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We report on the green up-conversion and near-infrared (NIR) emission in Er3+, MgO co-doped nearly stoichiometric LiNbO3 waveguides fabricated by femtosecond laser writing. The waveguides with so-called Type I geometry by laser writing support nearly single-mode propagation of light at NIR wavelength of 1.55 μm. In addition, it has been found that the guidance is only along the vertical (i.e., TM) polarization, which is due to the laser-induced positive changes of extraordinary index in the guiding core. The green up-conversion at 550 nm and 528 nm, corresponding to the transitions of 4S3/2 → 4I15/2, 2H11/2 → 4I15/2, as well as the NIR luminescence emission at C-band centered at 1550 nm under 4I13/2 → 4I15/2 transition have been realized in the waveguides, respectively. Our results have shown that the intensities of the guided-wave green up-conversion and NIR emissions are higher than those obtained from the bulk, which may be owing to the enhanced intracavity optical intensities of the waveguide with respect to the bulk.
© 2016 Optical Society of America
1. Introduction
Lithium niobate (LiNbO3) crystal is a multifunctional platform for a number of photonic applications [1, 2]. Recently, the rare-earth ion doped nearly stoichiometric LiNbO3 crystals (RE:NSLN, RE = Er3+, Tm3+, Pr3+, Nd3+, Yb3+ and Ho3+, etc.) have been recognized as promising candidates for nonlinear frequency conversion, laser operation and optical modulation, etc., since they could combine optical features of RE ions and diverse properties of the NSLN crystal [3–5]. In comparison to the congruent LiNbO3 (CLN, usually with [Li]/[Li + Nb] ≈48.5%) crystals, the NSLN (with ratio of [Li]/[Li + Nb] to a nearly stoichiometric value >49.5%) possess considerably improved nonlinear and electrooptic properties because the intrinsic defects in NSLN have been reduced significantly [6,7]. The Er3+-ion doped LiNbO3 crystals are particularly attractive for applications in modern telecommunication systems because the C-band luminescence emissions are used for optical amplifications [8]. In addition, Er3+ doped materials also offer possibilities for the realization of green and red luminescence via up-conversion processes under NIR excitation [9–11]. It has been ascertained that, as co-doped with MgO (Mg2+), the optical damage resistance of LiNbO3 could be significantly enhanced. In Er3+, MgO co-doped systems, as the concentration of Mg2+ is higher than its threshold (i.e., > ~4.6 mol% [12]), the transition probability at 1.5 µm of Er3+ in LiNbO3 crystal could be enhanced [13,14], whilst the lower co-doping of MgO is an effective way to improve properties of green up-conversion luminescence originating from Er3+ cluster sites [15]. In this sense, NSLN is superior to CLN crystals since the low-doping MgO (~0.3 mol%) in NSLN can effectively suppress the photorefractive effect [16–18] owing to the reduction of the density of non-stoichiometric defects inside the crystal. These features enable Er3+, MgO co-doped NSLN (Er3+:MgO:NSLN) intriguing candidates for the applications in both visible and NIR bands.
Miniaturized waveguide devices based on dielectric crystals are playing remarkable roles in the integrated circuits as passive and active components, benefiting from the combination of versatile bulk features and the compact geometries for light field confinement and tailoring. Efforts on the fabrication of Er:MgO:NSLN waveguide devices have predominantly focused on ion beam technologies, including light (He+ [19]) and heavy (O+ [8], C5+ [20]) ion implantation/irradiation for planar or channel waveguides, and Mg/Ti pre-diffusion in a pure congruent LN in sequence of local Er doping and post Li-rich vapor transport equilibration for strip waveguides [21]. Nevertheless, these approaches are difficult for three-dimensional (3D) waveguide fabrication since they generally suffers from complicated treatment procedures and hence inefficiency for complex geometries. The emergence of femtosecond-laser writing/inscription provides a powerful solution to this predicament [22–24]. As one of the earliest-investigated materials for laser writing, LiNbO3 has been verified to be a successful crystalline example for direct writing of 3D waveguiding structures. According to the refractive index changes of the laser-induced tracks, one can roughly categorize the configuration of laser-written waveguides into either Type I or Type II modification. For LiNbO3 crystal, waveguides based on Type I modification are correlated to positive change of extraordinary refractive index (ne) in the laser irradiated focal volume [25–29]. Such a mechanism enables direct-write of complex devices, e.g., 3D beam splitters, directional couplers, or waveguide arrays, for diverse applications of integrated photonics [30–32]. Nevertheless, as of yet, there has been no report on the fabrication and characterization of waveguides in Er3+:MgO:NSLN crystals by using femtosecond laser writing.
