Microcavity lasing has many excellent properties, such as a small mode volume, high quality factor (Q), high sensitivity, effortless manipulation, and good monochromaticity, which can be applied in sensing,1
Advanced Photonics, Volume. 6, Issue 3, 035001(2024)
Plasmon-assisted mode selection lasing in a lanthanide-based microcavity
Lanthanide-based microlasers have attracted considerable attention owing to their large anti-Stokes shifts, multiple emission bands, and narrow linewidths. Various applications of microlasers, such as optical communication, optical storage, and polarization imaging, require selecting the appropriate laser polarization mode and remote control of the laser properties. Here, we propose a unique plasmon-assisted method for the mode selection and remote control of microlasing using a lanthanide-based microcavity coupled with surface plasmon polaritons (SPPs) that propagate on a silver microplate. With this method, the transverse electrical (TE) mode of microlasers can be easily separated from the transverse magnetic (TM) mode. Because the SPPs excited on the silver microplate only support TM mode propagation, the reserved TE mode is resonance-enhanced in the microcavity and amplified by the local electromagnetic field. Meanwhile, lasing-mode splitting can be observed under the near-field excitation of SPPs due to the coherent coupling between the microcavity and mirror microcavity modes. Benefiting from the long-distance propagation characteristics of tens of micrometers of SPPs on a silver microplate, remote excitation and control of upconversion microlasing can also be realized. These plasmon-assisted polarization mode-optional and remote-controllable upconversion microlasers have promising prospects in on-chip optoelectronic devices, encrypted optical information transmission, and high-precision sensors.
1 Introduction
Microcavity lasing has many excellent properties, such as a small mode volume, high quality factor (Q), high sensitivity, effortless manipulation, and good monochromaticity, which can be applied in sensing,1
Surface plasmon polaritons (SPPs) propagate at the interface between a metal conductor and dielectric in the form of electromagnetic waves20 and can constrain and manipulate optical signals in subwavelength structures over long distances. Because of the limitations of the surface boundary conditions, only the transverse magnetic (TM) polarization mode surface waves of the SPPs can propagate at the metal–air interface. The unique properties of the SPPs provide an effective method for separating and selecting the polarization modes of lanthanide-based microlasers. However, traditional mode selection methods such as reducing cavity size, increasing mode selection structure, and Vernier efficacy-based methods, which adjust the laser mode by reducing the number of modes, cannot effectively select the laser polarization mode, and compared with pure silicon-based devices used for polarization beam splitters,21 SPP-based devices provide a compact size and a remote-control interface for on-chip applications.22
In this letter, a simple and effective plasmon-assisted mode selection method for a lanthanide-based whispering gallery mode (WGM) microlaser is proposed. A plasmon-assisted lanthanide-based microcavity composed of a silver microplate is used as a substrate, on which a UCNP-coated
2 Results and Discussion
In this letter, a plasmon-assisted mode selection lanthanide-based microcavity was realized by placing a single UCNP-coated
Figure 1.(a) Schematic diagram of the plasmon-assisted mode selection lanthanide-based microcavity. (b) Transmission electron microscope image of (i)
First, we investigated the characteristics of upconversion lasing from a single
The emission spectra were monitored at various excitation powers to confirm the generation of microlasing and characterize its behavior. In Fig. 2(a), the red and blue point plots represent the pumping power-dependent plots for the integrated emission intensity and spectral linewidth of the 802-nm lasing peak, respectively. The emission intensity is calculated by integrating the lasing peak, which exhibits a sharply increasing slope as the laser power increases, corresponding to a lasing threshold; the spectral linewidth narrows sharply at the lasing threshold, which proves the generation of a low-threshold and narrow-linewidth microlaser. To investigate the polarization anisotropy of the microlaser and distinguish between the TM and TE modes, the fluorescence emission spectra of the microlaser were collected at different polarization angles. In Fig. 2(b), the red triangles and blue spheres denote the polarization-dependent intensities of the 788- and 802-nm lasing, respectively, representing linear polarization-dependent emission vertically and horizontally. The polarization at 788 nm corresponds to the collected polarization angle of 90 deg, which corresponds to TM mode lasing. The 802-nm lasing corresponds to TE mode lasing owing to the lasing action, exhibiting strong polarization with the dominant optical feedback path perpendicular to the equatorial plane of the microcavity. The Q factor and full width at half-maximum (FWHM) of the 788- and 802-nm lasing were investigated while altering the polarization angle and were also found to be orthogonal (Fig. S2 in the Supplementary Material). It should be noted that the FWHM of the lasing lines 802 nm could reach a narrowness of 0.35 nm for these
Figure 2.(a) Pumping power-dependent plots of emission intensities and spectral linewidth narrowing of 802-nm lasing on the glass substrate exhibiting upconversion lasing emissions. (b) Polarization investigation of the 788- and 802-nm lasing lines using a polar plot of the intensities; the fitting curves were drawn by a cosine-square function.
