Experimental evidence of Bloch surface waves on photonic crystals with thin-film LiNbO3 as a top layer Download: 528次
1. INTRODUCTION
Lithium niobate (LN) is a high refractive index birefringent crystal with tunable optical properties. It is widely used for integrated optics due to its excellent ferroelectrical, piezoelectrical, and thermoelectrical properties, its transparency over a wide wavelength range (350–5200 nm), its nonlinear optical polarizability, and its Pockels effect [1]. In order to improve the performance of integrated optical devices, several research groups have developed different structures, such as ridge waveguides, photonic crystal waveguides, and periodically poled lithium niobate structures [2]. As a high refractive index material, LN as a top layer is used for enhanced light confinement for many devices [3] and thin-film
In this work, we propose two different novel architectures that can generate Bloch surface waves (BSWs) at TFLN layers. BSWs perform a strong field confinement at the interface between a periodic dielectric multilayer and a surrounding medium due to Bragg reflection and total internal reflection on two sides of the interface, respectively [4]. Light coupling can be easily achieved by a grating coupler [5] or by using the Kretschmann configuration [6,7]. Both coupling methods are simpler and more efficient in terms of coupling losses than the fiber-to-fiber coupling technique that is employed to horizontally excite a guided mode on TFLN [8].
Moreover, BSWs can be considered as an attractive alternative to surface plasmon polaritons due to the low loss features of dielectric materials in comparison with metals [6,9]. Therefore, these dielectric surface modes dramatically increase the light propagation length [7]. They were initially proposed for vapor sensing [10], biosensing [6], fluorescence studies [11], and for integrated optics [9,12]. The development of BSW-based devices has also profited a great deal from the development of different deposition techniques, such as atomic layer deposition [13,14] or plasma-enhanced chemical vapor deposition (PECVD) [15]. These techniques have allowed us to achieve the necessary precision in the manufacturing of subwavelength thickness layers required for the fabrication of BSW devices. Various materials constituting multilayer structures have been used in different designs of one-dimensional photonic crystal (1DPhC), offering a large panel of configurations for different applications at different wavelengths. The BSW propagation can be controlled by manipulating the refractive index inside the device [12].
In this work, we propose a 1DPhC with a TFLN as the top layer of the multilayer structure. The bonding into the 1DPhC structure brings anisotropy into the whole crystal, allowing the tunability of the BSW devices [16]. In previously studied 1DPhC with TFLN, the BSW is excited at the
Direct electric field or temperature application cannot introduce a shift of the
Additionally, nonlinear properties of BSWs are attractive topics for study. Recently, phase-matched third-harmonic generation via doubly resonant optical surface modes [18] was achieved on the base of 1DPhC coated with a 15 nm GaAs film. Unfortunately, GaAs is not transparent in the visible range of wavelengths and is not suitable for many optical applications. The use of
2. FABRICATION, EXPERIMENTAL DETAILS, AND DISCUSSION
We present a BSW-based device, which is able to sustain surface waves at the
As we have mentioned before, the 1DPhC that sustain surface waves requires subwavelength thickness layers. In the case of LN, this requirement is challenging, especially because it needs to maintain its crystallinity in order to use the nonlinear properties of the material. Current technologies, such as sputtering, evaporation, or epitaxial growth of
In order to achieve the desired LN properties and to avoid light scattering on the multilayer top surface a single-crystal TFLN bonded to an
A second configuration has been also fabricated using a single-crystal TFLN bonded to a Cr and Au layer on an Si substrate [22], which is thereafter bonded to the 1DPhC. For this geometry, the multilayer used was composed of Si/Au/Cr/Au/TFLN. The layer’s thicknesses were 500 μm for Si, 20 nm for both layers of Cr, 400 nm for the Au layer, and about 2 μm for the TFLN, respectively. In this case, the TFLN was manufactured by polishing of bulk
The 1DPhC was manufactured according to the steps shown in Fig.
