Cone emission induced by femtosecond excitation in rubidium vapor

We studied the emission from dense rubidium vapor under 100-fs pulsed laser excitation in the 730–820 nm range. Self-focusing along with a conical emission (CE) was observed for the excitation in the blue wing of $D2$ atomic resonance. Self-defocusing occurred for the red-wing excitation. This is in accordance with the theory of the intensity dependent nonlinear refraction index. In the case of the CE observation, our experimental results are attributed to the self-phase modulation of the laser beam interacting with the near-resonant atomic medium.

Figure 1

Experimental setup. GP: glass plate, FM: folding mirror, $z_f$: self-focusing length, ${\theta }_{\text{CE}}$: conical angle, $a_0$: beam radius, BBO: nonlinear crystal, $F$: filter.

Figure 2

Cone angle as a function of the number density of the rubidium atoms for the laser power of 650 mW and at ${\lambda }_0=755\text{nm}$.

Figure 3

(a) Transmission of the white light through the cell at $420°\text{C}$. (b) Cone angle dependence on central wavelength of the laser $T=420°\text{C}$ laser power is 650 mW.

Figure 4

(Color online) Spectral and angular distribution of the cone emission for the (a) ${\lambda }_0=755\text{nm}$ and (b) ${\lambda }_0=815\text{nm}$ at $420°\text{C}$ and at laser power of 650 mW.

Figure 5

(Color online) Spectral and angular distribution for (a) ${\lambda }_0=765\text{nm}$, (b) ${\lambda }_0=770\text{nm}$, and (c) ${\lambda }_0=778\text{nm}$ at $\text{42}0°\text{C}$ and at laser power of 650 mW.

Figure 6

Cone angle as a function of the power of the input beam, for ${\lambda }_0=755\text{nm}$ at $420°\text{C}$.

Figure 7

(a) Cross correlation of the cone emission for the ${\lambda }_0=755\text{nm}$ excitation at laser power of 650 mW for the temperatures of the cell $T=280$, 320, and $420°\text{C}$. Inset shows cross correlation through the cell at the room temperature. (b) CC for the ${\lambda }_0=815\text{nm}$ excitation at laser power of 650 mW and at $T=420°\text{C}$.

Figure 8

Self focusing length $z_f$ as a function of the input laser power, dots: experimental data, line: fit to Eq. (1). The temperature is $420°\text{C}$ and ${\lambda }_0=755\text{nm}$.

Figure 9

Cone angle as a function of the ${\lambda }_0$ detuning in the blue wing of the $D2$ resonance. The temperature is $420°\text{C}$ and the laser power is 650 mW.