Polarization In Thin Films Research Articles

Thin Film Polarizers: Properties and Technologies. Part 1

Thin Film Polarizers: Properties and Technologies. Part 1

Optical absorption in two-dimensional materials with tilted Dirac cones

The interband optical absorption of linearly polarized light by two-dimensional (2D) semimetals hosting tilted and anisotropic Dirac cones in the band structure is analysed theoretically. Supercritically tilted (type-II) Dirac cones are characterized by an absorption that is highly dependent on the incident photon polarization and frequency and is tunable by changing the Fermi level with a back-gate voltage. Type-II Dirac cones exhibit open Fermi surfaces and large regions of the Brillouin zone where the valence and conduction bands sit either above or below the Fermi level. As a consequence, unlike their subcritically tilted (type-I) counterparts, type-II Dirac cones have many states that are Pauli blocked even when the Fermi level is tuned to the level crossing point. We analyze the interplay of the tilt parameter with the Fermi velocity anisotropy, demonstrating that the optical response of a Dirac cone cannot be described by its tilt alone. As a special case of our general theory, we discuss the proposed 2D type-I semimetal 8-$Pmmn$ borophene. Guided by our in-depth analytics, we develop an optical recipe to fully characterize the tilt and Fermi velocity anisotropy of any 2D tilted Dirac cone solely from its absorption spectrum. We expect our paper to encourage Dirac cone engineering as a major route to create gate-tunable thin-film polarizers.

Contrast enhancement in near-infrared electro-optic imaging.

Access to subtle ultrafast effects of light-matter interaction often requires highly sensitive field detection schemes. Electro-optic sampling, being an exemplary technique in this regard, lacks high sensitivity in an imaging geometry. We demonstrate a straightforward method to significantly improve the contrast of electric field images in spatially resolved electro-optic sampling. A thin-film polarizer is shown to be an effective tool in enhancing the sensitivity of the electro-optic imaging system, enabling an adjustment of the spectral response. We show a further increase of the signal-to-noise ratio through the direct control of the carrier envelope phase of the imaged field.

Mechanical Nanoscale Polarization Control in Ferroelectric PVDF‐TrFE Films

AbstractFerroelectric polymer films offer strong advantages like mechanical flexibility, biocompatibility, optical transparency, and low‐cost processing. However, their dielectric or piezoelectric performance is often inferior to that of oxide ferroelectric materials. Key to the dielectric or piezoelectric performance of semicrystalline polymers is the enhancement of electric dipolar order that is naturally lower than in crystalline ferroelectrics. Here, reorientation and alignment of the electric polarization in thin films by the mechanical effect of a scanning unbiased force microscopy tip is demonstrated as a versatile tool for nanoscale domain writing. Thin films (50–150 nm) of PVDF‐TrFE (78:22) on graphite are prepared with dense (110)‐oriented β‐phase lamellae randomly oriented in the film plane. The in‐plane polarization can be poled “mechanically” along any deliberately chosen direction in the film plane after vertical electric poling. Domain patterns with resolution down to ≈50 nm are written with four (out of six possible) local polarization orientations. Written domains show excellent long‐time stability. The surface roughening from the mechanical treatment is moderate (rms roughness of 2–3 nm). A ferroelastic origin of the mechanical polarization switching is discussed. Finally, suggestions are made how to utilize the domain patterns in thin film devices.

Photoenhanced Electroresistance at Dislocation-Mediated Phase Boundary

Ferroelectric tunneling junctions have attracted intensive research interest due to their potential applications in high-density data storage and neural network computing. However, the prerequisite of an ultrathin ferroelectric tunneling barrier makes it a great challenge to simultaneously implement the robust polarization and negligible leakage current in a ferroelectric thin film, both of which are significant for ferroelectric tunneling junctions with reliable operating performance. Here, we observe a large tunneling electroresistance effect of ∼1.0 × 104% across the BiFeO3 nanoisland edge, where the intrinsic ferroelectric polarization of the nanoisland makes a major contribution to tuning the barrier height. This phenomenon is beneficial from the artificially designed tunneling barrier between the nanoscale top electrode and the inclined conducting phase boundary, which is located between the rhombohedral-island and tetragonal-film matrix and arranged with the dislocation array. More significantly, the tunneling electroresistance effect is further improved to ∼1.6 × 104% by the introduction of photoinduced carriers, which are separated by the flexoelectric field arising from the dislocations.

