Two-phonon Absorption Research Articles

Near-infrared-terahertz hyper-Raman spectroscopy of an excited silicon surface.

We recorded the hyper-Raman spectra resulting from the interaction of a near-infrared (near-IR) picosecond pulse and a terahertz (THz) ultrashort pulse at the surface of a (111) silicon sample. A simple model is proposed to analyze the evolution of the hyper-Raman spectra vs the time delay between the near-IR and THz pulses. It links the hyper-Raman spectra to the multi-phonon absorption in silicon. This approach makes it possible to demonstrate that, during carrier generation by the near-IR pulse, the two-phonon and three-phonon absorption bands are enhanced in modes involving optical phonons. This process results from the very rapid and strong population of the optical phonons induced by the photo-generated hot carriers. It occurs over a few hundreds of femtoseconds and lasts throughout the duration of the near-IR pulse.

Transmission and nanohardness studies of ternary GaAs1-xPx layers grown from the vapor phase by heteroepitaxy

Transmission measurements in the 0.5–25-μm spectral range are performed on thin (<350-μm) GaAsP layers grown by Hydride Vapor Phase Epitaxy (HVPE) on plain (100) GaAs substrates. Comparison with calculations taking into account multiple reflections reveals the role of the surface polishing quality on the clear transmission window and the wavelength dependent losses. A measurement of a 5-mm thick epitaxially grown GaP underlines more realistic spectral limitations on the application of orientation-patterned structures based on GaP and GaAsP for nonlinear optical frequency conversion. The mid-IR cut-off wavelength for the ternary GaAsP layers is almost independent of the composition and corresponds to the same two-phonon absorption limit observed in the binary GaP. Nanohardness and Young's modulus are measured for the same samples to evaluate their compositional dependence. The nanohardness dependence on the P-content obeys a second order polynomial law with a maximum around P = 0.8. Young's modulus depends linearly on the P-content, similar to the trend observed in other ternary systems, such as InxGa1−xAs and InxGa1-xP. The evolution of the band-gap, estimated from the transmission measurements, with the composition of the ternary compounds is linear in the range of 0–0.5 for the P content. In this same range the nanohardness can be considered to be linearly proportional to the band-gap.

Generation of 17-32 THz radiation from a CdSiP2 crystal.

A phase-resolved electric field pulse is produced through the second-order nonlinear process of intra-pulse difference frequency generation (DFG) in a (110) CdSiP2 chalcopyrite crystal. The generated electric field pulse exhibits a duration of several picoseconds and contains frequency components within the high-frequency terahertz regime of ∼17-32 THz. The intra-pulse DFG signal is shown to be influenced by single-phonon and two-phonon absorption, the nonlinear phase-matching criterion, and temporal spreading of the excitation electric field pulse. To date, this is the first investigation in which a CdSiP2 chalcopyrite crystal is used to produce radiation within the aforementioned spectral range.

Molecular overtones and two-phonon combination bands in the near-infrared spectra of talc, brucite and lizardite

Abstract. The near-infrared (NIR) spectra of hydrous minerals display absorption bands involving multiple excitations of vibrational modes. They usually involve OH stretching modes, but their interpretation is not straightforward due to the combined effects of bond anharmonicity and vibrational coupling. In the present study, the mid-infrared (MIR) and near-infrared spectra of well-ordered samples of trioctahedral layered hydrous minerals, talc, brucite and lizardite, have been measured on a spectral range extending from the fundamental vibrational modes to the second OH stretching overtones. The bands corresponding to molecular overtones are interpreted using an effective approach allowing us to infer the anharmonicity and coupling parameters controlling the OH stretching frequencies from spectroscopic data. They follow the usual relation between transition energy and quantum number of the excited state, which facilitates the comparison of NIR and MIR spectra. The results support the assignment of the main overtone bands to specific environments of OH groups and bring new constraints for the identification of the vibrational bands related to Fe and Al substitutions at octahedral sites in serpentines. The two-phonon absorption bands are theoretically analyzed at the density functional theory level by computing the absorption arising from the self-energy of the IR-active vibrational modes. The characteristics of the two-phonon OH stretching continuum between 7300 and 7400 cm−1 and of the combination bands between 4000 and 4800 cm−1 are related to the specificities of the one-phonon and two-phonon densities of states of the three minerals.

