Bidirectional Polarization Research Articles

Atomic-Thin WS2 Kirigami for Bidirectional Polarization Detection.

The assembly and patterning engineering in two-dimensional (2D) materials hold importance for chip-level designs incorporating multifunctional detectors. At present, the patterning and stacking methods of 2D materials inevitably introduce impurity instability and functional limitations. Here, the space-confined chemical vapor deposition method is employed to achieve state-of-the-art kirigami structures of self-assembled WS2, featuring various layer combinations and stacking configurations. With this technique as a foundation, the WS2 nano-kirigami is integrated with metasurface design, achieving a photodetector with bidirectional polarization-sensitive detection capability in the infrared spectrum. Nano-kirigami can eliminate some of the uncontrollable factors in the processing of 2D material devices, providing a freely designed platform for chip-level multifunctional detection across multiple modules.

Gas detection using cavity-enhanced Raman spectroscopy with a bidirectional multi-pass cell and polarization beam-splitting optical path

We demonstrate a substantial enhancement of gas Raman scattering using a bidirectional multi-pass cavity CERS system, which incorporates a polarization beam-splitting optical path. The system design allows the laser light to traverse the multi-pass cavity for four specific trips, satisfying the need for quick detection of various gas components. Our gas detection experiments using multi-pass cavities with different times of reflection indicate that the addition of polarization beam-splitting optical path gives 1.5 to 1.68 times enhancement of Raman signal compared with that of the system without polarization beam-splitting. For the detection of CH4, a limit of detection of 1.66 ppm was achieved with our system using a multi-pass cell with 41 times of reflection and an integration time of 30s. Our proposed design, which integrates a bidirectional multi-pass cavity with polarization beam-splitting optical path, gives an economical multicomponent gas detection system and a valuable tool for guiding the design and precise alignment of these cavities. This system shows significant promise for applications in e.g. human breath and environmental monitoring.

Bidirectional triple-band truly incident angle insensitive polarization converter using graphene-based transmissive metasurface for terahertz frequency

Abstract The design and response of a triple-band linear-to-circular polarization converter (LTCPC) in the terahertz frequency regime using a graphene-based transmission-type metasurface have been studied numerically and analytically. The unit cell of the converter is constructed using lossy silicon dioxide (SiO2) with a relative permittivity of 3.9 as the substrate. And two similar layers of graphene-based sub-wavelength structures are used at the top and bottom of it. Multiband linear to circular polarization conversion is achieved from 0.74 THz to 0.98 THz, 1.70 THz to 2.33 THz, and 4.06 THz to 5.19 THz, i.e., about 28 %, 31 %, and 24 % fractional bandwidths, respectively. As the same metasurface configuration is used at the top and bottom layers of the transmission-type unit cell, it can be used as a bidirectional polarization converter with identical responses from both sides. The geometrical optimization of the unit cell is done, and an equivalent lumped parameter circuit model is also proposed for the same, considering analogous responses. Tunability over the operating frequency is achieved by varying the chemical potential and relaxation time of graphene. Moreover, due to the ultrathin width and special types of identical configuration of the metasurfaces, the response of the system remains excellent over a wide range of incident angle variations up to 80° angle of incidence.

Bidirectional vortex beam and reflective polarization conversion terahertz metasurfaces

We design a terahertz metasurface with the functions of bidirectional vortex beams and reflection polarization conversion. The unit cell of the proposed metasurface consists of a vanadium dioxide metal mixed layer, a dielectric layer, a vanadium dioxide thin film, a dielectric layer, and a metal open ring. At room temperature, when a left-handed circularly polarized wave is incident, the metasurface generates a reflected mode vortex beam with topological charges of l=−1 and l=−2 at frequencies of 0.33 and 0.34 THz. Simultaneously, the metasurface produces transmitted mode vortex beams with topological charges of l=−1 and l=−2 at frequencies of 0.33 and 0.36 THz. When the temperature changes to 68°C and y-polarized waves are incident, the metasurface generates a reflected left-handed circularly polarized wave in the frequency bands of 0.6–0.65 THz and 1.75–1.93 THz, and a right-handed circularly polarized wave in the frequency band of 0.86–0.9 THz, achieving the function of converting linearly polarized waves into circularly polarized waves. This metasurface provides a new method for designing a single metasurface structure to achieve bidirectional multifunctional terahertz devices for transmission and reflection modes. In addition, the designed metasurface achieves a perfect polarization conversion function that can convert linearly polarized waves into circularly polarized waves.

