Dual dichroic mirrors

We have developed a novel ultrasensitive detection method, thermal lens microscopy (TLM), for nonfluorescent species [13]. TLM is photothermal spectroscopy under an optical microscope. Our thermal lens microscope (TLM) has a dual-beam configuration excitation and probe beams [13]. The wavelength of the excitation beam is selected to coincide with an absorption band of the target molecule and that of the probe beam is chosen to be where the sample solution (both solvent and solute) has no absorption. For example, in determination of methyl red dye in water, cyclohexane, and n-octanol, a 514-nm emission line of an argon-ion laser and a 633-nm emission line of a helium-neon laser were used as excitation and probe beams, respectively [21], Figure 4 shows the configuration and principle of TLM [13]. The excitation beam was modulated at 1 kHz by an optical chopper. After the beam diameters were expanded, the excitation and probe beams were made coaxial by a dichroic mirror just before they were introduced into an objective lens whose magnification and numerical aper-... 

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