Interband Absorption Research Articles

Nonlinear processes upon two-photon interband picosecond excitation of crystal

A new experimental method is proposed to study the dynamics of nonlinear processes occurring upon two-photon interband picosecond excitation of a lead tungstate crystal and upon its excitation by cw probe radiation in a temporal range from several nanoseconds to several seconds. The method is applied to the case of crystal excitation by a sequence of 25 high-power picosecond pulses with a wavelength of and cw probe radiation. Measuring the probe beam transmittance during crystal excitation, one can investigate the influence of two-photon interband absorption and the thermal nonlinearity of the refractive index on the dynamics of nonlinear processes in a wide range of times (from several nanoseconds to several seconds). The time resolution of the measuring system makes it possible to distinguish fast and slow nonlinear processes of electronic or thermal nature, including the generation of a thermal lens and thermal diffusion. An alternative method is proposed to study the dynamics of induced absorption transformation and, therefore, the dynamics of the development of nonlinear rocesses upon degenerate two-photon excitation of the crystal in the absence of external probe radiation.

Optical properties and electronic structure of BiTeCl and BiTeBr compounds

Optical properties of BiTeCl and BiTeBr compounds with a strong Rashba spin–orbit coupling are studied in the 0.08–5.0 eV range using the optical ellipsometry method. Fundamental characteristics of the electronic structure are obtained. Similarly to BiTeI, spectra of the imaginary part of dielectric permittivity constant e2(E) in the energy interval between the plasma edge and the threshold of an intense interband absorption (0.7 eV in BiTeCl and 0.6 eV in BiTeBr) display a fine structure of electronic transitions at 0.25 and 0.55 eV in BiTeCl and 0.20 and 0.50 eV in BiTeBr. These features are assigned to electronic transitions between the bulk conduction zones split by the Rashba spin–orbit interaction. The parameters of the electronic structure of BiTeCl and BiTeBr are compared with the BiTeI compound that was studied earlier. In the BiTeCl–BiTeBr–BiTeI row, the absorption edge and main features of the fundamental absorption exhibit a shift to low energies.

Suppression of Surface-Enhanced Raman Scattering on Gold Nanostructures by Metal Adhesion Layers

We report on the suppression of localized surface plasmon resonance (LSPR) and surface-enhanced Raman scattering (SERS) of 60 nm thick gold nanostructures by titanium adhesion layers. Significant plasmon damping is observed for titanium layer thicknesses up to t(Ti) = 5 nm, which cannot be fully explained by simulation models that account for the overlap of the electric field in the bimetal layer with the titanium layer. Furthermore, the relationship between the SERS enhancement factor and the LSPR quality-factor G proportional to Q(4) breaks down as t(T1), is increased beyond 1 nm. We attribute the LSPR and SERS suppression to plasmon damping and spectral broadening of interband absorption due to the field overlap with the titanium layer and titanium impurities that interdiffuse through the grain boundaries of the polycrystalline gold layer during the deposition of the Au-Ti bimetal. These observations and analyses serve as a guide for improving the reliability of gold nanoplasmonic sensors

Materials Pushing the Application Limits of Wire Grid Polarizers further into the Deep Ultraviolet Spectral Range

Wire grid polarizers (WGPs), periodic nano‐optical metasurfaces, are convenient polarizing elements for many optical applications. However, they are still inadequate in the deep ultraviolet spectral range. It is shown that to achieve high performance ultraviolet WGPs a material with large absolute value of the complex permittivity and extinction coefficient at the wavelength of interest has to be utilized. This requirement is compared to refractive index models considering intraband and interband absorption processes. It is elucidated why the extinction ratio of metallic WGPs intrinsically humble in the deep ultraviolet, whereas wide bandgap semiconductors are superior material candidates in this spectral range. To demonstrate this, the design, fabrication, and optical characterization of a titanium dioxide WGP are presented. At a wavelength of 193 nm an unprecedented extinction ratio of 384 and a transmittance of 10% is achieved.

Light sensitive memristor with bi-directional and wavelength-dependent conductance control

We report the optical control of localized charge on positioned quantum dots in an electro-photo-sensitive memristor. Interband absorption processes in the quantum dot barrier matrix lead to photo-generated electron-hole-pairs that, depending on the applied bias voltage, charge or discharge the quantum dots and hence decrease or increase the conductance. Wavelength-dependent conductance control is observed by illumination with red and infrared light, which leads to charging via interband and discharging via intraband absorption. The presented memristor enables optical conductance control and may thus be considered for sensory applications in artificial neural networks as light-sensitive synapses or optically tunable memories.