Tuning Molecular Conformations to Enhance Spontaneous Orientation Polarization in Organic Thin Films.

Three isomeric derivatives of 2,2',2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) bearing ethyl groups on the N-phenyl moieties were synthesized to elucidate the effects of intramolecular interactions on spontaneous orientation polarization (SOP) in thin films. The films of the TPBi derivatives displayed enhanced SOP with a surface potential change of up to 1.8 times that for TPBi, and the p-substituted derivative exhibited the largest potential change reported to date (+141.0 mV/nm). Density functional theory calculations and single-crystal structure analysis suggest that the introduction of the ethyl groups switched the stable molecular conformation from C1 to C3 symmetry. Through analysis of the structural anisotropy in the films by spectral ellipsometry and two-dimensional (2D) grazing-incidence wide-angle X-ray scattering, we conclude that the conformational change of the molecules was the major factor underlying the SOP enhancement.

Compact 80 W, 1 MHz Femtosecond Chirped Pulse Amplification Laser System Based on a Yb-Doped Fiber and a Yb:YAG Thin Rod

We present a hybrid chirped pulse amplification (CPA) laser system based on Yb-doped fiber and Yb:YAG thin rod. The laser system demonstrates a novel approach to generate high average power up to the 100-W level by using a Yb:YAG thin rod without pump guiding at room temperature. A Yb-doped all-fiber laser with an output power of 7 W and a repetition rate of 1 MHz is amplified to an output power of 126.2 W in simple and compact two-stage amplifiers consisting of a single fiber crystal (SCF) and a Yb:YAG thin rod. To the best of our knowledge, this is the highest output power of Yb:YAG thin-rod amplifier with good beam quality obtained without pump guiding at room temperature. After using a thin-film polarizer (TFP) to remove the thermal depolarization, a linearly polarized ultrashort pulse laser with an average power of 115.2 W is obtained. An output power of 80.6 W with a pulse width of 580 fs is achieved after pulse compression. The central pulse contains 84.1% of the total pulse energy, corresponding to a pulse peak power of 116.9 MW.

Role of oxygen vacancies in colossal polarization in SmFeO3-δ thin films.

The orthorhombic rare-earth manganates and ferrites multiferroics are promising candidates for the next generation multistate spintronic devices. However, their ferroelectric polarization is small, and transition temperature is far below room temperature (RT). The improvement of ferroelectricity remains challenging. Here, through the subtle strain and defect engineering, an RT colossal polarization of 4.14 μC/cm2 is achieved in SmFeO3-δ films, which is two orders of magnitude larger than its bulk and is also the largest one among the orthorhombic rare-earth manganite and ferrite family. Meanwhile, its RT magnetism is uniformly distributed in the film. Combining the integrated differential phase-contrast imaging and density functional theory calculations, we reveal the origin of this superior ferroelectricity in which the purposely introduced oxygen vacancies in the Fe-O layer distorts the FeO6 octahedral cage and drives the Fe ion away from its high-symmetry position. The present approach can be applied to improve ferroelectric properties for multiferroics.

