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Abstract
Cavity mode manipulation in lasers is urgent for the stable single-mode operation of a microring laser. Here, we propose and experimentally demonstrate the plasmonic whispering gallery mode microring laser for strong coupling between local plasmonic resonances and whispering gallery modes (WGM) on the microring cavity to achieve pure single-mode lasing. The proposed structure is fabricated based on integrated photonics circuits consisting of gold nanoparticles deposited on a single microring. Additionally, our numerical simulation provides deep insight into the interaction between the gold nanoparticles and WGM modes. The manufacture of microlasers for the advancement of lab-on-a-chip devices and all-optical detection of ultra-low analysts may benefit from our findings.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The microcavity laser is the latest evolutionary step ongoing since the birth of the laser [1–3]. Whispering gallery mode (WGM) optical cavities are structures that tailor light for a long time along equatorial trajectories close to the structure [2,3]. Therefore, they are a special selection for either classical or quantum communication still as integrated photonics due to their high photon lifetime with no dissipation in the cavity, resulting in a high-quality factor [4,5]. Nonetheless, the mode volume in the cavity is limited by diffraction. This value is used to calculate the electromagnetic field's spatial expansion [6]. Due to size reduction restrictions, most WGM microcavity lasers are multimode. The Vernier effect [7–10], the parity-time symmetry effect [11–13] in coupled microcavities, or more recently using cavity mode manipulation by loss engineering [14,15] or adding metal nanoparticles in plasmonic structures [16–20] are just a few of the structures that have been proposed to date to suppress the competing parasitic modes and thereby achieve single-mode lasing. Even though the majority of these techniques quench the sidebands between 13 and 20 dB [7,16,22], achieving a pure single mode is still out of reach.
Surface plasmons (SPs) are coherent collective oscillations of conduction electrons that have the ability to couple to electromagnetic waves and excite the surface plasmon resonance (SPR) at planar surfaces as well as the localized SPR (LSPR) at closed surfaces of nanoparticles. LSPR is a result of the strong increase in the confinement of electromagnetic waves close to the particle surface; the mode confinement decreases exponentially away from the surface. This phenomenon results in a small mode volume within the subwavelength, irrespective of the diffraction limit, at the cost of decreasing the quality factor because of absorption and diffraction loss within the metal.
The shortcomings of both photonic and plasmonic cavities have recently been addressed by the development of hybrid photonic-plasmonic microcavities [21–25]. Under specific conditions, placing metal nanoparticles in a large confinement field can impinge on the photonic and plasmonic modes. This also allows us to harness the single-mode lasing so that it can be suitably chosen for different sensing applications.
Various studies have been carried out regarding the coupling of nanoparticles to different microlasers. Some of these investigations have led to the suppression of side modes in the microresonator. One of the sources of optical losses in the cavity is the reabsorption of gain material, which prevented strong coupling from occurring [16]. The achievement of hybrid photonic-plasmonic or plasmonic microlasers is the result of additional research in this area. In [26], the concentration of nanoparticles placed in a dye-doped microlaser is investigated. However, the impact of nanoparticles on photonic modes hasn't been specifically discussed. An intriguing technique for the selective coupling of silver nanoparticles to the microtube cavity is put forward in [18]. In this method, by adjusting the laser power and irradiation time, the morphology of the nanoparticle is changed, and as a result, the degree of coupling of photon-plasmonic modes is adjusted. Here, we describe the direct writing lithography demonstration of a plasmonic polymer microring laser on a silica substrate. The purported technology is appropriate for the high-throughput, rapid, precise, and simple production of many types of microlasers with increasingly complex designs. To make the gain medium, we combined 1 mg of RhB dye with 1 mL of Gamma-Butyrolactone. 2.3 mg of SU-8 2002 was then added to the solution (detailed information is given in the Supplement 1). The microring with a 40 µm radius, 1.5 µm width, and 2 µm height is written on a silica glass substrate. SU-8 polymer is intrinsically hydrophobic. But intricate micropatterns significantly increased the sample surface's hydrophobicity. Therefore, to counteract the hydrophobicity, AC cold plasma is applied. Next, gold (Au) nanoparticles are deposited using the spin coating method on the surface of the microring. (See the Supplement 1). In order to prevent degradation of the sample surface, low-power plasma is used. The suggested structure's schematic is shown in Fig. 1(a). Figure 1(b) depicts an SEM image of a microring laser deposited by Au nanoparticles. 3-dimensional atomic force microscopy (AFM) was also performed to characterize the Au NPs on the mirroring’s surface which is indicated in Fig. 1(c).
