Indirect Band Gap Crystals Research Articles

Two-photon-absorption enhanced terahertz generation from KTP optically pumped in the visible-to-UV range.

By generating terahertz pulses in KTP crystals through optical rectification with a pump photon energy varying from below to above the bandgap, we observe a peak of the THz signal at the bandgap energy but also a second one around half the bandgap. This later one is attributed to a two-photon absorption enhanced nonlinearity, which is validated by the similarity of the two-photon absorption coefficient and THz peak amplitude data versus the pump photon energy. A careful analysis of the KTP sample absorption spectral dependence nearby the bandgap demonstrates that KTP is an indirect bandgap crystal, whose absorption below the bandgap involves emission of a phonon related to the symmetric Ti-O stretching vibration, i.e. the ν1 (A1g) mode.

First principles study of the electronic, elastic, infrared and electrical properties of LiZnPS4

LiZnPS4 is thought to be a promising mid-infrared optical crystal and lithium battery material, but its fundamental properties are still not well investigated. In this work, we performed a systematic study on its electronic, elastic, infrared and electrical properties. The results indicate that LiZnPS4 is an indirect band gap crystal. Calculated partial density of states reveals that the top of its valence band is formed by the S 3p orbital while the component of the bottom of the conduction band is complex. The Zn 4p orbital is not empty in LiZnPS4 owing to the tetrahedral coordination of the Zn atoms. Calculated elastic properties indicate that LiZnPS4 is a mechanically stable crystal with elastic anisotropy. It has a relatively low Debye temperature of 294.8 K. Its infrared absorption spectrum was simulated and the origins of the strong peaks in it were revealed. Vibrational modes at the Brillouin zone center were illustrated and Born effective charges were calculated and discussed. Computed dielectric constants uncover that electrons contribute more to the total dielectric constants than ions. Studies imply that LiZnPS4–Li+ battery has a high theoretical voltage, a low electric capacity and a slightly high activation barrier.

Archimedean (4,8)-tessellation of haeckelite ultrathin nanosheets composed of boron and aluminum-group V binary materials.

A compendium of unique haeckelite boron and aluminum-group V binary materials have been assessed for their fundamental thermodynamic and ground state electronic properties within density functional theory. We explore their thermodynamic stability relative to new bulk haeckelite crystal structures and find a number of stable polymorphs of planar and buckled ultrathin nanosheets. The bulk boron and aluminum haeckelite crystals display semiconducting and metallic behavior. From the dispersion curves, we predict the formation of both indirect and direct bandgap crystals. We also discover the existence of a five-coordinate aluminum antimonide crystal hitherto never before observed. Moreover, it is found that a number of the Archimedean four and eight membered ring tessellation planar nanosheets could form should synthesis be attempted. It is predicted that these nanosheets can attain two configurations - planar and buckled. From this work we find that combinations of elements such as boron and nitrogen or phosphorus, and aluminum and nitrogen will likely become true single-atom thick nanosheets. These materials show intrinsic indirect bandgap character, which spans the ultraviolet, visible, and infrared spectrum. In the boron series of these materials, the planar structures show double extrema in the bandstructures with van Hove singularities in the projected density of states at the Fermi energy suggesting strong light-matter interactions. The aluminum series we observe strong charge transfer and larger indirect bandgap nanosheets. This study serves as a starting point for a new class of inorganic bulk and ultrathin film materials, which can have many varied applications in nanotechnology.

Electronic, elastic properties and hardness of the novel tetragonal silicon

Silicon is an important material for technical applications. Recently, a long-puzzling metastable phase of silicon (T12-Si) has been theoretically identified. In this work, we used first-principles calculations to study the electronic and elastic properties and the hardness of this new silicon allotrope to enrich the relevant information. These properties of cubic Si (c-Si) were also calculated for comparison. The results show that the T12-Si is mechanically anisotropic and has a lower bulk modulus and shear modulus than c-Si. Its theoretical hardness is 10.3 GPa, smaller than the value of 13.5 GPa for c-Si. Analyses of the electronic properties reveal that T12-Si is an indirect band gap crystal with a gap value of 0.69 eV, making it a promising semiconductor for future technical applications and that covalent sp 3-hybridization is the main interaction in this crystal. The reason the calculated hardnesses of c-Si and T12-Si are rather small compared to that of diamond is also discussed.

