# Plasmonic Tweezers towards Biomolecular and Biomedical Applications
## 3 Major Designs
### Bowtie Nano-Apertures (BNA)  
Bowtie nanostructure is the most used plasmonic design in sensing and trapping with localized surface plasmons (LSP), since they can provide strong light confinements between the two sharp tips. They have been used to trap and manipulate nanoscale particles [62; 63; 64], but the heating accumulation could be an issue for stable trappings and the bio-sample integrity [65].  
Similar to the protruding bowtie nanostructures, bowtie nano-apertures (BNAs) also can be used to generate strong confinements which provide large near field gradients for optical trapping. Similar to monitoring transmission to detect trapping events, the second harmonic (SH) signal from the PTs was used to identify successful trappings by [PERSON] et al. [66]. The presence of the trapped nanoparticle enhances the field distribution near the tips and thus the SH signal is enhanced. The SH signal is quadratic dependent on the intensity and an optical filter which blocks the transmitted laser beam can be used to collect the SH signal. This methodology is more sensitive than the transmission measurement. The scheme of nanoparticle trapping and SH signal for monitoring purposes is shown in Figure 3a. The BNA was designed in a 100 nm thick gold film deposited on a SiO\({}_{2}\) substrate. Two apexes were tailored in 3D. The plasmonic resonance (\(\lambda_{\omega}\)) of this system in water was 1560 nm. A CW (continuous wave) laser at 1560 nm was used as the trapping light source and a femtosecond pulsed laser at 1560 nm was used as the SH pump source. The simulated near field distribution is shown in Figure 3b. The maximum enhancement is perpendicular to the surface of the gold thin film and the SH signal (\(\lambda_{2\omega}=780\) nm) can be generated from the gold surface where the inversion symmetry is broken. In Figure 3c, SEM images of a fabricated BNA are demonstrated. CdSe/ZnS (diameter \(\sim\)4.0 nm, \(\lambda_{em}\sim\)620 nm, not excited) quantum dots were successfully trapped. Figure 3d,e are the plots of SH signal intensity with fixed pump power at 4 mW when two trapping powers at 10 mW and 40 mW were used. Figure 3f,g are the corresponding transmission intensity plots. As expected, the presence of the quantum dot affected the distribution of the near field, and the SH signal and transmission signal were both modified. Three gaps of 5 nm, 8 nm, and 10 nm were tested. For 5 nm gap size BNA, the largest change in SH and transmission signal was observed. Spikes in both signals demonstrate that the quantum dot oscillated in the gap region.  
BNAs have also been applied in trapping 20 nm polystyrene particles with a SIBA effect [67] and quantum dots [68]. Although the transmission and SH signal from PTs can be monitored to identify trapping events for nonfluorescent particles, this type of detection methodology is not good enough since no comprehensive information of the trapped particle can be provided. Utilizing the enhanced near field, specific light-matter interactions, e.g., Raman scattering, can be applied to identify the trapped particle when a long, stable trapping can be achieved. Although the SH signal from PTs, not the trapped particle, was utilized, their research expands the monitoring methods for nanoscale particles trapping.