Elsevier

Optical Materials

Volume 88, February 2019, Pages 680-688
Optical Materials

Luminescent features in double-track type II waveguides made in Er/Yb:LiNbO3 by Ultrafast Laser Inscription

https://doi.org/10.1016/j.optmat.2018.12.035Get rights and content

Highlights

  • The first femtosecond laser written waveguides fabricated in Er/Yb Lithium Niobate crystals.

  • TM and TE modes supported at 1,55 μm guided wavelength.

  • Slight variations of emission lifetime between bulk and waveguide.

  • Confocal micro-luminescence studies on the waveguides cross section area.

  • Starting point to elucidate stress-induced morphological changes close to the filament.

Abstract

This work reports the first double-track type II waveguides fabricated by Ultrafast Laser Inscription (ULI) in a z-cut crystal of Er0.5% Yb0.5%LiNbO3. The light-guiding properties were explored and it was found that both polarization modes, TE and TM, are supported at a wavelength of 1550 nm. Also, a thorough spectroscopic characterization of these structures was performed. All luminescence bands of active ions and their corresponding lifetimes were measured and scanning confocal μ-photoluminescence maps were developed. Slight shifts and broadening were found, relative to the bulk, for some specific emission peaks in waveguides. Particularly, Yb3+ ions luminescence within a region up to 2 μm from to where the laser struck during ULI processing, exhibited changes of their positions, heights and widths. The contribution of ULI to the ion environment distortions could be detected by confocal microscopy and could be decoupled from multi-site effects. This procedure can be taken as a quality control tracking of waveguides fabricated by ULI, if one considers variations in micromachining parameters (energy per pulse, speed of writing, repetition rate or pulse duration) as well as in concentration ratios of Er/Yb, or even annealing treatments. Although spectral modifications were resolved, the bulk spectra of these ions are essentially preserved in the waveguide core and surroundings. Therefore, this gives rise to explore these Er/Yb co-doped waveguides as active photonics devices, considering the rich spectra of Er3+ ions: green, red and NIR (1550 nm) emissions and the doping with Yb3+ ions to improve its efficiency.

Introduction

ULTRAFAST Laser Inscription (ULI) is a well established technique for structuring optical waveguides in many transparent media by using femtosecond laser pulses [1,2]. In particular, many results such as good mode qualities and propagation performance have been demonstrated [1,3], for type II waveguides in LiNbO3 (LN) by forming double straight lines, called tracks. These, are paths written by ULI and they are characterized by a steep decrease in refractive index, separated from each other an adequate distance (∼ 20 μm) in order to allow optical guiding between them, where the refractive index is increased relative to the neat material. The search of new laser sources in photonics applications it is always a challenge and even more so when miniaturization is required. The cross sectional area of these of these kinds waveguides is on the order of a few hundred μm2. So, this technique provides the possibility of developing small cavities for laser action [4,5]. In this framework, the Er:LiNbO3 crystal is a proved host material for active integrated optoelectronic devices [6] and it combines excellent electro-optic, acousto-optic, and nonlinear optical properties of a LN crystal (with a laser gain of Er3+ ion) which is also suitable for laser diode pumping. For instance, efficient diode-pumped all-solid-state lasers and amplifiers at operation wavelengths of around 1.5 μm have already been established in Er3+ -doped bulk LN [7,8]. Although this ion presents a low absorption cross section in the laser diodes emission range (0.8 − 1.5 μm), it is possible to achieve a good improvement by co-doping the LN crystal with Yb. The sensitization of the Er3+:LiNbO3 system with Yb3+ ions is well justified by several advantages that include a stronger absorption cross section, a broader absorption band in the near infrared range of 880−1050 nm, which can offer the possibility of tuned excitation, as well as a large overlap between Yb emission and Er absorption that allows an efficient resonant energy transfer from Yb3+ ions to Er3+ ions [[9], [10], [11], [12]]. Yb ions do not modify the lifetime of the 4I13/2 erbium level (1.5 μm emission wavelength), whereas they induce a marked concentration-dependent change in the lifetime of the 2F5/2 (Yb3+) and 4S3/2 (Er3+) manifolds (corresponding to 1060 nm and 550 nm emission peaks, respectively) [11]. The sensitization works due to the process of energy transfer from Y b3+ to Er3+ [10], as illustrated by the dashed lines in Fig. 1.

