Closed-aperture Z-scan Research Articles

Nonlinear Refractive (NLR) Index of TiO2 NPs in E5CN7 Nematic Liquid Crystal under CW Laser Illumination

Nematic liquid crystals (NLCs) have proved to be excellent materials for nonlinear optics (NLO) and its applications because of their large nonresonant nonlinearity and their extended spectral transparency. The aim of this paper was to investigate nonlinear refraction index of eutectic mixture of heptylcyanobiphenyl 7CB and pentylcyanobiphenyl 5CB NLCs doped with TiO2 NPs (were synthesized by sol-gel method).The eutectic melt temperature of 273.45K and eutectic composition of 0.45 7CB were observed for the binary system 5CB, 7CB mixture in good agreement with theoretical predictions. The third-order nonlinear optical properties of prepared sample, which tested by close aperture Z-scan method with CW He:Ne laser beam at 632.8nm, are reported. Results suggest that TiO2 NPs in 5CB/7CB mixture is a promising material for third order nonlinear applications.

Z-scan studies of nonlinear optical properties of colloidal gold nanorods and nanoshells

A quantitative comparison of third-order nonlinear optical properties of colloidal gold nanoshells (NSs) and gold nanorods (NRs) in water solutions has been carried out using open- and closed-aperture Z-scan measurements, performed with femtosecond laser pulses over a broad range of wavelengths. Absorption saturation was found to be a dominant effect for all the studied nanoparticles; however, two-photon absorption (2PA) properties were also detected, and were clearly resolved especially at the shortest wavelengths used. The value of the merit factor σ2/M (2PA cross section scaled by the molecular weight) for the NRs (10×35 nm) at 530 nm is 7.5 (GM·mol/g), while for the NSs it is 1.9 (GM·mol/g) at the same wavelength.

Nonlinear optical responses and limiting behavior of sulfadiazine-chromotropic acid azo dye

The nonlinear optical properties of a new azo dye (1.8-Dihydroxy-naphthalin-3,6 disulfonic acid-[2-(4-azo)]-N-(2-pyrimiding)-benzene sulfonamide) in Dimethyl sulfoxide solvent were studied by Z-scan technique using cw diode laser at 532 nm. The nonlinear absorption coefficient and the nonlinear refractive index of the dye were calculated using the open aperture Z-scan and the closed aperture Z-scan techniques respectively. The sample showed a negative and large nonlinear refractive index value of $$-4.06\times 10^{-9}\,\hbox {cm}^{2}/\hbox {W}$$ , and reverse saturable absorption with high value of nonlinear absorption coefficient equal to $$1.37\times 10^{-4}\,\hbox {cm/W}$$ . Diffraction ring patterns were generated in the azo dye sample under cw laser beam irradiation together with optical limiting of the azo dye solution was demonstrated. These results indicate that the new dye is a promising candidate for applications in nonlinear optical devices.

Measurement of nonlinear refractive index in open-aperture Z-scan experiments

We present an experimental study on measurement of nonlinear refractive index (n 2) of organic liquids when the thermo-optic effects manifest into large nonlinear phase shifts (Δϕ 0) in an open-aperture Z-scan configuration. Although we do not obtain the familiar peak–valley normalized transmittance curve as in the case of closed-aperture Z-scan technique, we use a theoretical model using Gaussian beam decomposition (GD) technique to estimate the value of n 2. Using this recipe, we obtain the nonlinear refractive index n 2 = −(4.90 ± 1.20) × 10−15 cm2/W for toluene (organic solvent) and n 2 = −(10.60 ± 2.10) × 10−15 cm2/W for an organic polymer solution (10−4 M in toluene). By carrying out absorption measurements directly with an unfocussed Gaussian beam, we found nonlinear absorptions β tol = (2.42 ± 0.20) × 10−13 m/W and β poly = (2.79 ± 0.24) × 10−13 m/W which are close to the expected results.

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.

Gold nanoparticles in a polycarbonate matrix for optical limiting against a CW laser

The optical limiting behavior of thin polymer films doped with gold nanoparticles is investigated under continuous wave illumination at two different wavelengths: 488 and 514 nm. Closed aperture Z-scan measurements reveal a negative nonlinear refractive index of around 10–6 W cm−2, comparable with that reported for liquid samples, due to the non-local thermal refraction process. The effectiveness of the optical limiting action is assessed, for varying input powers, by measuring the transmitted irradiance in a 300 ms time interval, corresponding to the blinking time of the human eye. It is thus possible to evaluate the total fluence reaching the retina.

