Fig. 1. (Color online) The mechanism of phonon-assisted upconversion. (a) Stokes and anti-Stokes (AS) photoluminescence processes are represented by the energy structure of electronic and vibrational levels for color centers. (b) The Simplified Model of Optical Cooling. (a) Reproduced from Ref. [10], CC BY 4.0. (b) Reprinted with permission from Ref. [40]. Copyright © 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim.

Fig. 2. (Color online) Phonon-assisted upconversion photoluminescence in diamond. (a) Cooling of an NV-doped diamond suspended in vacuum. The cooling temperature as a function of quantum efficiency. Here it is assumed that non-radiative process heats the diamond. (b) The temperature dependence of intensity under Stokes and anti-Stokes excitation for SiV^– centers. (c) Anti-Stokes PLE spectra of SiV^– centers (ZPL: 738 nm) at room temperature. (d) Characterization of the anti-Stokes-based nanothermometer: the relative sensitivity versus temperature for several different systems. (a) Reprinted with permission from Ref. [24]. Copyright © 2017, American Physical Society. (b) and (c) Reprinted with permission from Ref. [25]. Copyright © 2018, American Chemical Society. (d) Reproduced from Ref. [10], CC BY 4.0.

Fig. 3. (Color online) Photoluminescence upconversion in low dimensional materials. (a) The photoluminescence excitation spectrum of the integrated ZPL (565 nm). The green dots and red squares correspond to Stokes and anti-Stokes excitations, respectively. The suppression of spectral diffusion under anti-Stokes excitation: the PL time series of a single emitter under (b) Stokes excitation (532 nm) and (c) anti-Stokes excitation (637 nm), respectively. The optical cooling of QDs. (d) The temperature changes of the QD sample under different excitation wavelengths. (e) The upconversion photoluminescence mechanism of QDs. (a) Reproduced from Ref. [43]. Copyright © 2018, American Chemical Society. (b) and (c) Reprinted with permission from Ref. [32]. Copyright © 2019, AIP Publishing. (d) and (e) Reproduced from Ref. [64], CC BY 4.0.

Fig. 4. (Color online) Quantum sensing under anti-Stokes excitation. (a) Charge state manipulation of NV centers: the charge state conversion process between NV⁰ and NV^– under different excitation wavelengths. (b) The decay process of NV^– centers at 4 K under anti-Stokes excitation. Robust coherent control of spin qubits using anti-Stokes excitation. (c) The excitation schemes of V_si center under Stokes excitation and anti-Stokes excitation. (d) Stokes and anti-Stokes excited ODMR of V_si centers for different external magnetic fields. (a) and (b) Reprinted with permission from Ref. [7]. Copyright © 2022, American Chemical Society. (c) and (d) Reproduced from Ref. [72], CC BY 4.0.

Fig. 5. (Color online) The single-photon source under anti-Stokes excitation. (a) Dual-resonance enhanced X-CX transition for highly pure single-photon emission. Temperature-dependent PL mapping of a QD in cavity. (b) Hanbury Brown and Twiss (HBT) measurements of single-photon purity under the dual-resonance enhanced intra-dot excitation. (c) Dual resonances enhanced upconverted excitation. (a)–(c) Reproduced from Ref. [38], CC BY 4.0.