Phonon Absorption Research Articles

Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO3.

Photonics in the frequency range of 5-15terahertz (THz) potentially open a new realm of quantum materials manipulation and biosensing. This range, sometimes called "the new terahertz gap", is traditionally difficult to access due to prevalent phonon absorption bands in solids. Low-loss phonon-polariton materials may realize sub-wavelength, on-chip photonic devices, but typically operate in mid-infrared frequencies with narrow bandwidths and are difficult to manufacture on a large scale. Here, for the first time, quantum paraelectric SrTiO3 enables broadband surface phonon-polaritonic devices in 7-13THz. As a proof of concept, polarization-independent field concentrators are designed and fabricated to locally enhance intense, multicycle THz pulses by a factor of 6 and increase the spectral intensity by over 90 times. The time-resolved electric field inside the concentrators is experimentally measured by THz-field-induced second harmonic generation. Illuminated by a table-top light source, the average field reaches 0.5GVm-1 over a large volume resolvable by far-field optics. Theseresults potentially enable scalable THz photonics with high breakdown fields made of various commercially available phonon-polariton crystals for studying driven phases in quantum materials and nonlinear molecular spectroscopy.

OPTICAL PROPERTIES OF HgGaInS4 CRYSTALS

Luminescence, transmission, reflection, and wavelength modulation transmission and wavelength modulation reflection spectra of HgGaInS4 crystals were researched in the temperature range of 300-10 K. This article discusses the various features observed in the spectra of HgGaInS4 crystals. The dependence on temperature of the energy gap is shown. Spectra of wavelength modulation transmission have minima a1, a2, and a3, which vary with temperature and are caused by indirect transitions L-Г with phonon absorption and emission. Spectra of thicker crystals have more features caused by the same indirect transitions. The photoluminescence show a strong maximum at 2.43 eV, attributed to radiative recombination of free excitons. These various features have significant implications for the understanding of the electronic and optical properties of HgGaInS4 crystals.

Detection of germanium isotopes in biosynthesized Ge nanocrystals through different optical spectroscopy techniques

The current work aims to investigate the isotopic effect in biosynthesized Germanium nanocrystals (Ge NCs) through Raman and IR absorption spectra. By treating bulk germanium crystal with microorganisms in a weak magnetic field, Ge NCs with great chemical and isotopic purity are produced on its surface. Using various techniques, including X-ray diffraction, photoluminescence spectroscopy, micro-Raman spectroscopy, and FTIR spectroscopy, the structural and optical characteristics of the synthesized Ge NCs are studied. The PL, Raman, and IR peaks of synthesized material, which are recorded at various time intervals, are seen to have shifted. The PL peak shift of fabricated material is seen as a result of the quantum confinement effect. Raman and IR absorption spectra analysis, which are conducted at room temperature reveals the possible presence of germanium isotopes. It is observed that the phonon absorption peaks of synthesized germanium NC material shift to longer wavelengths with increasing average atomic mass of germanium. The observed optical properties of the synthesized Ge NCs have provided great help to study the change in structural and optical properties of the synthesized Ge crystal.

DEPENDENCE OF THE POSITION OF PHONON IR ABSORPTION BANDS OF GERMANIUM ISOTOPES ON THEIR MASS NUMBER

For the first time, in a single experiment, the dependence of the position of the phonon absorption peaks in the IR spectrum (range 11–40 μm) for all five stable monoisotopic germanium single crystals was studied. The dependence obtained can be used to study the mass composition of germanium of various isotopic purity.

Ultrafast dynamics of bright and dark excitons in monolayer WSe2 and heterobilayer WSe2/MoS2

The energy landscape of optical excitations in mono- and few-layer transition metal dichalcogenides (TMDs) is dominated by optically bright and dark excitons. These excitons can be fully localized within a single TMD layer, or the electron- and the hole-component of the exciton can be charge-separated over multiple TMD layers. Such intra- or interlayer excitons have been characterized in detail using all-optical spectroscopies, and, more recently, photoemission spectroscopy. In addition, there are so-called hybrid excitons whose electron- and/or hole-component are delocalized over two or more TMD layers, and therefore provide a promising pathway to mediate charge-transfer processes across the TMD interface. Hence, an in-situ characterization of their energy landscape and dynamics is of vital interest. In this work, using femtosecond momentum microscopy combined with many-particle modeling, we quantitatively compare the dynamics of momentum-indirect intralayer excitons in monolayer WSe2 with the dynamics of momentum-indirect hybrid excitons in heterobilayer WSe2/MoS2, and draw three key conclusions: First, we find that the energy of hybrid excitons is reduced when compared to excitons with pure intralayer character. Second, we show that the momentum-indirect intralayer and hybrid excitons are formed via exciton-phonon scattering from optically excited bright excitons. And third, we demonstrate that the efficiency for phonon absorption and emission processes in the mono- and the heterobilayer is strongly dependent on the energy alignment of the intralayer and hybrid excitons with respect to the optically excited bright exciton. Overall, our work provides microscopic insights into exciton dynamics in TMD mono- and bilayers.

