Zero-phonon Line Research Articles

Sharp zero-phonon lines of single organic molecules on a hexagonal boron-nitride surface

Single fluorescent molecules embedded in the bulk of host crystals have proven to be sensitive probes of the dynamics in their nano environment, thanks to their narrow (about 30–50 MHz or 0.1–0.2 μeV) optical linewidth of the 0-0 zero-phonon line (0-0 ZPL) at cryogenic temperatures. However, the optical linewidths of the 0-0 ZPL have been found to increase dramatically as the single molecules are located closer to a surface or interface, while no 0-0 ZPL has been detected for single molecules on any surface. Here we study single terrylene molecules adsorbed on the surface of hexagonal boron-nitride (hBN) substrates. Our low-temperature results show that it is possible to observe the 0-0 ZPL of fluorescent molecules on a surface. We compare our results for molecules deposited on the surfaces of annealed and non-annealed hBN flakes and we see a marked improvement in the spectral stability of the emitters after annealing.

Relaxation processes among adiabatic potential energy surfaces of Tl+ and Au− centers in NaCl crystals

We investigated temperature dependencies of the temporal behaviors of luminescence bands attributed to the radiative transitions from adiabatic potential energy surfaces (APESs) of 1T1g, 3T2u+3Eu, 3T1g, and 3A1u of Tl+ and Au− centers in NaCl crystals. In addition to NaCl:Au−, a luminescence band with the sharp zero-phonon luminescence line was observed in NaCl:Tl+, a strong electron-lattice interaction system. A time-resolved study revealed that this luminescence band with the zero-phonon line in NaCl:Tl + exhibited the decay time of 4.3 ms at low temperatures attributed to the radiative transition from 3A1u APES. The characteristic temperature dependences of the decay time constant and luminescence intensity of the luminescence from the 3T1g and 3A1u APESs were discussed based on a configuration coordinate scheme. The intensities and decay time constants of luminescence bands caused by radiative transitions from the 1T1g, 3T2u+3Eu, 3T1g, and 3A1u APESs changed with temperature through thermally activated nonradiative transitions among APESs. The temperature dependencies of luminescence intensities and decay time constants were analyzed using rate equations that comprise radiative and nonradiative transitions from APESs and among APESs, respectively. The activation energies of APESs were estimated from the best-fit results. The characteristics of thermally activated processes among APESs are revealed in weak and strong electron-lattice interaction systems.

All‐Optical Thermometry with Infrared Emitting Defects in Diamond

AbstractDiamond color centers (optically active defects) can be used for all‐optical thermometry for non‐invasive and localized temperature measurements. The visible to near‐infrared photoluminescence of these defects is greatly attenuated in optical fibres and biological samples and therefore limits their use. A color center in Si‐doped diamond with emission coinciding with the O‐band and a major biological transparency window has recently been reported. It has a zero phonon line (ZPL) at 1221 nm and well‐resolved phonon side‐band features. In this work, a strong temperature dependence above 150 K is observed, allowing for accurate temperature measurements up to approximately 420 K. The temperature can be determined via spectral shifts or thermal broadening of the ZPL and through the intensity ratio between the ZPL and the phonon side‐band to a maximum temperature resolution of 0.57 K/. Thermometry using these micro‐diamonds is demonstrated for both electronic and biological applications highlighting their versatility. The potential for further enhancements in sensitivity is discussed.

A Red-Emitting Phosphor Rb2TaF7:Mn4+ for White Light-Emitting Diodes

Herein, a new red phosphor Rb2TaF7:Mn4+ was prepared by the co-precipitation method. Under excitation of the blue light at 468 nm, this phosphor exhibits a series of red-emitting peaks, due to the 2E→4A2 transitions of Mn4+. It is worth mentioning that an intense zero-phonon line (ZPL) of Mn4+ appears at 620 nm in this non-equivalent Mn4+ doping system. A white light-emitting diode (w-LED) based on the Rb2TaF7:Mn4+ (1.0 mol%) phosphor emits bright white light with a color temperature (T c) of 4387 K and a color rendering index (Ra) of 88, showing its potential application in w-LEDs.

