Ultrahigh-sensitivity optical power monitor for Si photonic circuits

TL;DR

This paper introduces a waveguide-coupled InGaAs phototransistor with ultrahigh responsivity (~10^6 A/W) that can detect extremely low optical power levels, making it highly suitable for Si photonic circuit monitoring.

Contribution

The development of a novel InGaAs ultrathin channel phototransistor with record-high responsivity for low-power optical detection in silicon photonics.

Findings

Achieved responsivity of approximately 10^6 A/W.

Detected optical power as low as 621 fW.

Response time of about 100 microseconds.

Abstract

A phototransistor is a promising candidate as an optical power monitor in Si photonic circuits since the internal gain of photocurrent enables high sensitivity. However, state-of-the-art waveguide-coupled phototransistors suffer from a responsivity of lower than $10^3$ A/W, which is insufficient for detecting very low power light. Here, we present a waveguide-coupled phototransistor consisting of an InGaAs ultrathin channel on a Si waveguide working as a gate electrode to increase the responsivity. The Si waveguide gate underneath the InGaAs ultrathin channel enables the effective control of transistor current without optical absorption by the gate metal. As a result, our phototransistor achieved the highest responsivity of approximately $10^6$ A/W among the waveguide-coupled phototransistors, allowing us to detect light of 621 fW propagating in the Si waveguide. The high responsivity and the reasonable response time of approximately 100 $\mu$s make our phototransistor promising as an effective optical power monitor in Si photonics circuits.