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Author(s): Chen, X; Yao, Z; Xu, S; McLeod, AS; Gilbert Corder, SN; Zhao, Y; Tsuneto, M; Bechtel, HA; Martin, MC; Carr, GL; Fogler, MM; Stanciu, SG; Basov, DN; Liu, M | Abstract: The underlying physics behind an experimental observation often lacks a simple analytical description. This is especially the case for scanning probe microscopy techniques, where the interaction between the probe and the sample is nontrivial. Realistic modeling to include the exact details of the probe is widely acknowledged as a challenge. Due to various complexity constraints, the probe is often only approximated in a simplified geometry, leading to a source for modeling inconsistencies. On the other hand, a well-trained artificial neural network based on real data can grasp the hidden correlation between the signal and the sample properties, circumventing the explicit probe modeling process. In this work we show that, via a combination of model calculation and experimental data acquisition, a physics-infused hybrid neural network can predict the probe-sample interaction in the widely used scattering-type scanning near-field optical microscope. This hybrid network provides a long-sought solution for accurate extraction of material properties from tip-specific raw data. The methodology can be extended to other scanning probe microscopy techniques as well as other data-oriented physical problems in general.

Author(s): Neal, SN; Kim, HS; O'Neal, KR; Haglund, AV; Smith, KA; Mandrus, DG; Bechtel, HA; Carr, GL; Haule, K; Vanderbilt, D; Musfeldt, JL | Abstract: We employ synchrotron-based near-field infrared spectroscopy to reveal the vibrational properties of bulk, few-sheet, and single-sheet members of the MPS3 (M=Mn, Fe, Ni) family of materials and compare our findings with complementary lattice dynamics calculations. MnPS3 and the Fe analog are similar in terms of their symmetry crossovers, from C2/m to P3¯1m, as the monolayer is approached. These states differ as to the presence of a C3 rotation around the metal center. On the other hand, NiPS3 does not show a symmetry crossover, and the lack of a Bu symmetry mode near 450 cm-1 suggests that C3 rotational symmetry is already present, even in the bulk material. We discuss these findings in terms of local symmetry and temperature effects as well as the curious relationship between these symmetry transformations and those that take place under pressure.

The honeycomb iridate ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ has received much attention as a candidate to realize a quantum spin liquid state, but the nature of its insulating state remains controversial. We found that the material exhibits structural transitions at 3 and 10 GPa. The former is accompanied by 166-meV suppression of the activation gap, but the energies for the low-lying interband transitions change by less than 10 meV. This can be reconciled in a picture in which the application of high pressure barely shifts the electronic bands, but rather merely broadens them. First-principles calculations uncover a strong correlation between the band gap and the $\ensuremath{\beta}$ angle of the monoclinic structure, indicating non-negligible interlayer coupling. These results offer clear evidence for a spin-orbit Mott insulating state in ${\mathrm{Na}}_{2}{\mathrm{IrO}}_{3}$ and are inconsistent with the quasimolecular orbital model.

We analyze the response of a BCS-type superconductor exposed to a strong THz pulse using the FDTD method in combination with a model time-dependent susceptibility having an explicit dependence on the superconductor's energy gap Δ. In the analysis, the energy gap is allowed to be time varying through its dependence on the current density, along with a relaxation time related to the inelastic electronic scattering rate. The model accounts for 3rd harmonic generation effects and can be applied to the case of an incident, strong-field, single or few cycle pulse for which significant nonlinear effects can occur.

When viewed at frequencies below about 8 THz (250 cm-1; 30 meV) the ab-plane optical conductivity of the cuprate superconductors (in their normal state) is well described by a Drude model. Examples include optimally-doped YBa2Cu3O7-δ and Bi2Sr2CaCu2O8; even the underdoped phases have a Drude character to their optical conductivity. A residual Drude-like normal fluid is seen in the superconducting state in most cases; the scattering rate of this quasiparticle contribution collapses at Tc.

