Polarization-resolved Measurements Research Articles

Polarization-resolved broadband time-resolved optical spectroscopy for complex materials: application to the case of MoTe2 polytypes.

Time-resolved optical spectroscopy (TR-OS) has emerged as a fundamental spectroscopic tool for probing complex materials, to both investigate ground-state-related properties and trigger phase transitions among different states with peculiar electronic and lattice structures. We describe a versatile approach to perform polarization-resolved TR-OS measurements, by combining broadband detection with the capability to simultaneously probe two orthogonal polarization states. This method allows us to probe, with femtoseconds resolution, the frequency-resolved reflectivity or transmittivity variations along two mutually orthogonal directions, matching the principal axis of the crystal structure of the material under scrutiny. We report on the results obtained by acquiring the polarization-dependent transient reflectivity of two polytypes of the MoTe2 compound, with 2H and 1T' crystal structures. We reveal marked anisotropies in the time-resolved reflectivity signal of 1T'-MoTe2, which are connected to the crystal structure of the compound. Polarization- and time- resolved spectroscopic measurements can thus provide information about the nature and dynamics of both the electronic and crystal lattice subsystems, advancing the comprehension of their inter-dependence, in particular in the case of photoinduced phase transitions; in addition, they provide a broadband measurement of transient polarization rotations.

A polarization-resolved light scattering method for eliminating the interference of water aerosol in industrial stack PM measurement

In this study we report polarization resolved intensity measurements for flyash and water aerosol, respectively, in an industrial stack like environment, for the purpose of developing a methodology...

Stationary and pulsating vector dissipative solitons in nonlinear multimode interference based fiber lasers.

Rapid progress in real-time spectroscopy uncovers the spatio-spectral scenarios of ultrashort pulses in dissipative systems. Varieties of transient soliton dynamics on different timescales have been revealed. Here, we report on an experimental observation of stationary and pulsating vector dissipative solitons in a nonlinear multimode interference (NL-MMI) based fiber laser with net normal dispersion. Polarization non-discrimination of the NL-MMI mode-locking facilitates the dissipative soliton trapping process. Two orthogonally polarized components are coupled together through oppositely shifting their central frequencies to form the group-velocity-locked vector dissipative solitons (GVLVDSs). Dispersive Fourier transform (DFT) based polarization resolved measurement enables insights into the transient polarization dynamics and the long-term evolution. Particularly, both stationary and pulsating GVLVDSs are obtained with appropriate parameter settings. It is found that the quasi-stationary pulsating manner is accompanied with recurrent spectral breathing and energy oscillation; the two orthogonally polarized components possess synchronous pulsating manners due to the cross-phase modulation induced trapping mechanism and the similar formation process. Additionally, chaotic pulsation is also captured in sense that the spectra cannot recover to their original profiles despite of the harmonic energy oscillation. All these findings can enhance our understanding towards soliton pulsation with the freedom of vectorial degree.

Discrimination between two distinct nonlinear effects by polarization-resolved Z-scan measurements.

We investigated how the Z-scan technique can be explored to distinguish two types of nonlinear refractive effects by employing two distinct laser polarizations. It is possible that pure electronic, molecular orientation and thermal nonlinear effects may occur simultaneously during light-matter interaction. We found a way to discriminate and quantify two distinct nonlinear processes from Z-scan signals measured with linear and circular polarizations. This paper provides analytical equations for nonlinear refractions and in order to test them, we carried out measurements in CS2 and in a rhodamine-B solution, where electronic and orientational, and electronic and thermal nonlinearities mixing, respectively, are present.

Polarized near-infrared intersubband absorptions in CdSe colloidal quantum wells

Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30–200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900–1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.

Plasmonic Cu nanostructures in ZnO as hyperbolic metamaterial thin films

Plasmonic metals, such as Cu and Al, have been considered as potential low-loss alternatives for Au and Ag for photonic structures and devices. However, challenges remain in the fabrication and applications of Cu nanostructures, because of its easy oxidation issues. In this work, a new metamaterial structure of plasmonic Cu nanostructures embedded in a dielectric ZnO matrix has been designed and successfully fabricated using a one-step thin film growth method. The Cu–ZnO hybrid thin films present excellent epitaxial quality and exotic optical properties, such as strong localized surface plasmon resonance in the visible regime, and, highly anisotropic and hyperbolic optical response, revealed by angular dependent and polarization resolved reflectivity measurements. This hyperbolic plasmonic metamaterial via the metal-in-oxide matrix form combining low-loss plasmonic Cu nanostructures and extraordinary anisotropic optical properties could be used towards various nanophotonic applications, such as plasmonic solar energy devices and hyperlens.

