Nonlinear optics at the nanoscale

We explore nonlinear optical phenomena at the nanoscale by launching femtosecond laser pulses into long silica nanowires. Silica nanowires [1] provide strong mode confinement in a simple cylindrical silica-core/air-cladding geometry, representing a model system for the study of the nonlinear propagation of short pulses inside fibers. We observe supercontinuum generation by femtosecond laser pulses in silica fiber tapers of minimum diameters as small as 90 nm and we used loops of these wires to demonstrate light-by-light modulation.

Supercontinuum generation refers to extreme spectral broadening of a laser pulse propagating in a nonlinear material. Qualitatively, the degree of broadening of the supercontinuum spectra can be understood in terms of the diameter-dependent dispersion and nonlinearity of the fiber. Contrary to supercontinuum generation by nanosecond pulses, for laser pulses propagating in negative dispersion regime, the observed spectrum is consistent with higher-order soliton formation and break-up [2]. Because of the interaction length is a mere 20 mm and because the energy threshold for supercontinuum generation in tapered fibers is only 1 nJ, these nanowires offer a viable approach for coherent white-light source in nanophotonics. The spectral broadening reveals an optimal fiber diameter to enhance nonlinear effects with minimal dispersion (around 500 nm). In addition, sub-200-nm diameter fibers have negligible dispersion and nonlinearity making these fibers ideal media for propagation of intense, short pulses with minimal distortion.

The low threshold energies required to generate supercontinuum in nanowires indicate that microphotonic devices can be constructed that take advantage of these nanoscale nonlinear effects. We present a microscopic nonlinear Sagnac interferometer that permits optical switching and a number of all-optical logic operations with femtosecond laser pulses in the nanojoule range. We fabricate Sagnac loops with lengths of about one millimeter from silica nanowires with diameters of 500–800 nm. Preliminary results show that we achieve light-on-light modulation. The data shows excellent agreement with predicted transmission behavior for a nonlinear Sagnac interferometer with a coupling parameter of 0.08. Our current research efforts are to increase the modulation depth of the transmission by controlling the coupling, and to further reduce the interaction length required for nonlinear phenomena.

[1] Subwavelength-diameter silica wires for low-loss optical wave guiding, Limin Tong, Rafael R. Gattass, Jonathan B. Ashcom, Sailing He, Jingyi Lou, Mengyan Shen, Iva Zaharieva Maxwell and Eric Mazur, Nature, 426, 816-819 (2003).
[2] Supercontinuum generation in submicrometer diameter silica fibers, Rafael R. Gattass, Geoffry Thomas Svacha, Limin Tong and Eric Mazur, Opt. Exp., 14, 9408-9414 (2006).