Longitudinal Optical Research Articles

Terahertz oscillations in gallium nitride quantum-well channels predicted by hot-electron noise temperature behavior at microwave frequency

The almost periodic streaming motion of accelerated electrons under moderate electric fields coupled with almost periodic emission of longitudinal optical (LO) phonons is studied in a gallium nitride quantum-well—a promising pathway for terahertz (THz) oscillations. The optimal conditions for the LO-phonon-terminated streaming depend, among others, on the density of the electron gas, the low-field electron mobility, the lattice temperature, and the electric field in a very specific way. The present manuscript exploited the electron noise temperature measured at an X band frequency as a marker for the oscillations at THz frequencies. The idea was tested on a deterministic model for a GaN two-dimensional electron gas (2DEG) through calculation of the electron noise temperature spectra in the Langevin approach for the frequency range from 1 GHz to 10 THz. The noise temperature at 10 GHz was found to be in a strong anticorrelation with the THz peaks in the noise temperature spectrum. In particular, a weaker dependence on the applied electric field at 10 GHz implies stronger THz oscillations. In an experiment, the microwave hot-electron noise measurements were carried out for AlGaN/AlN/GaN heterostructures with the 2DEG channel at 10 GHz under pulsed electric field conditions in order to mitigate the effect of Joule heating of the channel. The plateau-like behavior of the noise temperature, in its dependence on the electric field, was obtained for the 2DEG channels with rather low electron densities (2.5×1012 cm−2) in a good agreement with the model. The aforementioned plateau in the electron noise temperature observed at 10 GHz can be used as an indicator for the THz oscillations.

MORPHOLOGICAL, STRUCTURAL AND OPTICAL CHARACTERIZATIONS OF Zn-DOPED CdS BUFFER LAYER ELABORATED BY CHEMICAL BATH DEPOSITION

Zn-doped CdS layers, with various percentage ratios [Formula: see text] (= [Zn[Formula: see text]]/[Cd[Formula: see text]]%) were grown on glass substrates by chemical bath deposition (CBD). The effect of Zn-doping on different properties of CdS is studied by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, atomic force microscopy, energy dispersive X-ray analysis and UV-visible diffuse reflectance. The XRD patterns indicated polycrystalline films with (111) orientation and the insertion of Zn does not change the crystallinity of CdS. The Raman spectra show one major peak centered around 300[Formula: see text]cm[Formula: see text] assigned to the first-order longitudinal optic (LO) phonon modes of CdS. The surface morphology visualized by AFM and SEM analysis showed the influence of the Zn-doping on the morphology of the films, the surface roughness is found to decrease from 16.5 to 8.9[Formula: see text]nm with augmenting the ratio [Formula: see text] from 0 to 6%. In regard to the SEM analysis, the increase of [Formula: see text] yielded a lower porosity of the film and voids, and the films become more homogeneous. The EDAX spectra confirmed the existence of Zn in the doped samples. The bandgap decreases from 2.44 to 2.37[Formula: see text]eV, while the transmittance increases from 76 to 86% with augmenting the ratio [Formula: see text].

Exciton Localization and Enhancement of the Exciton–LO Phonon Interaction in a CsPbBr3 Single Crystal

This is the first study of lead halide perovskites (LHPs) that compares the coupling strengths between longitudinal optical (LO) phonons and free or localized excitons. In this research, we measure...

Site- and Spatial-Selective Integration of Non-noble Metal Ions into Quantum Dots for Robust Hydrogen Photogeneration

Summary Artificial photosystems consisting of semiconductor quantum dots (QDs) and non-noble metal ions (e.g., Fe2+, Co2+, Ni2+) show intrinsic advantages in efficient and cost-effective hydrogen (H2) evolution. However, well-controlled integration of these metal ions into ultra-small QDs is a major challenge. Here, we present a two-step strategy to realize site- and spatial-selective integration of earth-abundant Fe2+ ions in CdSe QDs; i.e., CdSe QDs are modified with a thin and anisotropic ZnS shell, and partial Zn2+ ions are selectively substituted by Fe2+ via cation exchange. The anisotropic ZnS layer not only passivates CdSe core but also works as an ideal medium to intimately anchor Fe2+. The multifunctional CdSe/Zn1−xFexS QD exhibits extraordinary activity and stability toward H2 photogeneration. Specifically, more than 880 mL (∼39,300 μmol) H2 gas is produced from 6 mL of aqueous solution during a consecutive 172 h (>1 week) experiment, thus achieving a turnover number of >600,000 per QD.

