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Klaas Wynne

University of Glasgow, Department of Chemistry, Faculty Member

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The main research goal of KW's group is the study of femtosecond chemical-reaction dynamics in the condensed phase and in particular the role of low frequency (terahertz) collective modes.
Address: Joseph Black Building
University Avenue
Glasgow, G12 8QQ
United Kingdom

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Xanthipe Jordanides

National Renewable Energy Laboratory

Krzysztof Kempa

Andreas Tortschanoff

CTR AG

Universidade Estadual de Campinas

Carmelo Corsaro

University of Groningen

AMOLF

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Papers by Klaas Wynne

Ultrafast Dipole Solvation Measured in the Far Infrared

Ultrafast visible and near-UV pump, far-IR (terahertz) probe spectroscopy on the solvation dynami... more Ultrafast visible and near-UV pump, far-IR (terahertz) probe spectroscopy on the solvation dynamics of two dyes in polar solution is reported. In both cases, the pump pulse produces an excited state in the dye with a permanent dipole moment that is significantly different from that in the ground state. This dipole moment change induces changes in the solvent librational spectrum. Coherent beats in the data indicate that the solvent response is partially diffusive and partially impulsive demonstrating the inhomogeneity of the response. [S0031-9007(97)04279-8] 33.20.Ea, 78.47. + p Low frequency translational and librational motions in liquids are crucial in determining the rates of condensed phase chemical and biological reactions . In principle, one needs all the positions and velocities of all the solvent molecules in the system in order to describe a condensed phase reaction. Although molecular dynamics calculations go a long way towards this end, this is often considered an impracticable task. Therefore, it is common to describe the effect of the solvent with approximate methods. It can be assumed that the solvent fluctuations will give rise to fluctuations in the energy levels of a solute with a Gaussian distribution by invoking the central limit theorem. This distribution of energies automatically leads to the solvation coordinate: The motion of a Brownian particle in a harmonic well also leads to such a distribution and therefore it is sensible to reduce the entire complexity of the solvent to a single harmonic mode described by a Langevin equation. It is frequently assumed that the spectrum of solvent fluctuations, the solvent spectral density (SSD), is white or, in other words, that the fluctuations are delta correlated. Simulations suggest that the SSD is actually bell shaped with a peak near zero frequency.

Superluminal terahertz pulses

In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dimensions of the order o... more In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dimensions of the order of the wavelength, it is observed that the T rays propagate faster than the vacuum speed of light. In the case of apertures this can be understood as a waveguide effect in which superluminal velocities are expected close to the cutoff frequency. However, the effect is also observed close to knife edges and in propagation past thin metal wires. 

Femtosecond Infrared Spectroscopy of Low-Lying Excited States in Reaction Centers of Rhodobacter sphaeroides

The mid-to near-IR difference spectra of optically pumped and unpumped reaction centers were reco... more The mid-to near-IR difference spectra of optically pumped and unpumped reaction centers were recorded over a time range circa 0.1-20 ps allowing us to isolate kinetically the transient electronic spectra of the excited special pair P* and its charge-separated form P + H-. The spectrum of P* between 1 and 5.2 µm exhibits two main bands, one at 5300 cm-1 and another at 2710 cm-1 , while P + H-exhibits two bands, one at 2600 cm-1 and another at 8000 cm-1. These new states do not appear as transitions of unexcited reaction centers at E(P*) + 5300 cm-1 or E(P*) + 2710 cm-1 (although the former is close to the Q X transition region of the special pair), suggesting that they derive almost exclusively from charge transfer (P L + P M-or P L-P M +) states. This conclusion is strengthened by the anisotropies obtained by pumping P* and probing these transitions with polarized light. The transition of P + Hat 2600 cm-1 corresponds to the known hole transfer transition, and this assignment is confirmed by kinetic and anisotropy measurements: the transition dipole is along the direction of charge transfer between P L and P M. The anisotropy for the transition at 8000 cm-1 is consistent with a transition dipole along the y-axis of P M , in agreement with theoretical predictions for a trip-doublet transition.

Femtosecond far-infrared pump-probe spectroscopy: a new tool for studying low-frequency vibrational dynamics in molecular condensed phases

Low-frequency (terahertz) motions that play an important part in reactive processes in condensed ... more Low-frequency (terahertz) motions that play an important part in reactive processes in condensed phases are directly probed, following an optical excitation. A voltage-biased semiconductor wafer, excited with a 70 femtosecond laser pulse, is employed as a source for intense terahertz pulses, which are used as the probe arm of a pump-probe experiment. Difficulties in using terahertz probe pulses are discussed. We then demonstrate the method with two examples. In the first, a terahertz signal is shown to follow the solvent response to charge transfer in betaine. In the second example, relaxation of sapphire lattice phonons, excited impulsively by a 405 nm pulse, is observed.

