Waveguide lasers are miniature light sources which, compared with the bulk systems, have reduced lasing thresholds and enhanced efficiencies due to the compact geometries of the waveguiding structures , , , , , , . The dielectric waveguide lasers are well investigated by researchers to realize diverse applications in a number of areas , , . These waveguides are constructed based on the platforms of laser materials, including crystals, glass and ceramics , , , . By using the optical pump technique, numerous high-performance waveguide lasers have been realized at lasing wavelength from visible to mid-infrared (MIR). In addition, both the continuous wave (CW) and pulsed lasers have also been operated successfully , , , , , . Recently, waveguide lasers in pulsed regimes have received great attention from researchers due to the possibility that on-chip nonlinear optical effects may be realized with ultrafast compact light sources , , . By applying the saturable absorbers (SAs) of low-dimensional materials, such as nanoparticles, carbon nanotubes, or two-dimensional (2D) materials (graphene, MoS2, black phosphorus, heterostructures, among others, the Q-switched, Q-switched mode-locked and CW mode-locked lasers have been achieved , , , , , . Particularly, the mode-locked waveguide lasers with pulse repetition frequencies up to the GHz level have attracted great attention for their applications in precision metrology, ultrafast nonlinear spectroscopy and high-speed optical communication , , , , . Owing to the high stability and spectral purity, GHz fundamental mode-locked lasers are ideal in meeting the demand of related practical applications. In most designs for the pulsed waveguide lasers, the SAs are coated on additional mirrors or glass plates, or deposited on the waveguide surfaces or end-facets. These designs require further alignments of the components with SAs in the system, and the abrasion of SAs may be possible during the experimental procedure. For more compact devices, one may require monolithic structures for the SAs and the waveguide structures. The monolithic structures can enhance the interaction of SAs and waveguide modal fields and further stabilize the system.
The metallic NPs are zero-dimensional materials that have applications in many areas , , , , , , . These metallic NPs could also be used as successive SAs in laser technology for laser pulse generation , , . For example, chemical techniques have been applied to synthesize Au NPs as SAs to implement ultrafast lasers in bulk or fiber systems . Ion implantation is a mature technique of synthesizing NPs that are embedded in the dielectric materials. Recently, it has been reported that a number of metallic NP-embedded crystals, such as Nd:YAG and LiNbO3, exhibit saturable absorption properties, which is due to the modulation of the optical nonlinearities of NPs onto the bulk crystals , , . This modification of bulk materials is considered to be correlated with the surface plasmon resonance (SPR) effect, which happens under light irradiation of NPs , , . By applying the crystal wafer embedded with metallic NPs as SAs, pulsed lasing has been realized in both visible and near-infrared (NIR) light bands. For example, the Q-switched mode-locked lasers have been implemented in a Nd:YVO4 waveguide cavity with an additional Au NP-embedded LiNbO3 wafer as SA, achieving 74-ps pulse duration and 6.4 GHz repetition rate . One of the advantages of the SAs of embedded NPs is the stabilized system, which offers non-abrasion interaction of SA materials with other optical elements . To further utilize this feature, one may consider synthesizing the embedded NPs in a laser material and constructing a “waveguide plus NPs” system in a single chip. In the current work, we propose a new design of ridge laser waveguide with embedded Ag NPs as SA to generate the Q-switched mode-locked lasers in a monolithic waveguide chip based on the Nd:YAG crystal, which is a widely used gain medium for the solid-state laser systems. The Ag NPs are embedded in Nd:YAG by direct Ag+ ion implantation to modulate the optical nonlinearity. The ridge waveguide is produced by 12 MeV O5+ ion implantation and subsequent diamond saw dicing, which is a recently developed technique for high-quality waveguides. With this processing, the waveguide lasers are operated in pulsed regime through the evanescent field interaction with the Ag NPs in the monolithic device.
2 Experimental section
2.1 Sample preparation
2.1.1 Synthesis of Ag NPs
Figure 1 shows the schematic plots of the monolithic waveguide fabrication of the Nd:YAG crystal embedded with Ag NPs. A cubic YAG crystal wafer doped by 1 atom % Nd3+ ions was cut to the following dimensions: 10×7×2 mm3. After the process of optical polishing, the Ag+ ions were implanted into the facet of 10×7 mm2 by using an analytical type ion-implanter LC22-1C0-01 at Wuhan University, with an energy of 200 keV and fluence of 5×1016 ions/cm2 at room temperature (Figure 1A). The randomly embedded Ag ions in the Nd:YAG crystal aggregated during ion implantation, resulting in the formation of embedded Ag NPs.
