  Second-order nonlinear optical interactions such as sum- and
difference-frequency generation are widely used for bioimaging and as selective
probes of interfacial environments. However, inefficient nonlinear optical
conversion often leads to poor signal-to-noise ratio and long signal
acquisition times. Here, we demonstrate the dramatic enhancement of weak
second-order nonlinear optical signals via stimulated sum- and
difference-frequency generation. We present a conceptual framework to
quantitatively describe the interaction and show that the process is highly
sensitive to the relative optical phase of the stimulating field. To emphasize
the utility of the technique, we demonstrate stimulated enhancement of second
harmonic generation (SHG) from bovine collagen-I fibrils. Using a stimulating
pulse fluence of only 3 nJ/cm2, we obtain an SHG enhancement >10^4 relative to
the spontaneous signal. The stimulation enhancement is greatest in situations
where spontaneous signals are the weakest - such as low laser power, small
sample volume, and weak nonlinear susceptibility - emphasizing the potential
for this technique to improve signal-to-noise ratios in biological imaging and
interfacial spectroscopy.