As is well-known, the intra-band fN −→ fN electronic transition of RE ions are almost independent of the doped host because of the shielding of 4fN electrons due to full-filled internals shells [13]. Nevertheless, although induced electric dipole transitions (ED) are strictly forbidden for centrosymmetric systems (according to the Judd Offelt theory [14]), the vibronic coupling mechanism between the f electrons and the degenerate vibrational modes makes the ED allowable [15]. Morover, the vibronic intensities show host dependence: higher covalence for the metal-ligand bond gives higher vibronic intensities (Meijerink et al., 1996). This has been illustrated for Pr3+ in several host lattices (de Mello Donegfi et al., 1992, 1995). Electron-phonon coupling is strong at the beginning (Pr3+) and at the end of the series (Tm3+), but weak at the center (Eu3+, Gd3+, Tb3+) (Ellens 1996, Ellens et al., 1997b). The Er3+ is located between the center and the end of the series. Intense vibronic transitions are expected if strong electron-phonon coupling is available. For noncentrosymmetric systems, Judd (1980a) concluded that vibrionic contributions are important for explaining the intensity of the so called hypersensitive transitions (p.238 [16]). These hypersensitive transitions were well studied and most of them are identified. They obey the selection rules |ΔS| = 0, |ΔL| ≤ 2 and |ΔJ| = 2. For the case of Er3+ ions,1 the one that corresponds to this type is 4H11/2 −→4 I15/2 (see table 25 of Ref. [16]). Following the background about host influence on RE, has already been demonstrated the existence of several distinguishable Er3+cluster sites in Er: LiNbO3 by site-selective spectroscopy [17]. The existence of these multi-sites is associated with the non-equivalent environment where the RE ions are located in the host. It is well-known (by Rutherford backscattering/channeling measurements) that both rare-earth ions Er3+ and Y b3+ enter into the lithium niobate at the lithium octahedron located in positions slightly shifted from the Li positions (corresponding to the undoped lattice), around 0.25Å [18]. But, additionally, due to many other results [[19], [20], [21], [22], [23]] it is commonly accepted that Er3+ ions are located in slightly different lattice environments which depend on the method of crystal preparation. This has also been observed in channel waveguides fabricated on (congruent) LN z-cut wafers when using Er and Ti in diffusion technique of [24]. The production of waveguide structures by diffusion of Ti4+ ions into the surface of an LN wafer creates within the wave-guiding zone specific conditions for Er3+ ions and their crystalline environments [24]. Moreover, it turns out that the up-conversion efficiency is very sensitive to the energetic position of the respective transitions [17,24,25]. Finally, the formation of clustered sites in Er3+-doped LN crystals gives rise to deleterious effects for laser and amplifier applications [25].