Remarkable electronic and optical anisotropy of layered 1T'-WTe2 2D materials.

Anisotropic 2D materials exhibit novel optical, electrical and thermoelectric properties that open possibilities for a great variety of angle-dependent devices. Recently, quantitative research on 1T’-WTe2 has been reported, revealing its fascinating physical properties such as non-saturating magnetoresistance, highly anisotropic crystalline structure and anisotropic optical/electrical response. Especially for its anisotropic properties, surging research interest devoted solely to understanding its structural and optical properties has been undertaken. Here we report quantitative, comprehensive work on the highly anisotropic, optical, electrical and optoelectronic properties of few-layer 1T’-WTe2 by azimuth-dependent reflectance difference microscopy, DC conductance measurements, as well as polarization-resolved and wavelength-dependent optoelectrical measurements. The electrical conductance anisotropic ratio is found to ≈103 for a thin 1T’-WTe2 film, while the optoelectronic anisotropic ratio is around 300 for this material. The polarization dependence of the photo-response is ascribed to the unique anisotropic in-plane crystal structure, consistent with the optical absorption anisotropy results. In general, 1T’-WTe2, with its highly anisotropic electrical and photoresponsivity reported here, demonstrates a route to exploit the intrinsic anisotropy of 2D materials and the possibility to open up new ways for applications of 2D materials for light polarization detection.

Giant nonreciprocal second-harmonic generation from antiferromagnetic bilayer CrI3.

Layered antiferromagnetism is the spatial arrangement of ferromagnetic layers with antiferromagnetic interlayer coupling. The van der Waals magnet chromium triiodide (CrI3) has been shown to be a layered antiferromagnetic insulator in its few-layer form1, opening up opportunities for various functionalities2-7 in electronic and optical devices. Here we report an emergent nonreciprocal second-order nonlinear optical effect in bilayer CrI3. The observed second-harmonic generation (SHG; a nonlinear optical process that converts two photons of the same frequency into one photon of twice the fundamental frequency) is several orders of magnitude larger than known magnetization-induced SHG8-11 and comparable to the SHG of the best (in terms of nonlinear susceptibility) two-dimensional nonlinear optical materials studied so far12,13 (for example, molybdenum disulfide). We show that although the parent lattice of bilayer CrI3 is centrosymmetric, and thus does not contribute to the SHG signal, the observed giant nonreciprocal SHG originates only from the layered antiferromagnetic order, which breaks both the spatial-inversion symmetry and the time-reversal symmetry. Furthermore, polarization-resolved measurements reveal underlying C2h crystallographic symmetry-and thus monoclinic stacking order-in bilayer CrI3, providing key structural information for the microscopic origin of layered antiferromagnetism14-18. Our results indicate that SHG is a highly sensitive probe of subtle magnetic orders and open up possibilities for the use of two-dimensional magnets in nonlinear and nonreciprocal optical devices.

Scalar and Vector Solitons in a Bidirectional Mode-Locked Fibre Laser

Recent discovery on scalar–vector features of ultrashort pulses sheds new light on the polarization dynamics of ultrafast fibre lasers. Here, we report on the generation of scalar and vector solitons in a bidirectional fibre laser based on nonlinear polarization rotation (NPR). Through introducing different polarization evolving mechanisms into the counter-propagation branches, the fibre laser respectively approaches the scalar-soliton regime and vector-soliton regime in the clockwise and counterclockwise directions. It is found that the strong local birefringence facilitates the energy to flow between two orthogonal polarization directions, consequently inducing the pulse evolution toward vector solitons even against obvious polarization discrimination effect introduced by the NPR mode-locking mechanism. Furthermore, the polarization resolved measurement is implemented to both analyze the scalar–vector feature of the emitted solitons and realize the manipulation of vector solitons. In addition, numerical simulations are carried out to clarify the manipulation process of the vector solitons. To our knowledge, this is the first observation of scalar–vector operation in a bidirectional fibre laser, which both enriches our understanding toward polarization dynamics of optical solitons and paves potential application scenarios of pulsed fibre lasers.

