Two-phonon Processes Research Articles

Nanoengineered diamond waveguide as a robust bright platform for nanomagnetometry using shallow nitrogen vacancy centers.

Photonic structures in diamond are key to most of its application in quantum technology. Here, we demonstrate tapered nanowaveguides structured directly onto the diamond substrate hosting shallow-implanted nitrogen vacancy (NV) centers. By optimization based on simulations and precise experimental control of the geometry of these pillar-shaped nanowaveguides, we achieve a net photon flux up to ∼ 1.7 × 10(6) s(-1). This presents the brightest monolithic bulk diamond structure based on single NV centers so far. We observe no impact on excited state lifetime and electronic spin dephasing time (T2) due to the nanofabrication process. Possessing such high brightness with low background in addition to preserved spin quality, this geometry can improve the current nanomagnetometry sensitivity ∼ 5 times. In addition, it facilitates a wide range of diamond defects-based magnetometry applications. As a demonstration, we measure the temperature dependency of T1 relaxation time of a single shallow NV center electronic spin. We observe the two-phonon Raman process to be negligible in comparison to the dominant two-phonon Orbach process.

Fast two-phonon relaxation process between the Landau levels of graphene on different polar substrates

Within the frame of Huang-Rhys's lattice relaxation model, we theoretically investigate the carrier relaxation mediated by two-phonon processes, which consists in polar surface optical phonon and longitudinal acoustic phonon emission between the Landau levels of graphene on different polar substrates. This two-phonon relaxation process is very fast within the picosecond scale and may seriously hinder the carrier multiplication based on effective Auger processes. Moreover, the polarizability of the polar substrate plays an important role in determining the relaxation time. These results could be helpful in designing graphene-based photoelectric and photodetector devices.

Analysis of two-phonon infrared spectral features of gallium arsenide and indium phosphide by first-principles calculations

We perform a self-consistent calculation based on density functional perturbation theory to analyze the infrared spectral features of GaAs and InP arising from two-phonon processes. The features are identified and assigned the critical points in the first Brillouin zone. Distribution of the critical points is investigated. The analysis demonstrates that collections of phonons of wave vectors around symmetry points and along symmetry lines are responsible for strong infrared features in two-phonon processes.

Theory of optomechanical interactions in superfluid He

A general theory is presented to describe optomechanical interactions of acoustic phonons, having extremely long lifetimes in superfluid $^{4}\mathrm{He}$, with optical photons in the medium placed in a suitable electromagnetic cavity. The acoustic nonlinearity in the fluid motion is included to consider processes beyond the usual linear process involving the absorption or emission of one phonon at a time. We first apply our formulation to the simplest one-phonon process involving the usual resonant anti-Stokes upconversion of an incident optical mode. However, when the allowed optical cavity modes are such that there is no single-phonon mode in the superfluid, which can give rise to a resonant allowed anti-Stokes mode, we must consider the possibility of two-phonon upconversion. For such a case, we show that the two-step two-phonon process could be dominant. We present arguments for a large two-step process and negligible single-step two-phonon contribution. The two-step process also shows interesting quantum interference among different transition pathways.

Entanglement rebirth of multi-trapped ions with trap phonon modes: entanglement sudden death with recovery

We consider a multi-qubit system consisting of two trapped ions coupled in a laser field. The ions are identical three-level electronic systems which interact with one another through the phonon modes of their relative or center of mass motions, and the system is tuned so that two-phonon processes dominate the electronic transitions. The resulting evolution of the system is studied theoretically with a focus on the entanglement properties of the system. A method of quantifying the entanglement is discussed, and the time dependence of these quantifications is determined. The cases of the two ions coupled to the same phonon field and to two different isolated phonon fields are compared for Fock cavity modes. Instances of the entanglement sudden death recovery are identified in these various systems.

Mechanisms of loss formation in nonlinear optical crystals ZnGeP2 in the terahertz frequency range

Physical mechanisms of formation of radiation losses in the terahertz range in ZnGeP2 crystals have been experimentally studied in the wave number range of 5–350 cm−1 at temperatures of 10–300 K. The dominant contribution of two-phonon difference processes to the loss formation in the given frequency range has been shown.

