Mouse Tissue imaging by random laser of Plasmonic two dimensional array

doi:10.21203/rs.3.rs-4921013/v1

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Mouse Tissue imaging by random laser of Plasmonic two dimensional array

https://doi.org/10.21203/rs.3.rs-4921013/v1

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Given the major applications of high resolution, and non-destructive bio-imaging, plasmonic waveguide assisted imaging system based on the random lasing is proposed here by helping micro ring arrays covered with the gold/ graphene layer and Rh6G dye. In order to achieve this objective, we employ a laser writing system to create micro ring arrays and subsequently cover them with a plasmonic gold thin film using a sputtering machine. Additionally, the chemical vapor deposition method is employed to generate the graphene layer. To use this medium as random laser active media, we cover it by Rh6G dye and PVP polymer as the top high index layer to get more localization of light. After theoretically and experimentally evaluating the plasmonic substrate, we use second harmonic generation of Nd: YAG laser as the source, and record the random lasing of the sample under 45 degree via spectrometer. Our results show the samples without PVP layer yield a coherent random laser with about 6 nm and 1.4 nm in the full width at half maximum (FWHM), threshold energy 3.17 mJ and 1.42 mJ for concentrations 10 − 5 and 10 − 4, respectively. While finding the laser threshold and FWHM are decreased by the sample with PVP layer reach from 2.62mJ and 5 nm to 1.95 mJ and 1.2 nm, respectively. This corresponded to the simulation part, in which PVP layer enhanced the field amplitude significantly. Finally, we record the images of mouse tissue by the CCD camera. These findings provided a simple and efficient way for the realization of low-threshold random lasers at low cost.

Physical sciences/Optics and photonics

Physical sciences/Physics/Optical physics

random laser

Plasmonic waveguide

laser writing

imaging

Plasmonic nanostructures in random lasers (RL) are an emerging field of research and interesting in recent years that combines the principles of plasmonics and random lasing and their applications. Plasmonics requests the interaction between electromagnetic waves and metallic nanostructures[1–3], known as plasmons, which can be confined to the nanostructures' surface and manipulate electromagnetic waves at the nanoscale in the active gain media to increase the local electromagnetic field intensity which leads to improving the efficiency of the random laser emission[4, 5].

In tissue imaging, plasmonic nanostructure random lasers can provide high-resolution, and sensitive imaging capabilities [6, 7]. These plasmonic nanostructures can be engineered to optimize the random laser emission and subsequently interact with specific biological molecules or structures within the tissue, thereby enabling targeted imaging and analysis [8, 9]. This enables the researchers to obtain detailed information about the cellular and molecular composition of the tissue, facilitating a better understanding of biological processes and disease mechanisms [10–13].

Incorporating random lasers into plasmonic waveguide nanostructures provides numerous opportunities for nanostructure development and applications [14, 15]. The combination of efficient propagation of electromagnetic waves, enhanced interaction with gain materials, and control of emission properties makes these nanostructures a promising platform for developing high-performance random lasers with customized functionality [16–19]. The dimensions are typically in the nanometer range, which enables intense light confinement and interaction with random laser material [20]. This can provide a variety of benefits, such as the enhancement of the random laser's emission spectrum [21–23]. This could be particularly useful for on-chip integrated photonics or nano-imaging applications, and enhance the interaction inside active media, leading to increased lasing efficiency, and reduced threshold. The confined light within the structure can efficiently excite the gain medium, resulting in an enhanced transition from incoherent to coherent random laser [24–27].

The work aimed to develop a novel imaging technique that utilizes plasmonic random lasers for high-resolution and sensitive imaging of tissues and focuses on leveraging the unique properties of plasmonic nanostructures and random lasing emission to enhance the imaging capabilities in biological samples using four different gain medium, two of the samples using different dye concentration (10^− 4 and 10^− 5 M) on the micro-rings array based on SU-8 material arranged on a glass substrate covered by 35 nm gold thin film by helping DC sputtering machine to active the thermo-plasmonics in each micro-cavity position after that covered with graphene layer which was fabricated by the chemical vapor deposition (CVD) method. Another two samples with the same mechanism, but covered with Polyvinylpyrrolidone (PVP) polymer on the dye laser to get waveguide structures.

A two-dimensional micro cavity array (2D-MCA) was prepared via direct laser writing experimental setup using 405 nm laser beam. The objective lens with a numerical aperture of 0.8 expanded and transferred the laser beam to the silica substrate. The array of microcavities was written onto the polymer after the substrate, which was coated with SU8 2002 polymer, was positioned on the motorized x-y stage. The laser power was set to 0.3 mW with the arrays consisting of 500 in 20 rings with width and radius set to 1 µm and 1.5 µm respectively. The gap between two rings was selected as 500 nm and the periodicity of the structure was assigned to 20 µm. After writing completion, the substances were covered with 35 nm gold thin film by helping DC sputtering machine. Next, the sample was covered with graphene layer which was fabricated via chemical vapor deposition (CVD) method as shown in the figure (1).

