Zero-phonon Line Research Articles

Unraveling the Emission Pathways in Copper Indium Sulfide Quantum Dots.

Semiconductor copper indium sulfide quantum dots are emerging as promising alternatives to cadmium- and lead-based chalcogenides in solar cells, luminescent solar concentrators, and deep-tissue bioimaging due to their inherently lower toxicity and outstanding photoluminescence properties. However, the nature of their emission pathways remains a subject of debate. Using low-temperature single quantum dot spectroscopy on core-shell copper indium sulfide nanocrystals, we observe two subpopulations of particles with distinct spectral features. The first class shows sharp resolution-limited emission lines that are attributed to zero-phonon recombination lines of a long-lived band-edge exciton. Such emission results from the perfect passivation of the copper indium sulfide core by the zinc sulfide shell and points to an inversion in the band-edge hole levels. The second class exhibits ultrabroad spectra regardless of the temperature, which is a signature of the extrinsic self-trapping of the hole assisted by defects in imperfectly passivated quantum dots.

First-Principles Predictions of Out-of-Plane Group IV and V Dimers as High-Symmetry, High-Spin Defects in Hexagonal Boron Nitride.

Hexagonal boron nitride (h-BN) has been recently found to host a variety of quantum point defects, which are promising candidates as single-photon sources for solid-state quantum nanophotonic applications. Most recently, optically addressable spin qubits in h-BN have been the focus of intensive research due to their unique potential in quantum computation, communication, and sensing. However, the number of high-symmetry, high-spin defects that are desirable for developing spin qubits in h-BN is highly limited. Here, we combine density functional theory (DFT) and quantum embedding theories to show that out-of-plane XNYi dimer defects (X, Y = C, N, P, and Si) form a new class of stable C3v spin-triplet defects in h-BN. We find that the dimer defects have a robust 3A2 ground state and 3E excited state, both of which are isolated from the h-BN bulk states. We show that 1E and 1A shelving states exist and they are positioned between the 3E and 3A2 states for all the dimer defects considered in this study. To support future experimental identification of the XNYi dimer defects, we provide extensive characterization of the defects in terms of their spin and optical properties. We predict that the zero-phonon line of the spin-triplet XNYi defects lies in the visible range (800 nm to 500 nm). We compute the zero-field splitting of the dimers' spin to range from 1.79 GHz (SiNPi0) to 29.5 GHz (CNNi0). Our results broaden the scope of high-spin defect candidates that would be useful for the development of spin-based solid-state quantum technologies in two-dimensional hexagonal boron nitride.

Spectral tuning of diamond photonic crystal slabs by deposition of a thin layer with silicon vacancy centers

Abstract The controlled extraction of light from diamond optical color centers is essential for their practical prospective applications as single photon sources in quantum communications and as biomedical sensors in biosensing. Photonic crystal (PhC) structures can be employed to enhance the collection efficiency from these centers by directing the extracted light towards the detector. However, PhCs must be fabricated with nanoscale precision, which is extremely challenging to achieve for current materials and nanostructuring technologies. Imperfections inherently lead to spectral mismatch of the extraction (leaky) modes with color center emission lines. Here, we demonstrate a new and simple two-step method for fabricating diamond PhC slabs with leaky modes overlapping the emission line of the silicon vacancy (SiV) centers. In the first step, the PhC structure with leaky modes blue shifted from the SiV emission line is fabricated in a nanocrystalline diamond without SiV centers. A thin layer of SiV-rich diamond is then deposited over the PhC slab so that the spectral position of the PhC leaky modes is adjusted to the emission line of the SiV centers, thereby avoiding the need for nanoscale precision of the structuring method. An intensity enhancement of the zero-phonon line of the SiV centers by a factor of nine is achieved. The color centers in the thin surface layer are beneficial for sensing applications and their properties can also be further controlled by the diamond surface chemistry. The demonstrated PhC tuning method can also be easily adapted to other optical centers and photonic structures of different types in diamond and other materials.

