Dye-doped Droplets Research Articles

Acoustically levitated whispering-gallery mode microlasers

Acoustic levitation has become a crucial technique for contactless manipulation in several fields, particularly in biological applications. However, its application in the photonics field remains largely unexplored. In this study, we implement an affordable and innovative phased-array levitator that enables stable trapping in the air of micrometer dye-doped droplets, thereby enabling the creation of microlasers. For the first time, this paper presents a detailed performance of the levitated microlaser cavity, supported by theoretical analysis concerning the hybrid technology based on the combination of whispering-gallery modes and acoustic fields. The pressure field distribution inside the acoustic cavity is numerically solved and qualitatively matched with the schlieren deflectometry technique. The optical lasing features of the levitated microlasers are highly comparable with those devices based On-a-Chip registering maximum Q-factors of ∼105, and minimum lasing thresholds ∼ 150 nJ cm−2. The emission comb is explained as a sum of multiple individual-supported whispering-gallery modes. The use of novel touchless micrometric lasers, produced with an acoustic levitator brings new technological opportunities based on photonic-acoustic technological platforms.

Scattering resonances observed in the lasing emission spectrum of large dye-doped droplets

High-resolution spectrum analysis of the lasing emission of mm3-sized pendant droplets that contain aqueous solutions of the Rhodamine 6G (Rh6G) dye is reported. Pumping was performed with a 532 nm pulsed laser beam. The lasing emission demonstrates a complex spectral structure that contains mode-cluster peaks spaced 2 nm apart and a ripple structure formed by periodic resonances, which are superimposed on the peak closest to the maximum of the dye gain curve. The periodic resonances are associated with the light-scattering resonances on a large sphere when light absorption in the sphere occurs.

Optical Properties of Methyl Orange-Doped Droplet and Photodynamic Therapy of Staphylococcus aureus.

Dye-doped droplets are known as mixtures of dyes with uniform solutions of water droplets in a continuous phase of oils with surfactants. To observe the relationship between water droplet structures and surfactant types on optical properties of dyes, a mixture of methyl orange (MO)-doped droplet prepared with benzane and hexane as oils and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as a surfactant was thus examined using Z-scan instrument, spectrophotometer, and fluorimeter in the present study. The findings revealed that nonlinear refractive (NLR) index, nonlinear absorption (NLA) coefficient, as well as fluorescence intensity of the MO had enhanced following a reduction in the droplet water content induced by changes in the non-centrosymmetric charge density distribution of this pH indicator. Moreover, the MO-doped droplet in a continuous phase of benzene investigated by 1H nuclear magnetic resonance (NMR) spectroscopy indicated that the MO had been located in the droplet in the vicinity of the hydrophilic part of the surfactant. Furthermore, the MO-doped droplets along with laser radiation were employed to perform antibacterial photodynamic therapy (APDT) of Staphylococcus aureus (S. aureus). It was ultimately concluded that the bacteria colony had also extremely diminished in the group treated by the MO-doped droplet.

Photophysics of Cationic and Anionic Dye Mixed by Sodium-2-Diethylhexyl Sulfosuccinate–Water–Hexane at Low Molar Ratio (X ≤ 8)

The fluorescence, absorption spectra, and Z-scan (open aperture) were studied in mixture of anionic dye (fluorescein sodium salt-FSS) and cationic dye (Rhodamine B-RhB) with low size of water droplets. The droplet was prepared by sodium-2-diethylhexyl sulfosuccinate/water/hexane microemulsion (MEs) at water to the surfactant molar ratio less than 8. The absorbance and fluorescence intensity was studied at different dye-doped droplet size and concentration. The results evidenced that the fluorescence spectra of FSS in MEs are less than water, but the fluorescence intensity of Rhodamine B in MEs is higher than bulk water. The results evidenced that the fluorescence depends on the size (molar ratio) of the droplet. The Rhodamine B in MEs with X = 3 (middle size) has a higher fluorescence intensity than other X. However, the fluorescence intensity of FSS reduces with reduction of droplet size. The red shift of fluorescence was studied by the Bilot and Kawski theory; the excited dipole moment over the ground state value (μe/μg) was determined. The results indicated that in the same dye in solution concentration, the μe/μg depends on molar ratio. The nonlinear optical properties of dye/sodium-2-diethylhexyl sulfosuccinate/hexane/water MEs for two types of dye (FSS and RhB) at constant X = 8 were investigated. The results show that two-photon absorption coefficient for both dyes increased with the increase of the droplet concentration.