Excited state dynamics of bacteriorhodopsin revealed by transient stimulated emission spectra

The femtosecond photoisomerization of bacteriorhodopsin (bR), pumped at either 550 nm or 405 nm, ... more The femtosecond photoisomerization of bacteriorhodopsin (bR), pumped at either 550 nm or 405 nm, was probed between 640 nm and 950 nm. The 550 nm pump spectra exhibit net gain signals peaking at > 950 nm. A transient absorption is proposed to partially cancel tbe emission. The 405 nm pump transients exhibit nonexponential kinetics and display this new absorption, due to the presence of hot molecules. The absence of time dependence of the shape of the stimulated emission spectra is inconsistent with conventional models invoking low-frequency motion out of the Franck-Condon region following excitation. The time independence is reminiscent of the photoisomerization of cis-stilbene.

200 ns pulse high-voltage supply for terahertz field emission

by Klaas Wynne and Gregor H Welsh

We present a method of generating 200 ns high-voltage up to 40 kV pulses operating at repetition ... more We present a method of generating 200 ns high-voltage up to 40 kV pulses operating at repetition rates of up to 100 kHz, which may be synchronized with laser pulses. These supplies are simple to make and were developed for ultrafast terahertz pulse generation from GaAs photoconductive antennas using a high-repetition-rate regeneratively amplified laser. We also show an improvement in signal-to-noise ratio over a continuous dc bias field and application of the supply to terahertz pulse generation.

Alternating high-voltage biasing for terahertz large-area photoconductive emitters

High-voltage biasing is necessary for efficient generation of terahertz radiation using large-are... more High-voltage biasing is necessary for efficient generation of terahertz radiation using large-area photoconductive emitters and for electric-field-oriented charge-transfer studies. Coherent detection of terahertz pulses allows ac biasing to be the basis of modulation for lock-in detection. Biasing emitters with an ac field also removes the need for a complete conduction path. The experimental advantages of this approach along with a simple resonant method of generating the high-voltage bias applicable to higher-repetition-rate up to a few hundred kilohertz regeneratively amplified systems are described.

Terahertz-Pulse Emission Through Laser Excitation of Surface Plasmons in a Metal Grating

The second-order processes of optical-rectification and photoconduction are well known and widely... more The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.

Noncausal Time Response in Frustrated Total Internal Reflection

Tunneling of photons in frustrated total internal reflection has been studied in the time domain ... more Tunneling of photons in frustrated total internal reflection has been studied in the time domain with single-cycle femtosecond pulses. It is seen that both the phase and energy of the pulse travel faster than the speed of light in vacuum. Theoretical analysis of the experiments shows that the time-response function for electromagnetic waves propagating in the air gap is noncausal. However, it is found that superluminal signal propagation is not possible in this case because of the inevitable diffractive spreading of the signal beam.

Optical Kerr-effect study of trans-and cis-1,2-dichloroethene: liquid–liquid transition or super-Arrhenius relaxation

The evidence that a molecular liquid in its thermodynamically-stable state can undergo a liquid–l... more The evidence that a molecular liquid in its thermodynamically-stable state can undergo a liquid–liquid transition (LLT) is still uncertain. Therefore, trans-1,2-dichloroethene is of interest due to reports of a LLT above the melting point [S. Kawanishi, T. Sasuga and M. Takehisa,. Ultrafast optical Kerr-effect (OKE) spectroscopy enables accurate measurement of the low-frequency modes arising from interactions in liquids and therefore should be sensitive to the change in liquid structure inherent in such a transition. In the OKE data presented here, no sharp transitions are discernible, nor are there any in calorimetry data. However, the same data do reveal that neither trans-nor cis-1,2-dichloroethene is a simple liquid: in each case, a non-Arrhenius temperature dependence (with a Debye lineshape) is observed for the a relaxation. This dependence can be fitted by the Vogel–Fulcher–Tammann (VFT) law over the measurable temperature range suggesting that at low temperature, cooperative relaxation, due to the formation of clusters or structure, is present. Accurate analysis of the OKE spectrum in the terahertz region is generally limited by approximations inherent in the models. Here the diffusional modes are convoluted with librational modes to give a more physically meaningful approximation to the inertial response.

