We report an investigation of waveguide inscription by femtosecond laser direct writing in two newly developed mid-infrared laser garnets. Usable guiding structures are obtained in one of these garnets but not in the other. This difference is mainly attributed to stimulated radiation, which is more intense in the latter garnet so that less absorbed energy of writing laser is involved in the process of waveguide creation. The mechanism for laser modification is ascribed to garnet intrinsic defects creation and crystalline lattices distortions. Their associated refractive index changes and stress field are essential conditions for waveguide formation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
The mid-infrared (MIR) wavelength band, typically ranging from 2.5 to 5 µm, is a strategically important region of the electromagnetic spectrum due to various potential applications in communications, biosensing, medical diagnostics, astronomy, environmental monitoring and industrial explorations [1–5]. With the fast development of diverse integrated techniques, increasing number of optical devices, including those operating at MIR band, will be integrated into small photonic chips. In optics, laser sources are indispensable devices. Indubitably, miniature MIR light sources are desirable for photonic integrated platforms.
So far, a number of structures have been successfully created into gain materials by using direct processing, such as proton beam writing , ion beam irradiation  or femtosecond laser direct writing (FLDW). Since first demonstrated in 1996 [8,9], the femtosecond (fs) laser micromachining technique has arisen as one of the most efficient technology for integrated optics due to outstanding performances like short material interaction time, high positioning precision, rapid prototyping and unique three-dimensional fabricating capability. These excellent properties of FLDW provide material and geometric manufacture freedom.
As the first steps toward miniature laser sources in all-on-chip-based optical platforms, FLDW technique has been used to fabricate waveguides into many laser media [10–22], and recent efforts further extend the radiation wavelength range of fs laser written waveguide lasers into the MIR region [23–30]. Compared with the lasing from bulk materials, waveguide lasers exhibit attractive advantages including compactness, environmental robustness, enhanced output performances and a low radiation threshold [31,32]. The waveguides fabricated by FLDW could be classified into 4 categories: Type I waveguides, Type II waveguides, cladding waveguides and ridge waveguides . A Type I waveguide is a single line of increased refractive index created in the laser focal region. A Type II waveguide typically consists of two filaments with decreased refractive index induced by laser exposure. A cladding waveguide usually has cross sections with large area of unmodified material that surrounded by decreased refractive index or damage tracks created by laser writing. A ridge waveguide ablated by FDLW technique on a planar waveguide. For many crystalline dielectrics, FLDW induces lattice amorphization, disorder or defects, results in compression and refractive index reductions near and in the laser modified region [34,35].
In this paper, we explore the prospect for waveguide fabrication by using FLDW technique for two newly developed MIR laser garnets. Usable guiding structures are obtained in only one of these MIR laser crystals but not in the other, though the femtosecond laser writing conditions applied on both materials are the same. The reason of this difference and the mechanism of laser induced changes in garnets are analyzed in view of the Raman spectra of laser modified and untreated materials. The results have implications for the design of future laser writing applications in this type of media. On the other hand, the results also enable the optimization of the material preparation to provide specific properties for laser process.
2. Material properties
The materials under study, known as Yb,Er,Ho:GYSGG and Er,Pr:GYSGG, are new MIR laser crystals developed by Chinese Academy of Sciences and shipped with a size of 2×2×5mm3 [36,37]. The former crystal was grown from a melt of congruent composition including 10 at. % Er3+, 1 at.% Yb3+ and 0.1 at.% Ho3+ by the Czochralski method. It has the chemical formula (Yb0.03Er0.3Ho0.003Gd1.17Y1.497)Sc2Ga3O12. The latter crystal was grown in the same way from a melt of congruent composition including 20 at.% Er3+ and 0.3 at.% Pr3+. Its chemical formula is (Er0.6Pr0.009Gd1.17Y1.221)Sc2Ga3O12. The hosts of both crystals are Gadolinium Yttrium Scandium Gallium Garnets (GYSGG), which attract increasing interests because of their excellent performance on emission endurance and dual-wavelength radiation. The refractive index of GYSGG at 800 nm (wavelength of laser for waveguide inscription) is about 1.9. The 2.79 µm MIR lasing originates from the 4I11/2 →4I13/2 transition of trivalent erbium (Er3+) ions. Codopants ytterbium (Yb), holmium (Ho) and praseodymium (Pr) are added to improve crystal properties. Yb3+ ions are sensitizers, Ho3+ and Pr3+ act as deactivators in the garnets. These bulk materials have been reported that show outstanding MIR laser performance at 2.79 µm and we expect they will also be good candidates for MIR waveguide lasers.