Long lifetime of the E1u in-plane infrared-active modes of h -BN

We present an infrared reflectivity study of the ${E}_{1u}$ in-plane phonons of hexagonal BN as a function of temperature in the 40--680 K range. The infrared reflectance spectra of high-quality lamellar single crystals are accurately fitted using Lowndes' factorized form of the dielectric response, where the longitudinal-optic (LO) frequency is an independent adjustable parameter. From this analysis we obtain reliable values for the phonon damping of the IR-active ${E}_{1u}$ phonons which couple to light and give rise to the phonon-polariton excitations in this hyperbolic material. Anharmonic coupling potentials are estimated from the temperature dependence of the damping parameters. The ${E}_{1u}(\text{LO})$ mode exhibits a substantially longer lifetime than its transverse-optic counterpart, as it is basically unaffected by isotopic-disorder scattering owing to the low density of phonon states around the ${E}_{1u}(\text{LO})$ frequency.

Oblique incidence infrared reflectance spectroscopy of phonons in cubic MgO, MnO, and NiO

The infrared (IR) reflectivity of the cubic metal oxides MgO, MnO, and NiO has been measured at room temperature using the technique of oblique incidence. The use of this technique at three angles of incidence provides multiple sets of spectra (including both s- and p-polarizations) to analyze when compared to the standard normal incidence case, which has been used extensively in the past for these compounds. It is shown that the transverse optic (TO) and longitudinal optic (LO) mode phonon parameters can be determined with greater accuracy by using the factorized model for the fits, as compared with the popular classical model used previously, and by fitting the derivative of the reflectivity. Our results for the phonon mode parameters are similar to those found earlier, as could be expected, but are generally more precise. An analysis of the difference in frequency of the TO mode in antiferromagnetic NiO at room temperature for the two polarizations of reflected light revealed a TO mode splitting of about 4 cm−1, with the p-polarized light TO mode having the higher frequency: This small splitting is in agreement with theoretical predictions. This more precise IR method for revealing the phonon mode behavior in such magnetically ordered cubic metal oxides, which are prototypes of strongly correlated electronic systems and have been found to be Mott-Hubbard insulators, may be readily applied to any similar antiferromagnetic system.

Defect induced ferromagnetism in NiO nanocrystals: Insight from experimental and DFT+U study

We investigate the defects induced magnetic properties of NiO nanocrystals prepared by mechanical alloying ball milling process. The average crystallite size reduces to ∼12nm after 40 hours of milling but the face centered cubic structure is stable both in pure and milled NiO powders. The unmilled pure NiO is anti-ferromagnetic in nature, whereas the as-milled nanoparticles are ferromagnetic at room temperature with saturation magnetization reaching a maximum of 0.85 emu/g. X-ray photoelectrons spectroscopy (XPS) shows two distinct peaks at 854.2 and 856.1 eV specifically related to the typical Ni-O bonds from Ni2+ and Ni3+ respectively. This is clearly supports the presence of non-stoichiometry in the milled samples due to decrease in Ni2+-O2−-Ni2+ spin correlation length caused by defects. These defects and crystallinity are characterized by Raman spectra which shows that the two-magnon band disappears, one-phonon (1P) longitudinal optical (LO) band broadens, two-phonon (2P) LO band exhibits red-shift and disappears eventually with decreasing of particle size. Magnetic hysteresis (M-H) loops reveals the ferro-magnetic nature of NiO nanocrystals. Considering these defects, we have also investigated the electronic and magnetic properties systematically within the framework of density functional theory. Our results suggest that the vacancies created by only oxygen as well as both nickel and oxygen are largely responsible for ferro-magnetism in nanoscale NiO powders.

Harnessing Hot Phonon Bottleneck in Metal Halide Perovskite Nanocrystals via Interfacial Electron–Phonon Coupling

Slow hot carrier (HC) cooling resulting from hot phonon bottleneck has been widely demonstrated in metal halide perovskites. Although manipulating HC kinetics in these materials is of both fundamental and technological importance, this task remains a daunting challenge. Here, via interfacial engineering, i.e., epitaxial growth of Cs4PbBr6 on CsPbBr3 nanocrystals (NCs), we have revealed an obvious shortening of HC cooling times, evidenced by transient absorption and ultrafast PL spectra. Collaborated with the longitudinal optical (LO) phonon model, theoretical calculations verify the breaking of the hot phonon bottleneck in CsPbBr3@Cs4PbBr6 and identify the interfacial electron-LO phonon coupling as the leading mechanism for the observed large tuning of HC cooling times. Especially, the participation of LO phonons from Cs4PbBr6 enables the efficient Klemens channel for hot phonon decay. Our findings establish an effective method to tailor HC dynamics in perovskite NCs, which could be conducive to improving the performance of optoelectronic applications.