Band gap tuning and nonlinear optical characterization of TiO2–SiO2 nanocomposites with respect to composition and phase

We report on the linear and nonlinear optical studies on TiO2–SiO2 nanocomposites with varying percentage ratio. It is found that optical band gap of the material varies with respect to the amount of the SiO2 in the composite. Nonlinear optical characterization of these samples was studied by using open as well as closed aperture Z-scan technique using an Nd:YAG laser (532nm, 7ns, 10Hz). The nanocomposites showed enhanced nonlinear optical properties than pure TiO2 and this can be attributed to the surface states and weak dielectric confinement of TiO2 nanoparticles by SiO2 matrix. The nanocomposites were thermally treated and similar studies were performed. The anatase form of TiO2 in the nanocomposites showed superior properties relative to the amorphous and rutile phase of the composite. The involved mechanism is explained by taking into account the dominant role played by the excitons in the TiO2 nanoparticles.

The numerical methods for analyzing the Z-scan data

In this paper, we are dedicated to exemplifying a two parameter curve fitting method and developing a Matlab-based simulation program to extract the nonlinear refractive index and nonlinear absorption coefficient from closed-aperture Z-scan or R(z) data without the need for performing open-aperture Z-scan measurement. It should be noted, however, that both approaches can only be applicable to a case for which the on-axis phase shift at the focus is small. In this way, we not only determine the nonlinear parameters quickly with reasonable accuracy, as well as save time, efforts and equipment in the process of Z-scan implementation, but also obtain an initial estimate in order to compare with the results of the open-aperture Z-scan measurement when needed.

Third-order nonlinear optical response of indigo carmine under 633 nm excitation for nonlinear optical applications

Abstract We report thermally induced third-order nonlinearity and optical limiting behaviour of Indigo Carmine dye. z-Scan technique was used to determine the sign and magnitude of absorptive and refractive nonlinearities. Continuous wave (CW) He–Ne laser operating at 633 nm was used as source of excitation. In open aperture z-scan experiments, samples exhibited reverse saturable absorption (RSA) process. For closed aperture z-scan experiments, samples revealed self-defocusing property. The presence of donor and acceptor groups in the structure increases the conjugation length and in turn increases the optical nonlinearity. Induced self-diffraction rings pattern was recorded for the samples and it is attributed to refractive index change and thermal lensing. Also, optical limiting and clamping studies were carried out for various input power. Optical clamping of about ~1 mW was observed. This endorses that the dye under investigation is a positive candidate for opto-electronic and photonic applications.

Controllable nonlinear refraction characteristics in hydrogenated nanocrystalline silicon

Nonlinear refraction (NLR) of hydrogenated nanocrystalline silicon (nc-Si:H) has been investigated through the close aperture Z-scan method. We demonstrate a significant NLR and a unique feature of controllable NLR characteristics between saturable and Kerr NLR with the incident photon energy. We numerically evaluate the proportion of these two mechanisms in different wavelengths by a modified NLR equation. The band tail of nc-Si:H appears to play a crucial role in such NLR responses.

Pulse-shaping of nanosecond pulse laser by means of thermal lens effect

With pump-probe separated closed aperture Z-scan technique, nonlinear refraction can be detected and taken into account with transmittance signal of probe beam. Because the transmittance change induced by nanosecond pump pulse is temporal, temporal profile of probe pulse also changes with the thermal lens effect rising. In this study, a new concept for pulse shaping of nanosecond laser with thermal lens effect is discussed in terms of nonlinear optics. The rising of thermal lens effect in nanosecond regime is demonstrated by time-resolved measurement and theoretical model. Theoretical model that accounts for unsteady heat conduction and generation of acoustic wave in liquid sample shows good agreement with experimental results. As pump beam waist gets narrow, rising time of thermal lens effect becomes faster. It means drastic temporal change of transmittance is obtainable and controllable. Using this knowledge, temporal pulse-shaping of nanosecond laser pulse is tried in terms of pulse compression. It is shown that 7 ns pulse can be compressed by 30.0% in the present configuration theoretically. Also, experimental results show 32.2 ns pulse can be compressed by 21.7%.

