Condensed matter physics aims to understand the behaviors of solid and liquid materials. Practitioners employ a variety of theoretical tools, from quantum mechanics to statistical mechanics, and a variety of experimental tools, from scattering to spectroscopy. Over the years Yodh has explored many problems in condensed matter physics. These problems have ranged from measurement and characterization of entropy-driven forces and self-assembly to nonlinear optical spectroscopy of electrons residing at the interface between two solids. Some of this research is described in this Web site.
Topics:
Soft Materials & Complex
Fluids
Premelting in Bulk Crystals
Melting of Layered Phases of Rods in Temperature Sensitive Polymer
Capillary Interactions Between Anisotropic Colloidal Particles
Biopolymers in Anistropic (Nematic) Fluids
Mechanical Responses of Soft Materials:
Macro- and Micro-rheology
Entropy-driven Colloidal
Interactions and Self-Assembly (Background)
Colloidal Interactions
Self-Assembly of Colloidal
Particles (Entropy-driven & Beyond)
Carbon Nanotubes
Solubilization of Single-Wall Carbon
Nanotubes in Aqueous Suspensions
Nematic Nanotube Gels
Optical Properties and Anistropy of Single-wall Carbon
Nanotubes
Structure and Rheology of Single-wall Carbon Nanotube
Suspensions
Single wall carbon nanotube epoxy composites
Hard Surfaces & Interfaces
Nonlinear Optical Spectroscopy of Interfaces, Surfaces and Particles
Ultrafast Spectroscopy of Surfaces and Surface Adsorbates
Soft Materials & Complex Fluids
Soft
condensed matter physics is concerned with materials such
as colloidal suspensions, emulsions, polymer solutions,
gels, membranes, liquid crystals and mixtures thereof,
whose structure and dynamics are strongly influenced
by entropy and by relatively weak mechanical forces.
They provide a fascinating testing ground for many-body
statistical physics, and for the investigation of entropy,
defects, phase transitions and other instabilities. Research
on these substances is technology driven too. Soft materials
are attractive starting media in photonics and lithography,
in high-tech ceramics and in biochemical sensing. Particle
additives also offer practical control of fluid rheologies,
improving the performances of materials ranging from
conventional paints and pastes to motor oils to food
and cosmetics. Finally, complex fluids experiments are
increasingly stimulated by analogies from cell biology,
in some cases providing critical insights about mechanisms
that arise in crowded cellular environments.
The Yodh lab has worked on a variety of scientific problems
in this field.
Selected recent research is described below.
Premelting in Bulk Crystals
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Slice through a three-dimensional colloidal crystal shows positions of 'colloidal atoms' and the onset of premelting at the grain boundary between two crystallites. |
The importance of melting in nature can hardly be overestimated, and yet a detailed understanding of the mechanisms that drive this transformation is still evolving. Scientists have speculated for more than a century about how crystalline solids melt, in the process generating microscopic models emphasizing the role of lattice vibrations, dislocations, grain boundaries, surfaces, dimensionality, and combinations thereof. In contrast to the continuous transitions that arise in ferromagnetism and liquid-vapor systems, a first principle theory of the solid-liquid transition is difficult because of long-range many-body effects, symmetry, and a lack of universality. Furthermore, experimental investigations to test underlying theoretical assumptions are extraordinarily difficult, because they must track motions of individual atoms or defects within crystals. Nevertheless, recent experiments and theory have shown convincingly that atomic crystal surfaces, at equilibrium below the bulk melting point, often form melted layers. This premelting lowers the energy barrier for liquid nucleation and effectively prevents superheating of the solid.
Theories have suggested a similar premelting occur at defects such as grain boundaries, stacking faults, and dislocations located within the bulk crystal, but these effects have not been observed. In recent work we imaged the motions of particles in three-dimensional colloidal crystals during the melting process. The images reveal premelting near grain boundaries and dislocations. Increased disorder is observed in crystalline regions bordering the defects as a function of defect type (e.g. grain boundaries, dislocations, vacancies), distance from the defect, and particle volume fraction.
Coexistance of solid and liquid in the colloidal crystal. (movie file) |
These observations answer longstanding fundamental questions about melting mechanisms, suggest that grain boundary and dislocation premelting is an important effect in the melting process, and introduce new quantitative measures of local disorder. Besides their intrinsic importance for colloid science and technology, all indications suggest interfacial free energy is the crucial parameter for premelting. Thus these results are also relevant for atomic scale materials.
