to interference between two scattering amplitudes: One is due to scattering within a continuum states (the background process) Second is due to an excitation of a discrete state (the resonant process) It helps for determining line shape of the energy level of atoms The scattering cross section (σ) of the Fano profile can be expressed as where q is the shape parameter and ε is the reduced energy E F is a resonant energy and Г is the width of the resonance. J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

Paper Outline Resonances in the photo absorption spectrum of the generating medium can modify the spectrum of high order harmonics. In particular, Fano resonances can reduce photo absorption within a narrow spectral region and, consequently, lead to an enhanced emission of high-order harmonics in absorption limited generation conditions. Phase matching of a large number of emitters gives rise to a coherent buildup of high-order harmonic (HH) emission along the propagation direction of the driving laser. J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

FIG. 1 (color online). Experimental setup for high harmonic generation: A few-cycle laser pulse (8fs), whose central wavelength is tunable between 780 and 860 nm, is focused upon an argon gas jet. An aluminum filter separates the fundamental laser radiation from the generated HH photons, which are analyzed spectrally and spatially by a grating- based spectrometer. Experimental Setup J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

Fig. 2 (a) Spectra of high order harmonics generated in argon gas in different backing pressure normalized to the peak of 19 th harmonic. (b) Logarithmic plot of the spectrum around the strongest enhanced line at 26.6 eV measured at 9bar (pink line) normalized to the 17 th harmonic (black line). (c) Measured continuous photoionization cross section which accounts for direct photoionization without including any resonances. Experimental Results J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

Fig. 3 (a) Spectral profiles of the observed resonances extracted from measurements for HH spectra at 7bar. (b) Photoionization cross section including 3s3p 6 np 1 p 1 series of window type resonances measured by Madden et al. Experimental Results J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

1-D Model J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014) M. Sayrac et al. “Pressure optimization of high harmonic generation in a differentially pumped Ar or H 2 gas jet”, Rev. Sci. Instrum. 86, (2015)

Fig. 4 Measured power spectral density at the center of 15 th to 19 th harmonic (dark blue, light blue, and pink dots) versus the applied backing pressure. All values are normalized to the maximum obtained for 17 th harmonic. The semi transparent lines is result of a simulation. Experimental Results J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

As summary, they demonstrate that resonances in the photo absorption spectrum of the generating medium can significantly affect and modify the spectrum of high- order harmonics under photo absorption-limited conditions. First experiments in argon show that window resonances due to virtual excitation of auto ionizing states give rise to enhanced macroscopic photon emission. In particular, the 3s3p 6 4p 1 P 1 resonance at 26.6 eV reduces reabsorption and, therefore, allows enhancing the conversion efficiency by a factor of 1.8 within a narrow spectral region. At highest backing pressure, this narrow line is further enhanced up to a factor of 30 relative to the underlying HH Conclusions J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

The presented mechanism provides a simple and easy to implement way of tailoring and enhancing high harmonics emission and can be applied to a huge variety of targets. In combination with high average power driving lasers it will enable tabletop XUV sources of unprecedented brightness and spectral purity. Thus, applications such as precision spectroscopy of highly charged ions or coherent diffractive imaging of nanoscale objects with unprecedented level of detail will be feasible in the future. Conclusions J. Rothhardt, et al. “Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances”, PRL 112, (2014)

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