Investigation of optical nonlinearity and diffraction ring patterns of carbon nanotubes

Abstract The third order nonlinear optical properties of multi-wall carbon nanotubes (MWNT), single-wall carbon nanotubes (SWNT) dissolved in 1,2-dichlorobenzene and SWNT doped poly(ethylacetylenecarboxylate) polymer have been studied using Z-scan technique. Experiments are performed using a CW diode laser at 635 nm wavelength with 26 mW power. It has been found that the values of nonlinear absorption coefficient β, nonlinear refractive index n2, Re χ3, and Im χ3 in SWNT are larger than in MWNT. It was noticed that the closed-aperture Z-scan is most effective when the linear transmittance of the aperture “S” is small. Laser-induced diffraction ring patterns were observed only for SWNT. Based on the number of diffraction rings, variation of refractive index Δn and the relative phase shift ∆ϕ(r) have been estimated at different input laser power: 5, 10, 15 and 25 mW.

Nonlinear optical properties of nematic liquid crystal doped with different compositional percentage of synthesis of Fe3O4 nanoparticles

In this research, Fe3O4 nanoparticles were synthesized and analyzed using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The synthesized Fe3O4 with different compositional percentage doped in an E7 nematic liquid crystal (NLC). Nonlinearity of pure and doped NLC with Fe3O4 nanoparticles were studied by z-scan technique (close aperture and open aperture z-scan). The experimental results show that the nonlinear optical responses of Fe3O4 nanoparticles doped E7 can be changed by varying the compositional percentage of Fe3O4 nanoparticles. A small compositional percentage of Fe3O4 nanoparticles, 1% W/W, dispersing in a NLC showed improved nonlinearity.

Thermochromic phase‐transitions of GafChromic films studied by z‐scan and temperature‐dependent absorbance measurements

Upon irradiation the active diacetylene layer of radiochromic films undergoes dose-dependent polymerization in the blue polydiacetylene form. Dose assessment is currently based on linear absorbance measurements. The scope of the present study was designed for the further understanding of the linear and, in particular, the nonlinear optical characteristics of such films and the utilization of these characteristics for devising alternative "reading" procedures. The nonlinear optical interactions of laser light with previously irradiated GafChromic(®) HD-810 and MD55-2 films were studied by z-scan techniques. The focused laser beam strikes on the film, which is located at various distances from the focal spot. The beam induces linear and nonlinear absorption and refraction. The measurement of the transmitted power (open-aperture z-scan) allows the study of the nonlinear absorption. On the other hand, the measurement of the axial region of the transmitted beam (closed-aperture z-scan) is related to both nonlinear absorption and refraction effects. A 10 mW, 633 nm, linearly polarized He/Ne laser was employed as the light source. The transmittance measurements were coupled with absorbance measurements carried out over the whole visible spectrum and at various reading temperatures, in an attempt to clarify the underlying mechanisms dictating the observed effects. These effects were incorporated to an open-aperture z-scan model, developed for the purpose of comparison to the corresponding experimental curves. The transmittance data obtained by both open- and closed-aperture z-scan were found to be dose-dependent, thus allowing such transmittance techniques to be employed for dose assessment. Low power open-aperture z-scan measurements (sensitive to absorptive nonlinearities) revealed a stepwise two-photon excitation of the active layer, through an intermediate state. At higher laser intensities (and/or absorbed dose), the shapes of the z-scan curves were found to be more complicated. These shapes were attributed to a temperature-dependent (thermochromic) transition from the blue form to the conformer red form of the active material, a transition which appears to be irreversible at high intensities. This transition was incorporated into the open-aperture z-scan model which reproduced satisfactorily the corresponding experimental curves. Finally, the use of a chopped laser beam in the closed-aperture z-scan technique allowed artifact-free, high spatial resolution dose measurements using a laser beam of relatively high power. The model developed in the present study elucidates some of the effects related to film reading and the causes of some of the artifacts reported in the literature. Two-dimensional dose distributions can be assessed by z-scan; however, the merits of such a technique relative to those currently in use have to be further investigated.