Two-photon absorption in gapped bilayer graphene with a tunable chemical potential

Despite the now vast body of two-dimensional materials under study, bilayer graphene remains unique in two ways: it hosts a simultaneously tunable band gap and electron density; and stems from simple fabrication methods. These two advantages underscore why bilayer graphene is critical as a material for optoelectronic applications. In the work that follows, we calculate the one- and two-photon absorption coefficients for degenerate interband absorption in a graphene bilayer hosting an asymmetry gap and adjustable chemical potential—all at finite temperature. Our analysis is comprehensive, characterizing one- and two-photon absorptive behavior over wide ranges of photon energy, gap, chemical potential, and thermal broadening. The two-photon absorption coefficient for bilayer graphene displays a rich structure as a function of photon energy and band gap due to the existence of multiple absorption pathways and the nontrivial dispersion of the low energy bands. This systematic work will prove integral to the design of bilayer-graphene-based nonlinear optical devices.

Visible and Near-Infrared Photothermal Catalyzed Hydrogenation of Gaseous CO2 over Nanostructured Pd@Nb2O5.

The reverse water gas shift (RWGS) reaction driven by Nb2O5 nanorod‐supported Pd nanocrystals without external heating using visible and near infrared (NIR) light is demonstrated. By measuring the dependence of the RWGS reaction rates on the intensity and spectral power distribution of filtered light incident onto the nanostructured Pd@Nb2O5 catalyst, it is determined that the RWGS reaction is activated photothermally. That is the RWGS reaction is initiated by heat generated from thermalization of charge carriers in the Pd nanocrystals that are excited by interband and intraband absorption of visible and NIR light. Taking advantage of this photothermal effect, a visible and NIR responsive Pd@Nb2O5 hybrid catalyst that efficiently hydrogenates CO2 to CO at an impressive rate as high as 1.8 mmol gcat−1 h−1 is developed. The mechanism of this photothermal reaction involves H2 dissociation on Pd nanocrystals and subsequent spillover of H to the Nb2O5 nanorods whereupon adsorbed CO2 is hydrogenated to CO. This work represents a significant enhancement in our understanding of the underlying mechanism of photothermally driven CO2 reduction and will help guide the way toward the development of highly efficient catalysts that exploit the full solar spectrum to convert gas‐phase CO2 to valuable chemicals and fuels.

Analysis of semiconductor coupled waveguides with interband absorption

The main goal of this paper is the discussion of the energy transfer mechanisms between two concentric waveguides. An analytical model is developed for the electromagnetic propagation in two coupled concentric semiconductor waveguides. This model includes the calculation of the semiconductor refractive index taking into account the carrier densities, temperature and frequency. The model allows the analysis of the mechanisms of energy transfer between the two waveguides. As expected two types of energy loss are present, one related to the curvature of the waveguides and the other related to the absorption of the radiation in the semiconductor. Finally, the analyzed structure was optimized with regards to the above parameters.

Pulse-width-dependent surface ablation of copper and silver by ultrashort laser pulses

The single-shot spallation thresholds for copper and silver surfaces demonstrate a considerable IR-laser (1030 nm) pulse-width dependent increase over a range of 0.2–12 ps for the former material and a very weak increase for the latter one, while the corresponding thresholds for visible (515 nm) laser pulses remain almost constant. The IR-laser increase of the ablation thresholds is related to two-photon interband (d–s) absorption in copper, contrasting with the linear absorption of visible laser pulses in this material. In silver, common weakly sublinear dependences on the laser pulsewidth were observed, ruling out possible multi-photon—either three(four)-photon in IR, or two-photon in the visible range—interband transitions in this material. Moreover, electron-lattice thermalization times of 1–2 ps were evaluated for these materials in the spallative ablation regime, contrasting strongly with the previously theoretically predicted multi-picosecond thermalization times.

Interband absorption strength in long-wave infrared type-II superlattices with small and large superlattice periods compared to bulk materials

The absorption spectra for the antimonide-based type-II superlattices (SLs) for detection in the long-wave infrared (LWIR) are calculated and compared to the measured data for SLs and bulk materials with the same energy gap (HgCdTe and InAsSb). We include the results for the metamorphic InAsSbx/InAsSby SLs with small periods as well as the more conventional strain-balanced InAs/Ga(In)Sb and InAs/InAsSb SLs on GaSb substrates. The absorption strength in small-period metamorphic SLs is similar to the bulk materials, while the SLs with an average lattice constant matched to GaSb have significantly lower absorption. This is because the electron-hole overlap in the strain-balanced type-II LWIR SLs occurs primarily in the hole well, which constitutes a relatively small fraction of the total thickness.

