Cavity cooling of silicon nanoparticles

Figure 1: silicon nanoparticles are launched via laser-induced acoustic break out/break-off. The traverse a high-finesse infrared (1560 nm) cavity field. Self-induced feedback tends to extract energy from the particles and cools them.

Dispersive Cavity Cooling

While one might think that laser irradiation leads to heating, it has been known from atomic physics that lasers can actually also be used for cooling the motional state  of matter. And while nanoparticles have no specific internal resonance that could be used to couple to the light field, atomic transitions can be effectively replaced by the resonance of an optical resonator (cavity).

When the cavity is filled with laser light, a standing light  wave is formed. This is a sinusoidal pattern of bright and dark regions between the mirrors. A nanoparticle that flies through this pattern always feels an attractive force towards the bright spots.
A nanoparticle that moves over the standing wave effectively alters the distance between the mirrors because it adds refractive index n>1. In this way, dependent on the position of the particle, it allows the cavity to be more or less resonant and it allows more or less light to enter the cavity. By choosing the right experimental parameters, the overall cavity intensity will then always be highest whenever the particle leaves a bright region of the standing wave and will be lowest when the particle approaches it. In this way it always feels a stronger slowing force than an accelerating one. This is a self-induced feedback mechanism that leads to the slowing of the particle.

 

The Sisyphus effect of Optomechanics

One can imagine a ball running over the valleys and hills of a mountainous region. While travelling along the optcial landscape the particle alters the steepness of its environment by its own motion. As illustrated in the simulation below, it therefore always has to climb higher than it runs down again. In this way some of its kinetic energy is transferred to the cavity light field and overall the particle is slowed.

Experimental realization

This effect has been recently seen for the first time in 2013 by two independent groups at the University of Vienna:

  • In the team of Markus Aspelmeyer and Nikolai Kiesel they realised cavity cooling of the centre of mass motion of a silica sphere, trapped in the cavity and assisted by buffer gas loading [6].  
  • In our QNP team around Markus Arndt we are able to observe cavity cooling of pure silicon nanoparticles, created in high vacuum and slowed while they propagating through the cavity [7], as depicted in Figure 1.
References
  • P. Horak, G. Hechenblaikner, K. M. Gheri, H. Stecher & H. Ritsch
    Cavity-induced atom cooling in the strong coupling regime. 
    Phys. Rev. Lett. 79, 4974–4977 (1997)
  • V. Vuletic & S. Chu
    Laser cooling of atoms, ions, or molecules by coherent scattering. 
    Phys. Rev. Lett. 84, 3787–3790 (2000)
  • N. Kiesel, F. Blaser, U. Delic, D. Grass, R. Kaltenbaek, M. Aspelmeyer
    Cavity cooling of an optically levitated nanoparticle. 
    Proc. Natl. Acad. Sci. USA 110, 14180 (2013)
  • P. Asenbaum, S. Kuhn, S. Nimmrichter, U. Sezer, M. Arndt 
    Cavity cooling of free silicon nanoparticles in high-vacuum 
    Nature Communications 4, 2743 (2013)