Phonon-Assisted Photoluminescence Up-Conversion of Silicon-Vacancy Centers in Diamond

Photo-Induced Charge State Dynamics of the Neutral and Negatively Charged Silicon Vacancy Centers in Room-Temperature Diamond

The silicon vacancy (SiV) center in diamond is drawing much attention due to its optical and spin properties, attractive for quantum information processing and sensing. Comparatively little is known, however, about the dynamics governing SiV charge state interconversion mainly due to challenges associated with generating, stabilizing, and characterizing all possible charge states, particularly at room temperature. Here, multi-color confocal microscopy and density functional theory are used to examine photo-induced SiV recombination - from neutral, to single-, to double-negatively charged - over a broad spectral window in chemical-vapor-deposition (CVD) diamond under ambient conditions. For the SiV0 to SiV- transition, a linear growth of the photo-recombination rate with laser power at all observed wavelengths is found, a hallmark of single photon dynamics. Laser excitation of SiV‒, on the other hand, yields only fractional recombination into SiV2‒, a finding that is interpreted in terms of a photo-activated electron tunneling process from proximal nitrogen atoms.

Phonon-Assisted Upconversion Photoluminescence of a Self-Trapped Exciton in the Rb2CuCl3 Single Crystal

Phonon-assisted upconversion photoluminescence (UCPL) plays an important role in a wide range of fields such as optical refrigeration, sensitive optical thermometry, quantum state control, and upconversion optoelectronics. High photoluminescence quantum yield (PLQY) and strong electron-phonon coupling are two basic prerequisites of efficient UCPL materials. The self-trapped exciton (STE) system with the above-mentioned advantages hints that it may be a good candidate for phonon-assisted UCPL. Here, we synthesized Rb₂CuCl₃ single crystals (SCs) which yield a high PLQY of the STE emission at 400 nm, and an efficient phonon-assisted UCPL was demonstrated at room temperature. By exponentially fitting the intensity of temperature-dependent UCPL spectra, we obtained an optical thermometry sensitivity of SCs up to 6 mK at 295 K. We also propose that net cooling would be possible if the PLQY is improved up to 91.5% with 345 nm excitation. Our results open a new door to explore laser cooling in STE systems.

Point Defects in Monolayer h-AlN as Candidates for Single-Photon Emission

A single-photon emission (SPE) system based on a solid state is one of the fundamental branches in quantum information and communication technologies. The traditional bulk semiconductors suffered limitations of difficult photon extraction and long radiative lifetime. Two-dimensional (2D) semiconductors with an entire open structure and low dielectric screening can overcome these shortcomings. In this work, we focus on monolayer h-AlN due to its wide band gap and the successful achievement of SPE compared to its bulk counterpart. We systematically investigate the properties of point defects, including vacancies, antisites, and impurities, in monolayer h-AlN by employing hybrid density functional theory calculations. The -1 charged Al vacancy (V_Al^-) and +1 charged nitrogen antisite (N_Al⁺) are predicted to achieve SPE with the zero-phonon lines of 0.77 and 1.40 eV, respectively. Moreover, the charged point-defect complex C_AlV_N⁺, which is composed of vacancies and carbon substitutions, also can be used for SPE. Our results extend the avenue for realizing SPE in 2D semiconductors.

Phonon-assisted up-conversion photoluminescence of quantum dots

Phonon-assisted up-conversion photoluminescence can boost energy of an emission photon to be higher than that of the excitation photon by absorbing vibration energy (or phonons) of the emitter. Here, up-conversion photoluminescence power-conversion efficiency (power ratio between the emission and excitation photons) for CdSe/CdS core/shell quantum dots is observed to be beyond unity. Instead of commonly known defect-assisted up-conversion photoluminescence for colloidal quantum dots, temperature-dependent measurements and single-dot spectroscopy reveal the up-conversion photoluminescence and conventional down-conversion photoluminescence share the same electron-phonon coupled electronic states. Ultrafast spectroscopy results imply the thermalized excitons for up-conversion photoluminescence form within 200 fs, which is 100,000 times faster than the radiative recombination rate of the exciton. Results suggest that colloidal quantum dots can be exploited as efficient, stable, and cost-effective emitters for up-conversion photoluminescence in various applications.

