Coherent energy migration in solids: Determination of the average coherence length in one-dimensional systems using tunable dye lasers

Abstract

The coherent nature of energy propagation in solids at low temperatures was established from the time resolved response of the crystal to short optical pulses obtained from a dye laser (pumped by a nitrogen gas laser). The trapping and detrapping of the energy by shallow defects (x traps) was evident in the spectra and enabled us to extract the coherence length: l≳700 ?=186 molecules for the one-dimensional triplet excitons of 1,2,4,5-tetrachlorobenzene crystals at T<4.2 °K. This length which clearly exceeds the stochastic random walk limit is related to the thermalization mechanisms in this coupled exciton–trap system, and its magnitude supports the notion that exciton–phonon coupling is responsible for the loss of coherence on very long molecular chains (trap concentration is 1/256 000).