Transmission, reflection, and absorption spectroscopy of graphene microribbons in the terahertz region

Both transmission and reflection spectra were obtained from graphene microribbons on a SiC substrate, which resonantly coupled with terahertz light through plasmon excitation. An absorption spectrum was also derived from the transmission and reflection spectra. Absorption by the confined intraband plasmons reached 5–10%/layer, which is considerably larger than the wavelength-independent interband absorption of 2.3%/layer. The absorption was found to be larger when the microribbons/substrate sample was illuminated from the back surface than from the ribbon surface because of an interference effect at the surface.

Solar energy conversion via internal photoemission in aluminum, copper, and silver: Band structure effects and theoretical efficiency estimates

Internal photoemission (IPE) across an n-type Schottky junction due to standard AM1.5G solar illumination is quantified with practical considerations for Cu, Ag, and Al under direct and fully nondirect transitions, all in the context of the constant matrix element approximation. Under direct transitions, photoemitted electrons from d bands dominate the photocurrent and exhibit a strong dependence on the barrier energy ΦB but are less sensitive to the change in the metal thickness. Photocurrent is shown to be nearly completely contributed by s-state electrons in the fully nondirect approximation that offers nearly identical results as in the direct transition for metals having a free-electron-like band structure. Compared with noble metals, Al-based IPE has the highest quantum yield up to about 5.4% at ΦB = 0.5 eV and a maximum power conversion efficiency of approximately 0.31% due mainly to its relatively uniform and wide Pexc energy spectral width. Metals (e.g., Ag) with a larger interband absorption edge are shown to outperform those with shallower d-bands (e.g., Cu and Au).

First principles study on defectives BN nanotubes for water splitting and hydrogen storage

First principles calculations within the spin polarized density functional approximation have been addressed to investigate the energetic stability, electronic and optical properties of defective BN nanotubes. Our results show that the presence of carbon impurities interacting with vacancies gives rise to defective electronic levels inside the nanotube band gap. By calculating the absorbance index, we have obtained a strong inter-band optical absorption in the visible region (around 2.1eV) showing that defective BN nanotubes could be an efficient catalytic semiconductor material to be used within solar energy for water splitting. In addition, we observe that the adsorption energy for one and two H2 molecules on the defective surface is in the desired window for the system to be useful as a hydrogen storage medium.

Energy spectra and optical transitions in germanene quantum dots

The band gap of buckled graphene-like materials, such as silicene and germanene, depends on external perpendicular electric field. Then a specially design profile of electric field can produce trapping potential for electrons. We study theoretically the energy spectrum and optical transitions for such designed quantum dots (QDs) in graphene-like materials. The energy spectra depend on the size of the QD and applied electric field in the region of the QD. The number of the states in the QD increases with increasing the size of the dot and the energies of the states have almost linear dependence on the applied electric field with the slope which increases with increasing the dot size. The optical properties of the QDs are characterized by two types of absorption spectra: interband (optical transitions between the states of the valence and conduction bands) and intraband (transitions between the states of conduction/valence band). The interband absorption spectra have triple-peak structure with peak separation around 10 meV, while intraband absorption spectra, which depend on the number of electrons in the dot, have double-peak structure.

Electronic and optical properties of a double quantum dot molecule with Kane’s dispersion law

In the framework of the adiabatic approximation the electronic states and direct interband absorption of light in the double quantum dot molecule (QDM) are discussed. Cases of both standard parabolic and Kane's dispersion law are considered. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are treated. The split of energy levels due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule is revealed. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge behavior characteristics depending on the QDs geometrical sizes and the width of the QDs connecting region are revealed as well.