We report the production of high power (20watts average, ∼ 1 Megawatt peak) broadbandTHz light based on coherent emission fromrelativistic electrons. Such sources areideal for imaging, for high power damagestudies and for studies of non-linearphenomena in this spectral range. Wedescribe the source, presenting theoreticalcalculations and their experimentalverification. For clarity we compare thissource with one based on ultrafast lasertechniques.

Terahertz (THz) radiation, which lies in the far-infrared region, is at the interface of electronics and photonics. Narrow-band THz radiation can be produced by free-electron lasers and fast diodes. Broadband THz radiation can be produced by thermal sources and, more recently, by table-top laser-driven sources and by short electron bunches in accelerators, but so far only with low power. Here we report calculations and measurements that confirm the production of high-power broadband THz radiation from subpicosecond electron bunches in an accelerator. The average power is nearly 20 watts, several orders of magnitude higher than any existing source, which could enable various new applications. In particular, many materials have distinct absorptive and dispersive properties in this spectral range, so that THz imaging could reveal interesting features. For example, it would be possible to image the distribution of specific proteins or water in tissue, or buried metal layers in semiconductors; the present source would allow full-field, real-time capture of such images. High peak and average power THz sources are also critical in driving new nonlinear phenomena and for pump-probe studies of dynamical properties of materials.

We measured the spectroscopic response of stripe- and vortex-containing ${\mathrm{ErMnO}}_{3}$ in order to uncover the electronic properties of the domain walls. We quantify Born effective charge and polarization differences using the lattice behavior, analyze the local rare earth environment from the $f$-manifold excitations, and reveal how shifts in the charge transfer excitations impact the band gap. The increased Born charge, polarization, and band gap in the vortex-containing material are brought together with a discussion of hybridization and wall density effects. The domain wall optical constants are strongly frequency dependent.

The characteristics of topological insulators are manifested in both their surface and bulk properties, but the latter remain to be explored. Here we report bulk signatures of pressure-induced band inversion and topological phase transitions in ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Sn}}_{x}\mathrm{Se}$ ($x=0.00$, 0.15, and 0.23). The results of infrared measurements as a function of pressure indicate the closing and the reopening of the band gap as well as a maximum in the free carrier spectral weight. The enhanced density of states near the band gap in the topological phase gives rise to a steep interband absorption edge. The change of density of states also yields a maximum in the pressure dependence of the Fermi level. Thus, our conclusive results provide a consistent picture of pressure-induced topological phase transitions and highlight the bulk origin of the novel properties in topological insulators.

We report the observation of multiparticle coherent emission in the very far infrared from bunched electrons in an electron storage ring. The emission occurs in quasi-periodic bursts, and only when the electron beam current exceeds a threshold value, suggesting an instability-driven modulation of the electron bunch density. For the operating conditions reported, the spectral content of the coherent emission is peaked near a wavelength of 7 mm. This wavelength is much shorter than the nominal electron bunch length, indicating the presence of a density modulation within the bunch.

The instrumentation and performance of the new infrared beamline U12IR at the National Synchrotron Light Source of Brookhaven National Laboratory is described. This beamline utilizes infrared synchrotron radiation from a bending magnet. A combination of beamline design features and spectroscopic instrumentation allows the facility to reach the extremely low frequency limit of ∼2 cm−1 (i.e., 60 GHz or a photon energy of 250 μeV). The infrared light from the synchrotron emission at U12IR is compared to standard thermal sources and reveals substantial benefits for the study of small samples. In particular, the intensity of the synchrotron radiation in the far infrared can be as much as 200 times greater than that from a blackbody when millimeter-sized samples are measured. The effects of diffraction and noise on beamline performance are also discussed.