Method to detect carbon in silicon crystals in the concentration range down to 5 × 1014 cm−3 by Fourier transform infrared absorption at room temperature

The procedure of Fourier transform infrared absorption (FT-IR) at room temperature for quantifying C impurities in Si crystals was reexamined to improve the detection limit down to 5 × 1014 cm−3. Since the substitutional C absorption peak overlaps with a strong two-phonon absorption, the difference spectroscopy is necessary with using a C-lean reference sample. A baseline flatness of less than 0.0005 in absorbance and a thickness uniformity of less than 0.001 mm are required to realize a detection limit of 5 × 1014 cm−3 for 2 mm thick samples. To check the flatness, we define the Si/Si baseline which is the difference in the absorbance spectra measured twice with the removal and attachment of the same Si sample. Four organizations participated in the FT-IR round-robin test for ten samples with the C concentration ranging from 3.6 × 1014 to 3.3 × 1015 cm−3. The obtained C concentrations were almost within 30% deviation from the values determined by reliable secondary ion mass spectroscopy.

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.

The optical properties of the nonlinear crystal BaGa4Se7

Large crystals BaGa4Se7 (BGSe), promising as a nonlinear optical converters in the mid-IR, were grown and investigated using methods of optical spectroscopy. The crystals are transparent in the range 0.465 ÷ 22 μm. The Tauc analysis showed that BGSe is a direct bandgap dielectric with Eg = 2.73 and 2.91 eV at 300 and 80 K, respectively, with dEg/dT = −8.07 × 10−4 eV/K. Vibrational modes are observed in the range 150–350 cm−1 in the Raman spectra: Thus, two-phonon absorption determines the transparency edge in the mid-IR. Main extended defects (the (001) microtwins and point defects (impurity centers such as Ga–O and different native centers) are revealed. BGSe crystals are photosensitive in the range of 1.5–4.0 eV. Luminescence in BGSe is quenched by two orders of intensity as temperature increases from 77 to 300 K. The possibility of controlling the luminescence parameters by post-growth annealing in a suitable atmosphere demonstrated. Pyroelectric luminescence in the 100–440 K range confirms the noncentrosymmetric structure of this crystal.

Terahertz Response from a Silicon Surface with Deposited Nanosized Gold Particles

Terahertz emission spectra of the surface of silicon crystals with different types of conductivity were experimentally recorded upon excitation by femtosecond laser pulses at various temperatures. The observed features in the terahertz spectra of the silicon surface correspond to the energy structure of the impurity centers determining the type of conductivity of the sample. Comparison was made with the results obtained in the case of deposition of gold nanoparticles on a semiconductor surface. The spectral features of the surface with deposited nanoparticles are discussed using the terahertz re-emission mechanism in two-phonon absorption.

Nonlinear optical switching behavior of nitrogen-doped carbon dots

Carbon dots (CDs), as an emerging class of carbon materials, have been recently demonstrated to possess nonlinear optical (NLO) behavior (e.g. nonlinear saturable absorption and reverse saturable absorption) which potentially expands CDs new application in optical limiting field. However, NLO switching behavior of CDs has not yet been reported in previous literature. In this paper, nitrogen-doped CDs (N-CDs) prepared from 2,3-diaminophenazine showed a switchable NLO behavior from saturable absorption (SA) to reverse saturable absorption (RSA) when the laser energy of incident picosecond pulse increased. By contrast, upon the irradiation of nanosecond pulse, the N-CDs exhibited only RSA behavior through excited state absorption (ESA) process. In mechanism, the switching behavior was probably resulted from competition between ground-state absorption and free carrier absorption (FCA) or two phonon absorption (TPA) processes, where both nitrogen doping and carbonyl groups of N-CDs were proved to be indispensable for the switching behavior.

