The development and some applications of holographic optical tweezers (HOT) are presented. Our HOT system uses a spatial light modulator (SLM) to control the location and properties of the optical trap. We have developed a method for optimizing the diffraction efficiency of a SLM that can be applied in situ and addresses the issues of nonlinear phase modulation and phase modulation less than 2[pi]. The method employs a one-dimensional blazed phase grating written on the SLM. For an ideal SLM, the phase shift is linear and covers 0-2[pi], yielding a first-order diffraction efficiency of unity. For a realistic SLM with nonlinear or reduced phase shift, the efficiency is approximately [eta] =1 - [sigma]2, where [sigma]2 is the variance of the phase error from the ideal case. Because each pixel contributes to the phase error independently, this suggests a method to maximize the efficiency by adjusting the phase encoding of the SLM pixel-by-pixel. In practice, we do this by adjusting the gray-scale of each pixel while measuring the first-order diffracted power. The collection of optimal gray values comprises the optimized gray-scale lookup table, which exhibits the nonlinearity required to produce a linear phase grating and the saturated phase encoding that maximizes the efficiency of phase limited SLMs. The optimized SLM enables strong trapping power, even when distributed among multiple traps, which is essential to enable our system to trap multiple nanosensors and simultaneously detect the sensors' fluorescence spectra with an imaging spectrometer. Such nanosensors are capable of detecting changes in their environment such as pH, ion concentration, temperature, and voltage by monitoring changes in the nanosensors' emitted fluorescence spectra. We have used streptavidin labeled quantum dots bound to the surface of biotin labeled polystyrene microspheres to measure temperature changes by observing a corresponding shift in the wavelength of the spectral peak, which is excited with a 532 nm wide field laser source. Particles with diameter greater than the wavelength of light exhibit Mie resonances in their fluorescence spectrum whose spectral locations are dependent on the size of the particle and the relative index of refraction between the particle and the surrounding medium. HOT also provides a useful platform to study the micromechanical properties of elastic materials such as collagen. Collagen gels are widely used in experiments on cell mechanics because collagen is the most abundant protein in the mammalian extracellular matrix and is the primary source of its mechanical properties. Collagen gels are often approximated as homogeneous elastic materials; however, variations in the collagen fiber microstructure and cell adhesion forces cause the mechanical propertiesto be inhomogeneous at the cellular scale. We study the mechanics of type I collagen on the scale of tens to hundreds of microns by using HOT to apply picoNewton forces to micron-sized particles embedded in the collagen fiber network. We measure the local compliance and elastic modulus of the collagen network and find that particle displacements are inhomogeneous, anisotropic and asymmetric. Confocal reflection microscopy is used to reveal the local fiber structure and a simulation treating the network as a triangular lattice is used for comparison to the HOT measurements. Collagen samples prepared at 21°C and 37°C show that gels formed at lower temperature are more inhomogeneous, anisotropic, and compliant than those formed at high temperature, and cellularized samples allow us to characterize the effects of cell adhesion forces on the network mechanics.

Three-dimensional optical manipulation of microparticles, cells, and biomolecules in a noncontact and noninvasive manner is crucial for biophotonic, nanophotonic, and biomedical fields. Optical tweezers, as a standard optical manipulation technique, have some limitations in precise manipulation of micro-objects in microfluidics and in vivo because of their bulky lens system and limited penetration depth. Moreover, when applied for trapping nanoscale objects, especially with sizes smaller than 100 nm, the strength of optical tweezers becomes significantly weak due to the diffraction limit of light. The emerging near-field methods, such as plasmon tweezers and photonic crystal resonators, have enabled surpassing of the diffraction limit. However, these methods msay lead to local heating effects that will damage the biological specimens and reduce the trapping stability. Furthermore, the available near-field techniques rely on complex nanostructures fixed on substrates, which are usually used for 2D manipulation. The optical tweezers are of great potential for the applications including nanostructure assembly, cancer cell sorting, targeted drug delivery, single-molecule studies, and biosensing.

We are pleased to present “Optical Trapping and Manipulation: From Fundamentals to Applications”, a Special Issue of Micromachines dedicated to the latest research in optical trapping. In recognition of the broad impact of optical manipulation techniques across disciplines, this Special Issue collected contributions related to all aspects of optical trapping and manipulation. Both theoretical and experimental studies were welcome, and applications of optical manipulation methods in fields including (but not limited to) single molecule biophysics, cell biology, nanotechnology, atmospheric chemistry, and fundamental optics were particularly welcome in order to showcase the breadth of the current research. The Special Issue accepted diverse forms of contributions, including research papers, short communications, methods, and review articles representing the state-of-the-art in optical trapping.

This book systematically introduces readers to the fundamental physics and a broad range of applications of acoustic levitation, one of the most promising techniques for the container-free handling of small solid particles and liquid droplets. As it does away with the need for solid walls and can easily be incorporated into analysis instruments, acoustic levitation has attracted considerable research interest in many fields, from fluid physics to material science. The book offers a comprehensive overview of acoustic levitation, including the history of acoustic radiation force; the design and development of acoustic levitators; the technology’s applications, ranging from drop dynamics studies to bio/chemical analysis; and the insightful perspectives that the technique provides. It also discusses the latest advances in the field, from experiments to numerical simulations. As such, the book provides readers with a clearer understanding of acoustic levitation, while also stimulating new research areas for scientists and engineers in physics, chemistry, biology, medicine and other related fields.