Nonresonant Spectral Hole Burning Research Articles

Mechanical hole-burning spectroscopy of PMMA deep in the glassy state.

Nonlinear mechanics of soft materials such as polymer melts or polymer solutions are frequently investigated by Large Amplitude Oscillatory Shear (LAOS) spectroscopy tests. Less work has been reported on the characterization of the nonlinear viscoelastic properties of glassy polymers within a similar framework. In the present work, we use an extension of LAOS, i.e., mechanical spectral hole burning (MSHB), to investigate the nonlinear dynamics of an amorphous polymer in the deep glassy state. MSHB was developed as an analog to non-resonant spectral hole burning developed by Schiener et al. [Science 274(5288), 752-754 (1996)], who attributed the presence of holes to dynamic heterogeneity. On the other hand, Qin et al. [J. Polym. Sci., Part B: Polym. Phys. 47(20), 2047-2062 (2009)] in work on polymer solutions of tailored heterogeneity have attributed the presence of holes to the type of dynamics (Rouse, rubbery, etc.) rather than to a specific spatial heterogeneity. Here, we have performed MSHB experiments on poly(methyl methacrylate) in the deep glassy state (at ambient temperature, which is near to the β-relaxation) to investigate the presence and origin of holes, if any. The effects of pump frequency and pump amplitude were investigated, and we find that vertical holes could be burned successfully for frequencies from 0.0098 Hz to 0.0728 Hz and for pump amplitudes from 2% to 9% strain. On the other hand, horizontal holes were incomplete at high pump amplitude and low frequency, where higher spectral modification is observed. The results are interpreted as being related to the dynamic heterogeneity corresponding to the β-relaxation rather than to the hysteresis energy absorbed in the large deformation pump.

Mechanical spectral hole burning of an entangled polymer solution in the stress-controlled domain.

Nonresonant spectral hole burning has proven to be a versatile method to characterize the dynamics of complex fluids, including polymers. Early work focused on dielectric susceptibility measurements in glass forming liquids, while recent work from our lab used mechanical viscoelastic measurements to investigate polymer melts and solutions. While the observed results were similar, the interpretations were different, with the former being interpreted by attributing the "holes" in the response as being due to dynamic heterogeneity in the system that is related to the glass or other transition, while the latter was interpreted in a way that suggested that the observed holes depend on the type of dynamics (Rouse, terminal, etc.) rather than an identifiable spatial heterogeneity. In this work, we have expanded mechanical spectral hole burning (MSHB) into the stress-controlled domain and carried out experiments in the rubbery regime of a polystyrene solution, similar to one which was tested previously with strain-controlled MSHB. The effects of pump stress amplitude, pump frequency, and waiting time were investigated. The mechanical holes in both directions (vertical and horizontal) were successfully burned, unlike the strain-controlled MSHB experiments on the same polystyrene solution, in which vertical holes were at best incomplete. The hole intensity exhibits a linear relationship with the amount of energy dissipated in the system during the large mechanical pump modification. The results suggest that the stress-controlled MSHB can be combined with strain-controlled MSHB to build a more complete framework to investigate the dynamics of polymeric materials and is consistent with the dynamic heterogeneity being related to the type of dynamics rather than to localized heating effects.

Nonresonant spectral-hole burning in erbium-doped fiber amplifiers

We report here, for the first time, observations of offset and collaterally burned holes in erbium-doped fiber amplifiers (EDFAs). This additional spectral-hole-burning structure in the 1.5-/spl mu/m region will impact the application of EDFAs in the wavelength-division-multiplexed environment as well as their descriptive models.