Electron-phonon coupling in suspended graphene: supercollisions by ripples.

Using electrical transport experiments and shot noise thermometry, we find strong evidence that "supercollision" scattering processes by flexural modes are the dominant electron-phonon energy transfer mechanism in high-quality, suspended graphene around room temperature. The power law dependence of the electron-phonon coupling changes from cubic to quintic with temperature. The change of the temperature exponent by two is reflected in the quadratic dependence on chemical potential, which is an inherent feature of two-phonon quantum processes.

Significance of an external magnetic field on two-phonon processes in gated lateral semiconductor quantum dots

Theoretical and numerical calculations of two-phonon processes on gated lateral semiconductor quantum dots (QDs) are outlined. A heterostructure made with two laterally coupled QDs, in the presence of an external magnetic field, has been employed in order to study the electron scattering rate due to two-phonon processes. The formalism is based on the acoustic phonon modes via the unscreened deformation potential and the piezoelectric interaction whenever the crystal lattice lacks a center of inversion symmetry. The rates are calculated by using second order perturbation theory. The strong dependence of the scattering rate on the external magnetic field, lattice temperature and QDs separation distance is presented.

Relaxation times of the two-phonon processes with spin-flip and spin-conserving in quantum dots

We perform a theoretical investigation on the two-phonon processes of the spin-flip and spin-conserving relaxation in quantum dots in the frame of the Huang-Rhys' lattice relaxation model. We find that the relaxation time of the spin-flip is two orders of magnitude longer than that of the spin-conserving, which is in agreement with previous experimental measurements. Moreover, the opposite variational trends of the relaxation time as a function of the energy separation for two-phonon processes are obtained in different temperature regime. The relaxation times display the oscillatory behaviors at the demarcation point with increasing magnetic field, where the energy separation matches the optical phonon energy and results in the optical phonon resonance. These results are useful in understanding the intraband levels' relaxation in quantum dots and could be helpful in designing photoelectric and spin-memory devices

Phonon-mediated decay of singlet-triplet qubits in double quantum dots

We study theoretically the phonon-induced relaxation ($T_1$) and decoherence times ($T_2$) of singlet-triplet qubits in lateral GaAs double quantum dots (DQDs). When the DQD is biased, Pauli exclusion enables strong dephasing via two-phonon processes. This mechanism requires neither hyperfine nor spin-orbit interaction and yields $T_2 \ll T_1$, in contrast to previous calculations of phonon-limited lifetimes. When the DQD is unbiased, we find $T_2 \simeq 2 T_1$ and much longer lifetimes than in the biased DQD. For typical setups, the decoherence and relaxation rates due to one-phonon processes are proportional to the temperature $T$, whereas the rates due to two-phonon processes reveal a transition from $T^2$ to higher powers as $T$ is decreased. Remarkably, both $T_1$ and $T_2$ exhibit a maximum when the external magnetic field is applied along a certain axis within the plane of the two-dimensional electron gas. We compare our results with recent experiments and analyze the dependence of $T_1$ and $T_2$ on system properties such as the detuning, the spin-orbit parameters, the hyperfine coupling, and the orientation of the DQD and the applied magnetic field with respect to the main crystallographic axes.

Multiple-timescale relaxation dynamics in CsGd(MoO4)2—a dipolar magnet with a highly anisotropic layered crystal structure

The dynamic properties of the dipolar magnet CsGd(MoO4)2 have been studied. The frequency and temperature dependence of the AC susceptibility investigated in the paramagnetic region above 2 K revealed the co-existence of magnetic field induced slow and fast relaxation channels with a timescale differing by three orders of magnitude. The slow relaxation is determined by the properties of the first coordination sphere of the Gd3+ ion and has the character of a two-phonon Orbach process. The fast relaxation is potentially attributed to a two-phonon Raman process realized via a localized phonon mode associated with the layered crystal structure. The temperature dependence of the phonon mean free path in zero magnetic field indicates significant phonon scattering below 1 K resulting from the combined effect of magnetic correlations and the scattering of dominant phonons with energies corresponding to the crystal-field levels.

