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The first hyperpolarizability of graphene quantum dots (GQDs) suspended in water was determined using the hyper-Rayleigh scattering (HRS) technique. To the best of our knowledge, this is the first application of the HRS technique to characterize GQDs. Two commercial GQDs (Acqua-Cyan and Acqua-Green) with different compositions were studied. The HRS experiments were performed with an excitation laser at 1064 nm. The measured hyperpolarizabilities were (1.0±0.1)×10-27 esu and (0.9±0.1)×10-27 esu for Acqua-Cyan and Acqua-Green, respectively. The results were used to estimate the hyperpolarizability per nanosheet obtained by assuming that each GQD has five nanosheets with 0.3 nm thickness. The two-level model, used to calculate the static hyperpolarizability per nanosheet, provides values of (2.4±0.1)×10-28 esu (Acqua-Cyan) and (0.5±0.1)×10-28 esu (Aqua-Green). The origin of the nonlinearity is discussed on the basis of polarized resolved HRS experiments, and electric quadrupolar behavior with a strong dependence on surface effects. The nontoxic characteristics and order of magnitude indicate that these GQDs may be useful for biological microscopy imaging.
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The nonlinear optical (NLO) response of photonic materials plays an important role in the understanding of light-matter interaction as well as pointing out a diversity of photonic and optoelectronic applications. Among the recently studied materials, 2D-LTMDs (bi-dimensional layered transition metal dichalcogenides) have appeared as a beyond-graphene nanomaterial with semiconducting and metallic optical properties. In this article, we review most of our work in studies of the NLO response of a series of 2D-LTMDs nanomaterials in suspension, using six different NLO techniques, namely hyper Rayleigh scattering, Z-scan, photoacoustic Z-scan, optical Kerr gate, and spatial self-phase modulation, besides the Fourier transform nonlinear optics technique, to infer the nonlinear optical response of semiconducting MoS2, MoSe2, MoTe2, WS2, semimetallic WTe2, ZrTe2, and metallic NbS2 and NbSe2. The nonlinear optical response from a thermal to non-thermal origin was studied, and the nonlinear refraction index and nonlinear absorption coefficient, where present, were measured. Theoretical support was given to explain the origin of the nonlinear responses, which is very dependent on the spectro-temporal regime of the optical source employed in the studies.
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We synthesized the mesoionic compound 2-(4-chlorophenyl)-3-methyl-4-(4-methylphenyl)-1,3-thiazole-5-thiolate and measured its refractive and absorptive nonlinear optical response in different temporal and spectral regimes. The experiments were performed by using the Z-scan technique with two pulsed light sources: the second harmonic (at 532 nm) of a mode-locked and Q-switched Nd-YAG laser (100 ps, 10 Hz) and a Ti: Sapphire laser system (100 fs, 1 kHz) operating at 800 nm. The observation and characterization of nonlinear refraction, two- and three-photon absorption, and excited state absorption of the mesoionic compound dissolved in dimethyl sulfoxide, in different concentrations, are presented and discussed with basis on the population redistribution in a three-energy-level model that allows the determination of the parameters which characterize the nonlinear response.
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We investigated the random lasing process and Replica Symmetry Breaking (RSB) phenomenon in neodymium ions (Nd3+) doped lead-germanate glass-ceramics (GCs) containing MgO. Glass samples were fabricated by conventional melt-quenching technique and the GCs were obtained by carefully devitrifying the parent glasses at 830 °C for different time intervals. The partial crystallization of the parent glasses was verified by X-ray diffraction. Photoluminescence (PL) enhancement of [Formula: see text] 500% relative to the parent glasses was observed for samples with a higher crystallinity degree (annealed during 5 h). Powders with grains having average size of 2 µm were prepared by griding the GCs samples. The Random Laser (RL) was excited at 808 nm, in resonance with the Nd3+ transition 4I9/2 â {4F5/2, 2H9/2}, and emitted at 1068 nm (transition 4F3/2 â 4I11/2). The RL performance was clearly enhanced for the sample with the highest crystallinity degree whose energy fluence excitation threshold (EFEth) was 0.25 mJ/mm2. The enhanced performance is attributed to the residence-time growth of photons inside the sample and the higher quantum efficiency of Nd3+ incorporated within the microcrystals, where radiative losses are reduced. Moreover, the phenomenon of Replica Symmetry Breaking (RSB), characteristic of a photonic-phase-transition, was detected by measuring the intensity fluctuations of the RL emission. The Parisi overlap parameter was determined for all samples, for excitation below and above the EFEth. This is the first time, for the best of the authors knowledge, that RL emission and RSB are reported for a glass-ceramic system.
