# Optical Effects Induced by Bloch Surface Waves in One-Dimensional Photonic Crystals
## 6 The BSW Application in Microparticle Optical Trapping  
The BSWs in photonic crystals are also promising in the field of optical control. Optical forces have been used for more than 40 years in laboratories for trapping, moving and studying nano- and microparticles, biological molecules and cells [57; 58; 59; 60; 61]. Single-beam gradient force optical traps, or optical tweezers, have become a routine tool for physical and biological research [62; 63]. The principle of operation of optical tweezers is based on the action of the ponderomotive force, which moves the particle with a refractive index higher than that of the surrounding medium along the intensity gradient of electromagnetic radiation. In optical tweezers, when the laser radiation is focused, this gradient is directed to the centre of the waist. However, an electromagnetic gradient can be created not only in the waist region of focused laser radiation. The possibility of trapping and moving nanoparticles and microparticles using an evanescent field that appeared with total internal reflection from a smooth surface and in optical waveguides was shown [64; 65; 66; 67]. Large values of optical field gradients can be achieved by using optical resonances of surface plasmons on metallic films and in plasmonic structures [68; 69; 70; 71; 72; 73]. However, local heating under the influence of optical radiation usually makes complicated the use of metal structures. The BSWs in photonic crystals can be an alternative to surface plasmons. They also provide a substantial increase of the optical field near the photonic crystal surface. At the same time, photonic crystals can be entirely made of transparent materials and devoid of heating problems. The force action of the BSW field in photonic crystals on micro-objects is considered in [74; 75; 76]. It is shown that this effect can be used for optical control. The force was experimentally determined using photon-force microscopy, which is based on the technique of optical tweezers. The method of photon-force microscopy consists of detecting the displacement of a test particle in an optical trap under the action of an external force: the higher the force, the greater the displacement. In fact, the trap acts as a dynamometer on the microscale in this method. The equilibrium position of the particle is determined by the balance between the external force and the restoring force, which tends to return the particle to the centre of the trap. In the experiments described in Refs. [74; 75; 76], a small amount of \(10^{-8}\) vol. % aqueous suspension of 1 \(\upmu\)m test particles of polystyrene was placed onto the surface of a photonic crystal on a glass substrate and hermetically covered with a cover glass, leaving a gap for the suspension with thickness of several tens of micrometers (see the inset in Figure 9). A particle from the suspension could be trapped at a controlled distance from the surface of the photonic crystal. The photonic crystal consisted of five pairs of ZrO\({}_{2}\)/SiO\({}_{2}\) layers with an optical thickness of each layer of 300 nm. The BSW at a wavelength of 532 nm could be excited at the photonic crystal/water boundary in a Kretschmann scheme of attenuated total internal reflection. To do this, \(s\)-polarized radiation from a continuous Nd:YAG laser with double frequency was directed to the inner surface of the photonic crystal through a glass prism. The power of the incident radiation was 1.6 kW/cm\({}^{2}\), with a beam divergence of 0.4 degrees. When the BSW was excited, the force from the BSW field began to act on a particle trapped near a photonic crystal surface. It caused a displacement of the average position of the particle in the optical trap. Figure 9 shows the experimentally obtained dependence of the force acting on the particle from the BSW. The BSW attracts the particle to the surface, and also pushes the particle along its propagation direction. The attraction is due to the gradient forces formed by the BSW electric field that exponentially decreases into the environment.