2.1.2 Fabrication of the optical waveguide
The optical waveguide in Nd:YAG was produced by 12 MeV oxygen ions (O5+) at a fluence of 4×1016 ions/cm2 through a 3 MV tandem accelerator at Helmholtz-Zentrum-Dresden-Rossendorf (HZDR) (Figure 1B). The ion beam was tilted by 7° off the normal plane of the sample surface to minimize the channeling effect. Then, the beam current density remained at a low level (<10 nA cm−2) to avoid charging and heating of the sample. After the O5+ ion irradiation, the planar optical waveguide was finally formed after the scanning of the ion beam. For comparison, we produced two Nd:YAG waveguide samples: sample A without Ag NPs and sample B with embedded Ag NPs.
The precise diamond blade dicing (DISCO Corp.) was used to manufacture the ridge optical waveguide (see Figure 1C). With a diameter of 55.56 mm and thickness of 12 μm, the diamond blade rotated and moved along the direction of the 7 mm side. The cutting speed and the rotating speed were set as 3 mm/s and 4500 r/min, respectively, to minimize the chipping and cracking of the edges. Several air grooves were formed with the depth of 8 μm, and the ridge optical waveguides were constructed with the ridge width of 10 μm.
2.2 Experimental characterization
2.2.1 Morphology characterization
Transmission electron microscopy (TEM) investigations were carried out by using Tecnai G2 F20 S-Twin (FEI) operated at an accelerating voltage of 200 kV. The pixel resolution and the line resolution were 0.24 and 0.102 nm, respectively, and the information resolution was less than 0.14 nm. For the high-resolution transmission electron microscope (HRTEM), the atomic number contrast imaging by HAADF-STEM was performed with the resolution of 0.2 nm. For the qualitative chemical analysis, EDXS was performed with a Li-drifted silicon detector (EDAX). Before TEM analysis, the sample was bonded by M-bond 610 and a classical cross-sectional TEM specimen was prepared by double side grinding method until the specimen was thinner than 20 μm. During the process, the voltage of the Gatan 691 type Ion Beam Thinner gradually decreased from 4.8 kV to 3 kV. At the same time, the angle decreased from 10 degrees to 4 degrees, resulting in the appropriate specimen for TEM observation.
In addition, for the purpose of visual observation, the cross-section of the ridge waveguide was investigated by microscope (Axio Imager) with the amplification times of 500× in the transmission mode.
2.2.2 SPR characterization
The linear absorption spectrum of sample A (the sample with waveguide only) and sample B (the sample with both the Ag NPs and waveguide) were measured by using a UV–vis–NIR spectrophotometer (Hitachi, U-4100) at room temperature in wavelengths ranging from 190 to 1100 nm. The SPR absorption of the Ag NPs was obtained by the deduction of sample A absorption to exclude the absorptions of the rare-earth ion (Nd3+) and waveguide.
A typical Z-scan system with an open aperture was implemented to investigate the nonlinear absorption of the Ag NP-embedded Nd:YAG crystal. With the pulse width of 340 fs, the mode-locked femtosecond laser operated at 1030 nm was focused by a lens with a 150 mm focal length. The repetition rate was adjusted to be 1 kHz. Meanwhile, the pulse energy was controlled by a calibrated neutral density filter. With the sample moved along the laser propagation direction by a linearly motorized translation stage, the transmittance of sample as a function of the optical intensity was detected.
2.2.3 Q-switched mode-locked laser operation
An end-face coupling system was utilized with a tunable CW Ti:sapphire laser as pump source. A half-wave plate was employed to control the polarization. The beam operated at 810 nm was focused by the spherical convex lens with the focal length of 30 mm. The sample was placed on the 3D-motorized stage and clamped tightly by a pair of mirrors (M1 and M2) to construct the lasing resonant cavity. The input mirror M1 was coated with films (high transmission of 98% at 810 nm and high reflectivity of 99% at 1064 nm) on two sides. The output mirror M2 was normal glass with transmission of 90% at 1064 nm. Both the flat mirrors M1 and M2 were directly butt-coupled to the waveguide chip. By coupling the pump light into the ridge waveguide structure with a coated Ag NP-embedded layer as the SA, the 1064 nm Q-switched mode-locked pulsed laser was generated. In addition, a filter was set before the detectors to filter the stray light. Finally, the output power and near fundamental modal distribution were investigated by using a power meter and CMOS, respectively. The mode-locked performance of the pulsed laser was monitored by using a digital oscilloscope (Tektronix MSO72504DX).
