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Printable planar lightwave circuits with a high refractive index

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 325302 (http://iopscience.iop.org/0957-4484/25/32/325302) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 25 (2014) 325302 (6pp)

doi:10.1088/0957-4484/25/32/325302

Printable planar lightwave circuits with a high refractive index Carlos Pina-Hernandez1, Alexander Koshelev2, Lucas Digianantonio3, Scott Dhuey3, Aleksandr Polyakov3, Giuseppe Calafiore1, Alexander Goltsov2, Vladimir Yankov2, Sergey Babin1, Stefano Cabrini3 and Christophe Peroz1 1

aBeam Technologies, 22290 Foothill Blvd, St. 2, Hayward, CA 94541, USA NanoOptic Devices, 2953 Bunker Hill Lane, Santa Clara, CA 95054, USA 3 The Molecular Foundry, LBNL, One Cyclotron Road, Berkeley, CA 94702, USA 2

E-mail: [email protected] Received 28 April 2014, revised 18 June 2014 Accepted for publication 23 June 2014 Published 25 July 2014 Abstract

We report a novel nanofabrication method to fabricate printable integrated circuits with a high refractive index working in the visible wavelength range. The printable planar ligthwave circuits are directly imprinted by ultra-violet nanoimprinting into functional TiO2-based resist on the top of planar waveguide core films. The printed photonic circuits are composed of several elementary components including ridge waveguides, light splitters and digital planar holograms. Multi-mode ridge waveguides with propagation losses around 40 dB cm−1 at 660 nm wavelength, and, on-chip demultiplexers operated in the visible range with 100 channels and a spectral channel spacing around 0.35 nm are successfully demonstrated. Keywords: nanoimprint, photonic integrated circuits, titanium dioxide, nanofabrication (Some figures may appear in colour only in the online journal) 1. Introduction

lithography techniques. Numerous optical components have been replicated by using NIL, from wire grid polarizers [11, 12], ring resonators [13], nanostructured antireflection coatings [14, 15], to on-chip fluidic sensors [16, 17]. A few commercial products integrating nanoimprinted structures were recently launched, the most popular of which are the high efficiency light emitting diodes [18] and the ultra-bright e-book readers [19]. As a step further, the NIL technology offers a unique and powerful route to fabricate printable devices by directly imprinting functional materials: the patterned films have the final desired optical functionality, and there is no need for additional plasma etching steps. This approach combines the advantages of both the top-down NIL process to fabricate micro/nanostructures with high control, and of the bottom-up synthetic chemistry approach to design and tune the properties of the patterned films. NIL was used to pattern a large variety of materials from functional polymers [20], and metals [21], to inorganic films [22]. In particular, inorganic materials with a high refractive index and high optical transparency are highly advantageous

Photonic integrated circuits, also named planar lightwave circuits (PLCs), promise the emergence of a broad range of novel applications from on-chip sensors [1, 2], and data storage [3] to quantum computing [4, 5]. Although PLC-based devices are currently utilized in optical data transmission [6, 7], the vast majority of the potential applications are still limited to research laboratories due to their complex and expensive fabrication. A new paradigm needs to be developed for the fabrication of PLC devices that combines high-resolution patterning (sub-wavelength feature sizes), processing of ‘non-conventional’, materials and substrates at low-cost and high-throughput. Nanoimprint lithography (NIL) [8–10] simultaneously satisfies all the requirements of the PLCs. NIL basically consists of stamping the resist material with a nanostructured template. In a ‘conventional’ fabrication schema similar to the one used for semiconductor processes, the imprinted resist is used as a sacrificial layer to transfer the pattern into the functional film by plasma etching. NIL is used as a low-cost replacement for the conventional 0957-4484/14/325302+06$33.00

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Figure 1. Direct imprinting of titania film with high refractive index. A hybrid organic/inorganic TiO2-based resist film is imprinted by direct ultra-violet nanoimprinting lithography. (a) Schematic of the process: functional resist is deposited on the substrate by spin-coating, a nanopatterned mold is pressed up to the resist and cross-linked under UV-light, the mold is finally detached, and the film on the substrate is annealed to reach the desired optical properties. (b) Schematic view (not in the scale) of the structure of the printed PLCs. (c) Simulation of the profile of the fundamental mode profile in a multi-mode ridge waveguide at a 650 nm wavelength. The imprinted TiO2 pattern has a height of 15 nm and the residual layer thickness underneath is 15 nm. The refractive index of the imprinted TiO2 film is equal to n = 2.05 at a 590 nm wavelength.