Generation of ultrafast terahertz radiation pulses on metallic nanostructured surfaces

A resonant " incoherent " rectification process is presented relying on the excitation of surface... more A resonant " incoherent " rectification process is presented relying on the excitation of surface plasmons on a nanostructured metal surface. Excitation of gold and silver films with 800-nm femtosecond laser pulses results in the emission of terahertz radiation with an angle-dependent efficiency and an approximately third-order power dependence. It is shown that the source of this terahertz pulse generation is the surface-plasmon-assisted multiphoton ionization and ponderomotive acceleration in the evanescent field of the surface plasmon. Simple models are used to understand the forces and dynamics near the surface.

An integrated description of terahertz generation through optical rectification, charge transfer, and current surge

Ultrafast terahertz pulses can be generated through a number of processes such as optical rectifi... more Ultrafast terahertz pulses can be generated through a number of processes such as optical rectification in an electro-optic crystal, a current surge in a biased photoconductive antenna, or charge transfer in aligned molecules. It will be shown that all these rectification processes can be described by a single analytical formula in the frequency domain. A recipe is given for the quick calculation of terahertz waveforms generated through any rectification process and detected through electro-optic sampling. This theory is compared with experimentally measured terahertz waveforms.

Causality and the nature of information

Superluminal propagation of electromagnetic radiation and photon tunnelling have been studied by ... more Superluminal propagation of electromagnetic radiation and photon tunnelling have been studied by a variety of direct and indirect techniques. Especially the use of femtosecond terahertz pulses, whose electric field can be measured directly, has resulted in the (re)discovery of a number of effects in which light propagates over small distances faster than the speed of light in vacuum. Naturally, this brings up the question whether information can be exchanged su-perluminally. It has been shown in nearly all cases studied that the principle of causality applies to the underlying physical processes. It has been argued, however, that the principle of causality might have no bearing on the question of superluminal information transfer. It will be shown here that all the confusion stems from a vague definition of the concept of information and from ignoring noise. Once the concept of information (and noise) has been defined properly, it can be shown that if the principle of causality applies then useful superluminal information exchange is strictly prohibited.

Tunneling of single-cycle terahertz pulses through waveguides

Propagation of single-cycle terahertz pulses through and past wavelength-sized metal structures h... more Propagation of single-cycle terahertz pulses through and past wavelength-sized metal structures has been studied experimentally. In waveguides close to cutoff, it is found that the phase velocity can become superluminal and even negative. Multiple reflections of evanescent waves inside the waveguide are found to be the cause of a negative phase velocity below the cutoff frequency. The centroid delay of terahertz pulses propagating past a thin metal wire is found to be advanced or delayed depending on the polarization with respect to the wire. In all cases of superluminal propagation described here, the principle of causality is preserved. In a restricted sense, exchange of information faster than the speed of light is found possible, however, the principle of causality ensures that information cannot advance by more than the inverse bandwidth of the signal. This eliminates causal-loop paradoxes and ensures that faster-than-light communication is not practical. q

Evanescent-wave acceleration of femtosecond electron bunches

A 150-fs 800-nm 1-J laser was used to excited surface plasmons in the Kretschmann geometry in a 5... more A 150-fs 800-nm 1-J laser was used to excited surface plasmons in the Kretschmann geometry in a 500-A s silver "lm. Multiphoton excitation results in the emission of femtosecond electron bunches (40 fC) as had been seen before. The electron beam is highly directional and perpendicular to the prism surface. A time-of-#ight setup has been used to measure the kinetic-energy distribution of the photoelectrons. Surprisingly, we "nd that this distribution extends to energies as high as 40 eV. Theoretical calculations show that these high energies may be due to acceleration in the evanescent laser "eld that extends from the silver "lm out into the vacuum. These results suggest that femtosecond pulses with more energy per pulse or longer wavelength may be used to accelerate electrons to the keV or even MeV level.

