Indirect Interband Transitions Research Articles

Cr-induced structural phase transformation in sputter deposited poly-AlN thin film from wurtzite to rocksalt structure and their effect on the optical properties

We report Cr doping-induced structural phase transformation (PT) in sputter-deposited polycrystalline-AlN thin film from wurtzite (w) to rocksalt (r) structure. Rietveld analysis shows that the steady substitution of Al ions by Cr ions in the w-AlN lattice distorts and compresses the wurtzite lattice along the c-axis. The chemical pressure exerted in the w-AlN lattice by the compression from the doped Cr ions causes the PT, similar to the high-pressure-induced PT in AlN. Post the PT, the r-Al1-xCrxN thin film continues to exhibit compressive residual stress, indicating that such sustained stress may also play a role in stabilizing the metastable r-Al1-xCrxN phase. The structural distortion and the PT also affected the optical absorption characteristics of the w-Al1-xCrxN thin films. The w-Al1-xCrxN exhibit a direct inter-band transition and the band gap (Eg) decreases from ∼ 6.06 eV for pure w-AlN to ∼ 4.15 eV for w-Al0.73Cr0.27N. After the PT, the r-Al0.61Cr0.39N thin film, on the other hand, exhibits an indirect inter-band transition with an Eg of ∼ 1.84 eV. In addition to the change in Eg, a prominent defect band appears at ∼ 4 eV at low Cr% and another band appears at ∼ 0.98 eV at higher Cr%.

Phonon-assisted carrier transport and indirect optical absorption of cubic boron nitride from first-principles

Inquiring the isotopically engineered carrier transport in polar materials remains an open question. Herein, the phonon-limited drift carrier mobility of single-crystal cubic boron nitride is presented using first-principles calculations. Natural c-BN has the predicted electron mobility of 1230 and 760 cm2/V s by solving the iterative Boltzmann transport equation and self-energy relaxation time approximation, respectively. The hole mobility under the Boltzmann transport equation and self-energy relaxation time approximation is 193 and 105 cm2/Vs, respectively. Subsequently, the electron and hole mobilities at the stable isotope levels of boron and nitride are predicted, and nitride isotopes are found to be more effective than boron for carrier mobility. Those carrier mobilities further decrease with increasing temperature due to the strengthened electron–phonon interactions. Moreover, the phonon-assisted indirect optical absorption of c-BN is investigated by considering the contribution of phonons to the indirect electronic inter-band transitions. The predicted imaginary part of the dielectric function is in better agreement with previous experiments. This work aims to understand the role of phonons in determining the carrier mobility and indirect optical absorption of c-BN.

Structural transition and photoluminescence behavior of (V2O5)1-x (Ag0.33V2O5)x (x = 0 to 0.1) nanocomposites

The two phase (V2O5)1-x(Ag0.33V2O5)x (x = 0, 0.05, 0.075 and 0.1) nanocomposites were synthesized through one step hydrothermal process. The X-ray diffraction followed by two phase Rietveld refinement confirms the crystallization of V2O5 and Ag0.33V2O5 phases in the Pmmn (orthorhombic) and C21 (monoclinic) symmetry respectively without any impurity phase. Furthermore, the FE-SEM, EDX, PL, UV–vis and XPS characterization were carried out for microstructural, optical and chemical studies. The formation of Ag0.33V2O5 nano-belts with average length and width of 3–4 μm and 40–50 nm respectively, were observed in the electron micrograph. Moreover, the XPS study confirms the constituent element are in expected oxidation states with Vanadium in only + 5 (V+5) for V2O5 and in mixed states (V+5 and V+4) for 10 % Ag0.33V2O5 nanocomposite. The photoluminescence spectra of these nanocomposite depicts two broad emission peaks located around 570 nm and 707 nm due to indirect inter-band and mid-state energy states transitions, respectively. PL also prove the increase in oxygen vacancies by loading of Ag in V2O5 matrix. The optical absorption of Ag0.33V2O5 represents a decrease in the energy band gap (2.10–1.97 eV) by adding Ag concentration in the V2O5, which also proved the semiconductor to metal transition (SMT).