In this work, for the first time to our knowledge, we have applied the direct femtosecond laser writing to produce the waveguides in Er3+:MgO:NSLN crystal based on the Type I modification of ne, and investigate the guided-wave green up-conversion and NIR luminescence properties at telecommunication wavelength band.
2. Experimental in details
The z-cut Er3+:MgO:NSLN crystal sample (doped with 0.2 mol% Er3+ and 1.5 mol% MgO, obtained from Crystalblue Co., Ltd., China, with [Li]/[Li + Nb] ratio of ~49.7%) was cut into dimensions of 10(x) × 10(y) × 1(z) mm3 and optically polished. Type I waveguides were fabricated by femtosecond-laser writing. During the microstructuring process, an optical fiber laser system (Origami-10 XP, OneFive, Switzerland) was utilized to generate linearly polarized laser (central wavelength of 1031 nm, pulse duration of 420 fs, repetition rate of 50 kHz, and maximum pulse energy of 65 μJ). A slit was inserted before the focusing objective to control the shape of the focused beam and the value of the pulse energy used to irradiate the sample. A 40 × microscope objective (N.A. = 0.6) was utilized to focus the laser beam at depth of ~150 μm beneath the upper 10 × 10 mm2 surface of the sample. During the irradiation process, the LiNbO3 sample was placed at a micro-positioning X-Y-Z motorized stage, moving at a constant velocity of 0.2mm/s along the 10-mm axis. In this work, the two groups of Type I waveguides (hereafter referring to WG1 and WG2) with the pulse energy incident on the sample setting to 0.54 μJ and 0.66 μJ were fabricated, respectively.
A microscope (Axio Imager, Carl Zeiss) was utilized to image the cross sections of the waveguides WG1 and WG2 in Er3+:MgO:NSLN crystal. To experimentally characterize the near-field modal profiles of the waveguides, we utilized an end-face coupling arrangement to investigate the modal profiles of the waveguides at 1064 nm and 1550 nm from a Nd:YVO4 solid-state laser and a semiconductor laser system (GCSLS-O, China Daheng Group, Inc.), respectively, as shown in Fig. 1. A half-wave plate was applied to control the polarization of the incident laser beam. Afterwards, a pair of microscope objective lens (N.A. = 0.4) were employed to couple the linearly polarized light (TE or TM mode) into and out of the waveguides, and finally a CCD camera was employed to observe and record the near-field intensity distributions at 1064 nm and 1550 nm. In order to determine the propagation losses of the waveguides, we measured the light powers coupled into and out of the waveguide end-facets.
We measured the green up-conversion emission and the near-infrared emission spectra of the waveguides and bulk in Er3+:MgO:NSLN based on the similar end-face coupling arrangement. A semiconductor laser operating at 980 nm wavelength was utilized as the excitation source. Before collecting the green up-conversion or NIR emission spectra, filters were employed to extract the green laser or NIR laser from the multi-wavelength mixed output beam. Afterward, two spectrometers (Omni-λ750, Zolix Instruments Co. Ltd., and NIR 1700, Shanghai Ideaopics Inc.) were used to measure the spectra of the up-conversion and NIR emissions of the waveguides and bulk in Er3+:MgO:NSLN, respectively. All the measurements were carried out at room temperature.
3. Results and discussion
Figure 2 shows the microscopic images of the cross section of the waveguides WG1 and WG2. As one can see, the Type I waveguide cores were located in the spatial locations of laser-induced tracks marked by dashed lines. As also shown in Fig. 2, the measured near-field intensity distributions of the waveguides WG1 and WG2 along TM polarization (corresponding to the ne polarization) exhibit fundamental modes, i.e., TM00 both at 1064 nm and 1550 nm wavelength. However, we cannot achieve the near-field modal profile along TE polarization (corresponding to the ordinary index (no) polarization) due to the weak ability in confining the light propagation, which is in agreement with the theoretical estimation from the guided-wave optics.
Fig. 2 The microscopic images of the cross section (left) and measured near-field intensity distributions at 1064 nm (middle) and 1550 nm (right) of the Type I waveguide WG1 and WG2 along TM polarization. The dashed lines indicate the spatial locations of laser-induced tracks.