The Ultrafast Dynamics of Hydrogen-Bonded Liquids: Molecular Structure-Dependent Occurrence of Normal Arrhenius or Fractional Stokes-Einstein-Debye Rotational Diffusive Relaxation

The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetam... more The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetamide (NMA), acetamide (Ac), and N,N-dimethylacetamide (DMA) have been studied as a function of temperature using optically heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Both NMA and Ac exhibit a non-Arrhenius temperature dependence of the rotational diffusive relaxation time. By contrast, the non-hydrogen-bonding DMA exhibits normal hydrodynamic behavior. The unusual dynamics of NMA and Ac are attributed to the decoupling of single-molecule rotational diffusive relaxation from the shear viscosity via a transition between stick and slip boundary conditions, which arises from local heterogeneity in the liquid due to the formation of hydrogen-bonded chains or clusters. This provides new insight into the structure and dynamics of an important peptide model compound and the first instance of such a phenomenon in a room-temperature liquid. The OHD-OKE responses of carboxylic acids acetic acid (AcOH) and dichloroacetic acid (DCA) are also reported. These, along with the terahertz Raman spectra, show no evidence of the effects observed in amide systems, but display trends consistent with the presence of an equilibrium between the linear and cyclic dimer structures at all temperatures and moderate-to-high mole fractions in aqueous solution. This equilibrium manifests itself as hydrodynamic behavior in the liquid phase.

Inter-and Intramolecular Hydrogen Bonding in Phenol Derivatives: A Model System for Poly-L-tyrosine

by Klaas Wynne and Andrew Turner

The ultrafast dynamics of solutions of phenol and two phenol derivativesshydroquinone (1,4-benzen... more The ultrafast dynamics of solutions of phenol and two phenol derivativesshydroquinone (1,4-benzenediol) and pyrocatechol (1,2-benzenediol)shave been studied with Optically Heterodyne-Detected Optical Kerr-Effect (OHD-OKE) spectroscopy. The solvents, methanol and acetonitrile, were selected to provide strong and weak solvent-solute hydrogen-bonding interactions, respectively, while pyrocatechol features an intramolecular hydrogen bond. Together these provide a series of model systems for polypeptides such as polytyrosine, which facilitate the direct study of inter-and intramolecular hydrogen bonding. A broad contribution to the Raman spectral density of the methanol solutions at frequencies between 150 and 300 cm-1 has been observed that is absent in acetonitrile. This contribution has been assigned to solvent-solute hydrogen-bond stretching vibrations. The OHD-OKE response of poly-L-tyrosine has been measured and was found to contain a similar contribution. Density functional theory geometry optimizations and normal mode calculations have been performed using the B3LYP hybrid functional and 6-311++G** basis set. These have yielded a complete assignment of the low-frequency Raman and far-infrared spectra of pyrocatechol for the first time, which has provided information on the nature of the intramolecular hydrogen bond of pyrocatechol.

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

The water molecule has the convenient property that its molecular polarizability tensor is nearly... more The water molecule has the convenient property that its molecular polarizability tensor is nearly isotropic
while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liquid water
is mostly collision-induced and therefore reports primarily translational motions while the far-infrared
(terahertz) and dielectric spectrum is dominated by rotational modes. Atomic and globular–molecular
liquids have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying
properties were exploited in a study of a number of liquids and solutions using ultrafast optical Kerr-effect
(OKE) spectroscopy combined with dielectric relaxation spectroscopy (DRS), terahertz time-domain
spectroscopy (THz-TDS), and terahertz field-induced second-harmonic generation (TFISH) spectroscopy.
For room-temperature ionic liquids (RTILs), liquid water, aqueous salt solutions, noble gas liquids, and
globular–molecular liquids it was found that, in each case, surprising structure and/or inhomogeneity is
observed, ranging from mesoscopic clustering in RTILs to stretched-exponential dynamics in the noble gas
liquids. For aqueous electrolyte solutions it is shown that the viscosity, normally described by the Jones–Dole
expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed
matter studies of glass transitions in colloidal suspensions.

Universal nonexponential relaxation: Complex dynamics in simple liquids

The dynamics of the noble-gas liquids underlies that of all liquids making them an important prot... more The dynamics of the noble-gas liquids underlies that of all liquids making them an important prototypical model system. Using optical Kerr-effect spectroscopy we show that for argon, krypton, and xenon, both the librational and diffusional contributions to the spectrum are surprisingly complex. The diffusional relaxation appears as a stretched-exponential, such as widely found in studies of structured e.g., glass-forming liquids and as predicted by mode-coupling theory. We show that this behavior is remarkably similar to that measured in water and suggest that it is a fundamental or universal property.

Glasslike behavior in aqueous electrolyte solutions

When salts are added to water, generally the viscosity increases, suggesting that the ions increa... more When salts are added to water, generally the viscosity increases, suggesting that the ions increase the strength of the water's hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules, implying no enhancement or breakdown of the hydrogen-bond network. Here, we report optical Kerr effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.

