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Erik Nelson,
Graduate Student in Materials Science and Engineering

B.S. degree in Materials Science and Engineering from Georgia Institute of Technology

My research is focused on optically interesting structures from the perspectives of both application driven research and fundamental or scientific discovery research. I am currently working with photonic crystals fabricated via colloidal self assembly. These colloidal crystals can be given functionality by having designed defects precisely placed in the structure followed by subsequent infilling with high refractive index materials such as Si or Ge. For optically active structures nonlinear optical materials can also be deposited. By controlling the shape and placement of the defect structures as well as the placement of nonlinear or high index materials many interesting properties can be achieved with photonic crystals that cannot be seen in other material systems or do not occur in nature.

My research involves designing structures to manipulate the optical properties of the constituent materials in order to achieve properties not available in natural materials. One such structure is a system with a periodic modulation of the refractive index on the length scale of the wavelength of light, generally referred to as a photonic crystal. For certain geometries, a photonic band gap (PBG) may open up if the dielectric contrast is sufficiently high. This is analogous to a semiconductor bandgap, only in this case certain frequencies of light are forbidden within the crystal, rather than certain electron energies. This property has promise for many applications, the most commonly discussed being low-threshold lasers, low-loss waveguides and all optical circuitry.

One simple and rapid method of fabricating such a structure is through colloidal self-assembly where spheres (usually silica or polystyrene) are assembled into an ordered FCC structure. These crystals can then be infilled with high index materials such as Silicon and the spheres removed by a chemical etch or burning. The remaining structure (referred to as an inverse opal) exhibits a modest bandgap. Photonic crystals are completely scalable as well, meaning that the periodicity of refractive index modulation can be changed to adjust the location of the bandgap. In our system the size of the colloids can be changed to tune the PBG to a wavelength of interest.

Figure 1. a) Laser scanning fluorescence confocal microscope image parallel to the substrate of two TPP features drawn through the top of the crystal and into the overlying monomer solution. Defects can be seen near the TPP feature as bright dots or lines. b) SEM image of the TPP features showing the high degree of lattice registration. c) SEM image of the narrower TPP feature highlighting the registration along two crystalline directions. d) SEM image of a TPP point feature placed between three colloids.

Designed defects can be added to photonic crystals to give them a desired functionality. Direct fabrication approaches such as lithography or direct laser writing allow the incorporation of defects during the assembly process. Colloidal crystals on the other hand require a method of writing defects after the assembly process is complete. One such technique used in our group is two-photon polymerization, where defects of arbitrary shape can be written in the colloidal crystal to create structures such as planar defects, waveguides or point defect cavities.

Two-photon polymerized structures are written on a confocal microscope with a Ti:Sapphire laser. The two-photon polymerization process requires two photons to interact with a photoinitiator within an extremely short time scale. In order to achieve this flux of photons a high energy pulsed laser source must be used. An advantage of this process is that polymerization only occurs within a small volume at the focal point of the laser. Additionally, thresholding agents can be added to allow features smaller than 100nm to be written. With the use of a compatible fluorescent dye we can align features almost perfectly with the crystal lattice. This level of control has been shown to be essential for many proposed device designs.

While the PBG is probably the most studied aspect of photonic crystals there are a number of other very important properties worth studying. One example is the tremendously reduced group velocity of light for certain frequencies in the crystal (specifically those near the edges of the PBG or at other flat bands in the band structure). This “slow light” results in an enhancement of the electric field as well as in increase in the density of states for these “slow light” frequencies. These properties offer tremendous possibilities for nonlinear optical processes in photonic crystals. By combining designed defects and nonlinear materials a number of device structures and scientific studies are enabled. In order to study these properties a fabrication route for nonlinear photonic crystals must be developed. There are a number of direct fabrication approaches such as lithography or direct laser writing that are compatible with some nonlinear materials. Low index contrast systems can also be made from nonlinear polymers. CVD approaches are advantageous because they allow an arbitrary template to be formed (either a colloidal crystal or holographic structure) that can later be infilled with a high index nonlinear material. We have demonstrated the ability to perform CVD of nonlinear materials in our colloidal crystals, enabling us to study these interesting properties.

Figure 2. (top) FTIR spectrum of a bare colloidal crystal and a crystal infilled with a nonlinear optical material. The shift in the spectra corresponds to a filling of approximately 86% of the pore volume (maximum possible filling fraction). (middle) SEM micrograph of the colloidal crystal in (top) infilled with a nonlinear optical material  (bottom) Inverse opal of the same nonlinear optical material as above.

Photonic crystals are also easily studied in simulation, both in terms of the band structure of the system and time-domain studies using the FDTD method. We can simulate our structures and material systems using MIT Photonic Bands (MPB) and MIT Electromagnetic Equation Propagation (MEEP) before building them, which is extremely beneficial for systems where there is substantial complexity in fabrication. Simulation aided research can often times lead to more rapid or fully developed results. My future interests and/or work in the early stages of development include metal-dielectric nonlinear photonic systems, plasmonics and nonlinear plasmonics and meta-material systems.

Figure 3. FDTD simulation of a 2D lattice with a planar defect using MEEP software. The lattice geometry is set to simulate the diffraction of a 3D FCC lattice. The planar defect creates a resonant mode near the center of the bandgap – the light corresponds to this resonant frequency and couples through the crystal

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