System Design and Setup

The experiment was performed by using a Ti:sapphire laser system with 10-Hz pulse repetition rate. The laser system located in Taiwan’s National Central University was based on the chirp-pulse amplification technique and consisted of three beam lines [38]. The first beam line generated laser pulses with a central wavelength of 810 nm, a minimum pulse duration of 30 fs, and a maximum peak power of 100 TW. The second beam line was of 810-nm central wavelength, 30-fs pulse duration, and 16-TW peak power. The

2.2. High-Brightness Ni-Like Krypton Lasing at 32.8 nm

Figure 2.3: Experimental layout for EUV lasing with a plasma waveguide [2].

CCD: charge-coupled device, TFP: thin-film polarizer, obj: objective, OAP:

off-axis parabolic mirror, QWP: quarter-wave plate. Four diagnostic tools were adopted, which are (a) on-line imaging system, (b) relayed imaging system, (c) interferometer, (d) flat-field spectrometer and (e) far-field pattern observing system.

third beam line was of 900-nm central wavelength, 35-fs pulse duration, and 6-TW peak power. Each laser beam can be further split into two beams with independent energy tuners, delay lines and pulse compressors. Figure 2.3 delineates the concise version of the whole experimental layout. A 30-fs, 2-J pump pulse from the 100-TW beam line was used to prepare the lasing ionization stage through optical-field ionization and heating of the plasma electrons. It was focused with an off-axis parabolic mirror of 30-cm focal length onto a krypton gas jet. The focal spot size of the pump pulse was 10-µm diameter in full width at half maximum (FWHM) with 71% energy enclosed in a Gaussian-fit profile, corresponding to a vacuum peak intensity of 4 × 10¹⁹ W/cm². A motorized quarter-wave plate in the pump beam path was used to vary the pump polarization. Two laser pulses from the 16-TW beam line, referred to as the ignitor and the heater pulses, were used to fabricate the plasma waveguide. The leading 35-fs, p-polarized ignitor pulse was 47 mJ in energy and was followed after 400-ps delay by the 210-ps s-polarized heater pulse with 208-mJ energy. After combined by a thin-film polarizer, the two pulses propagated collinearly and were then focused by an axicon of 30^◦ base angle to a line focus of > 2-cm length in FWHM. The line focus overlapped with the propagation path of the pump pulse in the gas

Figure 2.4: The upper half part illustrates the modified axicon-ignitor-heater pumping scheme and the relative temporal delays adopted in our experiment.

Located in front of the axicon, a convex lens of 150-cm focal length with a hole of 2-cm diameter at the center is used to focus the peripheral wave fields of the waveguide-fabricating beams on the line segment matching with the slit nozzle. The tranverse distributions of electron density correspond to the temporal positions (i), (ii), (iii) and (iv) on the timeline of the upper half illustration [2].

jet. A hole of 5-mm diameter at the center of the axicon allows passage of the pump pulse. To increase the efficiency of plasma waveguide fabrication, a convex lens of 150-cm focal length with a hole of 2-cm diameter at the center was installed in front of the axicon to concentrate the laser energy in a length of approximately 1 cm, matching with that of the gas jet, and to optimize the uniformity of the longitudinal intensity distribution. The modified axicon-ignitor-heater pumping scheme and the relative temporal delays of the waveguide fabricating pulses and the pump pulse are shown in Figure 2.4. Also in the figure, the chronological evolution of the radial electron density from (i) to (iv) illustrates the forming process of the plasma waveguide.

We use five optical diagnostic systems to check on every crucial step mainly during the waveguide fabrication and after EUV lasing, and they are deployed as shown in Figure 2.3. On-line imaging system uses a 20 X micro-scope objective to perform spatial overlaps between the waveguide-forming pulses and the pump pulse in the atmospheric environment and in the

vac-2.2. High-Brightness Ni-Like Krypton Lasing at 32.8 nm

Figure 2.5: Physical picture of the flat-field spectrometer. The cylindrical grating is aperiodically ruled with 1200-line/mm on a concave surface [3].

uum. The three-beam overlapping is imaged onto a 16-bit charge-coupled device (CCD) camera along the 1-cm-long waveguide-to-be lineal segment.

A relayed imaging system consisting of a retractable wedge, a pair of lenses, an objective lens, a 40-nm bandpass filter centered at 810 nm and another 16-bit CCD camera is used to measure the spatial profile of the pump pulse at the exit of the gas jet with an imaging resolution of 5.8 µm. Mach-Zehnder interferometry measures the density distribution of electrons with two compressed pulses consisting of a reference beam and a probe beam. The probe pulse of 15-mm diameter obtained from leakage of the pump pulse at a dielectric mirror passes transversely through the cluster jet to inter-fere with the unperturbed reinter-ference pulse on the other 16-bit CCD camera, providing decipherable interference fringes that relates to the local electron density. The primary spectral diagnostics for EUV radiation is a flat-field grazing-incidence x-ray spectrometer that is composed of an aperiodically ruled cylindrical grating with 1200 line/mm (001-0437, Hitachi) and a back-illuminated 16-bit x-ray CCD camera (DX420-BN, Andor Technology). The spectrometer is used to measure the spectrum and beam divergence of the EUV emission in the direction of laser propagation. As shown in Figure 2.5, such flat-field spectrometer (FFS) employs a concave grating with varied groove spacing to produce a flat image plane [39], which is exclusively useful for image acquisition of an x-ray CCD with a plane image sensor. Finally, an observing system for studying the far-field pattern of the lasing signal is introduced to evaluate the dependence of the guiding efficiency on the pump energy.

Two 0.25-µm-thick aluminum filters are used to block the transmitted

Figure 2.6: Interferograms of the plasma taken at 10 ps after the pump pulse passing through the gas jet. (a) Only the pump pulse under krypton atom density 8.0×10¹⁷cm⁻³. (b) Only the pump pulse under krypton atom density 1.6 × 10¹⁹cm⁻³. (c) The pump pulse guided by the waveguide under krypton atom density 1.6 × 10¹⁹cm⁻³. The beam profiles of the pump pulse before entering and after coming out of the gas jet are shown on the two sides of each interferograms [4].

pump laser pulse and attenuate EUV emission to avoid saturating the CCD camera. A dipole magnet with a magnetic field of 0.18 T is deployed in front of the FFS to deflect the electrons emitted from the interaction region to prevent false signals in the x-ray CCD camera. From the calibrated filter transmittance, the reflectivity of the concave grating and the CCD response with respect to the target wavelength, the approximate photon number of the EUV lasing is determined.