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We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO3 epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.

The 3rd harmonic system installed in the injector at FLASH (Free-Electron Laser at Hamburg) accelerator allows to achieve ultra short electron bunches with higher peak current, which pushed the limits of high brilliance laser light generation into the so-called water window. The 3rd harmonic system operates at 3.9GHz and requires a very precise phase and amplitude control of the accelerating field below 10fs, which is done by the Low-Level RF system. One of the key component is the RF field detector that converts the detected cavity field signal to an intermediate frequency for digital sampling. The paper presents an overview of the system, particularly the low phase noise reference generator, local oscillator and multichannel down-converter. System scheme and performance are demonstrated.

Until recently, the pulse energies and field strengths of terahertz (THz) pulses have been too low to be used to excite matter or to study nonlinear effects in the THz range. In this chapter we will describe methods of generation and nonlinear spectroscopies with intense single- and few-cycle THz pulses, reporting the current state of the art.

Terahertz (THz) pulses with energies up to 100 μJ and corresponding electric fields up to 1 MV/cm were generated by coherent transition radiation from 500 MeV electron bunches at the free-electron laser Freie-Elektronen-Laser in Hamburg (FLASH). The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs. High THz field strengths and quality of synchronization with an optical laser will enable observation of nonlinear THz phenomena.

Physical review letters 107, 175002 (2011). doi:10.1103/PhysRevLett.107.175002, Published by APS, College Park, Md.

Ultrashort terahertz pulses derived from femtosecond table-top sources have become a valuable tool for time-resolved spectroscopy during the last two decades. Until recently, the pulse energies and field strengths of these pulses have been generally too low to allow for the use as pump pulses or the study of nonlinear effects in the terahertz range. In this review article we will describe methods of generation of intense single cycle terahertz pulses with emphasis on optical rectification using the tilted-pulse-front pumping technique. We will also discuss some applications of these intense pulses in the emerging field of nonlinear terahertz spectroscopy.

One of the most intriguing features of some high-temperature cuprate superconductors is the interplay between one-dimensional “striped” spin order and charge order, and superconductivity. We used mid-infrared femtosecond pulses to transform one such stripe-ordered compound, nonsuperconducting La1.675Eu0.2Sr0.125CuO4, into a transient three-dimensional superconductor. The emergence of coherent interlayer transport was evidenced by the prompt appearance of a Josephson plasma resonance in the c-axis optical properties. An upper limit for the time scale needed to form the superconducting phase is estimated to be 1 to 2 picoseconds, which is significantly faster than expected. This places stringent new constraints on our understanding of stripe order and its relation to superconductivity.

THz pulses with energies up to 100 μJ and corresponding electric fields up to 1 MV/cm were generated by coherent transition radiation from 500 MeV electron bunches at the free electron laser FLASH. The pulses were characterized in the time domain by electro-optical sampling by a synchronized femtosecond laser with jitter of less than 100 fs.

We demonstrate saturable absorber behavior of n-type semiconductors GaAs, GaP and Ge in THz frequency range at room temperature using nonlinear THz spectroscopy. The saturation mechanism is based on a decrease in electron conductivity of semiconductors at high electron momentum states, due to conduction band nonparabolicity and scattering into satellite valleys in strong THz fields. Saturable absorber parameters, such as linear and non-saturable transmission, and saturation fluence, are extracted by fits to a classic saturable absorber model. Further, we observe THz pulse shortening, and an increase of the group refractive index of the samples at higher THz pulse peak fields., Submitted to Appl. Phys. Lett.

THz-induced optical birefringence was observed in the relaxor ferroelectrics KTN and KLTN. The dynamics observed may arise from the onset of ferroelectric nanodomains.

Saturated absorption and intervalley scattering in n-type semiconductors were observed using noncollinear THz-pump--THz probe spectroscopy with ultrabroadband probe pulses.