Decoupling absorption and radiative cooling in mid-wave infrared bolometric elements.

We present a spectrally selective, passively cooled mid-wave infrared bolometric absorber engineered to spatially and spectrally decouple infrared absorption and thermal emission. The structure leverages an antenna-coupled metal-insulator-metal resonance for mid-wave infrared normal incidence photon absorption and a long-wave infrared optical phonon absorption feature, aligned closer to peak room temperature thermal emission. The phonon-mediated resonant absorption enables a strong long-wave infrared thermal emission feature limited to grazing angles, leaving the mid-wave infrared absorption feature undisturbed. The two independently controlled absorption/emission phenomena demonstrate decoupling of the photon detection mechanism from radiative cooling and offer a new design approach enabling ultra-thin, passively cooled mid-wave infrared bolometers.

Quantum-Interference-Enhanced Phonon Laser in Cavity Optomechanics

Cavity-optomechanics-based phonon lasers are traditionally described in terms of three-mode resonance, while the energy nonconservation optomechanical terms are conventionally ignored. Here, we present a complete theory of phonon lasers, emphasizing the importance of the previously omitted optomechanical interaction. We show that this indispensable optomechanical interaction is equivalent to a Kerr nonlinear interaction. Analytical results show that the interference between this Kerr-type interaction and three-mode resonance adds a transition channel with no phonon emission or absorption, thus enhancing phonon lasing and realizing an ultralow-threshold phonon laser and even a phonon cooler. The interference effect produces an asymmetric gain profile with a Fano spectral shape in addition to the traditional Lorentzian shape. Our complete theory shows perfect agreement with existing experiments and provides a new degree of freedom to manipulate optomechanical phonon lasers.

Phonons of electronic crystals in two‐dimensional semiconductor moiré patterns

AbstractWe theoretically studied the phonon properties of the triangular‐, stripe‐, and honeycomb‐type electronic crystals recently found in two‐dimensional semiconductor moiré patterns. By analyzing the phonon dispersions, we found the interaction induced lattice deformation in the zigzag‐stripe crystal results in a much higher dynamical stability than in the linear‐stripe crystal. Moreover, chiral phonons with finite magnetizations and large Berry curvatures can emerge in triangular and honeycomb crystals under time‐reversal or inversion symmetry breaking. The small effective mass of the electrons allows the selective and efficient generation of chiral phonons from the optical activity of zone‐center phonons combined with the anharmonicity, facilitating the realization of the phonon Hall effect. These findings point to an exciting new platform for exploring chiral phonons and related topological phononic devices.Key points The phonons of the electronic crystals in semiconductor moiré systems are systematically analyzed, which can lead to the infrared absorption, Raman scattering, and Hall effect of chiral phonons. The small effective mass of the electrons as compared to their ionic counterparts facilitates tuning the chiral phonons through external fields, which may stimulate further studies about elementary excitations of the intriguing correlated insulators in moiré systems. Our study connects the two active research fields of strongly correlated physics and chiral phonons.

Absorption and emission of longitudinal optical phonons by confined electrons in Pöschl–Teller quantum wells under the effects of the aluminum concentration, temperature, and hydrostatic pressure

In this study, we investigated the simultaneous effects of the alloy concentration, electron temperature, and hydrostatic pressure on the magneto-optical transport properties (MOTPs) of GaAs/AlηGa1−ηAs Pöschl–Teller quantum wells (QW) by using the operator projection method and profile technique. The optical absorption power and full width at half maximum were calculated and compared for the optically detected magneto-phonon resonance peaks with phonon absorption and emission. In addition, the main results were compared with those for the rectangular potential QW. The results showed that the alloy concentration, electron temperature, and hydrostatic pressure greatly influenced the MOTPs of the GaAs/AlηGa1−ηAs Pöschl–Teller QW in both cases with the absorption and emission of phonons. In addition, these effects were greater when the degree of asymmetry was stronger for the Pöschl-Teller QW. Our results also demonstrated that the combined effects of the alloy concentration, electron temperature, and hydrostatic pressure on the MOTPs of the GaAs/AlηGa1−ηAs Pöschl-Teller QW were always greater in the phonon absorption case than the phonon emission case. Furthermore, the combined effects of these three parameters on the MOTPs of the Pöschl-Teller potential QW were greater than those on the rectangular potential QW in both the phonon emission and absorption processes.

