Longitudinal Optical Phonon Energy Research Articles

Damping effect of longitudinal optical phonon—plasmon coupling on thermal radiation from surface micro-gratings on direct and indirect electronic transition type semiconductors

Thermal radiation properties of surface micro-Au-line and space structures on direct and indirect electronic transition-type semiconductors of n-type (n-) GaAs and GaP, respectively, are studied. The electron densities range from 0.62 to 1.0 × 1018 cm−3. The emission spectra of samples on n-GaAs wafers show peak energies significantly shifted from the longitudinal optical (LO) phonon energy due to the LO-phonon−plasmon coupling (LOPC) at 630 K, while partial decoupling is observed. The thermal emission from the sample on n-GaP is peaked at 395.9 cm−1, shifted only by 1.5 cm−1 from the LO-phonon energy, with a linewidth of 12.5 cm−1 at 630 K. The effect of LOPC on the emission from the sample on n-GaP is mostly suppressed because of the electron localization in the X-valley, suggesting an advantage of indirect transition-type semiconductors in LO-phonon resonant infrared emitters using the phonons generated via the electron energy relaxation.

Temperature/laser dependence of N3 optical centers in natural type Ia diamonds through photoluminescence spectroscopy

In this work, low-temperature photoluminescence spectroscopy was employed to investigate the N3 optical color center in natural type Ia diamond. The optical properties of the N3 center with regular changes in testing temperature and laser power were studied, and the energy level transition was also discussed. The results showed that the PL intensity of the N3 center enhanced sub-linearly with laser power increased, indicating that the electron transition of the N3 center was mainly radiative recombination with weak Auger recombination participation. In addition, the physical model was employed to analyze the change in the zero phonon line with the increase in the test temperature; it obtained thermal quenching activation energy (25.2 meV), bond softening, and strong interaction with acoustical phonons of the N3 center. The theoretical analysis of the broadening parameters revealed that the longitudinal optical phonon energy was 55.1 meV, and the electron–optic phonon coupling strength was 2.3 meV.

Improved optical properties of lead-free double perovskite Cs2NaBiCl6 nanocrystal via K ions doping

Double perovskite has shown great potential to solve the toxicity and instability problems of lead halide perovskite in practical applications. However, most lead-free double perovskite nanocrystals have low photoluminescence quantum yield (PLQY). Metal ion doping is usually an effective method to improve the optical properties of lead-free double perovskite nanocrystals (NCs). Here, we have prepared monodisperse Cs2NaBiCl6 NCs with uniform morphology and good crystallinity, and a series of Cs2NaBiCl6 NCs with different K+ doping amounts were prepared by colloid synthesis strategy. As the amount of potassium ion doping increased, the absorption spectra of the perovskites shifted towards the red, and the bandgap decreased from 3.56 eV to 3.50 eV. At the same time, the introduction of K+ improved the optical properties of Cs2NaBiCl6 NCs. The PLQY of Cs2Na0.62K0.38BiCl6 NCs reached 7.06 %, which was 4.38 times higher than that of Cs2NaBiCl6 NCs. The lifetime of exciton was increased from 3.42 ns to 12.02 ns by 3.5 times. Thermogravimetric analysis showed that Cs2Na0.62K0.38BiCl6 NCs had better thermal stability than Cs2NaBiCl6 NCs. Temperature-dependent PL spectra showed that Cs2Na0.62K0.38BiCl6 had higher exciton binding energy and longitudinal optical phonon energy than Cs2NaBiCl6 NCs.