The colloidal crystals used for these studies are equilibrium systems composed of micron-size, nearly-hard-sphere particles. At high volume fraction these particles are driven entropically to condense into close-packed crystalline solids. A key feature of these measurements is our use of microgel particles, whose diameters depend on temperature. Thus by changing sample temperature slightly, we precisely vary the volume fraction of particles in the crystal over a significant range, driving the crystal from close-packing towards its melting point at lower volume fraction.
Melting of Layered Phases of Rods in Temperature Sensitive Polymer Solutions
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Melting of three-dimensional (3D) crystals is among the most ubiquitous phase transitions in nature. In contrast to freezing, melting of 3D crystals usually has no associated energy barrier. Bulk melting is initiated at the crystal surface. We have recently investigated the pathways for melting of lamellar phases. In contrast to crystals which have 3D order, lamellar phases have one-dimensional (1D) quasi-long-range order. Additionally while 3D crystals have only one surface with coexisting gas or liquid, the lamellar phase is a microphase separated state in which the entire bulk is spanned by interfaces between immiscible materials. Lamellar structures are typically built from amphiphilic molecules such as surfactants, lipids and block copolymers, and their phase transformations provide insight about membrane biophysics. Although lamellar phases are frequently studied, melting transitions of lamellar into nematic phases are rarely observed. We have created a novel system that exhibits such melting, and we have shown this process is fundamentally different from melting of 3D crystalline solids.
Lamellar, nematice and isotropic phases as a function of temperature in lyotropic rod-polymer solutions. |
We have developed a temperature sensitive colloidal suspension to carry out these studies. The suspension is composed of lyotropic monodisperse rods of fd-virus, and thermosensitive Poly(N-isopropylacrylamide) (NIPA) polymer. The thermotropic character of the fd/NIPA mixture stems from the temperature dependent solubility of NIPA polymer in water. The resultant mixture forms isotropic, lamellar and nematic phases depending on temperature and constituent concentration. The temperature dependence of our system enables us to study previously inaccessible phase transition kinetics between lamellar phases and coexisting isotropic or nematic phases. We also observe droplets of smectic and nematic phases in an isotropic background, and single membranes of rods in an isotropic background. The sequence of phases observed in these temperature-sensitive rod-polymer suspensions is shown in the figure above. The mechanism for melting of these droplets and membranes can be studied in real time by optical microscopy (click on movies below).
| Movies: |
Melting of a lamellar droplet to a nematic droplet. |
Melting of a single lamellae (one layer) of rods into a nematic droplet. |
Capillary Interactions Between Anisotropic Colloidal Particles
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Top two panels show assembled chain-like structures formed by micron-size ellipsoidal particles at a water-oil interface. The chain morphology depends on the type of particle used. Bottom panel show two ellipsoids attracted to one another by capillary forces. |
The behavior of particle monolayers at fluid interfaces has attracted interest for many years and across many scientific communities. From a practical viewpoint, the stability of thin liquid films and interfaces in the presence of particles affects the control of a variety of materials including emulsions, foams, and coatings. On the basic science side, particle monolayers form beautiful model systems for studies of fundamental issues in condensed matter physics. Nevertheless, questions remain about the mechanism of particle interaction at a fluid interface and about how these interactions lead to formation of ordered and disordered structures. Perhaps the most important interparticle interaction arising at a fluid interface is due to lateral capillary forces. For heavy particles, these capillary interactions stem from the overlap of interface deformations brought about by gravity. Surprisingly, lateral capillary forces also arise for lighter, micron-sized particles. In this case, interfacial deformations can be of electric origin or can arise from irregular wetting at the particle surface.
Most experimental investigations of interparticle interactions and assembly at fluid-fluid interfaces have focused on spherical or nearly spherical particles. We have explored the behavior of anisotropic particles, ellipsoids, at the oil-water interface. Direct measurements of attractive capillary interactions between the ellipsoids are reported (bottom panel of figure). The interaction energies are very large compared to thermal energies and compared to the interaction energies of spherical particles with the same surface chemistry. In addition, the pair interaction potential exhibits a power law behavior, which suggests a description in terms of capillary quadrupoles. These anisotropic capillary forces lead to the assembly of open structures and chains rather than crystals or random aggregates (top two panels of figure).
