Femtosecond nonlinear absorption and optical limiting action in nanoplatelet CuFe2O4-decorated rGO nanocomposites
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Using 800 nm infrared, 150 femtosecond (fs), 80 MHz high repetition rate laser pulse excitation, intensity-dependent (Io = 245–735 MW/cm2) measurements on the nonlinear absorption of copper ferrite hetero-architecture functionalized with three rGO contents (15, 25 and 40 wt%) were performed. Copper ferrite–rGO nanocomposite demonstrated a peculiar W-pattern [a peak (saturable absorption) with two valleys [reverse saturable absorption (RSA)] at extreme] in the open aperture Z-scan data for the decorated (15 wt%) rGO systems recorded at a low peak intensity (Io = 245 MW/cm2). The Z-scan traces were dominated by RSA characteristics due to two-photon absorption process at high peak intensities. The variation in intensity-dependent nonlinear absorption coefficient and ground-state absorption at laser excitation wavelength revealed the presence of excited-state absorption (ESA). The maximum two-photon absorption coefficient was recorded for CuFe2O4–(40 wt%) rGO at 735 MW/cm2 excitation, which is attributed to the combined contribution of rGO and CuFe2O4 that is four times higher than pure rGO and CuFe2O4. The presence of layer like CuFe2O4 nanoplatelets upon the layer-structured rGO offers an added advantage in enhancing the nonlinear absorption coefficient. Near-resonant ESA-based optical limiters comprising CuFe2O4–rGO can possibly be used as a versatile optical limiting material for eye safety against intense, high repetition rate fs laser pulses.
KeywordsReduced graphene oxide Nonlinear absorption Nonlinear refraction Optical limiting
Lasers these days are unavoidable tools in cutting edge technologies such as optics/photonics, and in particular, intense, femtosecond (fs)/picosecond (ps) laser pulses are in high demand in automobile, micro-fabrication and ultrafast measurement areas [1, 2, 3, 4, 5, 6, 7]. Such fs lasers, with capacity of delivering high intensities at short intervals of time, are capable of damaging the optical components, and therefore, numerous attempts are being made in preventing these laser-based accidents damaging the human eyes and precious sensors. The desideratum of ideal optical power limiters to delicate optical components (including the human eyes) with a low damage threshold due to deliberate or unintentional exposure to these high power lasers is one of the ultimate challenges for the optics community [8, 9, 10, 11]. Consequently, the development of optical limiting devices has shown tremendous advancement due to its possible utility in photonic application like laser safety devices, optical shutter, pulse shaping and light stabilizing sources [12, 13, 14]. However, the development of such materials (transparent at low intensity and translucent at high intensity) in ultrashort (fs) pulse time scale is still a work in progress. Several nonlinear mechanisms such as nonlinear absorption (NLA), nonlinear refraction (NLR) and nonlinear scattering (NLS) have been studied extensively in a variety of materials and have been engaged for achieving superior optical limiters [15, 16]. Among these diverse phenomena, materials that possess nonlinear absorption can yield effective energy-absorbing optical limiter for various laser pulses. Since the intensity-dependent NLA measurements can provide additional information on the nature of nonlinear absorption, Z-scan experiments with different input intensities of excitation were performed to measure the nonlinear transmissivity of the sample. Graphene possesses interesting nonlinear absorption and refraction properties because of the interband transitions that are independent of the broad wavelength. To improvise the nonlinearity with tunability, graphene oxides (cheaper alternates of graphene) are often decorated with noble metal atom, porphyrin, phthalocyanine or covalent functionalization groups [17, 18, 19, 20]. The remarkable light absorption and broad band emission of oxide derivatives of graphene (GO—graphene oxide, rGO—reduced graphene oxide) due to extended π-conjugate system have shown significant interest in laser damage protecting units. Here, the sp2 hybridization from oxygen-containing groups left out after exfoliation varies which leads to change in NLO properties of rGO. Also, these residual functional groups upon the surface of rGO prevent aggregation and percolate various metal oxides such as Fe2O3, ZnFe2O4, TiO2, ZnO and BBO [21, 22, 23, 24, 25]. Extensive efforts by various research groups have been extended toward understanding the nonlinear optical (NLO) properties of carbon-based graphene and graphene nanocomposites like graphene/epoxy resin , carbon black/epoxy resin , functionalized