Strong Coupling of Rattling Phonon to Conduction Electrons in Semimetallic Type-I Clathrate Ba8Ga16Sn30

We report studies of 117,119Sn and 69,71Ga nuclear magnetic resonance (NMR) to investigate electron–phonon interactions due to the off-center rattling of Ba atoms in the semi-metallic type-I Ba8Ga16Sn30 clathrate with \(n\)-type charge carriers in the temperature range of 0.4–200 K. The Ga NMR spin–lattice relaxation rate has an anomalous enhancement at approximately 10 K, indicating that the Ba atoms exhibit thermal rattling motion in an anharmonic potential. From calculations using a two-phonon Raman process model for a one-dimensional anharmonic Hamiltonian within a quasi-harmonic approximation, significant broadening of the phonon lifetime and softening of the phonon frequency are found to be essential to reproduce the temperature dependence of the NMR spin–lattice relaxation rate. The characteristic energy of the renormalized phonon frequency ωr≈21 K was obtained for the rattling state, in agreement with the values estimated by heat capacity analysis and other techniques. These results point to strong coupling of the rattling phonon to conduction electrons in semimetallic type-I Ba8Ga16Sn30.

Dynamics of negatively charged divacancies in neutron irradiated type Ib diamond

Thermal neutron irradiation of synthetic type Ib diamond, followed by annealing, results in a number of paramagnetic point defect centers. One of these is the composite defect designated W29. Previous EPR measurements have shown that the W29 center (S=3/2) is a charged divacancy [VV]− with spin Hamiltonian parameters similar to those of the R4/W6 neutral divacancy center [VV]0 (S=1) found in irradiated high purity type IIa diamond. The present EPR linewidth and spin-lattice relaxation rate measurements, made as a function of temperature on the W29 center, show that an Orbach two-phonon process, with a gap of ~20meV, plays a dominant role in determining the relaxation behavior above 20K. Below 20K a direct single phonon process provides the spin-lattice relaxation mechanism and the relaxation times become long, of the order of milliseconds. The linewidth behavior with temperature is accounted for in terms of changes in the spin dynamics.

Signatures of Nonlinear Cavity Optomechanics in the Weak Coupling Regime

We identify signatures of the intrinsic nonlinear interaction between light and mechanical motion in cavity optomechanical systems. These signatures are observable even when the cavity linewidth exceeds the optomechanical coupling rate. A strong laser drive red detuned by twice the mechanical frequency from the cavity resonance frequency makes two-phonon processes resonant, which leads to a nonlinear version of optomechanically induced transparency. This effect provides a new method of measuring the average phonon number of the mechanical oscillator. Furthermore, we show that if the strong laser drive is detuned by half the mechanical frequency, optomechanically induced transparency also occurs due to resonant two-photon processes. The cavity response to a second probe drive is in this case nonlinear in the probe power. These effects should be observable with optomechanical coupling strengths that have already been realized in experiments.

Optical characteristics of Er3+ ion in Er/Yb:LiNbO3 crystal: Comparison with the dissimilar effect of anti-photorefractive ions Zn2+, In3+ and Zr4+

The different influences of Zn2+, In3+ and Zr4+ ions on the optical characteristics of Er3+ ion in Er/Yb:LiNbO3 crystals were discussed. An enhanced 1.54μm emission was observed for Zr/Er/Yb:LiNbO3 crystal, but the Zn2+ tri-doping resulted in a decreased one, and the intensity of 1.54μm emission remained about same in In/Er/Yb:LiNbO3 crystal. The populations of the green emitting 4S3/2/2H11/2 states were achieved through the three-, two- and two-phonon processes in Zn/Er/Yb:LiNbO3, In/Er/Yb:LiNbO3 and Zr/Er/Yb:LiNbO3 crystals, respectively. Zn2+ and In3+ ions affected the optical characteristics of Er3+ ion via modifying the Er3+ ion occupancy in Er/Yb:LiNbO3 crystal. The formation of ErLi2+–ErNb2− ion pairs caused by the Zn2+ and In3+ ions could increase the rate of cross relaxation process. The OH− absorption spectra showed that the incorporation of Zr4+ ions increased OH− content, which increased the probability of the nonradiative relaxation process of 4I11/2→4I13/2 (Er) in Zr/Er/Yb:LiNbO3 crystal. The J–O intensity parameters Ωt (t=2, 4 and 6), the radiative lifetime (τrad) and fluorescence branching ratio (β) in Zr/Er/Yb:LiNbO3 crystal were predicted by Judd–Ofelt theory. Füchtbauer–Ladenburg and McCumber methods were carried out to calculate the emission cross-sections at 1.54μm emission. The gain cross-section, estimated as a function of the population inversion ratio, allowed us to evaluate a potential laser performance of Zr/Er/Yb:LiNbO3 crystal.

