Session R06: Instrumentation and Measurements III

High power terahertz photoconductive antenna with hybridized contact electrodes

In a photoconductive antenna, the photocurrents and antenna properties are greatly influenced by the type of electrodes, either Ohmic or Schottky contact electrodes fabricated on semi-insulating GaAs. In our theoretical model, Schottky contact, having a small chemical potential gradient, discourages electromigration of contact metal atoms whereas Ohmic contact tends to induce stronger photocurrents, generating a terahertz beam more efficiently for a given bias field. However, with actual electrodes fabricated on a semi-insulating GaAs substrate, the contact exhibits either Ohmic-dominant or Schottky-dominant hybridized behavior, instead of having either pure Ohmic or pure Schottky behavior. Interestingly, hybridized Schottky contacts suppress electromigration of contact metal atoms into the substrate and also improves the efficiency of terahertz beam generation, taking both advantages of Schottky and Ohmic contacts. Furthermore, due in part to the suppressed electromigration, the photoconductive antenna with Schottky electrodes becomes more durable and tolerate a high bias voltage that enables a more intense terahertz beam.   

High-resolution terahertz spectra of trace gases and aerosolized biological agents

High-resolution terahertz spectrometry can identify the gases or biological agents in real-time if the spectroscopy can be done over a short time with high signal-to-ratio (SNR) and superior resolution. We performed time-domain terahertz (THz) spectroscopy on gas (or vapor) phase samples of carbon monoxide, methanol, water, and acetonitrile at concentrations less than 10ppm, and demonstrated 50-60dB SNR on a single measurement. Also, we measured unambiguous terahertz spectra of various types of biological agents which were aerosolized. Using data measured at different sample concentration and terahertz probe beam power we investigated the interplay between SNR, probe beam power, sample concentration, and the electric dipole moment of molecules, for which we extended an absorbance theory to THz frequencies. Comparison of our data with references and a theoretical model revealed some discrepancy in certain spectral line intensities, suggesting that certain rotational resonance quantum state may have higher populations or transition probabilities than our model predicts. These results were possible largely due to our high power THz source capable of generating up to 3 mW, an order of magnitude greater than those available commercially.   

Study of Frequency-dependent Properties of SRO Using Terahertz Time-domain Spectroscopy

We studied four SrRuO₃ (SRO) films grown at the same time on different regions of an Al₂O₃ substrate using terahertz time-domain spectroscopy. The frequency-dependent transmittance values, real indices of refraction and imaginary indices of refraction at the THz range were reported. Real and imaginary conductivities were also reported with Drude fitting. The conductivities obtained from the experiment were in good agreement with the Drude model. The frequency-dependent properties of the four samples were found to be different from each other in terms of amplitude.   

Analysis of Local Structure about Zn in Zn doped Tetrahedrite using Extended X-ray Absorption Fine Structure (EXAFS) Spectroscopy

Tetrahedrites are a class of naturally occurring minerals with high
thermoelectric efficiency. Thermoelectric materials are needed in devices
aiming to convert heat to electricity directly via the Seebeck effect, and
also for cooling without a refrigerant via the Peltier effect. We examine
pure Cu₁₂Sb₄S₁₃ and zinc doped Cu₁₀Zn₂Sb₄S₁₃ tetrahedrite to better
understand the metal to semiconductor transition at T~95K in the pure
tetrahedrite. Understanding why the transition is suppressed upon Zn doping
will shed light on the structural properties of this material. To probe the
crystal structure, X-ray spectroscopy data at the Zn and Cu K edges were
collected at the Stanford Synchrotron Radiation Lightsource (SSRL). We use
the Real Space X-ray Absorption Program (RSXAP) to analyze the Extended
X-ray Absorption Fine Structure (EXAFS) region for both the Zn and Cu
absorption edges. The analysis shows that the Zn edge data are more ordered
than the Cu edge data and that Zn substitutes on both the Cu1 and Cu2 sites.   

Corrective Regridding Techniques for Non-Uniform Sampling in Time Domain Terahertz Spectroscopy

Time domain terahertz spectrometers using optical delay stages inherently suffer from non-uniform sampling positions. We review, simulate, and experimentally test the ability of corrective cubic spline and Shannon regridding algorithms to mitigate the inherent sampling position noise. Simulation and experimental results show that regridding increases the signal to noise ratio within the frequency range of 100 GHz to 2 THz. We apply these techniques to study cyclotron resonance of 2DEGs in AlGaAs/GaAs quantum wells and predict that regridding corrections will become increasingly important to both spectroscopy and imaging as THz technology continues to improve.   