To get efficient RL and use of this emission as the main light in tissue imaging, we fabricate four different gain medium based on 2D-MCA which covers by Gold/Graphene layer, Rhodamine 6G (by 10^-4 and 10^-5 M) and PVP respectively as shown in the Figure (1 c).

The prepared samples were tested using the second harmonic of Nd: YAG laser to excite the gain medium. At the same time, the detector is positioned at a 45° angle in the direction of the pump beam. The emission spectrum was recorded and analyzed by the spectrophotometer software on the computer as shown schematically in Fig. 1(d).

In a parallel way, 200 micrometers of mouse tissue were cut and fixed onto the glass substrate and put in the other direction from the output of the RL and at the same angle value as the detector position (45°) from two-dimensional micro cavity array gain media. During the imaging process, the RL recorded in varied pumping energies, and the primary comparison tools were the three extracted colors from each picture captured by the CCD camera. Besides, we should simulate light localization in terms of PVP top layer based on finite difference time domain (FDTD) method. The simulated structure consists of micro-rings array based on SU-8 material arranged on a glass substrate, which was coated by a thin gold layer (with a thickness of 35 nm), Rh6G coating, and PVP coating, respectively (Fig. 2(a)).

The mesh size in the x, y, and z directions was considered 100 nm, and the refractive index of the glass (SiO₂), SU-8 2000, Au, Rh6G, and PVP materials were considered from the refractive index database [1]. The periodic boundary condition was applied in x and y direction, and the perfectly matched layer (PML) boundary condition was applied in the z-direction.

The optical electric field distribution of the proposed structure was simulated at the wavelength and is shown in Fig. 2(b). As can be seen, the resonance phenomenon appeared clearly in the micro-ring structure, and the coupling among double microrings was observed.

Therefore, the field amplitude distribution inside the rhodamine layer was simulated and shown in Fig. 2(c). As seen, the field amplitude distribution in this layer is very weak, so PVP layer has enhanced the field amplitude significantly.

For a more detailed investigation, the field amplitude distribution in X-Z plane was calculated (Fig. 2(d)), where the boundary between the different layers also observed. As can be observed, light is trapped within the structure, and the resonance phenomena has happened, which may have a direct impact on the improved signal-to-noise ratio and resolution of imaging via RL.

The characteristic of a random laser in plasmonic nanostructures can be discussed in our results by changing the dye concentration in the top layer of the samples to study its effects on the lasing threshold, FWHM, and peak intensity which are considered the most important laser parameters.

The measured results represented in Fig (3) in two samples of two-dimensional microcavity array gain media, show that the emission spectrum increased by increasing the dye concentration

The reason for this can be attributed to increasing the concentration of dye in a thin film structure and the gain will increase until the net gain reaches its maximum, which can affect how light is scattered and amplified within the medium, potentially altering the dynamics of the random lasing process, and thus enhance the overall gain of the medium, potentially leading to stronger laser emission with lower threshold due to more available ions participating in the stimulated emission process. However, there is a limit to how much more dye concentration can be added before it starts to have a detrimental impact. If this limit is not effectively handled, increased heat production during operation might compromise the stability and performance of the random laser. In the lower dye concentration thin film structure, the percentage of gain is less than that of dye with a high concentration, this leads to a lower number of active ions available for stimulated emission, leading to reduced overall gain resulting in a reduction in the emission spectrum, and increased lasing threshold.

As we can see in Fig. 3(a), when pumping the sample with low dye concentration 10^-5 M by low energy of 0.85 mJ the emission spectrum of sample shows a broad spontaneous emission spectrum, and the full width at half maximum (FWHM) of this energy is approximately 12.5 nm and decreases slightly to reach 10 nm at 2.62 mJ. These results indicate that the pump energy is lower than the sample's laser threshold energy. When the pump energy is raised over the threshold energy of 3.17 mJ, the sample's emission spectrum exhibits sharp peaks with a smaller FWHM and higher intensity. This behavior continues as the energy pumping increases until FWHM reaches 6 nm with peak intensity 49.765 (a.u.) at higher energy pumping 5.11 mJ. Above threshold energy 3.17 mJ, the emission spectrum of sample shows sharp peaks with reduced FWHM and increased intensity of the emission spectrum. This behavior continues as the energy pumping increases until the FWHM reaches 6 nm with peak intensity 49.765 (a.u.) at higher energy pumping 5.11 mJ.

Compared to the emission spectrum of samples with dye concentration 10-5 M with samples with concentration 10^-4 M, the emission spectrum of last sample shows a significant red-shift of 3 nm. These transformations can be interpreted as the result of a process of re-absorption and re-emission that occurs when the transition between the ground state and the first excited singlet of dye molecule. In other words, the reabsorption efficiency of the dye is enhanced in terms of the presence of Au nanostructure.

