Polarization-dependent Optical Absorption Research Articles

Charge transfer excitons and directional fluorescence of in-plane lateral MoSe2-WSe2 heterostructures.

In this article, the linear and nonlinear optical properties of in-plane lateral MoSe2-WSe2 heterostructures are theoretically investigated. The polarization-dependent strongest optical absorption in one-photon absorption occurs in charge transfer excited states, where electrons transfer from WSe2 to MoSe2. This phenomenon is supported by the LUMO (lowest unoccupied molecular orbital) and HUMO (highest occupied molecular orbital) imaging obtained through scanning tunneling microscopy. The charge difference density and transition density matrix are used to interpret the electronic transitions, and these interpretations rely on the concept of transition density. The optical properties of two-photon absorption in its nonlinear optical process are significantly different from the excitation in one-photon absorption, where the strongest optical absorption is contributed from direct transition from the ground state to the final state without going through an intermediate excited state, due to the very large difference of permanent dipole moments between the excited and ground states. Our results also reveal directional fluorescence and physical mechanism of in-plane lateral MoSe2-WSe2 heterostructures. Our work can provide insights into the physical mechanism of the optical properties of in-plane lateral MoSe2-WSe2 heterostructures.

Exciton dispersion in two-dimensional organic perylene crystal indicates substantial charge-transfer exciton coupling

Two-dimensional, high-quality perylene single crystals were grown with a space-confined strategy method. The grown films crystallize in the $\ensuremath{\alpha}$ form, as is confirmed by a combination of techniques. Polarization-dependent optical absorption measurements show a strong anisotropy in very good agreement with the literature data, and the anisotropic mobility data in field-effect transistors document the very high crystalline order. Momentum-dependent studies using electron energy-loss spectroscopy reveal a negative dispersion of the first exciton along the crystal $b$ direction with an exciton bandwidth of 72 meV. We argue that this behavior is a result of charge-transfer exciton coupling between the perylene dimers in the unit cell.

Optical Anisotropy and Momentum-Dependent Excitons in Dibenzopentacene Single Crystals

High-quality single crystals of the organic semiconductor (1,2;8,9)-dibenzopentacene were grown via physical vapor transport. The crystal structure—unknown before—was determined by single-crystal X-ray diffraction; polarization-dependent optical absorption measurements display a large anisotropy in the ac plane of the crystals. The overall Davydov splitting is ∼110 meV, which is slightly lower than that in the close relative pentacene (120 meV). Momentum-dependent electron energy-loss spectroscopy measurements show a clear exciton dispersion of the Davydov components. An analysis of the dispersion using a simple 1D model indicates smaller electron- and hole-transfer integrals in dibenzopentacene as compared to pentacene. The spectral weight distribution of the excitation spectra is strongly momentum-dependent and demonstrates a strong momentum-dependent admixture of Frenkel excitons, charge-transfer excitons, and vibrational modes.

Polarization-dependent conversion efficiency of carbon nanotube-Si heterojunction solar cells based on aligned carbon nanotube films

We have fabricated solar cells that implement heterojunctions of Si and aligned carbon nanotube (CNT) films. Polarization-dependent optical absorption of highly aligned CNTs led to polarization-dependent conversion efficiencies, which in turn provided insight into the role of the CNT layer in the power generation mechanism in these heterojunction solar cells. When the incident light polarization was parallel to the CNT alignment direction so that the light absorption in the CNT layer was maximized, the short circuit photocurrent decreased by ∼25%. This indicates that electron–hole pair generation in the CNT layer does not directly lead to power generation and that the CNT-Si heterojunctions in our devices are Schottky junctions rather than p–n junctions.

Numerical investigation of excitonic resonances of few-layer black phosphorus based on field enhancement

The optical absorption characteristic, which is one of the key parameters of two-dimensional black phosphorus when functioning as an optoelectronic semiconductor device, can be affected by the exciton binding energy. Field enhancement achieved by metal nanostructures can be used to adjust the photoelectric properties of low-dimensional materials. Here, we propose a new coupled field enhancement method by using metal nanorod arrays and Fabry–Perot cavity to improve the exciton-dominated optical absorption in few-layer black phosphorus. Our simulation results indicate that the exciton-dominated optical absorption can be improved to 84% (increased by 5.8-fold), while achieving a polarization-dependent optical absorption ratio of more than 30. Meanwhile, full width at half maximum of the field-enhanced formant and the polarization ratio can be improved by optimizing the geometric parameters of metal nanostructures. Our results reveal that coupled field enhancement of few-layer black phosphorus could provide a potential new scheme for tunable or polarization-sensitive optoelectronic devices.

