| Popis: |
Laser cooling of matter through anti‐Stokes photoluminescence, where the emitted frequency of light exceeds that of the impinging laser light by virtue of absorption of thermal vibrational energy, has been successfully realized in condensed media, and in particular with rare‐earth‐doped systems achieving sub‐100 K solid‐state optical refrigeration. Studies suggest that laser cooling in semiconductors has the potential of achieving temperatures down to ≈10 K and that its direct integration can usher in unique high‐performance nanostructured semiconductor devices. While laser cooling of nanostructured II–VI semiconductors has been reported recently, laser cooling of indirect bandgap semiconductors such as group IV silicon and germanium remains a major challenge. Herein, the anomalous observation of dominant anti‐Stokes photoluminescence in germanium nanocrystals principally associated with plasmon coupling is reported. Specifically, this Raman anomaly to the confluence of ultrahigh‐purity nanocrystal germanium, generation of high density of electron–hole plasma, the inherent degeneracy of longitudinal and transverse optical phonons in nonpolar indirect bandgap semiconductors, and simultaneous spatial confinement effects are attributed. At high laser intensities, plasmon‐assisted laser cooling with lattice temperature as low as ≈50 K is inferred. |
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