Weak Two-photon Absorption Research Articles

Impact of photoinduced energy transfer and LSPR of Au and Ag nanoparticles on nonlinear optical response of methyl orange

In order to realize effective optical limiting (OL), it is necessary to utilize materials that exhibit distinct traits, such as a large and rapid nonlinear optical (NLO) response, high photothermal stability, and cost-effectiveness. In this work, the NLO properties of methyl orange (MO), a popular pH indicator, a potential nonlinear material, and an azo dye of great industrial importance are modulated by forming nanohybrids with Ag and Au nanoparticles. The NLO properties of the samples were investigated via the Z-scan technique using 7 ns laser pulses at a wavelength of 532 nm. A huge enhancement is observed in the nonlinear absorption (NLA) and OL performance MO-Ag and MO-Au nanohybrid compared to its constituent compounds, and it is attributable to photoinduced energy transfer between MO and metal NPs as well as the local field effect of NPs. The mechanisms responsible for the NLA and OL activity are primarily Excited State Absorption (ESA) with contributions from a weak Two-Photon Absorption (TPA). Notably, the nanohybrid system incorporating Au NPs outperforms the one with Ag NPs in terms of nonlinear optical properties under 532 nm excitation, primarily due to the better resonant energy transfer of Au compared to the Ag nanoparticle and intense local field effects of Au on exciting at 532 nm due to resonant absorption. The Closed Aperture (CA) Z-scan measurements were employed to investigate the nonlinear refraction property of the compounds, and both the pristine MO and its nanohybrid exhibited negative nonlinear refraction. These findings indicate that forming nanohybrids is an efficient way to modify the NLO response of MO. It also suggests MO has the potential for a diverse array of nonlinear optical applications, including but not limited to optical limiting.

Impact of two-photon absorption on two-dimensional solitons in silicon waveguides with a nonstationary potential*

We study the effects of weak two-photon absorption on two-dimensional (2D) solitons interaction in a silicon-based waveguide under a framework of coupled perturbed D nonlinear Schrödinger equations with a pure Kerr nonlinearity and a nonstationary potential. We extend the perturbative analysis for collisions of two one-dimensional solitons in silicon waveguides to derive the theoretical expressions for the amplitude evolution of a single perturbed soliton and for the abrupt amplitude downshift of a colliding soliton due to a fast interaction between two 2D solitons in silicon lattices. The theoretical calculations are then validated by the extensive simulations of the coupled perturbed D nonlinear Schrödinger model with periodic and non-periodic potentials. The simulations are implemented by using the accelerated imaginary-time evolution scheme and the split-step Fourier method. These results allow us to accurately capture the abrupt energy downshift of a colliding soliton in silicon-based two-dimensional waveguides.

Toward a Molecular Reorganization Energy-Based Analysis of Third-Order Nonlinear Optical Properties of Polymethine Dyes and J-Aggregates.

This work demonstrates the feasibility and applicability of the theory of extended multiphonon electron transitions for the description of nonlinear optical properties of polymethine dyes using quantum chemistry and model calculations. The transformation of a strong one-photon absorption band in dye monomers to a weak two-photon absorption band is rationalized from the electron-nuclear resonance condition. The power law fitting of the results of quantum chemical computations of nonlinear optical properties allows the predicting of the shift of the corresponding Egorov-like resonance curve to the shortest dye in the vinylogous series of dye monomers. The results presented provide an insight and guide for the rational molecular design and application of polymethine dyes.

Edge and bulk dissipative solitons in modulated PT-symmetric waveguide arrays.

We address dissipative soliton formation in modulated parity-time (PT)-symmetric continuous waveguide arrays composed from waveguides with amplifying and absorbing sections, whose density gradually increases (due to decreasing waveguide separation) either towards the center of the array or towards its edges. In such a structure, the level of gain/loss at which PT-symmetry gets broken depends on the direction of increase of waveguide density. Breakup of PT-symmetry occurs when eigenvalues of modes localized in the region where waveguide density is largest collide and move into a complex plane. In this regime of broken symmetry, the inclusion of focusing Kerr-type nonlinearity of the material and weak two-photon absorption allows to arrest the growth of amplitude of amplified modes and may lead to the appearance of stable attractors either in the center or at the edge of the waveguide array, depending on the type of array modulation. Such solitons can be stable; they acquire specific triangular shapes and notably broaden with increase of gain/loss level. Our results illustrate how spatial array modulation that breaks PT-symmetry "locally" can be used to control the specific location of dissipative solitons forming in the array.

