Person:

Woolf, David

The coupling of macroscopic objects via the optical near-field can generate strong attractive and repulsive forces. Here, I explore the static and dynamic optomechanical interactions that take place in a geometry consisting of a silicon nanomembrane patterned with a square-lattice photonic crystal suspended above a silicon-on-insulator substrate. This geometry supports a hybridized optical mode formed by the coupling of eigenmodes of the membrane and the silicon substrate layer. This system is capable of generating nanometer-scale deflections at low optical powers for membrane-substrate gaps of less than 200 nm due to the presence of an optical cavity created by the photonic crystal that enhances both the optical force and a force that arises from photo-thermal-mechanical properties of the system. Feedback between Brownian motion of the membrane and the optical and photo-thermal forces lead to dynamic interactions that perturb the mechanical frequency and linewidth in a process known as ``back-action.'' The static and dynamic properties of this system are responsible for optical bistability, mechanical cooling and regenerative oscillations under different initial conditions. Furthermore, solid objects separated by a small distance experience the Casimir force, which results from quantum fluctuations of the electromagnetic field (i.e. virtual photons).The Casimir force supplies a strong nonlinear perturbation to membrane motion when the membrane-substrate separation is less than 150 nm. Taken together, the unique properties of this system makes it an intriguing candidate for transduction, accelerometry, and sensing applications.

We present a scheme for obtaining stable Casimir suspension of dielectric nontouching objects immersed in a fluid, validated here in various geometries consisting of ethanol-separated dielectric spheres and semi-infinite slabs. Stability is induced by the dispersion properties of real dielectric (monolithic) materials. A consequence of this effect is the possibility of stable configurations (clusters) of compact objects, which we illustrate via a molecular two-sphere dicluster geometry consisting of two bound spheres levitated above a gold slab. Our calculations also reveal a strong interplay between material and geometric dispersion, and this is exemplified by the qualitatively different stability behavior observed in planar versus spherical geometries.

Selenium supersaturated silicon layers were fabricated by pulsed excimer laser induced liquid-phase mixing of thin Se films on Si(001) wafers. Sufficiently low Se coverage avoids destabilization of rapid epitaxial solidification, resulting in supersaturated solid solutions free of extended defects, as shown by transmission electron microscopy. The amount of retained Se depends on the original film thickness, the laser fluence, and the number of laser pulses irradiating the same spot on the surface. Using this method, Se has incorporated into the topmost 300 nm of the silicon with a concentration of 0.1 at.%. Channeling Rutherford backscattering spectrometry measurements show that the substitutional fraction can be as high as 75% of the total retained Se. These alloys exhibit strong sub-band-gap absorption with optical absorption coefficient ranging up to about 104 cm−1, thus making them potential candidates for applications in Si-based optoelectronic devices.