Transparent Conducting Oxide Based On-Chip Modulators for Applications at Cryogenic Temperatures

Optical datalinks, which feature unparalleled data transmission rates and energy efficiency, promise to provide the DoD with leap-ahead opportunities, such as surpassing their current frame rate, resolution, and bit depth limits on imaging sensors such as focal plane arrays (FPAs). Reduction in size, weight and power (SWaP) of optical transmitters is enabled through miniaturization of electro optic (EO) modulators on photonic integrated circuit (PIC) chips. Leading-edge thin film lithium niobate (TFLN) Mach-Zehnder modulators (MZMs) can achieve 100s of GHz BWs. However, the inherently weak EO interaction causes them to be large ( > 5 mm) and inefficient ( > 1000 fJ/bit). Resonance based modulators, like microring modulators (MRMs), are smaller ( ~ 10 um) and more efficient (~ 4 fJ/bit) than MZMs. However, due to their inherent wavelength selectivity and sensitivity to changes in temperature they require meticulous modeling, testing and control for stable operation. Therefore, they are less desirable for FPAs which require stable use at cryogenic temperatures.áTransparent conductive oxides (TCOs) have drawn increasing attention as an EO material due to the high tunability of their optical properties and transparency in the near-IR. TCO based electro absorption modulators (EAM) have demonstrated high extinction ratios (ER) exceeding 2.5 dB/um with theoretical EO bandwidths (BWs) exceeding 100 GHz. Therefore, high ER and BW operation can be achieved in a dramatically reduced footprint compared to MRMs (10 x smaller) or MZMs (100 - 10000 x smaller), enabling dense packaging of modulators. Moreover, TCO EAMs operate across a broad spectral range, from (1200 nm -1600 nm) while achieving sub 60 fJ/bit efficiency. Importantly, as temperature decreases TCOs experience increased mobility and decreased absorption. Therefore, TCO EAMs will exhibit lower loss and higher BW when operated at cryogenic temperatures. While TCO EAMs have shown great promise, both materials and designs must be further optimized to demonstrate their true potential.ááTo address the current limitations of EO modulators, RMD Inc. proposes a novel strategy which leverages the benefits of TCO EAMs and the unparalleled capabilities of atomic layer deposition (ALD) to provide a high BW and high efficiency modulator which is optimized for cryogenic operation. Wafer-scale precision of ALD will ensure that high quality TCO films can be manufactured in a scalable CMOS compatible manner. RMD Inc.ĺs proposed solution will enable high BW, efficient, and stable cryogenic EO modulation with a small footprint that allows for scalable and dense packaging in future devices.