Reverse Saturable Absorption Process Research Articles

Parametrically resonant surface plasmon polaritons

We present the theory of parametrically resonant surface plasmon polaritons (SPPs). We show that a temporal modulation of the dielectric properties of the medium adjacent to a metallic surface can lead to efficient energy injection into the SPP modes supported at the interface. When the permittivity modulation is induced by a pump field exceeding a certain threshold intensity, such a field undergoes a reverse saturable absorption process. We introduce a time-domain formalism to account for pump saturation and depletion effects. Finally, we discuss the viability of these effects for optical limiting applications.

Twisted intramolecular motion arrested in aggregated state emission and the nonlinear optical properties of pyrene pyrazoline derivatives

Heterocyclic pyrene pyrazoline moieties containing similar structures but with differences in thiophene (PPT), furan (PPF) and pyridine (PPP) substitutions at the terminal molecules were synthesized. Their aggregation behaviour in THF-water mixtures was investigated and results demonstrated that PPT and PPP exhibited aggregation-induced emission (AIE), whereas PPF exhibited aggregation-induced blue-shifted emission (AIBSE). PPT and PPP provided red-shifted emission, while PPF had observed blue-shifted emission at high water fractions of 70-90%, confirming that aggregation effects played a major role in the molecular structure. Two emission peaks from locally excited and twisted intramolecular charge transfer confirmed the twisted nature from the dihedral angle values of the free reorganized molecules that were completely restricted in high water fractions due to molecular aggregation. This was further confirmed from colour Commission Internationale de l'Eclairage values as well as dynamic light scattering analysis. Third-order nonlinear optical properties were studied using a Nd:Yag laser beam Z-scan technique at 532 nm. The open aperture Z-scan revealed that PPT and PPF towards the peak point endured strong saturable absorption, whereas PPP indicated a strong reverse saturable absorption process. The AIE and AIBSE mechanisms from undergoing restricted twisting intramolecular motion in the aggregated luminogens provide great insight into new developments in AIEgen materials for these optoelectronic materials.

Structural, photophysical and nonlinear optical limiting properties of sandwich phthalocyanines with different rare earth metals

Four sandwich phthalocyanines with different rare earth metals (La, Y, Yb and Sc) are synthesized to investigate the influence of aggregation on nonlinear optical limiting effect. Hyperchem fitting results show that the distance between the phthalocyanine rings of the four phthalocyanine compounds is 2.81, 2.69, 2.56 and 2.38 Å for LaPc (Tetra-β-(4-nitro) lanthanum phthalocyanine), YPc (Tetra-β-(4-nitro) yttrium phthalocyanine), YbPc (Tetra-β-(4-nitro) ytterbium phthalocyanine) and ScPc (Tetra-β-(4-nitro) scandium phthalocyanine), respectively. As the radius of rare earth ions decreases, the interlayer spacing between phthalocyanine rings decreases, which greatly increases the degree of aggregation between phthalocyanine rings. As a result, the photophysical and nonlinear absorption coefficients of different rare-earth phthalocyanines also show regular changes. The nonlinear absorption coefficients of the four rare-earth phthalocyanines were LaPc (β = 1.86ⅹ10−10 m/W), YPc (β = 9.78ⅹ10−11 m/W), YbPc (β = 6.63ⅹ10−11 m/W), ScPc (β = 7.43ⅹ10−11 m/W), respectively, indicating that the degree of aggregation between phthalocyanine rings gradually increases as the radius of rare earth metals changes from La, Y, Yb, to Sc, which inhibits the reverse saturable absorption process of the triplet states, resulting in a gradual decrease of the nonlinear absorption coefficient. The experimental results fully reveal the effect of the aggregation effect between phthalocyanine rings on photophysical and nonlinear optical properties, and would provide a theoretical basis for the design of high-performance nonlinear optical materials.

Influence of PVP polymer concentration on nonlinear absorption in silver nanoparticles at resonant excitation

Silver nanoparticles exhibit saturable absorption at resonant excitation, which upon addition of the polymer as a capping agent, exhibit reverse saturable absorption. These nonlinear optical processes play an important role in the overall nonlinear optical properties of silver nanoparticle. Thus, with this viewpoint, the nonlinear absorption of silver nanoparticles in deionized water in the presence of varying concentration polyvinylpyrrolidone polymer was investigated using femtosecond laser pulses at 400 nm. This study shows that the saturable absorption process is significantly suppressed at high concentration of the polymer. The silver nanoparticle with smaller size shows pure reverse saturable absorption process with the respective coefficient of $$\beta =7.55\times {10}^{-12}\, \mathrm{c}\mathrm{m}/\mathrm{W}$$. The change in the nonlinear optical response from combined saturable absorption and reverse saturable absorption to pure reverse saturable absorption process was observed as the concentration of polymer increases.

Effects of bovine serum albumin (BSA) on the excited-state properties of meso-tetrakis(sulfonatophenyl) porphyrin (TPPS4).

