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Abstract
We have reported a broadly tunable and dual-wavelength polarity amplified Nile red laser system. Combined with the method of traditional grating tuning, the broadband wavelength tuning could be realized by changing the polarity of the solvents in which Nile red dissolved, which would extend the family of organic dyes used as laser gain medium and play an important role in laser display technology as laser source. Then the dual-wavelength emission was obtained by pumping two distinct different dye solutions in series. The realization of the dual-wavelength lasing would open up an avenue to detection amplification technology, which also provided an alternative way to analyze trace amount of substances.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since the advent of lasers in 1960, the tunable laser has been always an important part of laser research. The core of tunable laser is tunable laser medium with broadband energy level structure. The most widely used tunable laser medium is organic dye. By choosing different types of laser dye, the output laser wavelength could be covered from near-ultraviolet, visible light to near-infrared [1,2]. After adding wavelength selection components in laser oscillation cavity, tunable narrow-band dye laser output could be achieved. Broadly tunable visible laser sources are needed for many applications, such as spectroscopy [3–5], or bio-chemo sensing [6–8]. Traditional tunable dye lasers are in the form of liquid state and use typical dispersion elements, such as gratings and Fabry-Perot etalons [9,10] to obtain the wavelength tuning of output laser. This method is simple to operate and has a wide tuning range, but it still has some problems, such as ignoring part of the wavelength range and needing precise operation, which could all limit the applications of dye laser in many areas. On the other hand, Lin et al. [11,12] studied the dye-doped cholesteric liquid crystals (DD-CLCs) and dye doped heavy chiral concentration liquid crystals (DD-HCCLCs), and obtained over 100 nm tuning output in visible range. Furthermore, dual-wavelength laser output could bring new ideas in micromeasurement, terahertz source, blood tests and medical treatments, etc. which has been achieved by different methods [13,14], and the most commonly used way is pumping two dye solutions in parallel [15]. Other scientists adopted grating tuning technology to achieve multi-wavelength output [16,17], but this needed sophisticated design and fine adjustments.
Whether an organic material is suitable for laser materials is determined by its energy level structure and physical/chemical characteristics [18]. Currently the most frequently used laser dye are Xanthene (Rhodamine and Fluorescein), Coumarin and Pyrromethene, etc. These laser dyes have high fluorescence (FL) quantum efficiency, and cover nearly most of the visible part of the spectrum. For example, F. J. Duarte et al. [19] adopted grating tuning method and obtained widely tunable green laser emission using Coumarin 545 tetramethyl dye as laser gain medium. However this method didn’t have the ability to be tuned without dispersion element for a certain dye.
In this letter, we proposed and demonstrated the realization of a broadly tunable and dual-wavelength polarity amplified Nile Red (NR) laser system. NR is a liposoluble dye which has good solubility in lipid organic solvents, such as ethanol, acetone and ethyl ether while there is almost no FL if dissolved in water [20,21]. In view of this polarity response characteristic, namely solvatochromism, we used 532 nm pulse pumping source to excite NR dissolved in different solvents and achieved output wavelength tuning by changing the polarity of the solution system and adding simple dispersion element. Moreover, dual-wavelength dye laser output could be achieved by pumping a tandem cuvette containing different dye solutions, and this method would find widespread applications in micromeasurement and extension of tuning range.
2. FL and laser spectra of NR solvents
NR, one of the most frequently mentioned dyes belonging to benzophenoxazine family, is a well-known lipophilic solvatochromic compound whose FL is significantly influenced by the polarity of its environment, which means the emission color varied with organic solvents and hydrophobic lipids drastically. The solvatochromism characteristic was presented in Table 1. The FL given blow was taken by dissolving NR in some common organic solvents. As Table 1 showed, the FL hypsochromically shifted from 634 nm in ethanol to 584 nm in ether, indicating its high sensitivity to polarity, which offered possibility to tunable laser.
