Optical tweezers are a noncontact method of 3D positioning applicable to the fields of micro- and nanomanipulation and assembly, among others. In these applications, the ability to manipulate particles over relatively long distances at high speed is essential in determining overall process efficiency and throughput. In order to maximize manipulation speeds, it is necessary to increase the trapping laser power, which is often accompanied by undesirable heating effects due to material absorption. As such, the majority of previous studies focus primarily on trapping large dielectric microspheres using slow movement speeds at low laser powers, over relatively short translation distances. In contrast, we push nanoparticle manipulation beyond the region in which maximum lateral movement speed is linearly proportional to laser power, and investigate the fundamental limits imposed by material absorption, thus quantifying maximum possible speeds attainable with optical tweezers. We find that gold and silver nanospheres of diameter 100 nm are limited to manipulation speeds of ∼0.15 mm/s, while polystyrene spheres of diameter 160 nm can reach speeds up to ∼0.17 mm/s, over distances ranging from 0.1 to 1 mm. When the laser power is increased beyond the values used for these maximum manipulation speeds, the nanoparticles are no longer stably trapped in 3D due to weak confinement as a result of material absorption, heating, microbubble formation, and enhanced Brownian motion. We compared this result to our theoretical model, incorporating optical forces in the Rayleigh regime, Stokes' drag, and absorption effects, and found good agreement. These results show that optical tweezers can be fast enough to compete with other common, serial rapid prototyping and nanofabrication approaches.

A low-loss and low-dispersive optical-fiber-like hybrid HE 11 mode is developed within a wide band in metallic hollow waveguides if their inner walls are coated with a thin dielectric layer. We investigate terahertz (THz) transmission losses from 0.5 to 5.5 THz and bending losses at 2.85 THz in a polystyrene-lined silver waveguides with core diameters small enough (1 mm) to minimize the number of undesired modes and to make the waveguide flexible, while keeping the transmission loss of the HE 11 mode low. The experimentally measured loss is below 10 dB/m for 2 < ν < 2.85 THz (~4-4.5 dB/m at 2.85 THz) and it is estimated to be below 3 dB/m for 3 < ν < 5 THz according to the numerical calculations. At ~1.25 THz, the waveguide shows an absorption peak of ~75 dB/m related to the transition between the TM 11 -like mode and the HE 11 mode. Numerical modeling reproduces the measured absorption spectrum but underestimates the losses at the absorption peak, suggesting imperfections in the waveguide walls and that the losses can be reduced further.

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Realization of single-mode low-loss waveguides for 1.0-2.0 THz remains a challenging problem due to large absorption in most dielectrics and ohmic losses in metals. To address this problem, we investigate dielectric-lined hollow metallic waveguides fabricated by coating 1-mm diameter 38-μm-thick polytetrafluoroethylene tubes with silver. These waveguides support a hybrid HE 11 mode, which exhibits low attenuation and low dispersion. Quasisingle-mode propagation is achieved in the band of 1.0-1.6 THz, in which the hybrid HE 11 mode is supported by the waveguide. In this band, the experimentally measured loss is 20 dB/m (~0.046 cm −1 ), whereas the numerically computed loss is~7 dB/m (~0.016 cm). The difference is attributed to additional losses in the dielectric layer, which can be reduced by using alternative polymers.