Figure 1. Linearly polarized light (E0) incident on a 2D array of dielectric chiral L-structures with embedded gold nanorods producing elliptically polarized transmitted light with the rotation of polarization azimuth Δ.
Research in the Furlani group involves the use of computational electromagnetic modeling for fundamental understanding of nanophotonic phenomena and for the development of novel nanostructured metamaterials and integrated microdevices for applications broadly in the areas of sensing, imaging and biomedical therapy.
Nanophotonics is an interdisciplinary field that involves the study and application of light-matter interactions at the nanometer length scale. Artificial nanostructured materials (metamaterials) with subwavelength constituent elements can be designed to confine, enhance, slow down, filter and generally manipulate light to enable unprecedented control that cannot be achieved using naturally occurring materials. Metamaterials are having a transformative impact in the fields of imaging, sensing, display, communications and computing. Research in the Furlani group involves the use of computational electromagnetic modeling for fundamental understanding of nanophotonic phenomena and for the development of novel nanostructured metamaterials and integrated microdevices for applications broadly in the areas of sensing, imaging and biomedical therapy. Current projects include the development of chiral materials and planar metasurfaces for controlling the polarization of light and applications of plasmonics for sensing, imaging, photothermal therapy and all optical manipulation of colloidal nanoparticles.
Figure 2. L metamolecule: (a) computational model for a unit cell and field analysis showing Ey in the transmitted field with the incident polarization along the x-axis (ϕpol = 0o), (b) plasmon-enhanced polarization Py in the short segment of the L, orthogonal to the incident polarization.
We have recently proposed a method for realizing metasurfaces that can be used to control the polarization state of light.*The metasurfaces are formed from arrays of planar chiral-patterned dielectric metamolecules with embedded achiral plasmonic nanostructures as shown in Fig.1. At plasmon resonance, the subwavelength plasmonic nanoinclusions induce enhanced polarization of the surrounding dielectric, which gives rise to rotation of the polarization azimuth in the transmitted field. We use full-wave electromagnetic analysis to investigate the optical response of the media. The analysis shows that the metamolecules can be tailored to control the polarization state of light and produce frequency selective giant rotation of the polarization azimuth exceeding 105 deg/mm in the visible (vis) to near-infrared (NIR) spectrum with relatively low loss. Such ultra-thin media open up opportunities for the miniaturization of optical components such as wave plates, polarization rotators, and circular polarizers, thereby enabling new micro-optic applications that cannot be realized using conventional optically-thick components.
Figure 3. Polarization rotation spectra: (a) for an array of right handed (solid blue line) and left handed (dashed red line) lossless dielectric L-structures on a substrate, and (b) for an array of right handed (solid blue line) and left handed (dashed red line) L-structures with embedded Au nanorod. The normalized absorption spectra is also plotted (dotted black line).
Figure 4. Gammadion metamolecule: (a) computational model and field analysis showing Ey in the transmitted field for a unit cell with the incident polarization along the x-axis (ϕpol = 0o), (b) plasmon-enhanced polarization Py, orthogonal to the incident polarization.
Figure 1 shows an example of a metasurface consisting of L-shaped metamolecules. The computational domain for a unit cell of this media is shown in Figure 2a. This contains a single metamolecule, i.e. a dielectric L-structure with an embedded gold nanorod. The dielectric is lossless and has an index of refraction essentially that of conventional negative photoresist SU8. The gold nanorod is embedded in the center of the long segment of the L-structure as shown. The L metamolecules reside on a lossless glass substrate. The optical response with the incident E field aligned with the long segment of the L-structure, i.e. along the x-axis as shown in Figure 2a (ϕpol = 0 in Figure 1). It is instructive to compare polarization rotation with and without the Au nanorod inclusion in the near infrared (NIR) spectral range. As shown in Figure 3a, the peak rotation due to an array of lossless L-structures occurs at 846 nm and is approximately 0.12 degrees for a metamolecule thickness of h = 200 nm. Note that the rotation for the right-handed (ΔRH) vs. left-handed (ΔLH) structures has an opposite sign as expected. The same dielectric structure with an embedded Au nanorod produces a peak rotation Δ= 24.1 degrees at 965 nm.
Figure 5. Gammadion metamolecule: rotation spectra and transmittance.
The same dielectric structure with an embedded Au nanorod produces a peak rotation Δ= 24.1 degrees at 965 nm, the plasmon resonance wavelength. This represents a plasmon-enhanced 2000 fold increase in polarization rotation. We also studied metasurfaces consisting of arrays of gammadion metamolecules that have an embedded achiral Au cross. The metamolecules have C4 rotational symmetry, which gives rise to a rotation Δ that is essentially independent of the incident polarization. The computational model for a unit cell is shown in Figure 4a. The rotation spectra and transmittance for this medium are shown in Figure 5.
*Alali, Fatema, et al. "Plasmon-enhanced Metasurfaces for Controlling Optical Polarization." ACS Photonics (2014).
