The performance of plasmonic titanium nitride (TiN) nanoantennas for the manipulation of fluidic flow and suspended particles in an optofluidic chip is studied. A unified theoretical framework is utilized to model the multidisciplinary problem that comprises optics, thermodynamics, and hydrodynamics. Using multiphysics finite element analysis, we simulate the temperature rise resulting from the photothermal heating of a plasmonic TiN bowtie nanoantenna (BNA) and the accompanying hydrodynamic flow generated in a microfluidic channel. We show that the TiN BNA enables over three times higher electrothermoplasmonic flow velocity in comparison to a gold BNA under similar signal conditions. Our analysis shows that TiN BNAs at near-IR biological transparency wavelengths can be utilized to initiate strong microfluidic flow for directed transport and trapping of target nanoscale objects. This makes TiN an excellent plasmonic material choice for optically controlling heat, fluidic dynamics and heat-induced forces in microfluidic devices.
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Fig. 1 Geometry of the simulation. A TiN BNA is placed on a TiN film embedded in a thick glass or sapphire substrate and immersed in water. The meshed domains with representative dimensions are shown. A smaller domain was used for the EM simulation, while the heat transfer and fluid dynamics simulations were performed in the larger domain.
Fig. 2 Near field enhancement and absorption cross section spectrum for TiN BNA and Au BNA. The tip-to-tip spacing between the dimer is 10.7 nm, and the thickness is 120 nm. (a) Distribution of the plasmonic hotspot around the TiN BNA on 120 nm thick TiN film on glass substrate with longitudinal polarization. (b) Distribution of the plasmonic hotspot around the TiN BNA with transverse polarization. (c) Local electric field intensity enhancement as a function of wavelength for TiN BNA and Au BNA. (d) Absorption cross section as a function of wavelength for TiN BNA and Au BNA. BNA is bowtie-nanoantenna.
Fig. 3 (a) Axial temperature field for TiN BNA on TiN film on a sapphire substrate. (b) Axial temperature field for TiN BNA on TiN film on a glass substrate. The glass substrate enables a better temperature field confinement and a higher temperature rise. The irradiation spot diameter is 1.12 μm. (c) Axial distribution of the temperature rises from the substrate, through the BNA, and into the fluid for TiN BNA on TiN film, and Au BNA on Au film on glass and sapphire substrates. The inset shows the geometry of the system relative to the temperature field. Scale bar is 1000 nm.
Fig. 4 (a) 2D velocity vector of the induced electrothermoplasmonic (ETP) flow in x-y plane for TiN BNA on TiN film on a glass substrate. (b) 2D velocity vector of the induced electrothermoplasmonic (ETP) flow in x-z plane for TiN BNA on TiN film on a glass substrate. (c) Comparison of the magnitude of the radial velocity of the ETP flow for TiN BNA and Au BNA on a glass substrate. (d) Radial velocity distribution for TiN BNA with transverse and longitudinal polarizations with an applied voltage of 2 V.
Fig. 5 (a) Temperature rise in the water medium along the x-direction at a distance of 10 μm from the surface of the TiN BNA. (b) Temperature gradient along the x-direction at a distance of 10 μm from the surface of the TiN BNA. The inset shows the x-direction along which the temperature was obtained relative to the BNA for Figs. 5(a) and 5(b). (c) Maximum radial velocity of the ETP flow under laser illumination and an AC voltage of 2 V and 6 V for TiN BNA on glass and sapphire substrates. (d) Variation of the product of maximum temperature gradient and voltage square with the maximum radial velocity of the ETP flow. A linear variation is obtained both for when the TiN BNA (on TiN film) is on a glass or sapphire substrate.