Magnon linewidth in hybrid quantum systems at milliKelvin temperatures

The dynamic properties of collective spin excitations (magnons) have been studied for decades. Only recently there has been progress towards studying the quantum properties of magnons at microwave powers where the average number of probe photons is less than one. Strongly coupled hybrid systems of microwave photons in superconducting quantum circuits and magnons may be suitable for application in quantum information processing.
We report on limiting effects of the intrinsic magnon linewidth, representing the coherence time of a quantum memory, at milliKelvin temperatures. Power dependent spectroscopic measurements on a hybrid system consisting of a YIG sphere and a 3D microwave cavity at temperatures of 50 mK to 1.5 K have been performed. The magnon frequency is swept across the cavity resonance by an external magnetic field. Using input-output formalism, we extracted the linewidths and coupling within the system. At lowest temperatures, the observed magnon linewidth is dominated by losses to near-resonance two-level systems (TLSs). Increasing the power from quantum excitation to multi-photon excitations, the TLSs get saturated and their contribution to the linewidth vanishes. At higher temperatures, the power influence decreases due to the presence of thermally saturated TLSs.   

Core-to-core RIXS of Transition Metals

Many technologically important compounds contain 3d transition metals, and their behavior depends on subtleties of their d electrons. X-ray emission and absorption spectroscopies at the metal L edge reveal information about the occupied and unoccupied d orbitals. However, the short penetration depth of these soft x-rays hinders measurements. Hard x-ray resonant inelastic x-ray scattering (RIXS) or high-energy resolution fluorescence detection (HERFD) techniques alleviate depth limitations while maintaining chemical sensitivity. These include measurements of 1s2p (Kα) fluorescence, 1s3p (Kβ) fluorescence, or valence RIXS. We have extended the Bethe-Salpeter equation approach within the ocean code to calculate core-to-core RIXS (1s2p and 1s3p). We present illustrative results for several transition metal compounds   

i-Al-Cu-Fe-Co Quasicrystals with High Co Content.

We developed a conceptual phase diagram of Al-Cu-(Fe+Co) quasicrystals and a method to grow stable Al-Cu-Fe-Co icosahedral single-grain quasicrystals with high cobalt content. Our original three-stage process includes (i) heating up a correct mixture of Al, Cu, Fe, and Co elements above the temperature of liquidus; (ii) fast cooling of the melt down to the quasicrystalline single phase region with rate of ~250 F/h to prevent growing of non-quasicrystalline phases in the melt; and (iii) slow cooling down with rate of ~3.5 F/h to grow large (mm-size) single-grain quasicrystals. The chemical composition of the grown quasicrystals was determined by the energy-dispersive X-ray analysis and inductively coupled plasma mass spectrometry and showed that Co substituted up to 23 % of Fe atoms. Powder XRD and selected area electron diffraction were carried out for the phase identification. Temperature dependencies of the electrical resistance were performed on oriented samples in the form of rectangular parallelepipeds by the four-probe method in the temperature range of 1.4 K - 300 K.   

Cross-Correlated Current Noise Spectroscopy and Detection of Shot Noise in Organic Light-Emitting Diodes

Noise spectroscopy is a powerful technique for probing electrical processes in materials and has been utilized to great effect to study inorganic devices. Organic semiconductors, however, present two problems: high resistance, due to molecular disorder; and high device capacitance, due to their planar structure. The associated high RC constants limit the bandwidth of voltage noise measurements. Current noise measurements allow one to circumvent the bandwidth issue, but the currents are often orders of magnitude smaller than the input noise of the amplifier itself. In this talk, we will show that cross-correlation methods present a way to overcome this obstacle. We will discuss the basic concepts of the cross-correlation technique and the system we have developed to implement it. We will then discuss its use in the current noise measurements of a variety of materials and devices, for instance phosphorescent and fluorescent organic light-emitting diodes. Finally, we will discuss our work using this technique to detect shot noise, which we have associated with percolative transport in organic materials.