Direct observation of cross-polarized excitons in aligned single-chirality single-wall carbon nanotubes

Optical properties of single-wall carbon nanotubes (SWCNTs) for light polarized parallel to the nanotube axis have been extensively studied, whereas their response to light polarized perpendicular to the nanotube axis has not been well explored. Here, by using a macroscopic film of highly aligned single-chirality (6,5) SWCNTs, we performed a systematic polarization-dependent optical absorption spectroscopy study. In addition to the commonly observed angular-momentum-conserving interband absorption of parallel-polarized light, which generates $E_{11}$ and $E_{22}$ excitons, we observed a small but unambiguous absorption peak whose intensity is maximum for perpendicular-polarized light. We attribute this feature to the lowest-energy cross-polarized interband absorption processes that change the angular momentum along the nanotube axis by $\pm 1$, generating $E_{12}$ and $E_{21}$ excitons. The energy difference between the $E_{12}$ and $E_{21}$ exciton peaks, expected from asymmetry between the conduction and valence bands, was smaller than the observed linewidth. Unlike previous observations of cross-polarized excitons in polarization-dependent photoluminescence and circular dichroism spectroscopy experiments, our direct observation using absorption spectroscopy allowed us to quantitatively analyze this resonance. Specifically, we determined the energy and oscillator strength of this resonance to be 1.54 and 0.05, respectively, compared with the values for the $E_{11}$ exciton peak. These values, in combination with comparison with theoretical calculations, in turn led to an assessment of the environmental effect on the strength of Coulomb interactions in this aligned single-chirality SWCNT film.

Determination of the number of graphene layers on one-dimensional photonic crystal by using polarization-dependent absorption effect

In this paper, we study polarization-dependent optical absorption of a graphene on the top of one-dimensional photonic crystal (1DPC), which offer a method to accurately count the number of graphene layers. The 1DPC is composed of 10 pairs of (SiO2/TiO2), and the two kinds of material is transparent, which is different from the commonly substrate with significant contrast to graphene. The absorption difference fitting formula is given by the 0–10 graphene layer. It may be applicable to other two-dimensional materials optically counting layers.

Controlling polarization-dependent optical absorption of graphene through its thickness

Graphene, a promising material with unique electric and photonic properties, has been reported to show polarization-dependent optical absorption in the visible spectral range. Here, we study further on it and demonstrate that this polarization-dependent optical absorption is highly related with the thickness of graphene, i.e., the number of graphene layers. Numerical simulation result shows the extinction ratio of the three-layer graphene polarizer, which is switchable to be s-polarized pass or p-polarized pass, is remarkably high, indicating a great potential in many applications such as broadband polarizer and optical sensor.

Spin-orbit coupling induced band structure change and orbital character of epitaxialIrO2films

We investigated the electronic structure of $\mathrm{Ir}{\mathrm{O}}_{2}$ to address the controversy regarding spin-orbit coupling (SOC) effects in metallic $5d$ transition-metal oxides. Two issues have come to the forefront: (1) SOC effects on electronic structure and physical properties of $\mathrm{Ir}{\mathrm{O}}_{2}$ and (2) the possible formation of a novel ground state in this material, the ${J}_{\mathrm{eff}}=1/2$ state. To better understand the SOC mechanism, we grew epitaxial $\mathrm{Ir}{\mathrm{O}}_{2}$ films whose $dc$ resistivity values were comparable with those of a single crystal. We obtained polarization-dependent optical and x-ray absorption spectra (XAS) and compared these results with those acquired using the generalized gradient approximation (GGA) and $\mathrm{GGA}+\mathrm{SOC}$ calculations. From the optical spectra, peak structures were identified at 0.4 and 2.0 eV, which could only be explained using the $\mathrm{GGA}+\mathrm{SOC}$ calculation. This suggests that SOC plays an important role in the electronic structure of $\mathrm{Ir}{\mathrm{O}}_{2}$. From the polarization-dependent $\mathrm{O}\phantom{\rule{0.16em}{0ex}}1s$ XAS spectra, we observed that the empty state near the Fermi level lacks involvement of an Ir ${d}_{xy}$ orbital. Despite the importance of SOC in $\mathrm{Ir}{\mathrm{O}}_{2}$, the ${J}_{\mathrm{eff}}=1/2$ state does not form in metallic $\mathrm{Ir}{\mathrm{O}}_{2}$.