We compare the observed strong saturation of the free carrier absorption in n-type semiconductors at 300 K in the terahertz frequency range when single-cycle pulses with intensities up to 150 MW/cm2 are used. In the case of germanium, a small increase of the absorption occurs at intermediate THz pulse energies. The recovery of the free carrier absorption was monitored by time-resolved THz-pump/THz-probe measurements. At short probe delay times, the frequency response of germanium cannot be fitted by the Drude model. We attribute these unique phenomena of Ge to dynamical overpopulation of the high mobility gamma conduction band valley., Comment: 4 figures

We demonstrate an instantaneous all-optical manipulation of optical absorption in InGaAs/GaAsquantum dots (QDs) via an electro-absorption effect induced by the electric field of an incidentfree-space terahertz signal. A terahertz signal with the full bandwidth of 3 THz was directly encodedonto an optical signal probing the absorption in QDs, resulting in the encoded temporal features asfast as 460 fs. The instantaneous nature of this effect enables femtosecond all-optical switching atvery high repetition rates, suggesting applications in terahertz-range wireless communicationsystems with data rates of at least 0.5 Tbit/s.

We have observed optical birefringence in liquids induced by single-cycle terahertz pulses with field strengths exceeding 100 kV/cm. The induced change in polarization is proportional to the square of the terahertz electric field. The time-dependent terahertz Kerr signal is composed of a fast electronic response that follows the individual cycles of the electric field and a slow exponential response associated with molecular orientation., United States. Office of Naval Research (ONR Grant. No. N00014–06–1–0459)

We demonstrate efficient generation of THz pulses by optical rectification of 1.03 um wavelength laser pulses in LiNbO3 using tilted pulse front excitation for velocity matching between the optical and THz fields. Pulse energies of 100 nJ with a spectral bandwidth of up to 2.5 THz were obtained at a pump energy of 400 uJ and 300 fs pulse duration. This conversion efficiency of 2.5×10-4 was an order of magnitude higher than that obtained with collinear optical recitification in GaP, and far higher still than that measured using ZnTe in an optimized geometry. Using a simple model we demonstrate that two- and three-photon absorption strongly limit the THz generation efficiency at high pump fluences in ZnTe and GaP respectively.

We use terahertz time-domain spectroscopy (THz-TDS) to measure the far-infrared dielectric function of two artificial RNA single strands, composed of polyadenylic acid (poly-A) and polycytidylic acid (poly-C). We find a significant difference in the absorption between the two types of RNA strands, and we show that we can use this difference to record images of spot arrays of the RNA strands. Under controlled conditions it is possible to use the THz image to distinguish between the two RNA strands. We discuss the requirements to sample preparation imposed by the lack of sharp spectral features in the absorption spectra.

Indium antimonide InSb is a model system for the study of hot-electron dynamics due to its low band gap of 170 meV at room temperature 1 and the fact that it has the highest electron mobility and saturation velocity among all known semiconductors. The wealth of nonequilibrium transport phenomena that have been observed in this material 2–4 is of special interest due to the large nonparabolicity of the conduction band, 5 which results in negative differential mobility

Nonlinear high intensity THz transmission measurements were made on Ge, showing nonlinear optical effects in the THz range including self-phase modulation and absorption saturation. THz-pump–THz-probe measurements were performed on n-type Ge, GaAs, and Si to follow the ultrafast free carrier dynamics that underlie the nonlinear effects seen in the transmission measurements. Complex relaxation behavior was observed, arising from inter-and intra-valley scattering in the conduction bands of these semiconductors

Self-phase-modulation of ultrashort THz pulses was observed in lithium niobate at the 100 MW/cm2 intensity level. The effect, observed in time and frequency domains, suggests 1000x larger nonlinear index n2 than at visible wavelengths.

Terahertz is nonlinearly upconverted to telecommunication wavelengths, resulting in detection with 4.5 pW/Hz1/2 noise equivalent power and nanosecond temporal resolution. Optical frequencies from an ultrashort pulse mix, generating 3 mW of broadband terahertz.

Single-cycle THz radiation was generated by optical rectification of Yb-fiber laser pulses with 250 fs duration and 10 muJ energy. We obtained an average power of 0.5 mW at 1 MHz repetition rate.

