Multicolour Laser Emission Based on the Polystyrene Dust

Abstract

Multicolour emission, independently of the considered mechanism, constitutes an appealing application for modern technologies and an interesting phenomenon for the various fields of science. Moreover, effective and easy colour tuning seems even more engaging. Through this contribution, we present a totally new approach to the organic-based system, which can provide effective and multicolour emission, including laser action. Polystyrene dust (PSd) particles play the role of the optically passive matrix for highly effective laser dyes (Coumarin 504, Coumarin 540, and DCM), which were successfully introduced into the polymer units by liquid crystals carrier. In such a way, various types of organic hybrid systems were created, starting from a single dye embedded in PSd volume, passing by two dye-component architectures, reaching the final and most complex network containing three chromophores, providing three basic tones (red, green, and blue). Moreover, by playing with an optical pumping magnitude, we were able to easily modulate colour tones/shades in a broad range, which is highly appreciated when considering human eye perception.

1. Introduction

The high scientific interest and the noticeable progress towards optically tunable laser sources contribute to modern technologies such as solid-state lighting, Li-Fi [1], efficient and high-resolution multiplexed imaging [2], novel displaying technologies, sensing [3], anti-counterfeiting [4], and many more [5]. The multicolour lasers can be realized with the investigation of both organic and inorganic materials [6,7], likewise forming the hybrid systems [8]. Undoubtedly, organic compounds hold great potential in multicolour lasing since they are characterized by feasible and low-cost processability, flexibility, and huge diversity [9,10]. Their properties can be tailored straightforwardly by molecular design and a well-planned synthesis process. These materials, indeed, revolutionized optoelectronic technologies by their exceptional compatibility, enabling the integration of different multicolour components into a single device [11].

The crucial role of effective lasing is the selection of host material providing the spatial confinement for the laser dyes. Namely, the choice of the matrix can affect the system in a very wide aspect: It can positively affect the system by reducing the lasing threshold, improving photostability upon pumping, and allowing for good spectral tunability. On the other hand, it can negatively affect the system by quenching the stimulated emission [12,13,14]. In the broad arena of organic materials, it is worth distinguishing the unusual type of soft matter represented by liquid crystals (LCs). Due to their nature, combining the long-range orientational order typical for solids, and the ability to flow, which is characteristic for liquids, this matter became the captivating candidate serving as the matrix for a great number of laser dyes [15,16,17]. Anisotropic, liquid crystalline molecules tend to reorient themselves under the influence of external stimuli, such as temperature, mechanical stress, and magnetic or electric fields; therefore, they may change the properties of the entire hybrid system, consequently affecting the chromophore dopants, namely, by modifying their absorption cross-section parameter. LCs can easily form size-controlled droplets, dispersed in the polymeric solutions [18], or fill variously constructed LC cells [19]. The concept of droplets or other irregular microscopic-size particles can lead to the achievement of the individual-colour lasing centres. When applied on one substrate, and combined in both homo-, and heterogenous systems, they can form a compact system providing tunable, multiband, and even white laser emission, which was shown in the literature recently [20].

In respect of the polychromatic lasing action, polymers indisputably show several unique features [21]. Exemplary, they are lightweight, flexible, and can be easily modified, doped, and combined with other optically active materials in the way of impregnation, soaking, coating, and many more [22,23,24,25,26,27]. Despite years of studies, it is not well-determined how many applications these compounds can offer. As great examples of laser devices investigating polymers, the micro-rings [28], micro-discs [29], one-dimensional nanowires [30] and the typical thin films fabricated of poly (vinyl alcohol) (PVA) [31], poly (methyl methacrylate) (PMMA) [32], or polystyrene (PS) [33] should be distinguished. Every host–guest system requires detailed planning and design to achieve relevant properties that can be of very high importance for the light amplification phenomena. The essential factors that determine effective, tunable, and multiband light emission are: analysis of dyes’ chemical structures in terms of their compatibility and appropriate fitting into the matrices, their basic spectroscopic features (absorbance and fluorescence), hydrophobic/hydrophilic nature of the selected materials, the possible energy transfer issue and its consequences, the quantum yields of chromophores, etc. [34,35].

