Generation of VUV, EUV, and X-ray light using VUV-UV-VIS lasers
A method for extending and enhancing bright coherent high-order harmonic generation into the VUV-EUV-X-ray regions of the spectrum involves a way of accomplishing phase matching or effective phase matching of extreme upconversion of laser light at high conversion efficiency, approaching 10 −3 in some spectral regions, and at significantly higher photon energies in a waveguide geometry, in a self-guiding geometry, a gas cell, or a loosely focusing geometry, containing nonlinear medium. The extension and enhancement of the coherent VUV, EUV, X-ray emission to high photon energies relies on using VUV-UV-VIS lasers of shorter wavelength. This leads to enhancement of macroscopic phase matching parameters due to stronger contribution of linear and nonlinear dispersion of both atoms and ions, combined with a strong microscopic single-atom yield.
1. The method of generating coherent emission within the VUV, EUV, and X-ray region of the spectrum comprising the steps of:
(a) providing a nonlinear medium comprising a liquid for high harmonic generation (HHG) of laser light;
(b) selecting the pressure of the nonlinear medium;
(c) generating a laser driving pulse having a selected wavelength within the VUV-UV-VIS region of the spectrum and a selected peak intensity above 5×10 14 W/cm 2 ; and
(d) focusing the laser driving pulse into the nonlinear medium to cause HHG upconversion resulting in a generated pulse within a selected range within the VUV, EUV, and X-ray region of the spectrum;
wherein steps (a)-(d) effectively phase match the laser driving pulse and the generated pulse sufficiently to result in a coherent generated pulse wherein the coherence length is approximately the adsorption length or a few absorption lengths.
2. The method of claim 1 wherein step (c) selects a wavelength and a selected peak intensity of the laser driving pulse resulting in significant ionization of the nonlinear medium.
3. The method of claim 2 wherein the ionization of the nonlinear medium is at least about 10%.
4. The method of claim 3 wherein the nonlinear medium is fully ionized.
5. The method of claim 3 wherein the nonlinear medium is multiply ionized.
6. The method of claim 5 wherein the ionization of the nonlinear medium exceeds 300%.
7. The method of claim 1 wherein the laser pulse has a wavelength below 0.7 μm.
8. The method of claim 7 wherein the laser pulse has a wavelength of at most about 0.4 μm.
9. The method of claim 7 wherein the laser pulse has a wavelength of at most about 0.27 μm.
10. The method of claim 1 wherein the nonlinear medium is multiply ionized and wherein the coherence length is at least about 1 mm.
11. The method of claim 1 wherein the coherence length is at least about 0.5 cm.
12. The method of claim 1 wherein the medium includes H 2 O.
13. The method of claim 12 wherein a mean charge Z is on the order of 1.
14. The method of claim 12 wherein the laser pulse has a wavelength of at most about 0.16 μm.
15. The method of claim 1 wherein the medium includes SiO 2 .
16. The method of claim 15 wherein the laser pulse has a wavelength of at most about 0.16 μm.
17. The method of claim 15 wherein a mean charge Z is on the order of 1.
18. The method of claim 1 wherein the medium includes He.
19. The method of claim 1 wherein the medium includes Ne.
20. A method for developing a high harmonic source comprising the steps of:
(a) evaluating a total index of refraction of neutral and ionic species of a medium as a function of the wavelength of a driving laser and evaluating an index of refraction of the corresponding harmonic emission;
(b) evaluating a total index of refraction of neutral and ionic species of a medium at a laser wavelength as a function of the interaction time between the driving laser pulse and the medium;
(c) evaluating (effective) HHG phase matching limits as a function of the driving laser wavelength, wherein (effective) phase matching means sufficiently phase matched such that the coherence length is approximately the absorption length or a few absorption lengths;
(d) determining optimal laser parameters as a function of the driving laser wavelength;
(e) evaluating the flux from a single emitter as a function of the driving laser parameters under (effective) phase matching conditions;
(f) evaluating the optimal density-length product of the medium as a function of the driving laser wavelength;
(g) combining steps (a)-(f) and calculating the macroscopic (effectively) phase-matched HHG emission as a function of the wavelengths of the generated light and the driving laser light; and
(h) finding a global maximum of flux at a desired HHG wavelength and selecting an optimal driving laser wavelength, optimal laser pulse parameters, optimal spatial and temporal pulse shape, optimal nonlinear medium and parameters of the nonlinear medium; and
(i) developing a high harmonic source device according to steps (a)-(h).
21. The method of claim 20 further comprising in step (a) the step of varying the driving laser wavelength.
22. The method of claim 21 , further comprising the step of using the high harmonic source device in imaging.