Intensity-dependent Refractive Index Research Articles

Intensity modulated optical transmission in a non-linear dielectric environment with an embedded mono-layer transition metal dichalcogenide

We study the optical behavior of an arrangement in which the interface between a linear and non-linear dielectric media is covered by an embedded mono-layer of transition metal dichalcogenides (TMDC). The optical behavior is qualitatively obtained through transmission and reflection coefficients which are a function of the third order non-linear susceptibility of the Kerr-type dielectric and the inter-band optical conductivity of the TMDC mono-layer. The inter-band optical conductivity of the TMDC mono-layer is calculated using the Kubo formalism from the linear response theory. In particular, we theoretically demonstrate that the optical response of this structure can be switched between the total internal reflection and a normal transmission regime by controlling the intensity of the incident radiation. The reflection and transmission functions are shown to be amenable to further control by altering the inter-band optical conductivity of the embedded TMDC mono-layer. The optical conductivity is directly related to its energy dispersion. We specifically choose two TMDC mono-layers, MoS2 and WSe2, which have nearly identical dispersion parameters apart from a much stronger spin-orbit coupling in the latter. The stronger spin-orbit coupling in WSe2 does not significantly alter the inter-band optical conductivity to manifest as an enhanced reflection spectrum. However, we find that application of an external perturbation such as strain could be effectively used to modulate the overall optical response. We conclude by discussing briefly the phenomenon of optical bistability which arises in materials exhibiting optical non-linearity via an intensity-dependent refractive index.

Crystal growth and DFT insight on sodium para-nitrophenolate para-nitrophenol dihydrate single crystal for NLO applications

Single crystals of sodium para-nitrophenolate para-nitrophenol dihydrate (SPPD) were grown by slow evaporation technique and its structure has been studied by FT-IR, FT-Raman and single crystal X-ray diffraction techniques. The optical and electrical properties were characterized by UV-Vis spectrum, and dielectric studies respectively. SPPD was thermally stable up to 128 °C as determined by TG-DTA curves. Using the Kurtz-Perry powder method, the second-harmonic generation efficiency was found to be five times to that of KDP. Third-order nonlinear response was studied using Z-scan technique with a He-Ne laser (632.8 nm) and NLO parameters such as intensity dependent refractive index, nonlinear absorption coefficient and third-order susceptibility were also estimated. The molecular geometry from X-ray experiment in the ground state has been compared using density functional theory (DFT) with appropriate basis set. The first-order hyperpolarizability also calculated using DFT approaches. Stability of the molecule arising from hyperconjugative interactions leading to its nonlinear optical activity and charge delocalization were analyzed using natural bond orbital technique. HOMO-LUMO energy gap value suggests the possibility of charge transfer within the molecule. Based on optimized ground state geometries, Natural bond orbital (NBO) analysis was performed to study donor-acceptor interactions.

Spectral broadening induced by intense ultra-short pulse in 4H–SiC crystals**Project supported by the National High Technology Research and Development Program of China (Grant No. 2014AA041402) and the National Natural Science Foundation of China (Grant Nos. 51272276 and 51322211).

We report the observation of spectral broadening induced by 200 femtosecond laser pulses with the repetition rate of 1 kHz at the wavelength of 532 nm in semi-insulating 4H–SiC single crystals. It is demonstrated that the full width at half maximum of output spectrum increases linearly with the light propagation length and the peak power density, reaching a maximum 870 cm−1 on a crystal of 19 mm long under an incident laser with a peak power density of 60.1 GW/cm2. Such spectral broadening can be well explained by the self-phase modulation model which correlates time-dependent phase change of pulses to intensity-dependent refractive index. The nonlinear refractive index n2 is estimated to be 1.88×10−15 cm2/W. The intensity-dependent refractive index is probably due to both the nonlinear optical polarizability of the bound electrons and the increase of free electrons induced by the two-photon absorption process. Super continuum spectra could arise as crystals are long enough to induce the self-focusing effect. The results show that SiC crystals may find applications in spectral broadening of high power lasers.