Luminescence and nonlinear optical properties of colloidal Ag2S quantum dots

The relationship of the spectral luminescent and nonlinear optical properties of Ag2S quantum dots (QDs), passivated with thioglycolic acid (Ag2S QDs/TGA) and Ag2S QDs, stabilized by gelatin (Ag2S QDs/Gel) with average sizes from 1.5 nm to 2.1 nm is found. Colloidal Ag2S QDs/TGA with different sizes, having the exciton (620 nm) and recombination (660 nm, 880 nm and 1205 nm) luminescence were studied using the methods of luminescent spectroscopy, luminescence decay, and Z-scans technique at the wavelengths of 532 nm using 10 ns pulses. Resonant two-photon transitions of reverse saturated absorption (RSA) were found for samples with recombination luminescence. In addition, the nonlinear refraction of thermal nature was observed using Z-scan method. Predominantly weak two-photon absorption (TPA) and nonlinear refraction (NR) of non-thermal nature occur for Ag2S/TGA quantum dots with exciton luminescence. The luminescence lifetimes support this conclusion. The differences in luminescence lifetimes of samples are associated with different state of interfaces for colloidal Ag2S QDs/TGA with recombination and exciton luminescence. This conclusion was confirmed by FTIR spectroscopy of colloidal Ag2S QDs/TGA solutions, synthesized under different conditions. RSA is characteristic of samples with a noticeable concentration of trap states acting as recombination luminescence centers (660 nm). The maximum effect was noted for Ag2S QDs with recombination luminescence in the region of 660 nm, 880 nm, 1205 nm. The fast component of this luminescence decay has a time constant of about 0.8 ns. It decay 5 times for the first 10 ns. In the case of Ag2S QDs/TGA with exciton luminescence (620 nm) there are no appropriate conditions for RSA. The fast component of luminescence decay has a time constant of 0.8 ns. The luminescence intensity decreases 50 times for the first 10 ns. In this case, TPA and nonlinear refraction of non-thermal nature take place, and RSA is absent.

Weak two-photon absorption applied to the rapid prototyping of cell scaffolds

Two-photon polymerization (TPP) has been employed to generate deep structures using the biocompatible and optically transparent monomer ethoxylated bisphenol A dimethacrylate (EO=6) (EBPADMA) and 4, 4'-Bis(diethylamino)benzophenon as the photoinitiator. The two-photon absorption cross section of the initiator was measured to be 1 GM (1 GM=1×10(-50) cm(4) s photon(-1)) in EBPADMA. Here we have explored a weak absorption regime whereby deep structures (∼300 μm) can be generated in a single pass. This allows rapid fabrication of structures suitable for cell scaffolds where the length scales are small, ∼10 μm, but are required over long ranges, ∼cm. The dependence of the TPP properties on the writing power, speed, exposure time and NA, of the focusing lens were studied in detail. Diffraction calculations for the focusing optics employed show that spherical aberration plays a significant role in determining the feature sizes achieved.

Synthesis, chemical–optical characterization and solvent interaction effect of novel fluorene-chromophores with D–A–D structure

