Fig. 2: Laser-scanning momentum-space-resolved spectroscopy with ultrahigh resolutions in wavelength and angle of signal.

a Schematic of the optical setup and how laser-scanning defines wavelength in momentum-space-resolved spectroscopy. L1-4, lens. BS, beamsplitter. OBJ, objective. CCD, charged-coupled device camera. More details in optical setup and measurements can be found in Methods. b–d Comparison of the measured momentum-space-resolved reflectance spectra under different wavelength and angle resolutions. Spectra of the device with P = 500 nm and L = 90 nm are presented as an example. A spectrometer with a 1200-lines/mm grating provides a wavelength resolution of ~0.05 nm/pixel in (b), while a resolution of ~2.1 pm/pixel is achieved in (c, d) via a tunable laser. The angle resolution is ~0.076 ̊ /pixel in (b) with a 10X objective, and ~0.044 ̊ /pixel in (c, d) with a 2X objective. Insets on top highlight the numbers of pixels summed along X axis on CCD when evaluating the momentum-dependent spectral response along Y axis. The more pixels summed, the more severe the dispersion-induced mode broadening is. e Reflectance spectra extracted at arbitrary k values in the momentum space from (d).