Synthesis and characterization of organic nonlinear optical material: urea para-nitrophenol

The organic nonlinear single crystals of urea para-nitrophenol (UPN) have been grown using slow evaporation solution growth technique. The formation of the organic product has been confirmed by CHN analysis. The structure of grown crystal was examined by single-crystal X-ray diffraction technique. The presences of functional groups were identified from FT-IR spectrum. The large transmittance properties of the crystal and its linear band gap energy of 3.18 eV inform that the material is suitable for optoelectronic applications. The TG–DTA curve confirms that the grown crystal was thermally stable up to 120 °C. Dielectric constant and dielectric loss values were also measured as a function of frequency between 5 Hz and 5 MHz and temperature from 40 to 100 °C. The second-harmonic generation efficiency of the UPN crystal is found to be 3.5 times greater than that of KDP crystal. The NLO parameters intensity dependent refractive index (n2), nonlinear absorption coefficient (β) and third-order susceptibility |χ3| were calculated using Z-scan techniques. The morphology of its UPN crystal was studied using SEM analysis.

Nonlinear characterization of conducting polymer and electrical study for application as solar cells and its antibacterial activity

A conducting polymer, poly(o-methoxyaniline) (POMA), has been synthesized by the chemical polymerization and characterized by FT-IR, AFM, Raman, and an UV–vis spectrometer. Measurements of the thermally induced optical nonlinearity of POMA thin film and orcein dye in a N-methyl-2-pyrrolidone (NMP) solvent were studied using a cw diode laser at 532nm as the source of excitation, both in solution and as a POMA solid film, respectively. The optical response was characterized by measuring the intensity-dependent refractive index n2 of the medium using the Z-scan technique. The optical properties of the thin films prepared were studied including recording the absorption spectroscopy for wavelengths of (300–900nm). The absorption coefficient and the forbidden energy gap to transmission directly were measured as well as the refraction index, n, extinction coefficient, k, dielectric constant ɛ. The optical constant represented by single oscillator energy, E0, dispersion energy, Ed, moment of dielectric constant (M−1, M−3,), nonlinear optical susceptibility, χ3, were determined. For the electronic application of prepared polymers POMA a solar cells were constructed by using n-type silicon wafers and different electrode from gold and aluminium. The electrical properties of the solar cell under the effect of the light equivalent to that of the sun and which falls vertically on the sample were studied. It is practically found that, the conversion efficiency is 0.4%. A better antibacterial activity was observed against Staphylococcus aureus (SA), Escherichia coli (EC), Streptococcus pyogenes, Klebsiella pneumoniae, and Proteus mirabilis by conducting polymer POMA thin film.

Third-Order Optical Nonlinearities in Organometallic Methylammonium Lead Iodide Perovskite Thin Films

With solar conversion efficiencies surpassing 20%, organometallic perovskites show tremendous promise for solar cell technology. Their high brightness has also led to demonstrations of lasing and power-efficient electroluminescence. Here we show that thin films of methylammonium lead iodide, prepared by solution processing at temperatures not exceeding 100 °C, exhibit a highly nonlinear intensity-dependent refractive index due to changes in the free-carrier concentration and for femtosecond excitation at higher intensities undergo saturation that can be attributed to the Pauli blocking effect. Nonlinear refractive index and nonlinear absorption coefficients were obtained by the Z-scan technique, performed simultaneously in open- and closed-aperture configurations. Both nanosecond- and femtosecond-pulsed lasers at multiple wavelengths were used in order to distinguish between the mechanisms inducing the nonlinearities. The magnitude and sign of the nonlinear refractive index n2 were determined. For resonan...

Vibrational corrections to the second hyperpolarizabilities of Al(n)P(n) clusters.

