Applications In Photonic Devices Research Articles

Atomically thin gallium telluride nanosheets: A new 2D material for efficient broadband nonlinear optical devices

Here, we experimentally demonstrate the nonlinear optical properties of atomically thin two-dimensional layered gallium telluride (GaTe). We performed open aperture and close aperture Z-scan measurements in the femtosecond regime for the spectral range of 520–700 nm to study the broadband nonlinear absorption and nonlinear refraction. Interestingly, exfoliated GaTe displays strong saturable absorption and high negative Kerr nonlinearity in this spectral range. We observed a high nonlinear refraction coefficient, n2I, of −(7.61±0.07)×10−1 cm2/GW with near band edge excitation at 700 nm. Additionally, it shows a significant nonlinear absorption coefficient, β, of −(18.02±0.20)×104 cm/GW at an intensity of 40 GW/cm2 with high modulation depth and low saturation intensity. Transient absorption spectroscopy measurement is also performed to investigate the carrier dynamics of exfoliated GaTe and to identify the physical mechanisms responsible for the optical nonlinearity, such as Pauli blocking. The measured nonlinear optical data of the exfoliated GaTe presented in this Letter will pave the way for potential application in ultrafast photonic devices like optical switching, passive Q-switching, and mode-locking, due to high Kerr nonlinearity and saturable absorption in the femtosecond regime.

Synthesis and the spectral, electrochemical, and nonlinear optical properties of β-dicyanovinyl-appended 'push-pull' porphyrins.

A series of "push-pull" porphyrins, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII), having triphenylamine (TPA) and dicyanovinyl (DCN) groups at antipodal positions were synthesized and characterised by UV-Vis, fluorescence and NMR spectroscopic techniques, MALDI-TOF mass spectrometry, cyclic voltammetry, DFT, and elemental analysis, which were then further utilized for third-order nonlinear optical measurements under mild conditions using femtosecond laser pulses. Remarkably, MTPP(MN)(TPA)2 (M = 2H, CuII, NiII, and ZnII) exhibited 21-48 nm and 38-80 nm bathochromic shifts in B and Qx(0,0) bands as compared to the corresponding MTPPs (M = 2H, CuII, NiII, and ZnII); the results are consistent with the effect of enhanced resonance due to TPA and -I effect of DCN moieties. In cyclic voltammetry, the push-pull porphyrins exhibited a cathodic shift (0.13-0.51 V) in their first oxidation potential as compared to the precursor owing to the presence of electron-donating TPA groups. The third-order nonlinear optical responses were recorded using a single-beam femtosecond Z-scan technique to retrieve information about the nonlinear absorption and nonlinear refraction of the samples. The two-photon absorption coefficients (β) are in the range of 0.87 × 10-13 to 4.28 × 10-13 m W-1 and the nonlinear refractive index (n2) in the range of 1.21 × 10-19 to 7.36 × 10-19 m2 W-1. The ultrafast absorption dynamics of the ground-state bleaching (GSB) and photo-induced absorption (PIA) are monitored by femtosecond broadband transient absorption studies. The strong nonlinearity of these push-pull porphyrins makes them potential candidates for nonlinear optical and photonic device applications.

Tunable multicolor and bright white emission in PEG modified β-NaGdF4 nanocrystals by systematic introduction of Ce3+ and Mn2+/Ln3.

In this paper, Mn2+/Ln3+-doped hexagonal phase (β-) NaGdF4:Ce (Ln = Tb, Dy, Eu) nanomaterials with subtly tuned multicolor output have been successfully synthesized by a typical simple hydrothermal method using polyethylene glycol (PEG) as a surface modifying agent. The crystal structures, morphology, luminescence performance, and energy transfer (ET) mechanism of the synthesized NaGdF4 nanoparticles (NPs) were investigated in detail. It is found that due to the effective ET between Ce3+ and Mn2+/Ln3+, the multicolor down-conversion (DC) emission phosphors can yield three major emission bands in the visible region including blue, green and red. Moreover, the white emission could be realized through manipulating the doping ratio of Ce3+, Dy3+ and Eu3+ with suitable concentration in β-NaGdF4 NPs through effective resonance-type ET under the irradiation of 273 nm. And the corresponding CIE1931 coordinates were calculated to be (0.31, 0.32), which is near the normative white emission (0.33, 0.33). All the multicolor tuning and white emission results evidently suggest that the present Ce3+ and Mn2+/Ln3+-doped β-NaGdF4 NPs are feasible phosphors for potential applications in white-light emitters, full-color displays and photonic devices.