To infer changes in the photophysical properties of porphyrins due to complexation with albumin, a combination of Z-scan and conventional spectroscopic techniques was employed. We measured the characteristics of excited states of meso-tetrakis(sulfonatophenyl) porphyrin bound to bovine serum albumin and observed that the binding reduces the intersystem crossing quantum yield and increases the internal conversion one. A reverse saturable absorption process was observed in the nanosecond timescale. These results are important for prediction of the efficiency of this complex in medical and optical applications, because associating porphyrins to proteins enables better accumulation in tumors and improves its stability in optical devices, but at the same time, decreases its triplet quantum yield.

Nonlinear spectroscopy of three-level cascade systems in the act of saturable and reverse saturable absorption

In the process of nonlinear absorption study using ultra-short laser pulses, it is imperative to know the population dynamics of excited molecules/atoms as a function of time. Using rate equation approach, we have numerically demonstrated the time-dependent population dynamics of three-level cascade systems under single photon resonant excitation by 125 fs single shot laser pulse with central wavelength at 532 nm. We present the effect of life-times, absorption cross-sections and photon flux on population redistribution among the energy levels by considering the saturable absorption (SA) and reverse saturable absorption process (RSA) in the wide range of spectroscopic parameters such as life-times and absorption cross-sections. In cascade energy level systems, ultra-short laser pulses have to be used to investigate higher energy levels absorption cross-section and life-times. Our results can be extended to higher energy levels of cascade systems by incorporating them into the rate equation models.

All-optical switching and limiting properties of a Ru (II) Schiff-base complex for nonlinear optical applications

A salen-based ruthenium (Ru) (II) complex was synthesized for possible use in nonlinear optical device applications. The Ru complex was doped in a polymer matrix to fabricate films using a low-cost spin-coating technique. The third-order nonlinear optical parameters of the complex were investigated by Z-scan and degenerate four-wave mixing techniques. The study reveals two-order enhancement of third-order optical susceptibility χ(3) and exhibits superior limiting capability due to a reverse saturable absorption process. All-optical switching action for the films indicates that the sample can function as an optical inverter or a NOT gate. Hence, the Ru (II) Schiff-base complex materializes as a possible candidate for use in nonlinear optical devices.

Enhanced third-order optical nonlinearity in Ce3+ ion-doped zinc sulfide–polyvinyl alcohol freestanding nanocomposite films

Undoped, 0, 2, and 5 mol% Ce-doped ZnS–PVA nanocomposite films have been prepared using in situ chemical method. X-ray diffraction patterns confirm that the prepared nanocomposite films are in cubic structure of ZnS. UV–Vis optical absorption spectra of Ce3+-doped ZnS–PVA films exhibit the red-shifted phenomenon with the increasing Ce dopant concentration in ZnS. Scanning electron microscope images reveal the morphology changes in the films with Ce doping. The high values of real and imaginary parts of dielectric constant at low frequencies are attributed to the space charge polarization, whereas the loss tangent of Ce-doped ZnS–PVA nanocomposite films at low frequency with the increasing Ce dopant concentration indicates the enhancement of optical quality of the films. The Z-scan technique exhibits a reverse saturable absorption process in nonlinear absorption studies and self-focusing optical nonlinearity in nonlinear refractive studies under the experimental conditions. The highest nonlinear optical parameters such as nonlinear absorption coefficient, nonlinear refractive index, and third-order nonlinear optical susceptibility are found to be about 9336.6 cm/GW, 1.782 × 10−4 cm2/GW, and 2.103 × 10−5 esu, respectively, for 5 mol% Ce3+-doped films. The estimated third-order nonlinear optical susceptibility is eight orders of magnitude larger than that of bulk ZnS, and four–five orders of magnitude higher than those of the some representative materials reported. Hence, the Ce3+-doped ZnS–PVA nanocomposite films investigated here emerge as promising candidates for the development of nonlinear optical devices.

Third-order nonlinear optical studies of anthraquinone dyes using a CW He–Ne laser

We present investigations on the third-order optical nonlinearity and optical power limiting of anthraquinone dyes. Z-scan measurements were performed using a continuous wave He–Ne laser at 633 nm wavelength as an excitation source. The nonlinear refraction studies exhibited self-defocusing behavior of the dyes. The nonlinear absorption in the dyes was dominated by a reverse saturable absorption process. Self-diffraction ring patterns were observed due to the change in refractive index and thermal lensing. Increase of the electron donating ability of the substituents resulted in enhanced values of the nonlinear optical parameters, establishing the structure–property relationship. The optical limiting study revealed that the dyes possess a lower limiting threshold and clamping level which is very important for eye and sensor protection. Hence, the dyes investigated here emerge as promising candidates for future opto-electronic and photonic device applications such as optical power limiters.