We review the properties of infrared synchrotron radiation and particularly those of the beamlines at the NSLS, Brookhaven National Laboratory. We show how this unique, pulsed broadband source with its 1000-fold brightness advantage over thermal sources has been utilized for research programs in surface science, solid-state physics and microscopy and discuss plans for 5 new beamlines at the NSLS.

We report the observation of two signatures of a pressure-induced topological quantum phase transition in the polar semiconductor BiTeI using x-ray powder diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI remains in its ambient-pressure structure up to 8 GPa. The lattice parameter ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis bonding through pz band crossing as expected during the transition. Over the same pressure range, the infrared spectra reveal a maximum in the optical spectral weight of the charge carriers, reflecting the closing and reopening of the semiconducting band gap. Both of these features are characteristics of a topological quantum phase transition, and are consistent with a recent theoretical proposal., revised final version

The vortex-state electrodynamics of s-wave superconductors has been studied by infrared spectroscopy. Far-infrared transmission and reflection spectra of superc onducting Nb0.5Ti0.5N and NbN thin films were measured in a magnetic field perpendicular to the film surface , and the optical conductivity was extracted. The data show clear reduction of superconducting signature. We consider the vortex state as a twocomponent effective medium of normal cores embedded in a BCS superconductor. The spectral features are well explained by the Maxwell-Garnett theory. Our analysis supports the presence of magnetic-fieldinduced pair-breaking effects in the superconducting component outside of the vortex cores. The vortex or Abrikosov state exists in type-II superconductors subjected to magnetic fields between Bc1 and Bc2. In this state the field penetrates the superconductor in the form of quantized tubes of flux, or vortices. The superconducting gap is zero inside the vortex cores and finite outside so that each vortex may be considered to have a core of normal metal, surrounded by superconductor [1, 2]. Because vortex quantization renders the material an inhomogeneous system, it necessarily affects the electrodynamics of the s uperconductor. The microwave response of the vortex state has been extensively studied theoretically [3‐5] and experimentally [6‐11]. However the picture is still incomplete in the infrared region spanning the superconducting gap [12, 13]. In this Letter we address the infrared electrodynamics of the vortex state. We obtain the complex optical conductivity of type-II superconductors and compare our results to calculations of a superconductor-normal metal mixture using the two key models for the effective conductivity of an inhomogeneous system: that of Garnett [14] (the so-called “Maxwell-Garnett theory” or MGT) and that of Bruggeman [15] (sometimes called the “effective-medium approximation” or EMA). We also compare our results to a theory of viscous motion of vortices driven by currents in the superconductor [3]. We find that only the MGT gives a good description of experiment, and then only when pairbreaking by the magnetic field [16‐18] is considered. That it does so is reasonable considering the topology of the vortex state: normal regions surrounded entirely by a connected superfluid. As pointed out some years ago [19], this is the topology of the MGT: the inclusions are embedded in a host medium and are correlated to stay apart. In contrast, the EMA allows percolation of the minority constituent at some critical concentration, something that does not happen in the vortex state until the upper critical field, when the entire material is in the normal state. We studied type-II superconducting thin films of BCS superconductors Nb0.5Ti0.5N and NbN, which are widely used in superconducting magnets [20], RF cavities [21], and photodetectors [22]. The 10 nm Nb0.5Ti0.5N film was grown on a quartz substrate in Ar and N2 gas with a NbTi target, and the 70 nm NbN film grown on a MgO substrate in N2 atmosphere using Nb, both by reactive magnetron sputtering [23, 24]. The substrates have negligible absorption in the spectral range of interest (10‐ 100 cm −1 ) for T < 20 K. We performed the experiment at Beamline U4IR of the National Synchrotron Light Source, Brookhaven National Laboratory. The beamline is equipped with a Bruker IFS 66v FT-IR spectrometer, modified to use synchrotron radiation and a superconducting magnet for low-temperature magneto-spectroscopy. A composite silicon bolometer operating at T ∼1.5 K detects far-infrared radiation with high sensitivity. Both sample s were cooled to 2 K (≪ Tc) in zero field, and their transmission and reflection measured in magnetic fields from 0‐10 T, with the field direction normal to the sample sur