Zero-Field ODMR and relaxation of Si-vacancy centers in 6H-SiC

Silicon vacancies in silicon carbide (SiC) have been proposed as interesting candidates for quantum technology applications such as quantum sensing and quantum repeaters. SiC exists in many polytypes with different plane stacking sequences, and in each polytype, the vacancies can occupy a variety of different lattice sites. In this work, we focus on the three important charged silicon vacancies in the 6H-SiC polytype. We record the photoluminescence and continuous-wave optically detected magnetic resonance (ODMR) spectra at different radio-frequency power levels and different temperatures. We individually select the zero-phonon lines of the different silicon vacancies at low temperatures and record the corresponding ODMR spectra. ODMR allows us to correlate optical and magnetic resonance spectra and thereby separate signals from V 1 and V 3. The results also explain the observed sign change of the ODMR signal as a function of temperature.

Germanium-vacancy centers in detonation nanodiamond for all-optical nanoscale thermometry

Nanodiamonds with group-IV color centers, such as silicon-vacancy centers and germanium-vacancy (GeV) centers, exhibit excellent properties, including a sharp and stable zero-phonon line, surface functionalization, and low cytotoxicity. Because the line peak wavelength shifts linearly with the temperature under ambient conditions, the nanodiamonds are promising candidates for all-optical nanoscale thermometry inside a living cell. However, the particles used for temperature measurements have been reported to be larger than a few hundred nanometers. Here, we report temperature sensing using GeV detonation nanodiamonds. The GeV nanodiamonds have a mean particle size of 20 nm. These are the smallest particles among GeV-based particles used in thermometry. The sensitivity of the single GeV centers in the detonation nanodiamond is estimated to be almost consistent with the reported ones of the single GeV centers in bulk diamonds, which can potentially reach sub-kelvin temperature accuracy. The GeV detonation nanodiamonds should function as good photoluminescence probes and intracellular temperature sensors.

Oxygen-vacancy defect in 4H-SiC as a near-infrared emitter: An ab initio study

Optically active spin defects in semiconductors can serve as spin-to-photon interfaces, key components in quantum technologies. Silicon carbide (SiC) is a promising host of spin defects thanks to its wide bandgap and well-established crystal growth and device technologies. In this study, we investigated the oxygen-vacancy complexes as potential spin defects in SiC by means of ab initio calculations. We found that the OCVSi defect has a substantially low formation energy compared with its counterpart, OSiVC, regardless of the Fermi level position. The OCVSi defect is stable in its neutral charge state with a high-spin ground state (S = 1) within a wide energy range near the midgap energy. The zero-phonon line (ZPL) of the OCVSi0 defect lies in the near-infrared regime, 1.11–1.24 eV (1004–1117 nm). The radiative lifetime for the ZPL transition of the defect in kk configuration is fairly short (12.5 ns). Furthermore, the estimated Debye–Waller factor for the optical transition is 13.4%, indicating a large weight of ZPL in the photoluminescence spectrum. All together, we conclude that the OCVSi0 defect possesses desirable spin and optical properties and thus is potentially attractive as a quantum bit.

Optimizing interstitial site luminescence via a sensitizer strategy towards green synthesis of Mn4+-activated fluoride phosphors

High water-soluble KHF2:Mn4+ (KHFM) is a potential precursor instead of K2MnF6 for preparation of Mn4+ doped fluoride materials, but it has a lower Mn4+ concentration in precursor causing a poor luminescence performance. Herein, we reported a Ti4+-doped KHFM phosphor with high luminous efficiency, high thermal stability via a sensitizer strategy. Significantly, the luminescence intensity of KHFM:xTi4+ (x = 0.8%) is increased by 4.6 times, besides, the maximum values of the internal quantum efficiency (IQE) and external quantum efficiency (EQE) increased to 81% and 49%, respectively. Further, the weaker zero phonon line (ZPL) at 621 nm (1.997 eV) is accordingly highlighted with the increase of PL intensity after codoping Ti4+ content. These results indicate that introducing suitable ion to interstitial site as sensitizer can promote the introduction of Mn4+ in ion exchange process. Notably, the obtained KHFM:Ti4+, as a particular green precursor material instead of K2MnF6, develops a novel green synthetic route for preparing Mn4+-activated K2SiF6 fluoride phosphors, and is helpful in developing high-performance warm WLED.