Incorporation sodium ions into monodisperse lead-free double perovskite Cs2AgBiCl6 nanocrystals to improve optical properties

Lead-free double perovskite nanocrystals (NCs) have emerged as a promising candidate in the optical field, owing to their non-toxic, good moist heat and chemical stability. However, their poor optical properties limited their application. To improve the optical properties of lead-free double perovskite NCs, metal ion doping or alloying had been suggested as a promising strategy. Here, we prepared monodisperse, uniformly sized, cubic morphology of Cs2AgBiCl6 NCs with different Na+ incorporation amounts via a simple hot-injection method. The Na+ incorporation broke the parity-forbidden transition by reducing the inversion symmetry of the electron wave function at the Ag site, which changed the parity of the self-trapped exciton wave function and thus allowed radiative recombination. As a result, the photoluminescence quantum yield (PLQY) of Na+-alloyed Cs2AgBiCl6 NCs (12.1%) was higher than that of Cs2AgBiCl6 NCs (2.4%), and the exciton lifetime of Na+-alloyed Cs2AgBiCl6 NCs increased to 36.98 ns from 17.58 ns for Cs2AgBiCl6 NCs. By adjusting the amount of Na+ incorporation, the band gap of Cs2AgBiCl6 NCs can be significantly tuned from ∼2.90 eV to ∼3.50 eV. Furthermore, the temperature-dependent photoluminescence spectra indicated that the Na+-alloyed Cs2AgBiCl6 NCs possessed higher longitudinal optical phonon energy and exciton binding energy compared to Cs2AgBiCl6 NCs. This suggested that there were strong exciton-phonon interactions during exciton recombination, a reduced probability of non-radiative processes, and excellent thermal stability. It offers a promising strategy for improving the optical properties of lead-free double perovskite NCs, and have the potential to replace traditional lead halide perovskite NCs in future optoelectronic applications.

Improved Optical Properties of Lead‐Free Double Perovskite Cs2AgBiBr6 Nanocrystals via Na Ions Doping

AbstractLead‐free double perovskite nanocrystals (NCs) are a promising alternative to lead‐based perovskite NCs for optoelectronic materials due to their nontoxic and good moist‐thermal stability. However, most of the lead‐free double perovskite NCs exhibit unsatisfying photoluminescence quantum yield (PLQY); the metal ions doping is an effective method to improve the optical properties. Here, monodisperse, homogeneous cubic morphology of Na+‐doped Cs2AgBiBr6 NCs is prepared via a colloidal synthesis strategy. When Na+ is doped into Cs2AgBiBr6 NCs, the inversion‐symmetry‐induced parity‐forbidden transition is broken. So, the wave function parity of the self‐trapped excitons at Ag atoms is changed, which allows the radiative transition to occur. This leads to a higher PLQY for Na+‐doped Cs2AgBiBr6 NCs (9.08%) than that for Cs2AgBiBr6 NCs (0.40%), and the exciton lifetime is extended from 1.87 ns for Cs2AgBiBr6 NCs to 17.15 ns for Na+‐doped Cs2AgBiBr6 NCs. The Na+ doping elevates the longitudinal optical phonon energy and exciton binding energy, which results in weaker nonradiative transitions during exciton recombination and improved NC thermal stability. So, the Na+‐doped Cs2AgBiBr6 NCs maybe possess potentially valuable applications in optoelectronic devices.

Two-photon absorption of FAPbBr3 perovskite nanocrystals

Perovskite nanocrystals (NCs) with high two-photon absorption (TPA) cross-section are of great interest due to their potential applications in three-dimensional optical data storage and multiphoton fluorescence microscopy. Among various perovskite materials, FAPbBr3 NCs show a better development prospect due to their excellent stability. However, there are few reports on their nonlinear optical properties. In this work, the nonlinear optical behavior of FAPbBr3 NCs is studied. The methods of multiphoton absorption photoluminescence saturation and open aperture Z-scan technique were applied to determine the TPA cross-section of FAPbBr3 NCs, which was around 2.76 × 10−45 cm4⋅s⋅photon−1 at 800 nm. In addition, temperature-dependent photoluminescence induced by TPA was investigated, and the small longitudinal optical phonon energy and electron–phonon coupling strength was obtained, which confirm the weak Pb–Br interaction. Meanwhile, it is found that the exciton binding energy in FAPbBr3 NCs was 69.668 meV, which may be ascribed to the strong hydrogen bond interaction. It is expected that our findings will promote the application of FAPbBr3 NCs in optoelectronic devices.