Technical note: A comparison of methods for estimating coccolith mass

Abstract. The fossil record of coccolithophores dates back approximately 225 million years, and the production of their calcite platelets (coccoliths) contributes to the global carbon cycle over short and geological timescales. Variations in coccolithophore parameters (e.g. community composition, morphology, size and coccolith mass) are a key factor for ocean biogeochemical dynamics (e.g. biological carbon pump) and have been used as a palaeoproxy to understand past oceanographic conditions. Coccolith mass has been frequently estimated with different methods with electron microscopy being the most applied. Here, we compared the electron microscopy (EM) method with the Coulter multisizer (CM) (i.e. electric field disturbance) and bidirectional circular polarization (BCP) methods to estimate coccolith masses (pg CaCO3) in controlled laboratory experiments with two ecotypes of Emiliania huxleyi. Average coccolith mass estimates were in good agreement with literature data. However, mass estimates from the CM were slightly overestimated compared to EM and BCP estimates, and a correction factor (cf=0.8) is suggested to compensate for this discrepancy. The relative change in coccolith mass triggered by morphotype-specific structures and environmental parameters (i.e. seawater carbonate chemistry) was suitably captured by each of the three techniques.

Analysis and research on polarized radiation characteristics of ship targets at sea

The traditional ship infrared detection method cannot solve the edge details when the ship target interacts flexibly with the sea surface background. Polarization detection technology has gradually become an important research direction for high-precision detection of marine targets because of its advantages of acquiring the intrinsic characteristics of targets. This paper selects the typical part panels and scaled models of ships as experimental research object, focusing on the analysis of the polarized radiation characteristics of marine ship targets. Starting from the principle of polarization detection, we extended the application of the bidirectional polarization reflection distribution function that can obtain the polarization spectral characteristics of the target. According to the principle of time-sharing polarization detection, we designed a simulation experiment scheme for the polarization characteristics of the typical parts of the ship and the overall model related to the sea surface background. Finally, we implemented the measurement experiment of multi-angle polarization characteristics in the field and performed data analysis. The results show that: (1) The polarization reflectivity and polarization degree of the typical parts of the ship tend to be consistent with the changes of the observation geometric conditions, both increase with the increase of the observation zenith angle, and the forward scattering area is larger than the back scattering area. (2) The observation direction has a great influence on the overall detection results of the ship. When using the 30°zenith angle of the forward scattering area of the main plane for observation, the contrast between the ship and the water body is obvious, and it is easy to detect the ship.

Three-Dimensional Reconfigurable Optical Singularities in Bilayer Photonic Crystals.

Metasurfaces and photonic crystals have revolutionized classical and quantum manipulation of light and opened the door to studying various optical singularities related to phases and polarization states. However, traditional nanophotonic devices lack reconfigurability, hindering the dynamic switching and optimization of optical singularities. This paper delves into the underexplored concept of tunable bilayer photonic crystals (BPhCs), which offer rich interlayer coupling effects. Utilizing silicon nitride-based BPhCs, we demonstrate tunable bidirectional and unidirectional polarization singularities, along with spatiotemporal phase singularities. Leveraging these tunable singularities, we achieve dynamic modulation of bound-state-in-continuum states, unidirectional guided resonances, and both longitudinal and transverse orbital angular momentum. Our work paves the way for multidimensional control over polarization and phase, inspiring new directions in ultrafast optics, optoelectronics, and quantum optics.

To be a transit link: Similarity in the structure of colonial system of integration and communication pores in autozooids and avicularia of Terminoflustra membranaceotruncata (Bryozoa: Cheilostomata).

