Spectroscopic ellipsometry is used to characterize the optical response of a series of ultrathin silicon-on-insulator (SOI) films with thickness ranging from 50 to 2 nm. Due to the potential barrier formed by the buried oxide layer and the interface with air, the electrons in the top silicon layer of the thinner samples experience dimensional confinement effects. Transition energies were extracted for the E-1 and E-2 critical points using reciprocal space analysis. This method offers improved accuracy over conventional regression-based techniques, such as line shape fitting, by eliminating error due to correlation among multiple fit parameters. It is shown that dimensional confinement increases the E-1 transition energy of similar to 3.375 eV for bulk silicon to 3.38, 3.393, 3.42, and 3.45 eV for the similar to 10, similar to 7, similar to 4, and similar to 2 nm SOI films, respectively. This trend resembles the 1/L-2 law predicted by quantum mechanical considerations for a confined particle. The dependence of these critical point transitions is investigated as a function of film thickness L. Additionally, the E-2 critical point transition energy is found to be much less affected by quantum confinement, contrary with theoretical predictions. (c) 2006 American Vacuum Society.