Noninvasive picoliter volume thermometry based on backscatter interferometry

Using the on-chip refractive index (RI) detector based on backscatter inter ferometry, sensitive, small volume, noninvasive thermometry can be performe d. The current optical configuration for the on-chip interferometric backsc atter detector (OCIBD) is quite simple and consists of an unfocused laser, an unaltered chip with a hemispherical channel and a photodetector. Alignme nt is straightforward with the only requirement being that the beam fully f ills the channel. The interaction of an unfocused laser beam with the uncoa ted etched channel with a curvature within the silica plate (chip) produces fringes whose positional changes scale with respect to the refractive inde x (RI), n, of the fluid in the channel. Due to the inherently high value of dn/dT for most fluids and the high sensitivity of OCIBD to RI changes, the measurement of small temperature variations in sub-nanoliter volumes is po ssible. Performing OCIBD with a 75 mum diameter laser beam on a silica chip that contains an etched channel with a 40 mum radius facilitates non invas ive thermometry on a N-(2-hydroxyethyl)piperazine-(2-ethanesulfonic acid) ( HEPES) solution in a 188 x 10(-12) L probe volume with a temperature resolu tion of 9.9 x 10(-4)degreesC, at the 99% confidence level.