The far-infrared (30--350 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) transmittance and reflectance of two ab-plane-oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films have been measured. These data show that the quasiparticle relaxation rate, 1/\ensuremath{\tau}, has a fast decrease below ${\mathit{T}}_{\mathit{c}}$ and saturates at T\ensuremath{\ll}${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$, 1/\ensuremath{\tau} exhibits a linear temperature dependence, in accord with the linear dc resistivity in the normal state. The fast decrease of 1/\ensuremath{\tau} is unique and intrinsic to the high-${\mathit{T}}_{\mathit{c}}$ cuprates; it does not occur in conventional BCS superconductors, where the scattering is from impurity or phonons. The low-frequency conductivity ${\mathrm{\ensuremath{\sigma}}}_{1}$ inferred from the far-infrared transmittance and reflectance measurements exhibits a peak just below ${\mathit{T}}_{\mathit{c}}$. The conductivity peak occurs at higher frequencies than those expected for BCS coherence effects. The peak in ${\mathrm{\ensuremath{\sigma}}}_{1}$ is attributed to the rapid drop in 1/\ensuremath{\tau} combined with a decreasing normal-fluid density. \textcopyright{} 1996 The American Physical Society.

Infrared reflection and transmission as a function of temperature have been measured on single crystals of Cu3Bi(SeO3)(2)O2Cl. The complex dielectric function and optical properties along all three principal axes of the orthorhombic cell were obtained via Kramers-Kronig analysis and by fits to a Drude-Lorentz model. Below 115 K, 16 additional modes [8(E parallel to(a) over cap)+ 6(E parallel to(b) over cap)+ 2(E parallel to(c) over cap)] appear in the phonon spectra; however, powder x-ray diffraction measurements do not detect a new structure at 85 K. Potential explanations for the new phonon modes are discussed. Transmission in the far infrared as a function of temperature has revealed magnetic excitations originating below the magnetic ordering temperature (T-c similar to 24 K). The origin of the excitations in the magnetically ordered state will be discussed in terms of their response to different polarizations of incident light, behavior in externally applied magnetic fields, and the anisotropic magnetic properties of Cu3Bi(SeO3)(2)O2Cl as determined by dc susceptibility measurements.

We demonstrate the generation of tunable, narrow-band, few-cycle and multi-cycle terahertz pulses from a temporally modulated relativistic electron beam through coherent transition radiation. OCIS codes: 320.0320

Quasiparticle recombination in a superconductor with an s-wave gap is typically dominated by a phonon bottleneck effect. We have studied how a magnetic field changes this recombination process in metallic thin-film superconductors, finding that the quasiparticle recombination process is significantly slowed as the field increases. While we observe this for all field orientations, we focus here on the results for a field applied parallel to the thin film surface, minimizing the influence of vortices. The magnetic field disrupts the time-reversal symmetry of the pairs, giving them a finite lifetime and decreasing the energy gap. The field could also polarize the quasiparticle spins, producing different populations of spin-up and spin-down quasiparticles. Both processes favor slower recombination; in our materials we conclude that strong spin-orbit scattering reduces the spin polarization, leaving the field-induced gap reduction as the dominant effect and accounting quantitatively for the observed recombination rate reduction.

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from $\ensuremath{\sim}0.26$ to 2.6 THz with a bandwidth of $\ensuremath{\sim}0.16\text{ }\text{ }\mathrm{THz}$.

We report the generation and characterization of single-, few-, and multi-cycle terahertz pulses from relativistic electrons.

We demonstrate that an intense single-cycle terahertz (THz) pulse can act as a time lens to phase modulate and compress a copropagating ultrashort laser pulse. By using the THz-induced phase modulation as a time lens and a glass plate as a group velocity dispersive element, we have compressed an unchirped $~165$ fs laser pulse to $~45$ fs.