Effect of Yb doping on the crystal structure, polarization dependent optical absorption and photoluminescence of Yb:YVO4 single crystal grown by optical floating zone technique

Single crystals of Yb doped YVO4 with dopant concentration in the range of 0.75–14.3 at.% were grown by optical floating zone technique. The lattice parameters of YVO4 remain unaffected up to 2.94 at.% doping of Yb but a contraction in the lattice volume was observed at higher concentrations. Spectral decomposition analysis of the broad absorption at 985 nm suggests that in addition to the contributions from three transitions from the ground state of the crystal field induced Stark multiplets of 2F7/2 and 2F5/2, it also has contribution from the transition from the first excited Stark level of 2F7/2 to the upper most Stark level of 2F5/2. Further, the absorption spectra exhibited significant polarization dependence. It was found that while for σ- (E⊥c) polarized light the absorption band at ∼972 dominates for π- (E||c) dominant absorption is at ∼985 nm. The absorption cross-section was found to be maximum at 2.94 at.% of doping for both the polarization signifying quenching at higher concentration. The emission spectra of the Yb doped crystals were characterized by two overlapped broad emissions centered at 985 and 1010 nm. The emission at 985 nm dominated over the emission at 1010 nm at lower doping concentrations which reversed at higher doping concentration. It is interesting to note that the PL intensity at 1010 nm is stronger for excitation at 972 nm in comparison to that for 985 nm.

Polarization-dependent optical absorption of MoS₂ for refractive index sensing.

As a noncentrosymmetric crystal with spin-polarized band structure, MoS2 nanomaterials have attracts increasing attention in many areas such as lithium ion batteries, flexible electronic devices, photoluminescence and valleytronics. The investigation of MoS2 is mainly focused on the electronics and spintronics instead of optics, which restrict its applications as key elements of photonics. In this work, we demonstrate the first observation of the polarization-dependent optical absorption of the MoS2 thin film, which is integrated onto an optical waveguide device. With this feature, a novel optical sensor combining MoS2 thin-film and a microfluidic structure has been constituted to achieve the sensitive monitoring of refractive index. Our work indicates the MoS2 thin film as a complementary material to graphene for the optical polarizer in the visible light range, and explores a new application direction of MoS2 nanomaterials for the construction of photonic circuits.

Large tunable optical absorption of CVD graphene under total internal reflection by strain engineering

We have developed a method to tune polarization-dependent optical absorption of large-scale chemical vapor deposition (CVD) graphene under total internal reflection (TIR) by strain engineering. Through control of the strain direction, the optical absorption of graphene for transverse magnetic or transverse electric waves can be separately tuned. Strain-induced modulation of the optical absorption has been theoretically expected when light is normally incident through graphene. Under TIR, however, we experimentally observed a significant increase in the strain-induced tunability of optical absorption for CVD graphene, with the modulation efficiency of optical absorption in monolayer graphene increasing by a factor of three times that for normal incidence. We conclude that the strain sensitivity of optical absorption of graphene under TIR offers significant potential for application in many areas such as ultra-thin optical devices and strain sensors.

A Comprehensive Study on the Optical Properties of Thin Gold-Doped Rhenium Disulphide Layered Single Crystals

We present a comprehensive study on the optical properties of gold-doped rhenium disulphide (ReS2:Au) layer crystal. Its anisotropic optical properties were studied by using the polarization-dependent optical absorption and photoconductivity (PC) measurements. Absorption measurements indicate that the absorption edge shifted toward high energy as the sample was slimmed down to a thin piece. For the first time, excitonic transitions have been observed by this method. The room temperature transition energies were evaluated to be 1.48 eV for E1ex and 1.516 eV for E2ex. E1ex exciton dominates the transition as the polarization is parallel to the b-axis of the layer crystal, while E2ex exciton is most present as the polarization is perpendicular. PC spectra are performed to check this anisotropic phenomenon. Thermoreflectance modulation (TR) and photoluminescence (PL) measurements have been performed in the temperature range of 42 to 300 K. The temperature dependence of the transition energies and broadening parameters were determined.

Polarization-dependent optical absorption of graphene under total internal reflection

It is shown that graphene exhibits strong polarization-dependent optical absorption under total internal reflection. Compared with universal absorbance of 2.3%, larger absorption was observed in monolayer, bilayer, and few-layer graphenes for transverse electric (TE) wave under total internal reflection. Our result indicates that reflectance ratio of transverse magnetic wave to TE waves can easily provide the information of number of graphene layers. Furthermore, the enhanced light-graphene coupling in a wide spectral range will be great potential in many applications such as photodetector, photovoltaics, and optical sensor.

Nonlinear and magneto-optical transmission studies on magnetic nanofluids ofnon-interacting metallic nickel nanoparticles

Oxide free stable metallic nanofluids have the potential for various applicationssuch as in thermal management and inkjet printing apart from being a candidatesystem for fundamental studies. A stable suspension of nickel nanoparticles of ∼ 5 nm size has been realized by a modified two-step synthesis route. Structural characterizationby x-ray diffraction and transmission electron microscopy shows that the nanoparticles aremetallic and are phase pure. The nanoparticles exhibited superparamagnetic properties.The magneto-optical transmission properties of the nickel nanofluid (Ni-F) wereinvestigated by linear optical dichroism measurements. The magnetic field dependent lighttransmission studies exhibited a polarization dependent optical absorption, known asoptical dichroism, indicating that the nanoparticles suspended in the fluid arenon-interacting and superparamagnetic in nature. The nonlinear optical limiting propertiesof Ni-F under high input optical fluence were then analyzed by an open aperturez-scan technique. The Ni-F exhibits a saturable absorption at moderate laser intensitieswhile effective two-photon absorption is evident at higher intensities. The Ni-Fappears to be a unique material for various optical devices such as field modulatedgratings and optical switches which can be controlled by an external magnetic field.