Broadband Optical Constants and Nonlinear Properties of SnS2 and SnSe2.

SnS2 and SnSe2 have recently been shown to have a wide range of applications in photonic and optoelectronic devices. However, because of incomplete knowledge about their optical characteristics, the use of SnS2 and SnSe2 in optical engineering remains challenging. Here, we addressed this problem by establishing SnS2 and SnSe2 linear and nonlinear optical properties in the broad (300–3300 nm) spectral range. Coupled with the first-principle calculations, our experimental study unveiled the full dielectric tensor of SnS2 and SnSe2. Furthermore, we established that SnS2 is a promising material for visible high refractive index nanophotonics. Meanwhile, SnSe2 demonstrates a stronger nonlinear response compared with SnS2. Our results create a solid ground for current and next-generation SnS2- and SnSe2-based devices.

Studies of Optical, Dielectric, Ferroelectric, and Structural Phase Transitions in 0.9[KNbO3]-0.1 [BaNi1/2Nb1/2O3−δ

The compound 0.9[KNbO3]-0.1[(BaNi1/2Nb1/2O3−δ] (KBNNO), a robust eco-friendly (lead-free) ferroelectric perovskite, has diverse applications in electronic and photonic devices. In this work, we report the dielectric, ferroelectric, and structural phase transitions behavior in the KBNNO compound using dielectric, X-ray diffraction, and Raman studies at ambient and as a function of temperature. Analyses of X-ray diffraction (XRD) data at room temperature (rtp) revealed the orthorhombic phase (sp. Gr. Amm2) of the compound with a minor secondary NiO cubic phase (sp. Gr. Fm3m). A direct optical band gap Eg of 1.66 eV was estimated at rtp from the UV–Vis reflectance spectrum analysis. Observation of non-saturated electric polarization loops were attributed to leakage current effects pertaining to oxygen vacancies in the compound. Magnetization studies showed ferromagnetism at room temperature (300 K) in this material. XRD studies on KBNNO at elevated temperatures revealed orthorhombic-to-tetragonal and tetragonal-to-cubic phase transitions at 523 and 713 K, respectively. Temperature-dependent dielectric response, being leaky, did not reveal any phase transition. Electrical conductivity data as a function of temperature obeyed Jonscher power law and satisfied the correlated barrier-hopping model, indicating dominance of the hopping conduction mechanism. Temperature-dependent Raman spectroscopic studies over a wide range of temperature (82–673 K) inferred the rhombohedral-to-orthorhombic and orthorhombic-to-tetragonal phase transitions at ~260, and 533 K, respectively. Several Raman bands were found to disappear, while a few Raman modes such as at 225, 270, 289, and 831 cm−1 exhibited discontinuity across the phase transitions at ~260 and 533 K.

“ZnS, an excellent material in photonics” - A review based on Z-scan study

Interest in the nonlinear optical properties of ZnS and the effects of alloying ZnS with other elements have been slowly increasing. Unlike ZnO, where tremendous work has been done, ZnS with similar properties is shadowed despite its prominent optical properties. Over the decade, ZnS has been successfully prepared using many techniques in its various forms such as nanoparticles, quantum dots, and thin films. Also, composite materials of ZnS have been developed to light on the enhancement in its properties. The possibility of tuning the properties of ZnS for the enhancement of nonlinear optical properties has been discussed by many reports. The suitability of ZnS for photonic device applications has been reviewed here using the Z-scan technique. Various methods of preparation, requirements, nonlinear behavior of various forms of ZnS in the past decade have been discussed in the present review article.