We demonstrate that an intense subpicosecond single-cycle terahertz pulse in an electro-optic medium can act as a temporal and spatial lens to phase modulate and focus a co-propagating ultrashort laser pulse.

The infrared absorption of oxide superconductors will be discussed. In the normal state, σ 1 (ω) consists of a strongly T -dependent part in the far infrared, above which is a weakly- T -dependent midinfrared absorption. In the superconducting state, there is absorption to very low frequencies, well below where the superconducting gap is expected to be. It is argued that this absorption is due to a second (“midinfrared”) component which is only weakly affected by the onset of superconductivity. The free carrier component is in the clean limit and most of its oscillator strength goes into the superfluid condensate zero-frequency delta function response.

In this paper we present the preliminary results from a new experimental technique to synchronize the pulses from a mode-locked Nd-YAG laser to the light pulses in the VUV storage ring at the National Synchrotron Light Source (NSLS). We describe a method to electronically change the delay time between the laser pulses and the synchrotron pulses. We also illustrate a method to overlap the synchrotron pulses with the laser pulses in space and time. Preliminary results will be presented for two experiments.

The low-frequency conductivity ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ has a Drude-like component below ${\mathit{T}}_{\mathit{c}}$. Interpreting the width of this component as the quasiparticle relaxation rate (${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$), we find that ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ decreases dramatically just below ${\mathit{T}}_{\mathit{c}}$, in sharp contrast with the T-linear ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ above ${\mathit{T}}_{\mathit{c}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6}$. This decrease causes a peak in ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}\ensuremath{\rightarrow}0,T) for T just below ${\mathit{T}}_{\mathit{c}}$, a peak which is due to the scattering rate and not to pair coherence effects, consistent with the lack of a coherence peak in the NMR relaxation rate. This result implies that the excitations which scatter the carriers are suppressed below ${\mathit{T}}_{\mathit{c}}$.

We demonstrate that the intense electric field of a subpicosecond single-cycle terahertz pulse can control and manipulate the temporal, spectral, and spatial characteristics of a copropagating ultrashort laser pulse through cross-phase modulation.

Infrared and far-infrared transmittance measurements were performed on Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} (2:2:1:2) and Bi{sub 2}Sr{sub 2}CuO{sub 6} (2:2:0:1). Below {ital T}{sub {ital c}}, a characteristic feature occurs at {omega}=700 cm{sup {minus}1} (12{ital k}{sub {ital B}T{ital c}}) in the 2:2:1:2 material. No similar feature is found in the 2:2:0:1 compound. The 2:2:1:2 material also showed substantial {ital ab}-plane anisotropy.

Linearly polarized spectra of far-infrared (IR) transmission in ${\text{HoMn}}_{2}{\text{O}}_{5}$ multiferroic single crystals have been studied in the frequency range between 8.5 and $105\text{ }{\text{cm}}^{\ensuremath{-}1}$ and for temperatures between 5 and 300 K. Polarization of IR-active excitations depends on the crystallographic directions in ${\text{HoMn}}_{2}{\text{O}}_{5}$ and is sensitive to the magnetic phase transitions. We attribute some of the infrared-active excitations to electric-dipole transitions between ligand-field (LF) split states of ${\text{Ho}}^{3+}$ ions. For light polarization along crystalline $b$ axis, the oscillator strength of electric dipoles at low frequencies (10.5, 13, and $18\text{ }{\text{cm}}^{\ensuremath{-}1}$) changes significantly at the commensurate-incommensurate antiferromagnetic phase transition at ${T}_{3}=19\text{ }\text{K}$. This effect shows a strong correlation with the pronounced steps of the $b$-directional static dielectric function. We propose that the LF on ${\text{Ho}}^{3+}$ connects the magnetism and dielectric properties of this compound through coupling with the Mn spin structure. We comment on the possibility for composite excitations of magnons and excited LF states.