Nature of the non-exponential primary relaxation in structural glass-formers probed by dynamically selective experiments

Several experimental methods feature the potential to distinguish between slow and fast contributions to the non-exponential, ensemble averaged primary response in glass-forming materials. Some of these techniques are based on the selection of subensembles using multi-dimensional nuclear magnetic resonance, optical bleaching, and non-resonant spectral hole burning. Others, such as the time-dependent solvation spectroscopy, measure microscopic responses induced by local perturbations. Using several of these methods it could be demonstrated for various glass-forming materials that the non-exponential relaxation results from a superposition of dynamically distinguishable entities. The experimental observation that subensembles can be selected efficiently indicates a large degree of heterogeneity. The intrinsic response is compatible with single exponential relaxation.

Experiments and theory of the nonexponential relaxation in liquids, glasses, polymers and crystals

More than 130 years ago it was first recognized that the observed response from many diverse substances may exhibit two distinct types of nonexponential relaxation. Although the Kohlrausch-Williams-Watts (KWW) stretched exponential and Curie-von Schweidler (CvS) power law have since been used to characterize the observed response from thousands of measurements, there is still no commonly accepted explanation for these empirical formulas. Only in the past few years have there been experimental techniques developed to answer the most basic question about the observed response: is it homogeneous where all regions of the sample exhibit intrinsic nonexponential behavior, or is it a result of a heterogeneous distribution of relaxation times? One technique, nonresonant spectral hole burning (NSHB), uses a large amplitude, low-frequency electric-or magnetic-field to selectively investigate the constituents in the net spectrum of response. An important advantage of NSHB is that the responding degrees of freedom are used directly as their own local probe. For all systems examined so far, including amorphous and crystalline materials, the nonexponential relaxation is found to result from a distribution of relaxation times. This raises the theoretical questions: why are there two types of nonexponential behavior, and why are they so “universal”? A possible answer comes from a mesoscopic model for nonexponential relaxation that provides a common physical basis for both universalities.

Nonresonant dielectric hole burning spectroscopy of supercooled liquids

The nonexponential response of propylene carbonate and glycerol near their glass transitions could be selectively altered using nonresonant spectral hole burning (NSHB) experiments. This observation provides evidence of the existence of a distribution of relaxation times in these supercooled liquids. NSHB is based on a pump, wait, and probe scheme and uses low-frequency large amplitude electrical fields to modify the dielectric relaxation. The temporal evolution of the polarization of the sample is then measured subsequent to a small voltage step. By variation of a recovery time inserted between pump and probe, the refilling of the spectral features could be monitored and was found to take place on the time scale set by the peak in the distribution. The recovery time and pump frequency dependences of the spectral modifications were successfully simulated using a set of coupled rate equations.

Nonresonant Spectral Hole Burning in the Slow Dielectric Response of Supercooled Liquids

Large-amplitude, low-frequency electric fields can be used to burn spectral holes in the dielectric response of supercooled propylene carbonate and glycerol. This ability to selectively modify the dielectric response establishes that the non-Debye behavior results from a distribution of relaxation times. Refilling of the spectral hole was consistent with a single recovery time that coincided with the peak in the distribution. Moreover, refilling occurred without significant broadening, which indicates negligible direct exchange between the degrees of freedom that responded to the field. Nonresonant spectral hole burning facilitates direct investigation of the intrinsic response of systems that exhibit nonexponential relaxation.

Slow Dielectric Relaxation of Supercooled Liqutos Investigated by Nonresonant Spectral Hole Burning

ABSTRACTWhen supercooled propylene carbonate and glycerol are subjected to a large-amplitude, low-frequency electric field, a spectral hole develops in their dielectric relaxation that is significantly narrower than their bulk response. This observation of nonresonant spectral hole burning establishes that the non-Debye response is due to a distribution of relaxation times. Refilling of the spectral hole occurs abruptly, indicative of a single recovery rate that corresponds to the peak in the distribution. The general shape of the spectral hole is preserved during recovery, indicating negligible interaction between the degrees of freedom that responded to the field. All relevant features in the behavior can be characterized by a model for independently relaxing domains that are selectively heated by the large oscillation, and which recover via connection to a common thermal bath, with no direct coupling between the domains.