Phonon Sidebands Research Articles

Reducing Phonon-Induced Decoherence in Solid-State Single-Photon Sources with Cavity Quantum Electrodynamics.

Solid-state emitters are excellent candidates for developing integrated sources of single photons. Yet, phonons degrade the photon indistinguishability both through pure dephasing of the zero-phonon line and through phonon-assisted emission. Here, we study theoretically and experimentally the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity as a function of temperature. We show that a large coupling to a high quality factor cavity can simultaneously reduce the effect of both phonon-induced sources of decoherence. It first limits the effect of pure dephasing on the zero-phonon line with indistinguishabilities above 97% up to 18K. Moreover, it efficiently redirects the phonon sidebands into the zero-phonon line and brings the indistinguishability of the full emission spectrum from 87% (24%) without cavity effect to more than 99% (76%) at 0K (20K). We provide guidelines for optimal cavity designs that further minimize the phonon-induced decoherence.

Probing Electron-Phonon Interaction through Two-Photon Interference in Resonantly Driven Semiconductor Quantum Dots.

We investigate the temperature dependence of photon coherence properties through two-photon interference (TPI) measurements from a single quantum dot (QD) under resonant excitation. We show that the loss of indistinguishability is related only to the electron-phonon coupling and is not affected by spectral diffusion. Through these measurements and a complementary microscopic theory, we identify two independent separate decoherence processes, both of which are associated with phonons. Below 10K, we find that the relaxation of the vibrational lattice is the dominant contribution to the loss of TPI visibility. This process is non-Markovian in nature and corresponds to real phonon transitions resulting in a broad phonon sideband in the QD emission spectra. Above 10K, virtual phonon transitions to higher lying excited states in the QD become the dominant dephasing mechanism, this leads to a broadening of the zero phonon line, and a corresponding rapid decay in the visibility. The microscopic theory we develop provides analytic expressions for the dephasing rates for both virtual phonon scattering and non-Markovian lattice relaxation.

Nonmagnetic Quantum Emitters in Boron Nitride with Ultranarrow and Sideband-Free Emission Spectra.

Hexagonal boron nitride (hBN) is an emerging material in nanophotonics and an attractive host for color centers for quantum photonic devices. Here, we show that optical emission from individual quantum emitters in hBN is spatially correlated with structural defects and can display ultranarrow zero-phonon line width down to 45 μeV if spectral diffusion is effectively eliminated by proper surface passivation. We demonstrate that undesired emission into phonon sidebands is largely absent for this type of emitter. In addition, magneto-optical characterization reveals cycling optical transitions with an upper bound for the g-factor of 0.2 ± 0.2. Spin-polarized density functional theory calculations predict possible commensurate transitions between like-spin electron states, which are in excellent agreement with the experimental nonmagnetic defect center emission. Our results constitute a step toward the realization of narrowband quantum light sources and the development of spin-photon interfaces within 2D materials for future chip-scale quantum networks.

Spectroscopic investigations and phonon side band analysis of Eu3+-doped multicomponent tellurite glasses

Novel and optically efficient multicomponent tellurite glasses doped with Eu3+ ions were prepared by melt quenching procedure. Absorption, photoluminescence excitation, emission and Raman spectra analyses of the Eu3+-doped glasses have been carried out. Spectroscopic studies were performed with the Judd-Ofelt and phonon side band analyses. The local vibrational mode around Eu3+ ions and the phonon energy of the prepared tellurite glasses was estimated by the phonon side band (PSB) associated with 7F0→5D2 transition of Eu3+ ion. Both the phonon energy (ћω) and the electron-phonon coupling constant (g) were derived from the phonon side band spectrum. Radiative properties such as spontaneous transition probabilities (AT), radiative lifetime (τR) and luminescence branching ratio (βR) ratio were calculated for different excited states. Five pronounced peaks have appeared in the photoluminescence spectra (PL) as a result of the transitions from the metastable 5D0 to the ground states 7FJ (J = 0 to 4) of Eu3+ ions. The red Eu3+ emission at 612 nm corresponding to the 5D0→7F2 transition is of particular importance and its emission intensity is observed to increase significantly with growing Eu3+ ion concentration, suggesting that these multicomponent tellurite glasses are suitable candidates for red laser source applications.

