Efficient optical frequency conversion in nonlinear crystals is made possible by phase matching - the coherent addition of the nonlinear response upon propagation through the crystal. Today, optical frequency conversion is exploited in many areas, ranging from laser fusion to laser pointers. However, in order for phase matching to occur a crystal needs to be many wavelengths in size, which is not compatible with nanoscale architectures. A strong local nonlinear response therefore relies on intrinsic material nonlinearities. This research program investigates the high optical nonlinearities of plasmonic nanostructures made of noble metals such as gold and silver. These structures make it possible to generate light locally at shifted frequencies and to exploit multispectral interactions in nanoscale devices, without the need of phase matching.
This research program investigates the nonlinear response of discrete and well-defined metal nanostructures, such as nanoparticles, nanorods and particle hybrids (dimers, trimers). These structures function as optical antennas - they localize and enhance the incident fields and they facilitate the release of the nonlinear response. The goal is to identify, understand and exploit configurations that yield high nonlinear efficiencies. Many nonlinear interactions, such as harmonic frequency generation or nonlinear four-wave mixing, are coherent processes and hence the phase-sensitive interplay of local fields is important for a strong response. The investigators study the interaction of frequency-converted fields with single quantum emitters, such as quantum dots, molecules, or ions. The ability to generate light locally through nonlinear processes provides new opportunities for nanophotonics and active plasmonics.
In this project we have studied nonlinear optical interactions in noble metal nanostructures. Nonlinear interactions give rise to frequency conversion. For example, an input light beam of red color (wavelength of 700 nm) can be converted to an output beam of ultraviolet color (wavelength 350 nm). Noble metals such as gold and silver exhibit extremely high optical nonlinearities. There nonlinear parameters are, for example, more than one hundered times higher than those of standard nonlinear optical crystals found in optoelectronic devices. However, for reasons of phase matching and absorption, metals are typically not used in nonlinear optics. Instead of using extended transparent laser crystals for frequency conversion we investigated the possibility of using the high intrinsic nonlinearities properties of metals for frequency conversion. Specifically, we have shaped the noble metals in antenna-like structures in order to take advantage of resonant enhancement effects . These enhancement effects are based on the excitation of surface plasmons, collective excitations of electron resonances in the metal. The study of nonlinear effects in metals based on plasmonic enhancement is termed "nonlinear plasmonics". Nonlinear plasmonics is a largely unexplored territory and only a few nonlinear interactions in noble metal nanostructures have been studied so far, including second-harmonic generation (SHG), third-harmonic generation (THG), two-photon excited luminescence (TPL), and four-wave mixing (4WM). In this project we studied the nonlinear response of discrete and well-defined metal nanostructures, such as nanoparticles, nanorods and particle hybrids (dimers, trimers). These structures functioned as optical antennas - they localize and enhance the incident fields and they facilitate the release of the nonlinear response. We identified and exploited configurations that yield a high nonlinear efficiency. One of the nonlinear plasmonic structures investigated was an optical antenna made of two arms, each resonant witha different input wavelength (see Figure 1). This multifrequency optical antenna was embedded in a nonlinear medium that acted as a receiver. Tuning of the antenna gap size strongly affected the nonlinear response. We expect that our research findings will have an impact on future nanoscale photonic devices. The ability to generate local frequency conversion with high efficiency opens the door for on-chip frequency-selective interactions across the entire visible and infrared spectrum while requiring only a single-frequency excitation. Frequency-converted fields can be employed to interact with nanoscale systems, such as single quantum dots, molecules, or ions.
