On-chip mid-IR graphene photodetector integrated with lithium niobate photonic circuit

Abstract

Graphene’s exceptional properties, such as high carrier mobility, zero-band gap, and compatibility, make it an ideal material for mid-infrared (mid-IR) photodetection and on-chip integration. Lithium niobite ($LiNbO_3$) is a promising material for chip-scale photonics due to its electro-optic properties and low optical loss in the mid-IR region. In this study, I present a mid-IR graphene photodetector (PD) integrated with a $LiNbO_3$ platform and investigate its potential applications in lab-on-a-chip systems. The mid-IR graphene PD is designed and fabricated with a $LiNbO_3$ waveguide. I also designed the electro-optic modulator based on the racetrack-resonator to integrate with graphene PD. I achieved mid-IR photodetection with a responsivity of $1.6 \sim 2.5 mA/W$ in the wavelength range from $3.2 \mu m$ to $3.9 \mu m$ at a bias voltage of $1 V$, which is comparable with other on-chip mid-IR graphene PDs, and with a minimum detectable power of $\sim 100 \mu W$. I conclude that our graphene PD is based on the mechanism of bolometric effect according to the negative photocurrent generation and the absolute value of resistance change when the light is on. In addition, I confirmed that our graphene PD can measure the photocurrent with a modulation frequency of $3.9 kHz$ and $50 kHz$ in the mid-IR and visible region, respectively, by passivating the graphene with $30 nm$ of $Al_2O_3$ layer. Thus, I expect that our graphene PD integrated with an electro-optic modulator has a chance to be utilized in on-chip lock-in photodetection, and applicable to on-chip Fourier-transform mid-IR spectroscopy with a small footprint.