An ultracompact hybrid plasmonic waveguide Bragg grating (HPWBG) with improved spectral properties of long-wavelength passband is proposed. A hollow HPW is introduced to suppress the entire loss, and a parabolic profiled sidewall is designed to optimize the spectral properties for specific wave bands. The transfer matrix method and finite element method are combined to ensure the efficiency of numerical research. The results show that the parabolic profile effectively reduces the reflection and strengthens the resonance of the mode in the long-wavelength passband, suppressing the oscillations and realizing significant smoothness and improvement in transmission. The optimized transmittance is greater than 99%, and insertion loss is as low as 0.017 dB. A wide bandgap of 103 nm is also attained. The structure also has a compactness with a length of 3.4 µm and exhibits good tolerance. This work provides a scheme for designing and optimizing wavelength selecting devices and has potential application value in integrated photonic devices.
The stimulated Raman scattering (SRS) instability of a left-handed circularly polarized (LH-CP) laser in strongly axially magnetized plasmas is investigated in detail with the help of one-dimensional (1D) and two-dimensional (2D) particle-in-cell (PIC) simulations. Since the LH-CP laser has a larger critical density in the axially magnetized plasmas, the SRS instability could be excited in over quarter-critical density plasmas, which is verified by the PIC simulations. This phenomenon could be used to amplify a seed with a frequency smaller than half of the laser frequency, which is impossible for traditional simulated Raman amplification. The simulation results also show that the scattered laser becomes right-handed circularly polarized. With this conclusion, we re-derive the temporal linear growth rate of the SRS instability of the LH-CP laser in the axially magnetized plasmas. The results show that the larger the external magnetic field is the smaller the temporal growth rate is. The suppression of the SRS by the external axial magnetic field in the linear region is verified by both 1D and 2D PIC simulations. The simulation results also show that the phase velocity of the electron plasma wave (EPW) will be decreased by the external magnetic field as expected by the theory, which makes it easier for the EPW to trap electrons and for the nonlinear frequency shift of the EPW to happen. As a result, not only the linear growth rate of SRS but also the saturation level of SRS is decreased by the external axial magnetic field. When the external magnetic field is strong enough, the saturation level of SRS can be suppressed by several times. So, this work also provides an efficient way of suppressing the SRS instability. Besides, the 2D simulation results show that some transverse instabilities of the electron plasma wave are also suppressed by the external magnetic field and this suppression will provide us with an electron plasma wave with a better structure, which may benefit the simulated Raman amplification.
A compact, low-loss, and high-polarized-extinction ratio TM-pass polarizer based on a graphene hybrid plasmonic waveguide (GHPW) has been demonstrated for the terahertz band. A ridge coated by a graphene layer and the hollow HPW with a semiround arch (SRA) Si core is introduced to improve structural compactness and suppress the loss. Based on this, a TM-pass polarizer has been designed that can effectively cut off the unwanted TE mode, and the TM mode passes with negligible loss. By optimizing the angle of the ridge, the height of the ridge, air gap height, and the length of the tapered mode converter, an optimum performance with a high polarization extinction ratio of 30.28 dB and a low insert loss of 0.4 dB is achieved in the 3 THz band. This work provides a scheme for the design and optimization of polarizers in the THz band, which has potential application value in integrated terahertz systems.
Laser interaction with an ultra-thin pre-structured target is investigated with the help of both two-dimensional and three-dimensional particle-in-cell simulations. With the existence of a periodic structure on the target surface, the laser seems to penetrate through the target at its fundamental frequency even if the plasma density of the target is much higher than the laser’s relativistically critical density. The particle-in-cell simulations show that the transmitted laser energy behind the pre-structured target is increased by about two orders of magnitude compared to that behind the flat target. Theoretical analyses show that the transmitted energy behind the pre-structured target is actually re-emitted by electron ‘islands’ formed by the surface plasma waves on the target surfaces. In other words, the radiation with the fundamental frequency is actually ‘surface emission’ on the target rear surface. Besides the intensity of the component with the fundamental frequency, the intensity of the high-order harmonics behind the pre-structured target is also much enhanced compared to that behind the flat target. The enhancement of the high-order harmonics is also related to the surface plasma waves generated on the target surfaces.