Effective sensitization of Eu3+ visible red emission by Sm3+ in thermally stable potassium zinc alumino borosilicate glasses for photonic device applications

Sm3+/Eu3+ co-doped thermally stable potassium zinc alumino borosilicate glasses prepared by melt quenching method have been characterized by using spectroscopic techniques to understand their luminescence behavior. For an undoped glass, the XRD pattern recorded confirms the amorphous nature whereas FT-IR and Raman spectrum confirms the functional groups present. Addition of Sm3+ ions as a sensitizer makes the as prepared glasses more efficient for display devices. The mechanism of the energy transfer from Sm3+ to Eu3+ ion has been observed through non-radiative process. The non-exponential time decay curves are giving decreasing lifetimes with increase in Eu3+ ion concentration confirms the energy transfer. The calculated CIE points for the glasses move towards the red region with increase in the concentration of Eu3+ ions under the UV excitation. All the results obtained contemplate the suitability of the as prepared glasses for photonic device applications such as w-LEDs, solar cells and display devices.

Dysprosium concentration-dependent fluorescent properties of antimony lead Oxyfluoroborate glasses

Different concentrations of dysprosium (Dy3+) ions doped antimony lead oxufluoroborate (SbPbOFB) glasses were synthesized through melt quenching method. These prepared glasses are aimed to study for their utility in photonic device applications such as white Light Emitting Diodes (w-LEDs) and visible lasers. Glassy nature and thermal stability of the undoped SbPbOFB glass was investigated by X-Ray Diffraction (XRD) and Thermal Gravimetry Analysis (TGA) techniques. Spectroscopic features were studied by diverse methods like optical absorption, photoluminescence (PL) excitation, PL emission and PL decay spectra. The Judd-Ofelt (J-O) analysis was used to understand the asymmetry and bonding nature in the as prepared glasses. The emission spectra recorded from 400 to 700 nm show three intense bands in blue (482 nm), yellow (575 nm) and red (663 nm) regions corresponds to the transitions 4F9/2 → 6H15/2, 4F9/2 → 6H13/2 and 4F9/2 → 6H11/2 respectively. Relatively higher values of emission cross-section (σse) along with branching ratio and quantum efficiency obtained for SbPbOFBDy1.0 glass for 4F9/2 → 6 H13/2 transition (yellow band) reveal its ability as potential candidate for visible yellow lasers. The Commission Internationale de I'Elcairage (CIE) color chromaticity coordinates estimated from the PL spectra are in proximity with the traditional white light coordinates (x = 0.333, y = 0.333) and illustrate the ability of the as prepared glasses to fabricate w-LEDs.

Phonon-Related Monochromatic THz Radiation and its Magneto-Modulation in 2D Ferromagnetic Cr2 Ge2 Te6.

Searching multiple types of terahertz (THz) irradiation source is crucial for the THz technology. In addition to the conventional fermionic cases, bosonic quasi‐/particles also promise energy‐efficient THz wave emission. Here, by utilizing a 2D ferromagnetic Cr2Ge2Te6 crystal, first a phonon‐related magneto‐tunable monochromatic THz irradiation source is demonstrated. With a low‐photonic‐energy broadband THz pump, a strong THz irradiation with frequency ≈0.9 THz and bandwidth ≈0.25 THz can be generated and its conversion efficiency could even reach 2.1% at 160 K. Moreover, it is intriguing to find that such monochromatic THz irradiation can be efficiently modulated by external magnetic field below 160 K. According to both experimental and theoretical analyses, the emergent THz irradiation is identified as the emission from the phonon‐polariton and its temperature and magnetic field dependent behaviors confirm the large spin‐lattice coupling in this 2D ferromagnetic crystal. These observations provide a new route for the creation of tunable monochromatic THz source which may have great practical interests in future applications in photonic and spintronic devices.