Multi-frequency complete optical isolation based on indirect interband transition

Indirect interband photonic transition is an important approach to achieve on-chip complete optical signal isolation. We demonstrate that the operational isolation bandwidth based on this effect can be increased by utilizing non-interfering transition channels obtained from the process of transition channel engineering. Multiple optical modes can be excited through non-interfering channels and be absorbed by matched filters without interfering with each other, thus achieving multi-frequency complete optical isolation. With two non-interfering transition channels, an optical isolator design that achieves complete isolation at two frequencies is proposed as an example. The potential design of multi-frequency isolator is also discussed. All the results are verified with finite-difference time-domain simulation.

Exciton Luminescence and Optical Properties of Nanocrystalline Cubic Y2O3 Films Prepared by Reactive Magnetron Sputtering.

This paper presents a comprehensive study of the energy structure, optical characteristics, and spectral-kinetic parameters of elementary excitations in a high-purity nanocrystalline cubic Y2O3 film synthesized by reactive magnetron sputtering. The optical transparency gaps for direct and indirect interband transitions were determined and discussed. The dispersion of the refractive index was established based on the analysis of interference effects. It was found that the refractive index of the Y2O3 film synthesized in this study is higher in order of magnitude than that of the films obtained with the help of other technologies. The intrinsic emission of Y2O3 film has been discussed and associated with the triplet–singlet radiative relaxation of self-trapped and bound excitons. We also studied the temperature behavior of the exciton luminescence of Y2O3 for the first time and determined thermal activation barriers. The optical energy and kinetic parameters of cubic Y2O3 films were analyzed in comparison with those of the monoclinic films of yttrium oxide. The main difference between the optical properties of cubic and monoclinic Y2O3 films was established, which allowed for a supposition of their application prospects.

Zinc gallate (ZnGa2O4) epitaxial thin films: determination of optical properties and bandgap estimation using spectroscopic ellipsometry

Electronic grade ZnGa2O4 epitaxial thin films were grown on c-plane sapphire substrates by metal-organic chemical vapor deposition and investigated using spectroscopic ellipsometry. Their thickness, roughness and optical properties were determined using a Multiple Sample Analysis based approach by the regression analysis of optical model and measured data. These samples were then compared to samples which had undergone ion etching, and it was observed that etching time up to four minutes had no discernible impact on its optical properties. Line shape analysis of resulting absorption coefficient dispersion revealed that ZnGa2O4exhibited both direct and indirect interband transitions. The modified Cody formalism was employed to determine their optical bandgaps. These values were found to be in good agreement with values obtained using other popular bandgap extrapolation procedures.

Temperature-dependent luminescence of intrinsic defects and excitons in nanocrystalline monoclinic Y2O3 films

This work presents a comprehensive study of the optical properties of Y2O3 film with a monoclinic structure obtained by magnetron sputtering on a silica glass substrate. The parameters of band energy structure and emission characteristics of the film were determined by absorption, transmission, and photoluminescence spectroscopy. The dispersion of refractive index was established and the values of transparency gaps for direct and indirect interband transitions were determined. Optically active intrinsic centers were identified as vacancy (F - centers) and interstitial type (Oi - centers) defects, which are characterized by distribution of radiative states over activation energy of emission quenching. Luminescence in the UV and visible spectral ranges is interpreted as triplet-singlet radiative transitions in self-trapped and bound excitons. The values of thermally activated barriers for flare up and quenching of exciton luminescence are determined, and the configuration-coordinate diagram is proposed.

Even-order harmonics in the nitrogen vacancy center in diamond from intertwined intraband and interband transitions

Different from odd-order harmonics, even-order harmonics are a result of broken inversion symmetry, where the second harmonic generation has long been used in the study of surface magnetism. Their underlying mechanisms are always intriguing. Here we reveal an unreported mechanism of even harmonics from the diamond with a nitrogen-vacancy (NV) center from the viewpoint of group theory. Two different types of interband transition channels are identified. One is the direct transitions between a valence band and a conduction band, called type-I interband transition. Type II is defined as the indirect interband transitions between two conduction bands or two valence bands. Once type-II transitions are involved only through the inversion symmetry breaking, even-order harmonics occur in the NV center in diamond. The physical origin of this new type of interband transitions is rooted in group theory, where the inversion symmetry forbids type-II transitions between two valence or conduction bands with the same irreducible representation. By contrast, the broken inversion symmetry in the NV center lifts this restriction, giving rise to even-order harmonics. Our study represents a paradigm shift in high harmonic generation.