Effect of stoichiometry and film thickness on the structural and magnetization dynamics behavior of Co2MnAl thin films cosputtered on Si(1 0 0)

Obtaining low gilbert damping (α) and high spin polarization (P) in Heusler alloy thin films is one of the key requirements from the device application point of view in spintronics. Both α and P are correlated, closely related to the density of states (DOS) at the Fermi level and need to be controlled for engineered material. We investigated the effect of stoichiometry (different Co to Mn atomic percent ratios, viz. 2.21, 4.53, 5.43, and 9.19) and film-thickness {(t) in the range of 24–90 nm} on the structural and magnetization dynamics behavior in co-sputtered Co2MnAl thin films grown over Si(100) substrates. While the stoichiometric (∼50 at.% of Co) films with t ≥ 38 nm were found to exhibit B2 order with low α, the off-stoichiometric films (t ∼ 50 nm) were found to stabilize in partially ordered A2 phase and exhibited higher α. The change in structural ordering due to change in stoichiometry and film-thickness in polycrystalline Co2MnAl (CMA) thin films helped in tuning α in the range of (4.11 ± 0.06) × 10-3 – (11.95 ± 0.28) × 10-3. The minimum value of α ∼ (4.11 ± 0.06) × 10-3 was achieved for stoichiometric CMA (t = 70 nm) thin films which is close to the literature value of epitaxially grown L21 and B2 ordered CMA thin films. The study shows that we can tune and attain low damping in polycrystalline CMA Heusler thin films by controlling the stoichiometry.

Polarization in pseudocubic epitaxial relaxed PMN-PT thin films

Understanding the relationship between structural characteristics and functional properties of complex relaxor ferroelectric thin films is of high interest for designing materials with high performances. In this work, the structure of epitaxial relaxed pulsed-laser-deposited Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT; x = 25, 33, and 40) thin films on LaNiO3/SrTiO3 substrates is analyzed using a variety of diffraction and spectroscopic techniques. While based on the data obtained from high-resolution x-ray diffraction and scanning transmission electron microscopy analysis, the average structure of the PMN-xPT films is metrically cubic, micro-Raman polarimetry measurements indicate the tetragonal-like ferroelectric phase with marked preference for the polarization perpendicular to the film for all three compositions. The results of the Raman scattering analysis are supported by electromechanical properties of the samples, which clearly show that the films have a locally non-centrosymmetric structure. Furthermore, only a gradual enhancement of the electrical properties from PMN-25PT to PMN-40PT is observed, which is attributed to small tetragonal distortions that are highly similar for all three compositions.

Efficient few-cycle Yb-doped laser oscillator with Watt-level average power.

So far, the operation of ultrafast bulk laser oscillators based on Yb-doped gain materials and directly emitting few-cycle pulses have been restricted to low optical-to-optical efficiencies and average output powers of only a few milliwatt. This performance limitation can be attributed to the commonly-applied standard collinear pumping scheme in which the optical pump is transmitted through a dichroic mirror whose spectral transmission and dispersion properties severely perturb the oscillating pulse when its optical spectrum extends towards the pump wavelength. In this study, we report on a novel pumping scheme relying on cross polarization that overcomes this challenge. In our concept, the pump transmitting mirror is highly transmissive for the pump light in p-polarization, while it is highly reflective for the laser light in s-polarization over a broad wavelength range, even covering the pump wavelength and beyond. In contrast to a standard thin-film polarizer featuring similar polarization dependent properties, it provides a low and flat dispersion profile over a broad spectral range for the s-polarization. Implementing this pumping scheme in a soft-aperture Kerr-lens mode-locked bulk laser oscillator based on the gain material Yb:CALGO, we achieve clean 22-fs soliton pulses at 729 mW of average output power and an optical-to-optical efficiency of 25%. In a second configuration optimized for the highest average output power, we demonstrate a high optical-to-optical efficiency of 36.6%, which was obtained for 31-fs pulses at 1.63 W of average output power. In a third configuration we experimentally confirm the limiting effect of a dichroic mirror commonly used in the standard collinear pumping scheme. All the results presented here and obtained in the first and second configuration generate pulses with a center wavelength ranging from 1030 nm to 1056 nm, well within the spectral region of high gain cross sections of Yb:CALGO. While this initial demonstration was realized using a commercial diffraction-limited fiber laser as pump source, the pump geometry appears also well suited for pumping with laser diodes coupled into multimode fibers. This novel approach opens up new opportunities for compact and cost-efficient high-power few-cycle bulk laser oscillators based on Yb-doped gain materials and can be applied to any gain material with small quantum defect.