Bright optical centre in diamond with narrow, highly polarised and nearly phonon-free fluorescence at room temperature

Using shallow implantation of ions and molecules with masses centred at 27 atomic mass units (amu) in diamond, a new artificial optical centre with unique properties has been created. The centre shows a linearly polarised fluorescence with a main narrow emission line mostly found at 582 nm, together with a weak vibronic sideband at room temperature. The fluorescence lifetime is ∼2 ns and the brightest centres are more than three times brighter than the nitrogen-vacancy centres. A majority of the centres shows stable fluorescence whereas some others present a blinking behaviour, at faster or slower rates. Furthermore, a second kind of optical centre has been simultaneously created in the same diamond sample, within the same ion implantation run. This centre has a narrow zero-phonon line (ZPL) at ∼546 nm and a broad phonon sideband at room temperature. Interestingly, optically detected magnetic resonance (ODMR) has been measured on several single 546 nm centres and two resonance peaks are found at 0.99 and 1.27 GHz. In view of their very similar ODMR and optical spectra, the 546 nm centre is likely to coincide with the ST1 centre, reported once (with a ZPL at 550 nm), but of still unknown nature. These new kinds of centres are promising for quantum information processing, sub-diffraction optical imaging or use as single-photon sources.

Stimulated emission from nitrogen-vacancy centres in diamond

Stimulated emission is the process fundamental to laser operation, thereby producing coherent photon output. Despite negatively charged nitrogen-vacancy (NV−) centres being discussed as a potential laser medium since the 1980s, there have been no definitive observations of stimulated emission from ensembles of NV− to date. Here we show both theoretical and experimental evidence for stimulated emission from NV− using light in the phonon sidebands around 700 nm. Furthermore, we show the transition from stimulated emission to photoionization as the stimulating laser wavelength is reduced from 700 to 620 nm. While lasing at the zero-phonon line is suppressed by ionization, our results open the possibility of diamond lasers based on NV− centres, tuneable over the phonon sideband. This broadens the applications of NV− magnetometers from single centre nanoscale sensors to a new generation of ultra-precise ensemble laser sensors, which exploit the contrast and signal amplification of a lasing system.

Resonant optical spectroscopy and coherent control ofCr4+spin ensembles in SiC and GaN

Spins bound to point defects are increasingly viewed as an important resource for solid-state implementations of quantum information technologies. In particular, there is a growing interest in the identification of new classes of defect spin that can be controlled optically. Here we demonstrate ensemble optical spin polarization and optically detected magnetic resonance (ODMR) of the S = 1 electronic ground state of chromium (Cr4+) impurities in silicon carbide (SiC) and gallium nitride (GaN). Spin polarization is made possible by the narrow optical linewidths of these ensembles (< 8.5 GHz), which are similar in magnitude to the ground state zero-field spin splitting energies of the ions at liquid helium temperatures. We therefore are able to optically resolve individual spin sublevels within the ensembles at low magnetic fields using resonant excitation from a cavity-stabilized, narrow-linewidth laser. Additionally, these near-infrared emitters possess exceptionally weak phonon sidebands, ensuring that > 73% of the overall optical emission is contained with the defects zero-phonon lines. These characteristics make this semiconductor-based, transition metal impurity system a promising target for further study in the ongoing effort to integrate optically active quantum states within common optoelectronic materials.

Temperature effect on the emission spectra of narrow band Mn4+ phosphors for application in LEDs.

Temperature dependence of the luminescence shape and decay time of narrow band Mn4+ fluoride phosphors: Rb2GeF6:Mn4+ and KNaSiF6:Mn4+ was investigated. The temperature changes in the relative intensity between the zero-phonon line and both phonon sidebands were observed in both samples. The sideband spectra consist of three lines related to interaction with three different phonon modes labeled ν3, ν4 and ν6. We present a comprehensive quantum theory which allows calculation of the luminescence intensity and the luminescence lifetime by simultaneously taking into account odd parity crystal fields, odd parity phonon modes and spin-orbit coupling. Since we include all modes, for which the respective interaction strengths and energies of the phonons are known, our approach does not involve any free parameters. We also discuss our results in relation to the temperature dependence of the lifetime of the 2Eg → 4A2g transition, taking into account the quantum efficiency of the system and the migration of the excitation energy. The presented model is applicable to all materials doped with Mn4+ ions and also to any narrow line emitting phosphor, where a zero-phonon line and phonon structure is simultaneously observed in the emission spectrum.

