A schematic illustration of the microscale-PDMS phase modulator incorporating plasmonic nanostructures and the process of direct frequency comb spectroscopy measuring the target absorption spectrum (IMAGE)
Caption
When the frequency comb (FC) passes through the microscale-PDMS (μ-PDMS), it undergoes acousto-optic phase modulation by the acoustic wave frequency of fm. This process generates new interleaved combs with a frequency spacing of fm. The phase-modulated FC is then transformed to the plasmonic mode, allowing more efficient interaction with the target materials on the Au surface. This plasmonic FC is subsequently transformed back into the photonic FC and combined with a reference FC whose frequency is shifted by fAOM (40 MHz), to produce a radio-frequency (RF) beat frequency. These RF beat spectrum possess the absorption spectral features of the target gas (right panel). The bottom left panel shows the scanning electron microscope (SEM) image of plasmonic nanostructures integrated μ-PDMS. SPP: surface plasmon polariton, BS: beam splitter, APD: avalanche photodetector.
Credit
Seungchul Kim
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