Second-Harmonic Generation: Hyper-Rayleigh Scattering

1. Second-Harmonic Generation: Hyper-Rayleigh Scattering NSF Grant DMR-0850037 Dillon Walker, Guangyao Li, Kenneth Singer Hyper-Rayleigh Scattering can be used in combination with a single-photon counting technique to measure the beta tensor of a given nonlinear optical material. This beta can provide important information about molecular alignment in various materials used in device applications. Such devices could be used for optical switching without need for conversion into electrical signal. Introduction Setup Hyper-Rayleigh Scattering is an efficient technique for measuring the β, or the coefficient of the second-harmonic in a given nonlinear optical material. Nonlinear optics is an increasingly relevant field of study as demand for ultra-fast data transfer and switching continues to increase. The β component is useful for determining the efficiency at which electrical data can be converted into optical signals and vice-versa. Applications include increased switching rates for fiber-optic cables and other optoelectronic devices. Although some extremely fast and efficient materials currently exist, price limits their use to industrial-size copper to fiber-optic switching stations. The materials currently being studied could decrease the cost exponentially, allowing individual computers to connect directly to fiber-optic cables for internet access without use of copper wire. Experiment Diagram In organic molecules, not only does Second-Harmonic Generation (SHG) require non-centrosymmetry on a molecular level, but also on a macroscopic level. Materials dissolved in a solvent are for the most part centrosymmetric. However, small fluctuations in the material result in a scattering of some second-harmonic signal. In order to measure this signal, the light must be focused into the photo-detector from a 90 degree angle and several filters must be used to make sure that only the doubled frequency is detected by the photo-multiplier tube. Because the photomultiplier tube has a dead-time of around twenty nanoseconds after a detection, it is impossible to measure the signal directly. However, because the pulses from the laser have a steady magnitude, there is a small constant probability that a photon will be detected by the photomultiplier tube. Provided that the probability is low (so that a second photon within the dead time is not ignored) , a histogram can be built that plots the number of photon detections at a certain point in time after a pulse is sent. Over the course of a few minutes, this can provide detailed information showing the relative beta strength of a given solution and any fluorescence that may be occurring. Theory Comparing Second-Harmonic Generation to Multi-Photon Fluorescence The dielectric polarization of a molecule can be written as the power series: χ(2) is a third-order tensor that determines the susceptibility of the material to second-harmonic generation. Because this is an even powered function of the electric field, the material must be non-centrosymmetric for a second-harmonic signal to exist. The signal manifests itself as radiation twice the wavelength of the input signal. Because the third-order tensor is quite small, measurable amounts of second-harmonic signal requires a high-intensity light source such as a laser. Unfortunately, with high-intensity light sources, there is also varying degrees of Multi-Photon fluorescence which can compete with the Hyper-Rayleigh signal. However, because SHG is a virtual process, it is instantaneous , unlike multi-photon fluorescence. Using a time-resolved single-photon counting technique, we can separate the Hyper-Rayleigh signal from the fluorescence as shown by the histogram .----------------------------------------- Analysis By taking measurements of the beta signal of the solvent and of various densities of Chomo1 in that solvent, one can calculate the beta of the molecule itself. Using the ratio of beta between parallel and perpendicular polarizations , the beta can be converted from the reference frame of the laboratory to the reference frame of the molecule. This is useful for calculating the beta that would occur in various macroscopic structures of the material. Example Single-Photon Counting Histogram Chemical Structure Material The molecule in this experiment (Chromo 1) is a non-centrosymmetric organic compound with hydrophilic and hydrophobic tails. With these tails, the molecules can be aligned between two planes in order to form a film-like material into which waveguides can be formed. The molecular beta and Second-Harmonic produced by an electric field can be used to measure how well the molecules are aligned. For an electro-optic switching device, an applied electric field can be used to change the refractive index of the nonlinear material to direct input light into varying paths. This process is many times faster than converting to an electrical signal to make the switch. Conclusion Preliminary Histogram for 2% pNAin Acetone Preliminary results show that the experiment has been set up properly and data is ready to be collected. A strong beta count with little to no fluorescence suggests that this molecule may be appropriate for continued study in preparation for device application.