Nonmetallic behaviour in half-Heusler phases YPdSb, YPtSb and LuPtSb

We present the results of combined macroscopic [magnetic susceptibility, heat capacity, electrical resistivity] and microscopic [nuclear magnetic resonance (NMR)] studies of half Heusler phases YPdSb, YPtSb and LuPtSb. The bulk results indicated diamagnetic and nonmetallic behaviours. The Solomon quadrupolar echoes revealed for all three materials the presence of some static electric field gradients, and thus some small deviation from the perfect cubic symmetry. The spin-lattice relaxation rates of the 121,123Sb nuclei were shown to be nonmetallic, quadrupolar in nature and due to the dominant two-phonon Raman processes.

Two-phonon relaxation processes of the graphene in the magnetic field

We theoretically investigate the electron relaxation mediated by two-phonon (LO+LA) scattering between Landau levels in graphene. We chose the transitions from LL n=1 to n=0 and from n=2 to n=−1 as examples to study the temperature and magnetic field dependence of two-phonon relaxation processes. We find that the relaxation time is very fast and is in the nanoseconds scale. Moreover, the temperature dependence of the relaxation rates is very different depending on the lattice relaxation strength. We hope that our results can be confirmed by many related experiments.

Tunable slow light in a quadratically coupled optomechanical system

We theoretically investigate the slow light in a quadratically coupled optomechanical system. Different from the linear coupling case, the slow light via quadratic coupling derives from a two-phonon process, and the fluctuation in displacement plays a vital role in nonlinear coherence. The numerical results show that the slow light can be realized in an extensive range of parameters even at high temperature, e.g., 200 K. We also find that the environment temperature which provides almost all of the phonon energy, together with the coupling field power, jointly drive the realization of slow light.

Les kystes hydatiques thoraciques extrapulmonaires

The infrared reflectivity and transmittance and Raman scattering in Ga3InSe4 layered crystals were investigated in the frequency ranges of 100–400, 400–4000 and 25–500 cm−1. The refractive and absorption indices, the frequencies of transverse and longitudinal optical modes, high- and low-frequency dielectric constants were obtained from the analysis of the IR reflectivity spectra. The bands observed in IR transmittance spectra were interpreted in terms of two-phonon absorption processes.

Upconversion Luminescence of Tb3+ and Yb3+ Co-doped Hexagonal and Monoclinic GdPO4 Nanophosphors

Tb3+ and Yb3+ codoped hexagonal GdPO4 nanorods have been prepared by a hydrothermal method at 220°C for 12 h and monoclinic GdPO4 nanoparticles have been obtained by sintering the hydrothermal products at 900°C for 5 h. Green upconversion luminescence of Tb3+ has been observed when pumped by 980 nm diodes due to efficiently cooperative energy transfer from codopant Yb3+. The luminescence of Tb3+ in both hexagonal and monoclinic GdPO4 nanophosphors has been comparatively studied. The dependence of the upconversion luminescence intensity on pump laser power shows that two-phonon processes involved in upconversion luminescence of the emission level 5D4 of Tb3+ is populated via two-photon process. Upconversion luminescence emission intensity and the energy transfer efficiency ηT from Yb3+ to Tb3+ ions upon the concentration of Tb3+ have been investigated.