Biopolymers in Anistropic (Nematic) Fluids
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Biopolymers (fluorescently labeled) in suspensions of rods. The rods are fd-virus. The left column shows biopolymers in isotropic suspensions of rods, and the right column shows these same biopolymers in nematic suspensions of rods. |
Polymer coils in solution exhibit a variety of conformational and dynamical behaviors depending on many factors, including polymer concentration, polymer stiffness, solvent quality, solvent flow, and mechanical stress. We have explored the conformations of polymer coils in anisotropic solutions, in particular nematic solvents composed of aligned rod-like macromolecules. We show by direct visualization that semi-flexible biopolymers dissolved in the nematic phases assume an elongated rod-like configuration aligned with the background nematic director. The coil-rod transition depends on the persistence length (i.e. stiffness) of the biopolymer and the size of the rod-like macromolecules. Using theoretical models that incorporate polymer bending, polymer coupling to the background nematic director fluctuations, and nematic elasticity, we were able to quantitatively explore the fluctuations of these semi-flexible polymers and find they cannot be described by a theory which treats the nematic background as a fixed external field.
Mechanical Responses of Soft Materials: Macro- and Microrheology
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The storage and dissipation of mechanical energy by soft materials is often surprisingly complex as a result of the hierarchical internal structure of these media. Elastic and viscous properties play an important role in the functioning soft material, and their precise origin is of fundamental interest. At PENN we are measuring these mechanical responses with conventional rheometry and with microrheology.
Relate links: Optical Imaging and Micromanipulation, Rheology Center
Microrheology has recently emerged as an important methodology for probing the viscoelastic properties of soft materials, providing access to miniscule sample volumes over a broader range of frequencies than conventional rheometry. In a typical microrheology experiment, the complex shear modulus of a material is derived by tracking the microscopic motions of tracer particles dispersed within the material. One-point microrheology is the traditional form of the microrheological measurement, wherein the mean-square displacements of single isolated particles are tracked. Two-point microrheology is based on measurement of the cross-correlated thermal motion of pairs of embedded tracer particles for determination of the complex shear modulus of the surrounding medium. By cross-correlating the thermal motion of pairs of embedded tracers, we isolate the motions resulting from wavelength undulations of the primary background complex fluid rather than the micro-scale inhomogeneities.
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In recent research we have used microrheology to explore the mechanical responses of thin layers that surround colloidal paricles in non-adsorbing semi-dilute polymer solutions of DNA (figure). Combining one-point (G1) and two-point (G2) microrheology with theory, we are able to assign a rheological size ( D ) to the thin layers surrounding the colloidal particles, by forcing the collapse of one-point data onto a single curve. The rheological layer was about twice as large as the polymer mesh size, indicating that there may exist a slip length between solvent and polymer that exceeds the polymer mesh size.
Current research themes are oriented towards macroscopic rheometry of suspensions of sticky rods (e.g. carbon nanotubes), and mechanical responses and noise spectra in active systems (e.g. bacterial baths and gels).
Entropy-driven Colloidal Interactions and Self-Assembly
(Background)
Entropic interactions are perhaps best understood in the context of suspensions of large- and small-diameter hard-spheres. Hard-sphere colloids lack attractive and long-range interactions, which typically compete with entropic effects to produce ordered phases. Nonetheless, as Asakura and Oosawa first noted in the 1950s, in mixtures of different size spherical particles an ordered arrangement of large spheres can increase the total entropy of the system by increasing the entropy of the small spheres. The basic idea underlying this phenomenon is depicted in Figure below. The box contains a few large spheres and many small spheres. The entropy of a small sphere depends on the number of positions it can occupy in the box, i.e. its free volume. More free volume means more entropy for the small spheres. Since the center of mass of the small sphere cannot penetrate within a/2 of the large sphere surface, a region of "excluded volume" surrounds each large sphere. Here, a(a L ) represents the small (large) sphere diameter. Thus, when the surfaces of the two large spheres approach within a small-sphere diameter, these excluded volume regions overlap one another, and the total volume accessible to the small spheres increases. The resulting increase in small sphere entropy induces the so-called attractive "depletion" or entropic force between the large spheres.
 |
Illustration of the entropic
interactions of hard sphere particles among themselves
and with the wall. The small sphere centers of mass
are excluded from the shaded blue regions. The red
regions correspond to the overlap of excluded volumes,
and thus represent the gain in volume for the small
spheres. Notice that the gain in volume near the
surface is larger than that between two particles
in the bulk. Note if figure is not colored, then
blue = light grey and red = dark grey. |
Colloidal Interactions
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We have developed an experimental apparatus to directly measure particle interaction potentials in suspension. Using the instrument we have studied interaction potentials between colloidal particles brought about by added particles, non-adsorbing polymers, adsorbing polymers and rods. Our observations reveal information about colloid interactions, and about physics of the background medium such as its osmotic pressure and structure.