We demonstrate the Kerr-effect induced by the electric field of single-cycle THz pulses in the relaxor ferroelectrics potassium-tantalum niobate KTa1-xNbxO3 (KTN) and K1-yLiyTa1-xNbxO3 (KLTN). We find a slow orientational relaxation with a time constant of 6 ps in KTN with x=1.8% at room temperature, that decreases upon approaching the transition temperature.

We observe impact ionization and saturation of free carrier absorption in indium antimonide at 200K and 80K. We employ a novel THz-pump-THz probe scheme with up to 100 kV/cm pump fields and 100 MW/cm2 intensity.

It is now possible to generate terahertz (THz) pulses with sufficient energy and field amplitude to enable versatile applications in THz nonlinear optics and spectroscopy. In addition, THz waveform shaping at high intensities promises wide ranging new capabilities in THz coherent control. We review recent progress in generation of high-power THz phonon-polariton waves in lithium niobate that can be coupled into free space THz radiation. A ldquopolaritonicsrdquo toolset for control and processing of the THz waves is also reviewed briefly. Recent demonstrations of THz nonlinear optics and spectroscopy are then presented.

Generation of near single-cycle pulses centered at 0.5 THz frequency with up to 10 muJ energy, 100 muW average power, and 5.0 MW peak power was demonstrated by tilting the intensity front of a femtosecond optical pump pulse from a 10 Hz Ti: sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal. THz pulse intensity as high as 10 MW/cm2 was achieved. The photon conversion efficiency was 45% and the calculated peak electric field strength at the focus of an off-axis parabolic mirror was 600 kV/cm.

There is a strong need for high energy ultrashort THz pulses. While optical rectification provides an accessible means to generate THz pulses, until recently, only pulses up to the 10 nJ energy level was demonstrated. This method needs velocity matching between optical and THz fields. In this contribution we present results on significant further up-scaling the energy.

For pump-probe experiments at VUV- and X-ray free- electron lasers, in the injector and bunch compression section the electron beam and timing reference must be in phase within 0.01 degree (rms) and have an amplitude stability within 10-4 (rms). The performance of the field detection and regulation of the acceleration RF critically influences the phase and amplitude stability. For the RF field control, a multichannel RF downconverter is used to detect the field vectors and control the vector sum of 32 cavities. In this paper a new design of an 8 channel downconverter is presented. The downconverter front-end consists of a passive RF double balanced mixer input stage, intermediate filters, and an integrated 16 Bit analog-to-digital converter (ADC). The design includes a digital motherboard for data pre-processing and communication with the controller. In addition, we characterize the downconverter performance in amplitude and phase jitter, temperature drifts, and channel cross-talk in a laboratory environment.

Generation of sub-μJ and 5 μJ single-cycle THz pulses is demonstrated through optical rectification of ultrashort pulses from 1 kHz and 10 Hz laser systems, respectively. Further scaling-up to 10 μJ levels is in progress.

THz pulses with over ten microjoules of energy are generated and used to drive nonlinear material responses. Nonlinear THz coherent spectroscopy and coherent control are illustrated.

A longitudinal broadband damper system to control coupled bunch instabilities (LMBF) has been installed in the 920 GeV proton accelerator HERA-p at the Deutsches Elektronen-Synchrotron DESY in Q4/2005. This was one of the attempts to increase the specific luminosity at HERA by reducing the bunch length. The feedback system is based on a single board FPGA (field programmable gate array) with two ADCs and 2 DACs (each 14 bits resolution) at 52 MHz sampling rate. It was fully automated, in order to relief the operator from manual control during system operation. During commissioning in Q1/2006 it turned out that the performance goals were reached and the noise introduced due to the feedback corrections and the 1 kW power amplifier was not as much a problem as expected. The proton bunch length is significantly reduced as well as the lengthening of the bunches over runtime since all formally occurring coupled bunch instabilities could sufficiently be suppressed during the enegergy ramp and the following luminosity run. System optimization points were found in automatic gain adjustment during acceleration ramp, oscillation level triggering and timing of kicker pulse to bunch.