In the present study, we investigated polystyrene dust (PSd), soaked with the LC nematic mixture E7 doped individually with the three separated laser dyes. As a gain media, we chose the commercially available chromophores: Coumarin504 (CM504), Coumarin540 (CM540), and DCM [36,37]. To mechanically stabilize the PSd, we propose the optimized, hydrophilic PVA/water solution, hosting the prepared colourful particles in individual, double-dye, and three-dye arrangements. All the systems were thoroughly characterized from the morphological, basic spectroscopic, and lasing points of view, accordingly. Among the other organic systems considered here, PSd playing the role of the matrix for different laser dyes and showing remarkable colour tunability is a novel, effective, and simple approach for multiwavelength lasing achievement. Moreover, it can serve as a promising idea towards the white lasing phenomenon. The current contribution can pave the way for the construction of the modern type of the organic-based materials dedicated to lighting, light amplification, or sensing systems.

2. Materials and Methods

2.1. Device Construction

The steps for the device realization are presented in Scheme 1. The commercially available polystyrene (M_w = 280 kDa, Sigma-Aldrich^®, Poznań, Poland) has its shape as two-dimensional objects (rods). These objects, understood as thin, flat, or round (in its diameter), long, and narrow and should be considered as the longitudinal structures. The initial macro-structured polymer was mechanically squashed up to the moment, where its size and shape corresponded to dust (one-dimensional heterogenous micro-objects, which refer to a collection of the small, solid particles with irregular surface, more similar to a cube). Subsequently, the laser dyes (CM504, CM540, and DCM) were individually mixed with the nematic liquid crystalline mixture (E7) in the ratio of 1.5 mg of dye and 0.3 mL of LC, and introduced to the PS dust volume. Due to the mechanical fragmentation of the polystyrene, the material appeared to have sharp edges and pores, and absorbed the mixture of the LC/dyes very effectively. The quantity of the obtained microparticles was constant for each sample containing the individual dye. For the two- and three-colour systems, the ratio of different colour PSd units was set for 1:1, or 1:1:1, accordingly. The next step was the dissolution of PVA (M_w = 124 kDa, Sigma-Aldrich^®) in deionized water at a concentration of 7.5% weight/weight (w/w) ratio. The PVA/H₂0 mixture was necessary to obtain a sufficiently viscous solution, in which its role was to coat the PSd, and stabilize it mechanically for further research. Then, such prepared material was manually deposited onto clean glass plates, creating colourful PS dust (photo taken under UV light illumination is shown on the right side of the Scheme 1). The photo shows all kinds of the investigated samples: the top line shows PS dust doped with single dyes (from the left: DCM, CM540, and CM504); the bottom line shows PS dust doped with dye pairs (from the left: CM540/DCM, CM504/DCM, and CM504/CM540); and in the middle (centre) is a triple-colour mixture of the PS dust units (PS/CM504, PS/CM540, and PS/DCM). The LC presence was necessary to provide sufficient dye and PS mixing (not feasible with some organic solvents that easily dissolve PSd). PVA polymer attendance was justified by the following reasons: (i) holding deposited PS dust units onto the substrate surface separately, and in that way preventing energy transfer between used dyes; and (ii) creating additional optically passive anti-air separator/barrier, which excludes emission decay due to the thermodynamic issues.