Temperature dependent temporal coherence of metallic-nanoparticle-induced single-photon emitters in a WSe2 monolayer

In recent years, much research has been undertaken to investigate the suitability of two-dimensional materials to act as single-photon sources with high optical and quantum optical quality. Amongst them, transition-metal dichalcogenides, especially WSe2, have been one of the subjects of intensive studies. Yet, their single-photon purity and photon indistinguishability remain the most significant challenges to compete with mature semiconducting systems such as self-assembled InGaAs quantum dots. In this work, we explore the emission properties of quantum emitters in a WSe2 monolayer which are induced by metallic nanoparticles. Under quasi-resonant pulsed excitation, we verify clean single-photon emission with a g (2)(0) = 0.036 ± 0.004. Furthermore, we determine the temperature dependent coherence time via Michelson interferometry, where a value of (13.5 ± 1.0) ps is extracted for the zero-phonon line at 4 K, which reduces to (9 ± 2) ps at 8 K. Associated time-resolved photoluminescence experiments reveal a decrease of the decay time from (2.4 ± 0.1) ns to (0.42 ± 0.05) ns. This change in decay time is explained by a model which considers a Förster-type resonant energy transfer process which yields a strong temperature induced energy loss from the single-photon emitters to the nearby Ag nanoparticle.

Enhanced photon extraction via cone structured waveguide from nitrogen vacancy center in diamond

We present a novel design of optical nanostructure for efficient light extraction from nitrogen vacancy (NV) center in diamond. Numerical simulations of the optimal structure exhibit significant collection efficiency along with a stable radiation profile. We achieved a photon collection efficiency of 70% even with a lower numerical aperture of 0.5. Two different approaches (simple cone and encapsulated cone) have been adopted to reach a high geometrical tolerance without affecting much to the efficiency and emission profile. Besides that, our structure adds an extra benefit in terms of emission enhancement near zero phonon line (ZPL) region of NV spectra. Overall, this cone shaped diamond structure provides a robust, highly efficient and directional emission which can be integrated with a quantum optical network for various applications.

Optical Lines in Europium and Terbium-Activated Yttrium Tantalate Phosphor: Combined Experimental and Group-Theoretical Analysis

The rare-earth ions in crystals such as terbium (YTaO4:Tb3+) and europium (YTaO4:Eu3+)-activated yttrium tantalate phosphors have a number of attractive features that predetermine their crucial role in practical application in contemporary optoelectronic devices. In this article, we employ the group-theoretical arguments aimed to reveal the group-theoretical classification of the crystal field levels and selection rules for the allowed optical transition between the crystal field components of Tb3+ and Eu3+ of the low symmetry crystal field in the activated yttrium tantalate phosphors. We also establish possible polarization rules for the lines corresponding to the allowed transitions. We deduce the symmetry-assisted results for the selection rules in the optical transitions accompanied by the absorption/emission of the vibrational quanta. The selection rules for the vibronic satellites of the zero-phonon lines are expected to be useful for the identification of the lines in the spectra of rare-earth ions with a weak vibronic coupling. The results of the low-temperature measurements of photoluminescence under the 325 nm excitation are in compliance with the group-theoretical analysis. The aim of the paper is to establish symmetry-assisted results that are the background of the quantitative crystal field theory based on the quantum-mechanical consideration.

Spectroscopic properties of Cr2+ ions in Zn1-xMnxSe solid solutions

The spectroscopic properties of Cr2+ ions in a range of cubic Zn1-xMnxSe (x = 0–0.4) solid solutions were investigated. An almost linear decrease in 5E level energy was observed, resulting in both absorption and fluorescence maxima shifts of approximately 30 cm−1 per 10 % of Mn. The temperature dependence of Cr2+ ion decay time was studied, and an increase in nonradiative quenching rate was revealed when Mn concentration (x) increased. The position of the Cr2+ ions zero-phonon line (about 4871 cm−1) in the Zn1-xMnxSe (x = 0.4) crystal was determined.

Computational search for efficient single-photon emitters among the substitutional doping defects in two-dimensional GaSe

Single photon emitters (SPE) in the near-infrared (NIR) spectrum range are critical elements in quantum communication technology. We apply here computational methods to reveal local defects in the GaSe monolayer with properties favorable for such SPEs. Based on an educated guess, we selected twelve defects YX in GaSe, where the Y dopant substitutes the X host atom, X = Ga or Se, and Y = C, Si, Ge, N, P, or As. Our calculations show that two defects, NGa, and PGa, are dynamically unstable. For the remaining defects, we apply the linear response GW method and Bethe-Salpeter equation (BSE) to calculate the electronic structure and optical excitation spectra. Some defects are found to have two sharp peaks (occupied and unoccupied) of the density of the electronic states within the band gap. The NSe-, PSe-, and AsSe-defects have intense sharp an optical excitation peak at energies around 0.8 eV (λ = 1550 nm). Analysis of the formation of the BSE eigenstates associated with these peaks suggests that these excitations can cause a narrow zero-phonon line emission. We thus propose the NSe-, PSe-, and AsSe-defects in the GaSe monolayer as promising SPE in NIR region.

