Frequency-Modulation SREF Imaging

The details of the frequency-modulation method we adopt here can be found in our previous publication^{24 (link)}. The main difference comes from the signal detection part. For FM-SREF detection, a non-resonance electro-optic modulator (EO-AM-NR-C2, Thorlabs) is used, and it is driven by a 50-kHz square wave amplified by a high-voltage amplifier (HVA200, Thorlabs) to achieve more than 90% modulation depth. The square wave is generated by our home-built lock-in photon counter for the convenience of phase control and signal synchronization. For filter set configuration, a 2-mm-thick shortpass dichroic mirror (T785spxxr-UF2, Chroma) was used for flatness consideration, all the other optical filters used for nitrile-band SREF signal detection are the same as those used in our previous publication^{20 (link)}. The same objective (UPLSAPO, 1.2NA, Olympus) and detector (SPCM-NIR-14-FC, Excelitas) were used for all measurements and imaging. And all imaging scanning and data acquisition (including the lock-in photon counter) are driven by a Multifunction I/O card (USB-6259, NI) controlled with a LabVIEW-based home-built software. The detailed construction of the three systems used in this research can be found in Fig. S1, Fig. S2c, and Fig. 4a. The lock-in photon counter can be coded by following the time sequence diagram shown in Fig. S3.