Microscopic modeling of the effect of phonons on the optical properties of solid-state emitters

Understanding the effect of vibrations in optically active nano systems is crucial for successfully implementing applications in molecular-based electro-optical devices, quantum information communications, single photon sources, and fluorescent markers for biological measurements. Here, we present a first-principles microscopic description of the role of phonons on the isotopic shift presented in the optical emission spectrum associated to the negatively charged silicon-vacancy color center in diamond. We use the spin-boson model and estimate the electron-phonon interactions using a symmetrized molecular description of the electronic states and a force-constant model to describe molecular vibrations. Group theoretical arguments and dynamical symmetry breaking are presented in order to explain the optical properties of the zero-phonon line and the isotopic shift of the phonon sideband.

Facile synthesis and luminescence studies of nanocrystalline red emitting Cr:ZnAl2O4 phosphor

A simple co-precipitation synthesis of Cr:ZnAl2O4 nanoparticles is reported for the first time. The materials were characterized by XRD, FE-SEM, HR-TEM, FT-IR, Diffuse reflectance and Photoluminescence spectroscopy. Annealing increased the transparency and material colour changed from green (500°C) to pink (1200°C) owing to the diffusion of Cr3+ into the ZnAl2O4 matrix. The intensity of two broad bands (540nm, 400nm) in the excitation spectra increased with annealing temperature. The asymmetric band at 400nm comprised of two bands attributed to the trigonal distortion. Cr3+ emissions obtained, comprised of a zero phonon line (R line) and multi phonon side bands on either side. Higher Dq/B (2.96), confirmed Cr3+ ions in a strong crystal field and thus the photoluminescence emission could be associated with spin-parity forbidden transitions. Photoluminescence lifetimes ranged from 20 to 30ms for samples annealed at higher temperatures. The reported work can be extended to other phosphors for bioimaging, waveguides and sensors.

Synthesis of SiV-diamond particulates via the microwave plasma chemical deposition of ultrananocrystalline diamond on soda-lime glass fibers

We report the synthesis of silicon-vacancy (SiV) incorporated spherical shaped ultrananocrystalline diamond (SiV-UNCD) particulates (size ∼1 μm) with bright luminescence at 738 nm. For this purpose, different granular structured polycrystalline diamond films and particulates were synthesized by using three different kinds of growth plasma conditions on the three types of substrate materials in the microwave plasma enhanced CVD process. The grain size dependent photoluminescence properties of nitrogen vacancy (NV) and SiV color centers have been investigated for different granular structured diamond samples. The luminescence of NV center and the associated phonon sidebands, which are usually observed in microcrystalline diamond and nanocrystalline diamond films, were effectively suppressed in UNCD films and UNCD particulates. Micron sized SiV-UNCD particulates with bright SiV emission has been attained by transfer of SiV-UNCD clusters on soda-lime glass fibers to inverted pyramidal cavities fabricated on Si substrates by the simple crushing of UNCD/soda-lime glass fibers in deionized water and ultrasonication. Such a plasma enhanced CVD process for synthesizing SiV-UNCD particulates with suppressed NV emission is simple and robust to attain the bright SiV-UNCD particulates to employ in practical applications.

Luminescence and phonon side band analysis of Eu3+-doped lead fluorosilicate glasses

Lead fluorosilicate (SPbKNLFEu) glasses doped with different concentrations of Eu3+ ions have been prepared by the melt quenching technique. The structural and spectroscopic analysis have been carried out by Raman, absorption, excitation, emission, phonon side band (PSB) spectra and decay time measurements. The Judd-Ofelt theory has been used to predict the radiative properties for the emission levels of Eu3+ ions. Local structure around the Eu3+ ions and the phonon energy of SPbKNLFEu glasses have been confirmed on the basis of PSB associated with the 7F0 → 5D2 transition. The decay curves of the 5D0 and 5D1 levels exhibit single exponential nature with a lifetime of 2240 μs and 20 μs, respectively. The multiphonon relaxation rates (Wmp) from the excited levels to the next lower level of Eu3+ ions have been calculated. The higher stimulated emission cross-section and the strong red emission at 613 nm corresponding to the 5D0 → 7F2 transition suggests that the present lead fluorosilicate glasses could be useful for the optical display devices.