Table 1. FL spectra of Nile red in different solvents
Schematic illustration of the laser setup was shown in Fig. 1. The second harmonic generation (SHG) of Q-switched Nd: YAG laser operating at 1064 nm with repetition rate of 10 Hz and pulse duration of 10 ns was used as the pump source. A simple laser oscillator cavity consisting of two plane mirrors with liquid-state laser dye as gain medium was designed to measure lasing spectra and slope efficiency. The cavity mirror was a dichroic mirror with high transmission (T > 95%) at 532 nm and high reflectance (R > 95%) from 560 nm to 700 nm, while the output coupler had partial reflectance (R = 85%) from 400 nm to 700 nm. NR solutions were put into a quartz cuvette with 5 mm optical length. The output spectrum was detected by a spectrometer (Ocean Optics QE65000) with resolution of 1 nm. The laser behavior was verified initially by pumping NR /ethanol (60 µg/mL). The blue line in Fig. 2(a) indicated that the broadband FL spectrum was centered at 642 nm. The input-output curve and full width at half maximum (FWHM) of the output spectrum were plotted in Fig. 2(d), which exhibited a threshold of 3.1 mJ, and a slope efficiency of 1.05% could be calculated from $\eta = \frac{{{E_{out}}}}{{{E_{in}}}}$, where ${E_{out}}$ and ${E_{in}}$ represented the output and the input energy. As the pump energy exceeded the threshold value, the FWHM would narrow from 46 nm to 6 nm dramatically. In view of the slope efficiency curve and the spectral narrowing, the generation of laser output in our system could be verified. The high loss of the plane-parallel optical cavity structure, the reflectivity of cavity mirrors, and the cavity length offered measurable influence on the slope efficiency here.
Fig. 1. Experimental setup. The half-wave plane and polarizer served as the controller of the pumping energy; KTP crystal served as the frequency doubling crystal. 10% of the pumping laser was transmitted into the cavity by the beam splitter as a direct pumping source, 90% of which was reflected to the energy meter. The dye solution was put in a quartz cuvette with an optical path of 5 mm; the cavity length was 50 mm.
Fig. 2. FL/Laser spectrum of NR in (a) ethanol, (b) acetone and (c) ethyl ether. (d) Slope efficiency curve and FWHM of output spectrum of NR/ethanol (60 µg/mL).
After the lasing behavior of NR/ethanol was observed, several other organic solvents, acetone and ethyl ether specifically, were further selected to verify the lasing characteristic of NR. As shown in Figs. 2(b) and 2(c), the laser emission peak of NR/acetone and NR/ethyl ether was observed at 620 nm and 608 nm, respectively. According to this, the good laser characteristic of NR in these organic solvents was revealed in our experiments.
3. Solvent-tuning and Dual-wavelength experiments
As mentioned previously, one common way of extending the tuning range in dye laser is adding several external optical devices, such as Fabry-Perot etalons, diffraction gratings, dispersing prisms, etc. These conventional methods may be convenient and easy to implement, while broad tuning range and dual-wavelength laser could be awkward. On account of the former shortage, we put forward a dye system of NR dissloveed in mixed solvents. Considering the specific and fixed polarity of each solvent, we mixed different solvents and by changing their concentration ratio, the polarity of the whole solvent system could be adjusted continuously. Figure 3(a) showed the tuning range of NR laser in mixed solvents of ethanol and ethyl ether at fixed concentration of 50 µg/mL. The volume ratio of two solvents (labelled on Z-axis) was set from 10:0 to 0:10 (0 means there was no corresponding solvent in this solution system). It provided legible graph of tuning spectrum in mixed solvents system which covered the area of 603 nm∼643 nm. The headmost and backmost row in Fig. 3(a) represented the laser spectrum of NR/ethyl ether (50 µg/mL) or NR/ethanol (50 µg/mL), respectively. To expand the tuning range, a blazed grating was adopted, and the laser configuration was described in Fig. 3(c).
