Thermal and non-thermal intensity dependent optical nonlinearities in ethanol at 800  nm, 1480  nm, and 1560  nm

Abstract

Intensity dependent self-action of a continuous wave (CW) or pulsed optical beam can lead to spatial or spectral effects upon propagation through a nonlinear medium, which can be described as an intensity dependence of the refractive index, known as self-phase modulation (SPM). In this work, we revisit the nonlinear optical propagation of a CW and a CW mode-locked (CW-ML) high repetition rate [ ∼ megahertz (MHz)] laser propagating through pure ethanol in regions of very low optical absorption (800 nm) or very high absorption (1480 nm, 1560 nm). Spatial and spectral SPM and Z -scan experiments were performed to clarify the origin of the third-order nonlinear optical response in the different optical excitation regimes. From spatial SPM and Z -scan at either CW or CW-ML MHz regime, a thermal nonlinear response was determined to be the origin of the nonlinearity at 800 nm or 1480 nm, 1560 nm region, with the nonlinear refractive index response of the order of 10 − 4 − 10 − 9 c m 2 / W . From the spectral SPM, the non-thermal origin of the nonlinearity, arising from electronic or nuclear processes in the ethanol, was determined, and values of the order of 10 − 13 − 10 − 16 c m 2 / W were obtained, depending upon the spectral region. The results were supported by theoretical calculations using the nonlinear Schrödinger equation for the spectral behavior or Fresnel–Kirchhoff integral for the spatial results and clarify some of the misinterpreted results reported in the literature, besides complementing other nonlinear refraction data available at different wavelengths from 355 nm to 1560 nm.

Intensity-Dependent Thermally Induced Nonlinear Optical Response of Two-Dimensional Layered Transition-Metal Dichalcogenides in Suspension

“…From the N vs I (number of rings versus intensity) curves, we can infer the nonlinear refractive index (n 2 ) related to the SSPM as ,, normaln 2 = N I λ L eff where N is the number of rings, I is the incident intensity, λ is the wavelength, and L eff is the sample effective length given by L eff = 1 − normale − α 0 L α 0 0.25em where L is the cuvette length and α 0 is the linear absorption coefficient. In this approach, we are neglecting the converging behavior of the beam throughout the sample and considering that its transversal area is approximately constant inside the cuvette.…”

Numerical Simulation of the Whole Thermal Lensing Process with Z-Scan-Based Methods Using Gaussian Beams

“…This is an area that deserves full attention and efforts to refine the nonlinear measurements where many existing artifacts are found in this field. Therefore, this paper paves the way to study from a metrological point of view the phase shift induced by the thermal lens effect that comes sometimes with the instantaneous Kerr induced ones as was found initially in [27] and more recently in [5].…”

“…Its diffracted beam-waist was measured from the simulation profile to be ω d = 2.8806 mm while theoretically ω d = λD/πω 0 f = 2.8846 mm. Scanning the cell from negative to positive z values, the peak intensity as shown in profiles ( 1) and ( 2) first increases and then suddenly decreases (3), and finally its value increases again in (4) and (5). This is typical behavior of the Z-scan method with negative phase shifts, which show a higher transmittance at pre-focal positions followed by a lower transmittance at post-focal positions [19].…”

“…Many applications use the thermal lens (TL) principle as an ultrasensitive spectrophotometric readout [1] to characterize different physical phenomena, for example (recently found in the literature) to investigate molecular/particle dynamics [2], to measure the photothermal parameters of opaque solids [3], to understand thermal lensing effects using Z-scan-based methods at multiple laser repetition rates and multiple average powers [4,5], to study the effect of highly localized thermal gradients on the catastrophic optical damage process of high-power laser diodes [6], to evaluate optically induced temperature changes in colloidal samples for photothermal therapy [7], to quantify very low concentrations in solutions [8], and to image single light-absorbing nanoparticles by photothermal microscopy [9]. Recently, we have demonstrated experimentally the feasibility of extracting an image of the phase shift induced by TL and applying this method to map an inhomogeneous thin film doped with different concentrations of silver nanoparticles transversally [10].…”

Time-Resolved cw Thermal Z-scan for Nanoparticles Scattering Evaluation in Liquid Suspension

“…In [ 8 ], the authors investigated the molecular/particle dynamics and, in [ 9 ], by measuring photothermal parameters of opaque solids. Moreover, Z-scan [ 10 ] based methods were performed at different regimes of excitation to understand the origin of the third-order nonlinear optical response [ 11 ] and the influence of the accumulated thermal effect, which can be an artifact in femtosecond closed-aperture Z-scan measurements [ 12 ]. Additionally, they can be used to evaluate optically-induced temperature changes in colloidal samples for photothermal therapy [ 13 ].…”