We present a novel plasmonic sensor configuration that allows the discrimination of chiral molecules. The sensor consists of handed gold nanostructures of gammadion shape, distributed in a racemic (50/50 mixture) matrix with C4 symmetry. Its optical response enhances the interaction with molecules thus circular dichroism can be measured in the visible range. The bare sensors exhibit a flat CD signal, providing background-free CD measurements for molecular detection. We have used a chiral molecular model based on L-, D-, and the racemic mixture of phenylalanine, which allows us to evaluate the opposite chiral effects while having a reference system. Additionally, we have used molecular thermal evaporation technique to deposit a dense molecular layer on top of the sensors in a controllable and reproducible way. Our results show the discrimination of phenylalanine enantiomers through positive or negative peaks while the racemic mixture shows a flat signal. In addition, we present preliminary results that show that this approach is also suitable for microfluidics systems with a much lower density of chiral molecules.
In the context of cavity optomechanics alternative techniques without the need of atomic resonances have widened the possibilities towards the cooling of macroscopic objects. Recently the radiation pressure cooling of mechanical oscillators, optomechanically induced transparency and ground state cooling have been demonstrated. Current progress in optomechanics has brought forward multiple experimental platforms of which many platforms necessitate complex cryogenic environments and suffer from clamping losses as major decoherence sources. An alternative approach is the cooling of levitated nanoparticles from room temperature, which have been suggested for probing quantum mechanics on the mesoscopic scale. In levitated systems collisions with residual gas molecules and photon recoil heating are now the remaining decoherence sources paving the way towards low phonon occupations. In the context of cavity optomechanics, resolved sideband cooling of a levitated nanoparticle has recently been realised. Here we demonstrate the resolved sideband cooling of a levitated nanoparticle within a high nesse cavity at high vacuum. Trapping the nanoparticle in an external optical tweezer allows on one hand the free positioning of the particle within the cavity fi eld and on the other hand the additional cooling via parametric feedback cooling. The combination with well-established resolved sideband cooling techniques creates a powerful platform for controlling the centre of mass motion (COM) of a mesoscopic object. By exploiting cavity enhanced Anti-Stokes scattering we all optically cool the COM to minimum temperatures of T ~ 100mK for a silica particle of 235nm diameter. Power dependent laser noise heating is observed, being the main current limitation in reaching lower temperatures. In the future laser noise suppression for resolved side band cooling brings low phonon occupation numbers of mesoscopic systems via passive cooling schemes within the reach of table top experiments at room temperatures.
Among the quantum systems capable of emitting single photons, the class of recently discovered defects in hexagonal boron nitride (hBN) is especially interesting, as these defects offer much desired characteristics such as narrow emission lines and photostability. Like for any new class of quantum emitters, the first challenges to solve are the understanding of their photophysics as well as to find ways to facilitate integration in photonics structures. Here, we will show our investigation of the optical transition in hBN with different methods: Employing excitation with a short laser pulse the emission properties in case of linear and non-linear excitation can be compared . The possibility to perform two-photon excitation makes this single photon emitter an interesting candidate as a biosensor. We further show the behaviour of defects in hBN when being excited with different wavelengths and deduce the consequences for its level scheme. Here, it is found that the quantum efficiency of the emitters varies strongly with excitation wavelength, a strong indication of a branched level system with different decay pathways.
 A W Schell et al., APL Photonics 1, 091302 (2016)
 A W Schell et al., arXiv:1706.08303 (2017)
In this talk we first introduce the use of a levitated nanoparticle in vacuum as a nano-optomechanical system with unprecedented performances. Subsequently, we focus on our efforts in cooling its motion towards mechanical ground state at room temperature. In particular, we present an experiment that combines active parametric feedback cooling with passive resolved side band cooling. We first demonstrate systematic transfer of a single trapped nanoparticle from a load lock to the main vacuum chamber hosting a high-finesse optical cavity and report our latest advances in cooling.
Micsospheres trapped in liquid by so called optical tweezers have been established as useful tools to study microscopic thermodynamics. Since the sphere is in direct contact with the liquid, it is strongly coupled to the thermal bath and its dynamics is dominated by thermal fluctuations. In contrast, here we use an optically trapped nanoparticle in vacuum to study fluctuations of a system that is coupled only weakly to the thermal bath. The weak coupling allows us to resolve the ballistic dynamics and to control its motion via modulation of the trapping beam, thereby preparing it in a highly non-thermal state. We develop a theory for the effective Hamiltonian that describes the system dynamics in this state and show that all the relevant parameters can be controlled in situ. This tunability allows us to study classical fluctuation theorems for different effective Hamiltonians and for varying coupling to the thermal bath ranging over several orders of magnitude.
The ultimate goal, however, is to completely suppress the effect of the thermal bath and to prepare the levitated nanoparticle in a quantum mechanical state. Our most recent result indicate that this regime is now within reach.