Fig. 2. Schematic of the membrane-based 1DPhC fabrication process: (a) bonding of bulk to Si with Cr and Au, (b) polishing, (c) photoresist deposition, (d) UV lithography of the photoresist, (e) DRIE etching of Si and wet etching of Cr and Au, (f) photoresist removal, and (g) multilayer deposition.
500 μm of an Si layer was removed by deep reactive ion etching (DRIE) [23] on SPTS equipment tooled by a dual plasma source (Rapier). Concerning the plasma etching process, a Bosch process with three sequences (Teflon deposition, Teflon removal in the trench bottom, and silicon etching) was used [24]. These three sequences were repeated in order to have an anisotropic wall’s profile (close to 90°); the Teflon deposition was performed by
Cr and Au were removed by a standard chemical wet etching process in Cr and Au etching solutions.
Concerning the multilayer fabrication, six periods of
Membranes of two different areas were manufactured:
Fig. 3. (a) Microscope images of the membranes. (b) Microscope images of the membranes after multilayer deposition.
Focused ion beam scanning electron microscope (FIB-SEM) images (Fig.
Fig. 4. (a) FIB-SEM image of the membrane. (b) FIB-SEM image of the 1DPhC (suspended membrane).
The manufacturing steps for the on-glass 1DPhC are shown in Fig.
Fig. 5. Schematic of the on-glass 1DPhC fabrication process: (a) obtaining TFLN with smart cut technology, (b) multilayer deposition, (c) UV glue bonding to the glass substrate, (d) protection of the sample with photoresist, (e) DRIE etching of Si and RIE etching of , and (f) photoresist removal.
The whole stack was protected by S1813 photoresist, and the 20 μm of Si was dry etched by DRIE. For this etching, the Bosch process was also employed. A 2 μm thickness layer of
After the manufacturing we obtained two different LN BSW-based devices. The multilayer of the membrane-based crystal was as follows: air/six pairs of
The multilayer of the glass-supported crystal was as follows: glass/UV glue/six pairs of
Fig. 6. (a) Dispersion curves for the on-membrane 1DPhC. (b) Dispersion curves for the on-glass 1DPhC.
In order to achieve experimental values for
Fig. 7. (a) Experimental setup for the on-membrane 1DPhC. (b) Experimental setup for the on-glass 1DPhC.
The images of the reflected light at the angle of BSW excitation were collected. In Fig.
Fig. 8. (a) Camera image intensity profile of the BWS-related reflectance dip for the membrane-based sample. (b) Camera image intensity profile of the BWS-related reflectance dip for the on-glass sample.
We can therefore conclude that the multilayer on the glass support is more stable and provides a bigger area for light manipulation that uses the BSW (in our case
In the case of the membrane configuration, we have limited access to the multilayer for the BSW coupling on the Kretschmann configuration (in our case, a maximum
In this work, we show theoretically and experimentally the excitation of BSWs at a TFLN/air interface, which introduces all the potentialities of the
3. CONCLUSIONS
We have demonstrated BSWs on top of TFLN in two different configurations. LN-based photonic crystals, which are able to sustain BSWs, were designed and fabricated at the base of a TFLN membrane and on a glass support. In order to compare two different 1DPhCs, far-field measurements were done. The designed 1DPhCs allowed us to obtain BSWs at the TFLN/air interface for TE-polarized light at a wavelength of 473 nm. Such designs of the 1DPhCs, together with the use of the nonlinear properties of
4 Acknowledgment
Acknowledgment. The authors thank the NANOLN company for providing thin-film LiNbO3 samples.
Article Outline
Tatiana Kovalevich, Djaffar Belharet, Laurent Robert, Myun-Sik Kim, Hans Peter Herzig, Thierry Grosjean, Maria-Pilar Bernal. Experimental evidence of Bloch surface waves on photonic crystals with thin-film LiNbO3 as a top layer[J]. Photonics Research, 2017, 5(6): 06000649.