Pressure dependence of direct optical transitions in ReS2 and ReSe2

The ReX2 system (X = S, Se) exhibits unique properties that differ from other transition metal dichalcogenides. Remarkably, its reduced crystal symmetry results in a complex electronic band structure that confers this material in-plane anisotropic properties. In addition, multilayered ReX2 presents a strong 2D character even in its bulk form. To fully understand the interlayer interaction in this system, it is necessary to obtain an accurate picture of the electronic band structure. Here, we present an experimental and theoretical study of the electronic band structure of ReS2 and ReSe2 at high-hydrostatic pressures. The experiments are performed by photoreflectance spectroscopy and are analyzed in terms of ab initio calculations within the density functional theory. Experimental pressure coefficients for the two most dominant excitonic transitions are obtained and compared with those predicted by the calculations. We assign the transitions to the Z k-point of the Brillouin zone and other k-points located away from high-symmetry points. The origin of the pressure coefficients of the measured direct transitions is discussed in terms of orbital analysis of the electronic structure and van der Waals interlayer interaction. The anisotropic optical properties are studied at high pressure by means of polarization-resolved photoreflectance measurements.

Anisotropic dependence of light extraction behavior on propagation path in AlGaN-based deep-ultraviolet light-emitting diodes.

The anisotropic extraction dependence of polarized light on propagation path in AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is investigated by simulations and photoluminescence (PL) measurements. Theoretical calculations based on k⋅p approximation and Monte Carol ray tracing indicate that there are two kinds of polarized sources with different angular distributions in ~280 nm AlGaN-based LEDs, s-polarized (spherical-shaped) and p-polarized (dumbbell-shaped) sources, which have different extraction behaviors. It is found that the total light extraction intensities are improved with decreasing the propagation path, and the lateral surface extraction gradually becomes dominant. Moreover, the extraction intensity of s-polarized light improves more than that of p-polarized light when the propagation path decreases, leading to a greater polarization degree. Polarization-resolved PL measurements show that the polarization degree of extracted light from lateral facet of the AlGaN multiple quantum well sample can be enhanced from 1% to 17% as the average propagation path reduces by 0.6 mm, which is consistent with the simulation results of the anisotropic dependence of light extraction on propagation path. Our results are significant for understanding and modulating the anisotropic extraction behavior of polarized light to realize high efficiency AlGaN-based DUV LEDs.

Highly pure single photon emission from spectrally uniform surface-curvature directed mesa top single quantum dot ordered array

Realizing ordered and spectrally uniform single photon source arrays integrable on-chip with light manipulating elements in a scalable architecture lies at the core of building monolithic quantum optical circuits (QOCs). We demonstrate here a spatially ordered 5 × 8 array of surface-curvature driven mesa-top GaAs(001)/InGaAs/GaAs single quantum dots (MTSQDs) that exhibit highly pure (∼99%) single photon emission as deduced from the measured g(2)(0) < 0.02 at 9.4 K. Polarization-independent and polarization-resolved high resolution photoluminescence measurements show that these ordered and spectrally uniform QDs have neutral exciton emission with an intrinsic linewidth ∼ 10 μeV and a fine structure splitting <10 μeV, an important figure of merit for the use of QDs in QOCs. The findings point to the high potential of using such MTSQD based single photon source arrays as a promising platform for on-chip scalable integration with light manipulating units (connected resonant cavity, waveguide, beam splitter, etc.) to enable constructing QOCs.

Exciton binding energy and hydrogenic Rydberg series in layered ReS2

Unlike monolayers of transition metal dichalcogenides such as MoS2, which possess high in-plane symmetry, layered ReS2 exhibits reduced in-plane crystal symmetry with a distorted 1 T structure. This unique symmetry leads to anisotropic optical properties, very promising for light polarization devices. Here, we report on low temperature polarization-resolved emission and absorption measurements of excitons in ReS2 from bulk to monolayer. In photoluminescence and reflectivity contrast spectra we distinguish two strongly polarized excitons X1 and X2 with dipole vectors along different crystal directions, which persist from bulk down to monolayer. Basing on the PL and RC spectra of bulk crystals we determine the energy of the ground and first four excited states of both excitons, which follow the usual hydrogenic Rydberg series of energy levels of 3D excitonic states (En = Ry*/n2). From the numerical fit we estimate that the energy gap is direct and equal to 1671.7 meV and binding energy of X1 and X2 is equal to 117.5 and 86.6 meV, respectively. In magneto-PL spectra of bulk ReS2 up to B = 10 T, the energy shift of all the states is below 2 meV. On reducing the crystal thickness from bulk to monolayer the ground state experience a strong blue shift.