2.2. Optical and Morphological Studies

The absorbance and emission spectra were recorded with the use of spectrophotometer JASCO V730, and spectrofluorometer Fluoromax-4 Horiba, respectively. The large accumulation of chosen dyes in PSd does not allow to obtain absorbance and emission spectra without the effect of supersaturation and deformation; therefore, the aforementioned tests were performed in the highly diluted dichloromethane solutions, in the following concentrations: CM504-1.6 × 10⁻⁵ mol/dm³, CM540-1.5 × 10⁻⁵ mol/dm³, and DCM-1.7 × 10⁻⁵ mol/dm³, accordingly.

For the light amplification experiment, the tripled frequency Nd:YAG pulsed laser (Surelite II, pulse duration 6 ns, repetition rate 10 Hz, Soliton, Gilching, Germany) was used as the source of optical pumping. We tuned the excitation wavelength useful for the investigation by using the optical parametric oscillator (Horizon, high-efficiency mid-band OPO by Continuum). The pumping wavelength was set for λ_ex = 440 nm to ensure effective excitation of all three laser dyes. The laser beam incident on the sample surface was formed into a strip by using a cylindrical lens and adjustable size slit. For the experiment, a 6.50 × 0.85 mm (height vs. width) longitudinal strip laser light shape was used to excite the widest possible fragment (in our setup) of the sample surface and provide directional emission, typical for laser action. The calibrated laser energy meter (Coherent Field Max II coupled with J-10MB-HE sensor) was used to control optical pumping conditions, whereas to collect optical signal generated from the edge of the sample’s volume, a high-resolution (0.1 nm) spectrometer (Shamrock 163) equipped with an optical fibre was used. Finally, the gathered data were sent to the computer and analysed by dedicated software (Origin Lab.^®, Origin Lab. 2021).

The fluorescence microscopic studies were realized with the investigation of the optical microscope (Olympus BX60 microscope, Warszawa, Poland), where two working modes were applied: transmission and luminescence. To have a deeper insight into the volume of the samples, the obtained organic-based devices were excited with wavelengths of 375 and 450 nm, accordingly.

3. Results and Discussion

The fundamental spectroscopic features were investigated by the measurements of absorbance and fluorescence bands, accordingly. At that point, dichloromethane solutions doped with proper dyes were measured (Figure 1a–c). The CM504 in dichloromethane mixture characterizes absorbance in the range of UV up to the visible part of the spectrum (~350–475 nm) and provides significant emission (fluorescence) only in the visible region (~425–575 nm) perceived as cyan (blue-green) (Figure 1a), which is consistent with the literature data [36,37]. The second coumarin (CM540) absorbs light from ~350 up to 500 nm with two significant maxima localized at 448.5 and 465.0 nm, and provides efficient fluorescence in the green region of the visible spectrum (precisely at ~450–600 nm, also in here with two distinguished maxima at 495.6 and 528.4 nm), which corresponds to the available literature data as well (Figure 1b) [37,38]. Finally, the third laser dye, DCM, shows a broad absorbance band starting from deep UV and present up to 550 nm (Figure 1c). The DCM dye provides efficient fluorescence perceived as reddish-orange colour, localized at the visible part of the spectrum in between ~510–700 nm, which is correct if compared with available spectroscopic data for that chromophore [37,39]. As the second step of the spectroscopic characterization, and by using an advanced laser setup, the stimulated emission (STE) spectra were observed for the final shape of considered dyes, their utilization in PS dust. In all cases, the STE character of emission was confirmed by spectral narrowing upon increased optical pumping and was acquired at the following position: 484.1 nm for PSd/CM504, 532.2 nm for PSd/CM540, and 612.4 nm for PSd/DCM, (Figure 1a–c). Importantly, the full width at half maximum (FWHM) parameter estimated for coumarins was less than 10 nanometres (8.9 and 8.7 nm for CM504 and CM540, accordingly), and less than 20 nm for DCM (FWHM_DCM = 18.3 nm), which is typical for the latter-mentioned dye and was described in the literature before [37,40]. The aforementioned optical output character places that kind of emission as the laser action. More detailed spectroscopic investigation results of the collected signals are presented and discussed in the next paragraphs of the article (Figure 2 and Figure 3).