Ultrafast Dipole Solvation Measured in the Far Infrared

Ultrafast visible and near-UV pump, far-IR (terahertz) probe spectroscopy on the solvation dynami... more Ultrafast visible and near-UV pump, far-IR (terahertz) probe spectroscopy on the solvation dynamics of two dyes in polar solution is reported. In both cases, the pump pulse produces an excited state in the dye with a permanent dipole moment that is significantly different from that in the ground state. This dipole moment change induces changes in the solvent librational spectrum. Coherent beats in the data indicate that the solvent response is partially diffusive and partially impulsive demonstrating the inhomogeneity of the response. [S0031-9007(97)04279-8] 33.20.Ea, 78.47. + p Low frequency translational and librational motions in liquids are crucial in determining the rates of condensed phase chemical and biological reactions . In principle, one needs all the positions and velocities of all the solvent molecules in the system in order to describe a condensed phase reaction. Although molecular dynamics calculations go a long way towards this end, this is often considered an impracticable task. Therefore, it is common to describe the effect of the solvent with approximate methods. It can be assumed that the solvent fluctuations will give rise to fluctuations in the energy levels of a solute with a Gaussian distribution by invoking the central limit theorem. This distribution of energies automatically leads to the solvation coordinate: The motion of a Brownian particle in a harmonic well also leads to such a distribution and therefore it is sensible to reduce the entire complexity of the solvent to a single harmonic mode described by a Langevin equation. It is frequently assumed that the spectrum of solvent fluctuations, the solvent spectral density (SSD), is white or, in other words, that the fluctuations are delta correlated. Simulations suggest that the SSD is actually bell shaped with a peak near zero frequency.

Superluminal terahertz pulses

In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dimensions of the order o... more In femtosecond terahertz-pulse (T-ray) imaging of metal structures with dimensions of the order of the wavelength, it is observed that the T rays propagate faster than the vacuum speed of light. In the case of apertures this can be understood as a waveguide effect in which superluminal velocities are expected close to the cutoff frequency. However, the effect is also observed close to knife edges and in propagation past thin metal wires. 

Femtosecond Infrared Spectroscopy of Low-Lying Excited States in Reaction Centers of Rhodobacter sphaeroides

The mid-to near-IR difference spectra of optically pumped and unpumped reaction centers were reco... more The mid-to near-IR difference spectra of optically pumped and unpumped reaction centers were recorded over a time range circa 0.1-20 ps allowing us to isolate kinetically the transient electronic spectra of the excited special pair P* and its charge-separated form P + H-. The spectrum of P* between 1 and 5.2 µm exhibits two main bands, one at 5300 cm-1 and another at 2710 cm-1 , while P + H-exhibits two bands, one at 2600 cm-1 and another at 8000 cm-1. These new states do not appear as transitions of unexcited reaction centers at E(P*) + 5300 cm-1 or E(P*) + 2710 cm-1 (although the former is close to the Q X transition region of the special pair), suggesting that they derive almost exclusively from charge transfer (P L + P M-or P L-P M +) states. This conclusion is strengthened by the anisotropies obtained by pumping P* and probing these transitions with polarized light. The transition of P + Hat 2600 cm-1 corresponds to the known hole transfer transition, and this assignment is confirmed by kinetic and anisotropy measurements: the transition dipole is along the direction of charge transfer between P L and P M. The anisotropy for the transition at 8000 cm-1 is consistent with a transition dipole along the y-axis of P M , in agreement with theoretical predictions for a trip-doublet transition.

Femtosecond far-infrared pump-probe spectroscopy: a new tool for studying low-frequency vibrational dynamics in molecular condensed phases

Low-frequency (terahertz) motions that play an important part in reactive processes in condensed ... more Low-frequency (terahertz) motions that play an important part in reactive processes in condensed phases are directly probed, following an optical excitation. A voltage-biased semiconductor wafer, excited with a 70 femtosecond laser pulse, is employed as a source for intense terahertz pulses, which are used as the probe arm of a pump-probe experiment. Difficulties in using terahertz probe pulses are discussed. We then demonstrate the method with two examples. In the first, a terahertz signal is shown to follow the solvent response to charge transfer in betaine. In the second example, relaxation of sapphire lattice phonons, excited impulsively by a 405 nm pulse, is observed.

