Abstract
Raman spectroscopy is well suited for readily revealing information about bio-samples. As such, this technique has been applied to a wide range of areas, particularly in bio-medical diagnostics. Raman scattering in bio-samples typically has a low signal level due to the nature of inelastic scattering of photons. To achieve a high signal level, usually a high numerical aperture objective is employed. One drawback with these objectives is that their working distance is very short. However, in many cases of clinical diagnostics, a long working distance is preferable. We propose a practical solution to this problem by enhancing the Raman signal using a parabolic reflector. The high signal level is achieved through the large light collection solid angle of the parabolic reflector while the long working distance is ensured by the novel design of our microscope. The enhancement capability of the microscope was demonstrated on four types of samples. Among these samples, we find that this microscope design is most suitable for turbid samples.
Highlights
Raman spectroscopy is well suited for readily revealing molecular information of samples
Abbreviations refer to the following: dichroic mirror (DM), beam expander (BE which consists of four lens, see Appendix 6.2 for the detailed design), long-pass filter (LPF), collection lens (CL), Raman photons (RP), objective lens (OBJ), and parabolic reflector (PR)
For the remaining three samples, we observed a much larger enhancement factor. This may not be surprising since all these samples are turbid and the translational symmetry is broken, meaning there is no rigorous relation between ωR and qìR, relaxing the restriction on the momentum Kìo of outgoing photons
Summary
Raman spectroscopy is well suited for readily revealing molecular information of samples. Abbreviations refer to the following: dichroic mirror (DM), beam expander (BE which consists of four lens, see Appendix 6.2 for the detailed design), long-pass filter (LPF), collection lens (CL), Raman photons (RP), objective lens (OBJ), and parabolic reflector (PR). We designed a beam expander for this purpose, the details of which are given in Appendix 6.2 Both portions of the scattered light, after passing through the dichroic mirror and a long-pass filter (LPF), are focused by a collection lens (CL, Thorlabs, Inc., f = 30mm, AC254-030-B-ML) into an optical fiber (Thorlabs, Inc., M200L02S-A, Ø=200μm, NA=0.22) coupled spectrometer (Princeton Instruments, Acton SP2100, not shown), and recorded by a CCD detector (Princeton Instruments, PIXIS 400, not shown). Assuming scattered photons are emitted nearly isotropically, the majority of photons can be collected, given the large solid angle of acceptance of the PR, resulting in an enhancement effect To ensure that these photons are reflected upwards, the alignment between the objective and the PR is critical. It is worth noting that in the statement above, we chose to ignore those photons emitted downwards (emission angle > 90◦) and reflected upwards by the back surface of a sample, since the samples that we used to demonstrate the enhancement effect are all opaque and have mean free paths or penetration depths much shorter than their thicknesses
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