Cavity-dumped nanosecond Cr:LiSAF laser in the 985\u20131030\xa0nm region for versatile seeding of Yb-based amplifiers

We report nanosecond (ns) cavity-dumped operation of a low-cost diode-pumped Cr:LiSAF laser around 1000 nm. The system is pumped with one 1-W single-emitter multimode diode at 665 nm. A Pockell cell (PC) and thin-film-polarizer (TFP) combination placed inside the cavity chops up an adjustable portion of the intracavity power and creates a variable time-dependent output coupler. Via adjusting the length and magnitude of the electrical signal going into the PC, output pulses with pulsewidths in the 2.5–500 ns range and with peak power levels above 10 W are generated at repetition rates up to 100 kHz. The central wavelength of the pulses could be smoothly tuned in the 985–1030 nm region, and is only limited by the anti-reflection coating bandwidth of the current PC and TFP. This versatile nanosecond source with 100 nJ level energies could serve as an attractive low-cost seed source for Yb-based amplifiers, including the cryogenic Yb:YLF systems.

Tunable Microwave Conductance of Nanodomains in Ferroelectric PbZr0.2Ti0.8O3 Thin Film

AbstractFerroelectric materials exhibit spontaneous polarization that can be switched by electric field. Beyond traditional applications as nonvolatile capacitive elements, the interplay between polarization and electronic transport in ferroelectric thin films has enabled a path to neuromorphic device applications involving resistive switching. A fundamental challenge, however, is that finite electronic conductivity may introduce considerable power dissipation and perhaps destabilize ferroelectricity itself. Here, tunable microwave frequency electronic response of domain walls injected into ferroelectric lead zirconate titanate (PbZr0.2Ti0.8O3) on the level of a single nanodomain is revealed. Tunable microwave response is detected through first‐order reversal curve spectroscopy combined with scanning microwave impedance microscopy measurements taken near 3 GHz. Contributions of film interfaces to the measured AC conduction through subtractive milling, where the film exhibited improved conduction properties after removal of surface layers, are investigated. Using statistical analysis and finite element modeling, we inferred that the mechanism of tunable microwave conductance is the variable area of the domain wall in the switching volume. These observations open the possibilities for ferroelectric memristors or volatile resistive switches, localized to several tens of nanometers and operating according to well‐defined dynamics under an applied field.

Role of Ca doping on oxygen vacancy production in modulating dielectric, ferroelectric and magnetic polarization in BaTiO3 thin films

Chemical bath deposition, being an application oriented electroless technique, is used for the deposition of calcium doped barium titanate (Ba1-xCaxTiO3) thin films using glass substrate. Ca concentration is varied as x = 0.0, 0.02, 0.04, 0.06, 0.08, 0.10 and 0.12. As-deposited thin films show amorphous behavior, whereas, annealed thin films show crystalline nature. X-ray diffraction (XRD) analysis of annealed thin films shows a pure tetragonal phase of Perovskite barium calcium titanate thin films up to x = 0.08 dopant concentration, while XRD peak corresponding to CaO is observed at x = 0.10 and 0.12. Maximum optical transmission of 65% is observed under undoped and as-deposited conditions. Annealed thin films show 83% transmission under undoped condition. While decrease in transmission is observed with the increase of dopant concentration due to the presence of sub bands. High value of dielectric constant (i.e. ∼3800 at log f = 1.3) and low value of tangent loss (i.e.∼0.15 at log f = 1.3) is observed for x = 0.08 dopant concentration. Cole–Cole plots reveal the presence of two electrically active grains and grain boundary regions. Maximum value of spontaneous polarization Pmax (18.46 μC/cm2) is observed for x = 0.08 dopant concentration. Mixed para-ferromagnetic response is observed under undoped condition. S-shaped magnetic response is observed for x = 0.02–0.04 whereas, ferromagnetic curves are observed for x = 0.06–0.12. It is important to mention here that tetragonal Perovskite calcium doped barium titanate thin films with U-shaped dielectric constant and good ferroelectric properties are hard to find in literature.

