Optical trapping and manipulation of micrometre-sized particles was first reported in 1970. Since then, it has been successfully implemented in two size ranges: the subnanometre scale, where light-matter mechanical coupling enables cooling of atoms, ions and molecules, and the micrometre scale, where the momentum transfer resulting from light scattering allows manipulation of microscopic objects such as cells. But it has been difficult to apply these techniques to the intermediate - nanoscale - range that includes structures such as quantum dots, nanowires, nanotubes, graphene and two-dimensional crystals, all of crucial importance for nanomaterials-based applications. Recently, however, several new approaches have been developed and demonstrated for trapping plasmonic nanoparticles, semiconductor nanowires and carbon nanostructures. Here we review the state-of-the-art in optical trapping at the nanoscale, with an emphasis on some of the most promising advances, such as controlled manipulation and assembly of individual and multiple nanostructures, force measurement with femtonewton resolution, and biosensors.

We study the Brownian motion (BM) of optically trapped graphene flakes. These orient orthogonal to the light polarization, due to the optical constants anisotropy. We explain the flake dynamics, measure force and torque constants and derive a full electromagnetic theory of optical trapping. The understanding of two dimensional BM paves the way to light-controlled manipulation and all-optical sorting of biological membranes and anisotropic macromolecules.The random motion of microscopic particles in a fluid was first observed in the late eighteenth century, and goes by the name of Brownian motion(BM) [1]. This was ascribed to thermal agitation[2], leading to Einstein's predictions of the resulting particle displacements [3]. BM is ubiquitous throughout physics, chemistry, biology, and even finance. It can be harnessed to produce directed motion [4]. It was also suggested that thermally activated BM may be responsible for the movement of molecular motors, such as myosin and kinesin [5]. When a Brownian particle (BP), i.e. a particle undergoing BM, is subject to an external field, e.g. a confining potential, the fluid damps the BM and, in a high damping regime, such as for a BP in water, the confining potential acts as a cut-off to the BM dynamics. This is free for short times (high frequency limit), while is frozen at longer times (low frequency limit) [8]. These processes have perfect ground in experiments with optical traps, where a BP is held by a focused laser beam, i.e. an optical tweezers [9]. In this context, BM can be utilized to investigate the properties of the surrounding environment [10,11], as well as of the trapped particle, and for accurate calibration of the spring constants of the optical harmonic potential [12,13].Dimensionality plays a special role in nature. From phase transitions [14], to transport phenomena[15], twodimensional (2d) systems often exhibit a strikingly different behavior [14]. Nanomaterials are an attractive target for optical trapping [16][17][18]. This can lead to top-down organization of composite nano-assemblies[16], sub-wavelength imaging by the excitation and scanning of nano-optical probes [17], photonic force microscopy with increased space and force resolution [18]. Graphene[19] is the prototype 2d material, and, as such, has unique mechanical, thermal, electronic and optical properties [20]. Here, we use graphene as prototype material to unravel the consequences of BM in 2d.Graphene is dispersed by processing graphite in a water-surfactant solution, Fig.1a. We do not use any functionalization nor oxidation, to retain the pristine electronic structure in the exfoliated monolayers [21][22][23]. We use di-hydroxy sodium deoxycholate surfactant. High resolution Transmission Electron Microscopy (HRTEM) shows∼10-40nm flakes. Fig.1b is an image of one such flake, with the typical honeycomb lattice. By analyzing over a hundred flakes, we find∼ 60% single-layer (SLG), much higher than previous aqueous [22] and nonaqueous dispersions [21], and the remainder bi-and trilayers (Fi...

We extract the distribution of both center-of-mass and angular fluctuations from three-dimensional tracking of optically trapped nanotubes. We measure the optical force and torque constants from autocorrelation and cross-correlation of the tracking signals. This allows us to isolate the angular Brownian motion. We demonstrate that nanotubes enable nanometer spatial and femtonewton force resolution in photonic force microscopy, the smallest to date. This has wide implications in nanotechnology, biotechnology, nanofluidics, and material science.

Combining state-of-the-art research with a strong pedagogic approach, this text provides a detailed and complete guide to the theory, practice and applications of optical tweezers. In-depth derivation of the theory of optical trapping and numerical modelling of optical forces are supported by a complete step-by-step design and construction guide for building optical tweezers, with detailed tutorials on collecting and analysing data. Also included are comprehensive reviews of optical tweezers research in fields ranging from cell biology to quantum physics. Featuring numerous exercises and problems throughout, this is an ideal self-contained learning package for advanced lecture and laboratory courses, and an invaluable guide to practitioners wanting to enter the field of optical manipulation. The text is supplemented by www.opticaltweezers.org, a forum for discussion and a source of additional material including free-to-download, customisable research-grade software (OTS) for calculation of optical forces, digital video microscopy, optical tweezers calibration and holographic optical tweezers.