Tuning Multi-wavelength Lasing in Rhodamine 640/Methylene Blue/Ag NWs Random Lasers

doi:10.21203/rs.3.rs-3767599/v1

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Tuning Multi-wavelength Lasing in Rhodamine 640/Methylene Blue/Ag NWs Random Lasers

https://doi.org/10.21203/rs.3.rs-3767599/v1

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This research investigates a type of random laser with coherent properties, designed and studied to understand how fluorescence resonance energy transfer affects the emission spectra and threshold of Rhodamine 640/methylene blue/Ag NWs lasers. We demonstrate that Rhodamine 640 enhances the laser output of methylene blue beyond 700 nm. Additionally, by adjusting the mixing ratio of the two dyes, we achieved multi-wavelength lasing with linewidths as narrow as 1 nm. Ag NWs serve as scatterers in this system, both enhancing light extraction efficiency in the visible range and lowering the operating threshold for multi-wavelength lasing.

Energy transfer

random lasing

silver nanowires

Rhodamine-640

Methylene blue

Energy transfer in a mixture of dyes has proven to be highly effective in several aspects of laser technology. By combining different dyes, we can extend the lasing wavelength, achieve wavelength tunability, increase emission wavelength range, enhance random laser power, and reduce the required pump power. This energy transfer process is characterized by the quenching of donor emission, increased emission of acceptor excited by the donor, reduced donor photobleaching rate, and decreased donor lifetime [1, 2].

In random lasers, organic dyes like rhodamine are commonly used due to their broad absorption spectra, high gain, and compatibility with available pump wavelengths, such as 532 nm. However, dyes like methylene blue and Nile blue are not compatible with blue and green pump light. To address this limitation, we combine multiple dyes in a single laser to extend the emission to near-infrared wavelengths and achieve multi-wavelength random laser operation through energy transfer. Efficient energy transfer between two dyes occurs when their spectral profiles overlap effectively [3, 4]. When the donor dye is excited, it transfers its excitation energy to the acceptor dye through a non-radiative process. This generates an oscillating dipole that resonates with an acceptor dipole in the near field [5].

There is a high demand for low-cost, multi-wavelength coherent laser sources in micro-optoelectronics and imaging applications [6, 7]. Random lasers offer promising solutions in this field. While significant effort has been devoted to developing low-threshold random lasers with single emission, only a few reports exist on multi-color random lasing systems induced by a single excitation. These systems typically exhibit incoherent emissions and lack the necessary coherent properties for certain applications [8–10].

The development of simultaneous multi-wavelength coherent random lasing faces two major challenges. The first challenge is finding a dye system that exhibits two or more emission bands with easily tunable wavelengths. To overcome this, we utilize the resonance energy transfer (RET) process between Rhodamine 640 and methylene blue. The absorption band of methylene blue partially overlaps with the emission band of Rhodamine 640, and the average distance between the donor and acceptor should not be smaller than the Förster distance to achieve moderate energy transfer efficiency [11, 12].

The second challenge involves selecting nanostructures as scatterers that possess a broadband plasmonic resonance (PR) spectrum and provide strong local electromagnetic field (LEF) enhancement at different wavelengths. Silver nanowires (NWs) are chosen for this purpose due to their broadband PR spectrum across the visible range and numerous nanogaps that introduce significant LEF enhancement factors at different wavelengths [21].

Stern-Volmer plots are useful for determining radiative and non-radiative rate constants [22, 23]. They provide insights into the efficiency of these processes in a system and are valuable for optimizing optoelectronic devices and understanding photophysical properties.

$$\begin{array}{c}\frac{{I}_{D}}{{I}_{DA}}=1+{\mathbf{K}}_{\mathbf{S}\mathbf{V}}\left[A\right]-----------------\left(1\right)\\ \frac{{{\varnothing}}_{\text{D}}}{{{\varnothing}}_{\text{D}\text{A}}}=1+{\mathbf{K}}_{\mathbf{E}\mathbf{T}}{\tau }_{\mathbf{D}}\left[\mathbf{A}\right] ----------------\left(2\right) \end{array}$$

By referring to equations (1) and (2), we can derive the following expressions:

$${\mathbf{K}}_{\mathbf{S}\mathbf{V}}={\mathbf{K}}_{\mathbf{E}\mathbf{T}}{\varvec{\tau }}_{\mathbf{D} }------------------\left(3\right)$$