Structure‐Dependent Spectrum Characteristics of Infrared Radiation from Metal–Semiconductor Surface Microstructures

The thermal radiation at 8.5 THz resonating with the longitudinal optical (LO) phonon is emitted from surface Au–GaAs microstripe structures on undoped GaAs wafers. The emission spectrum comprises a component originating from the LO phonon resonant electric dipole and another one of blackbody‐like emission subject to Planck's law for structures without or low mesa height. Narrow GaAs window yields thermal emission spectra with deep‐ and wide‐intensity drop at the higher‐energy side of the LO phonon energy. This intensity drop is formed by the absorption of the emitted radiation by generating surface phonon polaritons and forms the weak background emission in the region of 260–350 cm−1. An additional emission peak is observed at ≈350 cm−1 for high‐mesa structures. This peak is attributed to phonon–polariton‐related emission, for which electric dipoles are yielded by pairs of polarization charges in Au–GaAs–Au mesa structures.

Investigation of Two Photon Absorption of Ligand-Modified CsPbBr3 Quantum Dots.

The characteristics and application of nonlinear absorption from CsPbBr3 QDs film with the ligand-modified strategy have been investigated in this work. By means of a near-infrared fs Ti:sapphire laser as a light source, the up-conversion emission of CsPbBr3 QDs film of around 518 nm revealed a quadratic increase with the pump intensity. Through the temperature-dependent up-conversion emission, we obtained the binding energy and longitudinal optical (LO) phonon energy of CsPbBr3 QDs film of around 58.1 and 61.2 meV, respectively. Due to more active thermal coupling between the excited electron or hot phonon effect, the photon decay trace under two-photon excitation was prolonged at higher temperatures. The ligand-modified CsPbBr3 QDs film exhibits a relatively large TPA coefficient of around 28.6 cm/GW by the open aperture Z-scan measurement, and it has been demonstrated as a promising nonlinear medium to obtain the pulsewidth of ultrafast lasers.

Controlling the emission linewidths of alloy quantum dots with asymmetric strain

The emission linewidth of quantum dots (QDs) is one of the important optical properties, which is essential for the applications of QD lasers, high-quality displays, and biological imaging. However, we know less about controlling emission linewidth and its underlying mechanisms. Here we introduce a wurtzite ZnSe shell onto a wurtzite CdSe core to produce asymmetric strain due to their large, anisotropic lattice mismatch. Such asymmetric pressure induces significant splitting (ΔAB) between heavy-hole (hh) and light-hole (lh) in valence band (VB). We show that the emission intensity from the lh state (Elh) is significantly suppressed with the increasing ΔAB caused by the strong asymmetric strain. We demonstrate that the exciton-phonon coupling (EPC) is greatly inhibited under the anisotropic lattice strain. The alloying process between the core and shell occurs under the strong lattice strain and raises the longitudinal-optical (LO) phonon energy (ELO). Higher LO phonon energy declines LO phonon occupation numbers (NLO) and synergistically reduces the EPC. The asymmetrically strained alloy QDs ensemble exhibits highly bright emission with ultra-narrow linewidths of 13.8 nm (∼520 nm) and 15.8 nm (∼620 nm). This concept of band structure regulation via asymmetric strain can provide a new platform for high-quality QDs beyond the currently achieved.