Abstract The synthesis, chemical characterization and optical studies of three novel fluorene derivatives is reported. These compounds comprise a D–A–D architecture with fluorene moieties as donor groups and fluorenone or benzothiadiazole derivatives as acceptor groups. Theoretical analysis confirmed the existence and the nature of two principal electronic transitions (π → π* and intramolecular charge transfer). Spectroscopic studies in solution revealed that the intramolecular charge transfer character, and in turn the two-photon activity i.e., the fluorescence induced by two-photon absorption, is strongly affected by solvent polarity. The influence of specific solvent–solute interactions over emission properties was also studied through Lippert–Mataga plot. Evaluation of the two-photon absorption cross-sections, gave a maximum value of 105 GM (1 GM = 10−50 cm4 s) in toluene and a minimum value of 23 GM in THF solutions at 750 nm for the fluorenone derivative, a molecule with poor intramolecular charge transfer character and thus weak two-photon absorption; however the benzothiadiazole derivative, with stronger intramolecular charge transfer transition, produced a maximum value of 1000 GM in THF and a minimum value of 236 GM in methanol. Fluorescence quantum efficiency of these compounds was also affected by the medium, with fluorescence quenching in protic solvents such as methanol due to specific solvent interactions (i.e., hydrogen-bonding). Nevertheless, in a more polar medium such as water, nanoaggregates synthesized from the benzothiadiazole derivative exhibited good two-photon activity, i.e., ∼500 GM and fluorescence quantum efficiency of 0.83. Furthermore, these nanoaggregates exhibited more resistance against photodegradation processes than any of the organic solutions tested.

Interference-free two-photon LIF imaging of atomic hydrogen in flames using picosecond excitation

Interference-free, two-photon-excited planar laser-induced fluorescence (PLIF) imaging of atomic hydrogen is demonstrated in steady premixed methane flames. PLIF measurements of atomic hydrogen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the laser-induced fluorescence (LIF) signal on laser intensity. Previous two-photon LIF measurements of atomic hydrogen in hydrocarbon flames using nanosecond laser excitation have identified complications from photolytic production of hydrogen atoms by the excitation laser. Recent results from line-imaging studies in our laboratory indicate a significant advantage to using picosecond excitation for imaging atomic hydrogen with negligible photolytic interference. In the current study, we extend our capabilities for interference-free PLIF imaging of atomic hydrogen in steady premixed CH4/O2/N2 flames. Peak single-shot signal-to-noise ratios of approximately 6–8 are achieved, and the estimated single-shot detection limit is on the order of 1016cm-3. Avoidance of interference and stimulated emission and the effects of fluorescence quenching are discussed. Averaged composite PLIF images are generated by combining images from multiple axial locations in the flame. The images show enhanced number densities of atomic hydrogen near the flame tip, in accordance with numerical predictions of diffusional focusing of H-atoms resulting from the sharp curvature of the flame front.

Complex nonlinear refractive index of DNA

Femtosecond Z-scan technique was used to determine the dispersion of the complex cubic nonlinear susceptibility (related to the nonlinear refractive index n 2 and the two-photon absorption coefficient β 2) of deoxyribonucleic acid (DNA) in the range of wavelengths from 530–1300 nm. The nonlinear refractive index of DNA varies from ≈ 2 × 10 −15 to 1 × 10 −14 cm 2/W at λ = 1300–530 nm, respectively. Only a weak two-photon absorption is observed below λ = 600 nm with a nonlinear absorption coefficient β 2 of about 0.2 cm/GW at λ = 530 nm. The dispersion is approximated with a simple complex expression.

Two-photon LIF imaging of atomic oxygen in flames with picosecond excitation

Interference-free planar laser-induced fluorescence measurements of atomic oxygen are demonstrated in steady and unsteady laminar premixed methane flames. Two-dimensional LIF measurements of atomic oxygen present a challenge because of the relatively weak two-photon absorption cross-sections and the nonlinear dependence of the LIF signal on laser intensity. Previous two-photon LIF measurements of atomic oxygen in hydrocarbon flames using nanosecond laser excitation were complicated by significant interference from photolytically produced oxygen atoms. Recent results from line-imaging studies in our laboratory indicate that CO 2 is the dominant photolytic precursor. Here, we use picosecond excitation to image atomic oxygen with negligible photolytic interference. We demonstrate this capability in premixed CH 4/O 2/N 2 ( ϕ = 1.08 , X N 2 = 0.63 ) flames with an adiabatic flame temperature of 2480 K and an estimated peak O-atom mole fraction of 0.5%. Peak single-shot signal-to-noise ratios of 9 are achieved, and the estimated detection limit is approximately 80 ppm. Quantitative O-atom LIF measurements also require knowledge of the temperature-dependent quenching cross-sections, which are not currently available. The effects of collisional quenching on the O-atom LIF are predicted using two models for the temperature dependence of quenching cross-sections, and the results indicate that O-atom LIF signals can provide accurate measurements of O-atom profiles in flames. Measurements in an acoustically forced Bunsen flame illustrate the feasibility of using O-atom LIF to investigate flow–flame interactions.

