[en] We study the dynamical properties of the Mermin model, a simple quantum dissipative model with a monochromatic environment, using analytical and numerical methods. Our numerical results show that the model exhibits a second order phase transition to a localized state before which the system is effectively decoupled from the environment. In contrast to the spin-boson model, the Mermin model exhibits an ''orthogonality catastrophe,'' defining the critical point, before dissipation has destroyed all coherent behavior. An analytic approach based on the Liouvillian technique, although successful in describing the phase diagram of spin-boson and related models, fails to capture this essential feature of the Mermin model

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[en] In this work, we report for the first time, the nonlinear optical absorption properties of vanadium pentoxide (V_2O_5) nanoparticles in the femtosecond excitation regime. V_2O_5 nanoparticles were synthesized through solution combustion technique. The as-synthesized samples were further characterized using XRD, FESEM, EDAX, TEM and UV-visible spectroscopy. X-ray diffraction results revealed the crystalline nature of the nanoparticles. Electron microscopy studies showed the size of the nanoparticles to be ∝200 nm. Open-aperture z-scan technique was employed to study the nonlinear optical absorption behavior of the synthesized samples using a 100-fs laser pulses at 800 nm from a regeneratively amplified Ti: sapphire laser. The mechanism of nonlinear absorption was found to be a three-photon absorption process which was explained using the density of states of V_2O_5 obtained using density functional theory. These nanoparticles exhibit strong intensity-dependent nonlinear optical absorption and hence could be considered for optical-power-limiting applications. (orig.)

[en] PVDF based nanocomposite thin films have received great interest in energy harvesting, ferroelectric, pyroelectric and dielectric applications. In this novel study, we have exploited the electroactive β-phase (polar) formation in polyvinylidine fluoride–halloysite nanotube (PVDF–HNT) nanocomposite thin films fabricated by the spin coating technique for nonlinear optical applications. It was demonstrated that HNTs of different volume percentage loadings in the PVDF matrix were able to effectively nucleate PVDF in β (TTTT-all trans) conformation using X-ray diffraction and Infrared spectroscopy techniques. Closed aperture Z-scan measurements were performed for all the thin film samples with a CW laser as an excitation source at a wavelength of 632.8 nm. We observed a sign change in the nonlinear refractive index for PVDF. Nonlinear refractive index has a negative sign for pristine PVDF and a positive sign for HNT incorporated PVDF thin films. This anomalous behavior of change in the nonlinear refraction of PVDF is explained in our present work.

[en] Nanocrystalline Bismuth telluride and thallium (4 mol %) doped Bismuth telluride were synthesized through hydrothermal method. The as-prepared products were characterized using Powder X-ray Diffraction, High Resolution Transmission Electron Microscopy, Energy Dispersive X-Ray Spectroscopy, UV–Visible spectroscopy and Fourier Transform Infrared Spectroscopy. Powder XRD results revealed the crystalline nature of the obtained phases. HRTEM showed the particle-like morphology of the products. The decrease in the absorption coefficient due to thallium doping was observed in FTIR spectra. The intensity dependent nonlinear optical properties of nanocrystalline bismuth telluride and thallium doped bismuth telluride were studied using the Z-scan technique in open-aperture configuration. Bismuth telluride doped with thallium showed enhanced nonlinear optical response compared to pristine bismuth telluride and hence could be used as a potential candidate for optical power limiting applications. - Graphical Abstract: Nonlinear transmission (Z-scan) curves of nanocrystalline bismuth telluride (Δ) and thallium doped bismuth telluride (□). Thallium doped bismuth telluride showed enhanced nonlinear absorption compared to bismuth telluride. Inset: TEM micrograph of bismuth telluride nanocrystallites. Highlights: ► Synthesis of Nanocrystalline Bi₂Te₃ and Thallium doped Bi₂Te₃ through solvothermal method. ► Reduced absorption coefficient due to thallium doping found from IR spectroscopy. ► Open-aperture Z-scan technique for nonlinear optical studies. ► Two photon absorption based model for theoretical fitting of Z-scan data. ► Enhanced nonlinear absorption in Thallium doped Bi₂Te₃ – potential candidate for optical power limiting applications

[en] In this work, we report for the first time, the nonlinear optical absorption properties of antimony selenide (Sb₂Se₃) nanoparticles synthesized through solvothermal route. X-ray diffraction results revealed the crystalline nature of the nanoparticles. Electron microscopy studies revealed that the nanoparticles are in the range of 10 - 40 nm. Elemental analysis was performed using EDAX. By employing open aperture z-scan technique, we have evaluated the effective two-photon absorption coefficient of Sb₂Se₃ nanoparticles to be 5e-10 m/W at 532 nm. These nanoparticles exhibit strong intensity dependent nonlinear optical absorption and hence could be considered to have optical power limiting applications in the visible range

[en] KBiFe₂O₅ polycrystalline powder was synthesized via citrate assisted sol–gel route. Its structural, microstructural, linear, nonlinear optical and photocatalytic properties were investigated. X-ray diffraction analysis revealed its structure to be monoclinic phase at room temperature. Nonlinear optical absorption of this material was investigated using open aperture z-scan technique. A three-photon type absorption mechanism fitted well with the experimental data, and the corresponding nonlinear absorption coefficient was calculated to be 4 × 10⁻²³ m³/W². The band gap of this material as determined by diffuse reflectance spectroscopy was found to be 1.75 eV. This makes it suitable for visible light driven photocatalytic applications. The photocatalytic property of KBiFe₂O₅ was investigated by degradation of methylene blue under visible light irradiation at different pH. The degradation rate constant was found to be 0.031 min⁻¹ at a pH of 11.

[en] Lead-free perovskite materials with superior physical properties are currently explored for ferroelectric and optoelectronic applications. Ferroelectric materials that have large spontaneous polarization concomitantly possess large nonlinear optical response which is highly suitable for novel photonic applications. Ba_0.85Ca_0.15Zr_0.1Ti_0.9O₃ (BCZT) is one such novel lead-free ferroelectric material with large piezoelectric response arising from existence of morphotropic phase boundary. Conventional sol–gel technique was adopted for synthesizing nanostructured BCZT (nano-BCZT) powder using citrate precursor route. X-ray powder diffraction confirmed the phase purity and high-resolution transmission electron microscopy (HRTEM) proved that the as-synthesized BCZT was indeed nanostructured. Supportively, Raman vibrational analysis was employed to validate the site occupancies of dopants and structural correlations when compared to undoped and pristine barium titanate. Nanostructured barium titanate is extensively studied as biomarkers in second harmonic generation (SHG) microscopy for bio-medical applications. In our current work, we have explored both second- and third-order nonlinear optical response of nano-BCZT. These were found to exhibit stronger SHG signal than potassium di-hydrogen phosphate (KDP) which is a well-known SHG standard. Subsequently, we also have investigated their third-order nonlinear optical properties using open aperture Z-scan technique at 532-nm excitation wavelength in the nanosecond regime. Nano-BCZT was found to exhibit strong two-photon absorption behavior. Such materials with multiphoton absorption behavior are favorable for nonlinear photonics devices such as optical limiters and contrast agents in nonlinear optical microscopy.