Bryozoan colonies consist of zooids, which can differ in structure and function. Most heteromorphic zooids are unable to feed and autozooids supply them with nutrients. The structure of the tissues providing nutrient transfer is poorly investigated. Here, I present a detailed description of the colonial system of integration (CSI) and communication pores in autozooids and avicularia of the cheilosome bryozoan Terminoflustra membranaceotruncata. The CSI is the nutrient transport and distribution system in the colony. In both autozooids and avicularia it consists of a single cell type, that is, elongated cells, and has a variable branching pattern, except for the presence of a peripheral cord. The general similarity in the CSI structure in avicularia and autozooids is probably due to the interzooidal type of the avicularium. Interzooidal avicularia are likely to consume only a part of the nutrients delivered to them by the CSI, and they transit the rest of the nutrients further. The variability and irregularity of branching pattern of the CSI may be explained by the presence of single communication pores and their varying number. The structure of communication pores is similar regardless of their location (in the transverse or lateral wall) and the type of zooid in contact. Rosette complexes include a cincture cell, a few special cells, and a few limiting cells. Along each zooidal wall, there are communication pores with both unidirectional and bidirectional polarity of special cells. However, the total number of nucleus-containing lobes of special cells is approximately the same on each side of any zooidal wall. Supposing the polarity of special cells reflects the direction of nutrient transport, the pattern of special cells polarity is probably related to the need for bidirectional transport through each zooidal wall. The possibility for such transport is important in large perennial colonies with wide zones of autozooids undergoing polypide degeneration.

A Single-Enantiomer Emitter Enabled Superstructural Helix Inversion for Upconverting and Downshifting Luminescence with Bidirectional Circular Polarization.

The helical twisting tendency of liquid crystals (LCs) is generally governed by the inherent configuration of the chiral emitter. Here, we introduce the multistage inversion of supramolecular chirality as well as circularly polarized luminescence (CPL) by manipulating the ratio of single enantiomeric emitters (R-PCP) to LC monomers (5CB). Increasing the content of R-PCP from 1 wt % to 3 wt % inverted the helix of LCs from left-handed to right-handed, accompanying a CPL sign changed from positive to negative. The biaxiality of chiral emitters, as well as the steric effect of chiral-chiral and chiral-achiral interaction, were identified as the reasons for helical sense inversion. Due to the strong helical twisting power, 4 wt % R-PCP drove the photonic band gap (PBG) of chiral LCs to match up with their emission range, leading to an inversion of the CPL again with a high dissymmetry factor (≈1.2). Directly adjusting the PBG using chiral emitters is seldom achieved in cholesteric LCs. On this basis, an achiral sensitizer PtTPBP was assembled into the helical superstructure. The generation of triplet-triplet annihilation-induced upconverted CPL from R-PCP and the downshifting CPL from PtTPBP with opposite rotation was achieved in a single chiral LC system by tuning the position of the PBG.

Radial polarity in the first cranial neuromast of selected teleost fishes.

The neuromast is a sensory structure of the lateral line system in aquatic vertebrates, which consists of hair cells and supporting cells. Hair cells are mechanosensory cells, generally arranged with bidirectional polarity. Here, we describe a neuromast with hair cells arranged radially instead of bidirectionally in the first cranial neuromast of four teleost species: red seabream (Pagrus major), spotted halibut (Verasper variegatus), brown sole (Pseudopleuronectes herzensteini), and marbled sole (Pseudopleuronectes yokohamae). In these four species, this polarity was identified only in the first cranial neuromast, where it appeared at the rostral edge of the otic vesicle before hatching. We investigated the initial appearance and fate of this unique neuromast using scanning electron microscopy. We also assessed characteristics of radial neuromast pertaining to morphogenesis, development, and innervation using a vital fluorescent marker and immunohistochemistry in V. variegatus. The kinocilium initially appears at the center of each hair cell, then moves to its outer perimeter to form radial polarity by around 7 days postfertilization. However, hair cells arranged radially disappear about 15 days after hatching. This is followed by the appearance of bidirectionally arranged hair cells, indicating that polarity replacement from radial to bidirectional has occurred. In P. herzensteini, both afferent and efferent synapses between the nerve fibers and hair cells were observed by transmission electron microscopy, suggesting that radial neuromast is functional. Our discovery suggests that neuromasts with radial polarity could enable larval fish to assimilate multiaxial stimuli during this life stage, potentially assisting them in detecting small water vibrations or water pressure changes.

Colonial system of integration and communication pores in a polymorphic bryozoan Dendrobeania fruticosa (Bryozoa: Cheilostomata).