The master oscillator (MO) and phase reference system at FLASH must provide several RF reference frequencies to widely spread locations with low phase noise and small long term phase drifts. The phase noise requirement of the 1300 MHz reference is of the order of 0.1 deg. while the short and medium term phase stability are of of the order of 0.1 deg. and 1 deg. respectively. The frequency distribution system employs a temperature stabilized coaxial line for RF power distribution and a fiber optic system for the monitoring of phase drifts. Presented are the the concept, design and performance measured in the accelerator environment.

During implantable cardioverter defibrillator (ICD) implantation of an active can ICD several defibrillations with 20 J and 34 J as well as 360 J externally were ineffective. The implant criteria were finally met with a second defibrillation lead and reversed polarity. A left-sided pneumothorax due to subclavian vein puncture was detected soon after ICD implantation. It is assumed that especially in the active can alignment the developing pneumothorax made defibrillation current flow more difficult. In case of several unsuccessful defibrillations during active can ICD implantation in which the subclavian vein was punctured, the possibility of a pneumothorax should be considered.

Wolfgang Monch, Johannes Donauer, Bernd M. Fischer, Rudiger Frank, Gunter Gauglitz, Carsten Glasenapp, Hanspeter Helm, Paul Hing, Matthias Hoffmann, Peter Uhd Jepsen, Thomas Kleine-Ostmann, Martin Koch, Holger Krause, Nicolae Leopold, Tina Mutschler, Frank Rutz, Titus Sparna, Hans Zappe

We characterize the dielectric properties of RNA and DNA polymers, and perform THz imaging of biopolymer spot arrays. We discuss the design of a THz spectroscopy system for characterization of microscopic quantities of sample material.

We discuss the influence of electronic noise and laser fluctuations on the largest detectable absorption and data accuracy in terahertz time-domain spectroscopy. Error propagation from the time domain to the frequency domain is discussed.

We performed ultrafast time resolved near infrared pump resonant soft x ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic TbMnO3 at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of 22.3 ± 1.1 ps which is much slower than the ~1 ps melting times previously observed in other systems. To explain the data we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature we do not observe changes in the wave vector q of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions.We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin spiral wave vector on these time scales. DOI: 10.1103/PhysRevB.92.184429

WEISS, C., et al.: Can Radiofrequency Current Isthmus Ablation Damage the Right Coronary Artery? Histopathological Findings Following the Use of a Long (8 mm) Tip Electrode. This report describes the histopathological findings following successful RF isthmus ablation for common atrial flutter in a 68-years-old man using a long 8-mm tip ablation catheter. No acute complication was observed. The patient died 3 weeks after ablation due to severe heart failure and consecutive pneumonia. Lesion width (1.0 and 2.4 cm) and depth (0.4 and 0.8 cm) was measured. The right coronary artery showed an intramural hemorrhage adjacent to the side of the lesion. However, despite this finding no apparent injury of other layers of the coronary artery was detectable.

The time-resolved ultrafast electric field-driven response of crystalline and amorphous GeSbTe films has been measured all-optically, pumping with single-cycle terahertz pulses as a means of biasing phase-change materials on a sub-picosecond time-scale. Utilizing the near-band-gap transmission as a probe of the electronic and structural response below the switching threshold, we observe a field-induced heating of the carrier system and resolve the picosecond-time-scale energy relaxation processes and their dependence on the sample annealing condition in the crystalline phase. In the amorphous phase, an instantaneous electroabsorption response is observed, quadratic in the terahertz field, followed by field-driven lattice heating, with Ohmic behavior up to 200 kV/cm.