As the first step of creating the organic lasers based on the polystyrene dust, single-dye systems were obtained (Figure 2). The blue panel A of Figure 2 presents morphological as well as the advanced spectroscopic findings of the PSd/CM504 system. From the top, two photos acquired by the luminescent microscope were shown, the first one (Figure 2a) was achieved using a 375 nm laser line as the light excitation, while the second one (Figure 2b) shows the effect of light enhancement by 450 nm wavelength. Along different shapes and sizes of PS dust, the one with dimensions ~800–1000 × ~600 microns, is shown. It is clearly seen that this type of unit (PSd/CM504) can provide efficient blue and green emission, which seems advantageous for the multicolour light/laser systems. Then, a set of the emission spectra when optical pumping intensity increasing from 0.14 up to 29.6 mJ/cm², is presented (Figure 2c). Rapid signal intensity increase, as well as spectral narrowing, are observed, which are typical features for the laser action [41]. Moreover, the well-seen energy threshold (ρ_th) estimated by two various techniques confirms the nature of the observed emission (laser action). Two approaches were utilized to provide such proof, namely the Light IN/Light OUT (Li-Lo) method and FWHM parameter estimation (both described in detail elsewhere [20]). Briefly, the Li-Lo method relies on calculating the integral, i.e., the area of the lasing band, which is involved with particular pump intensity. Indeed, the spectra intensity and (occasionally) shape evolve with the increasing optical pumping energy density. The stimulated emission or lasing energy threshold is represented by the characteristic inflection point in the graph, and can be defined by two approximating lines. Then, similar assumptions are made by the method based on the measurement of the FWHM parameter. When reaching the pump energy intensity value equal to the lasing threshold, the full width at half maximum parameter value is significantly reduced, and the emission bandwidth becomes narrow. This is observed as a rapid decrease in the plot of the FWHM value according to the pumping energy density. Importantly, both techniques delivered similar values of the ρ_th parameter, which are equal to 5.9 mJ/cm², and 8.8 mJ/cm² for the PSd/CM504 system, respectively. Above the observed energy threshold, the emission changes its nature towards laser action, which is characterized by doubled intensity or even output intensity higher by one order of magnitude in that case as well as achieving an FWHM coefficient lower than 10 nanometres.

The analogous approach was adapted to panels B (green), and C (red), which represent CM540 and DCM laser dyes, respectively (Figure 2). In the case of PSd/CM540 units, the one with similar dimensions was previously found, and it was presented in two excitation modes of the luminescent microscope: emitting in green-blue (Figure 2a) in panel B, λ_ex = 375 nm), and in pure green (Figure 2b) in panel B, λ_ex = 450 nm. The presented PSd unit characterizes the following dimensions: ~1100 × 500–800 microns. Moreover, using the same experimental conditions provided by the nanosecond pulsed laser, the output signal magnitude was collected by a fibre spectrometer and a set of spectra was presented in Figure 2c in panel B. The intensity increased several times. Consequently, using the same theoretical approaches, the energy threshold value was estimated. Importantly, both techniques brought even more similar results of the ρ_th parameter, which are as follows: 4.0 mJ/cm², and 4.5 mJ/cm² from Light IN/Light OUT and FWHM models, respectively (Figure 2d, panel B).

