Electron Longitudinal Optical Phonon Coupling Research Articles

Engineering of electron–longitudinal optical phonon coupling strength in m-plane GaN terahertz quantum cascade lasers

In this work nonpolar (m-plane) GaN terahertz quantum cascade lasers are designed based on the non-equilibrium Green’s function method. The lasing quantum-level broadening that arises from electron–longitudinal optical (LO) phonon coupling is studied by tuning the coupling strength, and the changes of optical gain due to this broadening are demonstrated in a self-consistent way. It is found this coupling process largely widens the linewidth of intersubband transition and then quenches the lasing even at a low temperature of 10 K. The electron–LO phonon coupling strength, however, can be manually engineered by tailoring the quantum well structure. As a result, the optical gain will be recovered above the cavity threshold, indicating a potential for terahertz lasing from GaN semiconductors.

Temperature-Dependent Cationic Doping-Driven Phonon Dynamics Investigation in CdO Thin Films Using Raman Spectroscopy

In the present work, undoped cadmium oxide (CdO)- and 1% Tin (Sn)-doped CdO thin films were prepared by the sol–gel route. These samples have been analyzed by temperature (T)-dependent (80–500 K) Raman spectroscopy and studied for their lattice dynamics and vibrational density of states. Results indicate that the room T synthesized pure CdO thin film manifests two prime second order features, that is, 300.5 cm–1 transverse optical (TO) and 488 cm–1 longitudinal optical (LO) phonon modes. However, incorporation of cationic-assisted impurity (Sn) with larger ionic radii results in a softening of the high-frequency LO (480.5 cm–1) mode via lattice deformation scattering potential. In fact, selective Sn doping increases the carrier concentration in the host CdO matrix which subsequently mitigates intraionic anharmonicity owing to increased Coulomb screening, leading to disappearance of the low-frequency TO mode (300.5 cm–1). In the case of pure CdO thin films, surface electron-induced electron–LO phonon coupling causes the intensity enhancement in LO modes, while negligible four-phonon anharmonic coupling results in lowering of full width at half maxima below Debye T. On the other hand, Frühlich interaction in the polar LO phonon mode supersedes the impact of anharmonic decay and dominates the overall phonon decay process by impurity incorporation, via appropriate Sn doping. Theoretical phonon dispersion profiles throughout the Brillouin zone with increasing T suggests stronger TO phonon mode softening along with optical branch broadening followed by LO–TO splitting. The acoustic branch barely suffers any shift with changing T which cannot be observed in experimental spectra. However, the flexural phonon modes confer a direct indication of changed rigidity and bond stiffness with varying T. Overall, the present investigation provides experimental evidence regarding the significance of Debye T, below and above which the three- and four-phonon anharmonicity are feasible in phonon scattering in conjunction with theoretical insights.

Intraband relaxation process in PbSe quantum dot studied by lattice relaxation method

Within the frame of lattice relaxation model, several intraband relaxation processes between the three lowest excited states and ground state in a PbSe quantum dot are studied based on the electron-longitudinal optical phonon coupling via Fröhlich mechanism. We find that Huang-Rhys factors decrease with the radius of quantum dot increasing in different relaxation processes. More important is the fact that the obtained values of Huang-Rhys factors satisfy the experimental measurements in the strong coupling limit. These intraband relaxation processes follow the asymmetrical Gaussian distribution with respect to the radius, in which the probabilities with which these intraband relaxation transitions occur are different. Among these relaxation processes, two intraband transitions from the third excited state to ground state and to second excited states dominate the relaxation processes on a several nanometer scale of radius. Moreover, the temperature dependence for each of these relaxation processes can be modulated by the radius of quantum dot. These theoretical results are consistent with the experimental measurements and provide an important insight into the intraband relaxation in quantum dots in experiments.