Energy transfer rate and electron-phonon coupling properties in Eu3+ -doped nanophosphors.

A series of controllable emissions SrWO4 :Eu3+ and charge-compensated SrWO4 : Eum3+ (m=0.01 or 0.20) phosphors was successfully prepared via a simple co-precipitation method. The energy transfer mechanism was studied based on the Huang's theory. A low magnitude of Huang-Rhys factor (10-2 ) was calculated using phonon sideband spectra. The Judd-Ofelt parameters Ωλ (λ=2, 4 and 6) of Eu3+ -activated SrWO4 doped with charge compensation were obtained. The calculated Commission Internationale de l'Eclairage chromaticity coordinates were found to be about (0.67, 0.33) for SrWO4 : Eu0.203+ and charge-compensated SrWO4 : Eu0.203+ phosphors, which coincided with the National Television Standard Committee system standard values for red. A white light emission was obtained under 362nm excitation. The correlated color temperature was computed by a simple equation to characterize light sources. Thus, warm white light-emitting diodes with higher Ra can be constructed by combining as-prepared high efficiency, low correlated color temperature and high color purity phosphor.

Structural and optical characterization of Eu3+ doped polymer-zirconia composites

Eu3+ doped zirconia-poly ethylene glycol (ZrO2-PEG) composites were synthesized by non-hydrolytic sol-gel method. The structural and photoluminescence characteristics of the as-prepared samples were investigated by XRD, TGA-DTA, FTIR, Raman spectroscopy, excitation and emission spectroscopy. The coupling between vibrational modes and electronic transitions was studied by phonon side band (PSB) analysis. The vibrational modes found in the Raman and FTIR spectra are closely related to those in the PSB spectra. The role of dopant concentration on the emission properties of ZrO2-PEG: Eu3+ samples was studied and a concentration quenching of PL emission was also observed.

Coupling of Excitons and Discrete Acoustic Phonons in Vibrationally Isolated Quantum Emitters.

The photoluminescence emission by mesoscopic condensed matter is ultimately dictated by the fine-structure splitting of the fundamental exciton into optically allowed and dipole-forbidden states. In epitaxially grown semiconductor quantum dots, nonradiative equilibration between the fine-structure levels is mediated by bulk acoustic phonons, resulting in asymmetric spectral broadening of the excitonic luminescence. In isolated colloidal quantum dots, spatial confinement of the vibrational motion is expected to give rise to an interplay between the quantized electronic and phononic degrees of freedom. In most cases, however, zero-dimensional colloidal nanocrystals are strongly coupled to the substrate such that the charge relaxation processes are still effectively governed by the bulk properties. Here we show that encapsulation of single colloidal CdSe/CdS nanocrystals into individual organic polymer shells allows for systematic vibrational decoupling of the semiconductor nanospheres from the surroundings. In contrast to epitaxially grown quantum dots, simultaneous quantization of both electronic and vibrational degrees of freedom results in a series of strong and narrow acoustic phonon sidebands observed in the photoluminescence. Furthermore, an individual analysis of more than 200 compound particles reveals that enhancement or suppression of the radiative properties of the fundamental exciton is controlled by the interaction between fine-structure states via the discrete vibrational modes. For the first time, pronounced resonances in the scattering rate between the fine-structure states are directly observed, in good agreement with a quantum mechanical model. The unambiguous assignment of mediating acoustic modes to the observed scattering resonances complements the experimental findings. Thus, our results form an attractive basis for future studies on subterahertz quantum opto-mechanics and efficient laser cooling at the nanoscale.

Phonon-Photon Mapping in a Color Center in Hexagonal Boron Nitride.

We report on the ultraviolet optical response of a color center in hexagonal boron nitride. We demonstrate a mapping between the vibronic spectrum of the color center and the phonon dispersion in hexagonal boron nitride, with a striking suppression of the phonon assisted emission signal at the energy of the phonon gap. By means of nonperturbative calculations of the electron-phonon interaction in a strongly anisotropic phonon dispersion, we reach a quantitative interpretation of the acoustic phonon sidebands from cryogenic temperatures up to room temperature. Our analysis provides an original method for estimating the spatial extension of the electronic wave function in a point defect.

