Resumen de New Photonic devices based on NLO(non-linear optical) crystalline waveguides
RbTiOPO4 belongs to the KTiOPO4 family of the non-linear optical crystal. It has high electro-optic coefficient and its optical damage threshold (approximately 1.8 GW/cm2 for pulses with duration of 10nm at 1064nm) is twice as high as that of KTP crystals. These characteristics make it interesting for electro-optic applications such as modulators. Furthermore, there¿s a current interest in developing new dielectric based optical photonic components for integrated optics, identified as a topic of research by the Europe Horizon 2020. The aim of this doctoral thesis is to explore the magnificent properties of RbTiOPO4 single crystal as a platform for such dielectric based optical photonic platforms for integrated photonics, which can have applications in telecommunications and biosensing. The thesis focuses on the fabrication of optical waveguides based on the RbTiOPO4 (RTP) dielectric material. This fabrication involves the crystal growth as technique for producing the crystals, and after techniques such as reactive ion etching, ion exchange and laser writing to create the optical waveguides. The waveguides fabricated in this thesis are 2D channel waveguides fabricated in undoped RTP crystal and in epitaxial systems of (Nb,Yb):RTP grown on RTP. The waveguides fabricated are straight channel waveguides, Y-splitters and Mach-zehnder(MZ) structure. The MZ structures in turn can be converted into an intensity modulator by depositing electrodes over it, taking profit of the high electro-optical coefficient of the RTP crystal.
In this thesis is reported the successful grown the bulk single crystals of RTP, K:RTP and Na:RTP by using Top Seeded Solution Growth (TSSG) technique at high temperatures. The crystals obtained are suitable to be used as platforms to fabricate on them optical waveguides and also, to be used for substrates for posterior growth of the epitaxial layer. The crystallographic orientation of the substrate is a crucial parameter, which defines the physical properties to be explored later due to the anisotropy of the RTP crystals (RTP and KTP crystals belong to the orthorhombic system and are non centro-symmetric biaxial crystals). In this thesis, mainly we have been working on the (001) crystalline plane, in which is contained the phase matching direction for Type II second harmonic generation for 1 micron wavelength and also, because itis perpendicular to the ferroelectric vectors of thedomains, which makes it possible to produce a periodic inversionof them and realize Quasi Phase Matching. Epitaxial layers of (Yb,Nb):RTP were grown on RTP(001) substrate, RTP epitaxial layer were grown on KRTP(001) and KRTP(100) and finally KTP epitaxial layer were grown on Na:KTP(001) substrate by the Liquid phase epitaxy technique. This methodology allows obtaining a single crystalline layer crystallographically oriented, with high quality interface and with almost no crystalline defects. Generally, the thickness of the epitaxial layers obtained was around 20-50 microns. By this LPE technique, it has been also possible to grown cladding layers of RTP, in order to create crystalline systems such as RTP/ (Nb,Yb):RTP/RTP(001), in which the cladding and the substrate are the same material, so the refractive index contrast between the active core (the Nb,Yb:RTPepitaxial layer) and the substrate and the cladding are symmetrical.
The waveguide fabrication was performed by Reactive ion etching (RIE). The information of the etching process and recipes in these RTP materials was scarce in the literature, so an advancement in this chemical etching process on the RTP crystals is reported in this thesis. The obtained etch rate of 3.2-8.7nm/min depending on the substrate/epitaxial layer and the hard mask used. Al and Cr metal layers showed good adhesion on RTP substrates but Cr was selected as a hard mask due to its high selectivity. The surface roughness produced by RIE process was approximately 15nm. Moreover, Straight waveguides were fabricated on RTP(001) substrate by using ICP-RIE. The etch rate obtained was 127nm/min with the surface roughness of around 8 nm. This clearly shows that ICP-RIE has advantage over conventional RIE due to high etch rate and less surface roughness, which in turn reduce the fabrication cost and expected less propagation loss in the waveguide. The other kind of fabrication technique which was used to produce straight waveguides, bend waveguides, Y-splitter and MZ structures was through Cs+ ion exchange method. Due to the high ionic conductivity of the RTP compounds along the c crystallographic direction and the large anisotropy of this ionic conductivity, ion exchange is highly feasible and almost unidirectional. These kind of waveguides have graded refractive index, due to the graded Cs distribution along the c crystallographic direction. The diffusion is higher in c direction as compared to a and b due to the higher ionic conductivity. The diffusion process depends on the ferroelectric domains of the substrate which can highly influence the diffusion profile and lately refractive index gradient. It has been proven that the ion exchange is more favorable in the undoped RTP material than in the epitaxial (Yb,Nb):RTP epitaxial layers.
Waveguiding of the fabricated channel waveguides has been successful for all the fabricated waveguides.
For the channel Y-Splitter and MZ waveguides fabricated on the RTP/(Yb,Nb):RTP/RTP(100) crystals, by structuring the active layer or the substrate by RIE, the waveguides obtained were single mode in TM polarization at 1550nm.The propagation losses of these structures were around 3.5dB/cm. For the straight channel waveguides fabricated on the RTP/(Yb,Nb):RTP/RTP(100) crystals, by structuring the cladding layer by ICP- RIE, the obtained propagation losses were 0.376 dB/cm at 1550 nm. This low loss can be attributed to the fact that the propagation of light in the epitaxial layer far away from the etched surface. These waveguides are suitable for integrated optics due to their low losses and ease of fabrication.The straight waveguides, bend and MZ structures were fabricated on RTP(001) along b crystallographic direction by Cs+ ion diffusion process. The propagation losses of straight waveguide, S-bend and MZ were 11.3, 15.8 and 10.4 dB/cm at 633nm at TM polarization. Additionally, we have fabricated straight channel waveguides along the [180] crystallographic direction of RTP, which is the type II phase matching direction at 1050nm. The propagation losses of these waveguides were measured at TE polarization which was 5.7dB/cm, 18.8dB/cm, 6.6 dB/cm for 632,1064 and 1520nm, respectively. Also the propagation losses of these waveguides were measured at TM polarization which was 15.9 dB/cm, 26.1 dB/cm, 4.97 dB/cm for 632, 1064 and 1520nm, respectively. The straight channel waveguides were also fabricated along the [120] crystallographic direction of (Yb,Nb):RTP/RTP(001) which is the type II phase matching direction in (Yb,Nb):RTP at 1050nm. The propagation loss of these waveguides was 15.5 dB/cm measured at 632nm in TM polarization. By type II SHG, the 1050nm radiation was converted to the green light at 532nm.
It can be concluded from the results reported in the thesis; that RTP crystals and RTP epitaxial crystalline systems area powerful candidates as dielectric materials for dielectric based optical photonic platforms for integrated optics; due to the competitive physical properties of the material, the fact can be grown with high crystalline quality epitaxial systems with high step-index variation with a high crystalline quality interface, and that chemical etching techniques can be applied successfully to pattern complicated devices such Y-splitter and MZ; which confine the light with relatively low loss of propagat