Bryozoan colonies are composed of zooids, which can differ in structure and function. Autozooids supply heteromorphic zooids with nutrients, which are usually unable to feed. To date, the ultrastructure of the tissues providing nutrient transfer is almost unexplored. Here, we present a detailed description of the colonial system of integration (CSI) and the different types of pore plates in Dendrobeania fruticosa. All cells of the CSI are joined by tight junctions that isolate its lumen. The lumen of the CSI is not a single structure, but a dense network of small interstices filled with a heterogeneous matrix. In autozooids, the CSI is composed of two types of cells: elongated and stellate. Elongated cells form the central part of the CSI, including two main longitudinal cords and several main branches to the gut and pore plates. Stellate cells compose the peripheral part of the CSI, which is a delicate mesh starting from the central part and reaching various structures of autozooids. Autozooids have two tiny muscular funiculi, which start from the caecum apex and run to the basal wall. Each funiculus includes a central cord of extracellular matrix and two longitudinal muscle cells; together they are enveloped with a layer of cells. The rosette complexes of all types of pore plates in D.fruticosa display a similar cellular composition: a cincture cell and a few special cells; limiting cells are absent. Special cells have bidirectional polarity in interautozooidal and avicularian pore plates. This is probably due to the need for bidirectional transport of nutrients during degeneration-regeneration cycles. Cincture cells and epidermal cells of pore plates contain microtubules and inclusions resembling dense-cored vesicles, which are typical of neurons. Probably, cincture cells are involved in the signal transduction from one zooid to another and can be a part of the colony-wide nervous system.

Compact, Wideband, Dual-Band Polarization and Pattern Diversity Antenna for Vehicle Communications

A new design approach of dual-band polarization and pattern diversity antenna for vehicle communications is presented. This antenna consists of three essential parts: Monopole array, swastika-shaped gap-coupled patch, and customized L-shaped driven dipole. Firstly, the monopole array composed of four monopoles beneath the patch, is excited by dipole-induced patch mode of the swastika-shaped gap-coupled patch so as to realize bidirectional beam in the upper band. Secondly, the swastika-shaped gap-coupled patch is reused as the main radiator of the lower band to achieve wideband broadside right-handed circular polarization (RHCP) radiation, which is excited by a pair of customized parallel-lines-fed L-shaped driven dipoles. Finally, two short-circuited branches are etched in the long arm of the L-shaped driven dipole, bringing in the out-of-band suppression improvement between the lower and upper bands. In order to verify the developed design method, a dual-band antenna with different polarization and radiation properties over two bands was designed, fabricated, and tested. Measured results indicate that the developed antenna with a compact transversal size of 0.36λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> *0.36λ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> radiates broadside RHCP electromagnetic wave in 3.55-4.25GHz and bidirectional vertical polarization (VP) in 5.62-6.38GHz, which covers C-band and DSRC-band, and especially serves as a promising candidate for applications in both vehicle-to-satellite (V2S) and vehicle-to-everything (V2X) communications.

Bidirectionally polarizing surface chemistry of heteroatom-doped carbon matrix towards fast and longevous lithium-sulfur batteries

Herein, a bidirectional polarization strategy is proposed for hosting efficient and durable lithium-sulfur battery (Li-S) electrochemistry. By co-doping electronegative N and electropositive B in graphene matrix (BNrGO), the bidirectional electron redistribution enables a higher polysulfide affinity over its mono-doped counterparts, contributing to strong sulfur immobilization and fast conversion kinetics. As a result, BNrGO as the cathode host matrix realizes excellent cycling stability over 1000 cycles with a minimum capacity fading of 0.027% per cycle, and superb rate capability up to 10 C. Meanwhile, decent areal capacity (6.46 mAh/cm2) and cyclability (300 cycles) are also achievable under high sulfur loading and limited electrolyte. This work provides instructive insights into the interaction between doping engineering and sulfur electrochemistry for pursuing superior Li-S batteries.

2 μm soliton lasers in a bidirectional nonlinear polarization evolution Tm3+-doped fiber oscillator

We experimentally demonstrate a novel 2 μm soliton laser using a bidirectional nonlinear polarization evolution mode-locked Tm3+-doped fiber oscillator. The mode-locking operations between the laser signals propagating in clockwise and counterclockwise directions can be achieved through the intracavity polarization state adjustments independently, and the maximum achieved output powers are 332.5 and 367.8 mW with transfer efficiencies of 17.2 % and 18.3 %, respectively. Hysteresis in the output powers and spectra transformation between the two laser signals propagating in counter directions are observed and investigations are conducted in detail. Benefiting from the robust birefringence filtering effect, the central wavelength of this soliton laser can be continuously tuned from 1951.51 to 2015.89 nm. Stable high-order harmonic mode-locking (HML) operations are observed, and up to 633.02 MHz repetition rate with a central wavelength of 1992.55 nm is achieved. The monitored intensity spectra stability and average output power fluctuations exhibit excellent performance of this proposed 2 μm soliton fiber laser. The phase noise levels in two counter directions propagating light signals are characterized at different average output powers. Our experiment provides a potential technique for the generation of 2 μm solitons fiber laser that can be widely applied in ultrafast time-resolved measurements, mid-infrared frequency metrology, molecular spectroscopy, etc.

