Sensitive detection of electric field-induced second harmonic (E-FISH) signals

We demonstrate sensitive electric field measurements by coherent homodyne amplification of electric-field induced second harmonic generation (E-FISH) technique. In the process of E-FISH, an applied electric field breaks centrosymmetryof an otherwise homogeneous medium, in turn promoting generation of second harmonic frequency of an incident field. Due to weak third-order hyperpolarizability and requirement of an applied field to break the symmetry, the E-FISH technique has been mainly used to study high fields, also requiring strong optical field and sensitive detection. Here we superimpose the E-FISH signal with an auxilliary beam,also termed a local oscillator (LO), at double of the incident frequency. Coherent superposition of the LO and the E-FISH output allows for a homodyne amplification of the otherwise weak nonlinear signal. We have demonstrated an increase of signal to noise by a factor of 7, which results in a measurement time reduction of a factor of 49. This technique, local oscillator E-FISH, LOE-FISH, has a number of advantages: Detection with intensified detectors is not required. Furthermore, instead of mJ pulsed lasers, we can work with μJ pulsed lasers, which allows measuring at repetition rates of MHz and opens single shot and real time capability. The LOE-FISH technique increases in sensitivity at lower electric field. Our work is a demonstration of the principle. Already with our first results from the demonstration one can see the high potential of LOE-FISH.