Fig. 3. Tuning spectra of NR in mixed solvents using (a) polarity tuning method and (b) polarity tuning method plus grating tuning method. (c) Solvent tuning dye laser configuration adding traditional grating tuning method. The blazed wavelength of the grating (1200 grooves/mm) was 500 nm. KTP crystal served as the frequency doubling crystal.
The grating was a reflective blazed grating with size of 12.5 mm $\times $ 12.5 mm and 1200 grooves per mm. It was placed after the dye cuvette and reflected the first-order diffraction light of the incident laser while the zero-order diffraction laser acted as the laser output. By subtly adjusting the reflection angle of the grating, we could tune the wavelength of the output laser, and consequently a broadband tunable NR laser output was achieved in our system, as illustrated in Fig. 3(b). By rotating the grating for the purpose of changing incident angle of pumping laser in pure ethyl ether and ethanol solvents, the tuning floor can be dragged to 582 nm and the ceiling to 660 nm. In conclusion, on account of the lasing characteristic of NR in different solvents, we finally achieved a broadband tunable NR laser output in our system via polarity tuning which meant changing the concentration ratio of two organic solvents, and grating tuning to expand the tuning range. Comparing with previous work [9,10], this method of tuning dye laser output wavelength was innovative as it provided an additional novel approach of wavelength tuning in dye laser system. Moreover its tuning range covered almost all of the orange-red area, which would have great use in extending the color gamut of laser projection display technology. Laser projection display technology (LDT), also known as laser projection technology or laser display technology, is a display technology with red, green and blue (RGB) three-color laser as the light source, which could most realistically reproduce the rich and gorgeous colors of the objective world and provide more shocking exhibition. The broadly tunable NR laser based on solvatochromism and grating tuning could generate laser output with a spectral width of about 6∼10 nm, which effectively eliminated laser speckle caused by high coherence of laser. In addition, the optimum wavelength of red laser source could be selected via the tuning methods in this NR laser system. In a word, this broadly tunable NR laser system would play an important role in LDT.
After achieving the broadly tunable NR laser in our system, we began to turn our attention to the dual-wavelength laser output. As mentioned above, NR has the characteristic of solvatochromism. Martinez V et al. [22,23] elucidated the basic conception of solvatochromism and explained the factors that influenced this characteristic of NR, including polarity, temperature, pH, etc. In addition to this, the concentration of NR in organic solvents also showed the effect to its emitting spectrum, owing to the self-absorption of dye molecules. The trend was that increasing concentration caused the FL/laser spectrum peak to shift bathochromically, while decreasing concentration caused the spectrum peak to shift hypsochromically. This was verified in our experiment, as shown in Fig. 4(a), that different concentrations of NR/ethanol could lead to small shifting of its lasing peak. In ethanol, when the concentration increased from 10 µg/mL to 120 µg/mL, the lasing peak moved from 639.3 nm to 642.8 nm, covering about 3 nm tuning range. Therefore the potential of concentration tuning of NR was revealed although its range was narrow. For the sake of magnifying this tuning characteristic, we put forward a new concept called “Ratiometric laser” which was derived from “Ratiometric fluorescence”. As dual-wavelength emitting ratiometric fluorescence probe could be used as visual detection of some certain ions, such as Hg2+ [24], as a result of that one of its FL intensity was highly sensitive to the environment (like pH, temperature, polarity, etc.) while the other one remained stable regardless of the environmental changing. In view of above, the amount that needed to be detected in the environment could be calibrated by calculating the ratio of two peaks’ intensity. Considering this, we have implemented “Ratiometric laser” that was similar to “Ratiometric fluorescence” in our system.