High Temperature Mid‐IR Polarizer via Natural In‐Plane Hyperbolic Van der Waals Crystals

AbstractIntegration of conventional mid to long‐wavelength infrared (IR) polarizers with chip‐scale platforms is restricted by their bulky size and complex fabrication. Van der Waals materials based polarizer can address these challenges due to its nonlithographic fabrication, ease of integration with chip‐scale platforms, and room temperature operation. In the present work, mid‐IR optical response of the sub‐wavelength thin films of α‐phase molybdenum trioxide (α‐MoO3 ) is investigated for application toward high temperature mid‐IR transmission and reflection type thin film polarizer. To the authors’ knowledge, this is the first report of above room temperature mid‐IR optical response of α‐MoO3 to determine the thermal stability of the proposed device. It is found that the α‐MoO3 based polarizer retains high extinction ratio with peak value exceeding 10 dB, up to a temperature of 140 °C. The experimental findings are explained by natural in‐plane hyperbolic anisotropy of α‐MoO3 in the mid‐IR, high temperature X‐ray diffraction and Raman spectroscopic measurements. This work opens up new avenues for naturally in‐plane hyperbolic van der Waals thin‐films to realize sub‐wavelength IR optical components without lithographic constraints.

Abnormal Electromechanical Property of Nonlinearly Graded Lead‐Free Ferroelectric Thin Films

AbstractA phase field model for nonlinearly graded ferroelectric thin films is developed based on the Ginzburg–Landau theory. The developed phase field model is validated by comparing simulated results and available experiment data. Via phase field simulations, effects of gradient index on polarization field and electromechanical response are systematically investigated. Anomalously large electromechanical responses are explored in nonlinearly graded ferroelectric thin films made of BaSrxTiO3, where SrTiO3 mole fraction varies across the film thickness according to power‐law relationships. In addition, a large gradient of polarization can be stabilized in graded ferroelectric thin films, where both magnitude and gradient of polarization can be manipulated by controlling the gradient index of thin films. The remarkable enhancement of electromechanical properties originates from the large gradient of polarization in thin films, which makes the polarization field more susceptible to external excitation. An optimal gradient index for maximizing the electromechanical response is also identified. Furthermore, a consideration of energy properties of graded thin films suggests that both energy storage density and charge–discharge efficiency increase with increasing gradient index of thin films.

Crystal Orientations Dependent Polarization Reversal in Ferroelectric PbZr0.2Ti0.8O3 Thin Films for Multilevel Data Storage Applications

AbstractDeterministic creation of multiple ferroelectric states with variant values of polarization in ferroelectric thin films is promising for multilevel data storage applications. However, the intrinsic bi‐stability of ferroelectric switching makes it challenging to achieve. The deterministic ferroelectric states can be achieved by multistep switching through various means such as controlling domain structure and switching by inducing local disorder, and also by crystal orientations. However, to determine the effect of crystal directions on the stability of multistep polarization reversal requires investigation on different crystal directions. A high‐throughput approach namely combinatorial substrate epitaxy (CSE) is utilized to fabricate La0.7Sr0.3MnO3 (LSMO) polycrystalline substrate to grow PbZr0.2Ti0.8O3 (PZT) films epitaxially with different crystal directions in a single sample. The polarization reversal area is determined as a function of applied voltage along different <hkl>. From the analysis, it is found that all <hkl> exhibit multilevel polarization states due to multistep switching events. Further the degree of multilevel polarization states shows a sawtooth‐like oscillatory behavior with increase in [111] miscut angle, which is attributed to influence of local disorder on the domain structure. Therefore, the realization of multilevel data storage device based on deterministic polarization reversal can be achieved with appropriate crystal directed thin films.