Excited state dynamics of bacteriorhodopsin revealed by transient stimulated emission spectra

The femtosecond photoisomerization of bacteriorhodopsin (bR), pumped at either 550 nm or 405 nm, ... more The femtosecond photoisomerization of bacteriorhodopsin (bR), pumped at either 550 nm or 405 nm, was probed between 640 nm and 950 nm. The 550 nm pump spectra exhibit net gain signals peaking at > 950 nm. A transient absorption is proposed to partially cancel tbe emission. The 405 nm pump transients exhibit nonexponential kinetics and display this new absorption, due to the presence of hot molecules. The absence of time dependence of the shape of the stimulated emission spectra is inconsistent with conventional models invoking low-frequency motion out of the Franck-Condon region following excitation. The time independence is reminiscent of the photoisomerization of cis-stilbene.

200 ns pulse high-voltage supply for terahertz field emission

by Klaas Wynne and Gregor H Welsh

We present a method of generating 200 ns high-voltage up to 40 kV pulses operating at repetition ... more We present a method of generating 200 ns high-voltage up to 40 kV pulses operating at repetition rates of up to 100 kHz, which may be synchronized with laser pulses. These supplies are simple to make and were developed for ultrafast terahertz pulse generation from GaAs photoconductive antennas using a high-repetition-rate regeneratively amplified laser. We also show an improvement in signal-to-noise ratio over a continuous dc bias field and application of the supply to terahertz pulse generation.

Alternating high-voltage biasing for terahertz large-area photoconductive emitters

High-voltage biasing is necessary for efficient generation of terahertz radiation using large-are... more High-voltage biasing is necessary for efficient generation of terahertz radiation using large-area photoconductive emitters and for electric-field-oriented charge-transfer studies. Coherent detection of terahertz pulses allows ac biasing to be the basis of modulation for lock-in detection. Biasing emitters with an ac field also removes the need for a complete conduction path. The experimental advantages of this approach along with a simple resonant method of generating the high-voltage bias applicable to higher-repetition-rate up to a few hundred kilohertz regeneratively amplified systems are described.

Terahertz-Pulse Emission Through Laser Excitation of Surface Plasmons in a Metal Grating

The second-order processes of optical-rectification and photoconduction are well known and widely... more The second-order processes of optical-rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification that relies on the excitation of surface plasmons in metal films deposited on a shallow grating. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the surface plasmons results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher-order in the incident field.

Noncausal Time Response in Frustrated Total Internal Reflection

Tunneling of photons in frustrated total internal reflection has been studied in the time domain ... more Tunneling of photons in frustrated total internal reflection has been studied in the time domain with single-cycle femtosecond pulses. It is seen that both the phase and energy of the pulse travel faster than the speed of light in vacuum. Theoretical analysis of the experiments shows that the time-response function for electromagnetic waves propagating in the air gap is noncausal. However, it is found that superluminal signal propagation is not possible in this case because of the inevitable diffractive spreading of the signal beam.

Optical Kerr-effect study of trans-and cis-1,2-dichloroethene: liquid–liquid transition or super-Arrhenius relaxation

The evidence that a molecular liquid in its thermodynamically-stable state can undergo a liquid–l... more The evidence that a molecular liquid in its thermodynamically-stable state can undergo a liquid–liquid transition (LLT) is still uncertain. Therefore, trans-1,2-dichloroethene is of interest due to reports of a LLT above the melting point [S. Kawanishi, T. Sasuga and M. Takehisa,. Ultrafast optical Kerr-effect (OKE) spectroscopy enables accurate measurement of the low-frequency modes arising from interactions in liquids and therefore should be sensitive to the change in liquid structure inherent in such a transition. In the OKE data presented here, no sharp transitions are discernible, nor are there any in calorimetry data. However, the same data do reveal that neither trans-nor cis-1,2-dichloroethene is a simple liquid: in each case, a non-Arrhenius temperature dependence (with a Debye lineshape) is observed for the a relaxation. This dependence can be fitted by the Vogel–Fulcher–Tammann (VFT) law over the measurable temperature range suggesting that at low temperature, cooperative relaxation, due to the formation of clusters or structure, is present. Accurate analysis of the OKE spectrum in the terahertz region is generally limited by approximations inherent in the models. Here the diffusional modes are convoluted with librational modes to give a more physically meaningful approximation to the inertial response.

Generation of ultrafast terahertz radiation pulses on metallic nanostructured surfaces

A resonant " incoherent " rectification process is presented relying on the excitation of surface... more A resonant " incoherent " rectification process is presented relying on the excitation of surface plasmons on a nanostructured metal surface. Excitation of gold and silver films with 800-nm femtosecond laser pulses results in the emission of terahertz radiation with an angle-dependent efficiency and an approximately third-order power dependence. It is shown that the source of this terahertz pulse generation is the surface-plasmon-assisted multiphoton ionization and ponderomotive acceleration in the evanescent field of the surface plasmon. Simple models are used to understand the forces and dynamics near the surface.

