Session S21: Spectroscopic Techniques

Nanoscale imaging of nonequilibrium polymer films

In recent years there have been exciting advances in sub-diffraction limited imaging based on fluorescence microscopy. While most applications of super-resolution microscopy focus on static biological imaging, we are interested in extending these techniques to the study of polymer dynamics. To this end, we couple stimulated emission depletion (STED) with spectroscopic detection, relying on spectral features of fluorescence emission to serve as the imaging contrast agent. We aim to adapt fluorescent dyes responsive to environmental properties (polarity, mobility, current, temperature, ect.) to STED imaging. Using the fluorescent spectral response as a contrast agent allows for nanoscopic environments to be directly imaged without the need for specific labeling. Rapid acquisition of images allows for slow dynamic processes in nonequilibrium polymer films to be imaged in real time. We demonstrate the power of super-resolution spectroscopic imaging by directly imaging several topical problems in materials science.   

Nonlinear photothermal Mid-Infrared Microspectroscopy with Superresolution

We describe a nonlinear method for breaking the diffraction limit in mid-infrared microscopy using nonlinear photothermal microspectroscopy. A Quantum Cascade Laser (QCL) tuned to an infrared active vibrational molecular normal mode is used as the pump laser. A low-phase noise Erbium-doped fiber (EDFL) laser is used as the probe. When the incident intensity of the mid-infrared pump laser is increased past a critical threshold, a nanobubble is nucleated, strongly modulating the scatter of the probe beam, in agreement with prior work. Remarkably, we have also found that the photothermal spectral signature of the mid-infrared absorption bifurcates and is strongly narrowed, consistent with an effective ``mean-field'' theory of the observed pitchfork bifurcation. This ultrasharp narrowing can be exploited to obtain mid-infrared images with a resolution that breaks the diffraction limit, without the need of mechanical scanning near-field probes. The method provides a powerful new tool for hyperspectral label-free mid-infrared imaging and characterization of biological tissues and materials science and engineering.   

Novel Raman instrumentation for characterizing 2D nanomaterials

We have designed and constructed a unique Raman microscope system to enable diffraction limited measurements of graphene and two-dimensional transition-metal dichalcogenides (TMD). The design enables low frequency phonon measurements down to ten wavenumbers through a triple grating Raman spectrometer, as well as resonance Raman spectroscopy through multiple laser excitation lines throughout the visible region. Through coupling to a cryogen-free magnet system, Raman spectra can be collected while the sample is in fields up 9 Tesla and at temperatures from 4 K to 400 K. Uniquely, both Farady and Voight geometries are accessible. Furthermore, multiple electronic feedthroughs permit collecting Raman scatter from devices at varying voltages. Proof of concept measurements on TMDs will highlight the full capabilities of the instrumentation. Collaborations are sought to demonstrate the utility of the new instrumentation.   

Calculating Relative Ionization Probabilities of Plutonium for Resonance Ionization Mass Spectrometry to Support Nuclear Forensic Investigations

Ongoing work seeks to apply the technology of Resonance Ionization Mass Spectrometry (RIMS) to problems related to nuclear forensics and, in particular, to the analysis and quantification of debris from nuclear detonations. As part of this effort, modeling and simulation methods are being applied to analyze and predict the potential for ionization by laser excitation of isotopes of both uranium and plutonium. Early work focused on the ionization potential of isotopes of uranium, and the present effort has expanded and extended the previous work by identifying and integrating new data for plutonium isotopes. In addition to extending the effort to this important new element, we have implemented more~accurate descriptions of the spatial distribution of the laser beams to improve the~accuracy of model predictions compared with experiment results as well as an ability to readily incorporate new experimental data as they become available. The model is used to estimate ionization cross sections and to compare relative excitation on two isotopes as a function of wavelength. This allows the study of sensitivity of these measurements to fluctuations in laser wavelength, irradiance, and bandwidth. We also report on initial efforts to include predictions of americium ionization probabilities into our modeling package.   