Dy3+ ions as optical probes for studying structure of boro-tellurite glasses

Dy3+-doped glasses with various compositions (35+x)B2O3+ (45−x)TeO2+9.5ZnO+10Na2O+0.5Dy2O3 (x=0; 10 and 20) were prepared by a melt–quenching technique. The Dy3+ ions are used as an optical probe, of which the Judd–Ofelt parameters, the phonon-side band, and the Raman spectra were quantitatively estimated to search the change of glass structure (the change of the ratio of [BO4] to [BO3] units, formation of the non-bridging oxygens (NBO−), the change of [TeO3] to [TeO4] units) as a function of the B2O3 content. The Ω2 and Ω6 values of Dy3+-doped boro-tellurite samples are larger than that of Dy3+-doped borate or tellurite sample. The CIE chromaticity color coordinates were calculated for the luminescence spectra of Dy3+ ions of the glasses with the different compositions and they were all located in the vicinity of white light center of the color coordination diagram.

Dynamic vibronic coupling in InGaAs quantum dots [Invited

The electron-phonon coupling in self-assembled InGaAs quantum dots is relatively weak at low light intensities, which means that the zero-phonon line in emission is strong compared to the phonon sideband. However, the coupling to acoustic phonons can be dynamically enhanced in the presence of an intense optical pulse tuned within the phonon sideband. Recent experiments have shown that this dynamic vibronic coupling can enable population inversion to be achieved when pumping with a blue-shifted laser and for rapid de-excitation of an inverted state with red detuning. In this paper we confirm the incoherent nature of the phonon-assisted pumping process and explore the temperature dependence of the mechanism. We also show that a combination of blue- and red-shifted pulses can create and destroy an exciton within a timescale ~20 ps determined by the pulse duration and ultimately limited by the phonon thermalisation time.

Optical and transport properties of single crystal rubrene: A theoretical study

Optical and charge transport properties of single crystal rubrene are investigated using the multi-mode Brownian oscillator (MBO) model, the charge hopping model with quantum nuclear tunneling, and the Munn–Silbey approach. The MBO model is adopted to calculate absorption and photoluminescence spectra, yielding results in excellent agreement with measurements. In addition, temperature dependence of zero phonon lines (ZPL) and phonon sidebands (PSBs) of absorption spectra is also examined using the MBO model, revealing a nearly linear dependence of line widths of the ZPL and the PSBs on temperature. Model parameters obtained from MBO fitting and TD-DFT computation are then utilized for hole mobility calculations. It is found that temperature dependence of the calculated mobility is in general agreement with measurements, exhibiting “band-like” transport behavior.

(Invited) A Carbon Nanotube Based Single Photon Source

Carbon nanotubes have strong assets for future integrated single-photon sources since their working wavelength can easily be tuned to the telecom C band by choosing an appropriate diameter [1]. In addition, they show a pronounced antibunching both at low and room temperature[2,3]. Nevertheless, the practical implementation of such a single-photon source requires to embed the nanotube into a micro-cavity in order to funnel the emitted photons into a specific mode (and possibly to an optical fiber) and in order to beat the non-radiative recombination by means of Purcell brightening. In order to tackle the stringent spatial and spectral matching conditions, we developed a flexible micro-cavity design where the concave mirror is micro-engineered at the apex of a single mode optical fiber [4] that can be scanned at the surface of the back mirror where nanotubes has been deposited. By doing so, we are able to observe, both by means of photon counting and time-resolved measurements a Purcell effect of the order of 6, which allows us to funnel 90% of the photons into the mode [5]. In addition, by exploiting the cavity feeding effect on the phonon side-bands, we are able to tune the source over a 5 THz spectral window while keeping a high spectral purity (80 GHz) and an anti-bunching rate better than 0.01. [1] V. Ardizzone et al. Phys. Rev. B 91, 121410 (R) (2015) [2] Hogele et al. Phys. Rev. Lett. 100, 217401 (2008) [3] Ma et al. Nature nano. 10, 671 (2015) [4] Hunger et al. N. J. Phys 12, 065038 (2010) [5] Jeantet et al., arXiv1508:06297 (2015)