Tunable Polarization and Surface Relief Holographic Gratings in Azopolymer Nanocomposites with Incorporated Goethite (α-FeOOH) Nanorods

We employ two approaches to tune the properties of concurrently inscribed volume polarization and surface relief gratings in nanocomposite thin films containing the azopolymer PAZO (poly[1-4-(3-carboxy-4-hydrophenylazo)benzensulfonamido]-1,2-ethanediyl, sodium salt]) and goethite (α-FeOOH) nanorods. The first one is applied on the stage of sample preparation by varying the concentration of the goethite nanorods from 0% to 15%. Then, different angles between the recording beams are set in the holographic scheme, which allow us to obtain gratings with spatial periods in the range from 0.86 to 2.51 µm. Surface relief modulation close to 300 nm is achieved as well as total diffraction efficiency in the ±1 diffracted orders of more than 50%. The influence of the incorporated goethite nanorods on the properties of both volume birefringence and the surface relief grating are discussed.

Large remanent polarization in Ta-doped HfO2 thin films by reactive sputtering

Ta-doped HfO2 thin films grown on Pt/Ti/SiO2/Si by reactive sputtering possess strong ferroelectric properties, as detected by corresponding polarization vs electric field measurement and mesoscopic piezoresponse force microscopy. Capacitance/current-electric field curves are used to reveal the substances with ferroelectric properties in the thin films. The ferroelectricity of Ta-doped HfO2 is influenced by Ta contents. The remanent polarization Pr can reach ∼53 μC/cm2 by optimizing the Ta percentage. X-ray diffraction and Raman scattering analyses indicate only the presence of monoclinic phase peaks in undoped HfO2 thin films, while Ta doping promotes the formation of the ferroelectric phase (orthorhombic Pca21). X-ray photoelectron spectroscopy demonstrates that the incorporation of Ta5+ within the HfO2 lattice boosts the ferroelectric properties. A polydomain structure is observed on the bare surface of 16% Ta-doped HfO2. Moreover, the switching behavior of the domain is explored by applying an external voltage, demonstrating the potential for storage device applications.

Intrinsic Defect Limit to the Growth of Orthorhombic HfO2 and (Hf,Zr)O2 with Strong Ferroelectricity: First‐Principles Insights

AbstractIt is believed that promoting the fraction of ferroelectric orthorhombic phase (o‐phase) through O‐poor growth conditions can increase the spontaneous polarization of HfO2 and (Hf,Zr)O2 thin films. However, the first‐principles calculations show that the growth may be limited by the easy formation of point defects in the orthorhombic and tetragonal phases of HfO2, ZrO2, and (Hf,Zr)O2. Their dominant defects, O interstitial (Oi) under O‐rich conditions and O vacancy (VO) under O‐poor condition, have low formation energies and quite high density (1016–1019 cm−3 for 800–1400 K growth temperature). Especially, Oi has negative formation energy in tetragonal HfO2 under O‐rich condition, causing non‐stoichiometry and limiting the crystalline‐seed formation during o‐phase growth. High‐density defects can cause disordering of dipole moments and increase leakage current, both diminishing the polarization. These results explain the experimental puzzle that the measured polarization is much lower than the ideal value even in O‐poor thin films and highlight that controlling defects is as important as promoting the o‐phase fraction for enhancing ferroelectricity. The O‐intermediate condition (average of O‐rich and O‐poor conditions) and low growth temperature are proposed for fabricating HfO2 and (Hf,Zr)O2 with fewer defects, lower leakage current, and stronger ferroelectricity, which challenges the belief that O‐poor condition is optimal.