High power, high resolution terahertz spectroscopy technologies and its applications

Since a large number of molecules' resonance frequencies lie within terahertz frequencies, terahertz spectroscopy is a highly useful tool for scientific investigation of various materials. At the same time one can use the same technology for the identification of hidden materials. Despite these potential applications presently terahertz spectroscopy is largely underutilized, and it is mostly being used in the laboratory environment. This is in part largely due to the fact that no portable, high power, high resolution spectrometer is currently available. So we have been developing a high power, wideband terahertz source. The terahertz source is capable to produce a relatively high power (\textgreater 2 mW), wideband (0.1 -- 3 THz) terahertz beam. In addition to the source we have optimized and calibrated an electro-optic (EO) detector, of which sensitivity is 10$^{\mathrm{-13}}$ W/(Hz)$^{\mathrm{1/2}}$. Recently, by utilizing these terahertz source and detector, we have constructed a high power, high resolution terahertz spectrometer, and carried out various experiments to understand resonance spectra of water vapor, chemicals and ionized air. Also we constructed a modified terahertz spectrometer for a stand-off detection applications. In this presentation I will discuss our experimental achievements and progresses.   

High Power Terahertz Conductive Antenna with Chaotic Electrodes

Time domain terahertz spectroscopy (TDTS) is now widely adopted and being used for various purposes, including chemical and material analysis as well as detection of hazardous materials in the laboratories. While there are several different methods available to generate a wideband terahertz pulse for the TDTS, currently a terahertz photoconductive antenna may be the most popular one, as it can produce a wideband terahertz pulse very efficiently. However our experimental investigation indicates that the conventional photoconductive antenna with a pair of parallel electrodes can produce a terahertz pulse at most about 100 micro-Watts. When attempted to produce a higher power terahertz pulse the antenna may experience irrevocable failure. In order to overcome this problem we recently redesigned the photoconductive antenna and implemented electrodes that lead to a chaotic trajectories of charged particles. With the new electrodes we have demonstrated a high power (\textgreater 2 mW) coherent terahertz beam, and we found that the lifetime of the antenna is also substantially longer than that of the conventional antenna. In this talk I will present our experimental results and disclose some of our new antenna designs.   

Optical Properties of Magnetron sputtered Nickel Thin Films

The study of optical properties of Nickel (Ni) is important, given the pivotal role it plays in the semiconductor and nano-electronics technology. Ni films were made by DC and RF magnetron sputtering in an ATC Orion sputtering system of AJA on various substrates. The optical properties were studied ex situ by variable angle spectroscopic (220-1000 nm) ellipsometry at room temperature. The data were modeled and analyzed using the Woollam CompleteEase Software fitting ellipsometric and transmission data. Films sputtered at low pressure have optical properties similar to that of Palik [1]. Films sputtered at higher pressure however have a lower refraction index and extinction coefficient. It is expected from our results that the density of the sputtered films can be determined from the ellipsometric quantities. Our experiments also revealed that Ni is susceptible to a slow oxidation changing its optical properties over the course of several weeks. The optical properties of the native oxide differ from those of reactive sputtered NiO similar as found by [2]. Furthermore the oxidation process of our samples is characterized by at least two different time constants. \\[4pt] [1] Edward D.Palik, Handbook of Optical Constants of Solids, Academic Press (1998), p 313-323.\\[0pt] [2] Lina S. Abdallah, thesis New Mexico State University, August 2014.   

Practical Framework for an Electron Beam Induced Current Technique Based on a Numerical Optimization Approach

A practical framework for an electron beam induced current (EBIC) technique has been established for conductive materials based on a numerical optimization approach. Although the conventional EBIC technique is useful for evaluating the distributions of dopants or crystal defects in semiconductor transistors, issues related to the reproducibility and quantitative capability of measurements using this technique persist. For instance, it is difficult to acquire high-quality EBIC images throughout continuous tests due to variation in operator skill or test environment. Recently, due to the evaluation of EBIC equipment performance and the numerical optimization of equipment items, the constant acquisition of high contrast images has become possible, improving the reproducibility as well as yield regardless of operator skill or test environment. The technique proposed herein is even more sensitive and quantitative than scanning probe microscopy, an imaging technique that can possibly damage the sample. The new technique is expected to benefit the electrical evaluation of fragile or soft materials along with LSI materials.   