ZO phonon of a buffer layer and Raman mapping of hydrogenated buffer on SiC(0001)

AbstractWe have measured spatial Raman maps of hydrogen intercalated quasi‐free standing monolayer graphene (QFSMLG) on SiC(0001). We compare Raman spectra of QFSMLG with spectra of bare buffer layer, single‐layer graphene, and bare SiC substrate. We also present the evolution of QFSMLG Raman spectra with the temperature and duration of hydrogen intercalation. We present new Raman modes, and, on the basis of polarization resolved measurements, we attribute them to the totally symmetric out‐of‐plane optical phonon (ZO) modes of the buffer layer at the Γ and M points. We show that these modes are eliminated by hydrogen intercalation; thus, they indicate onset of buffer layer decoupling from the SiC substrate. The spatial mapping of Raman scattering reveals details of the optimal hydrogen intercalation at elevated temperatures. Further increase of the intercalation temperature leads to etching of the buffer layer and underlying SiC substrate. Therefore, we show that interplay between temperature and intercalation time is a promising route towards increased graphene grain size with reduced lattice strain.

Optical spin pumping induced pseudomagnetic field in two-dimensional heterostructures

Two dimensional heterostructures are likely to provide new avenues for the manipulation of magnetization that is crucial for spintronics or magnetoelectronics. Here, we demonstrate that optical spin pumping can generate a large effective magnetic field in two dimensional MoSe2/WSe2 heterostructures. We determine the strength of the generated field by polarization-resolved measurement of the interlayer exciton photoluminescence spectrum: the measured splitting exceeding 10 milli-electron volts (meV) between the emission originating from the two valleys corresponds to an effective magnetic field of ~ 30 T. The strength of this optically induced field can be controlled by the excitation light polarization. Our finding opens up new possibilities for optically controlled spintronic devices based on van der Waals heterostructures.

Second harmonic generation in strained transition metal dichalcogenide monolayers: MoS2, MoSe2, WS2, and WSe2

Second-harmonic generation (SHG) is a powerful measurement technique to analyze the symmetry properties of crystals. Mechanical strain can reduce the symmetry of a crystal and even weak strain can have a considerable impact on the SHG intensity along different polarization directions. The impact of strain on the SHG can be modeled with a second-order nonlinear photoelastic tensor. In this work, we determined the photoelastic tensors at a fundamental wavelength of 800 nm for four different transition metal dichalcogenide (TMD) monolayers: MoS2, MoSe2, WS2, and WSe2. Strain is applied using a three-point bending scheme, and the polarization-resolved SHG pattern is measured in backscattering geometry. Furthermore, we connected the strain dependent SHG with the strain dependence of the A-exciton energy. With the second-order nonlinear photoelastic tensor, full strain information can be accurately extracted from polarization-resolved SHG measurements. Accordingly, uniaxial strain, induced by polydimethylsiloxan (PDMS) exfoliation and transfer, is measured. We find that TMD monolayers fabricated with PDMS are strained by ∼0.2%. With the experimentally determined nonlinear photoelastic tensors, it will be possible to optically probe arbitrary strain fields in TMD monolayers.

A- and B-exciton photoluminescence intensity ratio as a measure of sample quality for transition metal dichalcogenide monolayers

The photoluminescence (PL) in monolayer transition metal dichalcogenides (TMDs) is dominated by the recombination of electrons in the conduction band with holes in the spin-orbit split valence bands, and there are two distinct emission features referred to as the A-peak (ground state exciton) and B-peak (higher spin-orbit split state). The intensity ratio of these two features varies widely, and several contradictory interpretations have been reported. In this work, we analyze the room temperature PL from MoS2, MoSe2, WS2, and WSe2 monolayers and identify the underlying cause of observed variations in emission profile. We determine that PL variations arise from differences in the non-radiative recombination associated with defect densities. Therefore, the relative intensities of the A- and B-emission features can be used to qualitatively assess the non-radiative recombination and a low B/A ratio is indicative of low defect density and high sample quality. We also performed polarization-resolved PL measurements. Emission from TMD monolayers is governed by unique optical selection rules which make them promising materials for valleytronic operations. We observe a notably higher valley polarization in the B-exciton relative to the A-exciton. The high polarization is a consequence of the shorter B-exciton lifetime resulting from rapid relaxation of excitons from the B-exciton to the A-exciton of the valence band. Our work clarifies disparities reported in the literature relating to the emission profile and provides a straightforward means to assess sample quality.

