Session S23: IR, THz, Optical and Neutron Measurements and Instrumentation

Quantitatively accurate numerical modeling of amplitude and phase contrast in broadband near-field infrared spectroscopy

Proper modeling of near field infrared spectroscopy data is critical to extracting useful material properties. Current analytical models make underlying assumptions about the probe geometry that makes their use applicable in only limited situations. When strong coupling between probe and sample exists, a more robust solution method must be used. A full-wave numerical method for calculating broadband demodulated near-field amplitude and phase contrast will be presented. Our method captures the probe geometry accurately and is thus essential for obtaining quantitative results free of underlying assumptions and tunable parameters. We will present simulation results on SiO₂ and SrTiO₃, both of which exhibit surface phonon-polariton modes, and will compare the simulation results to experimental data.   

Tunable narrow band sources for anisotropic THz spectroscopy

THz spectroscopy is well suited for material identification and characterization in many different fields due to unique molecular vibrational signatures in this range. Recently this has been extended to feature identification in large biomolecules like proteins and DNA, using the newly developed time domain Anisotropic Terahertz Microscopy (ATM)[1]. ATM measures polarization dependent changes in THz absorption and thus generates a comprehensive map of the vibrational modes, featuring their frequencies as well as their directions. Despite these advances, proliferation of THz systems is hindered by low dynamic ranges of measuring systems, large footprints and cost. Therefore, development of high power, compact THz sources and their characterization are critical. In this study we investigate the suitability of using quasi CW THz radiation generated by pumping Orientation Patterned Gallium Phosphide with a 1064 nm fiber laser for identifying anisotropic signatures of molecular crystals.THz generation from 0.5 to 4 THz is achieved. Results compared with that obtained using a broadband time domain spectrometer. The results show that new narrow band sources are suitable for high dynamic range THz systems.
1. Niessen, K.A., et al., Biophys. J., 2017
  

Terahertz Coherent Acoustic Phonons in SrRuO3/SrTiO3 Superlattices

We utilized time-resolved pump-probe spectroscopy to generate and detect coherent acoustic phonons (CAPs) in SrRuO₃/SrTiO₃ superlattices with frequencies according to the spatial modulation of the heterostructures. Thanks to the pulse laser deposition techniques, precise manipulation of atomic layers enables the fabrication of high-quality heterostructures and generation of CAPs in THz regime. Dynamics of CAPs from hundreds of GHz to 1 THz were investigated. By introducing a control pulse, coherent control of CAPs was also demonstrated. Since many complex oxides with perovskite structures can be deposited on the SrRuO3/SrTiO3 superlattices, our demonstrated technique can be used to study their coherent acoustic properties in THz regime.   

Near Field Optical-Pump-Terahertz-Probe Experiments on Graphene/InAs Heterostructure

We show that graphene is a perfect terahertz reflector in the high q regime with terahertz s-SNOM. We also demonstrate optical pump terahertz near-field probe (n-OPTP) and optical pump terahertz near field emission (n-OPTE) experiments on graphene/InAs heterostructure. Femtosecond pulse laser is used to both pump samples and to generate terahertz pulses. We show that in the near-field regime a single layer graphene is transparent to near-IR (800 nm) optical excitated terahertz emission from InAs but completely “screens” the photo-induced carrier dynamics in InAs probed by a seperate THz pulse.   

Developing Wide-angle Spherical Neutron Polarimetry at Oak Ridge National Laboratory

Spherical Neutron Polarimetry (SNP) analyzes complex magnetic structures through distinguishing contributions from nuclear-magnetic interference and chiral structure in addition to nuclear magnetic scattering separation. This analysis is achieved through determining all components in the polarization transfer process. Currently, wide-angle SNP is being realized at Oak Ridge National Laboratory (ORNL) for multiple beamlines including: the polarized triple-axis spectrometer (HB-1) and general-purpose small angle neutron scattering instrument (CG-2) at the High Flux Isotope Reactor (HFIR), as well as the hybrid spectrometer (HYSPEC) at the Spallation Neutron Source (SNS). The SNP device consists of three units: incoming/outgoing neutron polarization, sample environment and a zero-field chamber. The incoming/outgoing neutron polarization regions use high-Tc superconducting YBCO films and mu-metal to achieve full control of neutron polarization. The sample environment is a dilute refrigerator with a customized tail piece placed into the zero-field chamber. The device is under construction with performance simulations complete and calibration and demonstration experiments scheduled for next year.   

