We report infrared studies of the insulator-to-metal transition (IMT) in GaAs doped with either magnetic (Mn) or non-magnetic acceptors (Be). We observe a resonance with a natural assignment to impurity states in the insulating regime of Ga$_{1-x}$Mn$_x$As, which persists across the IMT to the highest doping (16%). Beyond the IMT boundary, behavior combining insulating and metallic trends also persists to the highest Mn doping. Be doped samples however, display conventional metallicity just above the critical IMT concentration, with features indicative of transport within the host valence band.

The band structure of a prototypical dilute magnetic semiconductor (DMS), Ga1-xMnxAs, is studied across the phase diagram via infrared and optical spectroscopy. We prove that the Fermi energy (EF) resides in a Mn-induced impurity band (IB). Specifically the changes in the frequency dependent optical conductivity [sigma1(omega)] with carrier density are only consistent with EF lying in an IB. Furthermore, the large effective mass (m*) of the carriers inferred from our analysis of sigma1(omega) supports this conclusion. Our findings demonstrate that the metal to insulator transition in this DMS is qualitatively different from other III-V semiconductors doped with nonmagnetic impurities. We also provide insights into the anomalous transport properties of Ga1-xMnxAs.

We report on the electromagnetic response of digital ferromagnetic heterostructures sDFHd: systems with d-doped MnAs layers separated by GaAs spacers of variable thickness syd. The gross features of the infrared conductivity of DFH samples are consistent with the notion that these digital structures are GaAs/ Ga 1˛xMnxAs superlattices. This conclusion is supported by a combination of spectral weight analysis and effective medium theory. The optical properties of DFH also provide insights into the evolution of their critical temperature with GaAs spacing. In DFH a low-lying gap materializes in the energy dependent conductivity, which is interpreted as a mobility gap resulting from Anderson localization.

We report on the infrared studies of the interlayer response for a series of YBa2Cu3Ox high-T-c superconductors with Pr, Ni, and Zn dopants, as well as for the optimally doped crystals of Bi2Sr2CaCu2Oz. These experimental results have motivated us to reexamine some of the long-standing issues in the interlayer electrodynamics of cuprates. Among them are the origins of the anomalous resonance specific to the conductivity of materials with more than one CuO2 plane per unit cell, as well as the microscopic roots of the notorious "semiconducting" behavior seen in a variety of cuprates. Our data for PryY1-yBa2Cu3O7-delta samples indicate that the suppression of the superfluid density (and normal-state conductivity) with Y-->Pr substitution occurs primarily due to changes in the electronic state of CuO2 planes and not because of the fragmentation of Cu-O chains. We also show that the transverse Josephson plasma model proposed to explain the anomalous mode in the interlayer response is not fully consistent with the totality of the experimental data for double-layered materials. We discuss alternative/complimentary scenario assigning this feature of the c-axis conductivity to lifted degeneracy between bonding and antibonding bands associated with the two constituents of the CuO2 bilayer (a so-called bilayer splitting effect).

The terahertz (THz) and sub-THz region of the electromagnetic spectrum bridges the infrared and the microwave. This boundary region is beyond the normal reach of optical and electronic measurement techniques normally associated with these better-known neighbors. Only over the past decade has this THz region become scientifically accessible with broadband sources of moderate intensity being produced by ultra-fast laser pulses incident on biased semiconductors or non-linear crystals [1, 2]. Very recently, a much higher power source of THz radiation was demonstrated: coherent synchrotron radiation (CSR) from short, relativistic electron bunches [3-5]. Coherent synchrotron radiation will open up new territory in the THz frequency range with intensities many orders of magnitude higher than previous sources. The energy range between microwave and the far infrared, 3 – 33 cm-1 (0.1 – 1 THz), has proven to be challenging to accesses and is therefore referred to as the " THz gap ". However, with the new CSR source at BESSY [3, 5] we have been able to extend traditional infrared measurements down into this sub-terahertz frequency range. This source is broad-band and is made up of longitudinally coherent single-cycle sub-picosecond pulses with a high repetition rate (100's of MHz). With the combination of high intensity and short pulse duration new opportunities for scientific research and applications are enabled across a diverse array of disciplines from condensed matter physics, to medical, technological, manufacturing, space and defense industries. Imaging, spectroscopy, femtosecond dynamics, and driving novel non-linear processes are all among the potential applications. The high average power of the CSR source allows one to extend experimental conditions to lower frequencies than have been possible with thermal and conventional synchrotron sources

Infrared (IR) spectroscopy has been employed to investigate the c axis reflectivity of Bi2Sr2CaCu2O8 in the sub-THz frequency region. In order to reach this challenging frequency range a synchrotron source has been employed. Working in a special low-momentum compaction mode of operation where the electron bunch shape is significantly shortened and distorted, stable broadband coherent (superradiant) very far-infrared radiation is produced with orders of magnitude more intensity than conventional thermal and synchrotron sources. Using this source for reflectivity measurements we have been able to observe the Josephson plasma resonance (JPR) in optimally doped Bi2Sr2CaCu2O8. Evidence is found for an inhomogenous distribution of superfluid density within the sample. This source allows us to investigate charge dynamics in this extremely anisotropic superconductor, and opens up the possibility to study other highly correlated systems in this critical low-energy region.