Atomic fluctuations in electronic materials revealed by dephasing

The microscopic origin and timescale of the fluctuations of the energies of electronic states has a significant impact on the properties of interest of electronic materials, with implication in fields ranging from photovoltaic devices to quantum information processing. Spectroscopic investigations of coherent dynamics provide a direct measurement of electronic fluctuations. Modern multidimensional spectroscopy techniques allow the mapping of coherent processes along multiple time or frequency axes and thus allow unprecedented discrimination between different sources of electronic dephasing. Exploiting modern abilities in coherence mapping in both amplitude and phase, we unravel dissipative processes of electronic coherences in the model system of CdSe quantum dots (QDs). The method allows the assignment of the nature of the observed coherence as vibrational or electronic. The expected coherence maps are obtained for the coherent longitudinal optical (LO) phonon, which serves as an internal standard and confirms the sensitivity of the technique. Fast dephasing is observed between the first two exciton states, despite their shared electron state and common environment. This result is contrary to predictions of the standard effective mass model for these materials, in which the exciton levels are strongly correlated through a common size dependence. In contrast, the experiment is in agreement with ab initio molecular dynamics of a single QD. Electronic dephasing in these materials is thus dominated by the realistic electronic structure arising from fluctuations at the atomic level rather than static size distribution. The analysis of electronic dephasing thereby uniquely enables the study of electronic fluctuations in complex materials.

Quantitative analysis of diffraction and infra-red spectra of composite cement/BaSO4/Fe3O4 for determining correlation between attenuation coefficient, structural and optical properties

To better understand the structural and optical properties of composite cement/BaSO4/Fe3O4 for various amount of BaSO4/Fe3O4, the X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy have been used to investigate the correlation between their structural and optical properties. The structural properties including crystallite size, micro strain, stress and energy deformation were analyzed from the quantitative analysis of XRD spectra by using size-strain plot (SSP) methods. The refractive index (n), extinction coefficient (k), dielectric functions (ε), and energy loss function (Im (−1/ε)) were analyzed from the quantitative analysis of FTIR spectra by using kramers-kronig (K–K) relations. The corresponding structures for high amount of BaSO4/Fe3O4 in composite cement/BaSO4/Fe3O4 become less stable which consistent with the distance between the wavenumber of transversal and longitudinal optical phonon vibration mode become shorter. For all composites cement/BaSO4/Fe3O4 in this study, we found that the distance wavenumber (Δ) between longitudinal optical (LO) and transversal optical (TO) phonon vibration decrease with increasing the crystallite size and linear attenuation coefficient. Our results indicated that the FTIR spectra could be useful for determining the optical phonon vibration, dielectric function, and energy loss function of composite cement/BaSO4/Fe3O4.

Long Carrier Diffusion Length and Slow Hot Carrier Cooling in Thin Film Mixed Halide Perovskite

The long diffusion length is the key to achieve remarkable power conversion efficiency and delayed hot carriers (HCs) relaxation helps to overcome the theoretical Shockley–Queisser limit in perovskite photovoltaic. Here, we investigate the incident photon density dependent bandedge bleaching and HCs dynamics in thin film methyl-ammonium lead iodide-chloride (mixed halide) perovskite using transient absorption spectroscopy. The bandedge bleaching dynamics shows the radiative recombination rate constant of the order of ∼10−11 cm−3 s−1 results in the maximum carrier diffusion length of the order of ∼10 μm. The HCs dynamics indicates cooling occurs by longitudinal-optical (LO) phonon emission in early ∼800 fs via Frohlich electron–phonon interaction at low-carrier concentration (∼1.7 × 1018 cm−3) and an efficient phonon bottleneck due to the delayed LO-phonon decay by two order of magnitude about ∼11 ± 1.3 ps at high carrier density (∼1019 cm−3). Our finding shows the potential of thin film mixed halide perovskite in HCs photovoltaic and light emission application.

