Optical vortex pulses

Optical vortex pulses. In addition to having spin angular momentum, light beams can also have orbital angular momentum (see the article by Miles Padgett, Johannes Courtial, and Les Allen in Physics Today, May 2004, page 35 ). Often called vortex beams, they have a helical wavefront and a doughnut-shaped profile with vanishing intensity along the beam axis, and they are being increasingly used in applications such as rotating particles in optical tweezers. Though most investigations with optical vortex beams have focused on continuous-wave (CW) operation, pulsed vortex beams could open up several additional applications in materials processing or nonlinear frequency conversion. Now Haohai Yu (Shandong University) and colleagues have demonstrated a new approach for directly and controllably generating pulsed vortex beams. To drive their pulsed laser, the researchers use a CW commercial laser diode that also has a doughnut-shaped profile. As the pump power increases, thermal effects in the pulsed laser cavity change the vortex mode that best couples to the input beam and gets preferentially pumped. Finally, a standard technique known as passive Q switching allows power to build up inside the cavity until it can be released in a short pulse. The net result is a train of stable, single-mode pulses with changeable vorticity. The pulses can also pack quite a punch: With roughly 10 W of pump power, the team demonstrated pulse energies up to 63 µJ and peak powers of more than 4.5 kW. (Y. Zhao et al., Appl. Phys. Lett. 101, 031113, 2012.)