Strain Engineering of the Electronic States of Silicon‐Based Quantum Emitters

AbstractLight‐emitting complex defects in silicon have been considered a potential platform for quantum technologies based on spin and photon degrees of freedom working at telecom wavelengths. Their integration in complex devices is still in its infancy and has been mostly focused on light extraction and guiding. Here the control of the electronic states of carbon‐related impurities (G‐centers) is addressed via strain engineering. By embedding them in patches of silicon on insulator and topping them with SiN, symmetry breaking along [001] and [110] directions is demonstrated, resulting in a controlled splitting of the zero phonon line (ZPL), as accounted for by the piezospectroscopic theoretical framework. The splitting can be as large as 18 meV, and it is finely tuned by selecting patch size or by moving in different positions on the patch. Some of the split, strained ZPLs are almost fully polarized, and their overall intensity is enhanced up to 7 times with respect to the flat areas, whereas their recombination dynamics is slightly affected accounting for the lack of Purcell effect. This technique can be extended to other impurities and Si‐based devices such as suspended bridges, photonic crystal microcavities, Mie resonators, and integrated photonic circuits.

Silicon Vacancy in Boron-Doped Nanodiamonds for Optical Temperature Sensing.

Boron-doped nanodiamonds (BNDs) have recently shown a promising potential in hyperthermia and thermoablation therapy, especially in heating tumor cells. To remotely monitor eigen temperature during such operations, diamond color centers have shown a sensitive optical temperature sensing. Nitrogen-vacancy (NV) color center in diamonds have shown the best sensitivity in nanothermometry; however, spin manipulation of the NV center with green laser and microwave-frequency excitations is still a huge challenge for biological applications. Silicon-vacancy (SiV) color center in nano/bulk diamonds has shown a great potential to be a good replacement of the NV center in diamond as it can be excited and detected within the biological transparency window and its thermometry operations depends only on its zero-phonon line (ZPL) shift as a function of temperature changes. In this work, BNDs were carefully etched on smooth diamond nanocrystals' sharp edges and implanted with silicon for optical temperature sensing. Optical temperature sensing using SiV color centers in BNDs was performed over a small range of temperature within the biological temperature window (296-308 K) with an excellent sensitivity of 0.2 K in 10 s integration time. These results indicate that there are likely to be better application of more biocompatible BNDs in hyperthermia and thermoablation therapy using a biocompatible diamond color center.

Theory of Photoluminescence Spectral Line Shapes of Semiconductor Nanocrystals.

Single-molecule photoluminescence (PL) spectroscopy of semiconductor nanocrystals (NCs) reveals the nature of exciton-phonon interactions in NCs. Understanding the homogeneous spectral line shapes and their temperature dependence remains an open problem. Here, we develop an atomistic model to describe the PL spectrum of NCs, accounting for excitonic effects, phonon dispersion relations, and exciton-phonon couplings. We validate our model using single-NC measurements on CdSe/CdS NCs from T = 4 to 290 K, and we find that the slightly asymmetric main peak at low temperatures is comprised of a narrow zero-phonon line (ZPL) and acoustic phonon sidebands. Furthermore, we identify the specific phonon modes that give rise to the optical phonon sidebands. At temperatures above 200 K, the spectral line width shows a stronger dependence upon the temperature, which we demonstrate to be correlated with higher order exciton-phonon couplings. We also identify the line width dependence upon reorganization energy, NC core sizes, and shell thicknesses.