In this work, we report results of vibrational corrections to the second hyperpolarizabilities of Al2P2, Al3P3, Al4P4, Al6P6, and Al9P9 clusters. The vibrational corrections were calculated through the perturbation theoretic method of Bishop and Kirtman and also using a variational methodology at the second order Møller-Plesset perturbation theory level with the aug-cc-pVDZ basis set. Results show that the vibrational corrections are important, accounting for more than half of the corresponding electronic second hyperpolarizabilities at the static limit. Comparisons between results obtained through both methods show very good agreements for the terms [α(2)] and [μβ] but significant differences for the term [μ(2)α]. Dynamic vibrational corrections to the second hyperpolarizabilities related to the dc-second harmonic generation, intensity dependent refractive index, and dc-Kerr nonlinear optical processes are also reported.

Optical bistability with film-coupled metasurfaces.

Metasurfaces comprising arrays of film-coupled, nanopatch antennas are a promising platform for low-energy, all-optical switches. The large field enhancements that can be achieved in the dielectric spacer region between the nanopatch and the metallic substrate can substantially enhance optical nonlinear processes. Here we consider a dielectric material that exhibits an optical Kerr effect as the spacer layer and numerically calculate the optical bistability of a metasurface using the finite element method (FEM). We expect the proposed method to be highly accurate compared with other numerical approaches, such as those based on graphical post-processing techniques, because it self-consistently solves for both the spatial field distribution and the intensity-dependent refractive index distribution of the spacer layer. This method offers an alternative approach to finite-difference time-domain (FDTD) modeling. We use this numerical tool to design a metasurface optical switch and our optimized design exhibits exceptionally low switching intensity of 33 kW/cm2, corresponding to switching energy on the order of tens of attojoules per resonator, a value much smaller than those found for most devices reported in the literature. We propose our method as a tool for designing all-optical switches and modulators.

Nonlinear optical properties and optical power limiting behavior of Leishman dye in solution and solid polymer film using z-scan

We have studied the nonlinear optical properties of Leishman dye in chloroform solution at different concentrations and solid film mixed with polymethylmethacrylate (PMMA) polymer using a continuous wave (CW) laser at 532nm as the excitation source. We have employed the z-scan technique to evaluate the sign and magnitude of the intensity dependent nonlinear refractive index (n2), the nonlinear absorption coefficient (β), and the third-order nonlinear optical susceptibility (χ(3)). The obtained results showed that the Leishman dye exhibits a negative nonlinear refraction (self-defocusing) and saturable absorption. It was found that the values of n2, β, and χ(3) are dependent on the dye concentration, they increases when the dye concentration increases. Optical power limiting characteristics of the dye at different concentrations in solution and solid film were also studied. Our results showed that the Leishman dye has large third-order optical nonlinearity, thus it could be a potential candidate for opto-electronic and photonic devices.

Thermal properties of a new dye compound measured by thermal lens effect and Z-scan technique

A new dye compound containing azomethine groups has been synthesized and characterized by FT-IR, 13C NMR, and an UV–visible spectrometer. Measurements of the thermally induced optical nonlinearity of dichloro bis[2-(2-hydroxybenzylideneamino)-5-methylphenyl] telluride platinum(II) in a chloroform solvent were studied using a cw diode laser at 487 nm as the source of excitation, both in solution and as a poly methyl methacrylate solid film, respectively. The optical response was characterized by measuring the intensity-dependent refractive index n2 of the medium using the Z-scan technique. The sample showed negative and large nonlinear refractive index values of the order of 10−7 cm2/W and reverse saturable absorption with high values of the nonlinear absorption coefficient of the order of 10−3 cm/W. The nonlinear refractive index was found to vary with the concentration. These results indicate that the dye is a promising candidate for applications in the nonlinear optic field. Thermal lens spectrometry was applied to investigate the thermo-optical properties and the nonlinear refractive index. In this technique, a pump and a probe beam were aligned collinearly. A localized change in the refractive index of the sample due to the thermal heating produced a thermal lens, which was then detected by studying the focusing and defocusing of the pump and probe beam.

