Mixed Frequency Generation in a Gold Antenna enables Double Blind Ultrafast Pulse Characterization

Ultrafast pulse characterization requires the analysis of correlation functions generated by frequency mixing of optical pulses in a nonlinear medium. A two-dimensional time versus wavelength spectrogram is compiled based on... [ view full abstract ]

Ultrafast pulse characterization requires the analysis of correlation functions generated by frequency mixing of optical pulses in a nonlinear medium. A two-dimensional time versus wavelength spectrogram is compiled based on either third order or second order correlation signals and analysed by an algorithm that reconstructs pulse shapes in time. In these techniques, the number of nonlinear processes involved dictates how many ultrashort pulses can be simultaneously retrieved. However, phase matching in a single nonlinear crystal will limit the types of nonlinear processes achievable and the spectral range where they can be efficient. Plasmonic optical antennas offer a promising route toward ultrathin nonlinear devices with little limitation on phase matching. These metallic nanostructures sustain strong resonant electromagnetic fields via electronic charge oscillations at their surfaces, which can simultaneously generate second (SHG) and third harmonic generation (THG), Sum Frequency Generation (SFG), and Four Wave Mixing (FWM), with interaction lengths shorter than the optical wavelength.

In this work, we exploit degenerate Four Wave Mixing (FWM) and Sum Frequency Generation (SFG) signals from an individual gold antenna to characterize two near IR ultrafast optical pulses from a Coherent Ti: Sapphire and Optical Parametric Oscillator (OPO) without the need of a known reference pulse. The multiresonant antenna consists of a gold bar and two gold disks with dipolar resonances detuned by about an octave, in this case near 1500 and 750 nm for the bar and disks respectively.  Due to narrow gaps between the disks and bar, a resonant near field coupling or “Fano” interference effect arises between the dipolar mode of the disks and a less-radiative second order mode of the bar. This coupling facilitates the various nonlinear mixing processes. By temporally-scanning the pump and signal laser pulses and recording spectra at each scan delay, we can produce a spectral intensity map of relative delay versus wavelength (i.e. a spectrogram) of the various frequency mixing processes. We can then characterize the chirp and spectra of our two optical pulses assuming secant square pulses demonstrating the viability of the technique.   The nonlinear mixing is efficient enough to retrieve pulses with energies in the picojoule range.