Multiphonon Sideband Research Articles

Multiphonon sidebands of Fröhich polaron in metal halide perovskite quantum dots

We theoretically study the optical absorption of Fröhlich polaron in metal halide peovskite quantum dots based on the well-known Huang–Rhys model. We find that multiphonon sidebands are appeared in the absorption spectra when the energy of incident photon equaling to the multiple LO phonons, where the orders of phonon sideband and the strongest sideband depend on the radius of quantum dot sensitively, which indicates the average phonon numbers around a polaron could be evaluated by the orders of phonon sideband. Moreover, we discuss the temperature dependences of these phonon sidebands in details, giving the enlightenment for analyzing the carriers mobility affected by LO phonons scattering in recently experiments. These theoretical results not only provide an effective way to prove Fröhlich polaron states, but also enrich the knowledge for understanding the basic properties of polaron in metal halide perovskite nanostructures.

Cr-doped ZnGa2O4: Simple synthesis of intense red-NIR emitting nanoparticles with enhanced quantum efficiency

Red-NIR emitting ZnGa2-xCrxO4 nanoparticles were synthesized using simple co-precipitation, and the effect of annealing was studied (600–1200 °C). Upon annealing, crystallite size increased (12 nm–73 nm), and color tone turned pink beyond 1000 °C, inferring dopant diffusion. Blue, green absorption bands in diffuse-reflectance spectra revealed Cr3+ in octahedral crystal field. Photoluminescence excitation spectra showed characteristic Cr3+ bands (∼400 nm and ∼550 nm), and the asymmetric 400 nm band suggested splitting the 4T1g(F) state due to trigonal distortion. Emission spectra consisted of a zero-phonon line (∼688 nm), along with associated multi-phonon sidebands. ZnGa1.98Cr0.02O4 showed intense red emission with λexc = 420 nm, while the tri-exponential fitting of the excited-state lifetime implied multiple decay processes due to Cr3+ occupying different environments. The internal quantum efficiency of the 1200 °C annealed sample was impressive (48%). Both UV and visible light up to 600 nm could induce persistent-luminescence (λem = 688 nm) for longer durations. The smaller size, intense emission, higher quantum yield, and visible light activation suggest that the nanoparticles are suitable for bioimaging applications.

Phonon sidebands of the optical spectrum for the defect structure GaN:C +O

We investigate the multi-phonon sidebands of the optical spectra of point defects in GaN based on the Huang-Rhys model, in which the localized phonon modes and the reorganization energy associated with the defect structure GaN: C N + O N are precisely calculated by ab initio approach. We present multi-phonon sidebands for both the localized phonon modes and intrinsic longitudinal optical phonon modes and give the comparisons between them in detail. Meanwhile, different types of the combinational phonon sidebands that are consisted of two localized phonon modes with different proportions are proposed. These multi-phonon sidebands not only could be used for the analysis of fine structure of the optical spectra, but also served as the fingerprints for the defect structures. • Multi-phonon sidebands of the optical spectra for point defects in GaN are present. • Localized phonon modes associated with the defect structure are calculated by ab initio approach. • Multi-phonon sidebands could be served as the fingerprints for the defect structures.

Effect of Zn substitution in Cr3+ doped MgAl2O4 mixed spinel nanoparticles on red/NIR emission properties

Red/Near-infrared (NIR) emitting Mg1-xZnxAl1.98Cr0.02O4 nanoparticles were synthesized by co-precipitation followed by firing(1200 °C). Gradual shift of XRD peaks with Zn-content confirmed complete solid-solution formation. HRTEM revealed hexagonal sheet-like structures and lattice fringes revealed their single-crystallinity. Owing to refractory nature of MgAl2O4, particles were smaller(∼38 nm) in Mg-rich samples and gradually increased(∼58 nm) with Zn-substitution. Due to trigonal distortion, the fine splitting of the 4T1g level of Cr3+ manifests as asymmetric band(400 nm) in the excitation spectrum. Cr3+ emission was clearly observed, comprising of R-line and multi-phonon side-bands on either side, with maximum R-line intensity for x = 0.5. With increasing x, side-bands were more pronounced probably due to the band-gap difference of end members. High Dq/B (3.5–3.9), suggested Cr3+ in a strong crystal-field. Racah-parameter(B) variations suggest increased electron delocalization over molecular orbitals for higher Mg2+ substitutions. Longer lifetimes (23–34 ms) are well-suited for bioimaging and optoelectronic applications. Such a simple synthesis can be easily extended to other important phosphors.

