Spectral shifts of stimulated Rayleigh-Mie scattering in Ag nanoparticle colloids

Stimulated Thermal Scattering in Two-Photon Absorbing Nanocolloids under Laser Radiation of Nanosecond-to-Picosecond Pulse Widths

Recent discoveries in nonlinear optical properties of nanoparticle colloids make actual the challenge to lower the energy threshold of phase conjugation and move it into the domain of shorter pulse widths. A novel effect of the stimulated Rayleigh-Mie scattering (SRMS) in two-photon absorbing nanocolloids is considered as a promising answer to this challenge. We report the results of experimental and theoretical study of the two-photon-assisted SRMS in Ag and ZnO nanocolloids in the nanosecond-to-picosecond pulse width domain. For 12 ns 0.527 μm laser pulses, the four-wave mixing SRMS scheme provides lasing and amplification of backscattered anti-Stokes signal in Ag nanocolloids in toluene at the threshold 0.2 mJ and the spectral shifts up to 150 MHz. For 100 ps 0.532 μm pulses, we observed for the first time efficient (over 50% in signal-to-pump ratio of pulse energies) SRMS backscattering of the anti-Stokes signal in Ag nanocolloids in toluene and predominantly Stokes signal in ZnO nanocolloids in water, with the spectral shifts up to 0.25 cm⁻¹. We develop the first order-in-perturbation model of the four-wave mixing two-photon absorption-assisted SRMS process which shows that at nanosecond pulses, amplification is predominantly due to the thermal-induced coherent oscillations of polarization while the slow temperature wave acts also as a dynamic spatial grating which provides a self-induced optical cavity inside the interaction region. At a picosecond pulse width, according to our model, the spectral overlap between pump and signal pulses results in formation of only the dynamic spatial temperature grating, and we succeeded at recovering the linear growth of the spectral shift with the pump power near the threshold.

Effect of pH and Nanoparticle Capping Agents on Cr (III) Monitoring in Water: A Kinetic Way to Control the Parameters of Ultrasensitive Environmental Detectors

In this work, we apply surface-enhanced Raman spectroscopy (SERS) to study the kinetics of chromium Cr (III) detection in solution using EDTA and silver nanoparticles (AgNPs). We examine for the first time the effect of pH and nanoparticles’ capping agent on the kinetic mechanism of Cr (III) detection using SERS temporal variations. The full width at half maximum (FWHM) and Raman shift variations show that the mechanism of detection is composed of two steps: a first one consisting of chemical coordination between Cr (III) and AgNPs that leads to exalted chemical and electromagnetic enhancement and the second one is an aggregation process with an important optical enhancement. The obtained results showed that the first step in the detection at lower pH was five times faster than in a basic medium using citrate capped silver nanoparticles (Cit-AgNPs). On the other hand, using a capping agent with dicarboxylate groups such as oxalate (Oxa-AgNPs) led to an important enhancement in SERS detection signal (more than 30 times) compared with Cit-AgNPs, although the detection kinetic’s mechanism was slower.

Mechanism of stimulated Mie scattering: Light-induced redistribution of self-assembled nanospheres of two-photon absorbing chromophore

We report the observation of backward stimulated Mie scattering (SMS) due to light-field induced spatial redistribution of self-assembled nanospheres of a two-photon resonant organic chromophore in water, pumped by ∼10-ns laser pulses of ∼816-nm wavelength. The pump-energy threshold for generating backward stimulated scattering in such a system is remarkably lower than that in pure water. The gain of backscattering originates from an induced Bragg grating that reflects partial energy from the pump beam into the backward Mie scattering beam. Based on the experimental fact that the time-delay of the SMS pulse onset depends on both the pump level and the viscosity of the solvent, a physical model of SMS generation is proposed. Our experimental results have shown that the major contribution to the formation of an induced Bragg grating is spatial redistribution of nanoparticles suspended in the liquid. These nanoparticles are driven by a force that is proportional to the intensity gradient of the standing-wave field resulting from interference between the forward pump beam and the backward Mie scattering beam. When the nanoparticle motion is frozen in a gel-like medium, no SMS is observed, which experimentally supports the validity of the proposed physical model.

Excited State Intramolecular Proton Transfer Plus Aggregation-Induced Emission-Based Diketopyrrolopyrrole Luminogen: Photophysical Properties and Simultaneously Discriminative Detection of Trace Water in Three Organic Solvents

Developing solid state near-IR (NIR) emitters and simultaneously discriminative detection of trace water in organic solvents has long been a significant challenge. In this work, a novel diketopyrrolopyrrole-based luminogen (DPP1) with excited state intramolecular proton transfer (ESIPT) and aggregation-induced emission (AIE) characteristics has been designed and synthesized. Its amorphous and crystal solids show red and NIR-emissive fluorescence at 625 and 675 nm, respectively. When DPP1 reacts with fluoride anion, the resulting system (DPP1·F) can discriminatively detect the water content in aprotic solvents with colorimetric and fluorescent dual modes. Distinct fluorescent responses of "turn-on", "ratiometric turn-off", and "ratiometric turn-on" and low limits of detection of 0.0064, 0.042, and 0. The water-induced sensitive and fast change in THF was applied to the determination of water in foodstuffs in practical solid state indicator paper strips.