We have the capability to design optical materials for multilayer coatings as well as nanocomposites for plasmonic or scattering-based applications, like in solar energy collection or waveguiding. For instance, collective oscillations of the conduction electrons present in the metal volume or surface are known as plasmons. The spectral location of the plasmon resonance is known to depend on metal- and dielectric-type as well as the shape, size and spacing of nanostructures. Therefore, plasmonics with metallic nanoparticles on surfaces or embedded in dielectric materials offers great promise in guiding light along nanoscale interconnects for ultrafast information processing as well for use in materials showing enhanced or tunable light harnessing. For instance, several groups have shown that single-metal nanoparticles on or encapsulated by dielectrics on Si photodiodes result in enhanced photocurrent at specific wavelengths that were correlated to the surface plasmon resonance. Therefore, broadband plasmonic absorbers could significantly improve solar energy harvesting of semiconductor solar cells, and thus could greatly help overcome the outstanding global challenge to improve solar energy harvesting. However, despite the obvious potential impact of plasmonic materials on science and technology, an efficient, cost-effective and reliable process to design and nanomanufacture plasmonic nanocomposites does not exist. In this modeling activity (depicted in Fig. 1)we collaborate with Prof. Hernando Garcia at Southern Illinois University to develop and use self-consistent optical mixing models to accurately predict the linear and non-linear optical and plasmonic behavior in multi-metal NCs.
We have capability to numerically model the time-temperature behavior of surfaces and multilayer structures under laser pulses. For example, the time-dependent temperature behavior during laser pulsing is critical towards an understanding of materials response, such as dewetting pattern formation via self-organization. As depicted in Fig. 2, we have developed accurate nanosecond laser thermal models for single and multilayer films This model calculates the full time-dependent thermal behavior of the laser heating based on all the physics of laser-material interactions, including multiple internal reflections, and absorption, as well as phase change and thermal transport in two dimensions. The model also incorporates nanoscale effects arising from the change in optical behavior in the nanoscale.
We have capability to numerically as well as analytically model fluid flow in ultrathin films. For example, the spontaneous pattern formation via the classical spinodal dewetting instability in ultrathin films is a nonlinear process. However, the physical manifestation of the instability in terms of characteristic length and time scales can be described by a linearized form of the initial conditions of the film’s dynamics. Alternately, the thermodynamic (TH) approach based on equating the rate of free energy decrease to the rate of frictional loss via viscous dissipation [de Gennes, C. R. Acad. Paris 298, 111 (1984)] can give similar information. We have developed a model to evaluate dewetting in the presence of film-thickness- (h) dependent thermocapillary forces. Such a situation can be found during pulsed laser melting of ultrathin metal films where nanoscale effects lead to a local h-dependent temperature. The TH approach provides an analytical description of this thermocapillary dewetting. The results of our technique agree with those from linear theory and experimental observations provided the minimum dissipation is equated to the rate of free energy decrease. The flow boundary condition that produces this minimum dissipation is when the film-substrate tangential stress is zero. The physical implication of this finding is that the spontaneous dewetting instability follows the path of minimum rate of energy loss.
We have capability to model the coupled optical and thermal transport in complex shapes. Due to the nature of our research we are often confronted width understanding the coupled optical, thermal, and fluid transport in complex shapes in the nanoscale. We are currently building expertise in this problem through FEM techniques based on open source software, primarily Fenics.
We are presently expanding our optical modeling capability to also solve for the scattering profile from complex shaped nanoparticles, especially those that are not easily solved ysing Mie theory. Often we encounter such shapes in our research, such as hemispherical metal drops on surfaces. Thus it is important that we have an accurate picture of optical absorption so that the thermal behavior under pulsed laser irradiation is properly modeled, and eventually the fluid dynamics is correctly obtained. Therefore, we are presently building capability by utilizing open source software to solve for the scattering from complex shapes using the discrete dipole analysis technique.
Send us a message by email to ramki @ utk dot edu!