Investigation of giant Kerr nonlinearity in quantum cascade lasers using mid-infrared femtosecond pulses

We study the Kerr nonlinearity of quantum cascade lasers (QCLs) by coupling resonant and off-resonant mid-infrared (mid-IR) femtosecond (fs) pulses into an active QCL waveguide. We observe an increase in the spectral width of the transmitted fs pulses as the coupled mid-infrared (mid-IR) pulse power increases. This is explained by the self-phase modulation effect due to the large Kerr nonlinearity of QCL waveguides. We further confirm this effect by observing the intensity dependent far-field profile of the transmitted mid-IR pulses, showing the pulses undergo self-focusing as they propagate through the active QCL due to the intensity dependent refractive index. We experimentally estimate the nonlinear refractive index n2 of a QCL to be ∼8 × 10−9 cm2/W using the far-field beam profile of the transmitted pulses. The finite-difference time-domain simulations of QCL waveguides with Kerr nonlinearity incorporated show similar behavior to the experimental results.

Analysis of optical instability in coupled microring resonators

We present energy coupling (EC) and power coupling (PC) formalisms for analyzing optical instability in coupled microring resonators having instantaneous intensity dependent nonlinear refractive index. Analysis of a chalcogenide double-microring resonator is performed to investigate and compare instability phenomena predicted by both formalisms. It is shown that the EC formalism fails to predict Ikeda instability in the double-microring resonator and generally yields results that drastically deviate from those of the more rigorous PC formalism at high input powers and large phase detunings. We also show that the input threshold powers for reaching self-pulsation and Ikeda instabilities in a double-microring resonator can be minimized by proper selection of the coupling parameters. In particular, self-pulsation can be reached in a chalcogenide double-microring with input powers as low as tens of milliwatts, while the threshold power for Ikeda instability can be reduced by more than 20% compared to the value required in a single microring.

Single beam Z-scan measurement of the third order optical nonlinearities of styryl7 dye

The optical nonlinearity of styryl7 dye in ethanol solution at different concentrations has been studied using pulsed Nd:YAG laser at 532nm as the source of excitation. The optical responses were characterized by measuring the intensity dependent refractive index (n2) of the medium using the Z-scan technique. The open aperture Z-scan trace of the dye in solution displayed saturable absorption. The closed aperture Z-scan trace of the dye exhibited a negative nonlinearity. The styryl7 dye at 1mM concentration exhibited nonlinear refractive co-efficient n2=−1.24×10−8cm2/W, nonlinear absorption coefficient β=−3.9×10−4cm/W and real and imaginary parts of third-order nonlinear optical susceptibility χ3=3.26×10−6esu in ethanol. These results showed that the dye has potential application in nonlinear optics.

Z-scan and optical limiting properties of Hibiscus Sabdariffa dye

The intensity-dependent refractive index n2 and the nonlinear susceptibility χ(3) of Hibiscus Sabdariffa dye solutions in the nanosecond regime at 532 nm are reported. More presicely, the variation of n2, β, and real and imaginary parts of χ(3) versus the natural dye extract concentration has been carried out by z-scan and optical limiting techniques. The third-order nonlinearity of the Hibiscus Sabdariffa dye solutions was found to be dominated by nonlinear refraction, which leads to strong optical limiting of laser.

Errata: Effect of concentration on the plasmonic absorption and optical nonlinearity of gold nanoparticles

The effect of concentration on the optical nonlinearity of gold nanoparticles is investigated. Gold nanoparticles were synthesized by nanosecond-pulsed laser ablation of a high-purity gold plate in distilled water. The size of nanoparticles was measured using dynamic light scattering (DLS) method and was calculated from full width half maximum of their plasmonic absorption peak using Mie theory. Different nonlinear characteristics of gold nanoparticles are observed by considering their effects under irradiation with a second harmonic beam of a continuous-wave low power Nd-YAG laser at 532 nm. Low-power optical limiting with low limiting threshold is obtained in the sample using different apertures at different points of the laser beam path. The nonlinear optical responses are characterized by measuring the intensity dependent refractive index of medium using the z-scan technique. Results show that the linear absorption coefficient and nonlinear refractive index of samples are increased with increasing the concentration of gold nanoparticles in distilled water. Increasing the intensity of the probe beam leads to increasing the magnitude of nonlinear refractive index.