Optical biosensing based on gold nanoparticles supporting localized surface plasmoncs (LSPR) potentially offers great opportunities for compact, sensitive and low cost diagnostic devices. While last two decades have witnessed a diversity of nanoplasmonic systems with outstanding sensitivity, the implementation of LSPR sensing into a real analytical device is only at its infancy. In this context, we present here our latest advances in the optical, label free detection of biomolecules based on gold nanoantennas integrated into a state-of-the-art microfluidic platform. We first demonstrate the capability of our platform to detect low concentrations (<1ng/ml) of protein cancer markers in human serum with low unspecific binding and high repeatability. In a second step we present a novel design that enables to simultaneously determine the absolute concentration of four different target molecules from an unknown sample. The system is validated in the context of breast cancer, as a strategy to assess the risk for brain metastasis. In the final part of the paper we discuss the use of LSPR sensing for the detection of other targets, including DNA and exosomes. Our research demonstrates the high potential of gold nanoparticles for the detection of different biomarkers in real biological samples and thus gets us closer to future LSPR-based point-of-care devices.
Bistable systems are ubiquitous in nature. Classical examples in chemistry and biology include relaxation kinetics in chemical reactions  and stochastic resonance processes such as neuron firing [2,3]. Likewise, bistable systems play a key role in signal processing and information handling at the nanoscale, giving rise to intriguing applications such as optical switches , coherent signal amplification [5,6] and weak forces detection .
The interest and applicability of bistable systems are intimately connected with the complexity of their dynamics, typically due to the presence of a large number of parameters and nonlinearities. Appropriate modeling is therefore challenging. Alternatively, the possibility to experimentally recreate bistable systems in a clean and controlled way has recently become very appealing, but elusive and complicated. With this aim, we combined optical tweezers with a novel active feedback-cooling scheme to develop a well-defined opto-mechanical platform reaching unprecedented performances in terms of Q-factor, frequency stability and force sensitivity [7,8]. Our experimental system consists of a single nanoparticle levitated in high vacuum with optical tweezers, which behaves as a non-linear (Duffing) oscillator under appropriate conditions. Here, we prove it to be an ideal tool for a deep study of bistability.
We demonstrate bistability of the nanoparticle by noise activated switching between two oscillation states, discussing our results in terms of a double-well potential model. We also show the flexibility of our system in shaping the potential at will, in order to meet the conditions prescribed by any bistable system that could therefore then be simulated with our setup.
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We review recent advances achieved in the field of integrated optical manipulations based on the control of surface
plasmons. In particular, we describe how intense and confined optical force fields can be precisely engineered in
the vicinity of transparent surfaces patterned with plasmonic metal structures.Recent progresses in the design
of such devices is presented together with the latest experiments that have demonstrated efficient trapping of
micro-objects under reduced laser intensity compared with conventional optical tweezers. Finally, we review other
proposals where the use of localized surface plasmons in coupled metal nanostructures opens new perspectives in
scaling down the trapping volumes well below the diffraction limit for the manipulation of single nano-objects.
Surface plasmon based photonic devices are promising candidates for highly integrated optics. An important effort in the
development of these devices is dedicated to the design of systems allowing the two dimensional control of surface
plasmon (SPP) propagation. Recently, it has been shown that Bragg mirrors consisting of gratings of metallic lines or
indentations on a metallic surface are very efficient tools to perform this task. Alternatively, using structured dielectric
layers on top of the metallic layer to build SPP optical elements based on the effective refractive index contrast has been
lately demonstrated. This kind of elements relies on the same principles as conventional optical elements. Here we
analyze the ability of gratings of dielectric ridges deposited on a metallic layer to act as dielectric SPP Bragg mirrors.
The dispersion relation of these systems shows the presence of a gap whose position can be approximately predicted by
the same relation as for standard optical Bragg mirrors. The properties of these dielectric based SPP Bragg mirrors have
been examined as a function of several structural grating parameters. The obtained results have been experimentally
confirmed by means of Fourier plane leakage radiation microscopy.
Two-photon induced photoluminescence (TPL) microscopy has been used to probe the local field of nanoantennas.
We demonstrate that TPL imaging is directly correlated to the antenna electromagnetic mode computed
with a full 3D solver. Furthermore, spectroscopic mode mapping while scanning the incident wavelength enables near-field spectroscopy of specific areas of the antenna response, providing a deeper insight into its
We report on the electromagnetic interactions between a two-dimensional periodic arrangement of resonant gold nanoparticles and a flat gold metallic film. We observe multi-peaks in the extinction spectra attributed to resonant modes of the hybrid system, resulting from the coupling between the Localized Plasmon of the nanoparticles with the underlying Surface Plasmon Mode. Simulations based on the Fourier modal method give good agreement with the experimental measurements and allow for the identification of the respective contributions.