Influence of magnetic field and Rashba spin–orbit coupling on strong-coupling magnetopolarons in quantum disks

On the basis of Lee–Low–Pines (LLP) unitary transformation, the influence of external magnetic field, Rashba spin–orbit coupling and quantum size effect on the ground-state interaction energy of strong-coupling magnetopolarons in quantum disks (QDs) is studied by using the Tokuda improved linear combine operator method. The results show that the ground-state interaction energy of magnetopolarons consists of four parts: the energy caused by the confinement potential of QDs, interaction energy between the electron and external magnetic field, electron and longitudinal-optical (LO) phonon interaction energy and additional term of Rashba effect originating from phonons. The electron–LO phonon interaction energy Ee- ph and additional term of Rashba effect are always negative; the absolute value |Ee- ph | increases with increasing transverse confinement strength ω0, cyclotron frequency of external magnetic field ωc and electron–LO phonon coupling strength α, but decreases with increasing the thickness of QDs L; the state properties of magnetopolarons are closely linked with the sign of the ground-state interaction energy of magnetopolarons E int and change of E int with ωc, ω0, α and L. In addition, the vibration frequency of magnetopolarons λ increases with increasing ωc, ω0 and α, but decreases with increasing L. For the ground state of magnetopolarons in QDs, the electron–LO phonon interaction plays a significant role, meanwhile, the influence of Rashba spin–orbit coupling effect cannot be ignored.

Gamma-irradiation effect on electron–phonon coupling in Ge-doped CdTe crystals: Raman and photoluminescence study

An impact of different doses (10–500kGy) of γ-irradiation on electron–phonon coupling in Ge-doped CdTe (CdTe:Ge) single crystals was investigated via low-temperature (T=5K) photoluminescence (LTPL) and resonant Raman scattering (RRS). In the present work electron–phonon coupling concerns longitudinal optical (LO) phonons and was described in terms of Huang–Rhys factor. It was obtained γ-irradiation in such dose range leads to the increases in the value of Huang–Rhys factor. At high doses (equal to 200kGy) the saturation of the increased at low doses Huang–Rhys factor was observed. Based on the features of radiation-stimulated changes in LTPL, increasing of electron–LO–phonon coupling can be explained by radiation-induced defect compensation process involving, with the great probability, Ge dopant atoms filling of native cadmium vacancies (VCd) and by other related mechanisms such as radiation-induced Ge clusters dissociation. Such compensation is resulted in the proving of the crystalline perfection of irradiated crystals. The latter was confirmed by resonant Raman scattering measurements. Resonant Raman conditions for near E0+Δ0 gap (2,54eV at 80K) were obtained by temperature tuning of the band gap keeping the excitation energy fixed for laser excitation energies near E0+Δ0. It was found good agreement in the dose dependencies of the values of Huang–Rhys factors and 2LO/1LO phonon modes ratio obtained from resonant Raman spectra. The increasing in the intensity of Fröhlich-induced 2LO-phonon modes with the increase of the irradiation dose and decreasing in impurity-induced 1LO-phonon intensity indicates about decreasing in the defect concentration in studied crystals. Relatively high saturation dose corresponding to the 2LO/1LO phonon modes ratio indicates high radiation strength of studied material.

Multiphonon Raman Scattering in Coupled Cd1–xMnxS Nanoparticles: Magnetic Doping and Thermal Annealing

We report on multiphonon Raman scattering in coupled Cd1–xMnxS nanoparticles grown within a glass matrix by the melting nucleation approach. Magnetic force microscopy identified two different-sized groups of nanoparticles coupled together. The high-frequency shoulder appearing above the longitudinal optical (LO) phonon mode has evidenced the acoustic–optical phonon coupling in nanoparticles, which depends upon the Mn doping. We found that the multiphonon Raman scattering and the electron–LO phonon coupling can be tuned by thermal annealing. Finally, we found that the magnetic doping induces changes on both the sp–d exchange interaction and the nanoparticles’ crystalline quality. We emphasize that our findings may have an impact on possible technological applications that request handling of the optically induced nonlinear features in nanoparticles which are favored by electron–phonon anharmonicities.

Effects of electron– and impurity-ion–LO phonon couples on the impurity states in cylindrical quantum wires

The variational method and the effective mass approximation are used to calculate the phonon effects on the hydrogenic impurity states in a cylindrical quantum wire with finite deep potential by taking both the couplings of the electron-confined bulk longitudinal optical (LO) phonons and the impurity-ion–LO phonons into account. The binding energies and the phonon contributions are calculated as functions of the transverse dimension of the quantum wire. The results show that the polaronic effect induced by the electron–LO phonon coupling and the screening effect induced by the impurity-ion–LO phonon coupling tend to compensate each other and the total effects reduce the impurity binding energies.