Variation-free baseline with p-polarized Brewster incidence for infrared measurement of carbon impurities in silicon wafer

In our previous infrared transmission study, the baseline variation [H. Saito and H. Shirai, J. Electrochem. Soc. 147, 1210 (2000)] appeared near the carbon impurity band at 605 cm−1 in the normal incidence infrared spectra of silicon. This occurs because of the diminishing of multiple reflections due to the strong two-phonon absorption in the range of 610–630 cm−1 and the difference in resistivity between the test and reference samples. To solve the problem of baseline variations, p-polarized Brewster incidence has been successfully applied to infrared measurements of carbon impurities in wafer-thick silicon samples. No baseline variations have appeared in this range, owing to the absence of multiple reflections.

Two-phonon Absorption Spectra in the Layered Honeycomb Compound α-RuCl3

We performed infrared absorption spectroscopy for α-RuCl3 with a two-dimensional honeycomb lattice of Ru3+ ions. In the mid-infrared range, we observed three absorption peaks corresponding to intra-atomic d–d transitions among Ru3+ ions in accordance with a previous report. In the far-infrared range, we observed conspicuous broad absorption structures near 600 cm−1 in addition to several sharp single phonon peaks. Our first-principles calculations for the isolate sandwich model together with the factor group analysis revealed that the peak structures near 600 cm−1 are absorptions related to a two-phonon process. Our results stimulate further study on the spin–lattice coupling in α-RuCl3 as well as the optical detection of Majorana excitations in Kitaev spin liquids.

Optical constants, band gap, and infrared-active phonons of (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) from spectroscopic ellipsometry

Using spectroscopic ellipsometry, the authors determined the optical constants (complex dielectric function) for (LaAlO3)0.3(Sr2AlTaO6)0.35 (LSAT) from 0.01 to 6.5 eV. Above 0.5 eV, the data were described with a sum of two Tauc-Lorentz oscillators and two poles. A direct gap of 5.8 ± 0.1 eV was found. An Urbach tail extends to even lower photon energies and makes the crystal opaque above 4.8 eV. Using Fourier-transform infrared ellipsometry, the lattice dynamics was studied. Nine pairs of transverse/longitudinal phonons were found and attributed to disorder in the La/Sr sublattice, ordering in the Al/Ta sublattice, and two-phonon absorption.

Optical Properties of Active Regions in Terahertz Quantum Cascade Lasers

In this work, AlGaAs/GaAs superlattice, with layers’ sequence and compositions imitating the active and injector regions of a quantum cascade laser designed for emission in the terahertz spectral range, was investigated. Three independent absorption-like optical spectroscopy techniques were employed in order to study the band structure of the minibands formed within the conduction band. Photoreflectance measurements provided information about interband transitions in the investigated system. Common transmission spectra revealed, in the target range of intraband transitions, mainly a number of lines associated with the phonon-related processes, including two-phonon absorption. In contrast, differential transmittance realized by means of Fourier-transform spectroscopy was utilized to probe the confined states of the conduction band. The obtained energy separation between the second and third confined electron levels, expected to be predominantly contributing to the lasing, was found to be ~9 meV. The optical spectroscopy measurements were supported by numerical calculations performed in the effective mass approximation and XRD measurements for layers’ width verification. The calculated energy spacings are in a good agreement with the experimental values.

Two-phonon absorption in LiF and NiO from infrared ellipsometry

Using Fourier-transform infrared spectroscopic ellipsometry, the infrared lattice absorption of LiF and NiO was studied in the reststrahlen region. The transverse optical (TO) and longitudinal optical phonon energies, broadenings, and amplitudes were determined. Both materials also show a weak two-phonon absorption, which modifies the shape of the reststrahlen bands. The authors did not find any evidence of a splitting of the TO phonon in NiO due to antiferromagnetic ordering and place an upper limit of 17 cm−1 on this splitting.