Related links: Light Forces, Center for Advanced Imaging and Micromanipulation
Potentials of mean force between two large colloidal spheres in a sea of small particles. Each curve represents a different background volume fraction of the small spheres. The data capture entropic (depletion) attractions at the smallest center-to-center separation, and oscillations due to the structural correlations of the 'fluid' of small spheres. |
An example of directly measured entropic potentials is shown in the figure to the right. The experimental sample consisted of suspensions of large and small hard spheres. We probe the interactions between two large spheres as a result of a sea of small spheres.
The most prominent feature in these potentials is the entropic attraction at short range. The liquid structure of the smaller particles in the background becomes important when their concentration is increased. Even at relatively low small-sphere volume fractions, there is a substantial entropic repulsion at separations of order one small sphere diameter from contact. Simple models do not predict this repulsion. Qualitatively the effect arises because the small spheres tend to form in layers around the large spheres. When the gap between the large spheres is commensurate with these layers, the free energy is lower; when the gap is incommensurate, the energy is higher.
Self-Assembly of Colloidal Particles (Entropy-driven & Beyond)
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The entropic excluded volume effects that induce attractions between particles, induce even stronger attractions between particles and walls. For example, in our lab we have demonstrated that these attractions between particles lead to crystallization and phase-separation in mixtures of particles of different sizes, particles and polymers, etc. We are using similar effects to position particles on substrates or move them in a predetermined ways. In particular, we have shown that geometric features on a surface can create entropic force fields that trap, repel, or induce drift of the larger particles in suspension. Large particles, moving in the vicinity of steps, grooves, or corners cut into a flat substrate, modify the small-sphere free volume in a manner dependent on large particle position and on the size and shape of the geometric features (see below figure for schematic examples). We have identified and quantified these phenomena at step edges, near corners, and in vesicles.
 |
| Schematic of the entropic 'depletion' effect in various geometries. Dark shaded region represents the gain of small sphere excluded volume (entropy). |
Building on this concept, we devised arrays of such structures in order to self-assemble particles in a chosen pattern, thus introducing a qualitatively new approach to colloidal epitaxy based on equilibrium thermodynamics and geometry. We used templated surfaces with spatially periodic 1-dimensional (1D) and 2-dimensional (2D) height profiles to create a periodic surface potential, and then to drive the growth of two- and three-dimensional (3D) particle structures nucleating from these templates. Some of the self-assembly effects we saw on these templates are shown in figure below.
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Colloidal assembly on cross-grating templates. Images show PMMA spheres. For all data the volume fraction of the PMMA particles is 0.25, and the concentration of polystyrene polymer is 0.1 mg/ml. In (E) we show the sequence of two-dimensional structures that nucleate on the grating template at particular ratios of the mean interparticle spacing, d, to the grating spatial period, p. The structures in (A), (B) and (C) are commensurate with the template. (D) is a 3D confocal image showing 20 layers of an FCC crystal.
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Current research explores self-assembly in microgravity, self-assembly of non-spherical particles on surfaces and in the bulk, convective assembly of particles on templated surfaces, and colloidal crystal templates filled with liquid crystalline material.
CARBON NANOTUBES
Isolated single-wall carbon nanotubes exhibit large electrical and thermal conductivities, extraordinary mechanical strength and rich optical spectra. Carbon nanotube complex fluids provide a diversity of new phenomenology in suspension, and are potentially of enormous technological utility. We have applied concepts and methods from soft condensed matter to control single-wall carbon nanotubes (SWNTs) in solution. Our initial research discovered a class of surfactants that stabilized SWNTs in aqueous suspensions extraordinarily well. This first observation led us to pursue several, initially unanticipated questions in nanotube science and application. Some of this work will be described below.
The Yodh lab has worked on a variety of scientific problems in this field. Selected recent research is described below.
