Detection of persistent spectral holes using ultrasonic modulation

Abstract

A useful zero-background method for the detection of persistent spectral holes, HUMPH (High-resolution Ultrasonic Modulation of Persistent Holes), is described and demonstrated. The method utilizes the fact that spectral holes are sensitive to strain fields and involves modulation of an ultrasonic field at kilohertz rates in order to induce a corresponding modulation in the shape of the spectral hole. The resulting time-varying sample transmission is detected with a lock-in amplifier. The technique is demonstrated for the case of photochemical holes in color centers in x-irradiated NaF and is shown to be effective for the detection of shallow strain-sensitive absorption features. A model for the process based on the interaction of the centers with uniaxial stress accounts for the observed line shapes. This method is expected to be generally applicable and quite sensitive for the following reasons: Spectral holes in photochemical and nonphotochemical systems are intrinsically strain sensitive, the technique involves modulation only of the effect under study, and the signals appear on zero background, in contrast to standard absorption and fluorescence excitation techniques.