Fig. 4. (a) Laser spectra of NR in single ethanol solvent under different concentrations (µg/mL). (b) Experimental setup of tandem cuvette system with R6G/ethanol and NR/ethanol. (c) Laser spectra of “Ratiometric laser” under different concentrations (µg/mL) of NR/ethanol. (d) Comparison of wavelength shifting range under different concentrations between single solution and dual solution systems.
As illustrated in Fig. 4(b), the laser setup was made of two dye solutions pumped by 532 nm laser in series here. The chosen dyes, Rhodamine 6G (R6G) and NR, were dissolved in ethanol respectively and put in a tandem cuvette. The tandem cuvette was made of two quartz cuvettes by gluing together, and R6G/ethanol was put in the former while NR/ethanol in the latter. R6G/ethanol was suitable in this system because of its high FL intensity under 532 nm pump and large distance of its laser peak (at 565 nm) away from NR/ethanol. The initial concentration of R6G and NR were 100 µg/mL and 120 µg/mL, respectively. By changing the concentration of NR/ethanol, we’ve obtained spectra of dual-wavelength with relative laser intensity. The laser peak of R6G was at about 565 nm which remained nearly unchanged as the concentration of NR changed, and its laser intensity was normalized to calibrate the laser wavelength and relative intensity of NR/ethanol, as shown in Fig. 4(c). By gradually decreasing the concentration of NR from 120 µg/mL to 10 µg/mL, the laser spectrum of NR/ethanol shifted from 645 nm to 610 nm, along with the decreasing laser intensity. That meant a certain concentration of NR solution corresponded to certain position and certain intensity of its laser spectrum in this dual-wavelength system, which could be defined as “Ratiometric laser”. Furthermore, the range of wavelength tuning by changing the concentration was very narrow in traditional single-dye or single-solvent system as stated earlier (nearly 3 nm tuning range by decreasing the concentration from 120 µg/mL to 10 µg/mL), while in this tandem cuvette system the tuning range of NR laser could be expanded to 35 nm under the same condition. This polarity amplification characteristic was revealed explicitly in Fig. 4(d). Besides, the ethanol in the latter part of the tandem cuvette which was used to dissolve NR could be substituted by other organic solvents, such as acetone, ethyl ether, etc. These solvents in this dual-wavelength system also showed similar characteristic of concentration tuning of NR laser, which provided wide applications of polarity amplification. In conclusion, this dual-wavelength laser output could be used to detect the concentration of NR/ethanol in this tandem cuvette system, given that the corresponding relation was provided. In addition, expanding the tuning range of NR laser in organic solvents was also achieved in this way, only by changing the concentration of NR in ethanol or other solvents, based on the characteristic of polarity amplification of NR.
4. Conclusions
In summary, we have proposed and demonstrated a broadly tunable and dual-wavelength polarity amplified NR laser. According to its solvatochromism property, we have achieved laser output in broadband tuning range (582 nm∼660 nm) by means of rotating the grating and changing the concentration ratio of hybrid solvents. This exploited a new method of wavelength tuning in dye laser system, and could be used as the laser source of laser display technology (LDT) for the purpose of expanding the color gamut and eliminating the speckle phenomenon. In addition to this, we provided a tandem cuvette system made of NR/ethanol and Rhodamine 6G/ethanol, and pumped in series to verify the characteristic of newly proposed “Ratiometric laser”. This polarity-based amplification could magnify the tuning range from 3 nm to 35 nm under the same condition, which would find applications in micromeasurement and expanding the tuning range of organic dye lasers. This kind of broadly tunable and dual-wavelength polarity amplified NR laser was likely to open up new ideas of achieving high-efficiency and wide-tuning-range laser output using organic dyes as gain medium.
Funding
National Natural Science Foundation of China (NSFC) (21574120, 21774115); the Basic Research Fund for the Central Universities (WK2060200025); the Science and Technological Fund of Anhui Province for Outstanding Youth (1608085J01); National Key Research and Development program of China (2016YFB0401901); Major science and technology special project in Anhui (17030901001).
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