Electro–optically tunable microring resonators in lithium niobate

Abstract

Optical microresonators have recently attracted a growing attention in the photonics community. 1 Their applications range from quantum electro-dynamics to sensors and filtering devices for optical telecommunication systems, where they are likely to become an essential building block. 2 The integration of nonlinear and electro-optical properties in the resonators represents a very stimulating challenge, as it would incorporate new and more advanced functionality. Lithium niobate is an excellent candidate material, being an established choice for electro-optic and nonlinear optical applications. Here we report on the first realization of optical microring resonators in submicrometric thin films of lithium niobate. The high index contrast films are produced by an improved crystal ion slicing and bonding technique using benzocyclobutene. The rings have radius R = 100 µm and their transmission spectrum has been tuned using the electro-optic effect. These results open new perspectives for the use of lithium niobate in chip-scale integrated optical devices and nonlinear optical microcavities.The established use of wavelength division multiplexed (WDM) for local area network systems has raised the demand for new filtering and switching functions. 3 In order to integrate these devices on a wafer scale, whispering gallery mode microresonators represent the most compact and efficient solution. They consist of a bus waveguide evanescently coupled to a micrometer-size ring resonator; the characteristic size-dependent frequency spectrum of the ring allows only selected wavelength channels to be transmitted or shifted to another waveguide. Small radii allow a large free spectral range -i.e. large separation between the filtered channels -but increase the propagation bending losses, 4 which can compromise the quality factor Q -i.e. the wavelength selectivity -of the device. To overcome this limitation, high refractive index contrast between the ring core and the surrounding materials is mandatory. A second, very important, requirement relates to the tunability. The possibility to electrically control the transmission spectrum, via electro-optic effect, would allow extremely compact and ultrafast modulation and switching. By integrating arrays of microring resonators on a single optical chip, the realization of complex functions would be feasible. 5 Besides, large-Q resonators based on non centro-symmetric materials would exploit the high amount of stored energy for enhancing the efficiency of nonlinear optical phenomena. 6 Several examples of microring resonators have been proposed and successfully realized in the last years in a variety of materials like semiconductors, 7-10 silica 11 and polymers. 12,13 The advanced structuring technology in semiconductor materials enables the realization of very high-Q resonators even for radii as small as 10 µm. Silicon-based resonators can be tuned by electrically-driven carriers arXiv:0705.2392v1 [physics.optics]

A strong electro-optically active lead-free ferroelectric integrated on silicon

“…However, today's silicon photonics technology does not exploit the linear electro-optical (EO) properties available in oxides, which for decades fuelled the progress in telecommunication based on fibre optics to extremely high modulation speeds with well-established devices 14 . In recent work, micro-scale EO devices have been presented 15 . These devices were based on LNO (LiNbO 3 ), which can be integrated as high-quality films on Si only via complex and costly layer-bonding approaches 16 .…”

Emerging heterogeneous integrated photonic platforms on silicon

“…Alternatively, crystal ion slicing has been successfully employed to obtain high-quality thin films of crystalline LiNbO 3 [162][163][164][165]. A combination of deep helium ion implantation, selective etching and epoxy-resin bonding has been applied to transfer 5-to-10-μm-thick LiNbO 3 layers to silicon substrates [162].…”

“…In another report, the adhesive polymer BCB, was used instead for wafer bonding and demonstrate electrooptically tunable LiNbO 3 -on-LiNbO 3 microring resonators [165]. Polymers, like epoxy-resin [162] or BCB [165], may be simple and straightforward solutions for lab demonstrations, but, as mentioned before, they are generally not reliable for practical applications due to the weak nature of the formed bonds.…”

“…Polymers, like epoxy-resin [162] or BCB [165], may be simple and straightforward solutions for lab demonstrations, but, as mentioned before, they are generally not reliable for practical applications due to the weak nature of the formed bonds.…”

Poling-inhibited ridge waveguides in lithium niobate crystals

“…Most of them utilize an etching step that defines the ridge geometry and a separate waveguide fabrication step [4][5][6] such as the commonly used ionindiffusion or proton-exchange processes. Delineation of the ridge is achieved either via wet etching in an acid mixture containing hydrofluoric acid (HF) or through dry etching processes such as ion beam milling 7 or plasma etching 8 . Domain-selective acid-based wet etching has also been reported as an alternative approach for defining ridges in domain engineered LN 9 .…”