Nonlinear Optical Switching

We have explored a variety of approaches to nonlinear optical switching, including new materials and new device structures as described below:

We have demonstrated a nonlinear mirror that consists of a multilayer of PVK and Co3O4 doped PVA; the Co3O4 nanoparticles were identified as materials with exceptionally large nonlinear refractive indices (Figure 1). 24 layer nonlinear mirrors have been fabricated, with individual layer thicknesses of 90nm, and the mirror demonstrated approximately 40% transmission at 532nm in the "off”state" and less than 5% transmission when "on" Figure 1. The transmission drops to 50% of its "off state" value at approximately 90mJ/cm². Many nonlinear mirrors have been made with similar performance, and the PVA:Co3O4 composite exhibits an exceptionally high damage threshold (> 1J/cm2 at 523nm, 5 nsec pulses).

Figure 1 - Transmission through 24 layer nonlinear mirror made from PVK and cobalt oxide doped PVA

The unique properties of one-dimensional (1D) Fibonacci chains of dielectric layers were experimentally demonstrated and exploited for the design of new mirrors with multiple reflection spectral windows (Figure 2). The new mirror structures are simple, straightforward to make and enable a wide variety of multiple spectral window device performance to be achieved. By changing the thickness of the layers or the order of the Fibonacci chain, tens or even hundreds of windows can be obtained with the same approximate reflectivity over a very broad spectral region. These mirrors have numerous applications in photonics and optoelectronics.

Figure 2 - Multiple window Fibonacci mirror spectrum and agreement with theory

A novel nonlinear switching device concept based on a nonlinear Fibonacci mirror has been developed. In this approach a multiple window linear Fibonacci mirror is put together in an etalon configuration with a nonlinear Fibonacci (Figure 3). At low intensities the two mirrors are designed to have identical reflectivities and windows. At high intensities, the spectrum of the nonlinear Fibonacci shifts in such a way that it blocks the windows of the linear mirror, leading to a broadband limiter.

Figure 3 - Nonlinear Fibonacci solid etalon schematic

We have formulated composites of lead (II) tetrakis (4-cumylphenoxy) phthalocyanine (PbTCPc) doped into nematic liquid crystal (LC), 40 -pentyl-4-biphenylcarbonitrile (5-CB) (Figure 4) that has received a 90° twisted alignment and investigated the nonlinear transmission properties using both pulsed (Nd:YLF 52 nm, 5 ns) and cw (532 nm) lasers. In the nanosecond regime, this compound is a reverse saturable absorber performing similarly to low-concentration solutions of PbTCPc. Under cw conditions, we observe optically self-activated polarization switching with low threshold input energy. Our results suggest the potential for an all-optical switch working from the nanosecond time scale to cw.

Figure 4 - Lead phthalocyanine doped liquid crystal optical switch

References

C. Sheng, R. A. Norwood, J. Wang, J. Thomas, D. Steeves, B. Kimball, and N. Peyghambarian, "Nonlinear optical transmission of lead phthalocyanine doped nematic liquid crystal composites for multiscale nonlinear switching from nanosecond to continuous wave," Applied Optics 48, 2731 (2009).
D. T. Nguyen, C. Sheng, J. Thomas, R. Norwood, B. Kimball, D. M.. Steeves, and N. Peyghambarian, "Observation of nonlinear transmission enhancement in cavities filled with nonlinear organic materials," Applied Optics 47, 5777 (2008).
C. Sheng, R. A. Norwood, J. Wang, J. Thomas, Y. Wu, Z. Zheng, N. Tabirian, D. M. Steeves, B. R. Kimball, and N. Peyghambarian, " Time resolved studies of photoinduced birefringence in azobenzene dye doped polymer films," Applied Optics 47, 5074 (2008).