First-principles study of geometric and electronic structures, and optical transition energies of Mn4+ impurity ions: K2SiF6 as a prototype

In this study, we present the results of first-principles calculations conducted within the density functional theory to investigate the geometric and electronic structures of the ground 4A2 and excited 2E and 4T2 states, as well as the optical transition energies between these states, in the prototype system of K2SiF6:Mn4+. To achieve this, we employed a comprehensive evaluation of the five representative exchange-correlation functionals, enabling the development of a state-of-art calculation scheme that effectively describes the geometric and electronic structures of the excited 2E and 4T2 states. The calculated excitation, emission and zero-phonon line energies of the optical transitions between the ground 4A2 state and the excited 2E and 4T2 states demonstrate a better agreement with experimental results. However, we observed that the conventional and widely-used approach based on the analysis of the electronic density of states diagrams derived from the ground state calculations failed to accurately evaluate the vertical optical transition energies from the 4A2 ground state to the excited 2E and 4T2 states when compared to experimental data. The calculation technique developed in this study holds the potential for widespread application to other combinations of host materials and impurity ion, which is of great importance for future developments in the field of optical materials.

A New Efficient Deep-Red-Emission Phosphor Y3Al4.9Sb0.1O12: Ce3+, Cr3+ with Application Potential in Plant Growth

Aiming at a system of more efficient plant growth, cationic substitutions for garnet-structure host crystal activated by Ce3+ and Cr3+ are investigated. A series of Y3Al5O12: Ce3+-Cr3+, red-emitting materials excited by near-ultraviolet and blue light with a peak emission near 693 nm, are synthesized using a solid-state method. The substitution of Sb3+ for Y3Al5O12: Ce3+, Cr3+ is found to increase the emission intensity of deep red about +80% because it improves the energy transfer from Ce3+ to Cr3+, where yellow emission of Ce3+ is depressed and deep red of Cr3+ increased. It also enhances the intensity-ratio of zero phonon line (ZPL) vs phonon assisted side band. It is induced that the substitution of Sb3+ having strong covalent property influences symmetric Ce3+O2– 6 octahedron, significantly increasing the intensity of 2E→4A2 ZPL of Cr3+ under the resonance energy transfer of Ce3+ to Cr3+. From the ratio of Ce3+ emission intensity in the absence and presence of Cr3+, the energy transfer has a mechanism of dipole-quadrupole interaction. By using Sb3+ substitution and flux AlF3, the original emission intensity of Y3Al5O12: Ce3+, Cr3+ is increased about +183%. The new phosphor has application potential for plant growth in both methods of using LED and converting Sunlight.

A Method of Evaluating NV centers Concentration by UV–Vis Transmittance Spectra

The ultraviolet-visible (UV-Vis) spectrum was not generally considered an available method for measuring the concentration of negatively charged nitrogen-vacancy (NV-, NV center). In this study, we propose the idea of evaluating the concentration of NV center by the slope of the UV-Vis spectrum. To establish this new method we synthesized diamonds with different NV center concentrations by the chemical vapor deposition (CVD) method. We then measured the zero-phonon line (ZPL) intensity of PL spectra at 637 nm and estimated the concentration of the NV centers as a baseline for comparison to our new UV-Vis method. The UV-Vis transmission spectra were then measured and transformed to relative absorption coefficient spectra. Slopes of the transformed UV-Vis spectra between 400 nm and 800 nm were calculated and compared with the ZPL intensities. The results show a strong positive correlation between the slopes of the absorption spectra and the ZPL intensities of PL spectra at 637 nm. Therefore, this paper demonstrates the feasibility of estimating the concentration of the NV centers from measuring the transmission UV-Vis spectrum.

Synthesis and Luminescent Properties of Multicomponent Garnets Y3MgGa3SiO12, Y3MgGa2AlSiO12, and Y3MgGaAl2SiO12 Doped with Cr3+ Ions

Ceramic samples of Y3MgGa3SiO12, Y3MgGa2AlSiO12, and Y3MgGaAl2SiO12 multicomponent garnets doped with 0.2 at % Cr3+ have been obtained by high-temperature solid-state synthesis. In the luminescence spectra of the synthesized garnet samples, overlapping broadband luminescence is observed in the far red spectral region caused by the 4T2 → 4A2 transition in Cr3+ ions, and a narrow band is observed in the range of 690–700 nm, corresponding to the zero-phonon line of the 2Е → 4A2 transition in Cr3+. The narrow-band and broad-band parts of the spectra are attributed to radiation from two different types of chromium centers, which are in octahedral coordination with different distortion degrees and strength of the crystal field. This results from the presence of two ions at the octahedral position of these garnets, which differ significantly in crystal chemical properties, namely, Mg2+ and Ga3+ (Al3+). The studied phosphors, which have broadband luminescence in the phytoactive far red region of the spectrum, have the potential for use in greenhouse LED lamps.