Revisiting longitudinal optical modes in materials with plasmon and plasmon-like absorptions – SrTiO3 and β-Ga2O3

We investigate exemplary the longitudinal optical (LO) mode order in compounds with a plasmon or plasmon-like phonon mode and additional phonon modes. When the oscillator strength of the plasmon or plasmon-like mode is gradually increased, a reordering of the modes takes place. Since it is not possible in crystals with orthorhombic or higher symmetry that a LO mode crosses a transverse optical (TO) mode's position, this reordering takes place via mode hybridization. During this mode hybridization, the plasmon or plasmon-like LO mode gradually becomes the originally higher situated LO mode while the latter morphs into the former. As a consequence, inner (LO-TO) and an outer (TO-LO) mode pairs are formed. These insights can be readily transferred to other semiconductors or many mode materials with reststrahlen bands, as long as their dielectric tensor can be (partly) diagonalized, and allow easily correct mode assignments.

Terahertz electromagnetic waves radiated from coherent longitudinal optical (LO) phonons and LO-phonon plasmon coupled modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals

We report on terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons and coherent LO-phonon plasmon coupled (LOPC) modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals. The coherent LO phonons and coherent LOPC modes are observed in the three samples. In contrast, coherent transverse optical (TO) phonons are not observed in the (110)- and (111)-oriented samples in spite of that the TO phonon is the Raman active mode. We conclude that only the coherent longitudinal oscillating polarization such as coherent LO phonons and coherent LOPC modes radiate the terahertz wave and that the Raman selection rule does not contribute to the terahertz-wave-radiation process. The photogenerated electron-density dependence of the frequencies of the coherent LOPC mode is almost the same in the three samples. This finding indicates that the coupling of the LO phonon with the plasmon is not influenced by the surface orientations of the three samples.

Study of optical constants and dielectric properties of nanocrystalline α-cordierite ceramic

ABSTRACT The nanocrystalline 2 Mg.2Al2O3.5SiO2 as amorphous-type α-cordierite was prepared by the modified sol-gel (or Pechini) technique. After calcination at 800ºC, the pressed powder as disk pellets was sintered at 1150ºC, 1250ºC, and 1350ºC. The structural information of α-cordierite was obtained by X-ray diffraction (XRD), scanning electron microscopy (SEM), micro-Raman scattering spectra, and Fourier transform infrared (FTIR) spectra. The XRD and Raman spectra showed that the nanocrystalline α-cordierite phases from pellets of glass-ceramic powders appeared when the temperature increased. The SEM results confirmed the rounded shape of the nanocrystalline α-cordierite. Using Kramers–Kronig (KK) method, the FTIR spectroscopy was evaluated. The increasing synthesized temperature increased the size of nanocrystalline. Furthermore, by increasing the synthesized temperatures the optical constants of nanocrystalline, both real and imaginary parts of the refractive index and dielectric function decreased. Consequently, the transversal optical (TO) and longitudinal optical (LO) phonon frequencies shifted to red frequencies by increasing reaction temperature.

Fano effect in resonant Raman spectrum of CdTe

In this work, we show a line-shape analysis of the resonant Raman scattering spectrum of un-doped CdTe single crystal. We found that the main peak correspondent to the longitudinal-optic (LO) phonon mode is asymmetrically broadened toward the high-frequency side. We attributed this asymmetry to an electron-phonon coupling effect, which was demonstrated using a Fano-type function to describe the spectrum line-shape. In our fitting we have additionally considered the weak contribution (in the low-frequency region) of two added phonons, whose presence is attributed to the rupture of the symmetry due to the surface. Results show an excellent agreement between the experimental data and the fitting model, where Fano asymmetry parameter was found to be q=4.5. The results obtained, are a tool that contributes to the understanding of the origin of LO phonon line-shape asymmetry of the Raman spectrum; and its use will be helpful to investigate the strength of electron-phonon coupling in semiconductors.

Infrared-active phonon modes in single-crystal thorium dioxide and uranium dioxide

The infrared-active phonon modes, in single-crystal samples of thorium dioxide (ThO2) and uranium dioxide (UO2), were investigated using spectroscopic ellipsometry and compared with density functional theory. Both ThO2 and UO2 are found to have one infrared-active phonon mode pair [consisting of one transverse optic (TO) and one associated longitudinal optic (LO) mode], which is responsible for the dominant features in the ellipsometric data. At room temperature, our results for the mode pair’s resonant frequencies and broadening parameters are comparable with previous reflectance spectroscopy characterizations and density functional theory predictions. For ThO2, our ellipsometry and density function theory results both show that the LO mode broadening parameter is larger than the TO mode broadening. This signifies mode anharmonicity, which can be attributed to the intrinsic phonon–phonon interaction. In addition to the main mode pair, a broad low-amplitude impurity-like vibrational mode pair is detected within the reststrahlen band for both ThO2 and UO2. Elevated temperature measurements were performed for ThO2 in order to study the mechanisms by which the phonon parameters evolve with increased heat. The observed change in the TO resonant frequency is in excellent agreement with previous density functional calculations, which only consider volume expansion of the crystal lattice. This suggests that the temperature-dependent change in the TO frequency is primarily due to volume expansion. The change in the main mode pair’s broadening parameters is nearly linear within the temperature range of this study, which indicates the intrinsic anharmonic scattering (via cubic anharmonicities) as the main decay mechanism.