The relationships can be expressed as follows, based on equations (1) and (2):

I_D and I_DA represent the fluorescence intensities of the donor in the absence and presence of the acceptor, respectively. KSV denotes the total transfer rate constant, while KET represents the nonradiative transfer rate constant. [A] refers to the concentration of the acceptor, and φD and φDA correspond to the quantum yields of the donor in the absence and presence of the acceptor, respectively. Additionally, τ_D signifies the fluorescence lifetime of the donor without the acceptor, which can be calculated using Bowen and Woks' relation [24, 25].

$$\frac{1}{{\tau }_{\text{D}}}=2.88\times {10}^{-9}\times {n}^{2}\times \left({\stackrel{-}{v}}^{2}\right)\int \epsilon \left(\stackrel{-}{v}\right)d\stackrel{-}{v}----------\left(4\right)$$

Here, the area under the curve of the molar extinction coefficient of Rh-640 versus wave number is denoted as ʃ Є(ν) d(ν). The refractive index is represented by n, and νo corresponds to the peak absorption spectrum value in cm^-1. By applying these values in the equation, we can calculate the critical transfer distance (Ro) between the excited donor and acceptor dye molecules. The critical transfer distance (Ro) between the excited donor and acceptor dye molecules can be determined using the following equation [26]:

$${\text{R}}^{0}=\frac{7.35}{{\left([\text{A}{]}_{1/2}\right)}^{1/3}}-------------------\left(5\right)$$

Where [A]_1/2 is the half quenching concentration which can be obtained under the condition [27, 28]:

$${I}_{DA }=\frac{{I}_{D}}{2}----------------------\left(6\right)$$

As it is widely recognized, in the presence of an acceptor, the fluorescence intensity of the donor is diminished from I_D to I_DA due to energy transfer to the acceptor. The total transfer efficiency, denoted as E, can be determined using the following equation [29]:

$$E=1-\frac{{I}_{DA}}{{I}_{D}}--------------------\left(7\right)$$

Ag nanowires (length: 50 microns, diameter: 50 nm) were purchased from the German PlasmaChem Company. Rhodamine 640 (Rh-640) and Methylene blue (MB) were obtained from Sigma Aldrich. The silver nanowires were dissolved in methanol by continuously stirring the solution for 30 minutes, resulting in a concentration of approximately 9 × 10¹⁷ cm³. It is important to note that no surfactant was added to the samples to preserve energy transfer efficiency. The concentration of Rh-640 and MB dyes was fixed at 1 × 10^–5 M and 1 × 10^–3 M, respectively.

To examine the energy transfer between the two dyes, three different samples were prepared by mixing Rh-640 and MB dyes in different ratios: S1 (1:1), S2 (1:2), and S3 (1:5). Additionally, three other samples were prepared to investigate random laser behavior by mixing fixed ratios of Rh-640/MB with varying concentrations of Ag nanowires.

During the experiments, each sample was placed in a glass cuvette with a 10 mm inner diameter, 1 mm thickness, and 45 mm height. They were illuminated by a pulsed Nd:YAG laser operating at second harmonic generation (SHG) with a wavelength of 532 nm to excite the Rh-640 dye solutions. The laser beam was directed toward the center of the cell. The emissions from the samples were collected using a fiber-coupled spectrometer (Ocean Optics USB2000 + UV-VS-ES) with a spectral resolution of 0.3 nm. The fiber probe was positioned at a fixed distance (typically 2 cm) and 30⁰ from the center axis of the cell using a rotatable arm. The pump laser power incident on the sample was controlled using an appropriate neutral density filter (NDF).

1.1 Absorption and fluorescence spectra of the donors and acceptors are analyzed.

Figure (2) shows the spectral overlapping area between the absorption and fluorescence emissions of the donor (Rh640) and acceptor (MB) spectra. It reveals a broad absorption and fluorescence emission band, with the fluorescence spectra of Rh 640 overlapping with the absorption spectra of MB. This suggests that energy can be transferred between these two dyes in both radiative and nonradiative ways.

