Lithium Niobate Vertical Cavity Electro-Optic Modulator

This paper introduces the vertical-cavity electro-optic modulator (EOM) based on lithium niobate (LN), specifically designed for free-space applications. The device leverages photonic crystal (PhC) mirrors to form a cavity that enhances light-matter interaction through defect-mode resonances, which are tuned by the Pockels effect to modulate light efficiently.

Key Contributions:

1. Device Architecture:

• The LN membrane is sandwiched between two PhC mirrors, each made of alternating TiO₂ and SiO₂ layers, forming a vertical cavity.

• Gold electrodes are integrated to apply a voltage that induces the Pockels effect in the z-polarized light to shift the resonance of the cavity.

• The Pockels effect allows the refractive index of LN to be modulated, which leads to shifts in the transmission spectrum of the device.

2. Modulation Performance:

• Experimental results show a modulation depth of 43% for z-polarized light at 787 nm under a drive voltage of ±50 V, with a modulation bandwidth of 5 MHz.

• The device achieved a quality factor (Q-factor) of 611 in its resonant cavity, showing sharp transmission resonances and a narrow linewidth of approximately 1.3 nm.

• Transmission spectra demonstrate efficient resonance shifts at specific wavelengths, confirming the capability of this vertical-cavity modulator to perform as an electro-optic device.

3. Free-Space Compatibility:

• Unlike traditional waveguide-based modulators, this device is optimized for free-space optical systems, making it suitable for applications in LiDAR, holography, and beam steering.

• The vertical-cavity design facilitates compact integration, offering a significant advantage over conventional in-plane devices that are typically used for guided-wave operations.

4. Fabrication Process:

• The modulator was fabricated using LN on insulator (LNOI) technology, with a 1 µm-thick LN film embedded between two PhC slabs.

• The device was fabricated through standard photolithography and metal deposition techniques, followed by the alignment of the LN film with the electrodes to form the vertical cavity structure.

• Optical microscopy and SEM imaging were used to inspect the structure and ensure precision in the alignment and electrode integration.

5. Applications:

• The device is intended for use in free-space optical systems requiring high-performance electro-optic modulation, including spatial light modulation and dynamic holography.

• The compact design and high modulation depth make it suitable for applications where size and power efficiency are critical, such as in beam steering and ultrafast optical communications.

6. Future Improvements:

• While the device demonstrated promising performance, there is room for improvement in terms of fabrication precision and measurement techniques to further enhance the modulation bandwidth and overall efficiency.

Conclusion:

The paper presents a novel and efficient design for a vertical-cavity electro-optic modulator using lithium niobate, demonstrating strong modulation capabilities with a high Q-factor and wide applicability in free-space optical systems. The integration of photonic crystal mirrors and Pockels effect offers a promising solution for compact, high-performance electro-optic devices, expanding the potential for future quantum and telecom-based applications.