Novel insights on the vibronic transitions in free base meso-tetrapyridyl porphyrin.

We present novel results on the free base 5,10,15,20-meso-tetra(pyridyl)-21H,23H-porphyrin (H2TPyP). This molecule presents complex electronic and vibrational properties and despite the vast literature reporting the transitions observed in its absorption and fluorescence spectra, a more accurate interpretation has been kept elusive. In particular, we show that the molecule's Q-band develops into many electronic and vibronic transitions, whose the well-known "four orbital model" finds it difficult to reconcile. Using distinct spectroscopy techniques, we conclude that both Qx- and Qy-bands comprise, in fact, two quasi-degenerated electronic states together with their respective vibronic progressions each. The analysis of the Huang-Rhys factors and complementary time- and polarization-resolved measurements reinforce the need for the proposed Q-band multi features remodeling.

Phonon Interaction and Phase Transition in Single Formamidinium Lead Bromide Quantum Dots.

Formamidinium lead bromide (FAPbBr3) quantum dots (QDs) are promising materials for light emitting applications in the visible spectral region because of their high photoluminescence (PL) quantum yield (QY) and the enhanced chemical stability as compared to, for instance, methylammonium based analogues. Toward practical harnessing of their compelling optical characteristics, the exciton recombination process, and in particular the exciton-phonon interaction and the impact of crystal phase transition, has to be understood in detail. This is addressed in this contribution by PL studies on single colloidal FAPbBr3 QDs. Polarization-resolved PL measurements reveal a fine structure splitting of excitonic transitions due to the Rashba effect. Distinct phonon replica have been observed within energetic distances of 4.3 ± 0.5, 8.6 ± 0.9, and 13.2 ± 1.1 meV from the zero phonon line, which we attribute to vibrational modes of the lead bromide lattice. Additional vibrational modes of 18.6 ± 0.3 and 38.8 ± 1.1 meV are found and related to liberation modes of the formamidinium (FA) cation. Temperature-dependent PL spectra reveal a line broadening of the emission caused by exciton phonon interaction as well an unusual energy shift which is attributed to a crystal phase transition within the single QD.

Solution-Synthesized High-Mobility Tellurium Nanoflakes for Short-Wave Infrared Photodetectors.

Two-dimensional (2D) materials, particularly black phosphorus (bP), have demonstrated themselves to be excellent candidates for high-performance infrared photodetectors and transistors. However, high-quality bP can be obtained only via mechanical exfoliation from high-temperature- and high-pressure-grown bulk crystals and degrades rapidly when exposed to ambient conditions. Here, we report solution-synthesized and air-stable quasi-2D tellurium (Te) nanoflakes for short-wave infrared (SWIR) photodetectors. We perform comprehensive optical characterization via polarization-resolved transmission and reflection measurements and report the absorbance and complex refractive index of Te crystals. It is found that this material is an indirect semiconductor with a band gap of 0.31 eV. From temperature-dependent electrical measurements, we confirm this band-gap value and find that 12 nm thick Te nanoflakes show high hole mobilities of 450 and 1430 cm2 V-1 s-1 at 300 and 77 K, respectively. Finally, we demonstrate that despite its indirect band gap, Te can be utilized for high-performance SWIR photodetectors by employing optical cavity substrates consisting of Au/Al2O3 to dramatically increase the absorption in the semiconductor. By changing the thickness of the Al2O3 cavity, the peak responsivity of Te photoconductors can be tuned from 1.4 μm (13 A/W) to 2.4 μm (8 A/W) with a cutoff wavelength of 3.4 μm, fully capturing the SWIR band. An optimized room-temperature specific detectivity ( D*) of 2 × 109 cm Hz1/2 W-1 is obtained at a wavelength of 1.7 μm.