Generation and Bistability of a Waveguide Nanoplasma Observed by Enhanced Extreme-Ultraviolet Fluorescence

We present a study of the highly nonlinear optical excitation of noble gases in tapered hollow waveguides using few-femtosecond laser pulses. The local plasmonic field enhancement induces the generation of a nanometric plasma, resulting in incoherent extreme-ultraviolet fluorescence from optical transitions of neutral and ionized xenon, argon, and neon. Despite sufficient intensity in the waveguide, high-order harmonic generation is not observed. The fluorescent emission exhibits a strong bistability manifest as an intensity hysteresis, giving strong indications for multistep collisional excitations.

Figure 1

(a) Schematics of the experimental setup. The generated radiation in a solid angle of $\pm 1.2°$ is spectrally analyzed with a flat-field EUV spectrometer. (b) Scanning electron micrograph of the gold platform containing the tapered waveguides. Upper insets: Close-up views of the entrance apertures of an elliptical and a circular waveguide. Lower inset: Exit aperture of the elliptical waveguide. (Scale bars: overview $50\mu \mathrm{m}$, top views $1\mu \mathrm{m}$, exit aperture 200 nm.) (c) Third harmonic signal generated in a waveguide without gas exposure.

Figure 2

(a) Nanostructure-enhanced EUV spectra (logarithmic scale) using xenon, argon, and neon gas (200 mbar backing pressure) excited in the waveguide displayed in Fig. 1b (inset, red frame). The argon and neon spectra are up shifted for better visibility. In all cases, dashed gray lines correspond to the same signal level (being the noise floor at $0.5\mathrm{counts}/(\mathrm{s}\mathrm{nm})$ in these measurements). Wavelength positions of triangles indicate the expected fluorescent transitions for neutral (filled) and singly ionized (open) atoms of the respective gas species [35]. Providing for better resolution, the following portions of the spectra were recorded in the second grating diffraction order: Ne ($>60\mathrm{nm}$), Ar (65–84 nm), Xe (84–100 nm). (b) Intensity-dependent series of argon spectra recorded for increasing incident laser power.

Figure 3

(a) Intensity of the 104.8-nm argon line as a function of incident linear laser polarization. Polarization parallel to the minor axis leads to maximum fluorescent emission. (b) Emission map obtained for argon gas. The EUV fluorescence is recorded as the waveguide (cf. inset) is scanned relative to the focal spot. The averaged profiles in the $x$ and $y$ directions (solid blue) both show a significantly narrowed width compared with the focal laser profiles (dashed red), which is caused by spatial mode overlap in conjunction with the emission nonlinearity.

Figure 4

(a), (b) Intensity-dependent strength of the 104.8-nm argon line excited in two different waveguides (pressure: 200 mbar). A pronounced intensity hysteresis is observed. (c) Pressure-dependent intensities of two prominent xenon and neon lines. (d) Degree of bistability in xenon and neon as a function of gas pressure, measured as the normalized area enclosed by the hysteresis curves [cf. shaded areas in (a) and (b)].