Anisotropic Optical Absorption Research Articles

Linear dichroism transition and polarization-sensitive photodetector of quasi-one-dimensional palladium bromide

Perpendicular optical reversal of the linear dichroism transition has promising applications in polarization-sensitive optoelectronic devices. We perform a systematical study on the in-plane optical anisotropy of quasi-one-dimensional PdBr2 by using combined measurements of the angle-resolved polarized Raman spectroscopy (ARPRS) and anisotropic optical absorption spectrum. The analyses of ARPRS data validate the anisotropic Raman properties of the PdBr2 flake. And anisotropic optical absorption spectrum of PdBr2 nanoflake demonstrates distinct optical linear dichroism reversal. Photodetector constructed by PdBr2 nanowire exhibits high responsivity of 747 A⋅W−1 and specific detectivity of 5.8 × 1012 Jones. And the photodetector demonstrates prominent polarization-sensitive photoresponsivity under 405-nm light irradiation with large photocurrent anisotropy ratio of 1.56, which is superior to those of most of previously reported quasi-one-dimensional counterparts. Our study offers fundamental insights into the strong optical anisotropy exhibited by PdBr2, establishing it as a promising candidate for miniaturization and integration trends of polarization-related applications.

Intercorrelated Ferroelectricity and Bulk Photovoltaic Effect in Two-Dimensional Sn2P2S6 Semiconductor for Polarization-Sensitive Photodetection.

A two-dimensional (2D) ferroelectric semiconductor, which is coupled with photosensitivity and room-temperature ferroelectricity, provides the possibility of coordinated conductance modulation by both electric field and light illumination and is promising for triggering the revolution of optoelectronics for monolithic multifunctional integration. Here, we report that semiconducting Sn2P2S6 crystals can be achieved in a 2D morphology using a chemical vapor transport approach with the assistant of space confinement and experimentally demonstrate the robust ferroelectricity in atomic-thin Sn2P2S6 nanosheet at room temperature. The intercorrelated programming of ferroelectric order along out-of-plane (OOP) and in-plane (IP) directions enables a tunable bulk photovoltaic (BPV) effect through multidirectional electrical control. By combining the capability of anisotropic in-plane optical absorption, a highly integrated Sn2P2S6 optoelectronic device vertically sandwiched with graphene electrodes yields the polarization-dependent open-circuit photovoltage with a dichroic ratio of 2.0 under 405 nm light illumination. The reintroduction of ferroelectric Sn2P2S6 to the 2D asymmetric semiconductor family provides possibilities to hardware implement of the self-powered polarization-sensitive photodetection and spotlights the promising applications for next-generation photovoltaic devices.

Black Arsenic Phosphorus Mid-Wave Infrared Barrier Detector with High Detectivity at Room Temperature.

The barrier structure is designed to enhance the operating temperature of the infrared detector, thereby improving the efficiency of collecting photogenerated carriers and reducing dark current generation, without suppressing the photocurrent. However, the development of barrier detectors using conventional materials is limited due to the strict requirements for lattice and band matching. In this study, a high-performance unipolar barrier detector is designed utilizing a black arsenic phosphorus/molybdenum disulfide/black phosphorus van der Waals heterojunction. The device exhibits a broad response bandwidth ranging from visible light to mid-wave infrared (520nm to 4.6 µm), with a blackbody detectivity of 2.7× 1010 cmHz-1/2 W-1 in the mid-wave infrared range at room temperature. Moreover, the optical absorption anisotropy of black arsenic phosphorus enables polarization resolution detection, achieving a polarization extinction ratio of 35.5 at 4.6 µm. Mid-wave infrared imaging of the device is successfully demonstrated at room temperature, highlighting the significant potential of barrier devices based on van der Waals heterojunctions in mid-wave infrared detection.

Effect of strain profiling on anisotropic opto-electronic properties of As2X3 (X =S, Te) monolayers from first principles

Strain Engineering is a widely adopted approach to modulate the opto-electronic performance of 2-Dimensional (2D) materials. Recently, anisotropic Van der Waals (vdW) based 2D As2S3 monolayer has gained significant attention within the scientific community due to its stability in ambient conditions. Similar compounds like As2Te3 have also been theoretically explored. However, its indirect bandgap nature limits its application in optical devices. In this study, a systematic study of compressive and tensile strain on three profiles–Uniaxial along a-axis, Uniaxial along b-axis and biaxial strain from −10% to +10%, is performed for As2S3 and As2Te3 monolayers. Certain strain profiles like Uniaxial tensile strain of 8% along b-axis results in transition to direct bandgap material. Similarly, for As2Te3, shear strain of (−10%, +8%) along (a, b) axis results in direct bandgap material. In addition, the anisotropic optical absorption spectrum is obtained for unstrained and strained monolayers within the random phase approximation (RPA).