At lower energy pumping of the sample with dye concentration 10^-4 where one can see that the value of (FWHM) is approximately 2.8 nm, and decreases slightly to reach 1.4 nm at 1.42 mJ, above this energy, reduced FWHM to 2 nm, and increased the intensity of emission spectrum, when energy pumping increases to 5.11 mJ the value of FWHM reaches 1.4nm with peak intensity 59.386 (a.u.) at higher energy pumping 5.11 mJ as shown in Fig (3 c).

On the one hand, the waveguide of 2D-MCA enhances the electromagnetic field in their vicinity of plasmonic excitation wavelength. In order to acquire a better understanding of the plasmonic improvement of random laser characteristics in MCAs with Au and G NPs, we cover the final samples with PVP polymer in the inset of Figure (1). The emission spectrum in this case is enhanced significantly in the vicinity of samples due to LSPR.

On the other hand, the scattering strength is enhanced by adding a PVP layer onto the samples, especially at the LSPR wavelength, the formation process of random laser and the description of the recurrent multiple scattering between the substrate and PVP layer.

Because of the recurrent multiple scattering in the waveguide structure, the diffusion coefficient is efficiently decreased to form spatially localized modes, in terms of multiple refractions, thus the dwelling time of the light is markedly increased. When the gain exceeds the loss, the emission spectrum of a two-dimensional microcavity array with a PVP layer at different pump energies is studied as shown in Fig. 4.

Where the random laser's emission spectrum (peak intensity, FWHM, and lasing threshold) appears to have improved following the addition of the PVP polymer. The increase in the emission intensity after adding the polymeric material for both two concentrations compared to previous results without polymer where the emission peak increases from 49.765 (a.u.) to 53.639 a.u. for the sample with dye concentration 10-5 M, while it increases from 59.386 (a.u.) to 67.731 a.u. for the sample with 10^-4 M. Also the effect of the polymeric material on narrowing the emission spectrum, as the FWHM decreases from 6 nm to 5 nm for the sample with dye concentration 10^-5 M while from 1.4 nm to 1.2 for the sample with dye concentration 10^-4 M. while confirms the decrease in the laser threshold when adding the polymeric PVP material to the solutions, so the laser threshold decreases from 2.62 mJ to 1.95 mJ for the sample with dye concentration 10^-5 M while it decreases from 1.95 to 1.42 mJ for the sample with dye concentration 10^-5 M.

The waveguide of polymeric materials is a critical component of random lasers, and it is responsible for the noticeable and tangible enhancements in their features.

Now after RL recording and in the same time, we record the images of mouse tissue by the CCD camera as mentioned above in experimental part for tissue 200 micrometer thicknesses under four different pumping energy by the best sample by PVP wave guiding effect. Figures 5 shows the CCD image of mouse tissue with 200 micrometer thickness for the same pumping intensity, and correspondence concentrations and structures of Fig. 4 (a to d). It is obvious that we have the best resolution, more hot spots in the mouse tissue imaging by the sample with PVP wave guiding effect and in 10^-4 concentrations as shown in Fig. 4(d).

In conclusion, we study plasmonic waveguide-assisted imaging system based on random lasing by helping micro ring arrays covered with the gold/ graphene layer and Rh6G dye. We find that the samples without PVP layer yield a coherent random laser about 6 nm and 1.4 nm in the full width at half maximum (FWHM), threshold energy 3.17 mJ and 1.42 mJ for concentrations 10^-5 and 10^-4, respectively, the peak intensity in the sample with PVP layer is higher than that without PVP layer, all these parameters correspond to the simulation part, in which the PVP layer enhanced the field amplitude significantly. The results and discussion presented in this paper offer a novel perspective on the plasmonic enhancement of random laser properties in micro ring arrays that are coated with the gold/graphene layer and Rh6G dye.

Sample preparation:

All of tissues were purchased from Pasteur Institute of Iran.

Sample guideline:

All experimental procedures were performed in accordance with guidelines and regulations approved by a

regional Institutional Animal Care and Use Ethics Committee of the "Ethical committee of Vice president of

research of Shahid Beheshti university/IR.SBU.REC.1405".

Conflict of interest:

There is no any conflict of interest.

Data Availability:

The authors declare that the data supporting the findings of this study are available within the paper and any extra files can be requested from corresponding author.

Author Contribution:

M. Lateeffabricates samples and measure RL and imaging process, W. A. Aldaim and S. F. Haddawi, control the process and results, S. M. Hamidi supervised the work and edit the final version of the file.

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No competing interests reported.

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Mouse Tissue imaging by random laser of Plasmonic two dimensional array

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I. Introduction

II. Experimental part

III. Results and Discussion

IV. Conclusion

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