Thermal radiation resonating with longitudinal optical phonon from surface micro-stripe structures on metal-gallium nitride and sapphire

Thermal emissions from metal/dielectric-material stripe structures on the surface of dielectric materials are observed. These emissions are attributed to electric-dipole emissions induced by coherent thermal lattice vibrations in the surface structures of a few micrometers. Thermal emission based on coherent lattice vibrations has been reported via surface phonon polariton (SPhP), where metal ingredients have been excluded, whereas the present emission is independent of SPhP. The structures on undoped GaAs shows the emission close to the longitudinal optical (LO) phonon energy, while the peak energy of the emission from the structures on GaN is located in between the LO and transverse optical (TO) mode energies. The present studies on several vibration modes of α-Al2O3 and the dependence of the emission peak energy on geometrical conditions of surface structures reveal that modification of electric permittivity by the local depolarization field induces additional zero points of the real part of the permittivity, which yields additional local LO-like modes and the shift of the main emission peak energy from the original LO mode. This shift is significant for materials with a large energy difference between the LO and TO modes, such as 270 cm−1 for a mode of α-Al2O3 and 170 cm−1 for GaN. This is contrasted with the case of GaAs with 30 cm−1 difference.

Ultranarrow lines in Raman spectra of quantum wells due to effective acoustic phonon selection by in-plane wave vector

An unusual fine spectrum of very narrow lines ($<1\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ overall) is observed in resonance Raman scattering from a CdTe quantum well under normal light incidence, when the laser excitation is about one longitudinal optical (LO) phonon energy higher than the quantum well exciton ground state. The visibility of this fine spectrum is improved by sample doping which reduces the quantum yield of the exciton photoluminescence. The appearance of the four sideway lines of the fine spectrum is explained by the Raman scattering on the combination of LO and longitudinal acoustic (LA) [transverse acoustic (TA)] phonons with the $e1hh1(2s)$ exciton serving as an intermediate state. The central line is due to elastic scattering of the $2s$ exciton to the hot $1s$-exciton state on a static random potential of the heterostructure followed by emission of a LO phonon.

The photothermal stability study of silica-coated CsPbBr3 perovskite nanocrystals

The lead halide perovskite (LHP) semiconductor material has many outstanding advantages and was widely used light-emitting device, however their low stability severely limited the practical application and further commercialization. We proposed a two-step method for synthesizing CsPbBr3/SiO2 composites to improve the stability of CsPbBr3 perovskite nanocrystals (PNCs). The synthesized CsPbBr3/SiO2 composites can maintain photothermal stability, and also have a narrower photoluminescence (PL) bandwidth of 20.2 ​± ​0.26 ​nm, and the PL peak (518 ​nm) hardly shifted during the two-step synthesizing process. Through the temperature-dependent PL spectroscopy, we show that the CsPbBr3/SiO2 composites have a high exciton binding energy 50.0 ​meV and longitudinal optical phonon energy 80.1 ​meV, suggesting that the robust excitons exist in CsPbBr3/SiO2 composites. The improved stability of the composites is demonstrated by exposure to harsh environments such as laser irradiation, heating and water. These findings are greatly significant for further applications of CsPbBr3 PNCs in optoelectronic devices.

Electron-LO-phonon intrasubband scattering rates in a hollow cylinder under the influence of a uniform axial applied magnetic field

Scattering rates arising from the interactions of electrons with bulk longitudinal optical (LO) phonon modes in a hollow cylinder are calculated as functions of the inner radius and the uniform axial applied magnetic field. Now, the specific nature of electron-phonon interactions mainly depends on the character of the energy spectrum of electrons. As is well known, in cylindrical quantum wires, the application of a parallel magnetic field lifts the double degeneracy of the non-zero azimuthal quantum number states; \(m \not = 0\); irrespective of all electron’s radial quantum number l states. In fact, this Zeeman splitting is such that the \(m < 0\) electron’s energy subbands initially decrease with the increase of the parallel applied magnetic field. In a solid cylinder, the lowest-order; {\(l = 1;\,m = 0\)} subband is always the ground state. In a hollow cylinder, however, as the axial applied magnetic field is increased, the electron’s energy subbands take turns at becoming the ground state; following the sequence \(\lbrace m=0,-1,-2...~ -N\rbrace\) of azimuthal quantum numbers. Furthermore, in a hollow cylinder, in general, the electron’s energy separations between any two subbands are less than the LO phonon energy except for exceptionally high magnetic fields, and some highest-order quantum number states. In view of this, the discussion of the energy relaxation here is focused mainly on intrasubband scattering of electrons and only within the lowest-order {\(l = 1;\,m = 0\)} electron’s energy subband. The intrasubband scattering rates are found to be characterized by shallow minima in their variations with the inner radius, again, for a fixed outer radius. This feature is a consequence of a balance between two seemingly conflicting effects of the electron’s confinement by the inner and outer walls of the hollow cylinder. First; increased confinement of the charge carriers generally leads to the enhancement of the rates. Second; the presence of a hole in a hollow cylinder leads to a significant suppression of the scattering rates. The intrasubband scattering rates also show a somewhat parabolic increase in their variations with the applied magnetic field; an increase which is more pronounced in a relatively thick hollow cylinder.