An integrated description of terahertz generation through optical rectification, charge transfer, and current surge

Ultrafast terahertz pulses can be generated through a number of processes such as optical rectifi... more Ultrafast terahertz pulses can be generated through a number of processes such as optical rectification in an electro-optic crystal, a current surge in a biased photoconductive antenna, or charge transfer in aligned molecules. It will be shown that all these rectification processes can be described by a single analytical formula in the frequency domain. A recipe is given for the quick calculation of terahertz waveforms generated through any rectification process and detected through electro-optic sampling. This theory is compared with experimentally measured terahertz waveforms.

Causality and the nature of information

Superluminal propagation of electromagnetic radiation and photon tunnelling have been studied by ... more Superluminal propagation of electromagnetic radiation and photon tunnelling have been studied by a variety of direct and indirect techniques. Especially the use of femtosecond terahertz pulses, whose electric field can be measured directly, has resulted in the (re)discovery of a number of effects in which light propagates over small distances faster than the speed of light in vacuum. Naturally, this brings up the question whether information can be exchanged su-perluminally. It has been shown in nearly all cases studied that the principle of causality applies to the underlying physical processes. It has been argued, however, that the principle of causality might have no bearing on the question of superluminal information transfer. It will be shown here that all the confusion stems from a vague definition of the concept of information and from ignoring noise. Once the concept of information (and noise) has been defined properly, it can be shown that if the principle of causality applies then useful superluminal information exchange is strictly prohibited.

Tunneling of single-cycle terahertz pulses through waveguides

Propagation of single-cycle terahertz pulses through and past wavelength-sized metal structures h... more Propagation of single-cycle terahertz pulses through and past wavelength-sized metal structures has been studied experimentally. In waveguides close to cutoff, it is found that the phase velocity can become superluminal and even negative. Multiple reflections of evanescent waves inside the waveguide are found to be the cause of a negative phase velocity below the cutoff frequency. The centroid delay of terahertz pulses propagating past a thin metal wire is found to be advanced or delayed depending on the polarization with respect to the wire. In all cases of superluminal propagation described here, the principle of causality is preserved. In a restricted sense, exchange of information faster than the speed of light is found possible, however, the principle of causality ensures that information cannot advance by more than the inverse bandwidth of the signal. This eliminates causal-loop paradoxes and ensures that faster-than-light communication is not practical. q

Evanescent-wave acceleration of femtosecond electron bunches

A 150-fs 800-nm 1-J laser was used to excited surface plasmons in the Kretschmann geometry in a 5... more A 150-fs 800-nm 1-J laser was used to excited surface plasmons in the Kretschmann geometry in a 500-A s silver "lm. Multiphoton excitation results in the emission of femtosecond electron bunches (40 fC) as had been seen before. The electron beam is highly directional and perpendicular to the prism surface. A time-of-#ight setup has been used to measure the kinetic-energy distribution of the photoelectrons. Surprisingly, we "nd that this distribution extends to energies as high as 40 eV. Theoretical calculations show that these high energies may be due to acceleration in the evanescent laser "eld that extends from the silver "lm out into the vacuum. These results suggest that femtosecond pulses with more energy per pulse or longer wavelength may be used to accelerate electrons to the keV or even MeV level.

The Ultrafast Dynamics of Hydrogen-Bonded Liquids: Molecular Structure-Dependent Occurrence of Normal Arrhenius or Fractional Stokes-Einstein-Debye Rotational Diffusive Relaxation

The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetam... more The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetamide (NMA), acetamide (Ac), and N,N-dimethylacetamide (DMA) have been studied as a function of temperature using optically heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Both NMA and Ac exhibit a non-Arrhenius temperature dependence of the rotational diffusive relaxation time. By contrast, the non-hydrogen-bonding DMA exhibits normal hydrodynamic behavior. The unusual dynamics of NMA and Ac are attributed to the decoupling of single-molecule rotational diffusive relaxation from the shear viscosity via a transition between stick and slip boundary conditions, which arises from local heterogeneity in the liquid due to the formation of hydrogen-bonded chains or clusters. This provides new insight into the structure and dynamics of an important peptide model compound and the first instance of such a phenomenon in a room-temperature liquid. The OHD-OKE responses of carboxylic acids acetic acid (AcOH) and dichloroacetic acid (DCA) are also reported. These, along with the terahertz Raman spectra, show no evidence of the effects observed in amide systems, but display trends consistent with the presence of an equilibrium between the linear and cyclic dimer structures at all temperatures and moderate-to-high mole fractions in aqueous solution. This equilibrium manifests itself as hydrodynamic behavior in the liquid phase.