Up-conversion photoluminescence in colloidal quantum dots is generally believed to be mediated by thermal activation from defect states. Here, the authors reveal that highly-efficient up-conversion photoluminescence instead is related to electron-phonon coupling.

Quantum Point Defects for Solid-State Laser Refrigeration

Herein, the role that point defects have played over the last two decades in realizing solid-state laser refrigeration is discussed. A brief introduction to the field of solid-state laser refrigeration is given with an emphasis on the fundamental physical phenomena and quantized electronic transitions that have made solid-state laser-cooling possible. Lanthanide-based point defects, such as trivalent ytterbium ions (Yb³⁺ ), have played a central role in the first demonstrations and subsequent development of advanced materials for solid-state laser refrigeration. Significant discussion is devoted to the quantum mechanical description of optical transitions in lanthanide ions, and their influence on laser cooling. Transition-metal point defects have been shown to generate substantial background absorption in ceramic materials, decreasing the overall efficiency of a particular laser refrigeration material. Other potential color centers based on fluoride vacancies with multiple potential charge states are also considered. In conclusion, novel materials for solid-state laser refrigeration, including color centers in diamond that have recently been proposed to realize the solid-state laser refrigeration of semiconducting materials, are discussed.

Robust coherent control of solid-state spin qubits using anti-Stokes excitation

Optically addressable solid-state color center spin qubits have become important platforms for quantum information processing, quantum networks and quantum sensing. The readout of color center spin states with optically detected magnetic resonance (ODMR) technology is traditionally based on Stokes excitation, where the energy of the exciting laser is higher than that of the emission photons. Here, we investigate an unconventional approach using anti-Stokes excitation to detect the ODMR signal of silicon vacancy defect spin in silicon carbide, where the exciting laser has lower energy than the emitted photons. Laser power, microwave power and temperature dependence of the anti-Stokes excited ODMR are systematically studied, in which the behavior of ODMR contrast and linewidth is shown to be similar to that of Stokes excitation. However, the ODMR contrast is several times that of the Stokes excitation. Coherent control of silicon vacancy spin under anti-Stokes excitation is then realized at room temperature. The spin coherence properties are the same as those of Stokes excitation, but with a signal contrast that is around three times greater. To illustrate the enhanced spin readout contrast under anti-Stokes excitation, we also provide a theoretical model. The experiments demonstrate that the current anti-Stokes excitation ODMR approach has promising applications in quantum information processing and quantum sensing.

Optically detected magnetic resonance of defect spins typically relies on Stokes excitation, in which the excitation energy is larger than that of the emitted photon. Here, the authors use the opposite regime of anti-Stokes excitation to detect Si vacancy spins in SiC, with a threefold improvement in signal contrast.

Humidity Sensors Realized via Negative Photoconductivity Effect in Nanodiamonds

Herein, the negative photoconductivity (NPC) effect has been observed in nanodiamonds (NDs) for the first time, and with illumination under a 660 nm laser lamp, the conductivity of the NDs decreases significantly. The NPC effect has been attributed to the trapping of carriers by the absorbed water molecules on the ND surfaces. A humidity sensor has been constructed based on the NPC effect of the NDs, and the sensitivity of the sensor can reach 10⁶%, which is the highest value ever reported for carbon-based humidity sensors.

Mechanism of Single-Photon Upconversion Photoluminescence in All-Inorganic Perovskite Nanocrystals: The Role of Self-Trapped Excitons

The efficient single-photon upconversion photoluminescence (UCPL) feature of lead halide perovskite semiconductors makes it promising for developing laser cooling devices. This is an attractive potential application, but the underlying physics still remains unclear so far. By using the all-inorganic CsPbX₃ (X = Br, I) nanocrystal samples, this phenomenon was investigated by photoluminescence (PL) and time-resolved PL under different temperatures and various excitation conditions. A broad emission band located at the low-energy side of the free exciton (FE) peak was detected and deduced to be from the self-trapped exciton (STE). The lifetime of STE emission was found to be 171 ns at 10 K, much longer than that of FE. The UCPL phenomenon was then attributed to thermal activation of transformation from STEs to FEs, and the energy barrier was derived to be 103.7 meV for CsPbBr₃ and 45.2 meV for CsPb(Br/I)₃, respectively. The transformation also can be seen from the fluorescence decay processes.