2. Experimental details

for building up integrated photonic chips. The high optical refractive index allows for the light to be controlled and bent at the nanometer scale, and the high optical transparency efficiently transmits the optical signal on the chip. One of the most promising materials for the visible wavelength range is titanium dioxide (TiO2), with its high optical refractive index (n ∼ 2.5 for the anatase form) and its good optical transmission (>90%) [23]. The strong nonlinear properties of titania films are also attractive for the development of all-optical logic devices [24]. In spite of all these advantages, only a few titania based photonic components (ridge waveguides (RWGs) [25], resonators [26] and photodetectors [27]) have been demonstrated due to the difficulty of patterning TiO2 films of a high quality by plasma etching [23]. As an alternative route, a few works have investigated the direct patterning of TiO2 films by NIL [28–30]. Titania-based NIL resists using sol–gel chemistry or hybrid organic–inorganic synthesis have been developed and have successfully demonstrated the patterning of feature sizes down to 5 nm [31]. Here, we extend these results and demonstrate for the first time that the direct imprinting of TiO2 films is suitable for fabricating PLCs with a high refractive index by direct NIL. The printed photonic circuits are composed of several elementary components including RWGs, light couplers and optical on-chip demultiplexers.

The process for direct imprinting of PLC devices into TiO2 films is described in figure 1(a) and is presented in detail in our previous publication [31]. The master molds consist of hydrogen silsesquioxane patterns defined by electron beam lithography (EBL) on silicon wafers. Ormostamp [32] and hard-polydimethylsiloxane bilayer NIL templates are then replicated from the master molds and treated with a 1H, 1H, 2H, 2H-perfluorooctyltrichlorosilane anti-sticking layer. Our hybrid organic–inorganic resist [31] is spin-coated and soft baked at 100 °C for 1 min to create uniform solvent-free films. Titania resist films are imprinted at low pressures (P < 2 bars) and exposed to ultra-violet light. Finally, a post-annealing step (400 ⩽ T ⩽ 500 °C) at ambient air is performed to control the structural phase of the resist material and precisely tune the optical properties of the imprinted structures. In this paper, all the devices were annealed at 400 °C to reach a refractive index (n) of around 2.05 at a 590 nm wavelength. During the post-annealing, the organic components of the resist material are decomposed and the dimensions of the printed structures shrink differently in the vertical and horizontal dimensions. The shrinkage of a grating is around 50% and 60% in the lateral and vertical dimensions, whereas a thin film will shrink around 80% at 400 °C [31]. These values of shrinkage need to be taken into consideration when designing the photonic components and can be used advantageously to minimize the residual layer thickness (RLT) underneath the 2

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Figure 2. Imprinted photonic components. (a) Optical top-view picture of an imprinted PLC chip including multiple RWGs, light splitters and DPHs. Scanning electron microscopy (SEM) pictures of (b) a light splitter and (c) a digital planar hologram after post-annealing at 400 °C. The inset picture shows the cross-section of one etched dash composing the DPH. The height is around 30 nm and the RLT is around 8 nm. The lateral and vertical dimensions shrink from 90 to 45 nm and from 85 to 30 nm, respectively.

imprinted components to create smaller feature sizes. We have designed a PLC integrating several components: multimode RWGs, Y-splitters [33], and digital planar holograms (DPH) [34, 35]. RWGs and their bending are simulated with the beam propagation method (BPM) [36], and show that 95% of the fundamental mode can be transmitted through 90° turn for radiuses larger than 900 μm. DPH are designed and simulated using the proprietary code Spectroplan [35]. The optical components are imprinted on the top of the Si/SiO2/ Si3N4 substrates, where a 8 μm thick SiO2 layer is used as a lower cladding, and a 156 nm thick Si3N4 film is used as the waveguide core (figures 1(b), (c)). The waveguide mode is mainly located into the Si3N4 waveguide core along both the horizontal and vertical dimensions, and benefits from the low absorption losses of the Si3N4 film [35], whereas the thickness variations of the TiO2 imprinted layer are used to guide the light (figure 1(c)). Finally, a 2 μm thick upper cladding is deposited by plasma-enhanced chemical vapor deposition according to [35], in order to ensure that there is good confinement of the optical modes inside the chip. The optical losses of the imprinted RWGs are measured with a homemade optical set-up. The light from a single mode fiber (630 HP) is coupled into the RWGs by directly attaching fiber to the edge. The output signal is measured, by collecting and focusing the light with a 40x objective onto a CCD camera. The input signal from the fiber was measured using the same optical setup for collecting light. The ratio of these two signals allows the insertion loss of the RWGs to be determined to be around 27 dB. The insertion loss consists of the sum of the coupling loss and of the propagation loss in 5 mm RWGs. Coupling loss is calculated to be 7 dB using the BPM method, thus

Figure 3. Light propagation in printable photonic circuits with a high refractive index. Optical top-view picture of the propagation of the light into a printed PLC annealed at 400 °C. The fabricated chip is illuminated at the input by a laser diode (λ = 660 nm), TE polarization. The light is guided and split in the circuit and can be seen at the output waveguides. The planar holograms appear illuminated due to the stray light associated with the coupling losses in the input edge of the chip.

propagation loss is around 40 dB cm−1. The planar holograms are tested using the optical setup described in detail in [34].

