Publication
Optical trapping was used for decades in biotechnology for a broad range of applications. It can be used for remote manipulation, application of pico-Newton forces and even study DNA processes. Nevertheless, optical trapping setups can be cumbersome. In order to reduce their footprint and be able to integrate them in lab-on-chips circuits, several techniques exist. This work explores the use of hollow cavities in two dimensional photonic crystals for integrated optical trapping in lab-on-chips. Photonic crystals are excellent candidates for this application thanks to their capacity to confine light below the diffraction limit. Photonic crystals sample were fabricated at EPFL, from silicon on insulator wafers. The crystals were made on the silicon membrane 220 nm thick. The holes of the photonic crystal lattice are 250 nm in diameter. The trapping occurs inside the hollow cavity, which is 700 nm in diameter. This hollow cavity introduced a localized mode with a hexagonal symmetry. Thanks to the gradient forces from the cavity mode, we are able to trap polystyrene nano spheres with sizes ranging from 500 nm down to 250 nm. The interaction between the trapped nano sphere and the mode gives rise to back action effects. Such resonant optical trapping differs greatly from classical optical tweezers. To understand this resonant trapping and find the most important parameters, we record the motion of single nano spheres trapped inside the photonic crystal cavity. The measurement is performed with back focal plane interferometry, which allows the measurement of the motion of the tracked object in three dimensions at large sampling frequency. To use this method of detection, a special sample and microfluidic membrane had to be designed and fabricated. The microfluidic membrane is made in polymer and glass to reproduces the thickness of a microscope coverslip. High-resolution imaging is performed through this microfluidic membrane, which is also used to carry the nano spheres in microchannels and control their flow with precision. The results show that ratio of the linewidth of the resonance of the cavity to the detuned resonance wavelength caused by the presence of the trapped object is a critical parameter. The analysis of 250 nm nano spheres displays a complex behaviour, where the trapped nano spheres are jumping from one lobe of the mode to another with a frequency depending of the power coupled to the cavity. Additionally, the measurement of the power transmitted through the photonic crystal after coupling to the cavity hold a considerable amount of information. It is possible to correlate the power coupled into the cavity with the trapping position. Moreover, the transmitted power can be used to identify the operation regime of the trapping cavity, as well as the size distribution of a solution of particles. It is also demonstrated that the trapping of several nano spheres is possible within a single cavity, while the nano spheres arrange themselves in complex structures.
Camille Sophie Brès, Marco Clementi, Jiaye Wu, Qian Li