The optomechanical coupling between a resonant optical field and a nanoparticle through trapping forces is demonstrated. Resonant optical trapping, when achieved in a hollow photonic crystal cavity is accompanied by cavity backaction effects that result from two mechanisms. First, the effect of the particle on the resonant field is measured as a shift in the cavity eigenfrequency. Second, the effect of the resonant field on the particle is shown as a wavelength-dependent trapping strength. The existence of two distinct trapping regimes, intrinsically particle specific, is also revealed. Long optical trapping (>10 min) of 500 nm dielectric particles is achieved with very low intracavity powers (<120 μW).

We fabricate and experimentally characterize an H0 photonic crystal slab nanocavity with a design optimized for maximal quality factor, Q = 1.7 × 106. The cavity, fabricated from a silicon slab, has a resonant mode at λ = 1.59 μm and a measured Q-factor of 400 000. It displays nonlinear effects, including high-contrast optical bistability, at a threshold power among the lowest ever reported for a silicon device. With a theoretical modal volume as small as V = 0.34(λ/n)3, this cavity ranks among those with the highest Q/V ratios ever demonstrated, while having a small footprint suited for integration in photonic circuits.

We demonstrate a resonant optical trapping mechanism based on two-dimensional hollow photonic crystal cavities. This approach benefits simultaneously from the resonant nature and unprecedented field overlap with the trapped specimen. The photonic crystal structures are implemented in a 30 mm 6 12 mm optofluidic chip consisting of a patterned silicon substrate and an ultrathin microfluidic membrane for particle injection and control. Firstly, we demonstrate permanent trapping of single 250 and 500 nm-sized particles with sub-mW powers. Secondly, the particle induces a large resonance shift of the cavity mode amounting up to several linewidths. This shift is exploited to detect the presence of a particle within the trap and to retrieve information on the trapped particle. The individual addressability of multiple cavities on a single photonic crystal device is also demonstrated.

We demonstrate the fabrication of a hybrid PDMS/glass microfluidic layer that can be placed on top of non-transparent samples and allows high-resolution optical microscopy through it. The layer mimics a glass coverslip to limit optical aberrations and can be applied on the sample without the use of permanent bonding. The bonding strength can withstand to hold up to 7 bars of injected pressure in the channel without leaking or breaking. We show that this process is compatible with multilayer soft lithography for the implementation of flexible valves. The benefits of this application is illustrated by optically trapping subwavelength particles and manipulate them around photonic nano-structures. Among others, we achieve close to diffraction limited imaging through the microfluidic assembly, full control on the flow with no dynamical deformations of the membrane and a 20-fold improvement on the stiffness of the trap at equivalent trapping power.Recent developments in microfluidics show an important trend in the use of polymers and thermoplastics instead of materials such as glass. Polydimethylsiloxane (PDMS) especially holds a privileged role in microfluidics because of several advantages compared to glass and other polymers. First, it is flexible and easy to fabricate by soft lithography. 1 It can be bonded permanently to PDMS or flat surfaces like glass or silicon by oxygen plasma activation. It can also be bonded temporarily by simple conformal contact thanks to van der Waals (VdW) forces, which form a water-tight seal between two flat surfaces. Second, PDMS is biocompatible and rather inexpensive. [2][3][4][5] It is common to use PDMS to fabricate channels for the immersion of nanostructure for sensing applications, such as ring resonators and photonic crystals. [6][7][8][9] Unfortunately, PDMS also has some limitations, 10 in particular when considering high-resolution imaging.In most silicon-based structures, the sample is opaque to visible light. Thus, the imaging has to be done through the microfluidic layer. This also applies to any other opaque samples, like metallic substrates. Mostly two options have been investigated so far. The first option is to use a transparent PDMS microfluidic layer on top of the silicon sample. Another option would be to use a bonded SU8 microfluidic layer. 13 This option is limited to the fabrication of simple, externally driven microfluidic channels and doesn't allow a precise control of the flow within the micro-channels. Precise control of the flow in microfluidic membranes is generally performed with flexible valves. It is necessary when working with small objects in the solution injected in the microchanels.High resolution imaging is generally performed with immersion objectives, which have very strict operational conditions for limiting aberrations. In particular, the refractive index n D and the thickness t of the glass coverslip used have to be as close as possible to the predefined values used to design the objective (typically n D = 1.523 and t = 170 μm). Because of t...

We report on the behaviour of singly optically trapped nanospheres inside a hollow, resonant photonic crystal cavity and measure experimentally the trapping constant using back-focal plane interferometry. We observe two trapping regimes arising from the back-action effect on the motion of the nanosphere in the optical cavity. The specific force profiles from these trapping regimes is measured.