Solubilization of Single-Wall Carbon Nanotubes in Aqueous
Suspensions
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Many envisioned applications employing the unique electronic, thermal, optical and mechanical properties of SWNTs require large-scale manipulation of stable suspensions at high weight-fraction. Tube solubilization provides access to solution-phase separation methodologies, facilitates chemical derivatization, controlled dispersion and deposition, microfluidics, fabrication of nanotube-based fibers and composites, and optical diagnostics. A barrier for these applications has been the substantial van der Waals attractions between tubes which causes aggregation.
Left-to-Right are single-wall carbon nanotube suspensions in water stabilized by surfactants SDS, Triton-X and NaDDBS respectively. |
We developed a methodology for creation of high weight-fraction suspensions of surfactant stabilized SWNTs in water, with a large fraction of single tubes. Our methodology is essentially a single step solubilization scheme, in which nanotubes are mixed with surfactants in low-power high-frequency sonicators for long time periods; the scheme enhanced disaggregation of bundles with dramatically reduced tube breakage. The critical ingredient was the surfactant sodium dodecylbenzene sulfonate (NaDDBS), consisting of a benzene ring moiety, a charged group, and an alkyl chain. Appropriate deployment of NaDDBS enhanced the stability of SWNTs in water by factors of tens to hundreds compared to commonly employed surfactants and polymers. New and quantitative insight about nanotube solubilization was derived from comparisons of single tube yield, measured by AFM, as a function of surfactant type, and concentration.
Current research uses these nanotube complex fluids as starting materials for deposition onto substrates for electrical devices, and for creation of mechanically strong composites.
Nematic Nanotube Gels
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Many of the interesting properties of single-wall carbon nanotubes are anisotropic due to the large length-to-diameter ratio of carbon nanotubes; responses measured parallel to the nanotube central axis differ from those measured in other directions. Thus many of the potential applications of SWNTs aim to produce composite materials with aligned nanotubes in order to take full advantage of these anisotropic responses. A nematic phase of SWNTs, similar to those of thermotropic and lyotropic liquid crystals, offers a natural route for creation of aligned composites.
Filaments (single-wall carbon nanotubes) are dispersed in a polymer gel, which is subsequently compressed to a fraction of its original size. The condensed gel creates a macroscopic aligned (nematic) phase, interesting topological defects, and potentially novel mechanical responses. |
To this end, we have created nematic nanotube gels. The gels contain large domains of oriented SWNTs that exhibit hallmark properties of a nematic: birefringence, anisotropy in optical absorption, and liquid crystalline defects. We created these nematic nanotube gels by embedding isolated surfactant coated nanotubes in a crosslinked gel at low density, and then inducing a volume phase transition (compression) of the gel by changing its temperature. Apparently the gel network prevented the close contact between parallel nanotubes that produces bundling, and compression produced mass densities of isolated nanotubes, not accessible in simple aqueous suspensions, that favor their alignment. In addition to the potential importance of this approach for nanotube technology, these lyotropic gels represent a new physical system different from both lyotropic hard-rod suspensions and from thermotropic elastomers, with which they share the property of nematic order in a randomly crosslinked gel. They exhibit an interesting interplay between local nematic alignment and local mechanical stresses and cracks. Finally, the general methodology we have introduced for making these materials provides a qualitatively new way to induce phase transitions in soft materials such as colloids, emulsions, and other complex fluids.
Optical Properties and Anistropy of Single-wall Carbon
Nanotubes
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Measured optical absorption cross-sections parallel and perpendicular to the carbon nanotube long axis as a function of photon energy. The lower panel shows theory (Kane, PENN) for these effects with and without field depolarization effects. |
The substantial length-to-diameter aspect ratio of SWNTs presents a fundamental structural anisotropy that influences optical absorption. In a recent experiment we (in close collaboration with the Kikkawa group at PENN) have obtained the polarized absorbance cross-sections of single-wall carbon nanotubes. To achieve this goal we weakly aligned surfactant (NaDDBS) stabilized SWNT suspensions in a magnetic field and subsequently locked them in place by suspension gelation using NIPA polymer. Polarized Raman scattering was then used to determine the nematic order parameter (i.e. SWNT alignment) for each sample. This alignment information was combined with polarization-sensitive absorbance data to recover the absolute wavelength-dependent SWNT absorption cross-section for light polarized parallel and perpendicular to the nanotube central axis. These data enabled us to rigorously test theories for absorptive processes, and provided the first direct confirmation that sub-band features are absent from the cross-polarized channel, as theoretically predicted. Moreover, these anisotropic spectra facilitate simple, rapid, and quantitative measurements of nanotube orientations in bulk dispersions and composites.
