# Holographic Optical Tweezers: Techniques and Biomedical Applications
###### Abstract
A precalculated CGH, containing amplitude and/or phase information [28; 29], can be used to induce the designed light fields on the target regions. By using a suitable device to encode CGH, the wavefront shaping technique can expand the OT's ability to the 3D real-time multiple manipulations. This technique is called holographic optical tweezers (HOT) [30; 31; 32; 33; 34]. Three primary research groups contributed to the early development of HOT [30; 31; 32; 33; 34]--[PERSON] et al., (Stuttgart University) [30; 31], [PERSON] et al., (Chicago University) [32; 33; 35; 37; 38; 41] and [PERSON] et al., (Glasgow University) [34; 40]. They adopted spatial light modulators (SLMs) to encode the CGH information and used it as the input of the optical system. In preliminary attempts, [PERSON] et al. [41] utilized a commercial diffractive pattern generator to produce a square array of spots on the trapping plane. Later, they simulated an optimized phase-only CGH to create arbitrary configurations of optical tweezers and fabricated a diffractive optical element (DOE), using the photolithographic technique [35]. This system achieved 3D manipulation of multiple particles; however, researchers usually require prefabricated DOEs. With further advancements of technology, programmable liquid crystal displays (LCDs) have been utilized as SLMs [42]. To the best of our knowledge, [PERSON] et al. [30] first introduced LCDs into the OT system. They calculated the CGH to generate desired light fields on the Fourier plane of the LCD. Then they projected the CGH by using transmissive LCD into OTs and implemented the lateral manipulation dynamically on the focal plane. Later, they performed axial manipulation [31] by combining a virtual lens function into the CGH simulation. [PERSON]'s group also conducted a similar study [32], but they applied the Gerchberg-Saxton algorithm (GSA) [43] to optimize the phase-only CGH. However, these CGH designs offered only a single defocus trapping plane. To shape a desired 3D field, [PERSON] et al. [34] developed the first HOT with multiple trapping planes. Using the iterative 2D-GSA [44] for multiple planes and summing the phases corresponding to the same pixels in each plane, they calculated a single phase-only CGH to produce multiple discrete trapping planes. Owing to its ability to manipulate multiple particles dynamically, HOT has been applied in various fields of assembly [45; 46], microstructure fabrication [47; 48; 49; 50], and transport [48; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60]. Another important application of HOT has been in the orientation control of microparticles [40; 61]. Additionally, it has been used in microscopy for detection from different perspectives. For instance, HOT is often incorporated with digital holographic microscopy (DHM) [62; 63] to perform holographic tomography (HT) [64; 65; 66; 67] to estimate the 3D refractive index information of the bio-samples. CGH can also be used to modulate the angular momentum and polarization of the light [68]. Trapping light with orbital angular momentum transfers the angular momentum to the trapping particle [69; 70; 71; 72; 73] and endows the trapping particle with rotational or spin motion. This technique is useful for absorptive particles because they cannot be constrained within the focal region of a Gaussian beam. Polarization control is often related to spin angular momentum [72]. Vectorial light can trap and arrange polarization-sensitive objects. This review focuses on the spatially modulated technique since it has more applications in biomedical issues. For more than ten years, our team has also been working on the development and bio-application of OT, HOT. We applied LTRS to study the vibration modes of the newly discovered living cyanobacterium [14] and oleaginous microorganisms [16], and the results provided the information for revealing the characteristics of these microbials.