Luminescence Fine Structures in Single Lead Halide Perovskite Nanocrystals: Size Dependence of the Exciton–Phonon Coupling

Lead halide perovskite nanocrystals (NCs) have superior photoluminescence (PL) properties, such as high PL quantum yields and wide PL wavelength tunability, for optoelectronic applications. Here, we report the PL spectra of single formamidinium lead halide perovskite FAPbX3 (X = Br, I) NCs examined by single-dot spectroscopy at low temperature. We found four PL peaks in the low-energy region below the strong exciton PL peak that originate from two longitudinal-optical (LO) phonon replicas of the exciton PL, biexcitons, and charged excitons (trions). The binding energies of the biexcitons and trions become larger as the NCs decrease in size. The LO phonon energies show no size dependence, but the Huang-Rhys factors, which reflect the strength of the exciton-phonon coupling, become larger for smaller NCs. Our findings provide important insights into the exciton properties of perovskite NCs.

Temperature dependence of the optical phonon reflection band in GaP

We explore the effect of temperatures between 80 and 720 K on the energy and linewidth of zone-center transverse (TO) and longitudinal (LO) optical phonons in bulk gallium phosphide (GaP) using Fourier transform infrared ellipsometry from 0.03 to 0.60 eV. We extract the optical phonon parameters of GaP by fitting the ellipsometric angles with the Lowndes–Gervais model, which applies two different broadening parameters to the TO and LO phonons. In GaP, the two-phonon density of states is larger for the decay of TO phonons than for LO phonons. Therefore, we observed a larger TO phonon broadening (compared to the LO phonon) and an asymmetric reststrahlen line shape. This would lead to a negative imaginary part of the dielectric function just above the LO phonon energy but the addition of two-phonon absorption avoids this. We find a temperature-dependent redshift and broadening of TO and LO phonons with increasing temperature due to thermal expansion and anharmonic phonon-phonon scattering, involving three and four phonon decay processes. We also investigate the temperature dependence of the high-frequency dielectric constant. Its variation is explained by thermal expansion and the temperature dependence of the Penn gap.

Manipulating hot carrier behavior of MAPbBr3 nanocrystal by photon flux and temperature

The temperature-dependent time-resolved spectroscopies are employed to investigate the hot carrier characteristics in MAPbBr3 nanocrystals (NCs). Simultaneously, the absorption and photoluminescence data exhibits that MAPbBr3 NCs pass through cubic phase and tetragonal phase in turn as the temperature drops. As the phase transition from the cubic phase to the tetragonal phase, the band gap narrows, the shallow defects population that participates in the carrier dynamics and the Longitudinal-optical phonon energy (ELO) decreases. We uncovered that the hot carrier can be induced in the MAPbBr3 NCs through changing the photon flux and the temperature. Moreover, the hot carrier cooling time and the carrier heating effect in tetragonal phase improved in comparison with those in cubic phase, owing to the influence of bandgap, shallow defect and ELO as mentioned above. These results can help us to further understand the hot carrier characteristics in the perovskite MAPbBr3 NCs for designing and optimizing the perovskite-based optoelectronic devices of high performance.

