Synchronous digitization for high dynamic range lock-in amplification inbeam-scanning microscopy

Digital lock-in amplification (LIA) with synchronous digitization (SD) is shown to provide significant signal to noise (S/N)and linear dynamic range advantages in beam-scanning microscopy measurements using pulsed laser sources. Direct comparisonsbetween SD-LIA and conventional LIA in homodyne second harmonic generation measurements resulted in S/N enhancementsconsistent with theoretical models. SD-LIA provided notably larger S/N enhancements in the limit oflow light intensities, through the smooth transition between photon counting and signal averagingdeveloped in previous work. Rapid beam scanning instrumentation with up to video rate acquisitionspeeds minimized photo-induced sample damage. The corresponding increased allowance for higher laserpower without sample damage is advantageous for increasing the observed signal content.