**Abstract**

Plasmonic nanoantennas provide an efficient platform to confine electromagnetic energy to the deeply subwavelength scales. The resonant absorption of light by the plasmonic nanoantennas provides the means to engineer heat distribution at the micro and nanoscales. We present thermoplasmonic metasurfaces for on-chip trapping, dynamic manipulation and sensing of micro and nanoscale objects. This ability to rapidly concentrate objects on the surface of the metasurface holds promise to overcome the diffusion limit in surface-based optical biosensors. This platform could be applied for the trapping and label-free sensing of viruses, biological cells and extracellular vesicles such as exosomes.

**Keywords:** metasurface, nano-antenna, thermoplasmonic, optofluidic, nano-tweezer

### **1. Introduction**

As an important category of metamaterials [1–4], which are artificial structures designed to achieve unique properties not occurring in nature, electromagnetic metasurfaces are able to provide unique electromagnetic responses for complete control over the phase, amplitude or polarization of light in a two-dimensional platform [5–9]. There are numerous kinds of applications that can be enabled by metasurfaces including holograms, metalens, near-eye displays, vortex beam generators and compact Spatial Light Modulators (SLM) [9–12] (**Figure 1**).

A meta-atom is the building block of a metasurface and it is usually a resonator made of either plasmonic or dielectric materials. In this section, we will focus on plasmonic metasurfaces to emphasize the significance of plasmonic metasurfaces for diverse applications, including on-chip optical trapping and optofluidic control. We will also present thermoplasmonic metasurfaces capable of inducing microfluidic motion in microchannels based on electrothermoplasmonic (ETP) effect, and discuss their role in on-chip nanoparticle trapping and manipulation [16–19].

Noble metals such as gold (Au) or silver (Ag) are typically used as plasmonic materials at visible or near-IR frequencies. Due to their ability to sustain plasmon oscillation, a well-designed metallic nanostructure can confine light in a very tiny volume, which generally creates a smaller mode volume than dielectric structures, especially at the resonance frequency. Hence, plasmonic resonators can tightly confine light in a tiny region so that they largely enhance the local electric field intensity. Furthermore, due to the Ohmic losses, the plasmonic materials generate heat under illumination. The enhanced local electric field results in efficient local

applied to simplifying the calculation of the optical force [24]. The detailed deriva-

In biological applications, target particles range in size from micrometer size scales such as cells and nanometer length scales such as viruses, protein molecules and vesicles. Trapping nanoscale particles is challenging in free-space optical tweezers. There are two main challenges faced by researchers using optical tweezers. The first challenge arises because the value of the gradient force is proportional to the third power of the radius of the particles in the quasi-static limit. Secondly, the optical gradient force is proportional to the gradient of light field intensity, and it is thus limited because the diffraction limit limits the achievable gradient in the light field intensity. One approach explored by Ashkin to increase the trapping stability for nanoscale objects in free-space optical tweezers involves the use of very high optical powers. However, this results in serious damage to the objects being

In order to overcome the limit on particle size that can be trapped in conventional optical tweezers, plasmonic optical tweezers have been proposed Novotny et al. [17, 26–33]. Plasmonic optical tweezers employ plasmonic nanoantenna, which can squeeze light to nanoscale volumes comparable to the size of the target particles, thus significantly enhancing the gradient force applied on nanoscale particles. Unlike the light-matter interaction between dielectric materials and electromagnetic wave, plasmonic materials uniquely react to the light field through freeelectron-photon coupling to generate a specific type of surface wave called surface plasmon polariton (SPP). Furthermore, a subwavelength plasmonic particle will efficiently couple to propagating light to generate localized surface plasmon resonance (LSPR) and further enhance the electric field due to this resonance. These phenomena permit light to be strongly localized at deep subwavelength scales. Besides, surface plasmon structures offer additional advantages on lab-on-a-chip manipulation due to its compatibility with integrated photonics devices [34, 35]. There are two main challenges with the use of plasmonic nanoantennas for nearfield nano-optical tweezers. These challenges include the Ohmic loss in the materials, which invariably results in loss-induced heating, and the need to rely on slow Brownian diffusion to transport particles towards the illuminated nanoantenna. The Ohmic loss in plasmonic materials at visible or near-IR is inevitable. It will not only hamper the efficiency of particle trapping due to the need to generate enough trapping potential energy to overcome the Brownian motion and possible

thermophoretic force, but the heat generated by Ohmic loss would be problematic in many experiments. Though the light field is tightly confined near an illuminated plasmonic nanoantenna leading to very high field intensity enhancement [16], which are advantageous for particle trapping, the damage from temperature rise due to this field enhancement affects the experimental trapping stability in several ways. Experimentally, bioparticles are vulnerable in an environment with increasing temperature. Furthermore, excessive photothermal heating could deform the plasmonic nanoantenna [36]. In aqueous solutions, this heating effect would even generate bubbles by boiling the water and disrupt the entire experiment. Wang et al. demonstrated that by integrating a heat sink in plasmonic tweezers made up of layers of high thermal conductivity materials, the adverse effects arising from the

Rather than treating the photothermal heating effect as detrimental in plasmonic

nanotweezer experiments, plasmonic heating can be harnessed to enable new

tion of MST and dipole approximation can be obtained in Ref. [25].

*Nanomanipulation with Designer Thermoplasmonic Metasurface*

*DOI: http://dx.doi.org/10.5772/intechopen.91880*

trapped, a process termed opticution.

**3. Plasmonic optical tweezers**

heating effect can be mitigated [37].

**99**

#### **Figure 1.**

*Various applications based on metasurfaces. (a) SEM image of metasurface lens composed of silicon nano-post array. (b) SEM image of a vortex beam generator made of silicon nano-post array. (c) A plasmonic metasurface hologram insensitive to light polarization states. (d) A vortex beam opto-multiplexer and demultiplexer made of gold at terahertz. (e) A titanium dioxide metasurface enhancing third harmonic generation to produce ultraviolet light. Images: (a) and (b) are adapted from [9] Copyright © 2016 American Chemical Society, (c): [13] Copyright © 2018 American Chemical Society, (d) [14] Copyright © 2017 American Chemical Society and (e) [15] Copyright © 2019 American Chemical Society.*

heating. Although, several research works in the literature have considered the loss in plasmonics as detrimental, it is now known that the loss in plasmonics can be beneficial to fast on-chip nano-particle manipulation [17, 20]. The loss-induced heating effect has given rise to the burgeoning field of thermoplasmonic metasurface.
