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Abstract
Angular dependence of the diffusive random laser (DRL) emission is assessed due to excitation of a highly concentrated solution of Rhodamine 6G (Rd6G) comprising monomers and dimers. Dimerization at extremely high concentrations leads to the random fluctuation of the dielectric constant in gain medium. As a result, aggregated dye molecules provide multiple scattering events for propagating photons which is confirmed by enhanced backscattering (EBS) test. This scattering feedback besides Försteŕs resonance energy transfer (FRET) from monomers to dimers provide RL spikes over low quantum yield dimeric fluorescence spectra. The unique spectral feature of RL emission is strong dependence on the angle of detection that results from anisotropic inner filter effect (IFE) within the gain volume due to local excitation of the medium by a pencil-like beam of laser. The results have a merit of importance in optical characterization of the media in which the fluorophores can aggregate significantly.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Coherent random laser (RL), as a potential probe for spectroscopic and imaging techniques, is characterized by discrete narrow spikes (FWHM∼0.5 nm) with random fluctuating intensities in time domain. It has been observed in both strong and weakly scattering media due to the disordered-induced multiple scattering of light within the gain volume [1–5]. In order to obtain RL in weak scattering regime, faint scattering feedback in medium should be compensated by high optical gain (i. e. increasing the concentration of chromophores) as well as the strong pumping [1]. However, in several studies regarding the Random lasing in hybrid (dye solution + scatterers) media, typical concentrations of dye solution are employed to provide utmost gain feedback for stimulated emission process [6,7] because of the concentration dependent debilitating effects on intensity i. e. self-quenching, reabsorption and deactivation processes [8–10]. It is well known that Rhodamine 6G molecules (Rd6G) exhibit high tendency of aggregation which likely occurs at high concentrations C > 10 mM [8]. Hence, proper choice of dye concentration is often accomplished well below the saturation threshold (typically in the range of 1 to 10 mM in the case of Rhodamine 6G dye) to acquire the highest gain feedback [2]. Literature survey ascertains that most of the studies regarding both localized and extended (including bichromatic emission) RL have examined dye solutions containing passive scattering structures [11–23]. To the best of our knowledge there are few reports on the coherent RL emission from pure dye solutions [24,25]. Wang et al. reported the coherent RL from a concentrated solution of π-conjugated MEH-PPV and PCPDTBT polymers after excitation by thin strips of 10 ns pulses of Nd:YAG laser at 532 nm wavelength [24]. They attributed their findings to the aggregation of polymers which results in spatial fluctuation of the refractive index. Yin et al. demonstrated how nano-scale aggregates of hybrid molecules can act as scatterers to form coherent RL in highly concentrated hybrid organic-inorganic molecular solutions under pumping by 532 nm Nd:YAG laser pulses [25]. Despite such valuable scientific reports, there is still a lack of closer look at the spectral characteristics of RL in highly concentrated gain media comprising monomers and aggregates.
Here, angular dependent RL emission is reported due to excitation of the highly concentrated pure ethanolic Rd6G solution (near the solubility threshold in ethanol i.e. 170 mM) without passive nanostructures. Local excitation of the active medium leads to the anisotropic IFE that modulates the size of gain volume within different detector’s field of view (FOV). It is shown that under proper conditions for extremely high concentrations of Rd6G dye solution, lasing spikes appear over the low quantum yield dimer fluorescence spectra.
2. Experiments
Rd6G (Acros Organics) with molar weight of 479.01 gr/mol was solved in ethanol (Merck, purity (GC) ≥ 99.9%). Notice that, though the aggregation of Rd6G molecules in water is much greater than that of alcohols, ethanol was used as the solvent because the fluorescence of Rd6G dimers in aqueous solutions is faint even at very high concentrations [8]. Solubility of the Rd6G in ethanol was determined to be ∼80 mg/ml at 25 °C. As demonstrated in Fig. 1, the measurements were accomplished at cylindrical configuration. Dye solution was inserted into the cylindrical glass cuvette with 10 mm inner diameter and 1 mm thick (to eliminate Fabry-Perot cavity effects). The cuvette was also slightly tilted to omit unwanted reflections of the laser light from the internal surface of the cell. To excite the Rd6G molecules, a Q-Switched Nd:YAG laser at 532 nm (2nd harmonic of 1064 nm) was utilized with a pulse duration of 10 nsec and 1-5 Hz repetition rate at constant spot diameter of ∼1 mm without focusing.
