Optical nanoparticles are nowadays one of the key elements of photonics. They do not only allow optical imaging of a plethora of systems (from cells to microelectronics), but, in many cases, they also behave as highly sensitive remote sensors. In recent years, it has been demonstrated the success of optical tweezers in isolating and manipulating individual optical nanoparticles. This has opened the door to high resolution single particle scanning and sensing. In this quickly growing field, it is now necessary to sum up what has been achieved so far to identify the appropriate system and experimental set-up required for each application. In this review article we summarize the most relevant results in the field of optical trapping of individual optical nanoparticles. After systematic bibliographic research, we identify the main families of optical nanoparticles in which optical trapping has been demonstrated. For each case, the main advances and applications have been described. Finally, we also include our critical opinion about the future of the field, identifying the challenges that we are facing.

Mechanical forces play an important role in the behaviour of cells, from differentiation to migration and the development of diseases. Optical tweezers provide a quantitative tool to study these forces and must be combined with other tools, such as phase contrast and fluorescence microscopy. Detecting the retro-reflected trap beam is a convenient way to monitor the force applied by optical tweezers, while freeing top access to the sample. Accurate in situ calibration is required especially for single cells close to a surface where viscosity varies rapidly with height. Here, we take advantage of the well contrasted interference rings in the back focal plane of the objective to find the height of a trapped bead above a cover slip. We thus map the viscous drag dependence close to the surface and find agreement between four different measurement techniques for the trap stiffness down to 2 μm above the surface. Combining this detection scheme with phase contrast microscopy, we show that the phase ring in the back focal plane of the objective must be deported in a conjugate plane on the imaging path. This simplifies implementation of optical tweezers in combination with other techniques for biomechanical studies.

We present and demonstrate a multifunctional single-fiber optical tweezer for particle trapping and transport. The fiber probe of fiber optical tweezers is constructed as a planar structure. Laser sources with wavelengths of 650 nm and 980 nm in a single-mode fiber excite the linearly polarized LP11 mode and LP01 mode beams, respectively. These two laser beams can achieve non-contact trapping and long-distance transport of particles after passing through a flat-facet fiber probe, respectively. This structure makes it possible to perform non-contact trapping and transport of particles by combining multiple wavelengths and multiple modes.

在液体中灵活操纵微粒或细胞，特别是将细胞或微粒运输到指定位置，已经被证明在细胞分析、疾病诊断、药物递送等方面有着至关重要的作用。针对细胞或微粒非接触光学捕获的灵活性受限于光纤光镊操纵距离短的问题，提出了一种结构简单且具有长距离非接触可控操纵微粒的新型光纤光镊。利用加热和拉伸技术制作了类锥形平口光纤探针，980 nm的激光经过光纤探针后会对微粒产生大的散射力，将微粒逐渐推离光纤端口，同时借助反向流体阻力，在不移动光纤探针的情况下通过调节激光器光源的输出功率，对轴向位置上直径为6 µm的聚苯乙烯微粒可以进行长达102.2 µm的可控往返操纵，应用有限元法仿真了光镊的光场强度分布，并采用麦克斯韦应力张量法分析了光镊对微粒的作用力。实验和仿真结果表明，所提出的类锥形平口光镊是可行的。