Sub-comb based gas sensing in a graphene functionalized microsphere

With high quality factor (Q) and compact size, the microcavity significantly enhances light-matter interactions, offering an ideal platform for biochemical sensing in photonics. So far, microcavity-based optical sensors have demonstrated advanced performance by utilizing mechanisms such as mode shifting, mode broadening, and mode splitting. More recently, the emergences of microlaser sensors and soliton microcomb sensors further provide new schemes for multispecies and ultrahigh resolution for gas detection. Among them, the microcomb-based sensing scheme offers high accuracy signals at coherent frequencies, illustrating unique advancements for tracing individual gas molecules in mixtures, however, it relies on the stability of the soliton excitation and maintaining, which determines the signal to noise ratio (SNR) in sensing operation. Such an acquisition of soliton states typically requires complex red detuning accessing, meanwhile the stability of soliton states needs strict control of environmental variables, which impairs the out-of-lab application of microcombs with high convenience and low cost. Besides, in gas sensing cases, due to the inertness of microcavity materials such as silica and silicon nitride, the absorption efficiency of gas molecules on microcavities is inhibited, limiting the sensitivity of gas detection. Here, a sub-comb based gas sensor in a graphene functionalized microsphere is demonstrated. Instead of using soliton states of a microcomb, we investigate sub-comb states, which appear in the blue-detuned region. Sub-comb is usually only regarded as an intermediate state in the soliton excitation process, although its theories and properties have been explored, it has never been applied to applications, to the best of our knowledge. In this work, the merge effect of sub-comb leveraged, providing frequency probe in the radio frequency domain for sensing, meanwhile, advantages owned by sub-comb are excavated, endowing the system with preponderances over its soliton counterpart. On one hand, it resides in thermal-locked regime, demonstrating high robustness. On the other hand, it does not require high precision double-balance control, so that it is much easier to obtain. For sensing, heterodyne signals generated from the sub-comb beating in their overlapping region, enable RF radio frequencies with SNR > 50 dB and linewidth < 5 kHz. The signals are sensitized by graphene, realizing gas detection limit down to 4 ppb level. This device allows plug-and-play operation, keeping the advantage of microcomb but avoiding complicated soliton access procedures. The combination of graphene materials and sub-comb in microcavity geometry paves a new paradigm for high performance miniature gas sensors. In summary, we demonstrate a sub-comb based gas sensor in a graphene functionalized microcavity, which shows 4 ppb detect limit and maximum sensitivity of 750 Hz/ppb to SO2 gas and the potential to be a plug and play device. It leverages the merge effect in the sub-comb overlapping region, where the complicated soliton access method is not in need. The sub-comb signals sensitized by graphene enable fast and highly recoverable light-gas responses spectrally. In this study, flexible microcomb formation, direct offset heterodyne and graphene optoelectronics are combined together, empowering a miniaturized, low-power consumption and easy-operation microcavity gas sensor. In the future, beyond microsphere-based gas sensing, this interdisciplinary principle also suggests a potential to open platform-independent approaches for wider sensing scenarios such as multispecies sensing and on-chip biomacromolecule sensing for antibody and antigen.