Sub-shot-noise high-sensitivity spectroscopy with optical parametric oscillator twin beams

Nondegenerate optical parametric oscillators generate above-threshold signal and idler beams that have intensity fluctuations correlated at the quantum level (twin beams). We describe what is to our knowledge the first high-sensitivity spectroscopy experiment using twin beams emitted by a cw optical parametric oscillator: a very weak two-photon absorption signal, in the 10(-7) range, is recorded on the 4S(1/2)-5S(1/2) transition of atomic potassium with a noise background that is reduced by 1.9 dB with respect to the shot-noise limit of the light used in the experiment.

Exact stationary photon distributions due to competition between one- and two-photon absorption and emission

We consider steady states of the one-mode quantized field interacting with two independent baths, each characterized by the one- and two-photon absorption and emission processes. In the absense of two-photon emission, using an exact analytical solution to the master equation for the diagonal elements of the density matrix in the Fock basis in terms of the confluent hypergeometric function, we obtain simple explicit expressions for the photon distribution function and for the factorial moments in the limiting cases of weak and strong two-photon absorption. If the two-photon absorption is strong enough, the steady state exhibits a sub-Poissonian photon statistics characterizing nonclassical behaviour, but Mandel's -parameter cannot be less . However, the distribution depends essentially on the temperature of the `one-photon bath'. For weak two-photon absorption, the stationary distribution is Gaussian, provided that the temperature of the `one-photon' bath is high enough. For an inversely populated `one-photon' bath, the -parameter is close to . In a generic case of nonzero two-photon emission probability, approximate asymptotic expressions for the factorial moments are found.

Is two-photon absorption a limitation to dark soliton switching?

It is shown that two-photon absorption does not always place a fundamental limitation on soliton devices: The steering angle, the key characteristic of switching devices based on the propagation of dark spatial solitons, is almost preserved as the intensity of the background wave decays in the presence of weak two-photon absorption.

Effect of two-photon absorption on bright spatial soliton switches

The effect of weak two-photon absorption on bright spatial soliton beams undergoing reflection, transmission, self-trapping, or breakup at interfaces separating nonlinear dielectrics is studied by using soliton perturbation techniques.

Coulomb correlations and two-photon spectra of conjugated polymers

Electron—electron correlations in conjugated polymers are shown to produce weak two-photon absorption (TPA) below the one-photon gap E g and intense TPA above E g. TPA intensities vary strongly with the alternation δ of transfer integrals t(1 ± δ) and with U/t in Hubbard models. Pariser—Parr—Pople (PPP) models reconcile TPA spectra of conjugated molecules and polymers.

Solitons and two-photon absorption

The effect of weak two-photon absorption on soliton propagation is investigated. It is found that fundamental solitons evolve adiabatically; expressions describing their broadening and attenuation are given. Higher-order solitons break up into two diverging fundamental solitons for a wide range of parameters.

Two-photon absorption spectra and cross-section measurements in liquids

THE recently developed high-sensitivity opto–acoustic (OA) absorption measurement technique1 has proved ideal for the measurement of a variety of weak absorption phenomena in liquids2–4. We report here the results of an investigation of two-photon absorption in liquid benzene using the OA technique. Weak two-photon spectra (cross-section ≲10−51 cm4 molecule−1 photon−1 s) in nonfluorescing liquids have been difficult to measure. The only previous published study for such a measurement used thermal blooming5, which is difficult to interpret theoretically and apparently produces a poor signal-to-noise ratio. We demonstrate here that far superior spectra can be obtained by the simple and sensitive two-photon OA spectroscopy (TPOAS) technique. Polarisation dependence, vibrational intervals and electronic origin are obtained, and two-photon absorption cross-sections are derived for the first time in this study for the case of a weak two-photon absorption band in liquid benzene. Note that Bonch-Bruevich et al.6 have recently attempted similar two-photon OA measurements, but they can only measure strong absorption (cross-section ≳10−48 cm4 molecule−1 s) with a poor signal-to-noise ratio.