3. Results and discussion Several PLC chips were successfully imprinted with a maximum total size around 6 × 6 mm2. Figure 2 displays an imprinted chip that includes, three 100-channel DPHs, and three Y-splitters (figure 2(b)) splitting the light towards the 3

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Figure 4. Optical responses of the printed on-chip demultiplexer. The planar holograms are printed into a film with a refractive index around

2.05 and have a 2 μm thick SiO2 upper-cladding. (a) Images of the output plane at the edge of a four channels DPH, in response to input laser excitations of λ = 624.7, 631.3, 638.5, 644.0 nm; (b) the spectral response of four adjacent channels in a four-channel DPH; (c) the spectral shape of a single channel of a 100-channel DPH; (d) the dispersion curve of 50 channels located to the left of the input spot for the 100-channel demultiplexer.

curved RWGs. The imprinting process was optimized to have a sub-10 nm RLT after post-annealing (figure 2(c)). The propagation of the light in the waveguide core layer is controlled by the thickness modulations of the imprinted titania pattern. The small thickness modulations −20 to 30 nm—of TiO2 films allows the light to be controlled and directed the light in the waveguide core and creates the photonic components. The shallow guiding structures beneficially decrease the requirements for a high vertical edge roughness associated with losses in RWGs [30]. They also increase the effective length of the Bragg grating and are used to develop highresolution on-chip devices based on DPH [35]. Figure 3 shows the propagation of the light in a printed circuit when it is illuminated at the input RWGs with a red laser light (λ = 660 nm). The light is guided and split over one centimeter long, demonstrating the ability of our method to fabricate printable PLCs. The propagation losses are measured to be around 40 ± 5 dB cm−1 at 660 nm wavelength. This value is similar to the ones reported in the literature reporting for TiO2 RWGs made by EBL and plasma etching: the propagation losses were around 30 dB cm−1 and 50 dB cm−1 for amorphous and anatase phases, respectively [23]. The absorption losses in our material have previously been measured to be around 3–4 dB cm−1 at a 635 nm wavelength in planar TiO2 waveguide films [31], which means that the main losses in the imprinted RWGs are due to the light scattering on the defects in the ridge waveguide.

As one step further, the DPH were successfully imprinted and demonstrated for on-chip wavelength demultiplexing. The DPH consists of a computer-designed planar hologram and involves millions of lines specifically located and oriented to direct the output light into focal channels according to its wavelength [34]. The geometry of the gratings (linewidth and height) is determined in accordance with the variation of the effective refractive index inside the guiding layer, and with the operating wavelength bandwidth. In a DPH-based demultiplexer, the back-diffracted light from the hologram is focused at different points (output channels) along the input–output edge of the planar chip. Over the last years, we have fabricated and demonstrated on-chip demultiplexers with a number of channels (up to 1024), and a spectral resolution down to 20 pm [34, 35]. The holographic chips were all fabricated by EBL and plasma etching into a waveguide core material (SiO2Gex, Si3N4) [32, 33]. Here, their fabrication is drastically simplified by directly imprinting them onto TiO2 films (see figure 2). Figure 4 displays the optical responses of two imprinted on-chip demultiplexers. Devices with 4 and 100 channels and a central wavelength of 634 and 645 nm respectively, were measured. The light reflected from the DPH is focused towards the discrete output channels according to the wavelength of the input light (figure 4(a)), and confirms the discrete response of the DPH. All the output channels of the printed chips are observed and the spectral channel width is measured to be around 0.35 nm 4

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(figure 4(c)), in good agreement with the design. The crosstalk between adjacent channels is around 10 dB (figure 4(b)) and the dispersion curves of the holograms are linear through the full spectral bandwidth (figure 4(d)) [35]. These results clearly demonstrate that our technology is suitable for the fabrication of printable photonic devices, both in terms of the pattern resolution and defects degree associated to the topdown NIL technique, and the optical properties of the imprinted films achieved by bottom-up synthetic chemistry.

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4. Conclusion The direct printing of a functional titania-based resist was used to fabricate the first printable photonic integrated devices with a high refractive index. PLCs integrating multimode RWGs, light splitters and 100-channel on-chip demultiplexers were successfully fabricated and measured. We believe that this work introduces a powerful and cost-effective route for the development of numerous nanophotonic devices and will lead to the emergence of new applications.

Acknowledgments The authors would like to thank S Sassolini and E Wood for their technical support. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the United States Department of Energy under contract DE-AC02-05CH11231. Efforts were sponsored by the Air Force Office of Scientific Research (AFOSR), Air Force Material Command, USAF, under grant/contract number FA9550-14-C-0020.

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Printable planar lightwave circuits with a high refractive index.

We report a novel nanofabrication method to fabricate printable integrated circuits with a high refractive index working in the visible wavelength ran...
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