Finally, the last used laser dye (DCM) was investigated and all morphological and spectroscopic insights were presented in the third red panel: C in Figure 2. Both introduced excitation laser lines (375 nm and 450 nm) in the luminescent microscope brought practically the same result, namely, orange-red emission coming from PSd/DCM units (Figure 2a,b in the C panel). The observed unit containing DCM dye characterizes the following dimensions: 900 × 800 microns. Subsequently, upon increasing optical pumping intensity, the emission coming from the PSd/DCM units started to change its nature from spontaneous to stimulated, which was shown in Figure 2c in the C panel. The generated laser action from the PSd/DCM system characterizes quite a broad profile with the FWHM parameter estimated at around 18 microns, which is typical for that particular laser dye and was discussed in literature before [37]. Independently of the abovementioned issue, the used dye provides efficient lasing action placed close to the 615 nm wavelength, which occurs as the reddish-orange colour from the point of view of human eye perception. After all, the energy threshold for laser action provided by the PSd/DCM system was estimated by the same both techniques as mentioned before (Figure 2d in C panel), and delivered the following ρ_th parameter values: 4.2 mJ/cm², and 4.3 mJ/cm², respectively. Once more, both utilized methods are consistent when thinking about the defined parameter and constitute reliable spectroscopic data analysis approaches.

Figure 3 presents dual-colour panels, which indeed reflect the next approach to the experiment of the multicolour lasing based on polystyrene dust, namely, three sets of the implemented pairs of the dyes. By simple two dyes composition, it was possible to achieve significant colour tuning of the laser action measured from the same dyes as before. Moreover, and what is also proven in the next Figure (Figure 4), an easy modulation of the provided multicolour laser action was observed.

Going into detail, the first dye pair was created by polystyrene dust soaked separately in two coumarins: CM504 and CM540 (Figure 3a–c). The microscopic photo, where the excitation laser line was set at 375 nm (the results obtained for λ_ex = 450 nm for all systems are shown in the Electronic Supplementary Information, ESI file, Figure S1) shows green, blue, and blue-green colours provided by the functionalized PSd units (Figure 3a). Moreover, a variety of the polystyrene dust dimensions are also visible, which is shown in the stimulated emission profile nature, where different laser modes are activated (comb-like spectra maxima observed also in another examples of the mixed dyes doping PSd structures, Figure 3b,h and Figure 4c). Then, spectral band narrowing, as well as its significant increase, were observed for the two regions, which come from the two utilized laser dyes, namely CM504 and CM540 (Figure 3b). Moreover, the energy threshold of the created multiband laser system was estimated. The Light IN/Light OUT methodology was implemented, where the values of integrated emission were taken into consideration (Figure 3c). The obtained value of ρ_th parameter (6.4 mJ/cm²) is quite similar to the ones defined before for the separated dyes, which means that even more sophisticated organic systems do not demand any significant additive of the optical pumping energy source to achieve a similar output effect. As the inset of Figure 3c, a macroscopic photo of the PSd/CM504/CM540 system under UV light illumination is provided. Once more blue, green, and blue-green emission was noticed.

The PS dust doped with (separately added) CM504 and DCM laser dyes provided a different emitting system (Figure 3d–f). Based on the photos acquired from the luminescent microscope (Figure 3d; λ_ex = 375 nm), the following collected colours can be listed as orange and blue. Even if in the case the laser action was not achieved by one of the components (PSd/DCM), the created polystyrene dust system provides efficient lasing influenced by the DCM additive in the context of colour tuning, which is better visible in Figure 4e. Nevertheless, the blue component arose rapidly along the optical pump and intensity was growing (Figure 3e). Moreover, its energy threshold (calculated by whole emission band integration) was successfully estimated to be equal to 4.5 mJ/cm² (Figure 3f). The inset provides clear insight into the UV photoinduced output colour coming from the PSd/CM504/DCM system, which is light blue (turquoise) and orange.