Current research (also with the Kikkawa group) is focussed on magnetic alignment and alignment anisotropies.
Structure and Rheology of Single-wall Carbon Nanotube
Suspensions
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Rigidity percolation of NaDDBS/Carbon-nanotube suspensions as a function of nanotube volume fraction. |
The mechanical properties of associating semi-flexible polymer and rod networks play a critical role in a variety of materials' contexts ranging from functioning cells to stress-bearing colloidal suspensions and polymeric composites. Generally the rheology of these networks depends on many factors including the bonds between rods, rod concentration, and rod flexibility. The relationship, however, between the microscopic structure and the macroscopic elasticity of associating networks of stiff rods remains essentially unexplored. We have investigated the viscoelastic properties of an associating rigid rod network: aqueous suspensions of surfactant stabilized single wall carbon nanotubes (SWNTs). The SWNT suspensions represent a fascinating model system, enabling us to isolate the contributions of bonding to the macroscopic elasticity of rod networks. Our experimental observations suggest nanotubes in suspension form an interconnected network with bonds that freely rotate and resist stretching. Suspension elasticity originates from bonds between nanotubes rather than from the stiffness or stretching of individual nanotubes. The suspensions exhibit rigidity percolation as a function of nanotube concentration. Our experiments provide new rheological understanding about solutions of carbon nanotubes. This new information may improve our ability to control the processing precursors of these novel composites.
We have also carried out small-angle neutron scattering (SANS) to probe the structure of these nanotube suspensions. The investigation provides strong evidence for individual SWNTs and very small bundles in suspension which behave like rigid rods. A crossover in the power law Q-dependence of scattered intensity, suggests that these isolated tubes and small bundles also form a loose three-dimensional network.
Single wall carbon nanotube epoxy composites
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Top right - Single-wall nanotube carbon nanotube composites as a function of nanotube concentration (black is most concentrated). Bottom right - Configuration for 4-probe electrical conductivity measurements. Left - Electrical percolation as a function of nanotube volume fraction. |
We (with Kikkawa group) have developed procedures to introduce single wall carbon nanotubes (SWNTs) into thermoset epoxies . We have characterized the morphology of the systems, and then explored the electrical conductivity of the resulting composites as a function of SWNT length, SWNT concentration, sample homogeneity, and sample preparation. We observe sharp (percolation-like) conductivity onsets, at very low SWNT concentrations. These lower thresholds result from our new synthetic methods, and by minimally altering mechanical properties of the epoxy may pave the way for important technological applications such as electrostatic charge dissipation, RF shielding, etc.
Current experiments (with Kikkawa group) probe thermal transport in similar composites, deducing interfacial thermal resistances.
Hard Surfaces & Interfaces
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The goals of these programs are to understand interfaces and their role in modifying the microscopic properties of adjoining bulk materials & adsorbed molecules.
The Yodh lab has worked on a variety of scientific problems in this field. Selected recent research is described below.
Nonlinear Optical Spectroscopy of Interfaces, Surfaces and Particles
Our group uses nonlinear optics (e.g. second-harmonic and sum-frequency generation) to probe level structure, charge dynamics, and defects at solid-solid interfaces. The experiments combine the intrinsic interface sensitivity of three-wave mixing processes with the long penetration depth of light to make possible a range of studies of buried solid-state interfaces. Such materials are very difficult to probe with conventional tools from surface science and linear optics. Scientific contributions include the discovery of intrinsic quantum well interface states at the ZnSe/GaAs interface and midgap defect states in Au/GaAs junctions. Nonlinear spectroscopy was also combined with photomodulation techniques to elucidate morphological properties associated with interface traps, trap lifetimes, strain and strain relaxation. More recently we have explored second-harmonic generation from colloidal particles, and we have been developing and using nonlinear microscopy for studies of complex media, including brain.
Ultrafast Spectroscopy of Surfaces and Surface Adsorbates
A second family of measurements employed ultrafast infrared methodologies to study vibrational dynamics of molecules on metal surfaces in ultrahigh vacuum. The objectives of this program were to resolve very fast processes such as energy transfer, dephasing, and the movement of adsorbed molecules, particularly following ultrafast laser excitation of the substrate. For example, we measured the energy transfer rate from metal substrate electrons and phonons to the very low frequency vibrations (i.e. the frustrated translation mode) of a chemisorbed diatomic molecule, following femtosecond visible light pulse excitation of the substrate.