Fig. 1. (a) Schematics of the LIF and RL setup (b) Schematics of the EBS measurement to evaluate the scattering strength of the solutions. P: linear polarizer, L: lens, BS: beam splitter.
According to the detailed study of Souza et al., at high Rhodamine concentrations dimer-to-monomer fluorescence intensity ratio enhances in the case of larger spot size of laser beam (tuned at monomer absorption peak wavelength) because of dominant monomer–dimer intercrossing reabsorption events compared with the monomer–monomer reabsorption rate [13]. Using proper ND filters, laser pulses at various energies ranging 1-60 mJ/pulse (quite below the degradation threshold of the Rd6G solution) were directed to the center of the cuvette surface to excite the Rd6G chromophores.
The emission was collected by a convex lens with 10 cm focal length and 1 cm diameter and detected by a spectrophotometer, Avantes AvaSpec-Mini 2048L, with spectral resolution of 0.1 nm and NA of 0.22. The fiber probe was placed at a fixed distance (typically 15 cm) from the surface of the cell. A notch filter was also utilized to prevent detection of the scattered laser photons from the medium sample. The emission spectra within fiber probe’s FOV were recorded at various detection angles (i. e. θ=10◦, 20◦, 30◦, 40◦, 50◦, 60◦, 70◦, 80◦ and 90◦) respect to the pump laser direction to investigate the angular dependence of the RL spectra.
Figure 2 compares typical deconvoluted monomer and dimer peaks before and after RL threshold of 50 mM Rd6G solution detected at particular detection angle θ=20◦. Dimer peak can be recognized after RL threshold due to the shrinkage of monomer emission.
The schematics of EBS setup for measuring the scattering strength of the solutions is depicted in Fig. 1(b). Output of continuous-wave He-Ne laser with 20 mW power at the wavelength of 632.8 nm illuminated the Rd6G solution in rectangular (1 cm) × (1 cm) × (4 cm) cuvette. Perpendicular polarizer plates were used to block the single scattering contribution. Gaussian TEM00 mode was achieved by using proper spatial filters. Afterwards, beam diameter of 10 mm was obtained by utilizing a beam expander. The enhanced backscattered cone was collected by a convex lens and recorded on a high resolution CCD (3840 × 2160 pixels, 30frame/second), located in the focal plane of the lens.
It is worthwhile to note that the spectral behavior of the RL emission was strongly dependent on the angle of detection and spikes were observed only within certain FOVs around the cylindrical cuvette. As RL spikes appeared for specific choice of the setup parameters, we investigated the evolution of the spectra based on a step-by-step parametric assessment.
3. Results and discussion
We first assessed the laser induced fluorescence (LIF) emission due to the excitation of typical 5 mM Rd6G solution (i. e. median concentration of dye solutions studied in RL reports). Figure 3 shows fluorescence spectra, peak intensity and corresponding full width at half maximum (FWHM) versus the pump energy. Fiber probe was positioned at θ=10◦ respect to the pump laser direction.
Fig. 3. (a) Schematics of the LIF and RL setup (b) Schematics of the EBS measurement to evaluate the scattering strength of the solutions. P: linear polarizer, L: lens, BS: beam splitter.
As the pump energy increases, the fluorescence intensity is amplified to reach a saturation level while the spectral bandwidth does not change significantly. This spectral behavior was expected for moderate Rd6G concentrations [10]. It is noticeable that similar trend was observed at all other detection angles (10◦<θ<90◦).
Afterwards, much higher concentrations (e. g. 50 mM) of the Rd6G solution were examined. Assay of detected spectra revealed RL features as well as lasing threshold for certain concentrated Rd6G solution. The characteristics of the lasing such as gain narrowing and amplification were observed for appropriate values of the pump energy. Remarkable fact was the significant dependence of those events as well as the RL spectral characteristics on the angle of detection. Figure 4 demonstrates typical emission spectra versus pump energy detected at various FOVs (detection angles between θ=20◦ to θ=90◦).
Fig. 4. Fluorescence spectra versus pump energy for detection angles (a) θ=20°, (b) θ=60° and (c) θ=90° (excited volume is qualitatively marked in the top view of the cylindrical cuvette).
