Graduate Student Seminar Scheduled for February 27

The Beckman Institute Spring 2008 Graduate Student Seminar Series continues on Wednesday, February 27 at Noon in Room 1005 at the Beckman Institute. The seminar will feature the following presentations: "Inorganic Photoresist Materials for Direct Fabrication of 3D Photonic Crystals Using Phase Mask Lithography" by Matt George; "The Unstoppable Juggernaut: Electrophysiology reveals the obligatory nature of semantic access" by Sarah Laszlo; and "Exchange Energy in Triple Quantum Dots: A Variational Monte Carlo Approach" by Jihan Kim. The hour-long seminar is open to the public and a pizza lunch will be served.

Inorganic Photoresist Materials for Direct Fabrication of 3D Photonic Crystals Using Phase Mask Lithography
Matt George

Direct fabrication of inorganic photonic crystals was achieved by using conformable phase mask elements to generate high contrast, 3D periodic interference patterns within two markedly different inorganic photoresists. Photonic crystals of varying lattice parameter and complexity were fabricated by varying the phase mask pitch and exposure wavelength and by utilizing both linear and 2-photon absorption. Photonic crystal structures were characterized via SEM and compared to the phase mask interference pattern predicted by RCWA. The two inorganic photoresists, one oxide based and the other a chalcogenide glass, have competing strengths. The chemically amplified oxide resist is based on the acid catalyzed condensation of poly(methyl silsesquioxane) end groups. We have demonstrated the oxide resists compatibility with nanoimprint interference lithography where the surface of the photoresist film is embossed to form the phase shifting element. After calcination, the silsesquioxane is thermally stable, allowing for infiltration with high index of refraction materials. The chalcogenide glass based photoresist is composed of As<sub>2</sub>S<sub>3</sub>. This material already has a high index of refraction, making further template infiltration and removal processing steps unnecessary. As<sub>2</sub>S<sub>3</sub> is also highly nonlinear with excellent sensitivity to pulsed near-IR radiation, allowing us to directly pattern high quality photonic crystals using multiphoton interference lithography.

The Unstoppable Juggernaut: Electrophysiology reveals the obligatory nature of semantic access
Sarah Laszlo

How does the sensory experience of written stimuli result in the experience of meaning in the brain? Past theories suggest that attempts at semantic access are gated by stimulus characteristics such as spelling-to-sound consistency or familiarity. However, past studies have not fully crossed the factors of consistency and familiarity and therefore have been unable to assess the independent contributions made to visual word recognition by those two factors. We completed this cross in a series of behavioral and electrophysiological experiments using familiar acronymsitems which are highly inconsistent as well as highly familiar. Our results suggest that written stimuli do not have to pass a filter on consistency or familiarity in order to cue the language processing system to attempt semantic access, but rather that semantic access is obligatorily attempted for all written stimuli.

Exchange Energy in Triple Quantum Dots: A Variational Monte Carlo Approach
Jihan Kim

Quantum dots are artificial zero-dimensional systems containing quantization in both the electron charge and the energy. Previous works indicate that coupled quantum dots are seen as promising nanostructures for realizing quantum gates, which are the basic elements of a quantum computer. In quantum dots, fundamental quantum logic operations (e.g. control-NOT operation) can be achieved by controlling the entanglement between electron spins by means of external electric and magnetic fields. The entanglement between the spins or qubits can be quantified by calculating the exchange energy, which is the energy difference between the lowest triplet and the singlet states. Proper operation of a quantum gate largely depends on the ability to control and to modulate the exchange energy with external voltages and magnetic fields. We use a variational Monte Carlo method to calculate the total energies of the two-electron system in a triple quantum dot structure. We break the symmetry of the potential and observe its effect on the behavior of the exchange energy.