3. Waveguide preparation and characterizations
The FLDW setup used to create waveguides into the GYSGG materials was a regeneratively amplified Ti:Sapphire femtosecond laser system (wavelength 800 nm, repetition rate 1 kHz, pulse width about 120 fs, linear-polarized). The morphological characteristics of structures fabricated under different focusing conditions at different laser energy levels were systematically investigated in our previous report . Based on the experimental results described in , we attempted to fabricate Type II waveguides into the new MIR laser crystals using a 100× (NA 0.90) objective. Femtosecond laser beam was focused via the objective into crystals at 150 µm depth beneath material surface. Then the crystals were translated through the laser focus perpendicularly at a speed of 3 mm/min. Four femtosecond laser energy levels, 100 nJ, 500 nJ, 1 µJ and 1.5 µJ, were selected for waveguide fabrication. The gap between the two filaments of type II waveguide was designed as 10 µm, 15 µm, 20µm, 30 µm, 40 µm and 50 µm respectively. During femtosecond laser writing, green light was observed in both crystals by naked eyes.
After fabrication, the end-faces of waveguides were polished for subsequent examinations. The completeness and cross sections of created structures were examined under a microscope. Light was coupled into waveguides on a 3-axis stage to study the guiding properties of waveguides. 532 nm laser was focused into the waveguide by using a 5× objective, the waveguide output was collected by using a 10× microscope and then imaged on a charge coupled device (CCD). Raman spectroscopy was applied to laser modified and untreated materials to investigate the mechanism of changes in both garnets induced by femtosecond laser inscription. The Raman analysis was performed using a Renishaw inVia confocal Raman microscope, which could attach to a 532 nm or a 785 nm excitation laser. The spectral resolution of the Raman spectrometer is 1 cm-1. The Raman scan range was set as 50 to 3200 cm-1 in wavenumber. Before measurements of garnets, test scans (excited by 532 nm or 785 nm laser respectively) were performed on a silicon and a silica sample separately to check the system alignment and to identify any light contamination in the environment.
3.1 Similar configurations but different guiding properties of structures created in two garnets
The form of laser modifications in both garnets was similar. For instance, Fig. 1(a) and (b) are the microscopy images of top-view and cross section of structures fabricated at 100 nJ energy level in Er,Pr:GYSGG. As shown in Fig. 1(b), the cross section of a waveguide consists of a pair of narrow filaments. The penetration depth of filaments was around 75 µm that was mainly due to the spherical aberration caused by the high numerical aperture of the focusing objective .
Although the morphological features of structures fabricated in both crystals were similar, their guidance properties were distinct. Waveguides were created in Er,Pr:GYSGG by using laser at 100 nJ and 500 nJ energy level with 15 µm, 20 µm, 30 µm, 40 µm and 50 µm gaps (Fig. 1(c) (d)). Although the waveguides should be multimode at 532 nm, transmitted light propagates in the fundamental TM mode in our experimental coupling situations. Structures with 10 µm gap could not confine light and those created at 1 µJ and 1.5 µJ laser energy levels showed no guidance due to cracks occurred in polish process. On the contrary, in Yb,Er,Ho:GYSGG crystal, no guidance but only some comet-like scatterings were observed beneath those created structures no matter what kind of writing situations were used.
3.2 Confinement improvement design
In order to improve light confinement of fabricated structures, we tried to overscan the modified region to further reduce the refractive index of this region and to increase the stress caused by laser writing in garnets. However, obvious cracks occurred immediately when we overscanned the laser modified region. On the other hand, we also designed depressed cladding waveguides that the claddings of different waveguides were comprised of different number of filaments. The separations between adjacent filaments were designed as 2 µm, 3µm and 4 µm respectively. For these designs, cracks started appearing when the 4th or 5th filament (hadn't yet formed a quarter of a circle) was writing in Er,Pr:GYSGG. In Yb,Er,Ho:GYSGG crystal, cracks emerged while the number of written filaments went beyond 12 (hadn't yet formed a semicircle) regardless of the design separation width.
4. Raman spectroscopy
As established above, the hosts (GYSGG) and the main dopant (Er) in both crystals were the same. The morphological features of laser modified regions in both crystals were similar. However, the guidance properties of structures created in two crystals were distinct. Cracks were more likely to occur in areas that near the laser exposed regions, especially the laser exposed areas in Er,Pr:GYSGG. To explain these phenomena and to understand the mechanism of laser produced modification in these two novel MIR laser crystals, we applied Raman spectroscopy to femtosecond laser modified regions and untreated materials.