The interest in nanoscale photothermal phenomena has grown steadily in recent years along with new applications in fields such as analytical and material chemistry, nanophotonics and biomedicine. One of the most promising areas of research in this field involves the use of plasmonics wherein laser light is used to remotely heat sub-wavelength noblel metal (e.g. Au and Ag) nanoparticles. Such particles have unique optical properties that make them well-suited for photothermal heating, most notably they exhibit localized surface plasmon resonance (LSPR).
Figure 6. Photonic analysis of a nanotorus with parallel alignment to the incident polarization: (a) computational domain and field analysis, (b) normalized absorbed power vs. wavelength at parallel orientation.
At plasmon resonance, there is a coherent oscillation of free electrons within the particles that gives rise to intense absorption and scattering of incident light, as well as highly localized field enhancement. The absorbed photon energy is efficiently converted to heat, which is ultimately transferred to the surrounding medium.
We use computational models to investigate fundamental photothermal effects associated with nanosecond-pulsed, laser-heated colloidal plasmonic nanoparticles.* We use a combination of numercal photonic and computational fluid dynamc (CFD) analysis to simulate energy conversion within the nanoparticles at plasmon resonance, heat transfer from the particle to the surrounding fluid and phase change of the fluid leading to homogenous bubble nucleation. We study various nanoparticle geometries such as spheres, rods, rings and tori. and show that process parameters such as the laser intensity, incident wavelength, pulse duration and shape of the nanoparticles can be tuned to optimize the photothermal process.
We use 3D full-wave time-harmonic field theory to study the absorption spectra of the metallic nanoparticles as a function of their dimensions, dielectric properties and orientation relative to the polarization of the incident field. A typical computational model is shown in Fig. 6. The illustrates an Au nanotorus being illuminated with a linearly polarized planewave with the E field parallel to the x-axis. Fig. 6b shows the absorption spectra for this nanoparticle, which indicates a plasmon resonance of approximately 780nm.
We use CFD analysis to study the thermofluidic behavior of laser-heated nanoparticles in fluid. This is used to predict thermal, pressure and f low effects including the temperature rise in the particle, heat transfer from the particle to the fluid, phase change with in the fluid leading to homog eneous bubble nucleation, the dynamic behavior of the bubble as it expands and collapses, and the temperature, pressure and flow throughout the fluid during the entire process. The Flow-3D CFD program (www.flow3d.com ) was used for this analysis.
Figure 7 shows the temperature of the torus throughout the photothermal process along with corresponding images of th e bubble dynamics. Initially the nanotorus is at ambient temperature. After 0.2 ns it is illuminated and its temperature begins to rise. During the first 3.4 ns of heating, its temperature gradually increases (Fig 7a) to the superheat temperature, at which point a bubble is nucleated around it. Once this occurs the torus is surrounded by vapor and its temperature increases rapidly as it is still absorbing energy. It reaches a peak temperature of approximately 1 000K, which occurs at the end of the heat pulse (4.1 ns), at which point it is completely surrounded by vapor (Fig. 7b). As soo n as the bubble has nucleated, it e xpands and reaches its maximum size at 5.4 ns after the onset of heating. At this time the bubble has a spherical shape, approximately 80nm in radius.
An interesting feature of this process is the residue of an isolated drop of heated fluid that forms in the middle of the torus during the bubble expansion as seen in Fig. 7c. Eventually, 8.7 ns after the onset of heating, the nanobubble collapses, bringing fluid back in contact with the torus (Fig. 7d). Consequently, it slowly cools to the ambient temperature as more of the fluid comes in contact with it (Fig. 7e). It is instructive to note that the capillary force that acts to collapse the bubble is relatively weak because of the relatively large radius of curvature that defines the fluid vapor interface as it gets closer to the torus. Thus, the nanobubble requires a substantial amount of time to completely collapse, compared to other geometries.
Figure 7. Photothermal heat cycle of a nanotorus (cross-sectional view): plot of nanotorus temperature vs. time, pulse duration indicated by red arrow and dashed line and inset plots showing various phases of the thermal cycle; (a) initial heating, (b) nanobubble formation, (c) nanobubble (maximum size), (d) nanobubble collapse, (e) cooling.
Figure 8 shows a CFD simulation of pulsed-laser bubble generation and dynamics shown in Fig. 7.
*E. P. Furlani and I. Karampelas and Q. Xie, “Analysis of Pulsed Laser Plasmon-assisted Photothermal Heating and Bubble Generation at the Nanoscale,” Lab on a Chip, 7;12(19):3707-19, DOI 10.1039/c2lc40495h, 2012.
The interest in compact, portable and inexpensive biosensors has grown dramatically in recent years due in part to advances in microfluidics, especially lab-on-a-chip technology. A relatively new and promising approach to biosensing involves optofluidicswhere optic and fluidic functionality are integrated into a microsystem to leverage their combined advantages . Microfluidic functionality enables compact and rapid processing of small biofluid samples, and optical functionality enables high detection sensitivity of target biomaterials within these samples. To date, various optofluidic sensing devices have been developed.