Observation of Backaction and Self-Induced Trapping in a Planar Hollow Photonic Crystal Cavity

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 mu W). DOI: 10.1103/PhysRevLett.110.123601

Single particle detection, manipulation and analysis with resonant optical trapping in photonic crystals

Nicolas Maxime André Descharmes, Ulagalandha Perumal Dharanipathy, Zhaolu Diao, Romuald Houdré, Mario Lucien Henri Tonin

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 x 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.

Royal Society of Chemistry2013

Resonant optical trapping in microfluidic-integrated hollow photonic crystal cavities

Nicolas Maxime André Descharmes

The recent years have highlighted the potential of photonic nanostructures to extend the capabilities of optical tweezers and to perform advanced optical manipulations. In this context, two-dimensional photonic crystal cavities have been regarded as promising candidates owing to their high quality factors and low mode volumes. The work presented in this thesis reports the setting up of a complete experimental platform and the demonstration of optical trapping using the field confined in a planar photonic crystal cavity. In this work, cavities formed by a large circular defect, 700 nm in the diameter, in a triangular lattice are studied. These cavities possess a high quality factor (up to 7000 in air, around 2000 in water) and a central region where the confined field overlaps with the immersion medium. This feature offers a singular advantage for the maximization of the interaction between the cavity field and a particle as compared with standard photonic crystal cavity. An ultra-thin PDMS microfluidic membrane (≈170 μm) is developed for the reliable immersion of the hollow cavities and the control of the flow of nanoparticles to be trapped. The hollow cavities are then integrated within a microchannel within the membrane. The characterization and excitation of the cavities is then performed on the developed optical bench. Fluorescent imaging of the polystyrene particles is performed along with position tracking. The micro manipulation of the particles in the neighborhood of the cavity is realized through a combination of microfluidic valves and optical tweezers. Based on this experimental platform, the resonant optical trapping of 250 and 500 nm polystyrene particles is demonstrated. Trapping times of the order of several tens of minutes are obtained at sub-milliwatt optical powers. Further investigations are then performed on the perturbation of the cavity mode induced by the presence of a particle. This perturbation is experimentally observed in the form of a resonance wavelength shift of more than two cavity mode linewidth in the case of a 500 nm particle. Finally, the existence of back-action between the trapped particle and the cavity mode is experimentally evidenced. The later allows to observe the presence of two distinct trapping regimes. A qualitative understanding of these regimes is proposed based on a finite element method analysis.

EPFL2013

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

Life sciences have a constantly growing need for novel methodological approaches suitable to investigate the intracellular environment with increased temporal and spatial resolutions. Recently, optical near-field probes, such as laser-irradiated pointed metal, uncoated/metal-coated tapered optical fibers, as well as nano-emitters, such as single molecules or nanoparticles, have attracted increasing attention as key components of high-resolution microscopes. In parallel, super-resolution fluorescence microscopy, operating in far-field regime and overcoming the light diffraction limit, has markedly improved imaging resolution. Although both near- and far-field optical approaches drastically improve image resolution, they still represent numerous drawbacks, such as, technical difficulties concerning probe preparation (near-field) or potential damaging through very high light intensities (far-field). The aforementioned shortcomings of high-resolution optical microscopy can be overcome to a great extent by photonic force microscopy (PFM). PFM employs a strongly focused near-infrared (NIR) laser light to hold a dielectric or metallic particle as a local probe. Such an optical trap enables one to ‘cage’ a mesoscopic particle and track its three-dimensional (3D) thermal fluctuations in the surrounding environment, e.g. a viscous liquid. Therefore, besides offering near-field 3D imaging, PFM reports also on other important quantities concerning local mechanical properties, including force and viscosity as well as dynamical properties of the surrounding medium. This additional information is derived from the careful analysis of the jittery motion (so-called Brownian motion) of the trapped single-particle probe that collides with thermally-activated surrounding molecules. In this thesis, we first study in detail the Brownian motion of a single spherical particle that is confined within the harmonic potential of the optical trap. To this end, we optimize the NIR light path and electronic noise floor of our custom-built PFM set-up for detecting and quantifying resonances in the Brownian motion. These resonances arise from the coupling between the hydrodynamic memory and strong strength of the optical trap. Due to the high sensitivity of the short-time dynamics, the size of the particle can be measured by simultaneous fitting of the velocity autocorrelation function and power spectral density of its thermal fluctuations. In order to choose the best-suitable spherical probe for a given experiment in PFM, computational modeling based on a Matlab toolbox is performed. The generalized Lorenz-Mie theory is computed using the T -matrix method for various experimental conditions, including changes in the size and refractive index of the sphere, as well as different laser polarization states. The axial equilibrium position is examined to predict its location compared with the position of the laser focus. Optical forces acting on the sphere are investigated in 3D to highlight, in particular, the limits of the perfectly harmonic potential assumption. These limits are quantified and discussed. Subsequently, we identify different types of inorganic nano/micro-sized particles that can be trapped by the NIR laser of the PFM and used simultaneously as mechanical and near-field light sources. To this end, we take advantage of the visible light emission from single probes confined in the optical trap and excited by the NIR laser light. The emphasis is on particles revealing nonlinear optical properties. In particular, nonlinear optical field enhancement resulting from excitation of surface plasmon resonance (SPR) on trapped gold particles of 200 nm is demonstrated by measuring the emission of that second-harmonic generation (SHG). Furthermore, we show, under optical trapping conditions, SHG emission from single potassium niobate (KNbO3) nano/micro-sized particles. We also report on the multicolor upconversion luminescence (UCL), at the single particle level, for randomly shaped crystals of sodium yttrium fluoride codoped with ytterbium and erbium, NaYF4:Yb,Er (UCC), when they are held in the optical trap. In parallel, 3D Brownian fluctuations of a single trapped particle are quantified. Moreover, luminescence resonance energy transfer (LRET) between a KNbO3 particle or an UCC and molecules of an organic dye (rose bengal) adsorbed on their surface is highlighted and analyzed. Finally, randomly shaped UCCs and hexagonal nanoplates are characterized in detail at the single particle level. The hexagonal β phase in the polycrystalline and monocrystalline crystals is identified, in both randomly and hexagonally shaped particles by electron microscopy. UCL emission from the trapped crystal is quantified in terms of power and found to be dependent on the laser power density, size and shape of the particle as well as its orientation within the trap. Most importantly, the different colors in the UCL spectrum do not vary with the same trend for different orientations of the randomly or hexagonally shaped crystal upon trapping. This suggests the existence of crystalline anisotropy-dependent UCL. Thereby, we should be able to design the best-suited particle probe for LRET experiments with subwavelength resolution.

EPFL2013

Developing luminescent Brownian probes for near-field investigations of the intracellular environment

Flavio Maurizio Mor

EPFL2013

Resonant optical trapping in microfluidic-integrated hollow photonic crystal cavities

Nicolas Maxime André Descharmes

EPFL2013