Rapid annealing: A novel processing technique for Cr:ZnAl2O4 nanoparticles

Nanocrystalline Cr:ZnAl2O4 was synthesized for the first time by rapid thermal annealing the corresponding hydroxide precipitate in just 10 min. The XRD patterns showed that synthesis between 500 °C and 700 °C had smaller crystallites (∼2.4 – 4 nm), while at 800 °C and 900 °C, had ∼10 nm crystallites. HRTEM images of the 900 °C sample showed the well-developed lattice fringes confirming the high crystallinity. The particles were polyhedral having an average size of ∼16 nm. The diffuse reflectance spectra showed characteristic 4A2g → 4T1g and 4A2g → 4T2g transitions, while the photoluminescence emission spectrum comprised of the zero phonon line (R-line) along with other multi-phonon side bands that are characteristic of the spin – parity forbidden transitions of the Cr3+ ion. The emission spectra and photoluminescence lifetimes are comparable with reports on sensing and imaging applications. Our work demonstrates the suitability of this technique to quickly synthesise phosphors for a variety of applications.

Nanocrystalline MgCrxAl2-xO4: Facile synthesis and thermal dependency of photoluminescence

MgCrxAl2-xO4 nanoparticles were synthesized by facile co-precipitation method. The x=0.02 sample showed intense emission and considered for studying the effect of annealing temperature (500–1200°C) on the structural and optical properties. On annealing, crystallinity improved and the material gradually turned pink at 1200°C. Hexagonal sheet-like nanoparticles (∼52nm) were observed by HRTEM and the lattice fringes suggested their single crystalline nature. Excitation spectra showed two peaks at 546 and 419nm where the latter was asymmetric, attributable to the trigonal distortion. The photoluminescence consisted of intense zero-phonon line and associated multi-phonon side bands, whose intensities increased with annealing. Higher Dq/B (3.5), confirmed strong crystal field for Cr3+, suggesting that the photoluminescence emission could be associated with spin-parity forbidden transitions. The emission lifetimes (λem=688nm) ranged between ∼20 and 24ms. Other oxide nanoparticles suitable for phosphors, biomarkers, etc. can be conveniently synthesized similarly.

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.

Investigation of visible emission induced by infrared femtosecond pulses in erbium-doped YVO4 and LuVO4 single crystals

Illumination of single crystal samples of erbium-doped YVO4 and LuVO4 by infrared femtosecond pulses brings about an intense green luminescence assigned to the 4S3/2 - 4I15/2 transition of Er3+ ions. When lowering the sample temperature an additional broad band in the blue related to the VO43− emission appears and next grows steadily. It has been observed that the variation of wavelength of incident infrared femtosecond pulses in the region 800nm–1600nm affects weakly the intensity of both the blue and green luminescence bands. Analysis of luminescence dynamics made it possible to reveal that upon infrared excitation the rise time of the 4S3/2 luminescence is longer by a factor of four, roughly as compared to that recorded upon direct excitation into high energy levels of Er3+. Observed temperature dependence of up-converted blue and green luminescence implies that the energy transfer from vanadate groups cannot be considered as a main mechanism involved in the excitation erbium ions. It has been supposed that erbium ions are likely to be excited by energy transfer from free electrons created in the conduction band of the host by multiphoton excitation and /or by non-resonant multi-step ETU process involving absorption of infrared light in multiphonon side bands of electronic transitions. Creation of free electrons has been corroborated by changes of electrical conductivity of crystals induced by an illumination with infrared femtosecond pulses.

Magnetic and optical properties of virgin arc furnace grown MgO crystals

Transition metal ions (Mn 2+, Cr 3+, Fe 2+/3+, V 2+) as substitutional impurities in cubic crystals play an important role due to their influence on the magneto-optical properties of the virgin crystals. The optical and magnetic properties of commercial MgO(001) single crystals were investigated. The PL spectra of 3 d impurity ions (Cr 3+ and V 2+) show narrow zero phonon lines with decay times in the range of milliseconds and a broad multiphonon sideband decaying in microseconds. The EPR study exhibits hyperfine components corresponding to Mn atoms and to V atoms located in orthorhombic symmetry. The intensity change of the EPR and PL spectra as well as the variation of the decay time with temperature can be explained by electron-spin lattice relaxation due to the Raman process.

