Plasmonic nanoapertures generate strong field gradients enabling efficient optical trapping of nanoobjects. However, because the infrared laser used for trapping is also partly absorbed into the metal leading to Joule heating, plasmonic nano-optical tweezers face the issue of local temperature increase.Here, we develop three independent methods based on molecular fluorescence to quantify the temperature increase induced by a 1064 nm trapping beam focused on single and double nanoholes milled in gold films. We show that the temperature in the nanohole can be increased by 10°C even at the moderate intensities of 2 mW/µm² used for nano-optical trapping. The temperature gain is found to be largely governed by the Ohmic losses into the metal layer, independently of the aperture size, double-nanohole gap or laser polarization. The techniques developed therein can be readily extended to other structures to improve our understanding of nano-optical tweezers and explore heatcontrolled chemical reactions in nanoapertures.

Plasmonic nano-tweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nano-tweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20× higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nano-tweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.

In this paper, we describe the implementation of leakage radiation microscopy (LRM) to probe the chirality of plasmonic nanostructures. We demonstrate experimentally spin-driven directional coupling as well as vortex generation of surface plasmon polaritons (SPPs) by nanostructures built with T-shaped and Λ-shaped apertures. Using this far-field method, quantitative inspections, including directivity and extinction ratio measurements, are achieved via polarization analysis in both image and Fourier planes. To support our experimental findings, we develop an analytical model based on a multidipolar representation of Λ-and T-shaped aperture plasmonic coupler allowing a theoretical explanation of both directionality and singular SPP formation. Furthermore, the roles of symmetry breaking and phases are emphasized in this work. This quantitative characterization of spin-orbit interactions paves the way for developing new directional couplers for subwavelength routing. PACS numbers:Chiral plasmonic nanostructures [1] exhibit unique optical properties, such as asymmetric optical transmission [2] and singular optical signatures, like vortices, visible in both the optical near-field [3,4] and in the far-field of twisted structures [5,6]. Motivated by fundamental questions as well as by their potentials ranging from highly integrated photonic circuits to quantum optics [1,7], interest in the field of chiral plasmonics has subsequently become a topic of intensive research. These peculiar optical effects stem from the spin-orbit interactions of light via plasmonic nanostructures, in which the photon spin couples to its spatial motion [3] and in particular to its orbital angular momentum. This leads to optical spin Hall effects [8,9], i.e., to a polarization-dependent photon shift evidenced with SPPs [3, 10, 11] which can be used for instance for inducing SPP directional coupling [12][13][14]. In this context, recent studies done in the optical near-field demonstrated that spin-controlled SPP directionality and vortex generation can be achieved with chiral nanostructures such as T-shaped aperture arrays, rings or spirals milled in metal films [15,16]. While the additional degree of freedom added by the incident spin enables tunable directionality, enhanced directional coupling is achieved by the broken symmetry of the plasmonic structures. Moreover, since controlling SPP propagation direction and rotational motion is essential for applications in integrated optics and optical trapping, it becomes urgent to develop sensitive imaging techniques to map plasmonic chirality not only in the near-field but also in the far-field. Due to the inherently confined SPP fields, near-field optical detection has been widely employed in the past to image SPP propagation (for a review see [17]). Indirect imaging via scattering of SPPs or via grating have also been used [18][19][20]. Here, we propose a different approach based on leakage radiation microscopy (LRM) [21][22][23]. As a complementary method for direct imaging of SPP propagation, L...

Single photon sources with high brightness and subnanosecond lifetimes are key components for quantum technologies. Optical nanoantennas can enhance the emission properties of single quantum emitters, but this approach requires accurate nanoscale positioning of the source at the plasmonic hotspot. Here, we use plasmonic nanoantennas to simultaneously trap single colloidal quantum dots and enhance their photoluminescence. The nano-optical trapping automatically locates the quantum emitter at the nanoantenna hotspot without further processing. Our dedicated nanoantenna design achieves a high trap stiffness of 0.6 (fN/nm)/mW for quantum dot trapping, together with a relatively low trapping power of 2 mW/μm 2 . The emission from the nanoantennatrapped single quantum dot shows 7× increased brightness, 50× reduced blinking, 2× shortened lifetime, and a clear antibunching below 0.5 demonstrating true single photon emission. Combining nano-optical tweezers with plasmonic enhancement is a promising route for quantum technologies and spectroscopy of single nano-objects.

*We study theoretically and experimentally coherent imaging of surface plasmon polaritons using either leakage radiation microscopy through a thin metal film or interference microscopy through a thick metal film. Using a rigorous modal formalism based on scalar Whittaker potentials we develop a systematic analytical and vectorial method adapted to the analysis of coherent imaging involving surface plasmon polaritons. The study includes geometrical aberrations due index mismatch which played an important role in the interpretation of recent experiments using leakage radiation microscopy. We compare our theory with experiments using classical or quantum near-field scanning optical microscopy probes and show that the approach leads to a full interpretation of the recorded optical images.

The gold adhesion layer can have a dramatic impact on the thermal response of plasmonic structures, offering new ways to promote or avoid the temperature increase in plasmonics.

We introduce a new paradigm: the chiral electromagnetic local density of states (LDOS) in a spiral plasmonic nanostructure. In both classical and quantum regimes, we reveal using scanning near-field optical microscopy (NSOM) in combination with spin analysis that a spiral cavity possesses spin-dependent local optical modes. We expect this work to lead to promising directions for future quantum plasmonic device development, highlighting the potentials of chirality in quantum information processing.

Plasmonic nano-optical tweezers enable the non-invasive manipulation of nano-objects under low illumination intensities, and have become a powerful tool for nanotechnology and biophysics. However, measuring the trap stiffness of nanotweezers...