Phase sensitive detection of persistent spectral holes using synchronous ultrasonic modulation

We present a new technique for the detection of spectral holes or other strain-sensitive spectral features using phase-sensitive ultrasonic modulation. The spectral feature is directly modulated at MHz rates in synchrony with an applied rf ultrasonic field, hence the method offers zero background and the potential for high-speed detection with sensitivity near the quantum limit. The technique is demonstrated for a model color center system, and the characteristic line shape is analyzed in detail. The method is more sensitive than laser FM (frequency modulation) spectroscopy for low modulation frequencies, and should be generally useful for the detection of strain-sensitive spectral features.