On the Structured Green Band in ZnO Nanowires

ZnO nanowires have been synthesized by using the vapor evaporation method. The material source was ZnO powder mixed with graphite. The nanowires have the shape of long rods with diameters ranging from 100 to 300 nm and lengths of several tens of micrometers. The photoluminescence (PL) was measured from 15 K to 300 K. At room temperature, the PL exhibited a relatively strong deep-level emission around 500 nm (green band) and a small UV emission peak at 380 nm (UV band). At low temperatures, the PL spectrum consisted of a group of sharp UV peaks and a smaller green band having the ne structure of a broad multiphonon side band. The ne structure was observed to include doublets at the higher energy side. The temperature dependences of the peak position and of the full width at half maximum height (FWHM) of the PL spectra for green band were investigated. The origin of the green band is attributed to a copper impurity.

Temperature-Dependent Localized Excitations of Doped Carriers in Superconducting Diamond

Laser-excited photoemission spectroscopy is used to show that the doped carriers in metallic or superconducting diamond couple strongly to the lattice via high-energy (approximately 150 meV) optical phonons, with direct observations of localized Franck-Condon multiphonon sidebands appearing as Fermi-edge replicas. It exhibits a temperature-dependent spectral weight transfer from higher to lower energy sidebands and zero-phonon Fermi-edge states. The quantified coupling strength shows a systematic increase on lowering temperature, implicating its relation to the normal state transport and superconductivity.

Multiphonon sideband intensities in rare earth ions in crystal

Vibronic sidebands may originate from two main classes of processes which have been called Δ- and M-processes. We will consider multiphonon transitions in the M-process case; vibronics are induced by the joint action of the electromagnetic field and the high-order optical anharmonicity. The contributions to multiphonon sidebands due to point-charge, inhomogeneous-dielectric and polarized-ligand mechanisms are considered. As a result, all mechanisms considered by Judd for the one photon–one phonon transitions are extended to the multiphonon case.

Two-photon excitation spectroscopy of Cr3+:K2NaScF6elpasolite: II. Theoretical models

Two-photon excitation (TPE) spectra of Cr3+:K2NaScF6, excited bya Raman-shifted, Nd:YAG-pumped tunable dye laser, exhibit severalunexpected features. A weak TPE spectrum of the symmetry-forbidden 4A2g→ 4T2g transition is observed without a zero-phononline. The symmetry-allowed TPE spectrum of the 4A2g→ 4T1ag transition has a multi-phonon side band with anomalously extendedvibrational progressions, and an anomalously weak, split zero-phonon linewith anomalous polarization anisotropy. These observations are explained,respectively, in terms of theoretical models involving phonon assistance,departures from the closure approximation that permitelectron-latticecoupling in intermediate states and a low-temperature phase transitioninvolving librational instability. Hypothetical line-shape simulations arecompared with observed TPE spectra.

Multiphonon sideband intensities in rare earth ions in crystal

Intermultiplet transitions involving emission (or absorption) of one photon and n phonons are theoretically investigated in trivalent rare earth (RE) ions in crystals. Transitions are induced by joint action of electromagnetic field and high-order optical anharmonicity (M-process). Hence, this work extends Judd's study of one photon–one phonon transitions to the multiphonon case. The processes under consideration give rise to the Stokes and anti-Stokes multiphonon sidebands of light emission or absorption. Besides, these processes can significantly increase the rate of intermultiplet transitions in the presence of stimulated emission. As far as we know the problem of multiphonon sidebands have not been considered previously in the theory of M-processes. Here we obtained general expression for transition probabilities. In doing this we used the crystal field model that takes into account both Coulomb and non-Coulomb interactions of 4f-electrons with ligands. (Previously we used this model for the calculation of multiphonon rates in RE ions.) The “electronic part” of obtained expression is presented exactly in the same form as the well-known Judd–Ofelt expression for radiative transition probability between multiplets. A simplified form of general expression is proposed. Other mechanisms of photon–phonon transitions are briefly discussed.

