Laser-scanning optical microscopy is widely used for the observation of microstructures and the analysis of molecular functions of samples with tightly focused light. Spectroscopic information is also available if a broadband light source is employed. General laser-scanning optical microscopy observes optical intensity by employing a sample- or laser-scanning system for the analysis of samples via reflectance, scattering, absorbance, and laser-induced phenomena. Another visualization method is using optical phase, which can enhance the image contrast of such high transparent materials and nano-step structures. However, broadband spectroscopic phase-contrast imaging with a laser-scanning configuration is slightly tricky due to the interferometric configuration is required to retrieve phase information of each wavelength. If the simultaneous measurement of amplitude and phase spectra is enabled in laser-scanning microscopy, it is possible to realize multivariate measurement to analyze more detailed information of samples based on such as complex refractive index, polarization characteristics, and so on with tightly focused light. To overcome these limitations, in this study, we proposed an optical-frequency-comb (OFC)-based laser scanning optical microscopy. The OFC technique enables fast Fourier transform spectroscopy by using well-defined two OFC lasers without any mechanical scan in the time domain. The combination of the laser scanning optical microscopy and the OFC technique realized the simultaneous and spectroscopic observation of quantitative amplitude and phase images with tight focusing down to the diffraction limit. Furthermore, we realized the analysis of polarization by the direct observation of the amplitude and phase of the orthogonal components. We applied the proposed method to the observation of nano-step structures, phase objects and anisotropic materials to provide a proof-of-principle demonstration of the proposed method. Our proposed approach will serve as a unique and powerful tool for characterizing the materials via complete characterization of optical information such as amplitude, phase, polarization and spectrum.
Dual-comb spectroscopy (DCS) is a powerful tool for gas spectroscopy due to high resolution, high accuracy, broadband spectral coverage, and rapid data acquisition, based on optical frequency comb (OFC) traceable to a frequency standard. In DCS, after a temporal waveform of interferogram is acquired in time domain, the corresponding mode-resolved OFC spectrum is obtained by fast Fourier transform (FFT) calculation of the acquired interferogram. However, FFT calculation of huge-sized temporal data spends significantly longer time than the acquisition time of interferogram, making it difficult to response the transient signal change. In this article, we demonstrate frequency-domain DCS by a combination of DCS with lock-in detection (LID), namely LID-DCS. LID-DCS directly extracts an arbitrary OFC mode from a vast number of OFC modes without the need for FFT calculation by the synchronous detection at a LID reference frequency while maintaining high resolution and high accuracy. Usefulness of LID-DCS is demonstrated in rapid monitoring of transient signal change and spectroscopy of hydrogen cyanide gas by comparing with usual DCS.