Theory of electronic states and direct light absorption in the biconical (superellipsoidal) quantum dot

The electronic states in the GaAs biconical (superellipsoidal) quantum dot (QD) in a strong and weak size quantization (SQ) regimes are theoretically investigated within the framework of the geometrical adiabatic approximation. An atypical linear term in the effective confining potential forms a family of non-equidistant sublevels in the electron energy spectrum, whereas the quadratic term leads to the appearance of the equidistant energy levels as a consequence of the symmetry of the QD. In the weak size quantization regime the motion of the exciton's center-of-gravity is quantized, which leads to the appearance of the additional Coulomb sub-levels in the energy spectrum of the QD. The direct interband absorption of light in the biconical QD is also considered in both strong and weak SQ regimes. The dependences of the absorption edge on QD geometrical parameters are revealed in the linear and quadratic approximations of the effective potential adiabatic terms. Corresponding selection rules for quantum transitions are obtained as well.

The tuning of light-matter coupling and dichroism in graphene for enhanced absorption: Implications for graphene-based optical absorption devices

The inter-band optical absorption in graphene characterized by its fine-structure constant has a universal value of 2.3% independent of the material parameters. However, for several graphene-based photonic applications, enhanced optical absorption is highly desired. In this work, we quantify the tunability of optical absorption in graphene via the Fermi level, angle of incidence of the incident polarized light, and the dielectric constants of the surrounding dielectric media in which graphene is embedded. The influence of impurities adsorbed on the surface of graphene on the Lorentzian broadening of the spectral function of the density of states is analytically evaluated within the equilibrium Green’s function formalism. In all the cases, we find that absorption of light graphene embedded in dielectric medium is significantly higher than 2.3%. We also compute the differential absorption of right and left circularly-polarized light in graphene that is uniaxially and optically strained. The preferential absorption or circular dichroism is investigated for armchair and zigzag strain and the interplay of k-space and velocity anisotropy is examined. Finally, we relate circular dichroism to the Berry curvature of gapped graphene and explain the connection to parameters that define the underlying Hamiltonian.

Effects of crossed electric and magnetic fields on the interband optical absorption spectra of variably spaced semiconductor superlattices

The interband optical absorption spectra of a GaAs–Ga1−xAlxAs variably spaced semiconductor superlattice under crossed in-plane magnetic and growth-direction applied electric fields are theoretically investigated. The electronic structure, transition strengths and interband absorption coefficients are analyzed within the weak and strong magnetic-field regimes. A dramatic quenching of the absorption coefficient is observed, in the weak magnetic-field regime, as the applied electric field is increased, in good agreement with previous experimental measurements performed in a similar system under growth-direction applied electric fields. A decrease of the resonant tunneling in the superlattice is also theoretically obtained in the strong magnetic-field regime. Moreover, in this case, we found an interband absorption coefficient weakly dependent on the applied electric field. Present theoretical results suggest that an in-plane magnetic field may be used to tune the optical properties of variably spaced semiconductor superlattices, with possible future applications in solar cells and magneto-optical devices.

Part 1: Wavelength dependent nanosecond laser patterning of very thin indium tin oxide films on glass substrates

Selective laser patterning of very thin (20nm) indium tin oxide (ITO) films on glass substrates using 266, 355, 532 and 1064nm nanosecond (ns) pulses is investigated. An ablative mechanism is observed at a laser wavelength of 266nm, where inter-band absorption enables direct vaporisation of the ITO material. Atomic force microscopy confirms selective film removal at longer wavelengths where the photon energies are less than that associated with the material׳s electronic band gap. Selective patterning at these longer wavelengths, close to the threshold fluence values, is partly attributed to a non-ablative thermally driven melt flow; the material flow is radial in direction, directed from the centre of the laser spot towards the crater edge. This molten flow leads to re-solidified material at the edge of the crater and can cause unwanted glass damage when subsequent overlapped pulses are used to selectively pattern these ultra-thin films. The re-solidified material can be minimised by increasing the spatial overlap of pulses. The experimental results are interpreted using a simple finite element laser heating model. The results are discussed in terms of how low fluence laser pulses can be applied to selectively pattern thin transparent conductive oxides for touch panel displays, and other large area electronic applications, with good electrical isolation and minimal damage to thin glass substrates.

Triple Surface Plasmon Resonances in Zn Nanoparticles in Silica

Theoretical calculations of optical absorption (OA) spectra of Zn nanoparticles (NPs) in silica glass, using a simple model, have been resulted in appearance of three OA bands, all of which are found to blueshift with decrease in NP radius due to quantum confinement effects of free electrons. The intensities of the OA bands are seen to increase with increase in incident light energy. Importantly, all the three OA bands have been found to satisfy, accurately, the surface plasmon resonance (SPR) condition. All these observations clearly indicate that the observed OA bands are originated from the SPR absorptions in Zn NPs, but not due to inter-band absorption.