Bias-Modulated Nanoscale Terahertz Linear and Nonlinear Spectroscopy
Abstract
In the last few years, imaging below the diffraction limit has become one of the most active topics in the field of terahertz science. A key development that has enabled these results is the use of a metal tip, such as the tip in an atomic force microscope (AFM). This tip can be held in close proximity (perhaps 10 nm) to a sample surface, and illuminated from the far field with terahertz radiation. The scattering of light off this tip-sample junction is sensitive to the dielectric properties of the region of the sample directly underneath the tip, so nanoscale dielectric information can be extracted from the signal. We have recently broadened the scope of this technique to include nonlinear terahertz spectroscopy in the near field. Instead of illuminating the tip with a terahertz signal, we use a near-infrared (800 nm) femtosecond pulse as the illumination source, and measure the THz radiation generated by the sample, in the vicinity of the AFM tip. Laser THz emission microscopy (LTEM) is a well-known and powerful tool for ultrafast spectroscopy. Our new results have translated this nonlinear technique into the nanoscale, as a complementary tool to our existing THz scattering microscope. Here, we propose an exploratory project under the Short-Term Innovative Research (STIR) program, to develop a potentially transformative advance for these existing nanoscopy tools. We will investigate the use of a DC bias applied to the AFM tip as a means for modulating the sampleÕs properties on the nanoscale. A large static DC field can have several different effects on the photophysics of condensed matter systems. In semiconductors, DC fields can induce carrier depletion as a field effect, or carrier multiplication through avalanche charge generation. In addition, a static electric field can cause a non-resonant modification of a sampleÕs dielectric function through the DC Kerr effect. DC fields can also cause transient alignment of molecules in solution, which could be extremely valuable for applications of THz near-field techniques to samples in the liquid state. The first phase of our 9-month research program will involve the experimental demonstration and theoretical exploration of the basic phenomena of bias-modulated tip scattering, using well-understood samples such as featureless doped semiconductor wafers. In the second phase, once we have a firm experimental and theoretical understanding of the results, we will apply the technique to a prototype nanostructured system: semiconducting nanowires. In a vertical orientation, these nanostructures will couple strongly to the vertically oriented dipole of the AFM tip, offering the possibility of observing bias-modulated THz emission from a single nanowire. In a horizontal orientation, the bias could induce carrier transport along an axis perpendicular to the nanowire axis, which can provide access to this poorly characterized aspect of these well-studied nanostructures. This geometry could be particularly interesting in the case of core-shell nanowires, where the heterojunction runs parallel to the axis of the wire, for probing spatially resolved carrier dynamics orthogonal to the wire axis.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Feb 14, 2019
- Source ID
- W911NF1810419
Entities
People
- Daniel Mittleman
Organizations
- Army Contracting Command
- Brown University
- United States Army