Abstract
Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30–200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900–1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.
Highlights
Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure
Recent advances in materials science have created several new classes of materials, which display a quantum well-like electronic structure but do not require epitaxial growth, including van der Waals two-dimensional materials[30], quasi-two-dimensional perovskites[31], and CQWs2
A combination of detailed analysis of static optical spectra, dynamics, and polarized spectroscopy confirms that the observed transitions are intersubband transitions of electrons from the first electronic shelf to the second electronic shelf of the Colloidal quantum wells (CQWs)
Summary
Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. CQWs are attractive candidates for light-emitting applications, in which they display nearly thermally limited spectral broadening and polarized emission[2,3,4,5,6,7], and as laser gain media, exhibit low-threshold, strong optical gain, and a large gain bandwidth[8,9,10,11,12] These advantageous properties emerge from the interband transitions of CQWs, but the electronic structure of CQWs naturally leads to well-defined electronic subbands[13,14,15]. The discretized electronic structure of epitaxial quantum wells results in narrow intersubband absorption features (between continuous subbands) in photoexcited or doped samples[13,16] These intersubband absorptions have been exploited extensively in mature infrared (IR) technologies, such as quantum cascade lasers and IR detectors[17,18,19], and they have been proposed for use in ultrafast all-optical switching[20,21]. This work demonstrates that intersubband absorption features can be exploited for sub-picosecond optical modulation
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