Coherent Water Window X Ray by Phase-Matched High-Order Harmonic Generation in Neutral Media

We demonstrate the generation of a coherent water window x ray by extending the plateau region of high-order harmonics under a neutral-medium condition. The maximum harmonic photon energies attained are 300 and 450 eV in Ne and He, respectively. Our proposed generation scheme, combining a $1.6\mu \mathrm{m}$ laser driver and a neutral Ne gas medium, is efficient and scalable in output yields of the water window x ray. Thus, the precept of the design parameter for a single-shot live-cell imaging by contact microscopy is presented.

Figure 1

Calculated harmonic spectra and effect of medium’s absorption. The green profile in the top panel depicts the calculated HH spectrum generated in He using a 35 fs, $0.8\mu \mathrm{m}$ pulse with an intensity of $5.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$. Because the ionization rate is fixed to less than 1%, the cutoff harmonic energy barely attains 150 eV. The red and blue profiles in the top panel show the calculated HH spectra generated by 35 fs, $1.6\mu \mathrm{m}$ pulse with an intensity of $4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ in Ne and $6\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ in He, respectively. The bottom panel shows the evolution of the square of the reciprocal of the imaginary component $f_2$ for three different gases [32], which indicates the effect of absorption.

Figure 2

Measured harmonic spectra from neutral Ne and evolution of the square of the reciprocal of imaginary component $f_2$. The solid blue line and the dashed blue line (left axis) correspond to the Ne spectrum obtained with a $2400\mathrm{\text{grooves}}/\mathrm{mm}$ grating and a $1200\mathrm{\text{grooves}}/\mathrm{mm}$ grating, respectively. Both HH spectra are normalized to the peak intensity. The inset shows the far-field spatial profile of the water window x ray (280–310 eV) obtained from neutral Ne gas. Since a toroidal mirror is not set in front of the spectrometer, we can directly observe the spatial profile of the HH beam (see inset of Fig. 3).

Figure 3

Measured harmonic spectra from neutral He. These spectra (left axis) are obtained with a $2400\mathrm{\text{grooves}}/\mathrm{mm}$ grating. The red line and the blue line, respectively, correspond to the spectra with and without a $1\mathrm{\text{−}}\mu \mathrm{m}$-thick Mylar filter (${\mathrm{C}}_{10}{\mathrm{H}}_8{\mathrm{O}}_4$). Both HH spectra are normalized to the peak intensity. The inset depicts a 2D HH spectrum image with a $1\mathrm{\text{−}}\mu \mathrm{m}$-thick Mylar filter on a microchannel plate. This image is obtained by the accumulation of 1000 shots.