Figure 12.2 — Basic configurations of spectroscopic optical tweezers Figure 12.2 — Basic configurations of spectroscopic optical tweezers. (a) Set-up integrating optical tweezers and spectroscopy. The trapping and excitation beams are combined using a dichroic mirror (DM) and focused through a high-numerical-aperture objective lens (OBJ). For scattering spectroscopy, the sample can also be excited using a halogen lamp focused by a dark-field condenser (C) from the top (Illumination); in this case, the signal is collected through the trapping objective lens. Notch/edge filters (F) are used to cut out the elastic scattering at the excitation/trapping wavelengths. A beam splitter (BS) divides the imaging light from the spectral signal. A grating spectrometer equipped with a CCD camera or an avalanche photodiode acquires the spectroscopic signal. (b) Simpler configuration based on a single beam and single objective. (c) More complex and versatile configuration using two independent objectives for the trapping and excitation beams.