Chiral splitting and polarization-dependent optical absorption of exciton for Rashba spin–orbit interaction

We study theoretically the effect of Rashba spin–orbit interaction (RSOI) on the chiral splitting of quasi-two-dimensional quantum well (QW) exciton. By a nonvariational method, entire energy spectrum is treated simultaneously, it does not require a great deal of insight to choose a good variational function. We show that, the coexistence of electron and hole RSOI introduces a four-energy system for both heavy-hole and light-hole exciton, the competition between them leads to an anticrossing exciton energy spectrum. We also show that the chiral splitting of energy spectrum leads to a polarization-dependent optical absorption. The results suggest a way for direct optical measurements of RSOI parameters.

Single-crystal film of a combination of organic materials involving a molecular salt and a laser dye; Electro-optic effect

Organic single-crystal films involving a combination of a molecular salt (DAST) and a laser dye (IR-125) have been grown on quartz substrates using the modified shear method. Polarized optical microscopy, polarization-dependent optical absorption, x-ray diffraction and electro-optic effect have been measured to characterize the film. The dipole axis of DAST is oriented close to parallel to the substrate. The IR-125 molecule appears to be oriented close to perpendicular to the dipole axis of DAST. The optical gap of the film is about 1.4 eV. The electro-optic coefficient ( r 11 ) as measured by the field-induced birefringence is ∼300 pm/V at 633 nm. Such films are promising for various photonic applications.

Impedance model for the polarization-dependent optical absorption of superconducting single-photon detectors

We measured the single-photon detection efficiency of NbN superconducting single-photon detectors as a function of the polarization state of the incident light for different wavelengths in the range from 488 nm to 1550 nm. The polarization contrast varies from ~5% at 488 nm to ~30% at 1550 nm, in good agreement with numerical calculations. We use an optical-impedance model to describe the absorption for polarization parallel to the wires of the detector. For the extremely lossy NbN material, the absorption can be kept constant by keeping the product of layer thickness and filling factor constant. As a consequence, the maximum possible absorption is independent of filling factor. By illuminating the detector through the substrate, an absorption efficiency of ~70% can be reached for a detector on Si or GaAs, without the need for an optical cavity.

Polarization dependent optical absorption properties of single-walled carbon nanotubes and methodology for the evaluation of their morphology

Polarization dependence of the optical absorption properties of SWNTs is presented and investigated in detail for the energy range 0.5–6 eV. We found that the absorption peaks in the UV region at approximately 4.5 and 5.25 eV exhibit remarkable and different dependencies on the morphology of the SWNT film, or equivalently, on the incident light polarization relative to the SWNT axis. An analytical pathway to evaluate the physical degree of SWNT alignment for a vertically aligned SWNT film is developed with both transition dipoles parallel and perpendicular to the SWNT axis taken into account. This analytical procedure, coupled with polarized optical absorption measurements performed on the vertically aligned SWNT film grown on substrates, leads to the determination of the bare optical absorption cross-section of SWNTs for both parallel and perpendicular to SWNT axis. In the end, the proposed methodology for evaluating the SWNT film morphology is applied to investigate the transient change of the degree of alignment in the growth process of our vertically aligned SWNT films.

In-plane anisotropy of the optical and electrical properties of ReS 2 and ReSe 2 layered crystals

ReS 2 and ReSe 2 belong to the family of transition-metal dichalcogenides crystallized in the distorted octahedral layer structure of triclinic symmetry. Crystals with triclinic symmetry are optically biaxial. The interband transitions of ReS 2 and ReSe 2 are expected to show anisotropic character for linearly polarized light, incident normal to the basal plane. The optical anisotropic effects for E‖ b and E⊥ b polarizations were studied through polarization-dependent piezoreflectance (PzR) and optical absorption measurements. The polarization dependence of the PzR spectra provides conclusive evidence that the features associated with the interband excitonic transitions are originated from different origins. The results of absorption measurements indicate that ReS 2 and ReSe 2 are indirect semiconductors, in which E parallel to b-axis polarization exhibits a smaller band gap and a single phonon makes an important contribution in assisting the indirect transitions. The electrical conductivity along the b-axis was shown to be several times higher than that perpendicular to b-axis for both crystals.