Dot lasers: isotropic droplets in a cholesteric matrix, and vice versa

We have measured lasing spectra from dye-doped droplets of an isotropic liquid located in a cholesteric matrix ('inverted' suspension) and an 'inverse' system, dye-doped cholesteric droplets in an isotropic liquid ('conventional' suspension), with the both systems involving the same cholesteric and isotropic liquid components. For the conventional suspension, we have collected the lasing spectra for about 200 droplets and found that the lasing lines are clearly separated into three groups located at the short-wavelength and long-wavelength edges of a photonic band gap (PhBG) and inside the PhBG. The latter ('forbidden') line is observed for all of the droplets under examination. Occasionally, it can coexist with one or both of the lines located at the short- or/and long-wavelength PhBG edges. We have shown for the first time that, for the inverted isotropic droplets in the cholesteric matrix, a single lasing line is located at the short-wavelength PhBG edge. This reveals a PhBG-nature of that line and corresponds to a zero scalar orientational order parameter for the dye molecules placed in the isotropic liquid.

Probing Microscopic Wetting Properties of Superhydrophobic Surfaces by Vibrated Micrometer-Sized Droplets

We determine contact angles of micrometer-sized NaCl-water droplets on superhydrophobic surfaces by analyzing their lowest-order axisymmetric vibrational resonances driven by vertical oscillations of the surface. Fluorescence spectra of the dye-doped droplets excited by laser light feature whispering-gallery modes (WGMs) whose spectral widths depend on the droplet vibration amplitude, thus enabling precise measurements of the droplet mechanical resonant frequency. Following droplet size determination by WGM mode-matching, we calculate the contact angles from the dependence of the measured mechanical resonant frequency on the droplet size for two surfaces with different superhydrophobicity levels, and find a good correlation with the values measured by direct imaging of millimeter-sized droplets.

Light-induced rotation of dye-doped liquid crystal droplets

We investigate both theoretically and experimentally the rotational dynamics of micrometric droplets of dye-doped and pure liquid crystal induced by circularly and elliptically polarized laser light. The droplets are dispersed in water and trapped in the focus of the laser beam. Since the optical torque acting on the molecular director is known to be strongly enhanced in light-absorbing dye-doped materials, the question arises whether a similar enhancement takes place also for the overall optical torque acting on the whole droplets. We searched for such enhancement by measuring and comparing the rotation speed of dye-doped droplets induced by a laser beam having a wavelength either inside or outside the dye absorption band, and also comparing it with the rotation of pure liquid crystal droplets. No enhancement was found, confirming that photoinduced dye effects are only associated with an internal exchange of angular momentum between orientational and translational degrees of freedom of matter. Our result provides also direct experimental proof of the existence of a photoinduced stress tensor in the illuminated dye-doped liquid crystal. Finally, peculiar photoinduced dynamical effects are predicted to occur in droplets in which the molecular director is not rigidly locked to the flow, but so far they could not be observed.

Parallel-plate ion trap useful for optical studies of microparticles

We report an electrodynamic ion trap composed of four disk electrodes with a hole at the center of each electrode. This ion trap has been developed in order to suspend single micrometer sized droplets. To levitate a droplet, an ac voltage was applied to the two middle electrodes, and the two end electrodes were used to form an additional dc field to balance the gravitational force on the droplet. By further applying compensating dc voltages to the rods which support the disks, the droplet could be maintained at the center of the ion trap with a positional fluctuation of less than 1 μm. It is shown here that this is useful for obtaining both spatial images and emission spectra of dye-doped droplets with higher spatial resolutions than those obtained in systems without such compensation.

Highly scattering Intralipid-10% assisted lasing from microdroplets with Acridine Orange dye

One order or greater of magnitude enhancement of lasing emission is confirmed experimentally from liquid microdroplets of Acridine Orange dye mixed with the fat emulsion Intralipid-10% suspension as highly scattering media, compared with pure dye-doped droplets. This novel method that makes use of a high-gain laser dye soft scatter microsystem allows for a wide range of lasing of microdropslets. Originally without lasing pure dye droplets enabled one to lase with a well-defined threshold and an appreciably increased emission intensity in suitable conditions. Spectral characteristics and emission peak intensities from these microdroplets are measured quantitatively as a function of volume content of Intralipid-10% solutions.

Lasing behavior in a liquid spherical dye laser containing highly scattering nanoparticles

With the addition of scattering nanoparticles to dye-doped spherical droplets, lasing has been observed with well-defined thresholds in input-output data. One-order or more magnitude enhancement of peak intensity from droplet emission has been obtained with certain (optimum) additive scattering particles compared with nonadditive scattering particles (neat dye-doped droplets). Characteristics of input-output intensities, emission spectra (with wavelength shifts), and spectral linewidths are reported experimentally, depending on additive quantities of scattering nanoparticles.

Time-resolved spectroscopy of laser emission from dye-doped droplets

Micrometer-sized droplets of Rhodamine 6G solution in water and ethanol are irradiated by high-intensity nanosecond pulses from a frequency-doubled Nd:YAG laser. Coupling of the spontaneous fluorescence emission with natural resonant modes of the spherical droplets results in stimulated emission, with each droplet behaving like a laser cavity. Spectral observations suggest that droplet lasing emission is supported by resonances of a single mode order. The emission exhibits faster rise times and is shorter lived than corresponding bulk-liquid fluorescence. Lasing in droplets is generally initiated almost simultaneously with elastic scattering, unlike stimulated Raman scattering, which is significantly delayed.