We study a special ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As heterostructure well that simulates a bimetallic junction between adjacent jellium films. Experimental transmission spectra and complementary calculations show that the two-density, ‘‘bimetallic’’ electron gas in such a well supports three types of collective modes. These evolve with gas width in a way that illustrates the changing roles of subband quantization and nonlocality in the transition from quasi-two-dimensional to three-dimensional collective response. Although comparison of experimental data with detailed calculations shows agreement in the frequencies of strong spectral features, observed and calculated linewidths and resonance strengths disagree. We discuss possible origins of these discrepancies, which may arise in part from fluctuations in the width of the electron layer. © 1996 The American Physical Society.

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