A possible superhard orthorhombic carbon

Carbon is a versatile element in the periodic table because of its ability to form various stable and metastable allotropes. In the present work, an orthorhombic carbon allotrope with ‘4-membered rectangles’ is predicated by Particle Swarm Optimization method at 0 GPa. Calculations indicate that this new phase is dynamically and mechanically stable although it has a high enthalpy. Its electronic and mechanical properties such as elastic constants, hardness, ideal tensile strength and shear strength are systematically studied. The results show that it is an indirect band gap crystal with a gap of 2.68 eV; in spite of its elastic anisotropy, the theoretical hardness of 76.2 GPa still makes it a potential superhard material; the obtained ideal tensile strength and shear strength are 69.0 GPa and 75.6 GPa, respectively, confirming its superhard character. The related microscopic deformation mechanisms are also detailedly analyzed through investigating the corresponding charge density, which reveals that the bonds arranged parallel to the b-axis are responsible for the breakdown of the crystal under the critical tension and shear deformations. These studies provide important information for the potential applications of the crystal once it is synthesized.

Determination of nonlinear absorption and refraction in direct and indirect band gap crystals by Z-scan method

A systematic investigation on nonlinear optical properties such as three photon absorption (3PA) wavelength dependent of Kerr type nonlinear refraction in direct and indirect band gap crystals has been reported in the present work. The Z-scan measurements are recorded for both ZnO and CdI2 with femtosecond laser pulses while the wavelength dependent of the Kerr nonlinearity are in agreement with a two band model. The wavelength dependence of the 3PA is determined by [(3Ephoton/Eg)–1]5/2[(3Ephoton/Eg)]−9 in the case of direct band gap crystal and [(3Ephoton±ℏΩ/Eg)-1]5/2[(3Ephoton±ℏΩ/Eg)]-9 in the case of indirect band gap crystal. In the present investigation the value of 3PA in the case of indirect band gap crystal is lower than the direct band gap crystal which is due to the phonon assisted transition. The materials of large band gap with optical nonlinearity and fast response speed should be dominating factor for further photonic devices such as optical limiters, optical switches and optical modulators. The higher order nonlinear optical effects have also been determined in the present study.

Intensity Dependent Nonlinear Absorption in Direct and Indirect Band Gap Crystals under Nano and Picosecond Z-Scan

The nonlinear absorption properties of direct (GaN) and indirect (CdI2) band gap crystals have been studied by using an open aperture Z-scan technique under fundamental (1064 nm) and frequency doubled (532 nm) wavelength respectively with 10 ns or 60 ps pulse durations. Direct band gap crystal exhibits two and three photon absorption at all input irradiances. On the other hand, at low input irradiance the indirect band gap crystal exhibits saturable absorption (SA). At higher input irradiances two and three photon absorption becomes dominant. A monotonic increase of the nonlinear absorption coefficients with increasing laser pulse duration from 60 ps to 10 ns is observed for GaN and CdI2 crystals.

Study of transmittance dependence closed-aperture Z-scan curves in the materials with nonlinear refraction and strong absorption

In this paper we report, the measurement of the nonlinear refraction and absorption in the bulk of CdS, ZnS, ZnO, GaN, and ZnSe (direct band gap crystals), CdI2 and PbI2 (indirect band gap crystals) with frequency double Nd:YAG laser. Further the characteristics of closed aperture (CA) Z-scan transmittance curves have been studied in detail when the third-order nonlinear refraction and absorption are simultaneously present, and both of which can be independently positive or negative. It was found that for the purely non linear absorption, the transmittance curve exhibits a completely symmetric shape and for nonlinear refraction the curve shape is anti-symmetric. We found that whether the peak–valley/valley–peak and valley or peak structure exists or not, depends on the coupling factor (ρ) which is the ratio of the imaginary part χI(3) to the real part χR(3) of the third-order nonlinear susceptibility χ(3).