New Polarized Neutron Imaging Capability at Oak Ridge National Laboratory

Polarized Neutron Imaging (PNI) visualizes magnetic field distribution through interactions between the neutron polarization and the magnetic field. The PNI technique creates unique imaging contrast related to magnetic properties in comparison to traditional absorption-based neutron imaging. Furthermore, quantitative PNI provides a nondestructive measurement on internal magnetic structures. At the Oak Ridge National Laboratory, a new PNI capability was implemented on the CG-1D neutron imaging beamline at the High Flux Isotope Reactor (HFIR). The PNI setup at ORNL uses an in-situ optical pumping ³He neutron spin filter to provide a non-distorted radiographs with a stable 90% neutron polarization. The polarized beamline operates in either polychromatic or monochromatic mode, with a field of view of 50mm×50mm and 350μm spatial resolution. Based on the new capability, a series of experiments were performed investigating the magnetic field distortion caused by Meissner effect of superconductors. The results of these experiments combined with modeling will be presented. The PNI team at ORNL is now seeking collaborations to utilize the PNI technique at CG-1D.   

Cross-Section and Measurement Considerations for Polarized Neutron Scattering from NMR Spin-Modulated Systems

We present a development of theoretically predicted cross sections, along with potential measurement scenarios, associated with the scattering of polarized thermal neutrons from target nuclei whose spins are modulated using nuclear magnetic resonance (NMR). Specifically, we investigate the modulation of the polarized differential scattering cross sections under the following two scenarios: (i) Measurements that take place during the time-interval between the application of radio-frequency (RF) pulses that repeatedly impart transverse rotations to selected sets of spin-1/2 target nuclei, and (ii) Measurements that take place during continuous RF irradiation of these nuclei. The aim of this work is to provide foundational knowledge and necessary considerations to explore, and ultimately achieve, the design of instrumentation that would provide enhanced scattering signals from selected nuclei in liquids and solutions by leveraging the use of NMR techniques. We discuss our in-progress calculations which are aimed at evaluating signal-to-noise for a prototypical molecular mixture in the context of NMR/neutron scattering protocols such as those outlined in our recent paper (1).

(1) M. Kotlarchyk and G.M. Thurston, J. Chem. Phys. 145 (2016) 244201   

Designs of neutron microscopes for high-resolution imaging

We demonstrate designs of two types of neutron microscopes. Like an optical microscope, the neutron microscope consists of a condenser and an image forming optics. Neutrons are focused at the sample by a condenser optic. An image forming optic focuses transmitted neutrons at the detector. The condenser optic is designed to maximize the neutron flux and to obtain desired beam divergence at the sample. The condenser consists of axisymmetric confocal paraboloid and a hyperboloid mirrors, which are concentrically nested. The image-forming optics are designed using two different types of Wolter mirrors, confocal nested ellipsoid and hyperboloid sections. The design of magnification-10 mirrors should achieve the spatial resolution of about 10 μm. Importantly, the resolution of the microscope is determined by the mirrors rather than by the beam collimation as in conventional pinhole imaging, leading to possible dramatic improvements in the signal rate and resolution. Also in contrast with pinhole imaging, in the microscope the samples are placed far from the detector to allow for bulky sample environment.   

Portable Mid-IR Spectrometer

We describe a mid-IR spectrometer by combining a commercial thermal imaging camera with a Diesel engine glow-plug and a micromachined hyperspectral filter. Our hyperspectral mid-IR filter consists of two silicon/air mirrors that form an optical cavity. A small angle between the two high-reflectivity mirrors provides us with a gradual variation of the cavity resonance peak and we obtain a position-dependent transmission filter. The linear variation of resonance frequency with position enables the ~40,000 pixels in the thermal bolometer camera to be converted into an optical multichannel spectrometer. Filters are made of multiple layers of silicon and air, and the high refractive index and transparent properties of silicon in the mid-IR region provide a wide free spectral range of up to 10 micrometers. Different wavelengths can be assigned to each pixel in the microbolometer array, enabling rapid spectral acquisition in our compact spectrometer. This allows us to measure the mid-IR spectra of various types of polymers, biomolecules and medications in a compact system. Here we show mid-IR molecular absorption spectra from different materials measured with this spectrometer and compare them to Fourier-Transform Infrared Spectroscopy (FTIR) results.