Arthur Ashkin was awarded the Nobel Prize in Physics 2018 for the invention of optical tweezers and their biological applications, which fulfilled the prediction of Ashkin "I think the field of biology may win a Nobel Prize for the great work done by optical tweezers". Optical tweezers have aroused extensive attention since they were born, which manipulate tiny objects by exerting optical forces. Till now, optical tweezers have made great achievements in the field of physics, chemistry, biology, medicine and nanotechnology.

In the process of interaction between light and matter, the transfer of linear momentum and angular momentum can generate optical forces, while the angular momentum of light can also induce optical torques. Optical torques have been widely used in optical manipulation for their advantages of non-contact, small size and high precision. Optical torques provide another degree of freedom in optical manipulation, finding wide applications in biomedical, physical and quantum sciences, including the fundamental mechanical properties of biopolymers and numerous molecular machines that drive the internal dynamics of cells.

On the one hand, optical torques endow biologists with a "microscopic hand" that can minimally manipulate organisms like a microscope because of their contactless nature. As a result, optical torques can control cells, viruses and other hard-to-control biological samples, avoiding potential sample damage and making many live experiments possible. Meanwhile, using non-contact capabilities, optical torques are able to drive the internal dynamics of cells without causing any damage. This can solve many thorny problems faced by biologists. On the other hand, optical torques are usually very small, down to the magnitude of pN?nm or even fN?nm, making them ideal for precise manipulation of small objects, such as rotating DNAs and proteins. With the support of such a precise tool, the characteristics of DNAs and proteins can be studied easier.

Except for biology, optical torques also have great potential applications in physical applications. As a cutting-edge technology, optical torque wrenches have been used to measure the Casimir torque and explore the quantum properties of gravity. Besides, it is known that optical torque wrenches are the basis of optical tweezers' quantitative experiments and have been used in various optical tweezers experiments, such as precise torque measurement and liquid viscosity determination.

In the past decade, the combination of optical torques with other technologies has further expanded the impact of optical torques in the field of biology and physics. For example, optical torques can be integrated with microfluidic systems. With the help of the transmission of fluid, optical torques assist optical tweezers capture the particle in the solution and transfer the angular momentum.

In this paper, we start by discussing the fundamentals and the conditions of two kinds of optical torques. By comparing the directions of optical torques and the angular momentum of the incident light, the optical torques can be classified as positive and negative optical torques. We discuss the mechanisms for the two kinds of optical torques in detail (Figs. 2-8). Then, we outline the traditional mechanisms for enhancing the optical torques, including spin-orbit coupling (Fig. 9), ring resonator (Fig. 10), and plasmonic structures (Fig. 11). Meanwhile, we introduce some leading-edge mechanisms for optical torques enhancement, like using super-hybrid modes from the combination of the electric toroidal dipole and magnetic multipoles (Fig. 12). Next, we review the physical and biological applications. In physical applications, we discuss the setup and theory of the optical torque wrench (Fig. 13) and micromechanics using optical torque, such as the Archimedes micro-screw (Fig. 14). In biological applications, we discuss the measurement of the biomolecule, like the characteristics of twisting in DNAs (Fig. 15), and we mention applications of optical torques in biorheology, biosensors, and micro-biorobots (Fig. 16). We culminate with a summary of the challenges in optical manipulation with optical torques, as well as the outlook of future developments, such as the torque sensing in biomedicine and light-driven biorobots.

The capabilities of optical torques to capture, control, and rotate particles have brought more opportunities for optical manipulation, compared with the translational movement of earlier optical tweezers technology. However, optical torques still face many challenges in optical manipulation. For example, the optical torque is generally small, so it is difficult to control large-sized particles, and the rotation efficiency is low, which brings some difficulties to the light-driven rotating micromechanics. Fortunately, nontrivial theoretical studies of topological optical forces have been proposed recently, which introduce topological concepts to optical forces at bound states in the continuum. This offers opportunities to realize optical force vortices, enhanced reversible forces, and even negative torque for manipulating nanoparticles and fluid flow. In the future, optical torques caused by topological optical force can be studied further to enhance the optical torques. In addition, by designing the diffraction characteristics of the metasurface, the direction control of the meta-robot can be realized, so as to realize the multi-dimensional steering of the light-driven robot. It can be predicted that optical torques will be more widely utilized in biomedical and physical sciences and other fields.