Adaptive polarization photoacoustic computed tomography for biological anisotropic tissue imaging

Most photoacoustic computed tomography (PACT) systems usually ignore the anisotropy of the tissue absorption coefficient, which will lead to the lack of information in reconstructed images. In this work, the effect is addressed of the possible optical absorption anisotropy of tissue on PACT images. The functional relationship is derived between the photoacoustic response and the polarization angle of the excitation light. An adaptive polarized light photoacoustic imaging (AP-PACT) approach is proposed and shown to make up for the lack of imaging information and achieve optimal image contrast when imaging samples with anisotropic optical absorption, by utilizing the standard deviation of photoacoustic response as the feedback signal in an adaptive data acquisition process. The method is implemented both on phantom and in vitro experiments, which show that AP-PACT can recover anisotropic absorption-related information from reconstructed images and thus significantly improve their quality.

Controllable preparation and photoelectric properties of oriented two-dimensional GeSe2 nanobelt arrays

Germanium diselenide (GeSe2) nanobelts are synthesized by atmospheric-pressure chemical vapor deposition under low temperature by using Se and Ge powders as precursor materials in a quartz tube furnace with double heating zones. The GeSe2 nanobelts thus prepared exhibit growth directionality. Unidirectional nanobelt clusters are tightly spaced and shaped as rectangular nanobelt arrays. Additionally, the thickness of the prepared GeSe2 material is less than 5 nm, and the area of a single array can attain 0.96 mm2. Our experimental results show that hydrogen directly affects the growth of GeSe2. First-principles calculations reveal the electronic properties and in-plane anisotropic optical absorption of the few-layer two-dimensional GeSe2 material. Optical absorbance measurements of GeSe2 nanobelt arrays reveal high ultraviolet absorbance of GeSe2 (200–400 nm). Photodetectors based on GeSe2 nanobelts are p-type, with high responsivity, superior detectivity, and a fast response time. These results show that GeSe2 is an excellent ultraviolet photoelectric material with potential photoelectronic applications.

Universal visualization of crystalline orientation for black phosphorus by angle-resolved polarized photoacoustic microscopy.

Anisotropic two-dimensional (2D) materials, such as black phosphorus (BP), normally possess unique directional in-plane electrical, optical, and thermal properties that are highly correlated with their crystalline orientations. Nondestructive visualization of their crystalline orientation is an indispensable premise for the 2D materials to harness their distinctive strengths in optoelectronic and thermoelectric applications. Here, by photoacoustically recording the anisotropic optical absorption variation under linearly polarized laser beams, an angle-resolved polarized photoacoustic microscopy (AnR-PPAM) is developed, capable of non-invasively determining and visualizing BP's crystalline orientation. We theoretically deduced the physical relationship between the crystalline orientation and polarized photoacoustic (PA) signals, and experimentally proved the ability of AnR-PPAM to universally visualize BP's crystalline orientation regardless of its thickness, substrate, and encapsulation layer. This method provides a new, to the best of our knowledge, strategy for crystalline orientation recognition of 2D materials with flexible measurement conditions, prefiguring important potential for the applications of anisotropic 2D materials.

External screening and lifetime of exciton population in single-layer ReSe2 probed by time- and angle-resolved photoemission spectroscopy

The semiconductor ${\mathrm{ReSe}}_{2}$ is characterized by a strongly anisotropic optical absorption and is therefore promising as an optically active component in two-dimensional heterostructures. However, the underlying femtosecond dynamics of photoinduced excitations in such materials has not been sufficiently explored. Here, we apply an infrared optical excitation to single-layer ${\mathrm{ReSe}}_{2}$ grown on a bilayer graphene substrate and monitor the temporal evolution of the excited state signal using time- and angle-resolved photoemission spectroscopy. We measure an optical gap of $(1.53\ifmmode\pm\else\textpm\fi{}0.02)\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, consistent with resonant excitation of the lowest exciton state. The exciton distribution is tunable via the linear polarization of the pump pulse and exhibits a biexponential decay with time constants given by ${\ensuremath{\tau}}_{1}=(110\ifmmode\pm\else\textpm\fi{}10)$ fs and ${\ensuremath{\tau}}_{2}=(650\ifmmode\pm\else\textpm\fi{}70)$ fs, facilitated by interlayer charge transfer to the underlying bilayer graphene and recombination via an in-gap state that is pinned at the Fermi level. By extracting the momentum-resolved exciton distribution we estimate its real-space radial extent to be greater than $(17\ifmmode\pm\else\textpm\fi{}1)$ \AA{}, implying significant spatial broadening of the distribution due to screening from the bilayer graphene substrate.

