Lithium niobate photonic crystal waveguides: Far field and near field characterisation

In this paper, we experimentally investigate photonic crystal waveguides in a X-cut lithium niobate substrate. The transmission response is measured through the CM direction of a triangular lattice structure and the results coincide with the theoretical predictions. In addition, a scanning near-field microscope is used in collection mode to map the optical intensity distribution inside the structure putting in evidence the guiding of the light through lines of defects. This study offers perspectives towards lithium niobate tunable photonic crystal devices. 2006 Published by Elsevier B.V.


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Photonic crystals (PCs), also known as photonic band-19 gap materials, are attractive optical materials for control-20 ling and manipulating the flow of light. Their structure 21 consists basically on periodic changes of the dielectric con-22 stant on a length scale comparable to optical wavelengths. 23 Multiple interference between scattered light waves can 24 eventually lead to some frequencies that are not allowed 25 to propagate, giving rise to forbidden and permitted bands, 26 similar to the electronic bandgap in a semiconductor. The 27 band structure depends on the geometry and the material 28 refractive index. Hence, an attractive feature of photonic 29 crystals consists in tuning the substrate refractive index con-30 trolling therefore the transmission response. With tunable 31 photonic crystals, the path is open towards high density 32 ultra-compact photonic circuits. This perspective has moti-33 vated various studies on tunable photonic devices [1][2][3][4][5]. 34 Among optical tunable materials, the combination of 35 excellent electro-optical, acousto-optical, non-linear optical 36 properties, electro-mechanical (piezoelectric) properties, 59 be tuned by changing the refractive index. We have already 60 theoretically shown [7] that in the case of a triangular array 61 of holes, the optimal sensitivity to the refractive index is 62 obtained when the direction of propagation is CM, and 63 the polarization of the electric field is TE (parallel to the sub-64 strate plane and perpendicular to the direction of the holes). 65 The CM propagation direction exhibits the additional 66 advantage of requiring a lower number of rows to obtain 67 a photonic gap. Indeed, we have shown in Ref.
[7] that the 68 CM direction requires only 15 rows to get a À12 dB extinc-69 tion ratio as opposed to the 30 rows that would be necessary 70 to achieve the same gap in the propagation direction CK. 71 Due to the well-known difficulty to etch lithium niobate, this 72 property has strongly motivated our choice. In addition, 73 with such a configuration we have experimentally demon-74 strated the existence of a photonic gap. 75 To complete the analysis of this configuration, we have 76 fabricated two alternative structures, based on the same 77 array as in our previous work, but with one (PCW1) or 78 three lines (PCW3) of defects. The aim is to investigate 79 the possibilities of a tunable guiding of the light through 80 the crystal. The geometrical parameters are chosen to get 81 a transmission zone around 1550 nm within the bandgap. 82 The photonic crystals are fabricated on a 0.3 mm thick X-83 cut LiNbO 3 wafer. In a first step, an optical gradient index 84 waveguide is fabricated by annealed proton exchange. This 85 step is realized through a SiO 2 mask in benzoic acid at 86 180°C during 1.5 h. The process is followed by an annealing 87 of the optical waveguide at 333°C for 9 h. These parameters 88 are chosen to position the core of the optical mode as close as 89 possible to the surface (approximately 1.4 lm) while keeping 90 single mode propagation at 1.55 lm. 91 The photonic crystal structure was fabricated in the cen-92 tral region on the optical channel waveguide as shown in 93 Fig. 1(a). It consists of a triangular lattice of 48 · 26 circular 94 holes. The lithium niobate substrate (300 microns thickness) 95 is metalised with a thin Cr layer (100 nm) to avoid charging 96 effects. This Cr layer is deposited by electron gun evapora-97 tion (Balzer, B510). The sample is g rounded with a conduc-98 tive paste before introduction in the FIB vacuum chamber 99 (10 À6 Torr). The FIB used is a FEI Dual Beam Strata 235.
100 Ga + ions are emitted with an accelerated voltage of 30 keV 101 and focused down with electrostatic lenses on the sample 102 with a probe current of 120 pA. The Gaussian beam shape 103 spot size is about 20 nm at the sample surface. The etching 104 time of the structures PCW1 and PCW3 (48 · 26 triangular 105 hole lattice, hole diameter = 255 nm, periodicity = 510 nm, 106 etching depth = 1500 nm) was 20 min each. We would like 107 to point out that the removal of material by FIB milling is 108 achieved without the use of a patterned resist mask and 109 therefore, high-precision complex structures can be directly 110 fabricated. A FIB image cross-section of the holes is shown 111 in Fig. 1(b). The angle between the FIB beam and the holes 112 axis is 52°.
113 In order to couple the conventional TE APE mode 114 (4 lm size) to the photonic crystal waveguides (approxi-

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115 mately 500 nm and 1500 nm wide), the optical mode is 116 smoothly guided through a photonic tapered structure 117 (see Fig. 1(a)). 118 The novel structures were first characterized by measur-119 ing their far field transmission. The experimental setup is 120 shown in Fig. 2(a). In order to get a spectrum as flat as pos-121 sible on a large range of wavelengths (1000-1700 nm), we 122 use a super-continuum light source. The white light is gen-123 erated by a sub-nanosecond microchip laser emitting at 124 1064 nm with 8 lJ energy per pulse [9]. The laser light is 125 coupled into a photonic crystal (PC) fibre, which enhances 126 the nonlinear effects required for the generation of a large 127 super-continuum. The resulting output spectrum for a 128 20 m long PC fibre is shown in Fig. 2 The optical transmission was measured through the two 130 photonic crystal waveguides, a photonic crystal without 131 defect lines, and through a standard optical waveguide, 132 fabricated on the same wafer and in the same conditions 133 as described above. The experimental results are shown 134 in Fig. 3. As it can be seen in the graph, the transmissions 135 through the photonic structures (filled triangle, empty 136 square, empty circle) exhibit a gap, which does not appear   Fig. 4(a) and (b) show the projected band dia-147 grams and the light line for the two photonic crystal wave-148 guides. For the PCW1 case, the diagram shows two guided 149 modes. In the PCW3 case all the modes are radiative 150 ( Fig. 4(b)). Experimentally (Fig. 3) undergo substantial modulations on length scales that are 165 much shorter than one wavelength. Therefore, it is impos-166 sible to resolve the spatial details of light propagation by 167 the far field transmission measurement described above.
172 In the work presented here, the instrument used is a 173 commercial scanning near-field optical microscope 174 (SNOM) (NT-MDT SMENA) in collection mode [18].
175 The near-field optical fibre probe is fabricated by heating 176 a single mode optical fibre with a CO 2 laser and then pull-177 ing it apart with a micropipette puller (Sutter Instrument 178 Co.) to obtain a sharp taper region with a small end face 179 ($100 nm). To obtain the SNOM images, one needs to 180 scatter the evanescent fields on the sample by raster scan-181 ning the sub-wavelength probe at a few nanometers from 182 the surface. A non-optical shear force feedback [19] is used 183 to keep the probe at a constant distance from the sample 184 surface. Both signals, the feedback and the optical one, 185 are simultaneously acquired to construct topographic and 186 SNOM collection images. 187 Fig. 5 shows the experimental set-up. To image the 188 transmitted mode through the LN photonic structure, 189 two different laser sources and two different optical detec-190 tors have been utilized. The first acquisition has been per-