Low-energy solar neutrino detection utilizing advanced germanium detectors

We explore the possibility to use advanced germanium (Ge) detectors as a low-energy solar neutrino observatory by means of neutrino-nucleus elastic scattering. A Ge detector utilizing internal charge amplification for the charge carriers created by the ionization of impurities is a novel technology with experimental sensitivity for detecting low-energy solar neutrinos. Ge internal charge amplification (GeICA) detectors will amplify the charge carriers induced by neutrino interacting with Ge atoms through the emission of phonons. It is those phonons that will create charge carriers through the ionization of impurities to achieve an extremely low energy threshold of ∼0.01 eV. We demonstrate the phonon absorption, excitation, and ionization probability of impurities in a Ge detector with impurity levels of 3 × 1010 cm−3, 9 × 1010 cm−3, and 2 × 1011 cm−3. We present the sensitivity of such a Ge experiment for detecting solar neutrinos in the low-energy region. We show that, if GeICA technology becomes available, then a new opportunity arises to observe pp and 7Be solar neutrinos. Such a novel detector with only 1 kg of high-purity Ge will give ∼10 events per year for pp neutrinos and ∼5 events per year for 7Be neutrinos with a detection energy threshold of 0.01 eV.

Effect of band-gap tuning on absorption of phonons and acoustoelectric current in graphene nanoribbon

We report the use of Boltzmann Transport Equation (BTE) in the hypersound regime for investigating the generation of acoustoelectric (AE) current in an armchair graphene nanoribbon (AGNR). The AE current obtained was deduced from the absorption due to the Landau damping of quantized sound waves (LD-QSW) energy. By stimulating the AGNR with a non-quantized electric field, the absorption and the AE current were analysed against the frequency, the wavenumber, the width of the bandgap and the drift velocity. The absorption was observed to switch to amplification as the drift velocity was varied. At high drift velocities, the amplitude of the AE current decreases as the band-gap (that depends on the applied electric field) was increased, suggesting a potential use of AGNR in the design of tunable acoustoelectric current devices. • Graphene Nanoribbon. • Landau Damping. • Absorption. • Acoustoelectric current.

Thermoelectric properties of granular Mg3Sb2 particles

This article presents the results obtained in the study of thermoelectric properties of granulated Mg3Sb2 particles. The results of the study show that the thermoelectric properties of granulated Mg3Sb2 particles mainly depend on the physical processes occurring in the interparticle boundary areas. As the temperature increases, the localized traps in the interparticle boundary areas are ionized, and the capture of charge carriers in them leads to a decrease in electrical conductivity (σ). The Seebeck coefficient (α) increases as the temperature difference occurs due to the potential difference and phonon absorption. Also, impurity thermal-voltaic effects appear with the formation of electron-hole pairs in impurity states with an energy level of Ein in the interparticle boundary regions. As a result, the total λ increases at the same time as the thermal conductivity of the two adjacent areas. The convergence of electrical conductivity and potential difference leads to a relatively stable change of λ. These processes lead to a change in the ZT index from ~0.021 to ~1.3 at T=300-700 K.

Influence of magnetic field on the dielectric characteristics in manganese sulfide substituted with thulium

The properties of a solid solution of thulium-substituted manganese sulfide prepared by melting the initial pure samples of manganese sulfide and thulium sulfide are studied. X-ray diffraction analysis was carried out. The face-centered cubic lattice is determined, and the decrease in the intensity of the peaks is found. Dielectric permittivity was measured in the frequency range 102106 Hz and temperatures 300-500 K in magnetic fields up to 12 kOe in TmxMn1xS solid solutions (0 x 0,15). From the frequency dependence of the permittivity, the relaxation time and the relaxation mechanism of the dipole moments are found. The leakage current is excluded, and the contribution of migration polarization due to charges in the region with chemical phase separation is estimated. Die real and imaginary components of the permittivity of TmxMn1xS samples are described in the Debye model. The maximum of the imaginary component of the permittivity shifts towards higher frequencies upon heating, and the relaxation time is described by the Arrhenius function. Dielectric losses are caused by electron-phonon interaction. The frequency of the crossover from Debye relaxation to relaxation conduction associated with the absorption and emission of phonons is found. Die crossover frequency increases as the sample is heated according to a power law. In a magnetic field, the permittivity decreases and the largest change of 2.5 % is achieved at T = 450 K. At other temperatures, the magnetocapacitance does not exceed 0.5 %. The shift of the maximum of the imaginary component of the permittivity to low frequencies in a magnetic field leads to a change in the sign of dielectric losses from positive to negative. A qualitative difference in () is established in the vicinity of the concentration of thulium ion flow through the FCC lattice, where the permittivity is not described in the Debye model and there is no magnetocapacitance. The mechanism for reducing dielectric losses in a magnetic field is determined.