Optically transparent silk fibroin nanofiber paper maintaining native β-sheet secondary structure obtained by cyclic mechanical nanofibrillation process

Optically transparent fibroin nanofiber (FNF) paper was obtained without using binders by passing a fibroin suspension through a mechanical nanofibrillation process using a water-jet. The optical transmittance of the nanofiber paper increased with the number of passes through the water-jet and reached 79.3% and 80.3% of light at 600 nm after 15 and 30 passes, respectively. The fibroin of the transparent nanopaper remained fibrous with a native β-sheet secondary structure. Therefore, the transparent nanopaper showed a high elastic modulus, high thermal durability, and low thermal expansion. This novel protein material, which can be fabricated without toxic chemicals or organic solvents, is bio-friendly and shows promise for new applications in biocompatible photonic devices.

A Proton‐Transfer Complex Containing 5‐Hydroxy‐isophthalic Acid with 3,3′‐(Piperazine‐1,4‐diylbis (methylene)) Dibenzonitrile: Structural Topology, Hirshfeld Analysis, NLO, and DFT Studies

AbstractThis work focuses on the study of a proton‐transfer complex (1) of 5‐hydroxy‐isophthalic acid (HIPA) and 3,3′‐(piperazine‐1,4‐diylbis(methylene)) dibenzonitrile (PBN). The single‐crystal XRD reveals that the HIPA and PBN molecules in complex (1) are mutually connected through intricate N+−H−O− H‐bonding interactions. Herein, close and open aperture measurement for the third‐order nonlinear optical response of the complex (1) has been investigated using the Z‐scan technique which shows its potential application in optoelectronic and photonic devices. Further, DFT calculations have been done for the HOMO‐LUMO energy gap and Mulliken atomic charge analysis which helps us to understand the proton‐transfer mechanism in the complex (1). The topological and Hirshfeld surface analysis were also achieved to understand the importance of non‐covalent interactions in the stability of the supramolecular structure and NLO properties.

Design of gate-tunable graphene electro-optical reflectors based on an optical slot-antenna coupled cavity

The unique properties of graphene offer an exciting opportunity towards tunable photonic surfaces for flexible devices. In this paper, we design a gate-tunable, free-space graphene electro-optical reflector based on cavity resonator structures. We firstly calculate the graphene refractive index n and k as a function of Fermi level and external gating voltage. Then, we designed the structure of the single-layer graphene reflective resonator by carefully selecting suitable materials and device parameters to maximize the reflectance differences before and after electro-optical tuning. We also developed a theoretical model to discuss this system based on the optical transition matrix method. Moreover, we used field enhancement to further increase the reflectance differences by incorporating Sn nanodots based optical slot-antenna coupled cavities. The maximum broadband, incident angle insensitive reflectance differences could reach 28% with an extinction ratio of 1.62 dB at a low insertion loss of 0.45 dB, and the spectral range is tunable by changing the optical cavity length. We also used an indium tin oxide layer as part of the optical cavity and the electrode simultaneously to reduce the voltage applied. To our best knowledge, this work is the first one on tunable two-dimensional (2D) material reflectors for free-space applications, apart from using liquid crystals or magnetic metasurfaces. This new design of tunable 2D electro-optical reflectors also reduces the complexity of fabrication steps, having promising applications in tunable flexible photonic surfaces and devices for variable optical attenuators and light detection and ranging systems.