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Nonlinear optical characterization of nanostructured layered transition metal dichalcogenides (LTMDs) is of fundamental interest for basic knowledge and applied purposes. In particular, second-order optical nonlinearities are the basis for second harmonic generation as well as sum or difference frequency generation and have been studied in some 2D TMDs, especially in those with a semiconducting character. Here we report, for the first time, on the second-order nonlinearity of the semi-metallic ZrTe2 monolayer in acetonitrile suspension (concentration of 4.9 × 1010 particles per cm3), synthesized via a modified redox exfoliation method and characterized using the Hyper-Rayleigh scattering technique in the nanosecond regime. The orientation-averaged first-hyperpolarizability was found to be ß(2ω) = (7.0 ± 0.3) × 10-24 esu per ZrTe2 monolayer flake, the largest reported so far. Polarization-resolved measurements were performed in the monolayer suspension and indicate the dipolar origin of the generated incoherent second harmonic wave.
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We demonstrate random laser emission from Rhodamine 6G with ZrTe2 transition metal dichalcogenide (TMD) as nanoscatters, both in powder and 2D nanoflakes liquid suspension. The 2D semimetal ZrTe2 was synthesized by a modified redox exfoliation method to provide single layer TMD, which was employed for the first time as the scatter medium to provide feedback in an organic gain medium random laser. In order to exploit random laser emission and its threshold value, replica symmetry breaking leading to a photonic paramagnetic to photonic spin glass transition in both 2D and 3D (powder) ZrTe2 was demonstrated. One important aspect of mixing organic dyes with ZrTe2 is that there is no chemical reaction leading to dye degradation, demonstrated by operating over more than 2 hours of pulsed (5 Hz) random laser emission.
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We report an alternative random laser (RL) architecture based on a flexible and ZnO-enriched cellulose acetate (CA) fiber matrix prepared by electrospinning. The electrospun fibers, mechanically reinforced by polyethylene oxide and impregnated with zinc oxide powder, were applied as an adsorbent surface to incorporate plasmonic centers (silver nanoprisms). The resulting structures - prepared in the absence (CA-ZnO) and in the presence of silver nanoparticles (CA-ZnO-Ag) - were developed to support light excitation, guiding and scattering prototypes of a RL. Both materials were excited by a pulsed (5 Hz, 5 ns) source at 355 nm and their fluorescence emission monitored at 387 nm. The results suggest that the addition of silver nanoprisms to the ZnO- enriched fiber matrix allows large improvement of the RL performance due to the plasmon resonance of the silver nanoprisms, with ~80% reduction in threshold energy. Besides the intensity and spectral analysis, the RL characterization included its spectral and intensity angular dependences. Bending the flexible RL did not affect the spectral characteristics of the device. No degradation was observed in the random laser emission for more than 10,000 shots of the pump laser.
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Phosphotellurite based glasses have interesting features such as low characteristic temperatures, high glass forming ability, high thermal stability against crystallization and a broad transparency window from ultraviolet (UV) to near-infrared (NIR), which makes them promising materials for photonic applications. In this work, phosphotellurite binary glasses, having a composition (100 - x)TeO2 - xBa(PO3)2 with x varying from 1 to 20 mol%, were synthesized by the conventional melt-quenching method in covered gold crucibles under air. Optical, physical and structural properties of the new glass samples were investigated by differential scanning calorimetry, X-ray diffraction, Raman spectroscopy, transmission electron microscopy, linear optical absorption from UV to NIR, IR transmittance, and optical limiting experiments. Transparent glass-ceramics in the visible range were obtained for phosphotellurite samples containing 2, 4 and 6 mol% of Ba(PO3)2 and the phase crystallization was investigated through Rietveld analysis and transmission electron microscopy. The incorporation of Ba(PO3)2 into the TeO2 network drastically increases the thermal stability against devitrification and helps to shift the infrared multiphonon absorption edge to longer wavelengths. Nonlinear measurements performed with a picosecond laser at 532 nm indicate large effective nonlinear absorption coefficients for all samples. In summary, the dependence of the spectroscopic properties on the compositions of the samples revealed promising transparent glass and glass-ceramics for photonic applications.