Ultrafast glimpses of the excitation energy-dependent exciton dynamics and charge carrier mobility in Cs2SnI6 nanocrystals.

Advancements in photovoltaic research suggest that tin-based perovskites are potential alternatives to traditional lead-based structures. Cs2SnI6, specifically, stands out as a notable candidate, exhibiting impressive performance. However, its complete potential remains untapped primarily owing to the limited understanding of its photophysics. In light of this, this study aims to bridge this knowledge gap. To commence our study, we first executed theoretical investigations to locate the energetically diverse excitons within the Brillouin zone. Building on this knowledge, we then utilized transient absorption spectroscopy to investigate their temporal evolution. Herein, we observed the formation of high-energy excitons even when the incident photon energy was below the necessary threshold, which is quite distinctive and intriguing. Of particular interest is the generation of ultraviolet (UV) domain exciton using visible photons, which implies that Cs2SnI6 has the potential for efficient solar light harvesting. Tracking the kinetics revealed that this unique finding arises due to the intertwined formation and decay pathways undertaken by the different excitons, aided by intervalley scattering and phonon absorption processes. In addition, we found that the decay of the UV exciton was unusually slow. Transient mobility investigations were undertaken to probe the carrier transport behavior that further established hot carriers (HCs) in Cs2SnI6 to be highly mobile and susceptible to polaron formation. Overall, our findings demonstrate that Cs2SnI6 is a strong candidate for HC-based photovoltaics because it possesses all the prerequisites desired for such applications.

Morphology-Controlled Reststrahlen Band and Infrared Plasmon Polariton in GaN Nanostructures.

Due to ultrabright and stable blue light emission, GaN has emerged as one of the most famous semiconductors of the modern era, useful for light-emitting diodes, power electronics, and optoelectronic applications. Extending GaN's optical resonance from visible to mid- and-far-infrared spectral ranges will enable novel applications in many emerging technologies. Here we show hexagonal honeycomb-shaped GaN nanowall networks and vertically standing nanorods exhibiting morphology-dependent Reststrahlen band and plasmon polaritons that could be harnessed for infrared nanophotonics. Surface-induced dipoles at the edges and asperities in molecular beam epitaxy-deposited nanostructures lead to phonon absorption inside the Reststrahlen band, altering its shape from rectangular to right-trapezoidal. Excitation of such surface polariton modes provides a novel pathway to achieve far-infrared optical resonance in GaN. Additionally, surface defects in nanostructures lead to high carrier concentrations, resulting in tunable mid-infrared plasmon polaritons with high-quality factors. Demonstration of morphology-controlled Reststrahlen band and plasmon polaritons make GaN nanostructures attractive for infrared nanophotonics.

Charge-State Dependent Vibrational Relaxation in a Single-Molecule Junction.

The outcome of an electron-transfer process is determined by the quantum-mechanical interplay between electronic and vibrational degrees of freedom. Nonequilibrium vibrational dynamics are known to direct electron-transfer mechanisms in molecular systems; however, the structural features of a molecule that lead to certain modes being pushed out of equilibrium are not well understood. Herein, we report on electron transport through a porphyrin dimer molecule, weakly coupled to graphene electrodes, that displays sequential tunneling within the Coulomb-blockade regime. The sequential transport is initiated by current-induced phonon absorption and proceeds by rapid sequential transport via a nonequilibrium vibrational distribution of low-energy modes, likely related to torsional molecular motions. We demonstrate that this is an experimental signature of slow vibrational dissipation, and obtain a lower bound for the vibrational relaxation time of 8ns, a value dependent on the molecular charge state.

Design and Fabrication of a C-Band Dielectric Resonator Antenna with Novel Temperature-Stable Ce(Nb1-xVx)NbO4 (x = 0-0.4) Microwave Ceramics.