High Resolution Electron Energy Loss Spectroscopy with Simultaneous Energy and Momentum Mapping

High resolution electron energy loss spectroscopy (HREELS) has been demonstrated as a powerful technique to probe vibrational and electronic surface excitations of solids. The dispersion relation of the surface excitations, i.e. energy as a function of momentum, can be obtained via the angle resolved measurements by rotating the sample or the analyzer in a conventional HREELS measurement. The sampling density in the momentum space and the detecting efficiency are restricted by the mechanical rotation. Here we introduce a new design of the HREELS system, by combining the traditional Ibach-type electron source with the mainstream hemispherical electron energy analyzer, which could simultaneously measure the energy and momentum of the scattered electrons without any mechanical rotation. The new system possesses higher efficiency and sampling density of momentum-resolved measurements by at least one order of magnitude than conventional spectrometers without deteriorating the resolution of energy and momentum. Using Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ as an example, we show that an energy loss spectrum can be scanned throughout the first Brillouin zone and a momentum-dependent spectral intensity distribution could be obtained in one measurement.   

Soft X-Ray Absorption Spectroscopy at an X-ray Free Electron Laser

X-ray free electron lasers, providing coherent, ultrafast, high intensity x-ray pulses, have enabled groundbreaking scattering experiments to probe the atomic structure of materials on femtosecond timescales. Nonetheless, x-ray absorption spectroscopy (XAS), one of the most fundamental and common x-ray techniques practiced at synchrotron light sources, has proven challenging to conduct with satisfactory signal-to-noise levels at soft x-ray energies using free electron laser sources. The ability to routinely collect high quality XAS spectra, especially in a time-resolved manner, will open many new scientific possibilities in the areas of ultrafast demagnetization, phase transitions and chemical dynamics to highlight a few. Here, we report how XAS using total fluorescence yield detection yields high signal-to-noise x-ray absorption spectra at an x-ray free electron laser source. Data were collected over multiple absorption edges on technologically relevant materials. These measurements were recorded on the Soft X-Ray Materials Science instrument at the Linac Coherent Light Source. The results are easily extendable to time-resolved measurements.   

Measurement of the low energy spectral contribution in coincidence with valence band (VB) energy levels of Ag(100) using VB-VB coincidence spectroscopy

A set of coincidence measurements were obtained for the study and measurement of the electron contribution arising from the inter-valence band (VB) transitions along with the inelastically scattered VB electron contribution. These Auger-unrelated contributions arise in the Auger spectrum ( Ag 4p NVV) obtained using Auger Photoelectron Coincidence Spectroscopy (APECS). The measured Auger-unrelated contribution can be eliminated from Auger spectrum to obtain the spectrum related to Auger. In our VB-VB coincidence measurement, a~photon beam of energy 180eV was used to probe the Ag(100) sample. The coincidence spectrum was obtained using two Cylindrical Mirror Analyzers (CMA's). The scan CMA measured the low energy electron contribution in the energy range 0-70eV in coincidence with VB electrons measured by the fixed CMA. In this talk, we present the data obtained for VB-VB coincidence at the valence band energy of 171eV along with the coincidence measurements in the energy range of 4p core and valence band.   

