Bound states in the continuum (BIC) refers to the non-radiative state located in the radiative continuum. BIC provides a novel method for the research and development of functional devices with ultra-high quality factor (Q) in the terahertz band. It has the potential to be used in several applications, including narrow linewidth filtering, terahertz slow light devices, and the enhanced interaction between terahertz waves and matter. In this study, terahertz BIC metasurfaces composed of classical metallic split ring resonators (SRRs) are proposed and numerically studied based on the symmetry protection principle of the structure. The leakage of BIC to the far field can be observed in the spectrum by changing the gap width of SRR to form an observable quasi BIC (QBIC) mode. Moreover, the influence of ohmic loss on the Q of QBIC is systematically studied by applying the Drude model. The proposed BIC and QBIC also have unique responses to the incident angle. The BIC based on SRR metasurface proposed in this study not only provides a new framework with clear mechanism and easy implementation for the development of high-Q terahertz functional devices, but also provides research ideas for subsequent studies on the terahertz BIC metasurface from the aspects of loss and tilted incidence.

The metasurfaces are composed of different superlattices based on classical metallic SRRs. A single unit cell is composed of either 2 or 4 SRRs. For the superlattices with two SRRs in the lattice, two adjacent SRRs with different orientations are arranged vertically [superlattice ① in Fig. 1(b)] or horizontally [superlattice ② in Fig. 1(c)] to form two types of superlattices. For the superlattices composed of four lattices, each SRR orients in a clockwise direction (superlattice ③ in Fig. 4). All metasurfaces have 2-μm-thick high resistivity silicon wafer as substrate. The refractive index of silicon is set as 3.4 and the SRR is set as perfect electric conductor (PEC). The structure is simulated in CST microwave Studio.

First, the BICs in superlattices ①, ②, and ③ are numerically investigated using the eigen-mode solver. Subsequently, the frequency solver is applied to calculate the transmission of the corresponding QBIC metasurfaces by breaking the structural symmetry of the BICs. The field monitor is used to observe the field distribution to clarify the relationship between a BIC and its derivative QBIC. The evolution from BIC to QBIC is effectively presented by changing the gap widths of the SRRs, and the Fano coupling mode is used to calculate the Q of the QBICs. The influence of ohmic loss on the QBICs is investigated by applying the Drude model to the SRRs. Tilted incidence is realized by changing the input and output directions of the ports in the frequency solver, and the unique dependence of the QBICs in superlattices ① and ② is obtained.

Only one BIC exists in superlattices ① and ②. For superlattice ③, which is composed of 4 SRRs, there are two different BICs existing in the metasurface. QBICs with Fano line shape appear in the transmission spectra when the symmetry of the superlattices is broken. The Q of QBIC exhibits an inverse quadratic correlation with the asymmetric parameter. The residual ohmic loss in the SRRs deteriorates the Q of the QBICs, in which the Q of the metasurface calculated using the Drude model drops to half compared to the result with PEC. Regarding the incidence dependence, a tilted incidence with transverse electric (TE) polarization induces a leakage of the BICs in superlattices ① and ②, in which the linewidth of the derivative QBICs is proportional to the oblique angle. However, the tilted incidence with transverse magnetic (TM) polarization will not perturb the BICs in the superlattices.

In this study, we construct symmetry-protected BICs in three superlattices based on SRRs. Subsequently, the bound states at Γ point in these superlattices are investigated via numerical simulation. When the structural symmetry is broken, BICs are converted to the corresponding QBICs, and the Q of the QBICs decreases with the increase in structural asymmetry. The Q of the QBIC is also strongly correlated to the ohmic loss in the SRRs, which was generally neglected in previous studies related to terahertz metasurfaces developed by SRRs. In addition, the superlattices ① and ② have a certain pitch angle dependence. Oblique incidence of TE polarization with an electric vector parallel to the gap can lead to the leakage of BIC. The Q of the formed QBIC decreases with the increase in incident angle, while the TM wave does not have a similar effect. The metasurface designed in this study has a clear mechanism and is conveniently fabricated, which provides a novel direction for the design of high-Q terahertz devices.

Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}

Since its invention, the laser has developed tremendously; stimulated important breakthroughs in numerous related fields such as physics, chemistry, biology, and information science; and played crucial roles in fundamental science and technology application research. The National Science Foundation of China compiles statistics on the funding of Key Programs, Major Programs, Research Programs of National Major Research Instruments, General Programs, and Youth Science Foundation Programs. Based on the statistics compiled between 2017 and 2021, this study analyzes hot words in titles and keywords of previously funded projects to summarize the key developments and challenges of laser science and technology in China and propose topics requiring further research and discussion.