High-gain bidirectional radiative circularly polarized antenna based on focusing metasurface

A high-gain bidirectional radiative circular polarization (CP) antenna loaded with a dual-mode focusing metasurface (MS) is proposed. The original microstrip patch antenna consists of two coaxially fed tangential patches with different sizes, enabling CP radiation at 8.15 GHz and 14.8 GHz with the gain of 5.4 dBic and 6.2 dBic, respectively. The unit-cell of the focusing MS is composed of two metallic layers separated by a dielectric substrate based on geometric phase principle, which can independently tailor the focusing effect of transmitted and reflected CP waves at 8.15 GHz and 14.8 GHz, respectively. Then, the high-gain bidirectional radiation antenna can be formed by combining the original CP microstrip antenna and focusing MS. The final designed antenna can achieve a CP radiation converge characteristic in transmission at 8.15 GHz with a peak gain of 15.9dBic, and at 14.8 GHz with a peak gain of 19.4dBic in refection, respectively. In order to verify the feasibility of the proposed design, the bidirectional radiative CP antenna with/without focusing MS were fabricated and measured, and the results are consistent basically with the simulated ones. The proposed bidirectional focusing MS antenna could provide an effective solution for dual-mode radiation and high-rate information transmission of the communication system.

2.7 Gb/s Secure Key Generation and Distribution Using Bidirectional Polarization Scrambler in Fiber

A physical-layer secure key generation and distribution (SKGD) scheme in classical fiber channel is proposed and demonstrated using a bidirectional wide-band polarization scrambler (WBPS), to alter rapidly and symmetrically the state of polarization (SOP) for the two counter-propagating optical signals in fiber. Due to the optical channel reciprocity, highly-consistent key bits can be extracted from the correlated SOP fluctuations, while the key secrecy is ensured by the fiber channel uniqueness. A final key generation rate (KGR) of 2.7 Gb/s is demonstrated over 10 km single mode fiber (SMF), where error-free and true randomness of the generated key are achieved after post-processing. The obtained KGR at Gb/s makes a further step towards the implementation of one-time-pad data encryption with perfect secrecy in fiber networks.

Technical note: A universal method for measuring the thickness of microscopic calcite crystals, based on bidirectional circular polarization

Abstract. Coccoliths are major contributors to the particulate inorganic carbon in the ocean that is a key part of the carbon cycle. The coccoliths are a few micrometres in length and weigh a few picogrammes. Their birefringence characteristics in polarized optical microscopy have been used to estimate their mass. This method is rapid and precise because camera sensors produce excellent measurements of light. However, the current method is limited because it requires a precise and replicable set-up and calibration of the light in the optical equipment. More precisely, the light intensity, the diaphragm opening, the position of the condenser and the exposure time of the camera have to be strictly identical during the calibration and the analysis of calcite crystal. Here we present a new method that is universal in the sense that the thickness estimations are independent from a calibration but result from a simple equation. It can be used with different cameras and microscope brands. Moreover, the light intensity used in the microscope does not have to be strictly and precisely controlled. This method permits the measurement of crystal thickness up to 1.7 µm. It is based on the use of one left circular polarizer and one right circular polarizer with a monochromatic light source using the following equation: d=λπΔnarctanILRILL, where d is the thickness, λ the wavelength of the light used, Δn the birefringence, and ILR and ILL the light intensity measured with a right and a left circular polarizer. Because of the alternative and rotational motion of the quarter-wave plate of the circular polarizer, we coined the name of this method “bidirectional circular polarization” (BCP).