Electromagnetic solitary waves generated by a two-color laser interaction with an underdense plasma are investigated. It is shown that, when the former wave packet of the two-color laser is intense enough, it will excite nonlinear wakefields and generate electron density cavities. The latter wave packets will beat with the nonlinear wakefield and generate both high-frequency and low-frequency components. When the peak density of the cavities exceeds the critical density of the low-frequency component, this part of the electromagnetic field will be trapped to generate electromagnetic solitary waves. By changing the laser and plasma parameters, we can control the wakefield generation, which will also control the generation of the solitary waves. One-dimensional particle-in-cell simulations are performed to prove the controlling of the solitary waves. The simulation results also show that solitary waves generated by higher laser intensities will become moving solitary waves. The two-dimensional particle-in-cell also shows the generation of the solitary waves. In the two-dimensional case, solitary waves are distributed in the transverse directions because of the filamentation instability.
Abstract Photonic nanojets (PNJs) and photonic hooks (PHs) are two significant effects in Mesotronics. However, it is difficult to analyze and control the two phenomena generated by diffraction-based structures, such as rectangles and right-angled trapezoids, using diffraction theory. This work focuses on the modulation of incident fields by edge diffraction and the reconstruction of energy distribution, and proposes a model based on energy flows and energy reconstruction, called the ‘energy-based model’, to analyze the formation of PNJs and PHs through such structures. This model reveals that the morphology of PNJ and PH originates from the contributions of different regions of the incident energy, especially the crucial influence of edge diffraction, and successfully clarifies the modulation mechanism of the near-field and far-field regions of PNJ, as well as the tailoring mechanism of the two arms of PH. On the one hand, the model provides reasonable and intuitive explanations for the control of energy flow paths resulting from edge diffraction in rectangles and their variants with different parameters on the generation of PNJs and PHs. On the other hand, it also serves as a basis for reverse design. By adjusting energy flow and energy reconstruction through alterations in incident conditions or structural shapes, PHJs and PHs can be tailored easily and flexibly. The model is also been validated to be applicable in explaining many reported works. The results indicate that the ‘energy-based model’, which describes the energy flow paths resulting from edge diffraction, offers intuitive, convenient, and predictive advantages in analyzing the morphological variations of PNJs and PHs generated by diffraction-based structures, such as rectangles, trapezoids, and their variants. This provides a valuable reference for relevant research on Mesotronics.
A potential terahertz (THz) radiation source generated by mode conversion from laser to surface plasma waves (SPWs) is proposed. The radiation is produced by a two-color laser interaction with a grating. It is shown that the frequency of the THz radiation can be precisely controlled by the frequency difference of the two-color laser. Two-dimensional (2D) particle-in-cell (PIC) simulations are used to verify this mechanism and the results show that the amplitude of the THz electric field increases nonlinearly with the amplitude of the laser electric field, which theoretically means, before the mechanism fails, the intensity of the radiated THz radiation may reach to a pretty high level as long as the driven laser is intense enough. For our simulations, the intensity of the THz radiation is even more than 10% of the laser. Besides, further analysis shows this mechanism also can be used to generate electromagnetic radiation with many other required frequencies by modifying the period of the grating.
A compact and ultra-low loss TE-to-TM polarization mode converter based on a hollow hybrid plasmonic waveguide is presented in this work. The incorporation of an air region in the hollow structure results in a refractive index as low as 1, enabling near-lossless mode propagation and conversion. Through the delicate design of multi-functional sections and the optimization of structural dimensions, efficient conversion from the TE mode to the TM mode is achieved. At the operating wavelength of 1550 nm, the converter demonstrates remarkable performance with a mode conversion efficiency of 0.88, a polarization conversion efficiency of 0.92, and an ultra-low insertion loss of 0.012 dB. Additionally, the device boasts a compact size of 8.5 µm while exhibiting excellent performance. This work offers a simple and efficient approach for realizing ultra-low-loss polarization mode conversion, holding promising potential for application in diverse integrated photonic devices.
This study explores the manipulation of photonic nanojets (PNJs) via axial illumination of cylindrical dielectric particles with cylindrical vector beams (CVBs). The edge diffraction effect of cylindrical particles is harnessed to achieve the near-field focusing of CVBs, minimizing the spherical aberration’s impact on the quality of the PNJ. By discussing how beam width, refractive index, and particle length affect PNJs under radially polarized incidence, a simple and effective approach is demonstrated to generate rod-like PNJs with uniform transmission distances and super-diffraction-limited PNJs with pure longitudinal polarization. Azimuthal polarization, on the other hand, generates tube-like PNJs. These PNJs maintain their performance across scale. Combining edge diffraction with CVBs offers innovative PNJ modulation schemes, paving the way for potential applications in particle trapping, super-resolution imaging, photo-lithography, and advancing mesotronics and related fields.
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