Results and Discussions The organic ligand dibenzoylmethane (DBM) exhibits a broad absorption band ranging from 285 nm to 450 nm; six narrow lines between 379 nm and 591 nm, belonging to the intrinsic absorption of Eu³⁺ ions from the ground states ⁷F₀ and ⁷F₁ to the excited states ⁵G₂, ⁵L₆, ⁵D₃, ⁵D₂, ⁵D₁,_{and ⁵D0, are observed for EuCl3. In the Eu(DBM)3Phen complex-doped PMMA film, the broad absorption of the organic ligands is significantly stronger than the intrinsic absorption of the Eu³⁺ ions (Fig. 1). A schematic of the intramolecular energy transfer and intrinsic absorption and emission of Eu³⁺ ions is presented (Fig. 2), based on the absorption and fluorescence emission of the doped film; the measured fluorescence lifetime of the ⁵D0 levelof Eu³⁺ ions in the PMMA host is 403 μs (Fig. 3). A ridge waveguide with a cross-section of 12 μm×5 μm can limit 93% of the signal laser and 95% of the pump light in the core layer. In the evanescent field waveguide with a cross-section of 4 μm×5 μm, the limitations in the core layer are 87% and 92% for the signal and pump light, respectively, owing to the smaller refractive index difference (Fig. 6). When pumping with the 405 nm LED, the relative gain in the ridge waveguide with a length of 1.5 cm increases from approximately 0.2 dB/cm to 1.9 dB/cm at 653 nm, as the pump power increases from 225 mW to 420 mW. For the evanescent field waveguide, a maximum gain of 1.5 dB/cm is obtained on a 2.0 cm-long waveguide under the excitation of the 420 mW 405 nm LED (Fig. 8); this demonstrates the possibility of the practical application of the evanescent-wave coupling method in PICs.}

Plastic optical fibers (POFs) have been widely used in Fiber to the Home (FTTH), automobile optical local area networks (LANs) and fiber-optic sensor fields owing to their large bandwidths, low prices, and easy coupling. POFs exhibit a low loss window in the red band around 650 nm; thus, it is considerably important to use optical waveguide amplifiers to compensate for the propagation loss at a wavelength of 650 nm. Furthermore, optical waveguide amplifiers can be integrated with optical switches, arrayed waveguide gratings, and optical sensors in photonic integrated circuits (PICs) to compensate for optical losses. Research on waveguide amplifiers has often utilized semiconductor lasers as pump sources to excite the intrinsic absorption bands of rare-earth ions. Consequently, the optical power density at the input side of the waveguide can reach approximately 10⁶ W/cm² with pumping power of 300 mW at a cross-section of 6 μm×5 μm for the waveguide, which leads to thermal damage in the waveguides and the up-conversion of rare-earth ions. Lanthanide ion complexes with organic ligands exhibit a continuous large absorption band in the blue-violet band, which is suitable for blue-violet light-emitting diode (LED) pumping. The energy absorbed by organic ligands can be effectively utilized to realize the radiative transition of rare-earth ions through intramolecular energy transfer. In addition, the LED pumping method can help improve the thermal stability of waveguides, which is expected to play an important role in optical integrated systems on chips.

The absorption spectra of organic ligands, EuCl₃ and Eu(DBM)₃Phen-doped polymethyl methacrylate (PMMA) films, are measured. The fluorescence emission and fluorescence lifetime of the Eu(DBM)₃Phen-doped PMMA film are characterized. Using an aluminum mask combined with inductively coupled plasma (ICP) etching and one-step photolithography, a ridge waveguide and an evanescent field waveguide are fabricated, respectively. Further, the film-forming properties of the doped film and the morphology of the waveguides are characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM), respectively. The optical field distribution of the signal laser in the waveguides is also simulated. Moreover, using a vertical top pumping mode with a 405 nm LED, the optical gains of the fabricated waveguides are measured at 653 nm.

In this study, the europium complex Eu(DBM)₃Phen is doped into a PMMA polymer as an active material to fabricate two types of polymer waveguide amplifiers—a ridge waveguide and an evanescent field waveguide—using an aluminum mask combined with ICP etching and one-step photolithography, respectively. Under the excitation of a 405 nm blue-violet LED, relative gains of 1.9 dB/cm and 1.5 dB/cm are obtained at 653 nm, respectively, for these waveguides. The UV absorption and fluorescence emission of the Eu(DBM)₃Phen-doped PMMA film are also characterized. The results show that the intramolecular energy transfer of organic ligands can realize the transition of Eu³⁺ ions from the ⁵D₀ energy level to the ⁷F₃ energy level under LED pumping. The relatively long fluorescence lifetime of the ⁵D₀ level_{of Eu³⁺ ions can facilitate high gains in optical amplifier systems.}