3 Results and discussion
As shown in Figure 2, a three-layer structure is constructed, with Ag NPs embedded in the surface layer of the optical waveguide structure based on the Nd:YAG platform. Sample A in Figure 2A is the reference sample with only the O5+ ion-implanted planar waveguide. Combined with the Ag ion implantation and the precise diamond blade dicing, ridge waveguides are formed on Sample B, of which the enlarged image is displayed in the insert of Figure 2A. As seen in the cross-sectional microscope images in Figure 2A, the width of the ridge waveguide is measured to be 10 μm, and the depth of both the ridge and planar waveguides is ~3 μm. To characterize the morphology of the NPs, TEM observation was carried out as shown in Figure 2B. The Ag NPs have been fabricated and distributed below the surface of sample with the depth of 120 nm, as shown in the insert of Figure 2B. The physical mechanism and the simulated ion distribution are depicted in Figure S1, which agrees well with experimental results. The statistical analysis of the TEM image is shown in Figure 2C. As we can see, the mean diameter of NPs is calculated to be 3 nm. Furthermore, the HRTEM micrographs of region A, B and C in Figure 2B are displayed in Figure 2D–F, which respectively reveal the detailed information of NPs, region at the end of the injection and the waveguide region without NPs. The enlarged images are shown in the upper inserts, and the Selected Area Electron Diffraction (SAED) images are displayed in the under inserts. As shown in Figure 2D, the Ag NPs show a polycrystalline state with lattice spacing of 0.2359 nm. With the increase of depth, the polycrystalline state of NPs is gradually replaced by the crystalline of the Nd:YAG crystal as seen in Figure 2E. At the end of the Ag ion injection range, the SAED image in Figure 2E shows two types of diffractogram patterns, which are respectively attributed to the Ag NPs and the Nd:YAG crystal. As for the waveguide region without the NPs, the HRTEM image shows a superior crystalline state with the lattice spacing of 0.8403 nm. It means that the implanted O5+ ions may have barely influenced the crystalline Nd:YAG crystal, which demonstrates that the crystalline state may be well reserved at the waveguide region. In addition, the dark field TEM image is depicted in Figure 2G, and the element mapping of Ag ions and the EDX spectrum are displayed in Figure 2H and I. The area within the orange frame in Figure 2G is the scanning region of Figure 2H. It further confirms the formation and distribution of the Ag NPs.
In order to verify whether the superior SPR response of the Ag NPs is well-remained in the waveguide, we measured the linear absorption of the Ag NPs by the deduction of sample A absorption to exclude the absorption of the Nd:YAG waveguide. As shown in Figure S2(a), the embedded Ag NPs have a wide SPR absorption band from 1 eV to 4 eV, and the absorption peak is located at 2.55 eV with an energy gap of around 1.7 eV. The near field intensity distribution of the NPs is simulated by numerically solving Maxwell’s equations using DDA, as implemented in the DDSCAT code . The excited wavelength is 2.55 eV according to the absorption peak shown in Figure S2(a), while a spherical model with diameter of 3 nm is employed based on the mean diameter in Figure 2C. As shown in Figure S2(b), the excited light is confined by the Ag NP, and the near field intensity around the NP is enhanced because of the strong surface plasmon resonance response. For the reason that the size effect is not considered in the DDSCAT software, we use the Mie theory combined with the classical mean-free path confinement effect to simulate the absorption of Ag NPs as follows :
where εm and εd are the complex dielectric constants of metal and insulator, respectively; and λ0 denote the imaginary part of εm and the wavelength of light in vacuum, respectively; p represents the volume fraction of the metal, where ωτ,0 denotes the bulk relaxation energy; νϝ denotes the Fermi velocity and d is the NP diameter. The calculated results are shown in Figure S2(c). With the increase of the NP’s diameter, the absorption peak will have an obvious increase accompanied by a slight red shift. It should be pointed out that when the diameter is 3 nm, the calculated peak and energy gap are located at 2.55 eV and around 1.7 eV, respectively, which are in good agreement with the measured results, as shown in the Figure S2(a).