Comparison of the spectra (a) and (b) in Fig. 4 reveals that the lasing threshold increases with increasing detection angle. There are three distinct angular regions where the RL emission spectra show significant changeover in terms of the pump energy. Notice that in general, boundaries of three angular regions were observed to be dependent on dye concentration and geometrical features of the experimental setup (e.g. laser beam diameter and cuvette size). In region I (θ<30°), the unique fluorescence peak regarding the monomer emission was amplified by increasing the pump energy.
Figure 5 (a) and (b) demonstrate LIF intensity and corresponding FWHM of monomer and dimer peaks against the pump energy at typical detection angles 20° (within region I). In accordance with Fig. 2, before RL threshold, monomer fluorescence covers dimer emission and emergence of dimeric fluorescence peak (which coincides with the RL threshold of the monomer peak) is possibly due to the spectral narrowing of the monomer peak. In region II (30°<θ<70°) there are no evidences of random lasing over monomer emission. Monomer peak intensity raises with increasing the pump pulse energy (up to ∼16.5 mJ) without spectral narrowing and subsequently faces a drastic decrease (see Fig. 5(c)).
Fig. 5. Emission intensity (red circle) and corresponding FWHM (blue triangle) versus excitation energy at typical detection angles 20° (within region I) and 60° (within region II). The dashed straight line indicates RL threshold on the horizontal scale. Yellow and pink colored areas denote pump pulse energies before and after dimeric coherent RL threshold respectively.
However, Fig. 5(d) reveals that there are two thresholds for dimer emission; First: RL threshold regarding the amplification and spectral narrowing of the dimer fluorescence peak (at Epulse ∼10 mJ), and second: RL threshold in which the narrow spikes appear over dimer fluorescence peak (at Epulse∼34 mJ). It should be noted that for detection angles up to 50°, spectral narrowing was not observed for both monomer and dimer peaks.
Finally in region III (70°<θ<90°), only gain narrowing and amplification were observed over the dimer peak. Here, unlike in the region II, monomer peak intensity increases monotonically against the pump energy and does not experience any inflection (Fig. 5(e)). In addition, the RL threshold for dimers occurs at much higher pump energies than the 1st RL threshold of dimers in region II. Peak intensity of dimers and corresponding spectral linewidth (FWHM) versus the excitation energy are demonstrated in Fig. 5(f).
In the following, we first assess the general angular dependence of the RL behavior, and then specifically investigate how the coherent RL spikes are formed in the angular region II (30°<θ<70°).
The essences of the above-mentioned angular dependent spectral behaviors are strong FRET from monomers to dimers besides the non-homogeneous pumping of medium by narrow laser beam. Figure 6 shows the schematic for distribution of the monomers and dimers in concentrated dye solution from the top view of a cylindrical container. Rd6G monomers in front of the medium are locally excited by a narrow laser beam at 532 nm (which adapts to the monomer maximum absorption wavelength).
Fig. 6. Schematic for distribution of monomers and dimers in a concentrated solution from the top view of the cuvette. Three different spectral regions as well as the absorption/emission processes regarding monomers and dimers inside and outside of the pumped area are illustrated.
Excited monomers emit fluorescence in all directions, which can be re-absorbed by other non-excited monomers as well as ground-state dimers. Figure 7(a)-(d) demonstrate emission-absorption spectra regarding the monomer and dimer components of 50 mM Rd6G solution. Spectra are recorded at θ=20° respect to the pump laser beam direction. Overlapping area between absorption and emission spectra are highlighted to reveal the likelihood of the reabsorption events.
Fig. 7. Emission-absorption spectra of monomer and dimer components in 50 mM Rd6G solution. Spectra are recorded at θ=20° respect to the pump laser beam direction (Crossover area is highlighted).
It is important to note that based on the exciton theory two monomers may be stacked in parallel plane (sandwich type or H-type with parallel monomer transition dipole moments) or form head-to-tail configuration in the same plane (J-type with perpendicular monomer transition dipole moments) [26]. Though the H-type dimer possesses strong absorption band (peaked at 490 nm in ethanol), its de-activation mainly takes place through the non-radiative internal conversions [27]. In fact, only J-type dimers (with peak of absorption at 530 nm in ethanol) are fluorescent howsoever with low quantum yield (∼ 0.4%) in low concentrations [27]. In addition, it has been found that J-type dimers (of oblique geometry) are most likely formed in weakly polar solvents such as ethanol (which is used in our experiments) rather than aqueous one [28].