Figure (2) shows the absorption spectra of Rh 640 (black curve) and MB (blue curve), as well as the fluorescence spectra of Rh 640 (red curve) and methylene blue (pink curve). The shaded area represents the spectral overlap.

1.2 Characteristics of the Silver Nanoparticles

Figures 3(a) and 3(b) illustrate the absorption spectrum of Ag nanowires dissolved in methanol at a concentration of (1.1× 10¹¹ cm^-3), both without and with Rh640/MB dye (at concentrations of 1×10^− 5 and 2×10^− 4, respectively). In Fig. 3(a), two absorption bands corresponding to the longitudinal and transverse Plasmon bands are visible. The longitudinal Plasmon band exhibits a center wavelength of 380 nm, while the transverse Plasmon band has a center wavelength of 348.8 nm.

1.3: The relationship between transfer efficiency and acceptor concentration

This section demonstrates the function of energy transfer in a binary dye mixture, utilizing Rh640 as the donor and MB as the acceptor, with an excitation wavelength of 532 nm. Figure 5(a-c) displays the fluorescence spectra of the dye mixture, consisting of Rh-640 (at a concentration of 1.65 x 10^− 3 M) and MB (at a concentration of 4.5 x 10^− 3 M), with different dye ratios: a-1:1, b-1:2, and c-1:5.

The efficiency of the Rh640 dye is significantly higher than that of MB, and it exhibits good absorbance at the excited wavelength of 532 nm. In contrast, the MB dye's absorbance at this wavelength can be neglected. Thus, the Rh-640 dye satisfies the requirements for the energy transfer process.

In the dye mixture with a ratio of 1:2, we observed the maximum intensity of Rh-640 and a reduced intensity of MB, indicating the occurrence of self-reabsorption at this ratio. This result is in agreement with reference [2]. In the dye mixture with a ratio of 1:5, we observed an improvement in the intensity spectra of MB, indicating efficient energy transfer in the mixture.

Finally, it should be noted that at the mixture ratio of 1:1, there are bleaching effects on the intensity of both Rh-640 and MB. The energy transfer from the 1:1 ratio is effective in providing dual-wavelength emission.

Figure (6) shows the I_D/I_DA ratio in relation to various MB dye acceptor concentrations. To calculate the Stern-Volmer constant, compare the fluorescence intensity of the donor (Rh-640) in the presence and absence of the acceptor (MB dye), or calculate the slope of the best-fit straight lines in the figure.

Equations (1, 3, and 5) are utilized to calculate the values of [A]_1/2, Ro, and KET, which are listed alongside the KSV value in Table (1). The radiative lifetime (τD) of Rh-640 at a concentration of 1 x 10^-5M was determined using Eq. (4) and found to be approximately 3.7 ns. The ∅_D/∅_DA, representing the quantum yields of the donor in the absence and presence of the acceptor, as well as the energy transfer (E), were calculated at varying concentrations of the acceptor dye (MB). Equations (2 and 6) were employed for these calculations.

Table (1) shows the energy transfer parameters for the Rh-640:MB dyes.

Donor Dye- Rh-640 - concentration ( 1×10^− 5 M)
K_SV (M^− 1)	[A]_1/2 (M)	R_o (A^o)	K_ET (M^− 1 Sec^− 1)	${\tau }_{D}$( ns)
4944	0.22×10^− 3	120	1.336×10¹²	3.7
Accepter Dye –MB- Concentration (mM)					$\raisebox{1ex}{${\varnothing }_{D}$}\!\left/ \!\raisebox{-1ex}{${\varnothing }_{DA}$}\right.$	E
0.2					2	0.73
0.4					3	0.77
0.6					4	0.8
0.8					5	0.83
1					6	0.85

According to the studies, it has been observed that for a fixed concentration of the donor molecule (Rh-640), the energy transfer efficiency exhibits an increasing trend with the concentration of the acceptor molecule (MB). As the acceptor concentration increases, the energy transfer efficiency approaches a value of 0.9. This indicates that at higher acceptor concentrations, the energy transfer mechanism becomes the dominant mechanism in the system.

These findings are in agreement with previous studies conducted by researchers such as Baha T. Chiad et al. [1] and Wan Zakiah Wan Ismail et al. [4]. Their research also demonstrated that as the acceptor concentration rises, the energy transfer efficiency rises accordingly. This consistency across multiple studies strengthens the understanding of the energy transfer process and supports the validity of the observed trends.