High repetition rate source of narrowband extreme-ultraviolet harmonics for time-resolved ARPES

We present a highly efficient table-top source of extreme ultraviolet (XUV) femtosecond pulses operating at 50-kHz repetition rate. A bright XUV source flux of 3x10$^{\mathrm{13}}$ photons/s is generated at 22.3 eV by driving high-harmonic generation with the ultraviolet second-harmonic of a laser amplifier focused tightly into Kr gas. The conversion efficiency (5x10$^{\mathrm{-5}})$ is enhanced by two orders-of-magnitude in this cascaded scheme, exceeding dipole wavelength scaling and evidencing enhanced phase matching conditions as confirmed by simulations. Importantly, the spectral structure enables the direct, high-contrast isolation of a single, narrowband harmonic with 72 meV linewidth. The high repetition rate, narrow bandwidth, and high flux (10$^{\mathrm{11}}$-10$^{\mathrm{12}}$ ph/s at the sample) of this source is ideal for time-resolved photoemission or nanoscale imaging. First applications in time- and angle-resolved photoemission (trARPES) will be discussed.   

Instrumentation for Cyclotron Resonance and Electron Spin Resonance in Pulsed Magnetic Fields

Electron spin resonance (ESR) and cyclotron resonance (CR) are vital in the study of electronic structure and magnetism of materials. For example, CR can yield the strength of electron-electron correlations via the dynamic mass, and determine the Fermi surface topology. However, very high magnetic fields are required to extend CR to heavy mass and/or disordered correlated-electron systems, and to cuprate superconductors, where the upper critical field must be exceeded. Similarly, magnetic fields $\sim$ 100~T in conjunction with high-frequency ESR could access the magnetic interactions in highly anisotropic spin-gap materials and molecular quantum magnets, probe phase transitions in $f-$electron systems, and examine the electronic structure of organic radicals. Pulsed magnets are therefore \textit{required} for such ultra-high-field CR and ESR experiments; but the resulting extreme environment presents challenges in resonant cavity and waveguide design. In this presentation, we describe probe designs tailored to CR and ESR experiments spanning fields up to 100 T and frequencies from 40 GHz to 4 THz. These new techniques will be illustrated using experimental ESR data from organic quantum magnets.   

High speed nonlinear optical harmonic generation rotational anisotropy measurements for sensitive detection of crystallographic and electronic symmetry breaking

The rotational anisotropy of optical nonlinear harmonic generation (NHG) from a crystalline material can be used to probe the symmetries of both its lattice structure and underlying ordered electronic phases. Presently however, low temperature experimental setups require both optics and detectors to be mechanically rotated during measurement [1], which makes the data collection slow and thus susceptible to low frequency sources of noise. We present a new method to perform rotational anisotropy measurements based on a triple beam-splitter setup and a spatially sensitive detection scheme. This method increases the data collection frequency by over three orders of magnitude by removing nearly all rotating parts from the experiment. We will report the improved sensitivity to symmetry changes in a material and discuss the potential to carry out wavelength dependent and time-resolved NHG measurements using this method. \\[4pt] [1] D. H. Torchinsky, H. Chu, T. Qi, G. Cao and D. Hsieh, Rev. Sci. Instrum. 85, 083102 (2014).   

Electron Energy-Loss Spectroscopy (EELS)Calculation in Finite-Difference Time-Domain (FDTD)Package: EELS-FDTD

Electron energy-loss spectroscopy (EELS) is a unique tool that is extensively used to investigate the plasmonic response of metallic nanostructures since the early works in the '50s. To be able to interpret and theoretically investigate EELS results, a myriad of different numerical techniques have been developed for EELS simulations (BEM, DDA, FEM, GDTD, Green dyadic functions). Although these techniques are able to predict and reproduce experimental results, they possess significant drawbacks and are often limited to highly symmetrical geometries, non-penetrating trajectories, small nanostructures, and free standing nanostructures. We present here a novel approach for EELS calculations using the Finite-difference time-domain (FDTD) method: EELS-FDTD. We benchmark our approach by direct comparison with results from the well-established boundary element method (BEM) and published experimental results. In particular, we compute EELS spectra for spherical nanoparticles, nanoparticle dimers, nanodisks supported by various substrates, and gold bowtie antennas on a silicon nitride substrate. Our EELS-FDTD implementation can be easily extended to more complex geometries and configurations and can be directly implemented within other numerical methods.