  

Endohedral fullerenes as frequency standards in portable atomic clocks

Stable frequency references underpin a range of navigation, communication, and sensing infrastructure. Over the past decade, advances in miniaturization have enabled the use of highly-stable atomic clocks in a range of portable applications. However, further miniaturization and improved stability are required to enable broader adoption of atomic frequency standards. The endohedral fullerene ¹⁵N@C₆₀ has been proposed as the basis for an alternative condensed-matter portable atomic clock that could address these challenges. Here, we measure the low-frequency electron paramagnetic resonance spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. From this experimental data, we infer a linewidth at the clock field of 100 kHz and estimate a bound on the clock’s projected frequency stability. We then discuss ways to improve the frequency stability of the proposed fullerene clock such that it is competitive with existing miniaturized portable atomic clocks.   

Electric-field control of the bound states of the window-coupled quantum waveguides

Energy spectrum of two straight quantum waveguides coupled through the opening in their common wall contains, in addition to the continuous part, discrete states with spatially localized orbitals number of which depends on the size a of the connecting window. Here, evolution of these bound levels is calculated when the transverse electric field is applied to the structure. It is shown that for the ground state, which for the voltage-free geometry exists at any nonzero a, the increase of the electric intensity shifts the energy that in the high-field regime approaches fundamental propagation threshold of each waveguide. It is predicted that applied voltage leads to the increase of the window critical widths at which new excited bound states emerge. Physical explanation of this electric-field control of the number of the localized levels is based on the analysis of the propagation thresholds in the opening and waveguides arms. This remarkable electric-field-induced switching of the bound states can be checked by the change of the optical absorption spectrum when the gate voltage is applied. A comparison between the 2D and 3D geometries is presented too.   

A low power Impulsive Stimulated Thermal Scattering apparatus for the measurement of acoustic wave propagation and thermal diffusion in solids and liquids

Impulsive Stimulated Thermal Scattering (ISTS) is an all-optical transient grating measurement that serves as a non-contact probe of micron lengthscale acoustic wave propagation and thermal transport in both solids and liquids. Current application of this technique requires either temporally limited mechanically delayed probing schemes or the use of potentially expensive continuous wave lasers that deposit several hundred mW of light into samples and necessitate extremely high bandwidth detectors. We describe the construction of an ISTS apparatus that does not rely upon either of these detection schemes, utilizes <1 μW of probe laser power and can simultaneously measure MHz-GHz frequency acoustic wave propagation and arbitrarily long-lived thermal diffusion. We demonstrate the efficacy of our apparatus in both liquid and solid systems and describe possible applications to the greater class of transient grating-based measurements.   

Conductivity of Hydrogen Rydberg Matter Phases on Top of Thin Film Platinum Surface

The Rydberg matter state of atoms (RM) was predicted by Manykin et. al. around 1980 and experimentally confirmed a few years later by Leif Holmlid’s work. LH published a review article about RM in 2012 [1]. In his later work LH has suggested that one form of RM of Hydrogen could be a superconductor and superfluid and has experimental results supporting formation of such phase. The aim of the talk is to report from current state of experiments to to measure electrical properties of Hydrogen Rydberg matter and transformed phases that LH has observed and he refers to as the ultra-dense state. There are no published reports by LH or anyone found in the scientific literature were this has been studied. A custom-built experimental setup has been constructed, programmed and tested at the Science Institute, University of Iceland since 2014. Here I report measurements with deuterium Rydberg matter that have shown very promising indications of electrical conductivity of such phases.

1. Experimental Studies and Observations of Clusters of Rydberg Matter and Its Extreme Forms Leif Holmlid. J. Clust Sci (2012) 23:5–34   

Modeling Refractive Index of Biomass Burning Aerosols

Tthere is a significant uncertainty regarding the Refractive Index RI of biomass burning (BB) aerosols. Experimentally measured cross sections for white pine BB aerosols were modeled using T-matrix theory and RI's extracted from these calculations. Experimental measurements were conducted using a cavity ring-down spectrometer and a nephelometer. BB aerosols were obtained by burning white pine using (1) open fire in a burn drum, where the aerosols were collected in distilled water using an impinger and (2) a tube furnace to directly introduce the BB aerosols into an indoor smog chamber, where BB aerosols were then sampled directly. Filter samples were collected and electron microscopy images were used to obtain morphology and size information for the calculations. For the particles generated in the tube furnace, the real part of the RI decreased with particle size and the imaginary part was much smaller and nearly constant. RI is sensitive to particle size and sampling method, but there was no wavelength dependence over the range considered (500-680 nm). Our values for the RI of fresh (White Pine) biomass burning aerosol range from 1.33 + i0.008 to 1.74 + i0.008 for 200, 300, and 400 nm diameter particles.