## Mentor: Dr. Chris Sorensen

My project is entitled "adaptive camouflage," and as you can imagine, there are any number of ways that I could take the project in. Over the past few weeks I have investigated several different methods of achieving this goal - I will explain each of them here. My most recent method, and the one that will most likely prevail until the end of the summer, entails nanoparticles and aerosol gels (aerogels).

I. Invisibility Through Metamaterials (Cloaking)

A. Basic Idea: Redirect incident light so that it curves half-way around the masked object before escaping and continuing on its path.

B. Method of Attack: Divert light around object by creating a gradient of the index of refraction (0 at the inner radius to 1 at the outer radius) using Metamaterials.

II. Invisibility Through the Ewald-Oseen Extinction Theorem

A. Basic Idea: Make the Extinction Coefficient equal to zero so that light will not interact with the object.

B. Method of Attack: Collapse the Optical Theorem so that the extinction coefficient is equal to zero.

1. If we wanted to make a material that was invisible, i.e. the extinction coefficient zero, we would have to find a way to make both the scattered and absorbed components be equal to zero.

2. There are several circumstances in which the extinction cross section will go to zero in the Optical Theorem. The one that is most relevant to our research is when

$\medium \mathbf{E}_{0}^{inc}^*\cdot\mathbf{E}_{1}^{sca}(\mathbf{\hat{n}}^{inc})=0$

3. This occurs only when the two elements of the scalar product are orthogonal to one another.

III. Camouflage Through Colloidal Gold Nanoparticles

A. Basic Idea: Create a material with tunable optical properties by controlling the intrinsic properties of colloidal gold nanoparticles.

B. Method of Attack: By taking advantage of the several trends that these gold nanoparticles have, and then controlling them in a flexible environment.

1. As the shell thickness is increased, transmission spectrum is redshifted.

a. This is the case of gold nanoparticles coated with Silicon Dioxide.

2. As the nanoparticles diameter is increased (for D >> 25nm) the Plasmon absorption maximum is redshifted.

3. As the interparticle distance is increased, the transmission spectrum is redshifted.

a. Possibly incorporate gold nanoparticles into a gelatinous medium that can be expanded/contracted on command.

b. Possibly use aerosol gels, since the index of refraction is similar to that of air.