We demonstrate that the intense electric field of a subpicosecond single-cycle terahertz pulse can control and manipulate the temporal, spectral, and spatial phase of a copropagating ultrashort laser pulse through the Pockels effect in an electro-optic crystal. In the temporal and spectral domains, the single-cycle THz pulse can impart either a positive or a negative quadratic phase modulation to the probe pulse, leading to a spectral shift, broadening, or pulse expansion or compression. While acting in the spatial domain, the THz-induced phase modulation induces a lenslike effect, providing focusing or defocusing of the copropagating probe beam. The experimental results are in good agreement with simulations. Our study gives a comprehensive picture of the nonlinear spatiotemporal dynamics in the high-field regime driven by the intense single-cycle THz pulse.

We report infrared synchrotron-radiation transmission measurements on \ensuremath{\sim}1-\ensuremath{\mu}m-thick films of high-${\mathit{T}}_{\mathit{c}}$ ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ from 40 to 290 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (5 to 36 meV). The films were deposited onto MgO substrates and showed resistive superconducting transitions with an onset at 83 K. Spectra below ${\mathit{T}}_{\mathit{c}}$ show superconducting behavior, including the characteristic 1/\ensuremath{\omega} supercurrent dependence for frequencies below 100 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The entire spectrum can be fit when a Mattis-Bardeen-type conductivity is included identifying increasing absorption beginning at a photon energy of 23 meV. However, a substantial residual absorption persists to the lowest measured frequencies. In addition, the appearance of phonons, some attributable to c-axis vibrations, suggests we are sampling a mixture of both a-b--plane and c-axis properties.

We have used electro-optic methods to investigate single-cycle THz pulses produced as coherent transition radiation from relativistic electron bunches. The pulses have energies up to 100 muJ such that electric fields approaching 100 MV/m should be attainable. We find that the combination of these large fields and their rapid variation (on a sub-ps time scale) results in significant time-dependent phase modulation of a laser pulse when co-propagated in a non-linear medium such as ZnTe.

We demonstrate that time-dependent electric field associated with intense single-cycle THz pulses can induce nonlinear phase modulation in electro-optical crystals, leading to spectral shift, broadening and modulation of co-propagating laser pulses.

We have demonstrated nonlinear cross-phase modulation in electro-optic crystals using intense, single-cycle terahertz (THz) radiation. Individual THz pulses, generated by coherent transition radiation emitted by subpicosecond electron bunches, have peak energies of up to 100 microJ per pulse. The time-dependent electric field of the intense THz pulses induces cross-phase modulation in electro-optic crystals through the Pockels effect, leading to spectral shifting, broadening, and modulation of copropagating laser pulses. The observed THz-induced cross-phase modulation agrees well with a time-dependent phase-shift model.

A bunched electron beam is an efficient source of coherent radiation at wavelengths longer than the bunch length. We describe an experiment demonstrating electron density modulation at 0.8 THz using an optically modulated drive laser to extract a prebunched beam from an RF photoinjector, which is preserved through acceleration to 75 MeV. Measurements of coherent THz transition radiation (CTTR) are presented at the prebunch frequency showing enhanced production over that produced from an unmodulated beam.

The University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory have been collaborating on a project that explores the use of electron beam pre-modulation at the cathode to control the longitudinal structure of the electron beam. This technique could be applied to creating deliberate modulations which can lead to the generation of terahertz radiation, or creating a smooth profile in order to suppress radiation. This paper focuses on simulations that explore some of the pre-modulated cases achieved experimentally.

The scattering process responsible for connecting the bands remains one of the last open questions on the physical properties of MgB2. Through the analysis of the equilibrium and photo-induced far-infrared properties as well as electron spin resonance of MgB2 we propose a phonon mediated energy transfer process between the bands based on the coupling of quasiparticles to an E2g phonon.