Influence of Tb3+ ions concentration and temperature on lithium bismuth alumino borosilicate glasses for green photonic device applications

Lithium bismuth alumino borosilicate (LiBiAlBSi) glasses doped with various concentrations of trivalent terbium (Tb3+) ions were prepared to explore their physical, optical, and photoluminescence (PL) characteristics features. Diffraction pattern recorded for an undoped as well as doped (1.5 mol% of Tb3+) LiBiAlBSi glass ascertained the amorphous nature. Absorption spectra reveal several peaks in n-UV and blue regions. The indirect optical band gap for Tb3+ doped LiBiAlBSi glasses was found in the 3.16-3.46 eV range. PL measurements such as emission (under 378 nm excitation) and excitation (under 542 nm emission) were recorded to study the usage of the as-prepared glasses for green photonic device applications. PL emission spectra reveal four emission bands of Tb3+ ions originating from the excited level of 5D4 to 7FJ (=3,4,5,6) levels. The intensity of the emission peak observed in the green region at 542 nm (5D4→7F5) escalates up to 1.5 mol% of Tb3+ ion concentration and declines beyond due to concentration quenching. Dexter theory applied to the PL emission spectral features reveals dipole-dipole interaction among Tb3+ ions responsible for the transfer of energy and thereby concentration quenching. The CIE coordinates and CCT values calculated for the as-prepared glasses are closure to the values of intense green emission given by the European Broadcasting Union illuminant. The experimental lifetime values (τexp) measured for the intense green emission (5D4→7F5) are reducing with the upsurge in the content of Tb3+ ions in the titled glasses. Correlation of τexp values with the PL emission spectral data facilitate the estimation of radiative properties. Relatively high activation energy value estimated from the temperature-dependent PL show the thermal stability of the optimized glass. The PL (excitation, emission, decay, temperature-dependent) studies along with colorimetric studies confirm the aptness of the LBiAlBSiTb15 glass for photonic applications such as laser and tricolour w-LEDs as a green emitting component.

Photoluminescence down-shifting studies of thermally stable Eu3+ ions doped borosilicate glasses for visible red photonic device applications

In this paper we report the synthesis of thermally stable borosilicate glasses doped with europium ions having chemical composition 35B2O3.20SiO2.(15-x)Al2O3.15ZnO.15Na2CO3.xEu2O3 (x = 0.5 to 2.5 mol%) using melt quench process. A broad hump without any sharp peaks observed in the XRD spectrum recorded for an undoped glass confirm its glassy nature. The DSC & TGA has been conducted on an undoped glass to understand thermal stability and aggregate weight loss. Raman spectrum recorded for an undoped glass reveal various functional groups present in the host glass along with its phonon energy. The absorption spectral features recorded for the as prepared glasses are used to estimate optical band gap. In the process of understanding the effective usage of the as prepared glasses in visible red photonic device applications, the spectral features such as photoluminescence (PL) excitation, PL emission and PL decay were recorded and analyzed. Under 393 nm sharp excitation, all the glass samples are showing red emission corresponds to 5D0 → 7F2 transition (612 nm) and whose intensity continuously increasing with Eu3+ ion concentration up to 2.5 mol%. The red to orange color ratio (R/O) estimated from the recorded PL spectral features varies from 3.62 to 3.92 within the variation limits of Eu3+ions from 0.5 to 2.5 mol% indicates relatively low symmetry around Eu3+ ions in the as prepared glasses. Relatively higher R/O ratio also reveals that the nature of bonding between Eu3+ ions and the surrounding ligands as covalent. The Judd-Ofelt theory has been applied to the emission spectral features to understand the nature of bonding between the doped RE ion and its surrounding ligands along with the radiative properties of the doped RE ion. Activation energy (0.175 eV) and percentage loss (82%) in PL intensity estimated for 2.5 mol% of Eu3+ions through temperature dependent PL (TDPL) studies reveal the superiority in thermal stability of the as prepared glasses. The PL, TDPL, PL decay studies conducted along with CIE coordinates estimated allows us to contemplate that, the as prepared glasses are quite useful in fabricating thermally stable visible red photonic devices.