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We demonstrate the replica symmetry breaking (RSB) phenomenon in the spontaneous mode-locking regime of a multimode Q-switched Nd:YAG laser. The underlying mechanism is quite distinct from that of the RSB recently observed in random lasers. Here, there is no random medium and the phase is not glassy with incoherently oscillating modes as in random lasers. Instead, in each pulse a specific subset of longitudinal modes are activated in a nondeterministic way, whose coherent oscillation dominates and frustrates the others. The emergence of RSB coincides with the onset of ultrashort pulse generation typical of the spontaneous mode-locking regime, both occurring at the laser threshold. On the other hand, when high losses are introduced, RSB is inhibited and only the amplified stimulated emission with replica symmetry is observed. Our results disclose the only theoretically predicted photonic phase with RSB that remained unobserved so far.
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Ultraviolet and blue light were obtained by nonlinear frequency conversion in a random laser (RL) based on Nd0.10Y0.90Al3(BO3)4 nanocrystalline powder. RL operation at 1062 nm, due to the (4)F3/2 â (4)I11/2 transition of neodymium ions (Nd(3+)), was achieved by exciting the Nd(3+) with a tunable beam from 680 to 920 nm covering the ground state absorption transitions to the (4)F9/2, ((4)F7/2,(4)S3/2), ((4)F5/2,(2)H9/2), and (4)F3/2 states. Light from 340 to 460 nm was obtained via the second-harmonic generation of the excitation beam while tunable blue light, from 417 to 486 nm, was generated by self-sum-frequency mixing between the excitation beam and the RL emission.
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Random lasers have been recently exploited as a photonic platform for studies of complex systems. This cross-disciplinary approach opened up new important avenues for the understanding of random-laser behavior, including Lévy-type distributions of strong intensity fluctuations and phase transitions to a photonic spin-glass phase. In this work, we employ the Nd:YBO random laser system to unveil, from a single set of measurements, the physical origin of the complex correspondence between the Lévy fluctuation regime and the replica-symmetry-breaking transition to the spin-glass phase. A novel unexpected finding is also reported: the trend to suppress the spin-glass behavior for high excitation pulse energies. The present description from first principles of this correspondence unfolds new possibilities to characterize other random lasers, such as random fiber lasers, nanolasers and small lasers, which include plasmonic-based, photonic-crystal and bio-derived nanodevices. The statistical nature of the emission provided by random lasers can also impact on their prominent use as sources for speckle-free laser imaging, which nowadays represents one of the most promising applications of random lasers, with expected progress even in cancer research.
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Various techniques to characterize the nonlinear (NL) optical response of centro-symmetric materials are presented and evaluated with emphasis on the relationship between the macroscopic measurable quantities and the microscopic properties of photonic materials. NL refraction and NL absorption of the materials are the phenomena of major interest. The dependence of the NL refraction and NL absorption coefficients on the nature of the materials was studied as well as on the laser excitation characteristics of wavelength, intensity, spatial profile, pulse duration and pulses repetition rate. Selected experimental results are discussed and illustrated. The various techniques currently available were compared and their relative advantages and drawbacks were evaluated. Critical comparisons among established techniques provided elements to evaluate their accuracies and sensitivities with respect to novel methods that present improvements with respect to the conventional techniques.
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Random lasers (RLs) based on neodymium ions (Nd(3+)) doped crystalline powders rely on multiple light scattering to sustain laser oscillation. Although Stokes and anti-Stokes Nd(3+) RLs have been demonstrated, the optical gain obtained up to now was possibly not large enough to produce self-frequency conversion. Here we demonstrate self-frequency upconversion from Nd(3+) doped YAl3(BO3)4 monocrystals excited at 806 nm, in resonance with the Nd(3+) transition (4)I9/2 â (4)F5/2. Besides the observation of the RL emission at 1062 nm, self-converted second-harmonic at 531 nm, and self-sum-frequency generated emission at 459 nm due to the RL and the excitation laser at 806 nm, are reported. Additionally, second-harmonic of the excitation laser at 403 nm was generated. These results exemplify the first multi-wavelength source of radiation owing to nonlinear optical effect in a Nd(3+) doped crystalline powder RL. Contrary to the RLs based on dyes, this multi-wavelength light source can be used in photonic devices due to the large durability of the gain medium.
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We report the observation and analysis of anti-Stokes coherent random laser (RL) emission from zinc oxide (ZnO) powders excited by one-, two- or three-photon femtosecond laser radiation. The ZnO powders were produced via a novel proteic sol-gel, low-cost and environmentally friendly route using coconut water in the polymerization step of the metal precursor. One- and two-photon excitation at 354 nm and 710 nm, respectively, generated single-band emissions centred at about 387 nm. For three-photon excitation, the emission spectra showed a strong ultraviolet (UV) band (380-396 nm) attributed to direct three-photon absorption from the valence band to the conduction band. The presence of an intensity threshold and a bandwidth narrowing of the UV band from about 20 to 4 nm are clear evidence of RL action. The observation of multiple sub-nanometre narrow peaks in the emission spectra for excitation above the RL threshold is consistent with random lasing by coherent feedback.
