Search

We report the wavelength dependencies of the two-photon absorption coefficients β as well as the nonlinear refractive index n2 of undoped β-Ga2O3 single crystal in the spectral range 350–515 nm (0.51 < Ephoto/Eg < 0.75). Femtosecond Z-scan measurements show that n2 decreases monotonically toward the shortwave side, while the maximum value of n2 is 5.0 × 10−19 m2/W near half of the bandgap, Eg. Interestingly, a second upward trend in β was observed when the incident photon energy is larger than 0.6Eg, indicating the impact of additional inter-band transitions with higher energy. A higher-energy band-to-band transition around 6.0 eV was determined by both femtosecond nondegenerate two-photon absorption spectroscopy and ab initio calculations. Our results could provide guidance for designing nonlinear β-Ga2O3 photonic devices in the UV-visible spectral range. [ABSTRACT FROM AUTHOR]

We report the spectral dependence of the three-photon absorption coefficient γ and third-order nonlinear refractive index n2 in pristine, Sn-doped and Fe-doped β-Ga2O3 crystals using 190 fs pulses in the range of 540–750 nm, i.e., photon energy between half and one-third of the bandgap Eg. The nonlinear coefficient n2 is found to be independent of Sn or Fe doping, and the maximum value of n2 is about 4 × 10−19 m2/W when the photon energy is near Eg/2. However, we observe that Fe doping can significantly enhance the γ value in the range of 540–600 nm (near Eg/2) while doping has negligible impact on γ for wavelength larger than 620 nm. Femtosecond pump-probe measurements show that Fe doping induced a fast (∼50 ps) carrier decay process after photo excitation, which could be attributed to the Fe induced deep trap states. We obtain three-photon figures of merit that allow operational intensities up to 100 GW/cm2. Our results indicate that β-Ga2O3 has great potential for all-optical switching applications in red and near IR regimes. [ABSTRACT FROM AUTHOR]

Gallium oxide (Ga2O3) is a promising candidate in future photonic integrated circuits devices due to its ultra-wide band gap and high stability. However, the nonlinear optical properties of Ga2O3 still remain elusive. In this work, we investigate the free-carrier absorption and refraction in pristine β-Ga2O3 single crystal via the degenerate pump-probe measurement with phase object technique at 355 nm wavelength. The free-carrier absorption cross-section and free-carrier refraction index of β-Ga2O3 on the sub-nanosecond time scale are determined to be ∼0.6 × 10−22 m2, and ∼−0.62 × 10−28 m3, respectively. And the two-photon absorption coefficient of β-Ga2O3 is characterized as ∼0.53 × 10−11 m W−1. It is found that the free-carrier absorption and free-carrier refraction effect in β-Ga2O3 are significantly smaller than that in other wide bandgap semiconductors such as GaN and ZnO. Our results can serve as guidelines for designing β-Ga2O3 based ultra-low loss waveguides and integrated photonic applications in UV spectral range. [ABSTRACT FROM AUTHOR]

The time-resolved nondegenerate pump-probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532 nm laser beam with 21 ps duration is used as the excitation and the laser beams of 600 and 680 nm with 10 ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump-probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined. [ABSTRACT FROM AUTHOR]

In this work, a novel soft-hard template method towards the direct fabrication of graphene films on silicon/silica substrate is developed via a tri-constituent self-assembly route. Using cetyl trimethyl ammonium bromide (CTAB) as a soft template, silica (SiO2) from tetramethoxysilane as a hard template, and pyrene as a carbon source, the self-assembly process allows the formation of a sandwich-like SiO2/CTAB/pyrene composite, which can be further converted to high quantity graphene films with a thickness of ~1 nm and a size of over 5 μm by thermal treatment. The morphology and thickness of the graphene films can be effectively controlled through the adjustment of the ratio of pyrene to CTAB. Furthermore, a high nonlinear refractive index n2 of ~10−12 m2 W−1 is measured from graphene/silica hybrid film, which is six orders of magnitude larger than that of silicon and comparable to the graphene from chemical vapor deposition process. [ABSTRACT FROM AUTHOR]

Two asymmetric pyrene derivatives (E)-1-(4-(dimethylamino)-3-(pyren-1-yl)prop-2-en-1-one) ( DAPP-1 ) and (E)-3-(4-(diemthylamino)phenyl)-1-(pyren-1-yl)prop-2-en-1-one ( DAPP-2 ) were synthesized to access lossless broadband nonlinear refraction. Femtoseconds Z-scan from 600 nm to 1030 nm showed positive third-order nonlinear refractive index with high linear transmission (>90%). Both compounds had ≥2.5 figure of merit (F) at 1030 nm, while DAPP-2 held >1 F value in a wide spectral region. Mechanism of ultrafast refraction under off & on-resonant regime were investigated via degenerate & non-degenerate pump-probe with phase object, respectively. Results indicate pure response of bound electrons under off-resonance and excited-state enhanced nonlinear refraction under on-resonance. All the study suggested that optical nonlinearity in pyrene derivatives has been underestimated and molecules like DAPP-2 possess the potential to be further optimized into superior broadband nonlinear refractive materials. [ABSTRACT FROM AUTHOR]

Abstract: The third-order nonlinear refractive index of carbon disulfide (CS2) with different thicknesses is measured by a nonlinear-imaging technique with a phase object (NIT-PO). The propagating characteristics of the laser beam within thick medium are analyzed by numerically solving the electromagnetic wave equation. By theoretically calculating, we study the relationship of the phase shift induced by nonlinear refraction versus medium thicknesses. Fitting the output image with the help of the effective interaction length, the nonlinear refractive index n 2 of CS2 can be determined, which suggests a one-single-shot method for the investigation of optical nonlinearity in thick medium. [Copyright &y& Elsevier]

Abstract: Signs change of nonlinear refractive index in ZnSe is observed by employing a modified double 4f imaging system at the wavelength of 800nm using picosecond pulses with different pulse energies. This process results from the competition of the bound electronic nonlinear refraction and the free carrier refraction. At low intensity, positive nonlinear refraction is obtained, which is attributed to bound electrons. As the increase of laser beam intensity, the nonlinear refractive index become small, and changes to negative. This is ascribed to free carriers generated by two-photon absorption. Additionally, the nonlinear refractive index of bound electron and the refractive index change of free carrier are determined unambiguously by a simple method. [Copyright &y& Elsevier]