The third-order nonlinear optical susceptibility of gold

We critically analyze reported measured values of the third-order nonlinear optical susceptibility χ(3) of bulk gold. Reported values of this quantity span a range of more than three orders of magnitude. Much of this variation results from the use of different measurement procedures which are sensitive to different contributions to the nonlinear optical response. For example, values measured through use of third-harmonic generation or non-degenerate four-wave mixing tend to be significantly lower than those obtained from measurements of the intensity-dependent refractive index. We ascribe this behavior to the fact that the first two processes respond only to “instantaneous” nonlinearities, whereas the nonlinear refractive index has a contribution from the much stronger but much slower “hot electron,” or “Fermi-smearing” mechanism, which has a response time of the order of picoseconds. The data also reveal that the hot-electron contribution has a strong dependence on laser wavelength, because of the turn-on of the 5d to 6sp transition at about 550nm. It is hoped that the compilation presented here will prove useful in establishing what value of χ(3) is most appropriate for adoption under various laboratory conditions.

Interactions in dye-microcavity photon condensates and the prospects for their observation

We derive the equation of motion for a Bose-Einstein condensate of photons in a dye-microcavity system, starting from Maxwell's equations. Our theory takes into account mirror shape, Kerr-type intensity-dependent refractive index and incoherent pumping and loss. The resulting equation is remarkably similar to the Gross-Pitaevskii equation for exciton-polariton condensates, despite the different microscopic origins. We calculate the incoherent photoluminescence spectrum of the photon condensate which shows the Bogoliubov-type excitations around the mean-field at thermal equilibrium. Both open and closed-system models are presented to account for, respectively dissipation and inhomogeneities. Considering realistic parameters and experimental resolution, we estimate that by observing the angle-resolved spectrum of incoherent photoluminescence it is possible to resolve dimensionless interaction parameters of order $10^{-5}$, two orders of magnitude below current estimates. Thus we expect that this technique will lead to accurate measurements of the interactions in photon condensates.

Measurement of the figure of merit of indigenously developed Nd-doped phosphate laser glass rods for use in high power lasers

High energy, high power (HEHP) Nd:glass laser systems are used for inertial confinement fusion and equation of state (EOS) studies of materials at high temperature and pressure. A program has been undertaken for the indigenous development of Nd-doped phosphate laser glass rods and discs for HEHP lasers. In this paper, we report the characterization of the Nd-doped phosphate laser glass rods produced under this program and compare the indigenously developed laser glass to LHG-8 laser glass of M/s Hoya, Japan. We experimentally measured the values of the stimulated emission cross-section (σ) and coefficient of intensity-dependent refractive index (n 2) and hence the figure of merit F = σ / n2 of the indigenous phosphate laser glass rods. This value is found comparable to the reported value of identically doped Nd:glass rods.

Ab initio study of nonlinear optical properties of aromatic fused rings

The nonlinear optical properties of benzene, naphthalene, anthracene, and pyrene are investigated. Static and dynamic polarizability, α(−ω;ω), and second hyperpolarizability, corresponding to dc electric-field-induced Kerr effect (DCKE), intensity dependent refractive index (IDRI), dc electric field-induced second harmonic generation (EFISHG), and third harmonic generation (THG) have been calculated. The calculated α and γ show good agreement with the available theoretical and gas-phase experimental data. The second hyperpolarizabilities exhibit the following trend: γ(THG)>γ(EFISHG)>γ(IDRI)∼γ(DCKE)>γ(static). We find that the second hyperpolarizabilities increase as the number of rings in the system increases in either dimension, but the magnitude of increase is more pronounced along the molecular chain axis.

Dispersion properties of slab waveguides with double negative material guiding layer and nonlinear substrate

The dispersion properties of transverse electric nonlinear waves in a three-layer slab waveguide which consists of a double negative material (DNM) guiding layer sandwiched between an intensity-dependent refractive index substrate and semi-infinite linear dielectric cover are investigated. The dispersion properties for self-focusing and self-defocusing substrate nonlinearity are presented. The effects of the negative parameters of the DNM on the dispersion characteristics are investigated.