Controlling the longitudinal distribution in an electron beam traveling through an accelerator can enhance system performance in various ways. Prebunching an electron beam in an accelerator can lead to enhanced radiation production from various emitters. Smoothing the shape of the electron beam pulse may suppress instabilities that result in beam breakup. This work, a collaboration between the University of Maryland and the Source Development Laboratory at Brookhaven National Laboratory, uses a Ti:Sapphire drive laser modulated at terahertz frequencies to extract a prebunched beam directly from the photocathode prior to acceleration in a linear RF accelerator. We have demonstrated that prebunched terahertz structure can be maintained after leaving the photocathode and through acceleration to high energy. Through the use of simulation and experiment, this work further explores the characteristics of an electron beam produced in this manner and examines the possibility of using this technology to develop a compact, powerful, narrowband terahertz emitter.

A new infrared beamline has been developed on a conventional dipole bending magnet port of the vacuum ultraviolet ring at the National Synchrotron Light Source. The port provides approximately 12 mrad horizontal and 8 mrad vertical aperture, which limits the useful spectral range to wavelengths less than 20 μm. Though the total flux across the midinfrared is less than that from a globar source, the calculated brightness is at least two orders of magnitude greater. Also, the synchrotron source delivers light in subnanosecond pulses. The developing experimental programs include studies of hydrogen and other materials at extremely high pressures, and time‐resolved studies of infrared sensor materials. The measurement results presented here, characterizing the actual brightness advantage and spectroscopic performance, demonstrate the synchrotron’s remarkable advantage for microspectroscopic studies.

A facility to perform infrared microspectroscopy is under development at the NSLS of Brookhaven National Laboratory. The high brightness infrared light produced as synchrotron radiation makes a nearly ideal source for microspectroscopy. High quality spectra from 10 μm sized areas can be acquired in less than 1 min. A description of the installation, microspectroscopy performance, and an example application are presented.

We report the production of high power (20 watts average, /spl sim/1 megawatt peak) broadband THz light based on coherent emission from relativistic electrons. We describe the source, presenting theoretical calculations and their experimental verification. For clarity we compare this source with one based on ultrafast laser techniques.

Bursts of coherent radiation are observed from the NSLS VUV ring near a wavelength of 7 mm. The bursts occur when the electron beam current (I) exceeds a threshold value (I/sub th/), which itself varies with ring operating conditions. Beyond threshold, the average intensity of the emission is found to increase as (I-I/sub th/)/sup 2/. With other parameters held nearly constant, the threshold current value is found to increase quadratically with synchrotron frequency, indicating a linear dependence on momentum compaction. It is believed that the coherent emission is a consequence of micro-bunching of the electron beam due to the microwave instability.

The far-infrared coherent emission from short electron bunches has been detected and partially characterized for two source locations at the NSLS Source Development Lab (SDL) accelerator system. One location is designed to extract dipole edge radiation while the other extracts transition radiation produced a short distance downstream of the SDL linac. The coherently radiated energy per pulse is 10's of nJ, and depends quadratically on bunch charge. The spectral content agrees with the longitudinal bunch density as determined from other measurements.

The DUVFEL electron linac is designed to produce sub-picosecond, high brightness electron bunches to drive an ultraviolet FEL. The accelerator consists of a 1.6 cell S-band photoinjector, variable pulse length Ti:Sapp laser, 4 SLAC-type S-band accelerating sections, and 4-dipole chicane bunch compressor. In preparation for FEL operation, the compressed electron beam has been fully characterized. Measurement of the beam parameters and simulation of the beam are presented.