Polarization of Light Reflected by Grass: Modeling Using Visible-Sunlit Areas

The Bidirectional polarization distribution function (BPDF) of land surfaces is important for studies of land surfaces and aerosol. With the availability of a huge number of polarization measurements, several semi-empirical BPDF models have been proposed. However, these models do not pay much attention to canopy structure, which is fundamental for generation of polarization. In this article, we propose a new BPDF model using canopy structure information, which is parameterized by visible-sunlit areas. It is evaluated over grassland using POLDER BPDF and MODIS leaf area index data sets. Experiments suggest that compared to Nadal–Bréon and Litvinov models, the new BPDF model reduces root-mean-square error by 7% and 10%, respectively. The new BPDF model also provides better performance when it is fitted using observations clustered by sun zenith angle. The new BPDF model thus provides an effective tool for the study of land surface polarization.

Modeling Bidirectional Polarization Distribution Function of Land Surfaces Using Machine Learning Techniques

Accurate estimation of polarized reflectance (Rp) of land surfaces is critical for remote sensing of aerosol optical properties. In the last two decades, many data-driven bidirectional polarization distribution function (BPDF) models have been proposed for accurate estimation of Rp, among which the generalized regression neural network (GRNN) based BPDF model has been reported to perform the best. GRNN is just a simple machine learning (ML) technique that can solve non-linear problems. Many ML techniques were reported to work well in solving non-linear problems and consequently may provide better performance in BPDF modeling. However, incorporating various ML techniques with BPDF modeling and comparing their performances have never been well documented. In this study, three widely used ML algorithms—i.e., support vector regression (SVR), K-nearest-neighbor (KNN), and random forest (RF)—were applied for BPDF modeling. Using measurements collected by the Polarization and Directionality of the Earth’s Reflectance onboard PARASOL satellite (POLDER/PARASOL), non-linear relationships between Rp and the input variables, i.e., Fresnel factor (Fp), scattering angle (SA), reflectance at 670 nm (R670) and 865 nm (R865), were built using these ML algorithms. Results showed that taking Fp, SA, R670, and R865 as input variables, the performance of the four ML-based BPDF models was quite similar. The KNN-based BPDF model provided slightly better results, and improved the accuracy of the semi-empirical BPDF models by 9.55% in terms of the overall root mean square error (RMSE). Experiments of different configuration of input variables suggested that using multi-band reflectance as input variables provided better results than using vegetation indices. The RF-based BPDF model using all reflectances at six bands as input variables produced the best results, improving the overall accuracy by 6.62% compared with the GRNN-based BPDF model. Among all the input variables, reflectance at absorbing spectral bands—e.g., 490 nm and 670 nm—played more significant roles in RF-based BPDF modeling due to the domination of polarized partition in total reflectance. Fresnel factor and scattering angle were also important for BPDF modeling. This study confirmed the feasibility of applying ML techniques to more accurate BPDF modeling, and the RF-based BPDF model proposed in this study can be used to increase the accuracy of remote sensing of the complete aerosol properties.

Distinguishing Mesoscale Polar Order (Unidirectional vs Bidirectional) of Cellulose Microfibrils in Plant Cell Walls Using Sum Frequency Generation Spectroscopy.

Cellulose in plant cell walls are synthesized as crystalline microfibrils with diameters of 3-4 nm and lengths of around 1-10 μm. These microfibrils are known to be the backbone of cell walls, and their multiscale three-dimensional organization plays a critical role in cell wall functions including plant growth and recalcitrance to degradation. The mesoscale organization of microfibrils over a 1-100 nm range in cell walls is challenging to resolve because most characterization techniques investigating this length scale suffer from low spatial resolution, sample preparation artifacts, or inaccessibility of specific cell types. Here, we report a sum frequency generation (SFG) study determining the mesoscale polarity of cellulose microfibrils in intact plant cell walls. SFG is a nonlinear optical spectroscopy technique sensitive to the molecular-to-mesoscale order of noncentrosymmetric domains in amorphous matrices. However, the quantitative theoretical model to unravel the effect of polarity in packing of noncentrosymmetric domains on SFG spectral features has remained unresolved. In this work, we show how the phase synchronization principle of the SFG process is used to predict the relative intensities of vibrational modes with different polar angles from the noncentrosymmetric domain axis. Applying this model calculation for the first time and employing SFG microscopy, we found that cellulose microfibrils in certain xylem cell walls are deposited unidirectionally (or biased in one direction) instead of the bidirectional polarity which was believed to be dominant in plant cell walls from volume-averaged characterizations of macroscopic samples. With this advancement in SFG analysis, one can now determine the relative polarity of noncentrosymmetric domains such as crystalline biopolymers interspersed in amorphous polymer matrices, which will open opportunities to study new questions that have not been conceived in the past.