1.5 Dual emission in Rh 640/MB random laser

Figures (7, 8, and 9) clearly show two intense spectral bands associated with Rh-640 and MB, which narrow down at different pump energies. Figure 7(a) displays the emission spectra of a dye solution without Ag NWs, while Fig. 7(b) shows a comparison where each peak in Fig. 7(a) appears wider compared to the peaks in Fig. 7(b).

Using a single-wavelength 532 nm pulsed laser beam, the Rh-640 dye is excited first, resulting in two peaks with center wavelengths of 580 nm and 700 nm, respectively. The excited spectra of Rh-640 exhibit a wide linewidth of approximately 42 nm, which is necessary for the excitation of the MB dye. This indicates that the energy transfer from the Rh-640 donor dye is sufficient to support random lasing from the MB acceptor dye, with the residual energy in Rh-640 being enhanced by scattering feedback. This phenomenon is also indicative of Rh-640 dye-stimulated emission. The linewidth of each peak is less than 1 nm, with the number of peaks approaching 20. These characteristics, including the linewidth and number of peaks appearing on the spectrum, indicate the typical coherent lasing behavior of a random laser.

It is observed that the effective concentration of Ag NWs for the emission from both dyes is not similar in terms of wavelength tunability. The donor Rh-640 experiences a blue shift range of (570–598) nm, while the acceptor MB exhibits a red shift range of (700–720) nm, influenced by increased birefringence due to optical elongation. However, both dyes show similar narrowing of the linewidth and number of peaks, as depicted in Fig. 8. The coherent emission spectra of the random laser, efficiently transmitted from the donor (Rh-640) to the acceptor (MB), are displayed in Fig. 9.

Due to the majority of the absorbed energy by the Rh-640 dye being transmitted to the MB dye, the emission spectra at 580 nm are significantly reduced (approximately 6 times lower compared to the 1 mM concentration of MB). This case involving two dyes with scatterers aims to create a single wavelength emission.

We also observed the effect of energy transfer on the lasing threshold. Increasing the acceptor concentration results in greater gain for lasing, leading to a reduced lasing threshold. Figures (10) and (11) illustrate the lasing thresholds of 0.7 mJ for the 1:1 ratio combination and 0.6 mJ for the 1:5 ratio combination, respectively.

Due to the spectral overlap between the absorption and fluorescence bands, the intensity of the fluorescence emission decreases gradually with increasing concentration of methyl bromide. It has been observed that the efficiency of Förster Resonance Energy Transfer (FRET) can be controlled by adjusting the donor-acceptor ratio. Furthermore, the combination of these two dyes leads to the generation of random laser emission with wavelengths exceeding 700 nm.

Introducing scatterers, such as Ag nanowires (Ag Nws), proves to be advantageous for achieving the laser threshold, enhancing emission laser linewidth intensities, improving contrast wavelengths, and providing positive feedback in the system.

Conflict of interest:

There is no any conflict of interest.

Author Contribution

The experiment design and planning was done by authors 1 and 2, sample preparation was done by authors 3 and 4, authors 1 and 3 carried out the experiment, and authors 3 produced the text with assistance from authors 1 and 2.The results were discussed and the manuscript was reviewed by all authors.

Acknowledgement:

The authors acknowledge Asst. Prof. ahmed sharba from Department of Laser Physics, College of Science for Women, University of Babylon, Hillah, Iraq for helpful advices given in the course of discussion in this work.

ORCID iDs

https://orcid.org/0000-0003-3806-3675

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No competing interests reported.

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Tuning Multi-wavelength Lasing in Rhodamine 640/Methylene Blue/Ag NWs Random Lasers

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Abstract

Figures

1- Introduction

2- Theoretical Part

Experiments

Result and discussions

1.1 Absorption and fluorescence spectra of the donors and acceptors are analyzed.

1.2 Characteristics of the Silver Nanoparticles

1.3: The relationship between transfer efficiency and acceptor concentration

1.5 Dual emission in Rh 640/MB random laser

Conclusions

Declarations

Conflict of interest:

Author Contribution

Acknowledgement:

References

Additional Declarations

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