Third- and fifth-order optical nonlinearity in PbO-BaO-Na2O-Nb2O5-SiO2 glass and glass-ceramic nanocomposite dielectrics

We investigate the optical nonlinearity in 6PbO–4BaO–20Na2O–40Nb2O5–30SiO2 glass and glass-ceramic containing ferroelectric Ba2NaNb5O15 nanocrystals and Pb2Nb2O7 nanocrystals using Z-scan method. The values of the third-order (n2) and fifth-order (n4) nonlinear refractive indices are quantitatively determined as 1.76 × 10−17 m2/W and −2.29 × 10−31 m4/W2 in glass phase, respectively, and 2.62 × 10−17 m2/W and −2.89 × 10−31 m4/W2 in glass-ceramic, respectively. The values of three-photon absorption coefficient of the glass and glass-ceramic are obtained as 8.60 × 10−27 m3/W2 and 1.22 × 10−26 m3/W2, respectively. The samples satisfy the criterion for all-optical switching and have potential applications for all-optical photonic devices.

Ultrafast Broadband Nonlinear Optical Response in Co-Doped Sb2Se3 Nanofilms at Near-Infrared

Transition metal-doped Sb2Se3 has become a heated topic caused by the strong nonlinear optical response and the ultrafast response time at high laser excitation. In this paper, the Co-doped Sb2Se3 with different doping amount (0.5, 1.0, and 1.5 W) nanofilms were prepared by magnetron sputtering technology, and the nonlinear behavior of Co-doped Sb2Se3 nanofilms at near infrared were systematically studied. The results of the femtosecond Z-Scan experiment indicate that the Co-doped Sb2Se3 nanofilms exhibit broadband nonlinear response properties owing to the free carrier absorption, the Kerr refraction, the two-photon absorption, and the free carrier refraction. The nonlinear absorption coefficients of Co-doped Sb2Se3 nanofilms are from 3.0 × 10−9 to 2.03 × 10−8 m/ W under excitation at 800, 980, and 1,030 nm, and the nonlinear refractive index of the Co-doped Sb2Se3 nanofilms is from 4.0 × 10−16 to -3.89 × 10−15 m2/ W at 800, 980, and 1,030 nm. More importantly, Co-doped Sb2Se3 (1.5 W) nanofilm exhibits ultrafast carrier absorption (<1 ps) and a stronger transient absorption intensity of ΔOD > 6.3. The Co-doping content can controllably tune the crystalline degree, the ultrafast carrier absorption, the intensity of the reverse saturation absorption, the broadband nonlinear optical response, and the carrier relaxation time of Co-doped Sb2Se3 nanofilms. These results are sufficient to support their applications in broadband nonlinear photonic devices.

Analogue of Electromagnetically Induced Transparency in an All-Dielectric Double-Layer Metasurface Based on Bound States in the Continuum.

Bound states in the continuum (BICs) have attracted much attention due to their infinite Q factor. However, the realization of the analogue of electromagnetically induced transparency (EIT) by near-field coupling with a dark BIC in metasurfaces remains challenging. Here, we propose and numerically demonstrate the realization of a high-quality factor EIT by the coupling of a bright electric dipole resonance and a dark toroidal dipole BIC in an all-dielectric double-layer metasurface. Thanks to the designed unique one-dimensional (D)–two-dimensional (2D) combination of the double-layer metasurface, the sensitivity of the EIT to the relative displacement between the two layer-structures is greatly reduced. Moreover, several designs for widely tunable EIT are proposed and discussed. We believe the proposed double-layer metasurface opens a new avenue for implementing BIC-based EIT with potential applications in filtering, sensing and other photonic devices.

Large bulk photovoltaic effect and second-harmonic generation in few-layer pentagonal semiconductors PdS2 and PdSe2