IR absorption and linear dichroism in BiFe0.5Co0.5O3 films

The optical density spectra D(hω) of BiFe0.5Co0.5O3 films with thickness d = 230 and 1400 nm on LaAlO3 substrate were studied in the infrared range from 0.09 to 1.65 eV in unpolarized and linear polarized light at room temperature and at T = 80 K in the magnetic field of 4.5 kOe and without the field. The absorption bands at hω ~0.2 and ~0.7 eV are revealed in the D(hω) spectra recorded in unpolarized light and attributed to combination of two-phonon and two-magnon absorption and to d–d transitions in Fe or Co ions, respectively. In the D(hω) spectra of BiFe0.5Co0.5O3 film with d = 230 nm recorded in linear polarized light, the additional bands in the range of 0.3 eV–1.0 eV are found and explained by the nonuniform charge state due to the existence of weakly conductive electric domains and domain walls with higher conductivity and by the scattering of the light on the domains. The position of the bands in the optical density spectra in polarized light D⊥(hω) and D∣∣(hω) depends on the direction of the magnetic field relative to the film and on the temperature, which is explained by the change of the electric domain sizes. The revealed dependence of the optical properties on the magnetic field direction allows recommending BiFe0.5Co0.5O3 films as new functional magneto-optical material.

The spread into terahertz radiation using FeCrAl alloys wires

FeCrAl alloys wires with different Cr contents were investigated as a broadband radiation source for the spectral region from 1100 to 100cm−1. Due to the change in temperature, the radiation intensity depends on the driven current. Compared with commercial mercury arc lamp and terahertz quantum cascade laser sources, the FeCrAl alloys wires source generates a pronounced THz broad spectrum from 400cm−1 to 100cm−1. Using FeCrAl alloys wires source, the optical phonon absorption and zone-edge two-phonon absorption were observed for typical GaAs substrate transmission. The measurement of the little absorption from 400cm−1 to 100cm−1 for polyethylene film further confirms the potential application of FeCrAl alloys wires source in terahertz field.

Dispersion model of two-phonon absorption: application to c-Si

A dispersion model describing two-phonon absorption is developed using several simplifications of the quasiparticle approach. The dielectric response is constructed from absorption bands corresponding to individual additive and subtractive combinations of phonon branches. The model also includes thermal effects, changes of the transition strength with temperature, originating in Bose-Einstein statistics, and the shift of phonon frequencies accompanying thermal expansion. The model is applied to the analysis of experimental data measured in the IR range on crystalline silicon. The modeled spectral dependencies of optical constants are capable of describing all features in the transmittance spectra 70–1000 cm−1 observable at 300 K for float-zone silicon. The phonon frequencies in the points of symmetry are obtained independently in good agreement with ab initio calculations. The model of thermal effects is verified using ellipsometric measurements 300–1000 cm−1 in the temperature range of 300–500 K. The agreement between the modeled and experimental data is good, except for the spectral range 750–850 cm−1, in which a better agreement at temperatures above 300 K would require including the three-phonon absorption. The analysis provides a reliable value of the thermal coefficient describing the phonon frequency shift and proves that changes of structure broadening with temperature are negligible within the temperature range of 300–500 K.

Radiation Damage Effects on Optical, Electrical, and Thermophysical Properties of CVD Diamond Films

Raman and IR spectroscopy and thermal conductivity were employed to investigate the effects of simultaneous vacuum annealing on defect transformation processes in diamonds irradiated by fast neutrons and nanocrystalline diamond films (DF) implanted with O or P ions. A correlation between the thermal conductivity at room temperature and the absorption coefficient near 13 μm was found in diamonds irradiated by neutron fluence 2⋅1019 cm–2. It was found that two-phonon absorption in diamond IR spectra was sensitive to destruction of long-range ordering in the diamond crystal lattice. An absorption band with a shape characteristic of localized centers in the band gap (activation energy ~0.25 eV) was identifi ed in IR spectra of DF irradiated by neutrons and annealed at temperatures <900°C. It was shown that the optical and transport properties of the studied radiation-damaged DF were determined by multi-vacancy complexes and inclusions of a non-diamond phase.

Time-resolved chirp properties of semiconductor optical amplifiers in high-speed all-optical switches

The chirp properties of semiconductor optical amplifiers in all-optical switches are numerically investigated using a field propagation model. The chirp dynamics in the blue-shift and red-shift sideband are analyzed under the injection of random optical pump pulses. We also analyze the impact of the blue-detuned filtering scheme that is used to eliminate the pattern effect and enhance the operating speed of the optical switching. The reason for overshoots in eye diagrams in the blue-detuned filtering scheme is explained. We find that overshoots result from the ultrafast blue chirp induced by carrier heating and two-phonon absorption. These results are very useful for semiconductor optical amplifier-based ultrafast all-optical signal processing.

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.