A Novel Red-Emitting Na2NbOF5:Mn4+ Phosphor with Ultrahigh Color Purity for Warm White Lighting and Wide-Gamut Backlight Displays.

In this work, a novel red-emitting oxyfluoride phosphor Na2NbOF5:Mn4+ with an ultra-intense zero-phonon line (ZPL) was successfully synthesized by hydrothermal method. The phase composition and luminescent properties of Na2NbOF5:Mn4+ were studied in detail. The photoluminescence excitation spectrum contains two intense excitation bands centered at 369 and 470 nm, which match well with commercial UV and blue light-emitting diode (LED) chips. When excited by 470 nm blue light, Na2NbOF5:Mn4+ exhibits red light emission dominated by ZPL. Notably, the color purity of the Na2NbOF5:Mn4+ red phosphor can reach 99.9%. Meanwhile, the Na2NbOF5:Mn4+ phosphor has a shorter fluorescence decay time than commercial K2SiF6:Mn4+, which is conducive to fast switching of images in display applications. Profiting from the intense ZPL, white light-emitting diode (WLED) with high color rendering index of Ra = 86.2 and low correlated color temperature of Tc = 3133 K is realized using yellow YAG:Ce3+ and red Na2NbOF5:Mn4+ phosphor. The WLED fabricated using CsPbBr3 quantum dots (QDs) and red Na2NbOF5:Mn4+ phosphor shows a wide color gamut of 127.56% NTSC (National Television Standard Committee). The results show that red-emitting Na2NbOF5:Mn4+ phosphor has potential application prospects in WLED lighting and display backlight.

Low-Temperature Spectroscopic Investigation of Lead-Vacancy Centers in Diamond Fabricated by High-Pressure and High-Temperature Treatment

We report the optical observation of lead-vacancy (PbV) centers in diamond fabricated by Pb ion implantation and subsequent high-temperature annealing (2100 {\deg}C) under high pressure (7.7 GPa). Their optical properties were characterized by photoluminescence at varying temperatures down to 5.7 K. We observed intense emission peaks at 550 and 554 nm with a large splitting of approximately 3900 GHz. The two lines are thought to correspond to the zero phonon line (ZPL) of PbV centers with split ground and excited states. A cubic trend of the ZPL width was observed while varying temperature. We performed polarization measurements of the two lines in a single PbV center, showing nearly orthogonal dipole polarizations. These optical measurements strongly indicate that the PbV center possesses D3d symmetry in the diamond lattice. The observed large ground state splitting significantly suppresses the phonon-mediated transition, which causes decoherence of the electron spin state of the group IV color centers in diamond, expecting a long spin coherence time at a temperature of approximately 8 K.

Extrinsic Effects on the Optical Properties of Surface Color Defects Generated in Hexagonal Boron Nitride Nanosheets.

Hexagonal boron nitride (hBN) is a wide-band gap van der Waals material able to host light-emitting centers behaving as single photon sources. Here, we report the generation of color defects in hBN nanosheets dispersed on different kinds of substrates by thermal treatment processes. The optical properties of these defects have been studied using microspectroscopy techniques and far-field simulations of their light emission. Using these techniques, we have found that subsequent ozone treatments of the deposited hBN nanosheets improve the optical emission properties of created defects, as revealed by their zero-phonon linewidth narrowing and reduction of background emission. Microlocalized color defects deposited on dielectric substrates show bright (≈1 MHz) and stable room-temperature light emission with zero-phonon line peak energy varying from 1.56 to 2.27 eV, being the most probable value 2.16 eV. In addition to this, we have observed a substrate dependence of the optical performance of the generated color defects. The energy range of the emitters prepared on gold substrates is strongly reduced, as compared to that observed in dielectric substrates or even alumina. We attribute this effect to the quenching of low-energy color defects (these of energies lower than 1.9 eV) when gold substrates are used, which reveals the surface nature of the defects created in hBN nanosheets. Results described here are important for future quantum light experiments and their integration in photonic chips.

