Low-Threshold Bidirectional Air Lasing

Air lasing refers to the remote optical pumping of the constituents of ambient air that results in a directional laserlike emission from the pumped region. Intense current investigations of this concept are motivated by the potential applications in remote atmospheric sensing. Different approaches to air lasing are being investigated, but, so far, only the approach based on dissociation and resonant two-photon pumping of air molecules by deep-UV laser pulses has produced measurable lasing energies in real air and in the backward direction, which is of the most relevance for applications. However, the emission had a high pumping threshold, in hundreds of $\mathrm{GW}/{\mathrm{cm}}^2$. We demonstrate that the threshold can be virtually eliminated through predissociation of air molecules with an additional nanosecond laser. We use a single tunable pump laser system to generate backward-propagating lasing in both oxygen and nitrogen in air, with energies of up to $1\mu \mathrm{J}$ per pulse.

Figure 1

(a) Energy level diagram for atomic oxygen showing relevant transitions for resonant two-photon pumping at a 226-nm wavelength and emission at 845 nm. (b) Same for atomic nitrogen. Two-photon pumping is resonant at a wavelength of around 211 nm and emission occurs on various lines at around 870 nm. Another option for pumping nitrogen exists, with resonant two-photon transition to the ${{}^{4}S}^0$ level instead of ${{}^{4}D}^0$. In that case, two-photon pumping is resonant at a 207-nm wavelength and the lasing occurs at 745 nm [14]. (c) Schematic of the experimental setup. DM: Dichroic Mirror.

Figure 2

Energy output of the backward and forward air lasing, vs energy of the deep-UV pump pulse, for the case of atmospheric oxygen. The predissociating Nd:YAG laser is turned off. The pump is tuned to a 226-nm wavelength. The spectrum of backward air lasing is shown in the inset.

Figure 3

Same as Fig. 2, but for the case of atmospheric nitrogen. The pump wavelength in this case is tuned to 211 nm. The predissociating nanosecond laser is turned off.

Figure 4

Log-scale plot of the energy of the backward-lasing signal as a function of the length of the gain region. Gain length is defined by the insertion of a tilted glass plate into the beam path, as described in the text. The two curves are for the cases of 226-nm pumping (oxygen lasing) and 211-nm pumping (nitrogen lasing). Fitting the exponential parts of the gain curves yields gain constants of $74{\mathrm{cm}}^{-1}$ and $71{\mathrm{cm}}^{-1}$, for the oxygen and nitrogen cases, respectively.

Figure 5

Energy output of backward-propagating air lasing, vs energy of the deep-UV pump pulse, for the case of atmospheric oxygen emission and with the predissociating Nd:YAG laser turned on. The wavelength of the tunable OPO pump is set to 226 nm. The threshold with respect to the energy of the deep-UV pump is essentially absent. The inset shows the far-field intensity profile of the generated backward-propagating laser beam with the angular FWHM beam divergence of about 6°.

Figure 6

Same as Fig. 5 but for the case of atmospheric nitrogen pumped at 211-nm or 207-nm wavelengths. The inset shows the far-field intensity profile of the generated backward-propagating laser beam.

Figure 7

Temporal waveforms of the forward and backward oxygen lasing with predissociation. In all plots, the solid lines show lasing waveforms and the dotted lines show the temporal waveform of the deep-UV pump pulse. Two bottom panels show temporal emission waveforms averaged over ten laser shots. The averaged forward and backward signals are out of phase, evidencing gain competition between the two signals.