By assumption of a step-like refractive index profile and measuring the numerical aperture of the waveguides, we can estimate the maximum refractive index change of the waveguide core using the formula [33]
where Θm is the maximum incident angular deflection at which no transmitted power change is occurring, while ne = 2.1555 is the refractive index of the bulk at 1064 nm wavelength. In this work, we calculated maximum refractive index change (Δne) ~9.1 × 10−4 and ~1.21 × 10−3 for the TM mode of Er3+:MgO:NSLN waveguides WG1 and WG2. It could be clearly seen that the two waveguides fabricated with the same parameters except energy (EWG1 < EWG2) also exhibit the positively related refractive index alternations (ΔnWG1 < ΔnWG2). Based on the end-face coupling system at 1064 nm, we measured the insertion losses (including propagation losses and coupling losses) of the waveguides WG1 and WG2 to be 4.26 dB and 2.83 dB, respectively. With consideration the overlap of the profiles of the incident light beam and waveguide modes, the coupling losses were estimated to be 0.48 dB and 0.61 dB, respectively. Finally, the propagation losses of WG1 and WG2 were determined to be 3.78 dB/cm and 2.22 dB/cm, respectively. It indicates that, as the pulse energy of the fs-laser increase, the refractive index alternation of the waveguide becomes larger, and the propagation loss decreases. Consequently, the propagation losses could be further reduced by optimizing the parameters (e.g., increasing the pulse energy) of the femtosecond laser writing.Figure 3 shows the all-angle light transmission of the waveguides WG1 and WG2 to investigate the thorough information of the polarization effects of the guidance at 1064 nm wavelength with the input light power of 100 mW. As one can see, the output light intensity will change accordingly with the change of the light polarization angle, which shows the sensitivity of polarization guidance of Er3+:MgO:NSLN Type I waveguides. Obviously, the output power approaches zero when the polarization angles are 0° and 180° corresponding to the TE polarization, while the output power reaches the maximum with 90° and 270° corresponding to the TM polarization. This behavior is in good agreement with the reported Type I waveguides in LiNbO3 [26]. It is also found that the output power of WG2 is greater than that of WG1, which accords with the measured propagation losses.
Fig. 3 The polar plots of the measured output power as function of output light polarization at transverse cross section for WG1 (black squares) and WG2 (red circles) at 1064 nm with the input light power of 100 mW. The solid lines are the corresponding fits of the experimental data.
Figures 4(a), 4(c) and 4(e) show the top-view of the photographs of green up-conversion emission in the waveguides WG1, WG2 and the bulk of Er3+:MgO:NSLN crystal, respectively. Correspondingly, Figs. 4(b), 4(d) and 4(f) show the near-field modal profiles of the green up-conversion emissions (eliminating 980 nm-wavelength), respectively. As one can see, the up-conversion fluorescence in the waveguides WG1 and WG2 is with TM00 mode, whilst the fluorescence in the bulk area experiences diffraction, leading to the output light undetected. It indicates that the laser written Type I waveguides can efficiently confine up-conversion fluorescence inside the guided cores.
Fig. 4 The top-view of the photographs of green up-conversion emission in the waveguides WG1 (a) and WG2 (c) and the bulk (e) of Er3+:MgO:NSLN crystal.
Figures 5(a) and 5(b) shows the emission spectra of green up-conversion and near infrared in the waveguides (WG1 and WG2) and the bulk of Er3+:MgO:NSLN crystal. The errors of fluorescent intensities in WG1 and WG2 are 0.9%, and 1.1%, respectively. The observed two emission bands around 550 nm and 528 nm and the wavelength range of 1450-1625 nm are assigned to Er3+ transitions of 4S3/2 → 4I15/2, 2H11/2 → 4I15/2 and 4I13/2 → 4I15/2 (1550 nm) involved, respectively. As one see, the fluorescent intensities are higher in the waveguide cores than in the bulk. For the green up-conversion emissions, the intracavity light intensities from the WG1 and WG2 are increased by 41.3% and 86.7% in comparison to the bulk, respectively. In addition, we have determined the output optical densities at 980-nm wavelength with the input light power of 1W, are ~2.87 × 107 W/m2, ~3.63 × 107 W/m2 and for Er3+:MgO:NSLN WG1 and WG2, respectively, corresponding to enhancement of 40.6% and 77.9% for the WG1 and WG2 to the bulk (~2.04 × 107 W/m2). One can therefore conclude that the enhancement of green up-conversion in the waveguides is due to the guiding effect of the waveguides for tight light confinement within extremely compressed volumes. Additionally, with the help of the Judd-Ofelt theory, the probability of spontaneous radiation A and excited-state radiative lifetime τJ [34]
where Sed and Smd are electric and magnetic dipole oscillator strengths, n is the refractive index along TM polarization. It indicates that, as refractive index increases, the spontaneous radiation A becomes higher with the decrease of excited-state radiative lifetime τJ, resulting in the fluorescence intensity enhancement. Liu et al. reported that the up-conversion fluorescence of the ion-implanted Er:MgO:LiNbO3 waveguides was quenched in the planar waveguide region compared with the bulk [20]. In the present work, the Type I Er:MgO:LiNbO3 waveguides inscribed by femtosecond laser exhibit higher up-conversion fluorescence efficiency than that of the bulk.Fig. 5 Emission spectra of green up-conversion (a) and near infrared (b) in the waveguides WG1 (black line) and WG2 (red line) and the bulk (blue line) of Er3+:MgO:NSLN crystal.