Fig. 1. (a) Schematic structure of the microring with deposition of Au nanoparticles. (b). SEM image of the microring laser after the Au nanoparticles’ deposition. (c) 3D atomic force microscopy (3D-AFM) of gold nanoparticles on a polymeric microring.
2. Results and Discussion
Polymeric microcavities owing to their high-quality factor are essential in the engineering of light-matter interaction and the realization of low-threshold lasers. These optical structures are very sensitive to any perturbation of the laser’s morphology. When noble metals are embedded in or implemented on the surface of the microcavities, they perform like a loss and gain medium when optically pumped close to the absorption wavelength [27]. Gold NPs stand out among other NPs for their potent absorption in the visible range. The size, quantity, and surrounding medium of Au NPs, among other factors, affect their optical characteristics [27]. Because of the plasmonic effect or loss engineering, the interaction of these nanoparticles with microcavity lasers might provoke appealing phenomena like single-mode lasers [16,27]. To support this assertion that the phenomena happen when nanoparticles interact with the microresonator, a numerical simulation is a very desirable solution.
For this simulation, we created a ring laser with a diameter of 12 µm and a width of 0.5 µm, with randomly distributed Au NPs of varying radii placed on the surface of the microring. In this regard, three-dimensional finite-difference time-domain (3D-FDTD) simulations are carried out by exciting the structure with a dipolar source, which is shown in Fig. 2(a). Numerical calculations of the spatial distribution of transverse modes in various cross-sections of the microring laser irradiation deposited by Au nanoparticles are shown in panels (b-d). We only see the plasmonic mode resonant at the resonant mode and on top of the nanoparticles’ height. Figure 2(e) illustrates the enlarged view of the yellow dashed square shown in Fig. 2(b). For observation of the exciting hybrid photonic-plasmonic modes, we show the E-field distribution at the interface of microring with Au NPs. The interaction of the plasmonic mode with the photonic mode is clearly seen from the enlarged view of Fig. 2(f). The E-field profile that represents the pure photonic mode inside the microlaser is shown in Fig. 2(g). The calculated distribution for an E-field intensity at the yz cross-section is illustrated in Fig. 2(h).
Fig. 2. (a) A plasmonic microcavity system, driven by a dipolar source (not-to-scale schematic). The cavity is represented by a ring supporting a high-quality factor in whispering gallery mode (WGM). The panel (b-g) depicts a numerical calculation of the spatial distribution of transverse modes in various cross-sections (xy-planes) of the microring laser irradiation deposited by Au NPs at z values of I, II, and III. (b). the corresponding plasmonic resonance at the height of the nanoparticles, (c). LSP resonance at the interface of microcavity laser and nanoparticles, and (d) is the microcavity transverse mode. Figs. (e-g) show the enlarged views of the indicated regions in Figs. (b-d), respectively. The yz cross-section and an enlarged view of the red dashed rectangle of the structure are shown in (h and i).
By evaluating the Au nanoparticles’ lasing performance experimentally, we first excited the microring laser without Au nanoparticles at a 3 Hz repetition rate, a 20 ns pulse width, and a 532 nm wavelength. The proposed setup in [7] is used to measure the lasing performance. Figure 3(a) illustrates the behavior of a single microring when exposed to various pump powers. The dominant mode is located at 603.3 nm with 0.5 nm full width at half maximum (FWHM). The microring laser has a very low threshold lasing as low as 1.5 µW/mm2.