Results and Discussions When T_OC=5%, the maximum continuous-wave output power reaches 20.4 W under an incident pump power of 70 W, resulting in optical-to-optical conversion efficiency of 29.1% and a slope efficiency of 32.5% (Fig. 2). Under the full output power, the beam quality factors are Mx2=1.65andMy2=1.81 (Fig. 3), and the power stability (root mean square) is 0.1% within 1 h. In addition, when T_OC =10%,the maximum output power reaches 19 W with an optical-to-optical efficiency of 27.2% and a slope efficiency of 32% (Fig. 2). After inserting an acousto-optic Q-switcher, when T_OC=5%, the average output power increases from 9.8 W at a pulse repetition frequency (PRF) of 1 kHz to 16.5 W at a PRF of 20 kHz, corresponding to a decrease in pulse energy from 9.8 mJ to 0.82 mJ (Fig. 6). The pulse duration increases from 119 ns at 1 kHz to 433 ns at 20 kHz, decreasing the peak power from 82.3 kW to 1.8 kW (Fig. 7). Under the full output power, the corresponding power stability (root mean square) within 1 h is 1.2% .

Lasers emitted in the 1.3 μm spectral region have received significant attention owing to increasing applications in remote sensing, timing systems, dermatologic procedures, and nonlinear frequency conversion. It is well known that Nd∶YLF is a promising material for generating high-energy 1.3 μm pulsed laser because of its extended upper-laser-level lifetime. However, the power scaling of 1.3 μm Nd∶YLF lasers is challenging because of their small stimulated emission cross-section and low thermal fracture limit. An end-pumped scheme with a broadband 880 nm laser diode (LD) is investigated to overcome these limitations. However, the power stability of the 1314 nm laser is reduced by the thermal wavelength shift and linewidth fluctuation of the broadband LD. When the broadband LD is used as the pump source, it is difficult to simultaneously improve the pump absorption efficiency, enhance the mode-to-pump overlap efficiency, and reduce the thermal stress of the laser crystal. Therefore, the high-power, high-efficiency laser output is greatly restricted. This paper introduces a wavelength-locked narrowband 880 nm LD as the pump source for generating a stable, efficient, and powerful 1314 nm laser.

Figure 1 shows the experimental setup. The pump source is a fiber Bragg grating (FBG) locked narrowband fiber-coupled LD with a numerical aperture of 0.22 μm and a core diameter of 200 μm. Its center wavelength is stabilized at 879.9 nm with a narrow spectral bandwidth of 0.2 nm. A pair of coupling lenses with 1:5 magnification is used to re-image the pump beam with a spot diameter of approximately 1 mm into the gain medium. An a-cut 1.0% (atomic fraction) Nd∶YLF crystal with a size of 3 mm×3 mm×30 mm is selected as the gain medium, which is coated for high transmission at 880 nm and 1047-1321 nm on the entrance surface and high transmission at 1047-1321 nm and partial reflectivity at 880 nm (reflectivity R≈60%) on the rear surface. Under non-lasing conditions, the pump absorption efficiency exceeds 90%. During the experiments, the gain medium is wrapped with indium foil and closely packed using a water-cooled copper holder at 16 °C. The Q-switched device is a 46-mm-long acousto-optic modulator plated with a 1314 nm antireflection coating on both surfaces and driven by a 27.12-MHz ultrasonic frequency generator operating at a 100 W radio frequency. The linear resonator is composed of a plano-concave mirror M1 with a radius of curvature of 500 mm and a plane output coupler M2. The input mirror M1 is coated for high transmission at 880 nm and 1047-1053 nm and high reflection at 1314-1321 nm, whereas the plane mirror M2 coated for partial reflectivity at 1314 nm (coupling output rate T_OC=5%, 10%) is employed as the output coupler. Considering the thermally induced diffraction loss and energy transfer upconversion (ETU) effect, the optimized mode-to-pump ratio is approximately 0.84. Consequently, the physical length of the resonator is set to approximately 250 mm based on the ABCD matrix theory.

A high-power end-pumped Nd∶YLF laser operating at 1314 nm is demonstrated using a wavelength-locked narrowband 880 nm laser diode. The optimized mode-to-pump ratio is approximately 0.84 considering the thermal and ETU effects. The Nd∶YLF laser delivers the maximum continuous-wave output power of 20.4 W with an optical-to-optical conversion efficiency of 29.1% and a slope efficiency of 32.5%. After Q-switching with an acousto-optic modulator, the laser system generates the maximum average output power of 16.5 W at 20 kHz and the maximum pulse energy of 9.8 mJ at 1 kHz. To the best of our knowledge, we demonstrate the highest average power and highest pulse energy from Q-switched end-pumped single-crystal 1.3 μm Nd∶YLF lasers. Future upgrades to achieve higher output power and pulse energy will involve a multisegment-doped or diffusion-bonded Nd∶YLF crystal and a double-end pumping scheme.