Double longitudinal-optical phonon intrawell depopulated terahertz quantum cascade structures: Electron transport modeling using a density matrix method

Terahertz quantum cascade structures using double longitudinal-optical phonon intrawell scattering for depopulation are theoretically studied. A density matrix Monte Carlo method is used to calculate the temperature dependent optical power, in double phonon structures with diagonal optical transitions. It is shown that using depopulation transitions greater than the resonant longitudinal-optical phonon energy ΔE &amp;gt; ℏωLO reduces the phonon absorption thermal backscatter, allowing for higher operating temperatures, with prospects for 300 K room temperature and beyond. Furthermore, results indicate that the temperature limit may also be improved in single phonon structures, by similarly increasing the depopulation transition.

Polaron Masses in CH_{3}NH_{3}PbX_{3} Perovskites Determined by Landau Level Spectroscopy in Low Magnetic Fields.

We investigate the electron-phonon coupling in CH_{3}NH_{3}PbX_{3} lead halide perovskites through the observation of Landau levels and high-order excitons at weak magnetic fields, where the cyclotron energy is significantly smaller than the longitudinal optical phonon energy. The reduced masses of the carriers and the exciton binding energies obtained from these data are clearly influenced by polaron formation. We analyze the field-dependent polaronic and excitonic properties, and show that they can be quantitatively reproduced by the Fröhlich large polaron model.

Computerized FDTD Method for Longitudinal Optical Phonon Energy on Semiconductor Hybrid Structure for High Power Devices Fabrication

The research problem in this study is the longitudinal optical phonon energy on metal/semiconductor interface for high performance semiconductor device. The research solution is to make the software model with finite difference time domain (FDTD) solution for transmission and reflection pulse between metal and semiconductor interface for carrier dynamics effects. The objective of this study is to find the quantum mechanics understanding on interface engineering for fabricating the high performance device for future semiconductor technology development. The analysis was carried out with the help of MATLAB. The quantum mechanical spatial field on metal-semiconductor stripe structure has been analyzed by FDTD techniques. This emission reveals a characteristic polar radiation distribution of electric dipoles and a wavelength independent of the structure size or the direction of emission; consequently, it is attributed to thermally generate electric dipoles resonating with the longitudinal optical phonon energy. Phonon energy occurs lattice vibration of material by the polarization of light, if the material has rigid structure reflect back the incident light. So, high reflective metal-semiconductor structure always use as photodectors devices in optical fiber communication. No lattice vibration material structure has no phonon effect, so this structure based devices can get high performance any other structure based devices. The transmission and reflection coefficient of metal-semiconductor GaN/Au layer structure compare with GaN/Ti and GaN/Pt structure. Parallel (P) and transverse (S) polarization of light incident on a metal-semiconductor nanolayer structure with IR wavelength. Efficient use of the layer by layer (LbL) method to fabricate nanofilms requires meeting certain conditions and limitations that were revealed in the course of research on model systems.

Strong Triplet-Exciton-LO-Phonon Coupling in Two-Dimensional Layered Organic-Inorganic Hybrid Perovskite Single Crystal Microflakes.

Two-dimensional (2D) layered hybrid perovskites provide an ideal platform for studying the properties of excitons. Here, we report on a strong triplet-exciton and longitudinal-optical (LO) phonon coupling in 2D (C6H5CH2CH2NH3, PEA)2PbBr4 perovskites. The triplet excitons exhibit strong photoluminescence (PL) in thick perovskite microflakes, and the PL is not detectable for monolayer microflakes. The coupling strength of the triplet exciton-LO phonon is approximately two to three times greater than that of the singlet exciton-LO phonon with a LO phonon energy of about 21 meV. This difference might due to the different locations of singlet excitons located in the well and triplet excitons located in the barrier in the 2D layered perovskite. Revealing the strong coupling of triplet exciton-LO phonon provides a fundamental understanding of many-body interaction in hybrid perovskites, which is useful to develop and optimize the optoelectronic devices based on 2D perovskites in the future.