Fig. 3. Lasing performance of microring laser before (a) and after (b) the Au nanoparticles deposition for various pump power irradiation.
Thenceforth, we prepared the Au nanoparticles by spark plasma discharge synthesis with a concentration of 1 µm2. To pinpoint the distribution of the Au nanoparticles, DLS analysis was also carried out. The peak distribution of Au NPs is within 220 nm (see the Supplement 1). Afterward, for 20 seconds at an 800 rpm speed, the Au nanoparticles are deposited using the spin coating process. The distribution of nanoparticles on the surface of the microring is random. The sample was dried at room temperature. When the Au nanoparticles are deposited on the surface of the microring, single-mode lasing is obtained. This performance is recorded at a higher pump power of 5.5 µW/mm2. In addition to single-mode lasing, we can see the blue shift in the spectra, which is shown in Fig. 3(b).
Using a polarizer, the amplified spontaneous emission (ASE) of the sample is collected with pulsed current injection at room temperature. Figure 4(a) displays the final outcome. The absorption spectrum of Au nanoparticles and the emission spectrum of microring wo/w Au NP are also shown in the same figure to assess the effectiveness of the structure. The absorption spectrum peak of Au NPs is located at 550 nm. As it’s clear from the figure, the excited modes in both cases, as is evident from the image, are located at the measured gain spectrum, and the blue shift in the microring with Au NPs trends to the gold nanoparticles’ absorption peak.
Fig. 4. (a) Absorption (red solid spectrum, the left vertical axis shows the absorption (%) of gold nanoparticles), ASE, and emission spectra of microring wo/w Au NP. (b) Lasing spectrum collected of CuO QD deposition on the surface of the microcavity. The resonant mode of the structure has experienced a redshift in comparison with the microring without NPs (c) Comparison of the integrated output intensity of microring, microring with Au and CuO NPs as a function of power irradiation. (d) Lasing spectra from microring with different compositions: Au NPs (green curve), without NPS (blue curve), and CuO NPs (brown curve). Insets: dark-field microscope image of the microring with Pin = 20 µW/mm2, Pin = 5 µW/mm2, and Pin = 15 µW/mm2 for different compositions.
By integrating the excitation power over the lasing peak and using the superliner curves, illustrated in Fig. 4(c), the lasing threshold is increased from 1.5 µW/mm2 to 5.5 µW/mm2 for Au NPs and 4.85 µW/mm2 for CuO NPs which are 4 and 3 times greater than the lasing threshold of microring without nanoparticles, respectively. Furthermore, the lasing slope fell after various compositions. Figure 4(d) demonstrates that CuO NPs experienced multimode red-shift and Au NPs had single-mode blue-shift.
In order to further validate the structure's performance, numerical simulations were run to examine how Au nanoparticles affected the microring laser. All-important parameters in the experiment, such as the gain medium, the real dimension of the microresonator, the actual size of nanoparticles, and their distribution, are mentioned in the simulation. The material's refractive index is expressed as $n = {n_r}\; - \; ig(\lambda )$, where ${n_r}$ is the real part of the index and $g(\lambda )$ is the gain medium. The ASE spectrum analysis, which can be expressed as a Gaussian function $g(\lambda )={-} {g_0}\ast \; exp\; ({({({\lambda - {\lambda_0}} )/\Delta \lambda } )\; 2\; } )$, is used to calculate the gain medium of the material in the WGM microring cavity. Here, ${g_0}\; $ denotes the fixed gain, ${\lambda _0}\; $ is the resonant wavelength of the WGM microcavity, and $\Delta \lambda $ represents an FWHM of the ASE spectrum (see the dotted spectrum in Fig. 4(a)).