The effective frequency in multiphonon processes: Differences for energy transfers or side-bands and non-radiative decay

We have found that the exponential parameters for multiphonon side-band excitation for Er 3+:ZBLAN and Er 3+:silica glasses are respectively 9.3 × 10 −3 and 8.3 × 10 −3cm for Stokes and 14 × 10 −3 and 13.4 × 10 −3 cm for anti-Stokes excitation. These values are much higher than the ones usually reported for non-radiative decay: 5.2 × 10 −3 cm for ZBLAN and 3.8 × 10 −3 cm for silica glass. Such behaviour is linked to a constant value of 1.2% for the multiphonon generation quantum efficiency.

Photon avalanche luminescence of Er 3+ ions in LiYF 4 crystal

Photon avalanche effect has been found in Er 3+:LiYF 4 crystal at room temperature when pumped at two wavelengths regions, 579 and 690 nm. These two pump schemes correspond to excitations energies from the ground state falling between 4S 3 2 and 4F 9 2 , and between 4F 9 2 and 4I 9 2 levels, respectively. Spectra and excited state dynamics of these nonlinear processes have been investigated. The link between the photon avalanche initial step and multiphonon side band absorption has been explored. Results show that the initial absorption region comes essentially from the anti-Stokes multiphonon contribution. The role of an independent injection of population into the metastable level is also investigated showing that the system may behave as a conditionally stable positive feed-back system. Temperature dependant measurements have shown that, a higher temperature is favorable for the avalanche emission in this system, which is different from other ion and crystals systems having displayed an avalanche effect to date.

Investigation of multiphonon assisted side band absorptions in rare-earth-doped halide glasses and crystals

Absorption outside the commonly considered electronic transition absorption regions in rare-earth-ion-doped ZBLAN fluoride glass have been investigated in detail and compared with those in halide crystals. Exponential decays with respect to energy have been found both in Stokes and anti-Stokes excitations, and the exponential parameters have been measured. By comparing with exponential parameters for multiphonon non-radiative decay, it is found that the phonons coupled to multiphonon side band absorption are generally smaller than the highest phonon cut off frequency which is known to be operative in non-radiative decay.

Multiphonon pumping in Er3+ ZBLAN bulk and fibre, the first step for the photon avalanche process

Based on the understanding of the role played by the multiphonon sideband absorption as the initial step in photon avalanche up-conversion, such an effect in Er3+ and in a ZBLAN glass and fibre at room temperature was obtained for the first time. The pump wavelength is far from any electronic absorptions from the ground state of the Er3+ ion, but corresponds to excited state absorption. The distinct characteristics such as rise time, threshold and non-linear dependence on pumping have been demonstrated. The dynamic process of the photon avalanche has been analyzed and mechanisms are proposed.

Room-temperature photon avalanche up-conversion in Er-doped fluoride glass and fibre pumped at 700 nm

A strong emission at 550 nm as well as a series of other wavelengths have been observed in Er-doped ZrF4-BaF2-LaF3-AlF3-NaF fibres and bulk glass at room temperature when the excitation wavelength is about 700 nm, which is far away from the absorptions from the ground state of Er3+ ions. Measurements of excitation, time delay and intensity dependence versus pump power suggest that these emissions are caused by the photon avalanche effect originating from the multiphonon side-band absorption. The excitation mechanisms of this photon avalanche have been analysed. Spectral measurement also show some strong emissions in the IR region, some features of which are probably linked with laser action.

Experimental determination of exponential parameters for multiphonon residual absorption in a ZBLAN: Er3+ fibre

Determination of exponential parameters for both Stokes and anti-Stokes sidebands by excitation of a ZBLAN: Er3+ fibre and bulk samples gives values of 9.4×10-3 cm and 14.1×10-3 cm, respectively. Observed values are larger than equivalent parameters usually found for nonradiative transitions. As examples, the measured parameters are used to determine the multiphonon sideband absorption between the 4I11/2 and the 4I13/2 electronic transitions of Er3+ and also between 1G4 and 3F4 of Pr3+. Available energy ranges for sideband pumping of distributed amplifiers are given as well as the minimum multiphonon background losses intrinsic to the rare-earth plus pure glass base combination.