Enhanced Raman Spectra in Femtosecond Laser Inscribed Yb:YVO4 Channel Waveguides

The femtosecond laser writing with double-line technique was employed to fabricate buried channel waveguides with different widths in Yb:YVO4 crystal. Model profiles of the waveguides were captured using the endface coupling setup at the wavelength of 633 nm under TE and TM polarization. Furthermore, the confocal micro-Raman spectra in bulk and waveguide areas were studied at the wavelength of 633 nm. The enhanced Raman intensity were performed in waveguide areas.


INTRODUCTION
Yttrium vanadate (YVO 4 ) is isostructural and crystallizes in the tetragonal space group D 19 4h with four molecules per unit cell (Z 4), which is an attractive laser host material for producing new highly efficient micro and diode-pumped systems [1][2][3]. YVO 4 crystals are largely fabricated at lower cost due to the excellent optical quality with larger hardness and better water insolubility [4][5][6]. Rareearth ions doped in YVO 4 exhibit excellent properties [4,6]. Nd ions doped with YVO 4 in a common diode-pumped solid state laser have been studied extensively [7]. Yb 3+ ion has a simple two-level electronic band structure. Just like Yb doped laser medium, Yb doped YVO 4 crystals possess strong absorption band near 985 nm, high thermal conductivity, and broad gain bandwidth [8,9], showing potential applications in continuous-wave, Q-switched and mode-locked [6]. Therefore, the study on the optical properties of Yb:YVO 4 crystals is essential and significant.
As a basics component of integrated photonics, optical waveguide structures can confine light propagation within small volumes. The optical properties perform better in waveguide areas in comparison with the substrate. More than one hundred materials are fabricated as optical guiding structures, including single crystals, polycrystalline ceramics, glasses, semiconductors, and organic materials [10][11][12][13][14]. As a fast, flexible, and cost-effective technique, femtosecond laser inscription has been used to fabricate diverse photonic structures on micro-or nano-scales [13]. Femtosecond laser inscription display controlled properties with ultrashort pulse width, extremely high peak intensity and other adequately inscription parameters [13]. So, channel waveguide structure with double lines in Nd:YVO 4 crystal was studied by femtosecond laser inscription [15]. As an excellent optical materials, waveguides in Nd:YVO 4 crystal have been also fabricated using ion irradiation with proton [16], N [17], Si [18], He [19], O [20] ions and swift Kr ions [21].
The Raman effect was discovered by C.V. Raman in 1928, which can be used to describe the inelastic scattering of photons on a quantized molecular system [22]. As an excellent Raman crystal, the Raman characteristic of YVO 4 is reported by experimental and simulated methods [23]. The Raman and infrared active modes of YVO 4 crystal were firstly theoretically analyzed and symmetry assigned in Ref. [1]. Nd: YVO 4 crystal has been studied using as self-Raman laser material, based on the Raman peak at ∼888 nm in YVO 4 host [17].
Although, Nd:YVO 4 waveguides have been well studied, the waveguide structure of Yb:YVO 4 still need more attentions. Here, channel buried optical waveguides in Yb:YVO 4 crystals were fabricated using femtosecond laser inscription with different widths. Due to the birefringence of uniaxial Yb:YVO 4 crystal, model profiles in the two-line waveguide structures at 633 nm under TE and TM polarization were investigated. The peaks in confocal micro-Raman spectra were recorded and assigned to the Raman modes in the Yb:YVO 4 waveguide areas and substrate.

EXPERIMENTS
The Yb:YVO 4 crystal with dimension 10 (x) × 6 (y) × 4 (z) mm 3 , was optically polished and applied to produce waveguides along y axis using the double-line technique. A Ti:sapphire amplifier system (Libra, Coherent Inc.) with repetition rate of 1 kHz, maximum pulse energy of 4 mJ, pulse width of 40 fs and central wavelength of 800 nm was used to fabricate the buried channel waveguide structures at the Shanghai Institute of Optics Fine Mechanics, China. A laser beam with a writing speed of 20 μm/s was focused 50 μm below the largest faces in each crystal having a lens with a numerical aperture of 0.55. Figure 1A shows the laser writing setup used in Yb:YVO 4 crystals. The details of the samples are shown in Table 1, including laser power and width of two -line varies from 6 to 24 μm with a step of 6 μm.
An endface coupling setup was employed to study the nearfield intensity profiles of the Yb:YVO 4 crystal waveguides with a He-Ne laser at the wavelength of 633 nm, as shown in Figure 1B. Furthermore, we used two microscope objective lens (× 25) to couple the 633 nm laser light through the waveguide structure. The near-field intensity profiles were then recorded using a charge-coupled device camera. A metallographic microscope using reflected polarized light was applied to catch the images of the waveguides at different magnifications. The Raman spectra of the bulk and centers of waveguide areas in the Yb:YVO 4 crystals were recorded with a micro-Raman spectrometer (Horiba/Jobin Yvon HR800) at the wavelength of 633 nm with the laser size of 1 μm at room temperature.