Recently, atomically thin PdSe2 semiconductors with rare pentagonal Se–Pd–Se monolayers were synthesized and were also found to possess superior properties such as ultrahigh air stability, tunable band gap and high carrier mobility, thus offering a new family of two-dimensional (2D) materials for exploration of 2D semiconductor physics and for applications in advanced opto-electronic and nonlinear photonic devices. In this work, we systematically study the nonlinear optical (NLO) responses [namely, bulk photovoltaic effect (BPVE), second-harmonic generation (SHG) and linear electric-optic (LEO) effect] of noncentrosymmetric bilayer (BL) and four-layer PdS2 and PdSe2 by applying the first-principles density functional theory with the generalized gradient approximation plus scissors-correction. First of all, we find that these few-layer PdX 2 (X = S and Se) exhibit prominent BPVE. In particular, the calculated shift current conductivity is in the order of 130 μA V−2, being very high compared to known BPVE materials. Similarly, their injection current susceptibilities are in the order of 100 × 108 A V−2 s−1, again being large. Secondly, the calculated SHG coefficients (χ (2)) of these materials are also large, being one order higher than that of the best-known few-layer group 6B transition metal dichalcogenides. For example, the maximum magnitude of χ (2) can reach 1.4 × 103 pm V−1 for BL PdSe2 at 1.9 eV and 1.2 × 103 pm V−1 at 3.1 eV for BL PdS2. Thirdly we find significant LEO coefficients for these structures in the low photon energy. All these indicate that 2D PdX 2 semiconductors will find promising NLO applications in light signal modulators, frequency converters, electro-optical switches and photovoltaic solar cells. Fourthly, we find that the large BPVE and SHG of the few-layer PdX 2 structures are due to strong intralayer directional covalent bonding and also 2D quantum confinement. Finally, we also discuss the prominent features of these NLO spectra of these materials in terms of their electronic structure and optical dielectric functions.

Activation energy and its fluctuations at grain boundaries of Er3+:BaTiO3 perovskite thin films: Effect of doping concentration and annealing temperature

An improved understanding of the grain growth kinetics in Erbium-doped Barium Titanate (BTE), has attracted great interest in its applications in photonic and electronic devices. To study the effects of doping and thermal treatment on the physical properties of films, the chemical surface, structural phase, grain size and fluctuations of the grain boundary of BTE films were analyzed and then their effects on the activation energy of the grain growth of the BTE films were revealed. BTE thin films were deposited on Si(100) substrates by electron beam physical vapor deposition (EB-PVD). XRD pattern analysis of the BTE films revealed a tetragonal crystal structure. Williamson–Hall analysis was used to confirm the tensile strain of the BTE thin films. The experimental results indicate that the Er doping in BaTiO3 show strong response by increasing the activation energy of the grain growth and increasing the grain size of the BTE films, while fluctuations of the surface roughness of the BTE films from 0.47 nm to 24.80 nm were also observed. Thin films: BTE2, BTE3, and BTE6 exhibit a grain growth exponent (n) of 5.71, 6.50, and 8.22 that respectively suggest a surface diffusion-controlled grain growth in these films. The activation energies for the grain growth came about 6.63, 6.95 and 19.7 kJ/mol for BTE2, BTE3 and BTE6, respectively, which were close to the values reported for the activation energy in BaTiO3. This was consistent with the high grain boundary density of the BTE films, which was responsible for the mechanism of surface diffusion and point defects. This behavior has been observed in case of BaTiO3, but for Erbium-doped barium titanate it has never been reported.

Manipulating Second Harmonic Generation in Higher‐Order Topological Photonic Crystals

AbstractHigher‐order topological photonic crystals (TPCs) have attracted much attention in the past several years since they do not obey the conventional bulk‐boundary correspondence in topological insulating phases. This characteristic offers a new dimension with which to trap and manipulate the flow of light. In this work, 2D higher‐order TPCs for engineering optical second harmonic generation (SHG) are designed. In the TPCs, a topological corner‐state is achieved and it is demonstrated that the high localization of photons at the corner‐state can significantly promote optical SHG. In addition, topological edge‐states are obtained by breaking the Dirac degeneracy point near the SHG frequency. They are also topologically protected and immune to bending resistance. Therefore, the enhanced SHG signal from the corner‐state can be manipulated precisely for transmitting through the designed interface with relatively low loss due to the topological‐protection edge waves. This design strategy combines the advantages of corner‐state and edge‐state in higher‐order TPCs, demonstrating potential applications of topological photonic devices, such as optical communication and optical computing technologies.