First-principle study on the influence of common impurities in diamond on the electronic structure of Ce-related defects

This paper determines the stable configuration and electronic structure of Ce-related defects (CeV) in diamonds doped with N, B, and Si impurities using the first-principle method based on density functional theory (DFT) and the Vienna ab-initio simulation package VASP software package. To this end, the zero-phonon line size of the color center of the doped diamond CeV is calculated and the corresponding fluorescence wavelength is measured. The results provide a theoretical explanation of the influence of various impurities on the fluorescence of the CeV color center in diamonds and provides a reference for their fabrication and application.

Temperature dependence of optical centers in 200 keV electron irradiated β-Ga2O3

In this work, the optical centers of β-Ga2O3 irradiated with 200 keV electrons were studied by photoluminescence (PL) spectroscopy. The oxygen vacancy-related center at 690 nm were enhanced after irradiation together with a new zero phonon line (ZPL) at 697 nm. The temperature dependence of these optical centers were carefully detailed and discussed. As the increase of measurement temperature, the ZPLs showed red-shift, intensity quenching and full width at half maximum (FWHM) increase. These data were fitted by the corresponding physical formula. Results showed compared with the 690 nm emission, the 697 nm emission had a similar intensity distribution, a weaker lattice relaxation strength, a weaker lattice vibration, and a lower thermal softness. These results indicated that the 697 nm emission was probably associated with the oxygen interstitials.

Electronic dephasing in mixed quantum–classical molecular systems using the spin-boson model

Pure electronic dephasing is investigated using the spin-boson Hamiltonian in mixed quantum–classical environment. The spin-boson model used here is a composite system made up of a quantum subsystem, an electronic 2-level subsystem linearly coupled to harmonic vibrations, interacting with a classical bath. Experimental results for a multitude of molecular systems indicate that the zero-phonon line (ZPL) profile is determined by electronic dephasing, which is not accounted for in the multimode Brownian oscillator (MBO) model due to the unphysical contribution from the MBO bath modes to the ZPL profile. Mixed quantum–classical dynamics formalism of non-equilibrium systems is employed to assess the contribution of the bath modes to pure electronic dephasing by probing the ZPL profile when coupled to a classical bath in the mixed quantum–classical condensed systems. Pure electronic dephasing is discussed in the context of mixed quantum–classical dynamics formalism which starts with mixed quantum–classical Liouville equation in a mixed quantum–classical environment. It is noteworthy, however, that the fundamental difference between the fully quantum MBO model and the mixed quantum–classical Brownian oscillator, is that the zero-phonon line calculated by the former shows unphysical asymmetry on the low-energy side as it has not been observed in real systems, whereas the ZPL reported herein eliminates this asymmetry. A systematic approach using matrix mechanics is developed to treat this phenomenon. To this end, a closed-form expression of linear and nonlinear optical electronic transition dipole moment time correlation functions in a dissipative media are derived. Linear absorption spectra and 4-wave mixing signals at various temperatures showing a sound thermal broadening, temporal decay, and accurate pure dephasing further ratify the applicability and correctness of the mixed quantum–classical dynamics approach to spectroscopy and dynamics are computed.

Optically Enhanced Electric Field Sensing Using Nitrogen-Vacancy Ensembles

Nitrogen-vacancy (NV) centers in diamond have shown promise as inherently localized electric-field sensors, capable of detecting individual charges with nanometer resolution. Working with NV ensembles, we demonstrate that a detailed understanding of the internal electric field environment enables enhanced sensitivity in the detection of external electric fields. We follow this logic along two complementary paths. First, using excitation tuned near the NV's zero-phonon line, we perform optically detected magnetic resonance (ODMR) spectroscopy at cryogenic temperatures in order to precisely measure the NV center's excited-state susceptibility to electric fields. In doing so, we demonstrate that the characteristically observed contrast inversion arises from an interplay between spin-selective optical pumping and the NV centers' local charge distribution. Second, motivated by this understanding, we propose and analyze a novel scheme for optically-enhanced electric-field sensing using NV ensembles; we estimate that our approach should enable order of magnitude improvements in the DC electric-field sensitivity.