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We show that by spatially arranging topological charges on a phase mask, it is possible to shape the spatial intensity profile of vortex beams in a controlled manner. As proof-of-principle experiments, we generated vortex beams with the spatial shape of straight lines, corners, and triangles. Potential applications for shaped beams include selective excitation of plasmonic modes, geometrically tunable Bose-Einstein condensates, and optical tweezers.
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The excitation of near-infrared (2+1)D solitons in liquid carbon disulfide is demonstrated due to the simultaneous contribution of the third- and fifth-order susceptibilities. Solitons propagating free from diffraction for more than 10 Rayleigh lengths although damped, were observed to support the proposed soliton behavior. Numerical calculations using a nonlinear Schrödinger-type equation were also performed.
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Glasses in the ternary system (70 - x)NaPO(3)-30WO(3)-xBi(2)O(3), with x = 0-30 mol %, were prepared by the conventional melt-quenching technique. X-ray diffraction (XRD) measurements were performed to confirm the noncrystalline nature of the samples. The influence of the Bi(2)O(3) on the thermal, structural, and optical properties was investigated. Differential scanning calorimetry analysis showed that the glass transition temperature, T(g), increases from 405 to 440 °C for 0 ≤ x ≤ 15 mol % and decreases to 417 °C for x = 30 mol %. The thermal stability against devitrification decreases from 156 to 67 °C with the increase of the Bi(2)O(3) content. The structural modifications were studied by Raman scattering, showing a bismuth insertion into the phosphate chains by Bi-O-P linkage. Furthermore, up to 15 mol % of Bi(2)O(3) formation of BiO(6) clusters is observed, associated with Bi-O-Bi linkage, resulting in a progressive break of the linear phosphate chains that leads to orthophosphate Q(0) units. The linear refractive index, n(0), was measured using the prism-coupler technique at 532, 633, and 1550 nm, whereas the nonlinear (NL) refractive index, n(2) was measured at 1064 nm using the Z-scan technique. Values of 1.58 ≤ n(0) ≤ 1.88, n(2) ≥ 10(-15) cm(2)/W and NL absorption coefficient, α(2) ≤ 0.01 cm/GW, were determined. The linear and NL refractive indices increase with the increase of the Bi(2)O(3) concentration. The large values of n(0) and n(2), as well as the very small α(2), indicate that these materials have large potential for all-optical switching applications in the near-infrared.
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We developed a new scheme for obtaining coherent random lasing based on a chip consisting of a polymer film doped with Rhodamine 6G, having as scatterers butterfly-like TiO(2) nanomembranes (TiO(2)-NM) supported on a glass substrate. The feedback mechanism for laser action is due to the multiple scattering of light by TiO(2)-NM rather than provided by localized variations of the refractive index in the polymer film. The above-threshold multiple spikes signature indicative of random laser emission with coherent feedback is confirmed. As nanomembranes are foreseen as new MEMS/NEMS building blocks, a new generation of combined active/passive photonic devices can be envisaged.
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An upconversion random laser (RL) operating in the ultraviolet is reported for Nd3+ doped fluoroindate glass powder pumped at 575 nm. The RL is obtained by the resonant excitation of the Nd3+ state 2G7/2 followed by energy transfer among two excited ions such that one ion in the pair decays to a lower energy state and the other is promoted to state 4D7/2 from where it decays emitting light at 381 nm. The RL threshold of 30 kW/cm2 was determined by monitoring the photoluminescence intensity as a function of the pump laser intensity. The RL pulses have time duration of 29 ns that is 50 times smaller than the decay time of the upconversion signal when the sample is pumped with intensities below the RL laser threshold.
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The nonlinear response of silica--gold nanoshells (SGNs) in chloroform was studied using laser pulses of 65 fs at 1560 nm. The experiments were performed using the thermally managed Z--scan technique that allows measurements of the electronic contribution for the nonlinear response, free from thermal influence. The results were analyzed using an analytical approach based on the quasi--static approximation that allowed extraction of the nonlinear susceptibility of a SGN from the data. High third--order susceptibility, χsh((3)) = - 1.5 x 10(-11) m(2)/V(2), approximately four orders of magnitude larger than for gold nanospheres in the visible, and large fifth--order susceptibility, χsh((5)) = - 1.4 x 10(-24) m(4)/V(4), were obtained. The present results offers new perspectives for nonlinear plasmonics in the near--infrared.