Inter-and Intramolecular Hydrogen Bonding in Phenol Derivatives: A Model System for Poly-L-tyrosine

by Klaas Wynne and Andrew Turner

The ultrafast dynamics of solutions of phenol and two phenol derivativesshydroquinone (1,4-benzen... more The ultrafast dynamics of solutions of phenol and two phenol derivativesshydroquinone (1,4-benzenediol) and pyrocatechol (1,2-benzenediol)shave been studied with Optically Heterodyne-Detected Optical Kerr-Effect (OHD-OKE) spectroscopy. The solvents, methanol and acetonitrile, were selected to provide strong and weak solvent-solute hydrogen-bonding interactions, respectively, while pyrocatechol features an intramolecular hydrogen bond. Together these provide a series of model systems for polypeptides such as polytyrosine, which facilitate the direct study of inter-and intramolecular hydrogen bonding. A broad contribution to the Raman spectral density of the methanol solutions at frequencies between 150 and 300 cm-1 has been observed that is absent in acetonitrile. This contribution has been assigned to solvent-solute hydrogen-bond stretching vibrations. The OHD-OKE response of poly-L-tyrosine has been measured and was found to contain a similar contribution. Density functional theory geometry optimizations and normal mode calculations have been performed using the B3LYP hybrid functional and 6-311++G** basis set. These have yielded a complete assignment of the low-frequency Raman and far-infrared spectra of pyrocatechol for the first time, which has provided information on the nature of the intramolecular hydrogen bond of pyrocatechol.

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

The water molecule has the convenient property that its molecular polarizability tensor is nearly... more The water molecule has the convenient property that its molecular polarizability tensor is nearly isotropic
while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liquid water
is mostly collision-induced and therefore reports primarily translational motions while the far-infrared
(terahertz) and dielectric spectrum is dominated by rotational modes. Atomic and globular–molecular
liquids have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying
properties were exploited in a study of a number of liquids and solutions using ultrafast optical Kerr-effect
(OKE) spectroscopy combined with dielectric relaxation spectroscopy (DRS), terahertz time-domain
spectroscopy (THz-TDS), and terahertz field-induced second-harmonic generation (TFISH) spectroscopy.
For room-temperature ionic liquids (RTILs), liquid water, aqueous salt solutions, noble gas liquids, and
globular–molecular liquids it was found that, in each case, surprising structure and/or inhomogeneity is
observed, ranging from mesoscopic clustering in RTILs to stretched-exponential dynamics in the noble gas
liquids. For aqueous electrolyte solutions it is shown that the viscosity, normally described by the Jones–Dole
expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed
matter studies of glass transitions in colloidal suspensions.

Universal nonexponential relaxation: Complex dynamics in simple liquids

The dynamics of the noble-gas liquids underlies that of all liquids making them an important prot... more The dynamics of the noble-gas liquids underlies that of all liquids making them an important prototypical model system. Using optical Kerr-effect spectroscopy we show that for argon, krypton, and xenon, both the librational and diffusional contributions to the spectrum are surprisingly complex. The diffusional relaxation appears as a stretched-exponential, such as widely found in studies of structured e.g., glass-forming liquids and as predicted by mode-coupling theory. We show that this behavior is remarkably similar to that measured in water and suggest that it is a fundamental or universal property.

Glasslike behavior in aqueous electrolyte solutions

When salts are added to water, generally the viscosity increases, suggesting that the ions increa... more When salts are added to water, generally the viscosity increases, suggesting that the ions increase the strength of the water's hydrogen-bond network. However, infrared pump-probe measurements on electrolyte solutions have found that ions have no influence on the rotational dynamics of water molecules, implying no enhancement or breakdown of the hydrogen-bond network. Here, we report optical Kerr effect and dielectric relaxation spectroscopic measurements, which have enabled us to separate the effects of rotational and transitional motions of the water molecules. These data show that electrolyte solutions behave like a supercooled liquid approaching a glass transition in which rotational and translational molecular motions are decoupled. It is now possible to understand previously conflicting viscosity data, nuclear magnetic resonance relaxation, and ultrafast infrared spectroscopy in a single unified picture.