4.1 Raman spectra (532 nm laser excitation)
Figure 2 shows the normalized Raman spectra of two garnets obtained under 532 nm laser excitation. The system baseline was subtracted from the original experimental data and then each processed spectrum was normalized to the peak at 738 cm-1 (the typical Raman peak). The scan range of Raman spectroscopy was set as 50 to 3200 cm-1, but only half range of measured spectra is plotted here because no Raman peak was observed in these spectra while the wavenumber went beyond 1600 cm-1. Although Er3+ can induce green upconversion luminescence (in a range of 492 to 577 nm that cover the pump laser wavelength and may cause overshadowing effect in spectra) via multiple absorption transition [40–42], no fluorescence feature appears in the Raman spectra obtained under 532 nm laser excitation.
As shown in Fig. 2, the Raman spectra of two garnets are similar although dopants and their concentrations in two garnets are distinct. This phenomenon confirms that the covalent networks of these two crystals are alike since both hosts are GYSGGs. We believe most of the rare-earth dopants (Er, Pr, Yb and Ho) exist in these crystals as ions. But the differences of dopants may affect the material covalent networks, that causes intensity differences and shifts of Raman peaks (compared the yellow curve with the blue one). The peaks between 200 to 738cm-1 may be ascribed to characteristics of the M-O (M = Gd, Sc, Y, Ga) vibrations in the garnets [43,44]. The peaks appeared beyond 738 cm-1 (last 3 sharp peaks) result from unavoidable room light contamination in our lab. In detail, the peaks at 283 cm-1, 383 cm-1 and 392 cm-1 can be attributed to distortional and bending vibrations of Gd-O bonds in octahedral GdO6 or tetrahedral GdO4. The peaks at 343 cm-1 correspond to symmetric and stretching vibrations of Gd-O bonds in dodecahedral GdO8. The small peaks at 371 cm-1 correspond to distortional and bending vibrations of Y-O bonds in YO8. The peaks around 420 cm-1 can be attributed to distortional and bending vibrations of Ga-O bonds in GaO4. The strong peaks at 738 cm-1 correspond to symmetric and dissymmetric stretching of Ga-O bonds in GaO4. The small peaks seen around 520 cm-1 correspond to symmetric and stretching vibrations of Sc-O bonds in ScO6.
Compared to the Raman spectra of untreated garnets, spectra of femtosecond laser exposed regions have obvious rising broad bands centered at approximately 580 cm-1. These bands can be attributed to intrinsic defects creation and crystalline lattices distortions (CLDs) [45,46]. For most transparent crystalline dielectrics, CLDs are usually characterized by the amorphization of the lattices and a decrease in refractive index . CLDs are also accompanied by a reduction of material density, a strong stress field and ionic rearrangement . The decrease of refractive index and stress field are the essential conditions for creation of Type II waveguides.
4.2 Raman spectra (785 nm laser excitation)
Unlike those spectra observed under 532 nm laser excitation, Raman spectra obtained under 785 nm laser excitation are largely affected by fluorescence, but this proves useful in its own right. As shown in Fig. 3 (unprocessed data), fluorescence occupies most part of the spectra ranging from 1800 cm-1 to 3050 cm-1. Information in the spectra range before 1800cm-1 are also affected by fluorescence background, but they are shown as flat lines in Fig. 3 due to enormous intensity contrast. The strongest fluorescence band in each spectrum is assigned to emission around 980 nm due to 4I11/2 - 4I15/2 transition of Er3+, and the fluctuations extended to 3050 cm-1 may owning to a near-infrared (NIR) tail of 980 nm Er3+ emission falls above 1100 nm . The intensity of Er3+ fluorescence band in the spectrum of femtosecond laser modified material is slightly weaker than that of untreated one, though the measurement parameter settings for all spectra are the same. This fluorescence intensity reduction may cause by Er3+ ionic rearrangement. A small fraction of Er3+ ions are photoionized by femtosecond laser irradiation, thus the lasing originated from Er3+ decreases. However, guiding light in type II waveguides mainly transmitted in the untreated region between two femtosecond laser written filaments (Fig. 1). Therefore, we believe that femtosecond laser irradiation has ignorable impact on the lasing properties of MIR waveguide laser. Compared with the spectrum of Er,Pr:GYSGG, the spectra of Yb,Er,Ho:GYSGG have additional fluorescence peaks around 2170 cm-1 that may correspond to 950 nm emission from Yb3+ . It is worth noting that the intensity of fluorescence in the spectra of Yb,Er,Ho:GYSGG is evidently stronger than that of Er,Pr:GYSGG under 785 nm laser excitation. This phenomenon illustrates that stimulated radiation is more intense in Yb,Er,Ho:GYSGG under NIR pump, though the Er3+ concentration in Er,Pr:GYSGG is twice that of Yb,Er,Ho:GYSGG.