Highly in-plane anisotropic optical properties of fullerene monolayers

Both the intrinsic anisotropic optical materials and fullerene-assembled 2D materials have attracted much interest in fundamental science and potential applications. The synthesis of a monolayer (ML) fullerene makes the combination of these two features plausible. In this work, using first-principles calculations, we systematically study the electronic structure and optical properties of quasi-hexagonal phase (qHP) ML and quasi-tetragonal phase (qTP) ML fullerenes. The calculations of qHP ML show that it is a semiconductor with small anisotropic optical absorption, which agrees with the recent experimental measurements. However, the results for qTP ML reveal that it is a semimetal with highly in-plane anisotropic absorption. The dichroic ratio, namely the absorption ratio of x- and y-polarized light αxx/αyy, is around 12 at photon energy of 0.29 eV. This anisotropy is much more pronounced when the photon energy is between 0.7 and 1.4 eV, where αxx becomes nearly zero while αyy is more than two orders of magnitude larger. This indicates qTP ML as a candidate for long-pursuit lossless metal and a potential material for atomically thin polarizer. We hope this will stimulate further experimental efforts in the study of qTP ML and other fullerene-assembled 2D materials.

Electronic structures, transport properties, and optical absorption of bilayer blue phosphorene nanoribbons.

Based on first-principles density functional theory and nonequilibrium Green's function, we study the electronic band structures, the electronic transport properties, and the optical absorption of bilayer blue phosphorene nanoribbons (BPNRs). Both bilayer armchair BPNRs (a-BPNRs) and zigzag BPNRs (z-BPNRs) behave as semiconductors in the narrow nanoribbon case and metals in the wide nanoribbon case, sharply different from their monolayer counterparts where the monolayer a-BPNRs (z-BPNRs) are always semiconducting (metallic). This indicates that interlayer couplings or the increasing layer number may induce the switching of the conductivity of the monolayer BPNRs, which is absent in graphene and phosphorene nanoribbons. Furthermore, we explore the edge states of the energy bands near Fermi energy, and find that there are almost no pure edge-state band branches in the bilayer BPNRs, which can be attributed to the interlayer couplings between the edge-states in one layer and the bulk-states in the other. Consequently, the resulting complex band structures cannot be directly analyzed any more in the framework of the two-body coupling picture just according to the simple band structures of the monolayer BPNRs. Finally, we present the current-voltage characteristics and the optical absorption of the bilayer a-BPNRs and z-BPNRs. The influences of the nanoribbon width and the interlayer couplings on the current and the anisotropic optical absorption can be understood based on the complex energy band structures. This research should be an important reference of extending the field of BPNRs from the monolayer to the bilayer case, and deepen the understanding of the difference between the monolayer and bilayer nanoribbons in different materials.

Decoupling light absorption and carrier transport via heterogeneous doping in Ta3N5 thin film photoanode

The trade-off between light absorption and carrier transport in semiconductor thin film photoelectrodes is a major limiting factor of their solar-to-hydrogen efficiency for photoelectrochemical water splitting. Herein, we develop a heterogeneous doping strategy that combines surface doping with bulk gradient doping to decouple light absorption and carrier transport in a thin film photoelectrode. Taking La and Mg doped Ta3N5 thin film photoanode as an example, enhanced light absorption is achieved by surface La doping through alleviating anisotropic optical absorption, while efficient carrier transport in the bulk is maintained by the gradient band structure induced by gradient Mg doping. Moreover, the homojunction formed between the La-doped layer and the gradient Mg-doped layer further promotes charge separation. As a result, the heterogeneously doped photoanode yields a half-cell solar-to-hydrogen conversion efficiency of 4.07%, which establishes Ta3N5 as a leading performer among visible‐light‐responsive photoanodes. The heterogeneous doping strategy could be extended to other semiconductor thin film light absorbers to break performance trade-offs by decoupling light absorption and carrier transport.

Dynamical Response of Nonlinear Optical Anisotropy in a Tin Sulfide Crystal under Ultrafast Photoexcitation.