Thermal Properties of Red Zero-Phonon Luminescence Line from Mn2+ Ions in Oxide Crystals

Temperature dependence of emission properties of sharp, zero-phonon line (ZPL) associated with 4T1g →6A1g transition of Mn2+ ions doped in LiMgPO4 is studied, together with the Mn2+-concentration dependence. The ZPL is observed at 611.8 nm at temperatures below 120 K. The intensity relative to the intensity of superimposed phonon sidebands is increased in proportion to T2 with increasing temperature from 18 K. The peak energy and full width at half maximum (FWHM) are increased in proportion to coth 1/2 (hν/2kBT). Unlike the phonon sideband emission which shows single exponential decay and decrease of decay time with increase of temperature, the ZPL emission has the fast and slow decay components and shows temperature-independent decay times. A blue shift of the ZPL peak energy is observed with increase of Mn2+ concentration x doped in LiMg1−x Mn x PO4 from 0.0015 to 0.0050 at 18 K, while a broadening of line width is observed with increase of the concentration. Each of the peak energy and FWHM is approximated by a quadratic equation in the form a+bx+cx 2. It is suggested that these results are understood by the interaction of the excited 4T1g state with phonons and crystal field, from comparison with the theoretically expected formulae.

Full understanding of KNaSiF6:Mn4+ properties under hydrostatic pressure based on the Tanabe−Sugano energy-level diagram

The aim of this study is to make clear the photoluminescence (PL) properties of Mn4+-activated phosphors under the application of hydrostatic pressure (p). A determination of the excited-state zero-phonon line (ZPL) energies in the 3d3-configuration ions is possible from a set of Racah (B and C) and crystal-field splitting parameters (Dq), and vice versa. It has been demonstrated that detailed analyses of PL and PL excitation spectra make possible exact determination of the excited-state ZPL energies, E (2Eg)ZPL and E (4T2g)ZPL, as a function of p. Introducing such determined ZPL energies into a newly proposed set of formulae, we obtain reliable Racah-related parameter values for 3d3 (Mn4+, Cr3+, etc.) activator ions in the red-emitting phosphors. Some examples of analyses are given on the Mn4+-activated KNaSiF6 fluoride phosphor. The present analysis model can be used to obtain broad perspectives and insight for deeply understanding metal−ligand chemistry and related phenomena in the various 3d3 ion-activated phosphors under the application of hydrostatic pressure.

Microscopic Insight into the Inhomogeneous Broadening of Zero-Phonon Lines of GeV– Color Centers in Chemical Vapor Deposition Diamond Films Synthesized from Gaseous Germane

Microscopic Insight into the Inhomogeneous Broadening of Zero-Phonon Lines of GeV^– Color Centers in Chemical Vapor Deposition Diamond Films Synthesized from Gaseous Germane

Theoretical and experimental study of the nitrogen-vacancy center in 4H-SiC

Using first-principles calculations and magnetic resonance experiments, we investigated the physical properties of the negatively charged nitrogen-vacancy (NV) center in 4H-SiC, a promising spin qubit. Our predictive theoretical model in conjunction with experimental measurements reveals a large sensitivity to strain and symmetry. The measured and computed zero-phonon lines (ZPLs) are in agreement and show a consistent trend as a function of the defect location in the crystal. The computed ZPLs are extremely sensitive to the geometrical configurations of the ground and excited states, and large supercells with more than 2 000 atoms are required to obtain accurate numerical results. We find that the computed decoherence time of the basal NV centers at zero magnetic field is substantially larger than that of the axial configurations. Furthermore, at natural nuclear spin abundance and zero field, the Hahn-echo coherence time of one of the basal configurations is similar to that of the axial divacancy in isotopically purified SiC.

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 (VAl-) and +1 charged nitrogen antisite (NAl+) are predicted to achieve SPE with the zero-phonon lines of 0.77 and 1.40 eV, respectively. Moreover, the charged point-defect complex CAlVN+, 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.