Scalable high-repetition-rate sub-half-cycle terahertz pulses from spatially indirect interband transitions

Intense phase-locked terahertz (THz) pulses are the bedrock of THz lightwave electronics, where the carrier field creates a transient bias to control electrons on sub-cycle time scales. Key applications such as THz scanning tunnelling microscopy or electronic devices operating at optical clock rates call for ultimately short, almost unipolar waveforms, at megahertz (MHz) repetition rates. Here, we present a flexible and scalable scheme for the generation of strong phase-locked THz pulses based on shift currents in type-II-aligned epitaxial semiconductor heterostructures. The measured THz waveforms exhibit only 0.45 optical cycles at their centre frequency within the full width at half maximum of the intensity envelope, peak fields above 1.1 kV cm−1 and spectral components up to the mid-infrared, at a repetition rate of 4 MHz. The only positive half-cycle of this waveform exceeds all negative half-cycles by almost four times, which is unexpected from shift currents alone. Our detailed analysis reveals that local charging dynamics induces the pronounced positive THz-emission peak as electrons and holes approach charge neutrality after separation by the optical pump pulse, also enabling ultrabroadband operation. Our unipolar emitters mark a milestone for flexibly scalable, next-generation high-repetition-rate sources of intense and strongly asymmetric electric field transients.

Indirect momentum excitation of graphene using high transversal modes of light in hyperbolic media.

Electrons in indirect semiconductors can optically transit between the valance and conduction band edges only when the momentum conservation is satisfied with help of a third quasi-particle, such as a phonon. In this report, we theoretically demonstrate that indirect interband transition of graphene electrons can be optically enabled only by light with highly enhanced transversal modes, which can be generated by scattering of point dipole radiation with periodic metal slits fabricated in a natural hyperbolic material. The light-matter interaction for graphene electrons is reformulated by using indirect transition matrix elements, and interband polarizations of graphene are obtained by solving quantum kinetic equations of motion in the semi-classical regime. The interband optical current density of graphene as a function of the polarization angle of the incident field shows clear hexagonal response to the high transversal modes of light, which results from the low dependence on dephasing rate and dominance of the indirect polarizations over the direct interband contributions.

Magnetic-free silicon nitride integrated optical isolator

Integrated photonics enables signal synthesis, modulation and conversion using photonic integrated circuits (PICs). Many materials have been developed, among which silicon nitride (Si3N4) has emerged as a leading platform particularly for nonlinear photonics. Low-loss Si3N4 PICs have been widely used for frequency comb generation, narrow-linewidth lasers, microwave photonics and photonic computing networks. Yet, among all demonstrated functionalities for Si3N4 integrated photonics, optical non-reciprocal devices such as isolators and circulators have not been achieved. Conventionally, they are realized based on the Faraday effect of magneto-optic materials under an external magnetic field; however, it has been challenging to integrate magneto-optic materials that are not compatible with complementary metal–oxide–semiconductors and that require bulky external magnet. Here we demonstrate a magnetic-free optical isolator based on aluminium nitride (AlN) piezoelectric modulators monolithically integrated on low-loss Si3N4 PICs. The transmission reciprocity is broken by spatio-temporal modulation of a Si3N4 microring resonator with three AlN bulk acoustic wave resonators that are driven with a rotational phase. This design creates an effective rotating acoustic wave that allows indirect interband transition in only one direction among a pair of strongly coupled optical modes. A maximum of 10 dB isolation is achieved under 300 mW total radiofrequency power applied to three actuators, with minimum insertion loss of 0.1 dB. An isolation bandwidth of 700 MHz is obtained, determined by the optical resonance linewidth. The isolation remains constant over nearly 30 dB dynamic range of optical input power, showing excellent optical linearity. Our integrated, linear, magnetic-free, electrically driven optical isolator could be a key building block for integrated lasers and optical interfaces for superconducting circuits. An electrically driven, magnetic-free optical isolator is demonstrated. The device, based on aluminium nitride piezoelectric modulators and a silicon nitride microring resonator, may be useful for integrated lasers and other opto-electric systems.

Size- and temperature-control optical direct/indirect band tuning in layered compounds: band gap engineering

The X-ray diffraction (XRD) is studied in thermally evaporated cadmium iodide (CdI2) thin films with various thicknesses (0.4–5.5 μm) and grain sizes (100–160 nm). The grain size, calculated from the XRD, is found to increase with increasing the thickness of the film, while the reflectivity and refractive index decease with increasing the wavelength of the exciting light. The optical absorption spectra show both allowed direct and indirect interband transitions across a fundamental gap in CdI2. It is found that both indirect and direct band gap (Eg) decrease with increasing the thickness of the film and that the indirect Eg is lower than the direct Eg by an amount of about 0.7 eV. The direct Eg is also decreased with increasing both the grain size and temperature. However, the temperature dependence of Eg follows the Varshni relation. Our results highlight the possibility of engineering or tuning the Eg of CdI2 by controlling the thickness of the film, grain size as well as temperature.

