Fig. 1. Real-color image of (a) a typical filament and (b) filament-guided CDs taken by a digital camera. The CD voltage and filamenting pulse energy were 50 kV and 5.4 mJ, respectively.

Fig. 2. Side schematic of the experimental setup for fluorescence detection in air. F1–F3 are focusing UV grade fused silica lenses with focal lengths of 30, 15 and 10 cm, respectively. E1 and E2 are electrodes. E1 was connected to a high-voltage power supply and E2 was floated. BD is a beam dump.

Fig. 3. Laser-guided leader type of CD. (a) Image of the tips of the two electrodes and typical fluorescence images of (b) CD, (c) FIL and (d) eFIL. The separation distance of the two tips was ${\sim}15$ mm. The filamenting pulse energy was 4.8 mJ and the CD voltage was 50 kV. The fluorescence length of the filament was ${\sim}10$ mm. It was situated at approximately 1–2 mm above the tips of the electrodes.

Fig. 4. Typical spectrum in the range (a) 290–440 nm (Ref. [37]) and (b) 500–1090 nm emitted by CD, FIL and eFIL, respectively. The intensity of CD fluorescence is magnified by 30 times in (b).

Fig. 5. (a) Resultant spectral intensity as a function of the pulse energy at 337 and 428 nm. The CD voltage was fixed at 50 kV. (b) Pseudocolor plot of pump energy versus wavelength corresponding to (a). (c) Resultant spectral intensity as a function of CD voltage when the pulse energy was fixed at 4.61 mJ. (d) Pseudocolor plot of voltage versus wavelength corresponding to (c).

Fig. 6. (a) Spectrum in the range 700–900 nm at different CD voltages: 5, 20, 30 and 50 kV, respectively. (b) Integrated spectral intensity as a function of CD voltage. The integrated spectral range is from 745 to 855 nm. The filamenting pulse energy was fixed at 4.61 mJ.