Trapping nanoparticles with slow Bloch mode cavity

We have shown that a new kind of optical nanotweezer based on slow Bloch mode cavity, could trap particle as small as 75 nm in diameter over a large area (25 µm2) using a wide (5µm) excitation beam.

Our approach relies on the use of slow Bloch mode in an extended PC cavity. Thanks to a new design based on a double-period photonic crystal, we have shown that high Q cavities can be realized that can be easily addressed using a free propagating Gaussian beam. The electromagnetic field is located within the holes of the photonic crystal, leading to multiple hot spots that can be used as efficient nanotweezers. Photonic crystal cavities with 5 x 5 µm2 size etched in SOI substrates have shown resonances with Q factors around 4000 in water. We have demonstrated the trapping of 200 nm, 100nm and 75 nm beads using these structures. The normalized stiffness of the trap is around 4 fN.nm-1.mW-1 for the 200 nm bead and 0.45 fN.nm-1.mW-1 for the 100 nm bead. This stiffness is comparable to results obtained for state of the art “classical” optical tweezers, whereas the trapping area is two orders of magnitude larger. This result is a major improvement, if we consider the integration of this kind of device in a microfluidic system where the channels are a few microns wide. In this perspective, we have defined a new factor of merit that take into account the size of the trapping surface. Using this criterion, we have shown that the approach developed here, is two orders of magnitude more efficient compared to photonic crystal nanocavities.

Contact: Taha BENYATTOU –

Référence: Engineering of slow Bloch modes for optical trapping
L. Milord, E. Gerelli, C. Jamois, A. Harouri, C. Chevalier, P. Viktorovitch, X. Letartre, and T.
Benyattou, Appl. Phys. Lett. 106,121110 (2015); doi: 10.1063/1.4916612

Collaboration: T. Grosjean,F Baida et A. El Eter (FEMTO-ST), D. Nedeljkovic, B. Dahmani (LOVALITE)

Artistic view showing a slow Bloch mode (light within the holes) in a photonic crystal microcavity and a 100nm bead trapped above one local maximum
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