Accueil > Recherche > Dispositifs Optiques Nonlinéaires (DON) > Thèmes de recherche > Nano-Optomécanique

Optical fiber taper coupling to a GaAs disk

publié le

GaAs disk whispering gallery resonators are of such a small extension (few microns in diameter, typically 200 nm in thickness) that optical near-field techniques must be used to address them in experiments. Approaching an optical fiber taper in the disk evanescent field is a way to couple light in and out in the resonator. In such tapered fiber, light flows in a silica guide with sub-micron lateral extension, resulting in a guided mode with an important evanescent part.

Optical fiber tapers manufacturing

In our laboratory, we have set-up an original technique for tapering standard telecom optical fibers : we use a thermo-electric heater to soften the silica fiber before pulling gently on both ends of the fiber. The high stability of the heater allows a high level of control on this fabrication process : our typical fiber tapers reproducibly achieve optical transmission over 95 %, with a 800 nm taper diameter.

Below is an histogram of transmissions achieved over 100 produced tapers.

See all the details in our paper :

L. Ding, C. Belacel, S. Ducci, G. Leo, and I. Favero. ’Ultralow loss single-mode silica tapers manufactured by a microheater’. Applied Optics 49, 2441 (2010).

Controlled optical coupling to a GaAs disk

Using our ultra-low loss fiber tapers, we routinely perfom evanescent coupling experiments on GaAs disks. We are able to control the different regimes of coupling : under-coupling, critical coupling, and over coupling, where the disk/taper optical coupling rate is respectively inferior, equal, and larger than the disk gallery mode optical loss rate. The critical coupling regime is often of interest in our experiments, since this regime allows a maximum number of photons to be injected in the whispering gallery cavity.

Below, we show the fiber taper transmission, with its on-resonance value (in red) and the width of the resonance (in black), as the disk/taper gap distance is varied. Critical coupling is obtained when the on-resonance transmission is minimal, reflecting the photons injection into the disk.

Gradient force actuation of an optical fiber motion

When photons are injected resonantly in a GaAs disk cavity, by means of evanescent coupling experiments as shown above, the value of the electric field in the disk increases. The fiber material, silica, is electrically polarizable and thus polarizes in the evanescent field of the disk. The polarized fiber gets attracted towards regions of high electric fields, that is attracted towards the disk. The force responsible for this attarction is called "optical gradient force", because it relates to the motion of a polarized object in an optical field with non-uniform optical density. This is a tiny force, as all optical forces, but it can give rise to eyes-visible effect if the fiber taper is mechanically soft enough. We have carried-out experiments that reveal in a stunning way the action of this optical gradient force, by giving rise to a micron-amplitude displacement of the fiber.

Below, we show an optical microscope top view of our experimental situation, where a very soft fiber taper is positioned in the close vicinity of a GaAs disk. The left picture is taken when the pump laser is off-resonance, the right picture corresponds to the on-resonant case, where the fiber is optically attracted towards the disk.

See all the details in our paper :

L. Ding, P. Senellart, A. Lemaitre, S. Ducci, G. Leo, and I. Favero. ’GaAs micro-nanodisks probed by a looped fiber taper for optomechanics applications’. Proceedings of the SPIE 7712, 771211 (2010). 8 pages.