Ultralow voltage, high-speed, and energy-efficient cryogenic electro-optic modulator

Photonic integrated circuits (PICs) at cryogenic temperatures enable a wide range of applications in scalable classical and quantum systems for computing and sensing. A promising application of cryogenic PICs is to provide optical interconnects by upconverting signals from the electrical to the optical domain, allowing a massive data transfer from 4 K superconducting (SC) electronics to the room temperature environment. Such a solution can overcome a major bottleneck in the scalability of cryogenic systems that currently rely on bulky coaxial cables that suffer from limited bandwidth, a large heat load, and poor scalability. A key element to realize a cryogenic-to-room temperature optical interconnect is a high-speed, electro-optic (EO) modulator operating at 4 K with a modulation voltage at the mV scale, compatible with SC electronics. Although several cryogenic EO modulators have been demonstrated, their driving voltages are substantially large (several hundred mV to a few V) compared to the mV scale voltage provided by SC circuits. Here, we demonstrate a cryogenic modulator with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>10</mml:mn> </mml:mrow> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">m</mml:mi> <mml:mi mathvariant="normal">V</mml:mi> </mml:mrow> </mml:math> peak-to-peak driving voltage and Gb/s data rate, with an ultralow electric energy consumption of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>10.4</mml:mn> </mml:mrow> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mi mathvariant="normal">a</mml:mi> <mml:mi mathvariant="normal">J</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">b</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">t</mml:mi> </mml:math> and an optical energy consumption of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>∼</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>213</mml:mn> </mml:mrow> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">J</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">b</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">t</mml:mi> </mml:math> . We achieve this record performance by designing and fabricating a compact optical ring resonator modulator in a heterogeneous InP-on-Si platform, where we optimize a multi-quantum-well layer of InAIGaAs to achieve a strong EO effect at 4 K. Unlike other semiconductors such as silicon, our platform benefits from the high-carrier mobility and minimal free-carrier freezing of III-V compounds at low temperatures, with a moderate doping level and a correspondingly low loss (intrinsic resonator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi>Q</mml:mi> </mml:mrow> <mml:mo>∼</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>272</mml:mn> </mml:mrow> <mml:mo>,</mml:mo> <mml:mspace width="negativethinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>000</mml:mn> </mml:mrow> </mml:math> ). These modulators can pave the path for complex cryogenic photonic functionalities and massive data transmission between cryogenic and room-temperature electronics.