For better understanding the mechanism, the powers of green up-converted luminescence (4S3/2 → 4I15/2) for the WG1 and WG2 were measured as a function of the excitation power on a log-log plot as shown in Fig. 6. During the up-conversion process, the green light output intensity IUC will be proportional to the infrared excitation intensity IIR [35],
where m is the number of IR photons absorbed per visible photon emitted. Therefore, a plot of log (IUC) versus log () should yield a straight line with the slope. According to Fig. 6, the slopes of 550 nm emission of the WG1 and WG2 are 2.08 and 2.17. Thus, we can propose that the green levels populated are resulting from a two-photon absorption under relatively lower excitation power in the up-conversion mechanism.Fig. 6 Pump power dependence of green up-conversion emission in the waveguides WG1 (black dots) and WG2 (red dots) of Er3+:MgO:NSLN crystal.
Based on the energy match situation dependency on pumping power and the Er3+ ion energy level diagram, the possible up-conversion mechanism was discussed as shown in Fig. 7, which has been well established in the literatures [33–40]. Under 980 nm excitation, two major steps occur in the processing of green up-conversion, including (i) the pumping laser operating at 980 nm excites electrons from the ground state 4I15/2 to the 4I11/2 excited state through ground state absorption (GSA) of exciting photons, i.e., 4I15/2 (Er3+) + a photon → 4I11/2 (Er3+); (ii) the 4S3/2 and 2H11/2 populated and produce up-conversion emission at 550 nm and 528 nm. Particularly in the step (ii), there may be two possible approaches to be populated 4S3/2 and 2H11/2: (a) excited state absorption (ESA): after a first excitation to the 4I11/2 level, a second photon is absorbed by the same ion, exciting it to 4F7/2 state, i.e., 4I11/2 (Er3+) + a photon → 4F7/2 (Er3+); (b) Energy transfer (ET): an excited ion relaxes from 4I11/2 state to the ground state 4I15/2 nonradiatively and transfer the energy to another neighboring one, promoting the latter to 4F7/2 state from 4I11/2 through the following channel: 4I11/2 (Er3+) + 4I11/2 (Er3+) → 4F7/2 (Er3+) + 4I15/2 (Er3+). Then the rapid nonradiative decay from 4F7/2 to 4S3/2 and 2H11/2 level results in the dominant population at 4S3/2 and 2H11/2 states.
4. Summary
We have successfully fabricated Type I waveguides in z-cut Er3+: MgO:NSLN crystal by direct femtosecond laser writing. The measured near-field modal profiles of waveguides exhibit fundamental transmission at 1064 nm and 1550 nm wavelengths. The green up-conversion emission at 550 nm and 528 nm and NIR emission at 1450-1625 nm of the waveguides have been obtained under excitation of 980 nm radiations. By data analysis and Judd-Ofelt theory, the analysis of two-photon absorption and up-conversion mechanism, the higher green up-conversion emission in waveguides area is explained and discussed. The obtained spectra results in the waveguides of Er3+:MgO:NSLN crystal indicate that the crystal is potential for miniaturized waveguide amplifiers and lasers.
Funding
National Natural Science Foundation of China (NSFC) (11274203 and 11511130017), and Project 111 of China (B13029).
Acknowledgment
The authors thank Mr. Q.M. Lu for polishing the samples in this work.
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