Electroanatomic vs Conventional Mapping for Atrial Flutter Ablation. Introduction: Three-dimensional electroanatomic (CARTO) activation mapping of the cavotricuspid isthmus can be helpful to guide atrial flutter ablation, but to date has not heen investigated in comparison to conventional strategies. The aim of the present study was to assess the efficacy of the CARTO navigation system, especially with respect to the fluoroscopy time required for successful atrial flutter ablation. Methods and Results: Eighty patients with recurrent common-type atrial flutter were randomly assigned to temperature-controlled radiofrequency (RF) catheter ablation, either guided by conventional criteria (group 1) or additionally oriented on electroanatomic mapping (group 2). In all patients, similar multipolar catheters were inserted into the coronary sinus and placed at the tricuspid annulus. respectively. In group 2, positioning of the mapping electrode and delivery of RF pulses within the cavotricuspid isthmus was mainly oriented on the CARTO map to achieve the most linear and continuous RF lesions. Abolition of intra-atrial conduction verified by conventional criteria (group 1) and electroanatomic mapping (group 2) could be verified in all patients. The overall number of RF pulses (group 1: 16.7 ± 6.5; group 2: 13.2 ± 5.3) and mean procedure duration (group 1: 172.5 ± 47.4 min; group 2: 169.3 ± 47.3 min) were not different between the two groups, but mean fluoroscopy time was significantly shorter when the CARTO tecbnology was used (group 1: 29.2 ± 9.4 min; group 2: 7.7 ± 2.8 min; F = 0.0001). Recurrence of atrial flutter was observed in 3 (9%) patients in each group after a mean follow-up of 8.5 ± 2.months. Conclusion: Atrial flutter can be abolished effectively using the conventional technique as well as oriented on electroanatomic mapping. However, overall X-ray exposure can he significantly reduced by the CARTO-guided approach without prolongation of procedure duration.

Catheter ablation orientated on the induction of a functional intraatrial block within the posterior isthmus of the tricuspid annulus has been shown to effectively abolish atrial flutter. In order to improve and simplify the current technique, a strategy based on an electrode catheter for combined right atrial and coronary sinus mapping and stimulation was explored prospectively. Twenty-four consecutive patients referred for catheter ablation of recurrent type I atrial flutter were included. A steerable 7 Fr catheter (Medtronic/Cardiorhythm) composed of two segments with 20 electrodes was used for right atrial and coronary sinus activation mapping and stimulation. Multiple steering mechanisms allowing intubation and positioning of the distal part within the coronary sinus were incorporated into the device. Adequate positioning of the mapping catheter was achieved solely via a transfemoral approach in all patients after 7.7 +/- 4.6 minutes, providing stable electrogram recordings during the entire ablation procedure. Radiofrequency current ablation (16.3 +/- 9.6 pulses) caused a significant bidirectional increase of the mean intraatrial conduction times via the posterior isthmus irrespective to the stimulation interval. Significant changes of intraatrial conduction properties were induced during ablation in 22 of 24 patients (bidirectional block: n = 18, unidirectional block: n = 3, conduction delay: n = 1, unchanged conduction: n = 2). Following ablation atrial flutter was noninducible in all patients. Twenty-two of 24 patients (92%) remained free of atrial flutter episodes during a follow-up of 12.5 +/- 5.7 months. Two of six patients without a bidirectional conduction block had a recurrence of atrial flutter. Atrial flutter ablation guided by the induction of an intraatrial conduction block can be effectively performed with this novel strategy for combined mapping of the posterior tricuspid isthmus, including coronary sinus and right atrial free wall. This transfemoral approach has a high accuracy with respect to the detection of radiofrequency current-induced changes of intraatrial conduction patterns.

Pumping n-type GaAs and InSb with ultrafast THz pulses having intensities higher than 150 MW/cm2 shows strong free carrier absorption saturation at temperatures of 300 K and 200 K respectively. If the energy imparted to the carriers exceeds the bandgap, impact ionization processes can occur. The dynamics of carrier cooling in GaAs and impact ionization in InSb were monitored using THz-pump/THz probe spectroscopy which provides both sub-bandgap excitation and probing, eliminating any direct optical electron-hole generation that complicates the evaluation of results in optical pump/THz probe experiments., 7 figures, J. Opt. Soc. Am. B. Special issue of THz wave photonics (2009)

Near single-cycle terahertz pulse generation from lithium niobate crystal by optical rectification of pulses from an Yb:fiber laser/amplifier system was investigated. The pump laser supplied 250 fs duration pulses with 14 μJ energy at up to 1 MHz repetition rate. The terahertz pulse energy and average power exceeded 0.25 nJ and 0.25 mW, respectively. The average power was more than ten times higher than corresponding values achieved by using GaP as the nonlinear crystal under similar pumping conditions.