LASER-INDUCED MULTIPHOTON PHOTOCONDUCTIVITY IN DIRECT AND INDIRECT BAND GAP CRYSTALS USING Z-SCAN TECHNIQUE

Nanosecond laser pulses have employed the photoconductive Z-scan technique. Photoconductivity traces measured by moving the sample across the laser beam waist were used for measuring two and three-photon absorption processes. The value of the three-photon absorption coefficient β3=9.6×10-10 cm 3/ GW 2 in the case of direct, and 8.96×10-12 cm 3/ GW 2 and 5.0×10-12 cm 3/ GW 2 in the case of indirect band gap crystals have been estimated from a comparison of traces measured by exciting the sample with the first and second harmonics of Nd : YAG laser. The low value of β3 in the case of indirect band gap crystals compared to direct band gap crystals is attributed to phonon-assisted transitions.

PHOTOEXCITED CARRIER LIFETIME IN DIRECT AND INDIRECT BAND GAP CRYSTALS ON THE Z-SCAN TECHNIQUE AT 532 nm

The photoexcited carrier lifetime in ZnSe and CdSe (direct band gap crystals) and CdI 2 and PbI 2 (indirect band gap crystals) has been investigated by the use of a single beam Z-scan technique at 532 nm. The carrier lifetime (τ) is determined to be (1.7 ± 0.5) ns in the case of direct band gap and (1.2 ± 0.03) ps in the case of indirect band gap crystals. Lower values of τ in the case of indirect band gap crystals can be attributed to the reduction in value of the carrier density (N).

Optical nonlinearities in direct and indirect band gap crystals on the [formula omitted]-scan technique with nanosecond laser

The free carrier optical nonlinearities in direct and indirect band gap crystals have been investigated by the use of a single beam Z -scan technique. The values of two-photon absorption coefficient ( β ) , nonlinear index n 2 ( w ) , the total free carrier absorption cross-section ( σ ab ) and the change in the refractive index per unit carrier density ( σ nr ) are determined in both direct and indirect band gap crystals. Smaller values of ( β ) and ( σ ab ) in the case of indirect band gap crystals have been attributed to phonon assisted transitions, while the smaller value of n 2 ( w ) and ( σ nr ) is due to the rise in saturation taking place in the absorption.

Effective-mass approximation for shallow donors in uniaxial indirect band-gap crystals and application to4H−SiC

The effective-mass theory is applied for description of the electronic states of shallow donors in indirect band-gap uniaxial crystals, which have three different components of the electron effective-mass tensor, and two different components of the tensor of the dielectric constant. The Hamiltonian in the resulting Schr\"odinger equation for the envelope function has ${D}_{2h}$ symmetry and, after proper parametrization, a nonvariational numerical method is used for its solution. Two particular cases of ${D}_{\ensuremath{\infty}h}$ symmetry are identified and discussed separately. The comparison between theory and experiment for the $4H$ polytype of silicon carbide is revised using the least-squares method to determine the binding energies of the ground state of the most shallow nitrogen donor in this material, its valley-orbit split-off counterpart, and the mean value of the dielectric constant, and completed with calculation of the theoretical transition probabilities. In addition, the lowest-lying binding energies of the states, between which optical transitions are allowed, are calculated on a grid of values of the two parameters describing the anisotropy and the tabulated values can be used for interpolation to describe other materials.

Pulsed Laser Induced Multiphoton Photoconductivity in an Indirect Band Gap Crystal: PbI2

The photoconducting properties of PbI2, an indirect band gap crystal have been studied using fundamental and frequency doubled Nd:YAG laser. The ratio of three and two photon absorption cross sections (γ3/γ2) have been calculated experimentally. The value of γ2 has been calculated by third order time dependent perturbation theory and that of γ3 has been estimated by making use of experimental results. Relatively lower values of the absorption cross-sections as compared to those of direct band gap crystals have been attributed to phonon-assisted transitions.

Multiphoton photoconductivity in an indirect band gap crystal: CdI 2

The photoconducting properties of CdI 2, an indirect band gap crystal, have been studied using fundamental and frequency doubled Nd : YAG laser. The ration of three and two photon absorption cross sections ( v 3 v 2 ) has been calculated with the help of experimental results. The value of two photon absorption cross section v 2 has been calculated by using third order time dependent perturbation theory and that of v 3 has been estimated by making use of the experimental results. Low values of the absorption cross sections obtained in the present case have been attributed to phonon assisted transitions.