Optical Response of an Interacting Polaron Gas in Strongly Polar Crystals

Optical conductivity of an interacting polaron gas is calculated within an extended random phase approximation which takes into account mixing of collective excitations of the electron gas with longitudinal optical (LO) phonons. This mixing is important for the optical response of strongly polar crystals where the static dielectric constant is rather high, as in the case of strontium titanate. The present calculation sheds light on unexplained features of experimentally observed optical conductivity spectra in n-doped SrTiO 3 . These features appear to be due to dynamic screening of the electron–electron interaction by polar optical phonons and hence do not require additional mechanisms for their explanation.

Probability Density, Transition Frequency and Spontaneous Emission Rate in the Asymmetric Gaussian Potential Two-Level System with Magnetic Field

The electronic probability density, the transition frequency and the longitudinal optical (LO) phonon spontaneous emission rate in the two-parameter asymmetric Gaussian (AG) potential two-level system with magnetic field are studied by using the Pekar variational method, and their single-parameter parabolic potential approximation is discussed. Numerical results show that the selecting the two-parameter AG potential to describe the restricted effects of electron in quantum dot can more accurately reveal the quantization characteristics of the wave property of electronic states, the statistical regularity of electron motion and the LO phonons’ spontaneous emission rate, and the results given by the single-parameter parabolic approximation are relatively rough. The influence of the magnetic field, dispersion and electron–phonon coupling of materials on the probability distribution, quantum transition frequency and the LO spontaneous emission rate in the two-level system cannot be ignored.

Coherent band-edge oscillations and dynamic longitudinal-optical phonon mode splitting as evidence for polarons in perovskites

The coherence of collective modes, such as phonons, and their modulation of the electronic states are long sought in complex systems, which is a cross-cutting issue in photovoltaics and quantum electronics. In photovoltaic cells and lasers based on metal halide perovskites, the presence of polaronic coupling, i.e., photocarriers dressed by the macroscopic motion of charged lattice, assisted by terahertz (THz) longitudinal optical (LO) phonons, has been intensely studied yet still debated. This may be key for explaining the remarkable properties of the perovskite materials, e.g., defect tolerance, long charge lifetimes and diffusion length. Here we use the intense single-cycle THz pulse with the peak electric field up to $E_{THz}=$1000\,kV/cm to drive coherent band-edge oscillations at room temperature in CH$_3$NH$_3$PbI$_3$. We reveal the oscillatory behavior dominantly to a specific quantized lattice vibration mode at $\omega_{\mathrm{LO}}\sim$4 THz, being both dipole and momentum forbidden. THz-driven coherent dynamics exhibits distinguishing features: the room temperature coherent oscillations at $\omega_{\mathrm{LO}}$ longer than 1 ps in both single crystals and thin films; the {\em mode-selective} modulation of different band edge states assisted by electron-phonon ($e$-$ph$) interaction; {\em dynamic mode splitting} controlled by temperature due to entropy and anharmonicity of organic cations. Our results demonstrate intense THz-driven coherent band-edge modulation as a powerful probe of electron-lattice coupling phenomena and provide compelling implications for polaron correlations in perovskites.

Influence of Dispersion and Polarization on the Two-Parameter Asymmetric Gaussian Confinement Potential Two-Level System with Electric Field

In this paper, the influence of dispersion and polarization on the electronic probability density, the transition frequency and the longitudinal optical (LO) phonon spontaneous emission rate in the two-parameter asymmetric Gaussian confinement (AGC) potential two-level system with the electric field are studied by using the Pekar-type variational method. Numerical results show that the selecting the two-parameter AGC potential to describe the restricted effects of electron in quantum dot can more accurately reveal the quantization characteristics of of the wave property of electronic states, the statistical regularity of electron motion and LO phonons spontaneous emission rate. The influence of the electric field, dispersion and electron -phonon coupling of materials on the probability distribution, transition frequency and LO spontaneous emission rate in the two-level system cannot be ignored.