5. Mechanisms for laser induced-effects in garnets
We have clearly established that femtosecond laser writing can induce CLDs in both MIR garnets under study. The formation of CLDs may be associated with a reduction of material density, ionic rearrangement and a strong stress field. CLDs are usually characterized by crystal lattices amorphization and a decreased refractive index. The stress field and the decrease of refractive index are essential conditions for Type II waveguide creation. However, the generation of CLDs and accompanied stress field may be affected by stimulated radiation during femtosecond laser (wavelength 800 nm) writing, because both garnets can be excited by NIR light and emit luminescence. In other words, part of the femtosecond laser energy absorbed by the material will participate in stimulated radiation rather than waveguide creation. The NIR-excited green emission was observed during femtosecond laser writing. The Raman spectra obtained under 785 nm laser excitation (Fig. 3) also verify the stimulated radiation produced by NIR pump. In addition, these experimental data indicate that the stimulated radiation is much more intense in Yb,Er,Ho:GYSGG. According to Ref.  and , the absorption coefficient of Yb,Er,Ho:GYSGG to femtosecond laser is lower than that of Er,Pr:GYSGG. For all these analyses, we believe that, compared with the FLDW process in Er,Pr:GYSGG, less energy contributes to CLDs’ generation in Yb,Er,Ho:GYSGG due to lower absorption and stronger stimulated radiation. Therefore, guiding structures can be fabricated in Er,Pr:GYSGG but not in Yb,Er,Ho:GYSGG. Furthermore, in Yb,Er,Ho:GYSGG, each fs laser scan produces less stress inside and near the laser exposed area, thus the region near laser written filaments in this crystal can withstand more subsequent laser scans and cracks emerge later (see details in Section 3.2).
Based on the discussion above, we further increased the energy of femtosecond laser for waveguide fabrication and expected that guiding structures could be obtained in Yb,Er,Ho:GYSGG. However, micro-filaments (Fig. 4(a)), which hadn’t been observed in previous experiments, appeared during FLDW process while laser energy exceeded 2 µJ. These micro-filaments are clearly different from the cracks caused by stress. The micro-filaments have nearly the same extending direction. The appearance of these micro-filament was related to the direction of laser writing. For instance, the two scans in Fig. 4(a) were written by using laser at the same energy level, but the moving direction of laser focus was left to right for the top scan and was opposite for the bottom one. Obviously, this is a complicated issue and additional researches, beyond the scope of this paper, is needed.
We applied FLDW technique on two types of MIR laser garnets to explore the possibility for potential MIR waveguide lasers. Functional Type II waveguides were obtained in Er,Pr:GYSGG but not in Yb,Er,Ho:GYSGG, though the applied writing conditions and the morphological characters of laser modified regions were similar in both garnets. Stimulated radiation, which was more intense in Yb,Er,Ho:GYSGG under NIR excitation, likely play a major role in impeding waveguide creation. Depressed cladding waveguides were also designed and attempted to fabricate into garnets, but cracks appeared soon due to stress overlapping while laser scanned the regions near previous laser written filaments. The mechanism for laser induced changes in both garnets is attributed to material intrinsic defects creation and CLDs. The associated effects of CLDs, including decreased refractive index and stress field, provide essential conditions for usable guidance in fabricated structures. The influence of femtosecond laser writing on Er3+ ions and their lasing characteristics is considered to be acceptable since light mainly propagates in the unmodified area between two fabricated filaments. The results may act as a guide for the design of future generations of MIR laser crystals that might exhibit surpassing performance in laser inscribing processes. Following this investigation, we will study the causes of micro-filaments. We will also try to improve waveguide confinement by translating the sample through the laser focus along a helical trajectory during laser writing. Complete MIR waveguide lasers can be also proceeded to the next stage of development.
National Natural Science Foundation of China (61661004, 61965003); Natural Science Foundation of Guangxi Zhuang Autonomous Region (2017GXNSFAA198227, 2018GXNSFAA294133).
We gratefully acknowledge Chinese Academy of Sciences for kindly providing the MIR laser crystals. We would like to thank Tao Lin, Hang Zhang and Zaijin Fang for helpful discussions.
The authors have no competing financial interests.
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