The production of stable, high power, coherent synchrotron radiation at sub-terahertz frequency at the electron storage ring BESSY opens a new region in the electromagnetic spectrum to explore physical properties of materials. Just as conventional synchrotron radiation has been a boon to x-ray science, coherent synchrotron radiation may lead to many new innovations and discoveries in THz physics. With this new accelerator-based radiation source we have been able to extend traditional infrared measurements down into the experimentally poorly accessible sub-THz frequency range. The feasibility of using the coherent synchrotron radiation in scientific applications was demonstrated in a series of experiments: We investigated shallow single acceptor transitions in stressed and unstressed Ge:Ga by means of photoconductance measurements below 1 THz. We have directly measured the Josephson plasma resonance in optimally doped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} for the first time and finally we succeeded to confine the sub-THz radiation for spectral near-field imaging on biological samples such as leaves and human teeth.

Superconductivity is a phenomenon of matter where below a defined temperature (Tc) the electrical resistance drops dramatically and the specific resistance is zero. Therefore, electric currents can propagate for many years without energy loss. ”Classical” superconductors, with Tc between 4 23 K, require liquid helium for cooling which is not feasible for many technological applications. High-temperature superconductors with Tc of around 100 K would be far more suitable since liquid nitrogen can be used for cooling. However, the production of these compounds for technological applications is still unsolved since they are ceramic oxides which cannot be manufactured like metal wires.

We report on the design and implementation of an instrument for spectroscopic studies of materials at sub-terahertz (THz) frequencies at temperatures down to 340 mK. We achieved consistent operation under these rather extreme conditions by coupling a modified Martin–Puplett interferometer to a single cryogenic unit housing two independently controlled He-3 platforms: one as a sample stage and the other for bolometric detectors. Both the optical scheme of the interferometer and detector layout are tailored for the use of the two-channel data acquisition mode which is especially advantageous for measurement of absolute values of reflectance as well as for high-resolution spectroscopy. We document the reliable performance of the sub-THz apparatus with several experiments exploring electrodynamics of both conventional and high-Tc superconductors.

Infrared spectroscopy is used to study the doping and temperature dependence of the intragap absorption in the ferromagnetic semiconductor Ga1-xMnxAs, from a paramagnetic, x=0.017 sample to a heavily doped, x=0.079 sample. Transmission and reflectance measurements coupled with a Kramers-Kronig analysis allow us to determine the optical constants of the thin films. All ferromagnetic samples show a broad absorption resonance near 200 meV, within the GaAs band gap. We present a critical analysis of possible origins of this feature, including a Mn-induced impurity band and intervalence band transitions. The overall magnitude of the real part of the frequency dependent conductivity grows with increasing Mn doping, and reaches a maximum in the x=0.052 sample where T-C saturates at the highest value (similar to70 K) for the series. We observe spectroscopic signatures of compensation and track its impact on the electronic and magnetic state across the Mn phase diagram. The temperature dependence of the far infrared spectrum reveals a significant decrease in the effective mass of itinerant carriers in the ferromagnetic state. A simple scaling relation between changes in the mass and the sample magnetization suggest that the itinerant carriers play a key role in producing the ferromagnetism in this system.

Infrared (IR) spectroscopy has been employed to investigate the c axis reflectivity of Bi2Sr2CaCu2O8 in the sub-THz frequency region.In order to reach this challenging frequency range a synchrotron source has been employed. Working in a special low-momentum compaction mode of operation where the electron bunch shape is significantly shortened and distorted, stable broadband coherent (super radiant) very far-infrared radiation is produced with orders of magnitude more intensity than conventional thermal and synchrotron sources. Using this source for reflectivity measurements we have been able to observe the Josephs on plasma resonance (JPR) in optimally doped Bi2Sr2CaCu2O8. Evidence is found for an in homogeneous distribution of superfluid density within the sample. This source allows us to investigate charge dynamics in this extremely anisotropic superconductor, and opens up the possibility to study other highly correlated systems in this critical low-energy region.

Infrared spectroscopy is used to study the doping and temperature dependence of the intragap absorption in the ferromagnetic semiconductor Ga1-xMnxAs, from a paramagnetic, x=0.017 sample to a heavily doped, x=0.079 sample. Transmission and reflectance measurements coupled with a Kramers-Kronig analysis allow us to determine the optical constants of the thin films. All ferromagnetic samples show a broad absorption resonance near 200 meV, within the GaAs band gap. We present a critical analysis of possible origins of this feature, including a Mn-induced impurity band and intervalence band transitions. The overall magnitude of the real part of the frequency dependent conductivity grows with increasing Mn doping, and reaches a maximum in the x=0.052 sample where T-C saturates at the highest value (similar to70 K) for the series. We observe spectroscopic signatures of compensation and track its impact on the electronic and magnetic state across the Mn phase diagram. The temperature dependence of the far infrared spectrum reveals a significant decrease in the effective mass of itinerant carriers in the ferromagnetic state. A simple scaling relation between changes in the mass and the sample magnetization suggest that the itinerant carriers play a key role in producing the ferromagnetism in this system.