Fluorescence lifetime imaging microscopy (FLIM) is a powerful tool for quantitative fluorescence imaging because fluorescence lifetime is independent of concentration of fluorescent molecules or excitation/detection efficiency and is robust to photobleaching. However, since most FLIMs are based on point-to-point measurements, mechanical scanning of a focal spot is needed for forming an image, which hampers rapid imaging. Here, we demonstrate scan-less full-field FLIM based on a one-to-one correspondence between two-dimensional (2D) image pixels and frequency-multiplexed radio frequency (RF) signals. A vast number of dual-comb optical beats between dual optical frequency combs are effectively adopted for 2D spectral mapping and high-density frequency multiplexing in the RF region. Bimodal images of fluorescence amplitude and lifetime are obtained with high quantitativeness from amplitude and phase spectra of fluorescence RF comb modes without the need for mechanical scanning. The parallelized FLIM will be useful for rapid quantitative fluorescence imaging in life science.

Dual-comb microscopy (DCM), based on a combination of dual-comb spectroscopy (DCS) with twodimensional spectral encoding (2D-SE), is a promising method for scan-less confocal laser microscopy giving an amplitude and phase image contrast with the confocality. However, signal loss in a 2D-SE optical system hampers increase in image acquisition rate due to decreased signal-to-noise ratio. In this article, we demonstrated optical image amplification in DCM with an erbium-doped fiber amplifier (EDFA). Combined use of the image-encoded DCS interferogram and the EDFA benefits from not only the batch amplification of amplitude and phase images but also significant rejection of amplified spontaneous emission (ASE) background. Effectiveness of the optical-image-amplified DCM is highlighted in the single-shot quantitative nanometer-order surface topography and the real-time movie of polystyrene beads dynamics under water convection. The proposed method will be a powerful tool for real-time observation of surface topography and fast dynamic phenomena. An optical frequency comb (OFC) 1-3 is a unique optical spectrum composed of a vast number of discrete, regularly spaced optical frequency modes, and the optical frequency and phase of all OFC modes are secured to a frequency standard by active laser control of carrier-envelope-offset frequency f ceo and a frequency spacing or repetition frequency f rep. Dual-comb spectroscopy (DCS) 4-7 has appeared as a new mode to make full use of OFC as an optical frequency ruler for broadband high-precision spectroscopy. Use of two OFCs with slightly different frequency spacings (signal OFC, f rep1 ; local OFC, f rep2 = f rep1 + ∆f rep) enables us to make a replica of the signal OFC in radio-frequency (RF) region based on a frequency scale of 1:(f rep1 /∆f rep), typically 1:10 5. The resulting mode-resolved OFC spectra of amplitude and phase have been used for broadband high-precision spectroscopy of gas 8,9 , solid 10 , and thin film 11. Also, such DCS is available in the broad spectral range of ultraviolet 12 , visible 13 , mid-infrared 14,15 , and terahertz 6,16 , due to wavelength diversity of OFC and DCS 17. Recently, a new door of application has opened for DCS: spectro-imaging 18-20 and dual-comb imaging (DCI) 21-27. In the spectro-imaging, a combination of DCS with point-scanning imaging 18,19 or camera-based imaging 20 enables the hyperspectral imaging based on OFC. In DCI, OFC is regarded as an optical carrier of amplitude and phase with a vast number of discrete frequency channels in place of optical frequency ruler. Then, image pixels to be measured is spectrally encoded into OFC modes by space-to-wavelength conversion or spectral encoding (SE). Finally, image is decoded all at once from the mode-resolved spectrum of the image-encoded OFC acquired by DCS, based on one-to-one correspondence between images pixels and OFC modes. Due to the scan-less imaging capability in DCI and the simultaneous acquisition capability of amplitude and phase spectra in DCS, combination of DCI with c...

A mode-locked fiber comb equipped with a multimode interference fiber sensor functions as a high-precision refractive-index (RI) sensor benefitting from precise RF measurement. However, its dynamic range and repeatability are hampered by the inherent characteristics of nonlinearpolarization-rotation mode-locking oscillation. In this article, we introduce saturable-absorber-mirror mode-locking for RI sensing with a wide dynamic range and high repeatability. While the RI dynamic range was expanded to 41.4 dB due to high robustness against cavity disturbance, the self-starting capability without the need for polarization control improves the RI sensing repeatability to 1.10 × 10 −8 for each mode-locking activation. The improved dynamic range and repeatability will be useful for enhancing the performance of RI sensing.

We proposed a refractive index (RI) sensing method with temperature compensation by using an optical frequency comb (OFC) sensing cavity employing a multimode-interference (MMI) fiber, namely, the MMI-OFC sensing cavity. The MMI-OFC sensing cavity enables simultaneous measurement of material-dependent RI and sample temperature by decoding from the comb spacing frequency shift and the wavelength shift of the OFC. We realized the simultaneous and continuous measurement of RI-related concentration of a liquid sample and its temperature with precisions of 1.6×10 -4 RIU and 0.08 ºC. The proposed method would be a useful means for the various applications based on RI sensing.

We report a near-infrared surface plasmon resonance (SPR) system to achieve highly sensitive, unlabeled detection of the SARS-CoV-2 nucleocapsid protein antigen. Use of the near-infrared light in SPR makes the SPR dip of the angular spectrum sharp and causes a large change of the reflected light intensity at a fixed incident angle. The present SPR system achieves the resolution of 10−5 refractive index unit in the refractive index measurement of glycerol solution samples. Additionally, we measured the nucleocapsid protein antigen of SARS-CoV-2 down to a molar concentration of 1 fM by immobilizing its corresponding antibody on the SPR sensor surface. This demonstration indicates a high potential of the present system for highly sensitive biosensing in medical diagnostics.