The last example of the easily achievable multicolour laser tuning was realized thanks to the combination of Coumarin540 and DCM dyes, introduced separately to the polystyrene dust (Figure 3g–i). In the photograph coming from the luminescence microscope shown in Figure 3g, two colours can be distinguished: green (light and dark) and orange. Interestingly, a spectral investigation of the optically pumped PSd/CM540/DCM system shows significant band narrowing of the two regions, and several times signal intensity increase for both of them, which is presented in Figure 3h. The obtained result shows differences between the systems where two various coumarins (CM504 or CM540) were used to functionalize the PS dust. The emission nature changed, and the latter example sustained its stimulated emission character, such as in the case of the PSd/CM504/CM540 system. Consequently, the estimated energy threshold (ρ_th = 4.1 mJ/cm²) seems to be similar to the previously defined ones, and it is in good agreement when singular dyes are considered for a reliable comparison (Figure 3i). Importantly, such dye combination with the PS dust delivers an interesting output colour configuration, which is shown in the macroscopic photo (inset of Figure 3i). Namely, several colours are easily detected: green (dark and light), orange, and yellow, which significantly enriched features of the multicolour laser system based on the polystyrene dust.

Finally, a composition of the three dyes embedded separately into the polystyrene dust was created and investigated (Figure 4). When looking at the photos acquired from a luminescence microscope, with two applied settings of excitation laser wavelengths (Figure 4a, λ_ex = 375 nm, and Figure 4b, λ_ex = 450 nm), all colours can be distinguished, namely: blue, blue-green, orange, yellow, and green. This means that the possible colour generation from a hybrid system is feasible and easily achievable by a simple collection of variously doped polystyrene dust microparticles. Moreover, the laser nature of the three-colour system is sustained, which is well-seen in Figure 4c,d where a rapid increase in the range of two emission bands is observed, and an energy threshold is well-defined by the same Light IN/Light OUT methodology. Indeed, only the first two laser bands (blue and green) are promoted, which was also proved by spectral deep insight into them and shown in the ESI file (Figure S2). The last one, the most shifted towards the infrared region, is not visible, probably due to the energy consumption by two other dyes, coumarins, for which absorbance bands are shifted more towards the UV region and with higher probability (absorption cross-section coefficient value) to absorb the light. In that mechanism they achieve effectively Boltzmann energy distribution inversion and finally transform it into radiative decay in the way of stimulated emission; however, the highest and hardest to achieve, high Stokes shift, which is present in the DCM dye, and such energy conversion is not achievable. Moreover, in the current circumstances, where energy transfer is limited (PS dust units are separated and held by PVA thin layer), the such explanation seems justified. The inset of Figure 4d shows the great potential of such created hybrid PSd system, which provides in fact multicolour emission under UV illumination. Finally, the CIE diagram is presented in Figure 4e to clarify how the human eye perceives colours achieved by an organic hybrid multicolour laser system. Starting from the points representing emission (laser action) from the single dyes, an appealing colour modulation is shown for the dye pairs (by red, green, and blue arrows), and the triple-dye PSd system (by empty, black contoured arrow). In all cases, a significant and highly efficient modulation is observed. Such achievements indicate the proposed approach as an easy and cheap way to construct a tunable multicolour laser system based on organic material (polystyrene dust).

4. Conclusions

Several differently constructed organic hybrid systems were reported in the article. The first kind of device consisted of the individually embedded dyes (CM504, CM540, or DCM) into the polystyrene dust structures, which characterize productive laser action with well-defined energy thresholds ranging between 4.0 and 8.8 mJ/cm² for each dye. Dual-dye equipped PSd microparticles constitute the second type of the investigated organic hybrid system, which allowed not only for effective multicolour laser action emission, but also efficient colour shade tuning, which was achieved easily by applying various optical pump intensities. Finally, the RGB polystyrene dust system delivers effective multimode lasing emission, which chromaticity can be easily tuned by optical pumping range, as postulated for the dual-colour devices. By playing with the dimensional and shape parameters it was possible to design and construct a versatile organic-based laser system, which characterizes various and mixed colour emission, and the easy way of fabrication. Such device realization faces current requirements for technologically advanced and simple solutions, which provide efficient and advanced features, such as multispectral light emitters (in here, even lasers), precise colour adjustment availability, and bio-friendly character. The postulated systems can be successfully implemented into various lighting, light amplification, or sensing systems.