A diagnostic undulator is used to measure relative electron bunch lengths. The radiation produced by the short undulator is observed off-axis so that the beam is not interrupted. Wavelengths radiated which are longer than the bunch length are coherent and may be more intense than the shorter wavelengths by a factor of up to 10/sup 9/. The total energy radiated thus changes as the bunch length changes, and a measurement of the radiated energy provides a robust beam diagnostic. This non-disturbing diagnostic would be useful in applications where the length of short electron bunches is critical, such as single-pass FELs and linear colliders. For example, real-time bunch length measurements could be used to optimize performance of a single-pass FEL. The experiment is performed at the Source Development Lab (SDL) at Brookhaven National Laboratory. The theory and description of the experiment are reviewed, and the first experimental results are presented.

The terahertz (THz) region of the electromagnetic spectrum is attracting interest for a broad range of applications ranging from diagnosing electron beams to biological imaging. Most sources of short pulse THz radiation utilize excitation of biased semiconductors or electro-optic crystals by high peak power lasers. For example, this was done by using an un-doped InAs wafer irradiated by a femtosecond free-electron laser (FEL) at the Thomas Jefferson National Accelerator Facility. Microwatt levels of THz radiation were detected when excited with FEL pulses at 1.06 mm wavelength and 10W average power. Recently substantially higher powers of femtosecond THz pulses produced by synchrotron emission were extracted from the electron beamline. Calculations and measurements confirm the production of coherent broadband THz radiation from relativistic electrons with an average power of nearly 20W, a world record in this wavelength range by a factor of 10,000. We describe the source, presenting theoretical calculations and their experimental verification. Potential applications of this exciting new source include driving new non-linear phenomena, performing pump-probe studies of dynamical properties of novel materials, and studying molecular vibrations and rotations, low frequency protein motions, phonons, superconductor band gaps, electronic scattering, collective electronic excitations (e.g., charge density waves), and spintronics.

We report the production of high power (20 watts average, ~;1 Megawatt peak) broadband THz light based on coherent emission from relativistic electrons. We describe the source, presenting theoretical calculations and their experimental verification. For clarity we compare this source with one based on ultrafast laser techniques, and in fact the radiation has qualities closely analogous to that produced by such sources, namely that it is spatially coherent, and comprises short duration pulses with transform-limited spectral content. In contrast to conventional THz radiation, however, the intensity is many orders of magnitude greater due to the relativistic enhancement.

Temperature dependent optical conductivities and dc resistivity of c-axis oriented superconducting ( ${T}_{c}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}39.6\mathrm{K}$) ${\mathrm{MgB}}_{2}$ films ( $\ensuremath{\sim}450\mathrm{nm}$) have been measured. The normal state ab-plane optical conductivities can be described by the Drude model with a temperature independent Drude plasma frequency of ${\ensuremath{\omega}}_{p,D}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}13600\ifmmode\pm\else\textpm\fi{}100{\mathrm{cm}}^{\ensuremath{-}1}$ or $1.68\ifmmode\pm\else\textpm\fi{}0.01\mathrm{eV}$. The normal state resistivity is fitted by the Bloch-Gr\"uneisen formula with an electron-phonon coupling constant ${\ensuremath{\lambda}}_{\mathrm{tr}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.13\ifmmode\pm\else\textpm\fi{}0.02$. The optical conductivity spectra below ${T}_{c}$ of these films suggest that ${\mathrm{MgB}}_{2}$ is a multigap superconductor.

The DUVFEL electron linac is designed to produce sub-picosecond, high brightness electron bunches for driving a short wavelength FEL. Four experiments have been commissioned to address the challenge of accurately measuring bunch lengths on this timescale. In the frequency domain, a short 12 period undulator is used to produce both off-axis coherent emission and on-axis incoherent single-shot spectra. The total coherent infrared power scales inversely with the bunch length and the spectral cutoff is an indication of bunch length. The density of the power spikes in the single-shot visible spectrum may also be used to estimate the bunch length. In the time domain, the linac accelerating sections and a bending magnet are used to implement the RF-zero phasing method, and a subpicosecond streak camera is also installed. The beam measurements with comparisons of these methods are reported.