The significant crossover of the monomer emission with the monomer/dimer absorption (highlighted area in Fig. 7(a) and Fig. 7(d)) indicates the possibility of reabsorption of monomer emission by other ground state species i.e. monomers and J-type dimers. However, negligible overlapping area of dimer emission and either monomer/dimer absorption quantum distribution (highlighted area in Fig. 7(b) and Fig. 7(c)) suggests that dimers do not experience significant reabsorption events during propagation in the medium. Figure 7(c) reveals that 532 nm laser photons can also be absorbed by J-type dimers. Table 1) summarizes the general emission/absorption properties of Rd6G monomer, J-type and H-type dimers.
Table 1. Emission/Absorption properties of Rd6G monomer, J/H-type dimer
Accordingly, one can write relations regarding the wavelength dependent gain and loss terms for monomers and dimers as follows:
Based on the Eqs. (1)–(4), in addition to the absorption/stimulated emission cross-section, the abundance of the species in ground state/excited state is a definitive parameter in determining the amount of gain or loss in active medium. In the case of the absorption cross-section, the curves in Fig. 7 are decisive. However, further investigation is needed to determine the population distribution of the ground state/excited state species in the medium. Looking back at Fig. 6 it is deduced that the incident laser beam strongly excites ground-state monomers and J-type dimers in front of the cuvette. Therefore, the front area of the medium is populated by excited-state molecules that cannot re-absorb the emission of surrounding monomers. On the other hand, strong absorption of laser beam in front region allows fewer photons to reach farther areas of the medium. As a result, as the detection angle increases, $N_M^{Exc.S}$ within the detector’s field of view (FOV) increases. But this is not the case for J-type dimers. Because they can also be excited by fluorescence photons emitted by monomers in all directions. In fact, $N_D^{Exc.S}$ slightly reduces by increasing the detection angle. It is clear that the more ${N^{Exc.S}}$ there are, the less ${N^{G.S.}}$ there are. Accordingly, by increasing detection angle, $los{s_M}$ raises and both $gai{n_M}$ and $gai{n_D}$ fall, while $los{s_D}$ would not alters. Such an angular dependence in loss and gain of species is the reason for the differences in spectral characteristics of the emission spectra of different regions. Table (2) qualitatively provides relative distribution of the excited state population of monomer/dimer within three different regions, as well as the corresponding loss/gain weights achieved by emitted photons before leaving the medium.
Table 2. Relative distribution of the ground/excited state population of monomer/dimer and the corresponding loss/gain weights achieved by emitted photons within the volume specified by the detector field of view in the four different regions. The dominant RL mode is specified in the last two column.
In region I, though gain exceeds the loss for both monomers and dimers, however, $N_M^{Exc}$ is much larger than $N_D^{Exc}$. Hence, the competition for lasing ends in favor of the monomers. In region III, not only $N_D^{Exc}$ is more abundant than $N_M^{Exc}$, but also lossM exceeds the gainM benefit to formation of RL over dimer peak. In Fig. 8, emission spectra detected at typical detection angle ${\mathrm{\theta}}$=60° (within region III) due to excitation of 50 mM pure Rd6G solution by pump energies ranging 1-56 mJ per pulse are depicted (further pump energies led to saturation of the detected intensities). At low pumping energies, the fluorescence emission features a broad spectrum that peaks at the monomer vibronic transition wavelength (∼560 nm). For pulse energies more than 18 mJ, dimer fluorescence peak becomes dominant as the pump energy increases. This event coincides with a sudden decrease in the intensity of the monomer due to resonance energy transfer from monomer to nearby dimers within the Förster distance (this event is unidirectional and is not allowed in opposite direction) [8].
Fig. 8. 2D and 3D presentation of the emission spectra due to excitation of 50 mM pure Rd6G solution by different pump pulse energies, detected at typical detection angle ${\mathrm{\theta}}$=60°.