Analogous to black phosphorus, SnS processes folded structure that shows a strongly anisotropic optical absorption. Herein, by using ultrafast two-color pump and probe spectroscopy, the azimuthal angle dependence of nonlinear optical anisotropy in SnS is investigated. After 390 nm photoexcitation, the reflectivity of the 780 nm probe beam is first reduced significantly, followed by a complex alternation with the rotation of the sample along the c-axis. The relaxation of reflectivity consisted of two components: a 1-3 ps fast process that shows azimuthal angle and pump fluence dependence, which arises from electron-phonon coupling. The slow process shows strongly azimuthal angle dependence, which arises from the recovery of a photoinduced structural change, i.e., from the photoinduced metastable state with Cmcm-like symmetry to the initial state with Pnma symmetry. In addition, a coherent acoustic phonon with a frequency of 40 GHz is also identified, which originates from the temperature gradient-induced strain wave in the SnS crystal.

Anisotropic photoelectric properties in GaN/AlN quantum dots under terahertz laser polarization

The linear and nonlinear anisotropic optical absorption coefficients(OACs) and refractive index changes(RICs) of GaN/AlN cubical quantum dots(CQDs) subjected to terahertz laser field(TLF) are investigated. The results show that under the polarization of TLF, with the increase of laser dressing parameters, the peak of OACs and RICs increases nonlinearly and have a redshift. More importantly, our work indicated that with the enhancement of TLF polarization, the redistribution of electron excited state wave function will lead to the “loss” of optical properties in the specific frequency band. The redistribution of electron ground state wave function has a significant modulation effect on the magnitude of OACs(RICs) peak. • The enhancement of the refractive index changes was observed at low frequencies. • The phenomenon of wave function exchange in laser field modulation is found. • The modulation of laser field on optical coefficient cannot cover the whole frequency band.

High Anisotropic Optoelectronics in Monolayer Binary M8X12 (M = Mo, W; X = S, Se, Te).

Exploring high performance and excellent ambient stability in two-dimensional (2D) monolayer photoelectric materials is motivated by not only practical applications but also scientific interest. Here, a new 2D monolayer W8Se12 structure is synthesized via in situ electron-beam irradiation on 2D WSe2. Moreover, we systematically studied the photoelectric properties of the class of monolayer M8X12 (M = Mo, W; X = S, Se, and Te) materials by first principles. The results indicated that Mo8S12, Mo8Se12, W8S12, and W8Se12 monolayers possess desirable direct band gaps and remarkable anisotropic optical absorption in visible light, while Mo8Te12 and W8Te12 monolayers are metals. Impressively, the monolayer W8Se12 can result in a direct-indirect-metal transition under uniaxial strain. In addition, they show high anisotropic carrier mobilities (up to 104 cm2 V-1 s-1), significantly over those of transition-metal dichalcogenides. These new binary monolayer M8X12 structures can effectively broaden the 2D material family and may provide four potential candidates in photoelectric applications.

Cation‐Alloying‐Induced Blue‐Shifted and Wide‐Spectrum Polarization‐Sensitive Photodetection in Quasi‐1D SbBiS3

Quasi‐1D ternary semiconductors have attracted considerable attention recently because of their additional bandgap engineering deriving from the variable stoichiometry. Herein, SbBiS3single crystals are successfully synthesized by chemical vapor transport (CVT) method. The basic characterizations combined with theoretical calculations reveal that SbBiS3semiconductor has an intrinsically low symmetry structure and in‐plane anisotropy of optical absorption. Significantly, the bandgap of SbBiS3shows a blue‐shifted compared to the bandgaps of binary Sb2S3and Bi2S3. Moreover, the quasi‐1D SbBiS3‐based photodetector shows a wide‐spectrum photosensitivity (360–1064 nm), a fast response speed (τrise≈ 10 μs andτdecay ≈ 94 μs) at 532 nm, a high photoresponsivity (6.82 A W−1) at 360 nm. In addition, the photodetector exhibits an anisotropic photocurrent ratio up to 1.12 at 808 nm. The work demonstrates the SbBiS3prepared by cationic alloy is a promising candidate for the application of polarized optoelectronics.