Generation of near single-cycle pulses centered at 0.5THz frequency with up to 10μJ energy, 100μW average power, and 5.0MW peak power was demonstrated by tilting the intensity front of a femtosecond optical pump pulse from a 10Hz Ti:sapphire laser to match the phonon-polariton phase velocity to the group velocity of the pump pulses in a lithium niobate crystal. Terahertz pulse intensity as high as 10MW∕cm2 was achieved. The photon conversion efficiency was 45% and the calculated peak electric field strength at the focus of an off-axis parabolic mirror was 250kV∕cm.

Ultrafast mid-infrared and terahertz pulses open a new avenue to control complex solids by directly accessing their low-energy excitations. Here we discuss recent results in inducing and manipulating superconductivity in cuprate compounds using strong fields.

The principles and most-recent results of high-power THz generation through optical rectification using a tilted optical pulse front are described. Single-cycle THz pulses of multimicrojoule energies are generated at kHz repetition rates, and average THz power levels exceeding 1 mW can be generated at kHz-MHz repetition rates. Applications in nonlinear THz spectroscopy and THz coherent control are discussed.

This paper describes the development of new instrumentation at the Linac Coherent Light Source for conducting THz excitation experiments in an ultra high vacuum environment probed by soft X-ray diffraction. This consists of a cantilevered, fully motorized mirror system which can provide 600 kV cm−1 electric field strengths across the sample and an X-ray detector that can span the full Ewald sphere with in-vacuum motion. The scientific applications motivated by this development, the details of the instrument, and spectra demonstrating the field strengths achieved using this newly developed system are discussed., Journal of Synchrotron Radiation, 22 (3), ISSN:0909-0495, ISSN:1600-5775

We present a comprehensive experimental and numerical study of magnetization dynamics triggered in a thin metallic film by single-cycle terahertz pulses of $\sim20$ MV/m electric field amplitude and $\sim1$ ps duration. The experimental dynamics is probed using the femtosecond magneto-optical Kerr effect (MOKE), and it is reproduced numerically using macrospin simulations. The magnetization dynamics can be decomposed in three distinct processes: a coherent precession of the magnetization around the terahertz magnetic field, an ultrafast demagnetization that suddenly changes the anisotropy of the film, and a uniform precession around the equilibrium effective field that is relaxed on the nanosecond time scale, consistent with a Gilbert damping process. Macrospin simulations quantitatively reproduce the observed dynamics, and allow us to predict that novel nonlinear magnetization dynamics regimes can be attained with existing table-top terahertz sources., 6 pages, 4 figures

In a THz pump - optical probe experiment we demonstrate an instantaneous electro-absorption effect in InGaAs/GaAs quantum dots, induced by the electric field of a single-cycle THz pulse with 3 THz bandwidth and with free-space peak electric field reaching 220 kV/cm. The transient modulation of QD ground state optical absorption at 1040 nm coherently follows the evolution of the absolute value of THz electric field. The optical modulation signal was found to be as short as 460 fs at FWHM, and retained the 3 THz bandwidth of the THz pulse. Optical absorption modulation in QDs by the THz field is dominated by absorption quenching effect due to the electron- and hole wavefunction separation in THz field, rather than by the Stark shift away from the optical probe.

Optical rectification of ultrashort laser pulses in LiNbO3 by tilted-pulse-front excitation is a powerful way to generate near single-cycle terahertz (THz) pulses. Motivated by various applications, calculations were carried out to optimize the THz peak electric field strength. The results predict THz output with peak electric field strength on the MV/cm level in the 0.3-1.5 THz frequency range by using optimal pump pulse duration of about 500 fs, optimal crystal length, and cryogenic temperatures for reducing THz absorption in LiNbO3. The THz electric field strength can be increased further to tens of MV/cm by focusing. Using optimal conditions together with the contact grating technique THz pulses with 100 MV/cm field strength and energies on the tens-of-mJ scale are feasible., Comment: 7 pages, 4 figures