graphene fluoride , graphene families , graphene oxide , cobalt phthalocyanine , layered GO , polymer–GO , GO–Fe2O3  and graphene . Most of these materials have demonstrated superior NLO coefficients, which is essential for photonic applications. Particularly, metals incorporated in graphene nanocomposites depict impressive NLO and optical limiting performance owing to the combination of different mechanisms. Among metal oxide loading, ferrite stands high and recent work on ferrite-decorated rGO showed remarkable ultrafast nonlinearity characteristics. With intensity-dependent high repetition rate fs laser excitation, CdFe2O4–rGO nanocomposite illustrated a shifting of RSA to SA because of the involvement of various molecules with respect to excitation intensity . Impressed by this factor, we have extended our earlier investigations on ferrite-decorated rGO systems. Earlier reports showcase that the combination of rGO with CuFe2O4 can yield potential systems with strong nonlinearity . Also ferrites are investigated in different optoelectronic applications as it possess the advantages of (a) owning 3d10 4 s1 configuration in the valence shell, (b) available at low cost and (c) environmentally benign [37, 38, 39]. So using the different intensities (Io = 245, 490 and 735 MW/cm2) of Ti:Sapphire (150 fs, 800 nm and 80 MHz) laser pulses, NLO activities of a series of CuFe2O4–rGO nanocomposites were measured. It is well known that inclusion of copper ferrite can induce excited states near the excitation wavelength, and thus, there is a possibility to demonstrate ESA-based optical limiters. Therefore, the motivation of the work is to demonstrate a near-resonant excited-state absorption-based optical limiter comprising CuFe2O4–rGO with improved NLO performance for eye safety against intense, high repetition rate fs laser pulses. Interesting transition of nonlinear absorption mechanism (SA, W-pattern, sequential 2PA) with respect to the composition of samples was observed. Thus, the basic of understanding the influence of elemental composition (Cu) and morphology in altering the nonlinear optical performance of the material is analyzed.
2 Experimental details
2.1 Preparation of solar exfoliated reduced graphene oxide
Reduced graphene oxide (rGO) that resembles the internal structure of graphene is chosen as host element since graphene is expensive to be produced in bulk quantities. Using inexpensive graphite as a starting precursor, graphene oxide was prepared by modified Hummer’s method . The process of oxidation was achieved by reacting graphite (1 g, Alfa Aeser) with sodium nitrate (1 g, EMPLURA) in the presence of sulfuric acid (4 ml). The process of oxidation was continued by the addition of potassium permanganate (6 g, EMPLURA), and the reaction being exothermic excess of heat was released. The solution was immersed in ice bath to control the temperature well below 20 °C. Hydrogen peroxide (6 ml) hydrated solution was used to terminate the oxidation process, and the obtained black power was washed (with HCl and H2O), centrifuged (8000 rpm, 10 min) and dried (60 °C, 12 h). The best way to remove the attached functional groups without destructing the layered structure of graphene is to reduce graphite oxide using solar radiation. The process of exfoliation was activated by focusing solar radiation upon prepared graphene oxide using convex lens of focal length 100 cm. The transition of light brown to dark black accompanied by the sudden increase in the volume ascertains the formation of solar exfoliated reduced graphene oxide . Here the process of de-oxygenation in graphene oxide occurred due to localized thermal excitation (150–200 °C in 1–2 s) arising from the photoacoustic effect originated through focused solar radiation.
2.2 Preparation of CuFe2O4–rGO nanocomposite
The decoration of copper ferrite with different load contents upon reduced graphene oxide to form CuFe2O4–rGO nanocomposites was achieved by a one-step hydrothermal method. Here, nitrates of copper and ferric (1:2 molar ratio) were used as precursor to form CuFe2O4. Upon reduced graphene oxide (rGO) with various contents (15, 25 and 40 wt%), copper ferrite metal oxide was formed. The oxygen functionalities on rGO sheets serve as nucleation sites to accommodate copper and ferrous oxides dissociating from the respective nitrates and thus forming CuFe2O4–rGO nanocomposites. The reaction was carried out in an autoclave unit (volume of 150 mL) with rGO dispersed metal nitrate solutions being heated at 180 °C under ambient pressure. Solvable byproducts in the precipitate were removed with water washing and then dried in air atmosphere (60 °C, 24 h). The above procedure was carried out for 0, 15, 25, 40 wt% of rGO to attain pure CuFe2O4 nanoparticles and CuFe2O4–rGO nanocomposites.