In the case of Er/Y b co-doped LN systems there is still missing a full spectroscopic characterization of all allowed transitions as well as studies of the effect of the crystalline field surrounding the RE ions dopants. Changes in the ions environment could take place by different means: for instance external pressure, laser processing, removing defects after annealing treatment and by added impurities. In the kind of structures that we are researching, distortion of the crystalline field could probably arise from induced anisotropic stress owing to plasma generated by the ultrahigh intensities reached with ULI [26,27]. Induced stress, thus, is related to the increment of the refractive index that allows light-wave propagation. While the latter parameter is a macroscopic property of a crystal, distortion of the crystalline environment might be considered a microscopic one. If, in some way we would be able to uncover the kind of distortion induced by ULI in transparent media, we would be going forward to better understand the mechanisms by which the refractive index increment takes place, extending pioneering modeling [3,28]. In this sense, our work introduces a first approach to allow this issue to be addressed. In particular, if Yb co-doped Er:LiNbO3 crystals are considered in order to apply ULI technology for photonics devices, the characterization of the influence of induced stress on the emission spectra (inter-Stark transitions (2S+1L0J0 −→2S+1 LJ) would let us inquire into the crystalline field distortion of the octahedral environment of Li site substitutions. Even though confocal microscopy provides optical spatial resolution over the mesoscopic scale, which is far from the molecular scale, it could still be a first step to better understand the possible systematic distortions through analysis of the spectral maps of the RE ions. An elasto-optic model is discussed in Ref. [29] from μ -Raman analysis. So, the hypothesis is that, if there is any characteristic effect of ULI over the unit cell, possibly spatially dependent relative to the laser focus zone, dopant environment distortions averaged over many unit cells could be detected through spectral changes.

To analyze spectra, the reported lines [30] corresponding to the inter-Stark transition are very useful to estimate the peaks to be fitted in the emission spectra measurements which are greater than the expected number of (J0 +1/2) × (J +1/2) from transitions LJ2S+1LJ2S+1. But, it is worth noting that the thermal effects on sharp emission lines associated with the inter-Stark energy levels (see Ref. [31] and references therein) can be described by line width broadening arising from the 1) crystal inhomogeneity 2) direct one-phonon process 3) multiphonon process and 4) Raman phonon scattering process. The nature of these contributions as well as their lineshape (Gaussian/Lorentzian) has been discussed for the case of Nd3+ in a different host [31]. A rigorous spectra analysis must consider this inter-Stark transitions line width broadening.

Finally, despite the results achieved up until now, ULI has not been applied to Er3+/Y b3+ co-doped lithium niobate crystals. In this work, it is the first time that type II waveguides fabricated by ULI in Er3+/Y b3+ co-doped LN z-cut are reported. The main motivation of this work is to proceed in the use of Er3+/Y b3+ co-doped LN crystals applying ULI and so advance in its potentiality in photonics by elucidating the possible effects of induced stress in the well-known spectral characteristics of bulk. We looked for well localized distortions as well as for possible trends along the waveguides cross sectional area.

Section snippets

Fabrication

The fabrication process was performed by structuring double track type II waveguides by focusing femtosecond laser pulses centred at wavelength of 796 nm in a z-cut rectangular sample of Er0.5%/Yb0.5%:LiNbO3 using a 3D−micro−positioning stage. The sample was moved at a fixed speed set at 30 μm/s perpendicular to the laser beam which struck the sample in a direction parallel to the c−axis. Laser pulses were produced by a Ti: sapphire Chirped Pulse Amplification (CPA) system from Spectral Physics

Light propagation

According to section II-B, we show in Fig. 3 the spatial intensity profile of the best propagated mode corresponding to a double-line waveguide made with a delivered energy of 1 μJ per pulse and longitudinal polarization of writing. Light was well confined using 115 mW of power from the DFB laser. An integration time of 1/2000s and a gain of 6.3 dB were set in the 8-bit CCD device. The inset shows TM and TE propagation modes in a monochrome scale, corresponding to auto-confined luminescence of π

Conclusion

In this work is demonstrated the feasibility of double-track waveguides using Ultrafast Laser Inscription applied in Eb0.5/Yb0.5:LNB co-doped z-cut crystals which up to nowadays has never been reported. The emission of luminescence of the Er3+ at around 1.5 μm resulted to be well confined, and both polarization modes, TE and TM, were supported, with the latter being stronger than the former. Additionally, the lifetime of up-conversion (Er3+) emission, and NIR emissions of Er3+ and Yb3+ were

Acknowledgment

This work was partially supported by the National Agency of Scientific and Technology Promotion (Argentina) under project PICT-2016-4086. G.A.T and D.B are members of CONICET (Argentina). This manuscript was revised by Christopher Young (Faculty of Engineering, National University of La Plata).

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