RESULTS AND DISCUSSION
In this study, channel waveguide structures were fabricated by laser writing using the double-line technique in Yb:YVO 4 crystal, as shown in Table 1. In order to limit the transmission of light, different writing energies were used in fabricating waveguide structures with different widths. Figure 2 contains photographs and model profiles of the buried channel waveguide structures in Yb:YVO 4 crystal at the wavelength of 633 nm under TE and TM polarization. The photographs collected using a microscope at cross-section of S 1 (6 µm), S 2 (12 µm), S 3 (18 µm), and S 4 (24 µm) were shown in Figure 2A. Many flaws at the junction of the crystal and air originate from optical polishing. Areas between the laser writing two lines can be clearly seen at the endface photographs of S 1 , S 2 , S 3, and S 4 , indicating the possible formation of the waveguide structures with refractive index variation. The model profiles of S 2 (12 µm), S 3 (18 µm), and S 4 (24 µm) that were measured at the wavelength of 633 nm under TE and TM polarization were shown in Figure 2B. The dashed lines mean the damage track in S 2 (12 µm), S 3 (18 µm), and S 4 (24 µm). The results show that S 1 cannot confine the 633 nm laser light under both TE and TM polarization, while S 2 only confines the 633 nm laser light under TM polarization. 12 µm (S 2 ) maybe the lower limit width of mode transmission in Yb:  To investigate the effect on the laser writing, the confocal micro-Raman spectra in S 0 , S 1 , S 2 , S 3 , and S 4 were obtained by using a confocal micro-Raman system with an excitation wavelength of 633 nm at room temperature, as shown in Figure 3A, which corresponds to the symmetry species (A 1g + B 1g ) [24]. The spectra were measured vary from 100 to 1,000 cm −1 at the smallest end face of the samples. Raman peak positions were listed in Table 2, where none extra peak and peak shift existence in waveguide areas and Yb:YVO 4 substrate comparing with the results in Refs. [16,17,23]. The experiment modes vary with different wavelengths.
The Raman spectra can provide information about the phonon modes, from the energy of the laser photons shifted by the interaction of the laser light with phonons or other excitations. As reported in Refs. [25,26], the Raman peaks corresponds to the internal vibrations into VO 4 3− group and external vibrations of complex VO 4 3− and Y 3+ ions in YVO 4 unit cell. The symmetric stretch (ν 1 ), symmetric bending (ν 2 ), antisymmetric stretch (ν 3 ) and anti-symmetric bending (ν 4 ) of the VO 4 "molecules" are four distinct internal vibrational modes in VO 4 tetrahedron [19,26,27]. The peaks at 153, 485, and 813 cm −1 are assigned to B 1g mode; the peaks at 375 and 888 cm −1 are to A 1g mode; 258 and 836 cm −1 are belongs to the Eg modes [1,17,23]. As reported by Zhang et al. [28], the change of peak position in crystal Raman spectrum could disclose the stress and strain in the sample, while the change of full width half maximum (FWHM) indicates crystal quality change. In our work, the peak position and width are both none change, meaning the waveguide areas keep good crystal structures. While one can see that all peak intensities are enhanced in the waveguide areas comparing with the Yb:YVO 4 substrate, as shown in Figure 3B. Especially, the enhanced intensity of confocal micro-Raman peaks at 888, 836, and 485 cm −1 were performed with increasing waveguide width in S 1 , S 2 , S 3 , and S 4 , making the obtained waveguides promising candidates for the development of integrated self-Raman laser sources.

CONCLUSION
We fabricated buried channel waveguides in Yb:YVO 4 crystal using the femtosecond laser writing with double-line technique. The photographs of waveguides structures with the widths of 6, 12, 18, and 24 µm were captured using a metallographic microscope with reflected polarized light. Model profiles at TE Observed modes (cm −1 )@633 nm Experimental modes (cm −1 )@473 nm [16] Experimental modes (cm −1 )@514.5 nm [17] Experimental modes (cm −1 )@514.5 nm [8] Assigned Raman modes [8,16,17]   Frontiers in Physics | www.frontiersin.org September 2021 | Volume 9 | Article 758071 and TM polarization were captured using the endface coupling setup at the wavelength of 633 nm. The results imply that the width of two lines fabricated using the laser writing influences the light guiding. The confocal micro-Raman spectra of the bulk and waveguide areas in the Yb:YVO 4 crystal were studied at 633 nm. The enhanced Raman intensity at 888, 836, and 485 cm −1 were performed with increasing waveguide width in waveguide areas.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.