Z-scan theory for nonlocal nonlinear media with simultaneous nonlinear refraction and nonlinear absorption

In this paper, the Z-scan theory for nonlocal nonlinear media has been further developed when nonlinear absorption and nonlinear refraction appear simultaneously. To this end, the nonlinear photoinduced phase shift between the impinging and outgoing Gaussian beams from a nonlocal nonlinear sample has been generalized. It is shown that this kind of phase shift will reduce correctly to its known counterpart for the case of pure refractive nonlinearity. Using this generalized form of phase shift, the basic formulas for closed- and open-aperture beam transmittances in the far field have been provided, and a simple procedure for interpreting the Z-scan results has been proposed. In this procedure, by separately performing open- and closed-aperture Z-scan experiments and using the represented relations for the far-field transmittances, one can measure the nonlinear absorption coefficient and nonlinear index of refraction as well as the order of nonlocality. Theoretically, it is shown that when the absorptive nonlinearity is present in addition to the refractive nonlinearity, the sample nonlocal response can noticeably suppress the peak and enhance the valley of the Z-scan closed-aperture transmittance curves, which is due to the nonlocal action's ability to change the beam transverse dimensions.

Thermoreflection Measurement of Magnetic Thin Films

With recent developments in heat assisted magnetic recording (HAMR), there is growing intense interest in understanding the thermal-optical properties of the HAMR media. We study the phase change of the optical beam as it reflects from a FePt thin film under strong laser irradiation by carrying out close-aperture reflection Z-scan experiments. It is found that the far field spatial distribution of the reflected beam is sensitively dependent on the thermal field distribution in the media. We also carry out a temperature dependent pump-probe experiment to study the variation of the differential thermoreflection signal with different media temperature. It is found that the differential signal is sensitively dependent on the media temperature. The present studies show potential to enable noncontact temperature measurement of HAMR media and detect the spatial distribution of the thermal field in the HAMR media.

Ultrafast photoinduced enhancement of nonlinear optical response in 15-atom gold clusters on indium tin oxide conducting film

We show that the third order optical nonlinearity of 15-atom gold clusters is significantly enhanced when in contact with indium tin oxide (ITO) conducting film. Open and close aperture z-scan experiments together with non-degenerate pump-probe differential transmission experiments were done using 80 fs laser pulses centered at 395 nm and 790 nm on gold clusters encased inside cyclodextrin cavities. We show that two photon absorption coefficient is enhanced by an order of magnitude as compared to that when the clusters are on pristine glass plate. The enhancement for the nonlinear optical refraction coefficient is ~3 times. The photo-induced excited state absorption using pump-probe experiments at pump wavelength of 395 nm and probe at 790 nm also show an enhancement by an order of magnitude. These results attributed to the excited state energy transfer in the coupled gold cluster-ITO system are different from the enhancement seen so far in charge donor-acceptor complexes and nanoparticle-conjugate polymer composites.

Nonlinear refractive index of Hibiscus Rosa-Sinensis by CW laser

Nonlinear refractive index n 2 , of the calyces of Hibiscus Rosa-Sinensis, in aqua solution, has been measured by varying the incident laser intensity from a continuous wave (CW) Ar-ion laser. The closed aperture Z-scan technique has been used to measure n 2 with incident laser wavelengths of 457, 488 and 514 nm. The results obtained are found to be of the order of ~10−14 esu.

Enhancement of nonlinear optical properties of BaTiO_3 nanoparticles by the addition of silver seeds

Barium titanate (BaTiO3) is a technologically important material because of its nonlinear properties, such as its strong second harmonic generation and high third order susceptibility. While many nonlinear effects have been extensively studied on the bulk scale, there are still questions regarding the strength of nonlinear effects in nanoparticles. The nonlinear properties of BaTiO3 nanoparticles and nanorods have been studied using the closed aperture z-scan technique. Silver was then grown photochemically on the surface of the BaTiO3 nanoparticles, and it was found that the third order susceptibility increases dramatically.

The Influence of Coherent Transient Energy Transfer on Femtosecond Time-Resolved Z-Scan Measurements

We investigate the influence of the coherent effect on femtosecond time-resolved Z-scan measurements using a degenerate pump-probe Z-scan technique. The time response of the light-induced transient grating (LITG) of Bi2O3-B2O3-SiO2 (BI) glass shows a valley-peak variation, which has an obvious influence on the time-resolved Z-scan measurements. The valley-peak variation of the LITG signals with the delay time in BI glass is due to coherent transient energy transfer between positively chirped pump and probe pulses. The influence of the LITG effect on the closed-aperture time-resolved Z-scan measurements could be reduced by adjusting the position of the aperture transversely.