The loss in sensitivity of two-photon excited fluorescence, when switching from a 1 cm cell format to square and cylindrical capillaries, is described quantitatively. Sensitivities are determined with calibration curves, and beam shapes are computed with optical design software. The decreased sensitivity with square capillary (75 μm i.d.) is shown to be the simple result of a truncated pathlength resulting in a factor of ~ 2 reduction in sensitivity from a 1 cm cell. Cylindrical capillary of the same i.d. is characterized by astigmatism caused by the curved refractive index boundary in addition to the truncation. The astigmatism is determined to be smaller in the thin-walled capillary, allowing for better performance than the thick-walled capillary. In fact, the thin-walled capillary is found to have similar performance to the square capillary.
S. A. Zugel and F. E. Lytle, "Two-Photon Excitation in Square and Cylindrical Capillaries," Appl. Spectrosc. 54, 1203-1207 (2000)