We investigate the local field spectroscopy of gold dimers by Two-Photon Photoluminescence (TPL) microscopy. A direct comparison with far-field scattering measurements shows that TPL provides additional data on the structure modes of major importance for their use in SERS, enhanced fluorescence and sensing.
Following the recent advances in nano-optics, optical manipulation by evanescent fields instead of conventional propagating fields has recently awaken an increasing interest. The main advantages of using low dimensionality fields are (i) the possibility of integrating on a chip applications involving optical forces but also (ii) the absence of limitation by the diffraction limit for the trapping volume. Previous works have investigated theoretically and experimentally the guiding of dielectric and metallic beads at an interface sustaining an extended surface wave. In this work, we study theoretically the radiation forces exerted on Rayleigh dielectric beads under local evanescent illumination. Especially, we consider the configuration where a three-dimensional Gaussian beam is totally reflected at the interface of a glass prism. The results point out the illumination parameters where the gradient forces exceed the scattering force and allow for a stable trapping. The effect of the Goos-Haenchen shift on the location of the trapping site is also discussed.
We report quantitative measurements of the radiation forces exerted on a micrometer dielectric sphere by a Surface Plasmon Polariton (SPP) excited at a gold/water interface. We separate the contributions of the two constituents of the plasmon wave - the electromagnetic field and the charge-density oscillations - to the total radiation force. Measurements performed with a Photonic Force Microscope (PFM) show an enhanced attraction to the surface compared to a conventional evanescent wave at the dielectric interface (10<sup>2</sup> enhancement factor).
The trapping of micro-objects by optical radiation forces, so-called optical tweezers, has become widely used in physical, chemical and biological experiments where accurate and non-invasive manipulation is required. Recent advances in beam shaping render it possible for instance to rotate or to dynamically manipulate independently several elements. Today, one of the remaining challenges of conventional optical tweezers is the direct manipulation of systems with sizes belonging to the sub-wavelength or Rayleigh regime. Indeed, the diffraction limit prevents in that case from achieving a commensurable trapping volume and thus does not allow for minimizing the fluctuations in position of the trapped object due to its strong Brownian motion. In order to overcome this limitation, it has been proposed to use evanescent fields instead of the usual propagating fields. Recent advances in optics of noble metal nano-structures have recently provided new configurations to achieve nano-optical tweezers. Especially, tightly localized modes resulting from the coupling between resonant noble metal nanostructures may offer the gradient forces able to trap and manipulate Rayleigh objects. In this work, we calculate the radiation forces exerted on a nanometric dielectric sphere when exposed to a patterned optical near-field landscape at an interface decorated with resonant gold nanostructures. By comparing their magnitude with other forces that affect the movement of the particle, we discuss the practical ability of our configuration for multiple parallel optical manipulation.
Resonant noble metal nanoparticles with dimensions of a few tens of nanometers and sustaining Localized Surface Plasmon (LSP) modes have been recently proposed as good candidates for increasing both integration and sensitivity compared to conventional extended thin metal films. Very recently several groups have reported results of sensing with a single nano-particle.
The study we present contains two main parts. First, using randomly distributed colloidal gold spheres, we demonstrate the ability of LSP sensors for monitoring quantitatively and without the use of any label, the binding between small organic molecules and antibodies with real-life applications. In a second part, the Fourier Modal Method (FMM) is used to model controlled geometries of particles that allow for optimizing the sensor properties. In particular, we show that the electromagnetic coupling within a periodic 2D particle array can be optimized to increase the field localization and thus the sensitivity of the sensor.
In the ongoing general trend for miniaturization, there is an
increasing interest in the manipulation of electromagnetic fields
at the nanometer scale. A main obstacle to this goal is the
diffraction limit that prevents from focussing light down to
volumes much smaller than the incident wavelength. In order to
overcome this limitation, it has been proposed to deal with
evanescent fields instead of the conventional propagating beams.
Especially, plasmon fields bound at a noble metal interface or
around metal nanostructures have shown to be very suitable to
control the light confinement down to the nanometer scale. In this
work we investigate the near-field coupling in finite metal
particle chain geometry. The Green Dyadic method is used to
demonstrate that high enhancement factors can be achieved by
exploiting the in-plane forward scattering of the particles, with
no need for cumbersome geometries with nanometer features.
We report the sub-wavelength patterning of the optical near-field
by total internal reflection illumination of a regular array of
resonant gold nano-particles. Under appropriate conditions, the
in-plane coupling between localized surface plasmon fields
gives rise to sub-wavelength light spots between the structures.
Measurements performed with an Apertureless Scanning Near-Field
Optical Microscope (ASNOM) show good agreement with theoretical
predictions based on the Green dyadic method. This concept might
offer a convenient way to elaborate extended optical trap
landscapes for manipulation of sub-micrometer systems.