Femtosecond-laser-excited luminescence of the A-band in natural diamond and its thermal control

Ultrafast visible-range excitation in natural diamond produces UV-VIS A-band photoluminescence with marginal zero-phonon line and intense regular multi-peak optical-phonon progression. The A-band photoexcitation can occur via band-center, center-center and band-band transitions, being related to two-photon (center-terminated), or interband three-photon and impact-ionization processes. Minor thermal pre-heating of the diamond (< 55 °C) demonstrates strong damping of A-band progression intensities with two different activation energies.

Resolving Jahn-Teller induced vibronic fine structure of silicon vacancy quantum emission in silicon carbide

Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from $n$-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with $1.45\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ and $1.59\phantom{\rule{0.28em}{0ex}}\mathrm{meV}$ energy spacing after H and He irradiation and increase linearly in intensity with fluence---reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect.

ADAQ: Automatic workflows for magneto-optical properties of point defects in semiconductors

Automatic Defect Analysis and Qualification (ADAQ) is a collection of automatic workflows developed for high-throughput simulations of magneto-optical properties of point defects in semiconductors. These workflows handle the vast number of defects by automating the processes to relax the unit cell of the host material, construct supercells, create point defect clusters, and execute calculations in both the electronic ground and excited states. The main outputs are the magneto-optical properties which include zero-phonon lines, zero-field splitting, and hyperfine coupling parameters. In addition, the formation energies are calculated. We demonstrate the capability of ADAQ by performing a complete characterization of the silicon vacancy in silicon carbide in the polytype 4H (4H-SiC).

Investigation on Anomalous Thermal Quenching of Mn4+ Luminescence in A2XF6:Mn4+

Studying luminescence properties of phosphor materials is not only of scientific interest, but also technological importance. Here, we investigate luminescence intensity as a function of temperature for Mn4+-activated fluoride and oxide phosphors. A recent study suggested that an anomalous increase in Mn4+ luminescence intensity (I PL) with increasing lattice temperature (T) in various fluoride phosphors is likely a pitfall caused by a diminishment in the optical path lengths of the spectrofluorometer stemming from lattice thermal expansion. We show that such anomalous enhancement of I PL is due to the increased phonon number that makes it possible to gain the parity and spin-forbidden 2 E g → 4 A 2g transitions rather than an extrinsic effect of lattice thermal expansion. The temperature dependence of the zero-phonon line emission intensity I ZPL may be a good indicator for deciding whether an observed anomaly of I PL is due to an intrinsic or an extrinsic effect because its intensity should in principle not be dependent on T. The experimental data on the Rb2GeF6:Mn4+ fluoride and Cs2WO2F4:Mn4+ oxyfluoride phosphors are presented as a direct verification of this. A discussion is also given on the 2 E g → 4 A 2g transition properties in connection with the inelastic light scattering spectroscopy.

High-pressure synthesis and characterization of diamond from europium containing systems

Experimental studies have been carried out on diamond synthesis in Mg-based systems with additions of europium in various charge states, including Eu0, Eu2+, and Eu3+. The experiments were carried out at a pressure of 7.8 GPa and a temperature of 1800 °C. Metallic Eu, Eu-oxalate, Eu-carbonate, Eu2O3, EuCl3, EuF3, and EuF2 were used as additives. The influence of the content of Eu-containing compounds on diamond crystallization and the photoluminescence characteristics of synthesized diamonds was determined. The addition of europium compounds to Mg-based systems in all cases had an integral inhibitory effect on the diamond crystallization process, which manifested in a decrease in the degree of graphite conversion to diamond, a decrease in the diamond growth rate, and the appearance of metastable graphite. It was found that the inhibiting ability of europium compounds decreased in the following sequence: Eu-oxalate > Eu-carbonate > EuCl3 > Eu2O3 > metallic Eu > EuF3 > EuF2. Spectroscopic studies of the synthesized diamonds revealed a new defect center manifesting in photoluminescence as an optical system with a zero-phonon line structure at 502 nm. The center was tentatively assigned to defects involving Eu impurities and the main factors controlling the formation of the 520 nm center were determined.