Multi-Functional Materials and DevicesProjects:
- Electrical-Mechanical Coupling in Multi-functional Architectures
- Ultra-Thin Si Nanopillar Arrays for Polarization-Independent Spectral Filters in the Near-IR
Electrical-Mechanical Coupling in Multi-functional Architectures
Personnel: Max Lifson (Ph.D. student in Materials Science)
Piezoelectric materials are a class of materials that exhibit a charge separation due to an anisotropy in the electron distribution of the atoms with applied strain. To characterize this property and to understand how it changes at small dimensions, we design and fabricate nanolattices out of piezoelectric materials, i.e. ZnO and quartz, and perform nanomechanical experiments to determine the piezoelectric coefficients d or e. These coefficients describe the extent to which these materials deform with an applied electric field or create a charge separation with an applied strain, and how effective the material can be when used in sensors, actuators, and other devices.
Another aspect of electrical-mechanical coupling is realized through studying dielectric materials, which are electrical insulators that alter the capacitance of the material stack by affecting the permittivity of the electric field through it. Through our collaboration with the Gwangju Institute of Science and Technology (GIST), we are studying the effect of nanoarchitecture on the dielectric constant κ (sometimes written as the permittivity ε). For semiconductor applications, it is particularly important to decrease the capacitance of the device stack, so as to decrease the signal delay. This has led to an industry-wide push to discover and characterize new low-k materials, to help further speed the electronic devices we use every day.
Ultra-Thin Si Nanopillar Arrays for Polarization-Independent Spectral Filters in the Near-IR
Personnel: Ryan Ng (Ph.D. student in Chemical Engineering)
Spectral filters have a wide range of sensing applications ranging from environmental (hazardous waste, oil, etc) to surveillance. In sensing, detectors are sensitive to anywhere from several to hundreds of electromagnetic bands. Based on the number of bands and bandwidth, these systems are separated into multispectral and hyperspectral imaging systems with multispectral systems capturing under 10 bands and hyperspectral imaging capturing hundreds to thousands of bands of narrow width (around 10-20 nm) that allow for a continuous measurement across a spectrum.
Subwavelength dielectric nanopillar arrays have potential for such spectral filtering applications as band pass and notch filters. In these arrays, rapid spectral variations in reflectivity and transmission are observed when incident light couples via a grating vector to a leaky waveguide mode propagating perpendicular to the surface and are reradiated, leading to sharp near-unity reflectivity resonances. The band width, amplitude, and peak wavelength are easily controlled through array fabrication parameters such as the pillar height, radius, and array periodicity.
Personnel: Victoria Chernow (Graduate student in Materials Science)
3-dimensional photonic crystals (PhCs) have unique light interaction and propagation properties which make them applicable in numerous areas including low-loss mirrors, sensors, optical elements, and structures for light management in solar cells. The utility of photonic crystals for particular applications may, however, be limited by their optical response. The optical response, otherwise defined as the position of the photonic bandgap of the PhC, is usually preset at the time of fabrication and constrains the photonic bandgap to a narrow wavelength range. Applied mechanical deformation can be used to alter the dimensions and periodicity of a PhC, which will modify its optical bandwidth and increase the wavelength range of the photonic bandgap.
We have previously explored the relationship between uniaxial compressive strain, ε, and photonic bandgap, λstopband, in 3-dimensional polymeric nanolattices subjected to uniaxial compression. Polymer octahedron nanolattices are fabricated using Two Photon Lithography-Direct Laser Writing (TPL-DLW), with unit cell sizes on the order of 4 microns. Uniaxial compression experiments were performed in-situ, inside of a scanning electron microscope (SEM), which enabled us to simultaneously collect mechanical data and observe deformation of the photonic crystal. Optical characterization was performed using Fourier Transform Infrared (FTIR) Spectroscopy where reflection spectra were collected for nanolattices under varying amounts of strain. Reflection spectra were used to identify the wavelength position of the photonic bandgap as a function of compressive strain, and revealed that λstopband blueshifts as the periodicity of the lattice decreases with increasing strain. Plotting the photonic bandgap position versus strain revealed a linear relationship between λstopband and ε. We also demonstrated that bandgap shifts on the order of 3 microns could be achieved when the octahedron nanolattice photonic crystals were compressed by ~40%.
Currently, we are exploring 3D photonic crystal designs which display unusual diffraction phenomena including self-collimation and negative refraction. Vast interest in negative refraction has been motivated by the possibility of creating a “superlens” as proposed by Pendry (Phys. Rev. Lett. 85, 3966 (2000)) – his theoretical work showing that a material capable of negative refraction amplifies evanescent waves, allowing this material to act as a lens with a resolution not limited by working wavelength. While theory and experiment have shown that certain metamaterials and photonic crystals (PhCs) can act as superlenses, actually achieving negative refraction in the optical and infrared range remains a challenge; most metamaterials employ lossy metal elements limiting their application, and most PhC structures found to exhibit negative refraction, though made of positive index dielectric materials, are 2D. The subwavelength imaging of a 3D object requires a 3D PhC capable of negative refraction, and based on the numerical simulations of Luo, Johnson, and Joannopoulos (Appl. Phys. Lett. 81, 2352 (2002)), we demonstrate the fabrication and characterization of a polymer-Germanium core-shell 3D photonic crystal lattice which exhibit nearly all angle negative refraction over a frequency range in the mid-infrared. The 3D photonic crystal resembles a BCC lattice of air cubes in dielectric media and was fabricated using two-photon lithography direct laser writing followed by sputtering. The band gap properties of the lattice are verified through FTIR spectroscopy reflectance measurements, and negative refraction is demonstrated through the use of angle resolved mid-IR transmission measurements.