First-principles study of electronic and optical properties of small edge-functionalized penta-graphene quantum dots

Tailoring the optoelectronic properties of semiconductor quantum dots is essential for designing functionalized nanoscale devices. In this work, we use first-principles calculations to study the optoelectronic properties of small penta-graphene quantum dots (PGQDs) with various edge-functionalized groups, including hydrogen, halogen (fluorine, chlorine, and bromine), and hydroxyl functional groups. It is evident that these quantum dots, especially those passivated by hydrogen atoms, are thermally stable in vacuum. Moreover, the larger the quantum dots, the more negative the formation energy on stability could reach, thus forming thermodynamically more stable quantum dots. All investigated PGQDs exhibit semiconductor properties. Their bandgaps decrease with an increase in the size of the quantum dots, resulting from the hybridization of sp2 and sp3 carbon atoms and from the charge depletion or accumulation between the passivated atoms and the principal components upon interactions. Concurrently, this study aims to explain the optical absorption anisotropy induced by the edge-functionalized groups of PGQDs under multiple incident light polarizations. These results highlight the use of edge-functionalized groups to develop the next generation of optoelectronic devices.

Nanoimaging of the Edge-Dependent Optical Polarization Anisotropy of Black Phosphorus.

The electronic structure and functionality of 2D materials is highly sensitive to structural morphology, not only opening the possibility for manipulating material properties but also making predictable and reproducible functionality challenging. Black phosphorus (BP), a corrugated orthorhombic 2D material, has in-plane optical absorption anisotropy critical for applications, such as directional photonics, plasmonics, and waveguides. Here, we use polarization-dependent photoemission electron microscopy to visualize the anisotropic optical absorption of BP with 54 nm spatial resolution. We find the edges of BP flakes have a shift in their optical polarization anisotropy from the flake interior due to the 1D confinement and symmetry reduction at flake edges that alter the electronic charge distributions and transition dipole moments of edge electronic states, confirmed with first-principles calculations. These results uncover previously hidden modification of the polarization-dependent absorbance at the edges of BP, highlighting the opportunity for selective excitation of edge states of 2D materials with polarized light.

Anisotropic and tunable optical conductivity of a two-dimensional semi-Dirac system in the presence of elliptically polarized radiation

We investigate the effect of ellipticity ratio of the polarized radiation field on optoelectronic properties of a two-dimensional (2D) semi-Dirac (SD) system. The optical conductivity is calculated within the energy balance equation approach derived from the semiclassical Boltzmann equation. We find that there exists the anisotropic optical absorption induced via both the intra- and interband electronic transition channels in the perpendicular $xx$ and $yy$ directions. Furthermore, we examine the effects of the ellipticity ratio, the temperature, the carrier density, and the band-gap parameter on the optical conductivity of the 2D SD system placed in transverse and vertical directions, respectively. It is shown that the ellipticity ratio, temperature, carrier density, and band-gap parameter can play the important roles in tuning the strength, peak position, and shape of the optical conductivity spectrum. The results obtained from this study indicate that the 2D SD system can be a promising anisotropic and tunable optical and optoelectronic material for applications in innovative 2D optical and optoelectronic devices, which are active in the infrared and terahertz bandwidths.

Electronic and optical properties of cubic quantum dots subjected to terahertz laser field

This study introduces the influence of linearly polarized terahertz laser field (TLF) on electronic and anisotropic optical absorption coefficients (OACs) related to the cubical quantum dots (CQDs). The energy levels and wave functions of the system are obtained by solving the Schrödinger equation within the framework of the effective-mass. The OACs are inferred from the scheme of compact density-matrix formalism. The results show that the laser correction parameters and polarization angle of TLF have a significant influence on the optical properties. As a key result, the modulation of anisotropic OACs of CQDs by terahertz laser field with different parameters is given.

Strain engineering of electronic and optical properties of monolayer diboron dinitride

We studied the effect of strain engineering on the electronic, structural, mechanical, and optical properties of orthorhombic diboron dinitride (o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$) through first-principles calculations. The 1.7-eV direct band gap observed in the unstrained o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ can be tuned up to 3 eV or down to 1 eV by applying 12% tensile strain in armchair and zigzag directions, respectively. Ultimate strain values of o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ were found to be comparable with that of graphene. Our calculations revealed that the partial alignment of the band edges with the redox potentials of water in pristine o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ can be tuned into a full alignment under the armchair and biaxial tensile strains. The anisotropic charge carrier mobility found in o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ prolongs the average lifetime of the carrier drift, creating a suitable condition for photoinduced catalytic reactions on its surface. Finally, we found that even in extreme straining regimes, the highly anisotropic optical absorption of o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ with strong absorption in the visible range is preserved. Having strong visible light absorption and prolonged carrier migration time, we propose that strain engineering is an effective route to tune the band gap energy and band alignment of o-${\mathrm{B}}_{2}{\mathrm{N}}_{2}$ and turn this two-dimensional material into a promising photocatalyst for efficient hydrogen production from water splitting.