THOR spectrograph with patented technology for spectroscopic camera tilt around fused silica transmitting grating. Technology allows spectral range tuning.
The laser beam delivery layout consists of two geometries: the first geometry is a point illumination mode and the second geometry is for producing the DLL and WL sample illumination modes. The DLL‐focus mode is achieved when a sample is illuminated by a diffraction limited laser line, that is, dDLL = 2.8 µm in width using a 785 nm laser excitation wavelength and 10× magnification lens. The WL mode is obtained by expanding the width of the laser line, that is, dWL > 2.8 µm and up to 64 µm, which is achieved by adjusting the position of a cylindrical lens in the laser delivery path. The laser beam profile is modified using a set of cylindrical lenses including laser line generator lens.
In order to image the entire ≈2 mm long DLL/WL laser profile on a sample, the aperture for the laser beam was designed to fit the field of view (FOV = 2.2 mm) of the 10× near‐infrared (NIR) microscope objective.
The sample surface is imaged by a camera while collecting SERS spectra, which simplifies the measurement procedure. This was realized by combining the incident laser beam with visible light from a light emitting diode (LED). To ensure that the white light source is not contributing to the recorded SERS spectra in the 785–1000 nm wavelength range, the NIR emission from the LED source was suppressed (on the order ≈106) by an edge filter.
The camera projection optics is designed to enable imaging of the entire microscope objective FOV with a homogeneous sample illumination. This is usually not the case for most commercial Raman microscopes that only utilize the central part of FOV.
The optical path from sample to spectrograph slit is designed to transmit an aberration‐free image of the DLL or WL laser profile. This is achieved by combining a low numerical aperture (NA) collimating lens with a high NA focusing lens in the imaging spectrograph. The low NA collimating lens allows one to insert the lens with equal NA. Therefore, the focal length of the pre-slit lens is sufficiently long for delivering the DLL/WL image from the sample surface to the slit, and with correct input angles at the spectrometer. In this way, beam cutting inside the spectrograph is avoided. Alternatively, the image delivery can be obtained using two intermediate lenses between a microscope objective and a lens; however, this leads to losses in the Raman signal and increases aberrations.
Due to the low NA of the slit and collimating lenses, even at dslit = 100 µm slit, the pixel size limited spectral resolution is ≈1.8–2.5 cm−1 throughout the Raman Stokes shift range of 350–2200 cm−1.
WL SERS microscope: a) WL SERS microscope optical design, b) point, c) DLL, and d) WL laser illumination modes in SERS microscope. The orientation of EM vectors (blue arrows in (b)–(d)) represents depolarized laser irradiation. Laser polarization in WL mode is parallel to the surface of the SERS chip