Several studies have reported that the fluorescence peak of J-type dimer dominates over the monomeric peak in highly concentrated Rhodamine solutions [13,26,27]. As a novel result, we report here that the discrete narrow spikes (FWHM∼0.4 nm) appeared over the J type dimer fluorescence peak at excitation energies more than 34 mJ which their intensities grew up with increasing the pump pulse energy. It is noticeable that below 18 mJ energy per pulse, RL spikes did not appear even for Rd6G concentrations near the solubility limit.
In Fig. 9 (a), spectral evolution of the emission at the pump energy of 48 mJ is plotted against the Rd6G concentration. Above a certain dye concentration, the RL spikes gained substantial enhancement. This suggests that for sufficient pumping energy, there is a certain concentration threshold that yields stable coherent RL emission.
Fig. 9. (a) Emission spectra due to excitation by laser pulses of 48 mJ energy against the Rd6G concentration ranging 1-150 mM. (b) Successive spectra after excitation of typical 100 mM Rd6G solution by consecutive 40 mJ laser pulses. (c) Intensity of three different RL spikes (with high Q-factor) versus pump pulse energy that excites 100 mM Rd6G solution.
As depicted in Fig. 9 (b), lasing frequencies with nearly constant spacing do not change shot to shot while the relative intensities exhibit stochastic fluctuation. This repulsive behavior of the spikes besides suddenly increasing Iout/Iin (as shown in Fig. 9 (c)) just near a given pumping threshold (depending on the Rd6G concentration) detracts the contingency of spikes to be ASE signals. Since the concentrated solution comprises both monomers and dimers with non-specified population ratio and spatial distribution, enhanced backscattering profile of the 650 nm CW diode laser beam from the 100 mM and 1 mM Rd6G solutions was measured to investigate the likelihood of the multiple-scattering events. EBS setup was arranged according to Ref. [29]. Figure 10 illustrates the corresponding angular distribution of the backscattered intensity. Both FWHM of the EBS cone and corresponding enhancement factor are significantly greater for concentrated dye solution than dilute one.
Fig. 10. Recorded EBS cone on CCD camera (not denoised) in the case of (a) 50 mM. and (b) 1 mM Rd6G solutions. (c) Angular distribution of the backscattered intensity due to illumination of the pure 1 mM and 50 mM Rd6G solutions by a 650 nm diode laser beam.
Hence, it is elucidated that the scattering strength of the concentrated solution is much more than dilute one. The event implies that the highly concentrated Rd6G solutions contain scattering centers.
In this way, we suggest that associated dye molecules (mainly dimers) provide scattering events for the dimer fluorescence emission. Note that this feedback was not expected for monomer fluorescence emission because both dimer types strongly reabsorb the respective emitted photons.
We now try to answer the question why the spikes appear over the broad dimer fluorescence spectrum. Several studies revealed the occurrence of spikes in weakly scattering systems with high optical gain and the strong pumping [30]. Another possibility is the reduction of the gain volume by either local pumping or inner filter of the emission outside the pumped area [1,31,32]. In regions II and III, dimers are most likely excited due to reabsorption of the monomer’s emission. However, dimers in the back side of the medium (within region III) gain much less excitation energy compared to that in region II because the excited monomers (as the source of excitation) are accumulated in front of the medium (region I).
4. Conclusions
To summarize, we present that the coherent spikes of the random laser over low quantum yield dimeric peak of the Rd6G fluorescence emission could be attained by the local excitation of pure Rd6G solution using nanosecond laser pulses. In those super concentrated dye solutions, dimerization occurs which strongly disturbs the loss/gain balance of the monomer fluorescence emission. Dimers as quencher re-absorb the monomer emission and re-emit light with different quantum distribution which does not overlap with the monomer’s absorption spectrum. As a consequence, loss for monomers leads to gain for dimers. Strong dependence of the spikes on detection angle (due to non-homogeneous pumping of the medium by narrow laser beam) was described according to the modulation of loss/gain weights in different regions. Assey of consecutive spectra together with the coherent backscattering (CBS) experiment attest that lasing spikes are competitive modes formed by weak resonant feedback of large aggregated molecules (mainly dimers). The result would be useful in the field of optical sensing and imaging where conventional conditions for coherent random lasing are not attainable.
Acknowledgements
The authors are grateful of Dr. Parviz Parvin from optics & laser laboratory, physics department of Amirkabir University of technology, Tehran, Iran, for his kind collaboration.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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