2.3 Z-scan experiment
Open aperture Z-scan experiments were carried out on CuFe2O4–(15, 25 and 40 wt%) rGO nanocomposite to study the nonlinear absorption at various input intensities. An excitation source of Ti:Sapphire laser oscillator (Chameleon, M/s Coherent) was used providing output at a wavelength of 800 nm, pulses with duration of 150 fs at 80 MHz repetition rate. The laser pulses were focused using 100-mm focus lens, and the corresponding beam waist was estimated to be ~ 25.5 µm at 800 nm, and all the experiments were performed with various input peak intensities (245–735 MW/cm2). From the recorded open aperture Z-scan data, the variation in normalized transmittance with respect to light fluence [F(z)] was derived using standard relations .
3 Results and discussion
3.1 Preliminary confirmation and structural studies
3.2 Textural, magnetic and linear optical properties
3.3 Nonlinear refraction
3.4 Intensity-dependent nonlinear absorption
Estimated nonlinear absorption coefficient and onset optical limiting threshold values
800 nm, 150 fs, 80 MHz excitation
Nonlinear absorption coefficient (β2PA) × 10−10m/W
Onset optical limiting threshold (µJ/cm2)
Io = 245 MW/cm2
Io = 490 MW/cm2
Io = 735 MW/cm2
Io = 245 MW/cm2
Io = 490 MW/cm2
Io = 735 MW/cm2
2.1 ± 0.10
2.5 ± 0.12
2.7 ± 0.13
0.7 ± 0.03
1.0 ± 0.05
3.3 ± 0.16
CuFe2O4–(15 wt%) rGO
0.17 ± 0.01
0.9 ± 0.04
1.1 ± 0.05
CuFe2O4–(25 wt%) rGO
1.0 ± 0.05
1.4 ± 0.14
6.7 ± 0.33
CuFe2O4–(40 wt%) rGO
1.3 ± 0.06
1.7 ± 1.70
12.7 ± 0.63
Comparison of the two-photon absorption coefficients obtained for the carbon and metal/semiconductor nanoparticles [Ti:Sapphire (~ 800 nm)]
3.5 Optical limiting behavior
CuFe2O4 was made to anchor upon (15, 25 and 40 wt%) rGO contents by hydrothermal method where electrostatic force plays an important role in distributing and holding various sizes of particles. The decoration of CuFe2O4 nanostructures upon rGO nanosheets was confirmed by analyzing their XPS, XRD, FTIR, VSM, Raman, SEM, TEM and UV–Vis absorption studies. Higher content of rGO with CuFe2O4 nanoplatelets exhibited superior self-defocusing (NLR) and 2PA (NLA) process than other lower rGO content hybrid structure. Intensity-dependent nonlinear absorption studies showcase the following features of CuFe2O4–rGO composites (1) CuFe2O4–(15 wt%) rGO at lower intensity of excitation exhibits W-pattern with dominant saturable absorption behavior arising from the influence of Pauli blocking rGO, (2) CuFe2O4–rGO composite exhibits near-resonant reverse saturable absorption and is ascribed to be sequential 2PA (1PA + ESA) process and (3) CuFe2O4 nanoplatelets decorated upon 40 wt% rGO exhibited superior self-defocusing (NLR) and 2PA (NLA) process than other lower rGO content hybrid structure due to the interaction between the layered morphologies of host and decorating molecules. The prepared CuFe2O4 nanoplatelet-decorated rGO hybrid layer structure provides an insight on the possible way to enhance NLO properties by anchoring layered complex oxides upon carbonaceous-layered materials.
TCS acknowledges financial assistance from CSIR-India [03(1375)/16/EMR-II] and S.V acknowledges the DRDO-India financial support.
Compliance with ethical standards
Conflict of interest
The author(s) declare that they have no competing interests.
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