Based on the high-efficiency narrow-linewidth upconversion lasing, a single lanthanide-based microcavity was placed on a single silver microplate using a micromanipulation transfer technique to realize plasmon-assisted mode selection. As shown in the upper left of Fig. 3(a), a lanthanide-based microcavity was successfully transferred onto a silver microplate. The fluorescence emission spectra of the
Figure 3.(a) Fluorescence emission spectra of
To reveal the mechanism of the plasmon-assisted TE mode selection, an electromagnetic field analysis of the silver microplate-coupled microcavity was performed. A point dipole source with a wavelength of 800 nm was used to represent the fluorescence emission of the UCNPs, which was placed at the position of the strongest electric field in the silver microplate and microcavity coupling system for subsequent simulation. We first simulated the far-field radiation spectra of the dipole source in the microcavity and silver microplate-coupled microcavity, as shown in Fig. 3(c). The black line represents the far-field radiation spectrum of the dipole source in the microcavity, and the intensities of each pair of the TE and TM modes are equal. The red line represents the far-field radiation of the dipole source in the silver microplate-coupled microcavity. The overall intensity of the TE and TM modes was enhanced relative to the dipole source in the microcavity, whereas the TM mode intensity was weaker than the TE mode intensity. The results showed the same TE mode selection effect as that in the experiment. We further simulated the charge distribution on the silver microplate surface using a microcavity-coupled silver microplate system. Because the fluorescence of UCNPs has no polarization characteristics, the luminescence of UCNPs was simulated using different dipole electric field orientations. We defined the
The lasing mode was also split on the silver microplate when investigating plasmon-assisted mode selection. As shown in Fig. 4(a), the TE mode lasing peak at 798 nm splits into two peaks. We represent the modes of these two peaks as modes 1 and 2. Polarization-dependent spectra were collected to distinguish between the two modes. As shown in Fig. 4(b), when the polarization was collected at 0 deg, the right peak was dominant, whereas when collected at 90 deg, the left peak prevailed. The intensities of the split left and right peaks were compared, resulting in a drafted table (see Table S2 in the Supplementary Material). The intensity ratio of modes 1 and 2 changed from 1.43 at 0 deg to 0.63 at 90 deg, indicating that the polarizations of modes 1 and 2 were orthogonal. Interestingly, the plasmon-assisted microlaser was different from the conventional WGM microlaser when a mirror microcavity was introduced by the silver microplate, and the two microcavity modes were coherently coupled to form two orthogonal modes, as shown in Fig. 4(a), resulting in the phenomenon of modes 1 and 2 splitting.30 To explain the mechanism by which the microsphere creates a mirror cavity on the silver microplate and splits the lasing peaks, we used a two-sphere system to simulate the electric field distribution profile (see Fig. S13 in the Supplementary Material). Dipoles with different orientations were placed between the two spheres owing to coupling with the second microsphere once the degenerate mode was split into two different modes. Polarization degeneracy in excitation lifted, and modes 1 and 2 were excited using orthogonal input polarizations. Both modes exhibited a continuous total internal reflection along the periphery of the sphere. Because the input polarizations of the excited modes were orthogonal, the outcoupled radiation preserved this orthogonality.
Figure 4.(a) Enlarged view of the lasing mode peak near 798 nm on the silver microplate. (b) Fluorescence spectrum of the corresponding lasing peak in panel (a) when the polarizer in the collected optical path was at 0 and 90 deg.
In addition to inducing lasing mode selection, SPPs propagating over long distances on a silver microplate also present an opportunity for remote manipulation of microlasers. As shown in the bright-field image in Fig. 5(a), the red circle represents the excitation position, and
Figure 5.(a) Bright-field microscopy image of a microlaser remotely excited by SPPs. The red and blue circles represent the excitation and collection positions, respectively. The angle
3 Conclusion
In summary, we designed a simple and effective plasmon-assisted lanthanide-based microcavity that achieved TE polarization mode-selective enhancement and remote lasing excitation. Selective enhancement of the TE polarization mode was achieved through the introduction of local electromagnetic field amplification and the assistance of SPPs propagating on the surface of a silver microplate. Meanwhile, laser mode splitting was observed on the silver microplate owing to the coherent coupling between the microcavity and mirror microcavity modes. Moreover, because of the tens-of-micrometers long-distance propagation characteristics of SPPs on silver microplates, remote excitation and laser control were realized. These plasmon-assisted polarization-mode optional and remote-controllable upconversion microlasers hold significant potential in nonlinear hybrid nanophotonics, stochastic laser applications, and nanooptical sensing.
[18] L. Zhao et al. High quality two-photon pumped whispering-gallery-mode lasing from ultrathin CdS microflakes. J. Mater. Chem., 7, 12869-12875(2019).
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Lei Guo, Min Ji, Bowen Kang, Min Zhang, Xin Xie, Zihao Wu, Huan Chen, Volker Deckert, Zhenglong Zhang, "Plasmon-assisted mode selection lasing in a lanthanide-based microcavity," Adv. Photon. 6, 035001 (2024)
Category: Letters
Received: Dec. 31, 2023
Accepted: Apr. 7, 2024
Posted: Apr. 8, 2024
Published Online: May. 9, 2024
The Author Email: Chen Huan (hchen@snnu.edu.cn), Zhang Zhenglong (zlzhang@snnu.edu.cn)