Plasmons in MoS2 studied via experimental and theoretical correlation of energy loss spectra.

This paper takes a fundamental view of the electron energy loss spectra of monolayer and few layer MoS2 . The dielectric function of monolayer MoS2 is compared to the experimental spectra to give clear criteria for the nature of different signals. Kramers-Krönig analysis allows a direct extraction of the dielectric function from the experimental data. However this analysis is sensitive to slight changes in the normalisation step of the data pretreatment. Density functional theory provides simulations of the dielectric function for comparison and validation of experimental findings. Simulated and experimental spectra are compared to isolate the π and π + σ surface plasmon modes in monolayer MoS2 . Single-particle excitations obscure the plasmons in the monolayer spectrum and momentum resolved measurements give indication of indirect interband transitions that are excited due to the large convergence and collection angles used in the experiment. LAY DESCRIPTION: Two-dimensional materials offer a path forward for smaller and more efficient devices. Their optical and electronic properties give way to beat the limits set in place by Moore's Law. Plasmon are the collective oscillations of electrons and can confine light to dimensions much smaller than its wavelength. In this work we explore the plasmonic properties of MoS2 , a representational candidate from a family of 2D materials known as transition metal dichalcogenides. High resolution electron microscopy and spectroscopy provide insights in the plasmonic properties of MoS2 down to an atomic scale. Experimental results show the relationship between plasmons and interband transitions in the electron energy loss spectrum. Density functional theory provides a theoretical support for the experimental findings and provides commentary on the fundamental underlying physics.

Electronic structure and related optical, thermoelectric and dynamical properties of Lilianite-type Pb7Bi4Se13: Ab-initio and Boltzmann transport theory

The electronic, optical, thermoelectric, and dynamic properties of Pb7Bi4Se13 have been investigated using first principles calculations. The existence of heavy elements, bismuth and lead, has required to consider the spin–orbit coupling as implemented in the second variational procedure. It was observed that the upper of the valence band region appears predominantly from Se-p states with an admixture of Bi-p and Pb-p states, while the CBM comes mainly from Bi-p states only. The calculated optical properties illustrate a small manifold direct and indirect inter-band transitions emerging in the visible region. Semi classic Boltzmann theory was then employed to determine the Seebeck coefficient, electrical conductivity, thermal conductivity, and power factor. The phonon dispersion curve has been computed and explained. It can be deduced that the slight structural instability observed from phonon calculations is related to lower lattice thermal conductivity. Our findings were compared with the available experimental measurements, showing a good agreement.

On the Band Structure of Bi2Te3

For p-Bi2Te3 crystals grown by the Czochralski method, the temperature dependences of the conductivity, Hall coefficient, thermoelectric power (α), and transverse Nernst–Ettingshausen coefficient are obtained experimentally in the temperature range 77–450 K. The transmittance spectrum in the range 400–5250 cm–1 is recorded at room temperature. It is shown that, to interpret the temperature dependences of the scattering parameter r and the ratio of the thermoelectric power to temperature (α/T), it is essential to take into account the complex valence-band structure and the contribution of heavy holes to transport phenomena. Estimations of the energy band parameters in the context of the two-band model give a hole effective mass close to the free electron mass and the energy gap between nonequivalent extrema at a level of several hundredths of eV. In the absorption spectrum derived from the transmittance spectrum, a sharp increase in absorption defined by indirect interband transitions with the band gap Eg ≈ 0.14 eV is observed in the region of frequencies ν ≥ 1000 cm–1. The absorption spectrum calculated from the reflectance data using the Kramers–Kronig relations is in agreement with the experimental absorption spectrum.