Studies on third-order optical nonlinearity and power limiting of conducting polymers using the z-scan technique for nonlinear optical applications

We present the synthesis and characterization of third-order optical nonlinearity and optical limiting of the conducting polymers poly (aniline-co-o-anisidine) and poly (aniline-co-pyrrole). Nonlinear optical studies were carried out by employing the z-scan technique using a He–Ne laser operating in continuous wave mode at 633 nm. The copolymers exhibited a reverse saturable absorption process and self-defocusing properties under the experimental conditions. The estimated values of βeff, n2 and χ(3) were found to be of the order of 10−2 cm W−1, 10-5 esu and 10−7 esu respectively. Self-diffraction rings were observed due to refractive index change when exposed to the laser beam. The copolymers possess a lower limiting threshold and clamping level, which is essential to a great extent for power limiting devices. Therefore, copolymers of aniline emerge as a potential candidate for nonlinear optical device applications.

Excited State Dynamics and Nonlinear Absorption of a Pyrazinoporphyrazine Macrocycle Carrying Externally Appended Pyridine Rings

The multiphoton absorption properties of the tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazinato(monoacquo)-Mg(II) complex [Py(8)TPyzPzMg(H(2)O)] (1) are reported and interpreted. The nonlinear optical behavior of 1 and the characterization of the excited states important for the nonlinear absorption process were studied at the pump frequency of the second harmonic generation of a Nd:YAG laser in the nanosecond time regime. It was found that complex 1 shows a very good optical limiting performance at 532 nm, which derives from two processes: (a) a reverse-saturable absorption process, which involves a triplet excited state at low intensities, and (b) a two-photon absorption process at higher intensities, which is due to the formation of the radical monoanion of 1, [Py(8)TPyzPzMg(H2O)](.-), during the photoreduction of the triplet state. The participation of a monoanion in determining the overall nonlinear absorption behavior of 1 is found, for the first time, for a tetrapyrrolic system. One can deduce that the involvement of the monoanion derives from the electron-withdrawing effect of the dipyridinopyrazino fragments externally attached to the porphyrazine core which make the reduced form of 1 easily accessible. These results suggest a modification of tetrapyrrolic systems with new nonlinear absorption properties.

Surface-Enhanced Fluorescence and Reverse Saturable Absorption on Silver Nanoparticles

Langmuir isotherm is applied to analyze the laser-induced fluorescence from solutions containing silver nanoparticles and tris(2,2‘-bipyridyl)ruthenium(II) chloride. Fluorescence enhancement factors based on the simple adsorption model are found to be 47−90, depending on the nanoparticle concentration. The enhancement is attributed to the laser-induced free-carrier absorption on the silver surfaces, which is a reverse saturable absorption process, at the excitation wavelength of 532 nm.

Reverse saturable absorption in Er+3 doped systems

This work reports on reverse saturable absorption (RSA) from Er3+ doped systems. RSA is characterized by a decrease in the transmitted intensity as the incident intensity increases. This effect is interesting due to its potential application in optical devices such as optical inverters, optical limiters and switches. Particularly, Er doped systems pumped at wavelengths around 830nm may exhibit RSA. In this case, the RSA process is achieved because the resonant excited-state (4I13/2–4S3/2) absorption cross section is larger than the quasiresonant ground state (4I15/2–4I9/2) absorption cross section. We have simulated the transmitted intensity of Er doped samples as a function of laser intensity, laser wavelength and Er concentration. Our results indicate that Er concentration plays an important role in the process. RSA is intensified as cross relaxation between Er ions increases. Moreover, the effect of optical limiting is observed and is dependent on the cross relaxation among Er ions.

Dynamic saturable optical nonlinearities in free base tetrapyridylporphyrin

Dynamic optical nonlinearities in free base tetrapyridylporphyrin ( H 2 TPyP ) solutions were investigated at 532 nm with the Z-scan technique. We observed a reverse saturable absorption process that was found to have a fast contribution related to the singlet population and a slow accumulative contribution arising from the triplet population. The optical excitations and subsequent relaxations can be interpreted with a five-energy-level diagram that allows determination of the excited states photophysical parameters such as triplet and singlet absorption cross-sections and the intersystem crossing time.

Chemical dynamics and reverse saturable absorption in di-furfuryl ether solutions

We report on the chemical dynamics and nonlinear optical absorption of di-furfuryl ether (DFE) solutions. DFE molecules were derivatized from the room temperature furfuryl alcohol self-reaction, whose reaction kinetics was investigated with the Z-scan technique and laser-induced fluorescence. These molecules are shown to exhibit a pronounced reverse saturable absorption process. The magnitude of the excited-state absorption cross-sections and the intersystem crossing lifetime have been evaluated with 8 ns pulses at 532 nm.

Characterization of dynamic optical nonlinearities in ytterbium bis-phthalocyanine solution

Dynamic optical nonlinearities of ytterbium bis-phthalocyanine (YbPc 2) dissolved in chloroform were characterized using the Z-scan technique with pulse trains. Two contributions of opposite signs were observed for the nonlinear refraction: a fast process related to the singlet population, and a slow accumulative contribution arising from the triplet population and thermal lensing. As the fluence increases, the nonlinear absorption first presents a weak saturation, followed by a reverse saturable absorption process. We used a six-energy-level diagram to explain these results and to obtain spectroscopic parameters such as the excited state cross-sections and the intersystem crossing lifetime.