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.W h e n subjected to a n external perturbation, lnany materials, including polynlers (1), protelns ( 2 ) , and supercooled liquids ( 3 ) , exhibit nonesponent~al relaxation. T h e response of thousands of different sul~stances (4) has been character~zed by the Kohlrausch-Will~a~ms-Watts (KWW) stretched exponential, exp[-(t/r)@], where 9 < 1, t is time, and 7 IS a character~stic relaxation t~m e .It has long been Jehated (5-8) whether t h~s dvnam~cal complexity is intrinsic, \ \~t h all repions of the salnole exhihitine a silllilar nor-Llebve response, or ~vhether it is the result of heterogenelty, r i t h localized degrees of freeilom relaxing exponentially hut w~t h a distribution of relaxat~on times that y~elds the net lxhavior. Several itudies (9-1 1 ) of supercooled liquids near t h e~r calori~netrlc glass transition tenlperature T,, have concludeii that heterogeneity occurs on length scales of 1 to 5 nm. Recent lnagnetlc resonance measurements 11 2 ) h a w s h o~v n that these domains are not static, but that their d~stinct relaxation rate. persist long enough to cause the broadened response. Such dvnalnic heterogeneity, n h~c h evolves as the saml?le responds, has not previousl\-been observed directly in the slonr d~electric rebponse.Spectral hole burning (SHB) map be the no st direct t e c h n i q~~e for investijiating the constituents in the net response of a macroscopic sample. O h s e r~. a t~o n of S H B g e rerally requires a heterogeneously broadened spectnlm, a strong external sigllal that can modify selected parts of the spectrum, and a resulting "hole" that is persistent enough to allo\v subsequent measurement of the modified response. A n in-portant advantage of the technic~ue is that the responding de-B Schiener and A Loid;, lnst~tut fur Festkorperpi/sk,

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Dielectric spectroscopy has been performed on supercooled glycerol for frequencies 3 µHz ≤ ν ≤ 40 GHz and temperatures between 100 K ≤ T ≤ 500 K. Hence, the absorptive part of the dielectric susceptibility was measured at comparable frequencies as the dynamic susceptibility obtained by neutron and light scattering techniques. The characteristic timescales obtained from all experimental techniques essentially agree. However, the dielectric data as measured on the high-frequency wing of the loss spectra are not consistent with neutron and light scattering results which probe density fluctuations. We conclude that in glycerol at high frequencies the density fluctuations are dominated by local vibration excitations which are fully decoupled from the dipolar reorientations. The temperature dependence of the dielectric loss traces that of the spin-lattice relaxation times measured with NMR techniques and exhibits almost frequency-independent signatures near the calorimetric glass transition.

Dielectric spectroscopy covering 17 decades of frequency up to 380 GHz has been performed on the glass formers [Ca(N03)2]o.4[KN03]o.6, [Ca(N03)2]o.4[RbN03]o.6, and propylene-carbonate. Special attention is given to the dielectric loss c 11 in the crossover regime from the a-relaxation to the far infrared response. Our data cannot be described by a simple crossover from the a-process to the far infrared bands, instead we obtain clear evidence for additional processes prevailing in this frequency range. We compare the results for c"(v) with our previous results on glycerol and with the predictions of the mode coupling theory of the glass transition.