О зонной структуре Bi-=SUB=-2-=/SUB=-Te-=SUB=-3-=/SUB=-

AbstractFor p -Bi_2Te_3 crystals grown by the Czochralski method, the temperature dependences of the conductivity, Hall coefficient, thermoelectric power (α), and transverse Nernst–Ettingshausen coefficient are obtained experimentally in the temperature range 77–450 K. The transmittance spectrum in the range 400–5250 cm^–1 is recorded at room temperature. It is shown that, to interpret the temperature dependences of the scattering parameter r and the ratio of the thermoelectric power to temperature (α/ T ), it is essential to take into account the complex valence-band structure and the contribution of heavy holes to transport phenomena. Estimations of the energy band parameters in the context of the two-band model give a hole effective mass close to the free electron mass and the energy gap between nonequivalent extrema at a level of several hundredths of eV. In the absorption spectrum derived from the transmittance spectrum, a sharp increase in absorption defined by indirect interband transitions with the band gap E _ g ≈ 0.14 eV is observed in the region of frequencies ν ≥ 1000 cm^–1. The absorption spectrum calculated from the reflectance data using the Kramers–Kronig relations is in agreement with the experimental absorption spectrum.

Тип оптических переходов на краю фундаментального поглощения кристаллов TlGaSe-=SUB=-2-=/SUB=- и TInS-=SUB=-2-=/SUB=-, подвергнутых γ-облучению

The effect of gamma irradiation on the optical properties of layered TlGaSe2 and TlInS2 crystals in the wavelength range of 400–1,100 nm at 300 K is studied. From the analysis of optical absorption spectra, the energies of direct and indirect optical interband transitions before and after gamma irradiation are determined. It is shown that as the gamma-irradiation dose is accumulated in the range of 0–25 Mrad by TlGaSe2 and TlInS2 single crystals, the energies of direct and indirect allowed optical transitions increase from Egd = 2.06 eV and Egi = 1.90 eV at D = 0 Mrad to Egd = 2.11 eV and Egi = 1.98 eV at D = 25 Mrad for TlGaSe2 and Egd = 2.32 eV crystals and Egi = 2.27 eV at D = 0 Mrad to Egd = 2.35 and Egi = 2.32 eV at D = 25 Mrad for TlInS2 crystals. A decrease in the transmittance at doses from 0 to 5 Mrad and a further increase in the transmittance at the radiation dose D = 25 Mrad are observed.

Three-photon absorption due to indirect interband transitions in crystals

An approximate analytical expression is obtained for the probability of three-photon indirect interband transitions involving longitudinal acoustic phonons in crystals. These probabilities are compared with the probabilities of direct three- and four-photon interband transitions.

Study on some linear and nonlinear optical parameters of glycine hydrofluoride single crystals

Abstract Single crystal of glycine hydroflruoride (GHF) was grown from aqueous solution by slow evaporation technique. The structure of the grown crystal was tested and analyzed through X-ray powder diffraction. The functional groups have been identified from the FT-IR spectra. Slabs cut normal to the b-axis from the grown crystal were subjected to incident radiation with a wavelength range of 200 nm to 800 nm to investigate the transmittance and reflectance spectra. Linear optical parameters such as extinction coefficient k, refractive index n and both the real and imaginary parts: ∊real and ∊im of the dielectric permittivity were calculated as functions of the incident photon energy. The dispersion of the refractive index was fitted in terms of Cauchy formula and Wemple-DiDomenico single oscillator model. GHF crystals exhibited indirect optical interband transition and the optical energy gap Eg was determined by using Tauc plot. The indirect band gaps at elevated temperatures were determined and their temperature dependence was estimated. Optical band gap Eg values were found to decrease with an increase in crystal temperature; however, the band tail width exhibited opposite behavior. The nonlinear optical potential was examined by the second harmonic generation (SHG) test.

Absorption coefficients of silicon: A theoretical treatment

A theoretical model with explicit formulas for calculating the optical absorption and gain coefficients of silicon is presented. It incorporates direct and indirect interband transitions and considers the effects of occupied/unoccupied carrier states. The indirect interband transition is calculated from the second-order time-independent perturbation theory of quantum mechanics by incorporating all eight possible routes of absorption or emission of photons and phonons. Absorption coefficients of silicon are calculated from these formulas. The agreements and discrepancies among the calculated results, the Rajkanan-Singh-Shewchun (RSS) formula, and Green's data are investigated and discussed. For example, the RSS formula tends to overestimate the contributions of indirect transitions for cases with high photon energy. The results show that the state occupied/unoccupied effect is almost negligible for silicon absorption coefficients up to the onset of the optical gain condition where the energy separation of Quasi-Femi levels between electrons and holes is larger than the band-gap energy. The usefulness of using the physics-based formulas, rather than semi-empirical fitting ones, for absorption coefficients in theoretical studies of photovoltaic devices is also discussed.