Vanadium(V)-substituted cerium niobate [Ce(Nb1-xVx)O4, CNVx] ceramics were prepared to explore their structure-microwave (MW) property relations and application in C-band dielectric resonator antennas (DRAs). X-ray diffraction and Raman spectroscopy revealed that CNVx (0.0 ≤ x ≤ 0.4) ceramics exhibited a ferroelastic phase transition at a critical content of V (xc = 0.3) from a monoclinic fergusonite structure to a tetragonal scheelite structure (TF-S), which decreased in temperature as a function of x according to thermal expansion analysis. Optimum microwave dielectric performance was obtained for CNV0.3 with permittivity (εr) of ∼16.81, microwave quality factor (Qf) of ∼41 300 GHz (at ∼8.7 GHz), and temperature coefficient of the resonant frequency (TCF) of ∼ -3.5 ppm/°C. εr is dominated by Ce-O phonon absorption in the microwave band; Qf is mainly determined by the porosity, grain size, and proximity of TF-S; and TCF is controlled by the structural distortions associated with TF-S. Terahertz (THz) (0.20-2.00 THz, εr ∼ 12.52 ± 0.70, and tan δ ∼ 0.39 ± 0.17) and infrared measurements are consistent, demonstrating that CNVx (0.0 ≤ x ≤ 0.4) ceramics are effective in the sub-millimeter as well as MW regime. A cylindrical DRA prototype antenna fabricated from CNV0.3 resonated at 7.02 GHz (|S11| = -28.8 dB), matching simulations, with >90% radiation efficiency and 3.34-5.93 dB gain.

Size-Dependent Phonon-Assisted Anti-Stokes Photoluminescence in Nanocrystals of Organometal Perovskites.

Anti-Stokes photoluminescence (ASPL), which is an up-conversion phonon-assisted process of the radiative recombination of photoexcited charge carriers, was investigated in methylammonium lead bromide (MALB) perovskite nanocrystals (NCs) with mean sizes that varied from about 6 to 120 nm. The structure properties of the MALB NCs were investigated by means of the scanning and transmission electron microscopy, X-ray diffraction and Raman spectroscopy. ASPL spectra of MALB NCs were measured under near-resonant laser excitation with a photon energy of 2.33 eV and they were compared with the results of the photoluminescence (PL) measurements under non-resonant excitation at 3.06 eV to reveal a contribution of phonon-assisted processes in ASPL. MALB NCs with a mean size of about 6 nm were found to demonstrate the most efficient ASPL, which is explained by an enhanced contribution of the phonon absorption process during the photoexcitation of small NCs. The obtained results can be useful for the application of nanocrystalline organometal perovskites in optoelectronic and all-optical solid-state cooling devices.

Theory of ultrasound propagation in LuCo3 near the low-spin–high-spin crossover

The possibility of experimental observation of the ultrasonic attenuation in the ferromagnet $\mathrm{LuCo}{}_{3}$ near the low-spin-high-spin crossover is discussed. We show that ultrasound propagation gives rise to transitions between states of the magnon band due to absorption of phonons, and this process is highly sensitive to the value of magnetization. The high magnetic field, which governs the crossover, alters the ultrasound propagation regime from off-resonant to resonant and we formulate a criterion of the change. Calculated temperature and field dependences of the ultrasonic wave number and attenuation clearly demonstrate anomalies in these characteristics in the vicinity of the crossover at intermediate temperatures far below the Curie temperature.

Terahertz acoustic phonon Cerenkov emission in bilayer graphene

We present a theoretical investigation on the generation of Cerenkov emission of terahertz acoustic phonons in bilayer graphene (BLG) in the presence of a driving dc electric field. We have numerically and analytically studied the Cerenkov phonon emission spectrum, Pspectrum(ωp,θ), and phonon intensity, Pintensity(θ), dependence on the phonon frequency ωp, drift velocity vd, electron temperature Te, concentration n, and phonon emission angle θ in BLG with and without considering the chirality of the charge carriers. We find that the magnitude of Pspectrum(ωp,θ) increases at larger drift velocities and applied electric fields with the peak of the spectrum shifting toward the higher frequency side. The spectrum magnitude in BLG is found to be much enhanced as compared to conventional 2D semiconductors and transition metal dichalcogenides, which makes it viable for SASER and other practical device applications. The chiral nature of carriers strongly influences the Pspectrum(ωp,θ) behavior and sharpens the spectrum peak but with a decrease in the magnitude. The chirality favors the negative emission spectrum caused by the absorption of acoustic phonons. Pspectrum(ωp,θ) and Pintensity(θ) are found to be strongly dependent on temperature but independent of carrier concentration in the equipartition regime. The study is significant from the point of application of BLG as an acousto/optoelectronic device and high-frequency phonon spectrometers.