Probing unfolded acoustic phonons with X rays.

Ultrafast laser excitation of an InGaAs/InAlAs superlattice (SL) creates coherent folded acoustic phonons that subsequently leak into the bulk (InP) substrate. Upon transmission, the phonons become "unfolded" into bulk modes and acquire a wave vector much larger than that of the light. We show that time-resolved x-ray diffraction is sensitive to this large-wave vector excitation in the substrate. Comparison with dynamical diffraction simulations of propagating strain supports our interpretation.

Observation of surface-avoiding waves: a new class of extended states in periodic media.

Coherent time-domain optical experiments on GaAs-AlAs superlattices reveal the existence of an unusually long-lived acoustic mode at approximately 0.6 THz which couples weakly to the environment by evading the sample boundaries. Classical as well as quantum states that steer clear of surfaces are generally shown to occur in the spectrum of periodic structures, for most boundary conditions. These surface-avoiding waves are associated with frequencies outside forbidden gaps and wave vectors in the vicinity of the center and edge of the Brillouin zone. Possible consequences for surface science and resonant-cavity applications are discussed.

Generation and propagation of a picosecond acoustic pulse at a buried interface: time-resolved x-ray diffraction measurements.

We report on the propagation of coherent acoustic wave packets in (001) surface oriented Al0.3Ga0.7As/GaAs heterostructure, generated through localized femtosecond photoexcitation of the GaAs. Transient structural changes in both the substrate and film are measured with picosecond time-resolved x-ray diffraction. The data indicate an elastic response consisting of unipolar compression pulses of a few hundred picosecond duration traveling along [001] and [001] directions that are produced by predominately impulsive stress. The transmission and reflection of the strain pulses are in agreement with an acoustic mismatch model of the heterostructure and free-space interfaces.

A TSM sensor investigation of low crystallinity cellulose films.

A thickness shear mode (TSM) quartz sensor has been used to characterize the substantivity, viscoelasticity, and mucoadhesive properties of low crystallinity cellulose (LCC) films. LCC is a novel pharmaceutical excipient that has been attributed with mucoadhesive properties. Thin films of LCC were deposited onto TSM sensors by a spin coating technique. The films were treated by passing water or 1.0% w/v mucin solution (pH 3.7 or 7.0) over the surface. Changes in the mass and viscosity of the film were observed by monitoring changes in the impedance spectra of the coated TSM sensors. Scanning electron micrographs (SEMs) of each film were used to assist the interpretation of the TSM sensor data. This study showed that LCC forms highly tenacious and viscoelastic films able to withstand prolonged (approximately 1 h) exposure to both water and mucin solution. Furthermore, these results indicate that the films may have mucoadhesive properties as LCC was found to bind significant (P<0.05) amounts of mucin in comparison with control measurements. Mucin binding to the LCC sensor was greater at pH 3.7 (P<0.05) than at pH 7.0, suggesting that the LCC formulation is mucoadhesive under these conditions.

Characterisation of fatty acid multilayers using a TSM biosensor.

Thickness shear mode (TSM) biosensors have many potential applications within the pharmaceutical sciences as a means of measuring mass changes in the nanogram range, film thickness, viscosity and shear moduli. This study addresses the possible use of the TSM sensor as a biosensor for measuring drug partition coefficients. In order to realise this potential, some fundamental understanding is required of the behaviour of lipid films on the sensor. The present study characterises the behaviour of fatty acid multilayers as a suitable model chemical system. Frequency shifts and impedance spectra are presented for multilayers of three fatty acid films coated on to the sensor using a Langmuir-Blodgett trough. The results indicate that the frequency shift is non-linear at lower numbers of fatty acid layers but the response is Sauerbrey-like at higher numbers of layers. Also at high numbers of layers, changes in the impedance spectra indicate viscoelastic behaviour in thicker membranes. An inverse relationship is observed between chain length and frequency shift, which is attributed to variations in the topography of the sensor surface. This work demonstrates the importance of fully characterising the physical behaviour of the lipid multilayers prior to using these systems for the measurement of drug partition coefficients.