We measured the polarization-dependence of Sample A and Sample B by coupling the 1064 nm laser into the optical waveguide based on the end-face coupling system. As shown in Figure 3, both the waveguides with NPs and without NPs present a strong polarization-dependence at 1064 nm: only guided at TM polarization. The waveguide loss was calculated by Eq. 3  as follows:
where Pin and Pout are the in-coupled and output light powers, respectively; L is the length of light propagating in the sample; R is the reflectance and η is the coupling efficiency. The waveguide loss at the TM polarization is calculated to be 0.8 dB/cm, which suggests the superior waveguiding properties at 1064 nm. In general, traditional optical waveguides based on the cubic Nd:YAG crystals show polarization-independence because of the similar absorption cross-section of pure crystal at both the TE and TM polarizations . However, the depth of the optical waveguide in this work is much smaller than the reported Nd:YAG waveguide by previous studies, and the center of near-field modal distribution is closer to the substrate. It is speculated that the laser with the TE polarization may be more sensitive to the depth of this kind of waveguide with constant refractive index at the TE direction. In this way, laser with the TE polarization may be easier leaked to the substrate.
As one of the most attractive optical responses of SPR, the ultrafast nonlinear optical absorption of the Ag NPs was also measured by the Z-scan measurement, which is depicted in Figure 3C. It can be seen that the transmittance is maintained at a constant 89% with probe intensity lower than 0.4 GW/cm2, indicating that the nonlinear absorption response does not appear at a low probe intensity. With the probe intensity increasing to ~0.4 GW/cm2, the transmittance is enhanced gradually and remains at a value of 91.7% at high probe intensity. Furthermore, considering the contribution of the intraband transitions and free-carrier absorption, the saturation intensity is obtained by fitting the experimental data with the combination of equations expressed as 
where α(I) is the optical intensity-dependent nonlinear absorption coefficient; L is the thickness of the Ag NP layer , ,  and α0 and αNS denote the saturable absorption coefficient and non-saturable absorption coefficient, respectively. In addition, η and Isat represent the free-carrier absorption coefficient and the saturation intensity, respectively. The corresponding values of Isat, αNS and η are determined to be 0.63 GW/cm2, 4210.86 cm−1 and 82.13 cm·GW−1. At the same time, the saturable absorption coefficient α0 and the modulation depth ΔT are calculated to be 11510 cm−1 and 2.7%, respectively. Compared with the nonlinear optical properties of other metal NPs embedded in the crystal materials, the modulation depth in this work is higher and the saturation intensity is much lower , . It should be noted that the sample without Ag NPs shows the two-photon absorption property at the same condition as that presented in Figure S3. It is demonstrated that the embedded Ag NPs with superior ultrafast saturable absorption at the NIR are well integrated with the Nd:YAG waveguide chip.
Based on the ultrafast saturable absorption response, the passively Q-switched mode-locked laser is achieved on the integrated platform by the end-face coupling system as depicted in Figure 4 (see the Experimental Section for detailed description). The schematic diagram of the laser configuration is shown in Figure 5A. As can be seen, the SA (Ag NPs) is integrated on the gain medium (Nd:YAG waveguide). With the combination of M1 and M2, the laser resonant cavity is constructed for the NIR Q-switched mode-locked laser generation. The 810 nm pump laser is coupled into the integrated waveguide, and the output pulsed laser is obtained owing to the evanescent-field coupling effect. Figure 5B reveals the corresponding spectra of the pump laser and output laser. The polarization-independent continuous wave pump laser is located at 810 nm; nevertheless, the pulsed output laser at 1064 nm shows the polarization-dependence due to the different waveguiding properties at the different polarization as mentioned above. The near-field modal profiles under the condition of the TM pump show superior fundamental mode as seen in Figure 5C.