The WGM mode spectrum before the Au NP is plotted with a solid black line in Fig. 5(b). The free spectral range (FSR) of WGM modes in both simulation and experimental results is around 0.821 nm, which is plotted in the top inset of Fig. 5(a). A uniformly random particle distribution with varying sizes and overlap allowances is used to define the Au NP in the simulation method (see the DLS distribution in Supplement 1). The Claussius-Mossotti theory, which states that the Froehlich resonance is independent of the form and size of nanoparticles, provides a detailed explanation of the electromagnetic properties of metallic nanoparticles [28,29]. Despite the fact that the resonance spectral width is influenced by nanoparticle size. According to the classical or quantum descriptions of the electrons in nanoparticles, the intrinsic and extrinsic broadening caused by the size of the nanoparticles on the resonance spectral width can be distinguished [30]. In the same figure, the plasmonic microcavity simulation result is shown. The results of the simulation and the experiment are very consistent. Additionally, the simulation clearly shows the same wavelength shift as the experiment. As depicted in the inset of the top inset of Fig. 5(b), by exciting the LSPR mode in the laser, we can see the broadening in resonance spectral.
Fig. 5. Experimental (a) and simulation (b) spectra of the WGM microring wo/w Au nanoparticles. The inset of (a) shows the comparison of the FSR between experimental and simulation results. The comparison of FWHM between experimental and simulation results for the exciting plasmonic mode is illustrated in the inset of (b).
To precisely explain the microresonator performance, Figs. 6(a) and (b) show the emission spectra of the microring before and after Au NP deposition when exposed to a 5 µW/mm2 and 20 µW/mm2 pump power, respectively. There are at least five modes that contribute to lasing before the Au NP deposition. After the Au NP deposition, only one mode participates in a lasing, and the central peak shifts by 7.4 nm which is shifted toward an absorption peak of Au NP.
Fig. 6. Experimental observation of the lasing performance of microring laser (a) Emission spectrum of single microring laser (radius 40 µm, width, and height 2 µm) when exposed with a 5 µW/mm2 pump power. (b) Single-mode spectrum intensity was obtained from the microring laser deposited by Au nanoparticles with 20 µW/mm2 pump power. (c) The corresponding intensity pattern is obtained from (a) when its sensing performance is measured with water. (d) Performance evaluation of the proposed structure in (b) for the measurement of water.
Meanwhile, we observed the relatively robust emission from the Au NP under the microscope, which indicates a strong coupling between the Au NP and the microring. It can be inferred that the localized surface plasmon resonance occurred at the interface of the microresonator with Au NP. LSPR instigated the enhanced electric field, which may intensify the photon flux of the incident emission. Therefore, the origin of the blue shift can be expressed as the change of interface of the WGM mode cavity (photonics mode) with the excited LSPR mode (plasmonic mode). Additionally, considering the 5 times raised lasing threshold and the size of Au NP, it can be stated that the absorption loss occurred at Au NP, and therefore the microlaser is excited at a higher pump power.
LSPR is very sensitive to the nature of the interface and the refractive index (RI) of the environment. A wavelength shift is caused by a change in RI. If a thin layer of analyte is located on the interface, the resonant condition changes, even though the change may be very slight, but is measurable. In Figs. 6(c) and (d), a thin layer of DI water are poured on the surface of the microresonator before and after the Au NP deposition, respectively. This thin layer acts as a sensing medium. The refractive index of the environment is changed by Δn = 0.33, and the wavelength shift is expected to be significant in both cases. The central wavelengths in both cases shifted by 6 nm and 3.3 nm respectively and higher wavelengths. The wavelength shift relations for dielectric cavities and plasmonic cavities are given in [31–33].These results are very important in sensing applications.
3. Conclusion
In conclusion, we demonstrate a simple method to manipulate the WGM microring cavity by deposition of the Au nanoparticles. The random distribution of Au NP on the surface of the microresonator intensifies the localized surface plasmon resonances. The local fields coupled with WGM modes and generated hybrid plasmonics modes. The experiments revealed the single-mode lasing without any side mode and subharmonic generation. For further investigation, a very precise and numerical simulation by taking into account the gain medium in laser structures is proposed. The performance of various types of microcavity lasers can be accurately predicted by the results of this method.
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
Supplemental document
See Supplement 1 for supporting content.
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