Figure 6 summarizes the typical characteristics of the mode-locked laser at TM polarization. As shown in Figure 6A and B, the Q-switched envelope and mode-locked pulse trains are recorded on the nanosecond (40 ns/div) and picosecond (400 ps/div) time scales, respectively. The maximum repetition rate of the Q-switching is measured to be 31.3 MHz, which is enhanced by 2 orders of magnitude compared with that of graphene-based SA by evanescent field interaction , . This result demonstrates the higher evanescent field absorption efficiency of the embedded plasmonic Ag NPs compared with that of the surface-coated graphene layer. Figure 6C depicts the single mode-locked pulse trace with the pulse duration of 29.5 ps, whereas the radio frequency (RF) spectrum is shown in Figure 6D. As can be seen, the measured fundamental repetition rate is 10.53 GHz with signal-to-noise ratio of 44.6 dB. As for the operation of the mode-locked waveguide lasers in the Q-switched mode-locked regime, this work obtains the relatively higher repetition rate compared with those reported in some past works , , , , 52], , . Compared with the mode-locked laser obtained by attaching the separate graphene SA on a Nd:YAG waveguide system (pulse duration of 16 ps, repetition rate of 11.3 GHz and maximum output power of 12 mW), the combined system with the encapsulated Ag NPs proposed in the current work exhibits comparable mode-locking performances (e.g. repetition rate and pulse duration) with much enhanced laser efficiency and much higher output power (168 mW) . As for the mode locking by single-wall carbon nanotubes (pulse duration of 1.6 ps, repetition rate of 16.7 MHz), the pulsed laser in the present work owns shorter pulse duration but much higher repetition rate . The SESAM-based mode-locked laser has the highest repetition rate (15.2 GHz) and shorter pulse duration (738 fs), but much lower average power (60 mW) . In addition, this work shows superior laser properties compared with the mode locking by ITO (pulse duration of 526 ns, repetition rate of 6.4 GHz, output power of 28.6 mW) . The concept of encapsulating plasmonic nanoparticles in the laser microstructures has shown its great potential to be a composite monolithic platform for future ultrafast on-chip applications, with performance that is comparable to laser systems with separate 2D materials and waveguide platforms.
The fundamental repetition frequency f and the stability criterion of mode-locking based on the linear Fabry-Perot cavity EP,c can be calculated by the following equations :
where c is the speed of light, n is the refractive index of the waveguide and l is the length of cavity. In addition, Esat,L is the saturation energy of gain medium, and the Esat,A denotes the absorber saturation energy of the SA. Based on Eq. 6, the fundamental repetition frequency is estimated to be 10.54 GHz, which is consistent with the experimental result. With the modulation depth ΔT of 2.7%, the EP,c is determined to be 5.59×102 nJ. Considering the larger EP,c value compared with the typical intracavity pulse energy of waveguide laser, the use of laser system optimization (e.g. adding Gires-Tournois interferometer) to realize CW mode-locking shall be the subject of our further work. Moreover, the Q-switched mode-locked pulsed laser in this work shows superior polarization-maintaining behavior: only the pulsed laser with TM polarization is obtained, which suggests applications in designing devices of monolithic polarized waveguide light sources.
We have produced a monolithic waveguide laser device by encapsulating the Ag NPs into the Nd:YAG ridge waveguide using the ion implantation and diamond saw dicing. The Ag NPs in Nd:YAG matrix show superior SPR absorption at the peak of 2.55 eV, which is in agreement with the calculation by the combination of classical mean-free path confinement effect and Mie theory. Furthermore, the near-field intensity distribution is simulated by DDA algorithm, indicating the enhanced near-field intensity distribution due to the SPR response. Particularly, the encapsulation of the plasmonic Ag NPs has endowed the fabricated monolithic ridge device with ultrafast saturable absorption properties at 1 μm. With this feature, the stable Q-switched mode-locked laser at 1064 nm is achieved by evanescent field interaction under optical pump, reaching maximum repetition rate of 10.53 GHz and minimum pulsed width of 29.5 ps. Moreover, the Q-switching repetition rate (31.3 MHz) is enhanced by 2 orders of magnitude compared with that of the graphene-based SA. Our work paves the way for the realization of polarization-maintaining multi-gigahertz mode-locked lasers based on the monolithic waveguide chip.
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The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2019-0035).
About the article
Published Online: 2019-03-23
Funding Source: National Natural Science Foundation of China
Award identifier / Grant number: 11535008
Award identifier / Grant number: 61522510
The authors acknowledge the financial support provided by the National Natural Science Foundation of China (NSFC), Funder Id: http://dx.doi.org/10.13039/501100001809 (Grant Nos. 11535008, 61522510), Project 111 of China (Grant No. B13029) and the STCSM Excellent Academic Leader of Shanghai (Grant No. 17XD1403900).
Citation Information: Nanophotonics, Volume 8, Issue 5, Pages 859–868, ISSN (Online) 2192-8614, DOI: https://doi.org/10.1515/nanoph-2019-0035.
©2019 Feng Chen et al., published by De Gruyter, Berlin/Boston. This work is licensed under the Creative Commons Attribution 4.0 Public License. BY 4.0