Brewster's angle flow cell for cavity ring-down spectroscopy

A container to be positioned in a ring-down optical cavity for containing a sample having a first index of refraction, the container to be used to facilitate measurement of light absorption in the sample, the container comprising:

• an outer surface having opposing first and second outer faces that are oriented to receive and transmit a light beam, the container including a first portion forming the first outer face having a second index of refraction and a second portion forming the second outer face having a third index of refraction;
• an inner surface forming a void for containing the sample, the inner surface having opposing first and second inner faces that are oriented to receive and transmit the light beam;
• the first and second outer faces of the container forming first and second cavity/container interfaces;
• and
• the first and second inner faces of the container forming first and second container/sample interfaces,
• wherein the first and second outer faces of the outer surface of the container are to be oriented in the optical cavity such that the first and second cavity/container interfaces transmit and receive the light beam at an incidence angle approximately equal to the Brewster's angle of the respective first and second cavity/container interfaces, and
• wherein the first and second inner faces of the inner surface are oriented at first and second angles, respectively, with respect to the first and second outer faces of the outer surface such that the first and second container/sample interfaces transmit and receive the light beam at an incidence angle approximately equal to the Brewster's angle of the respective first and second container/sample interfaces.

A method for measuring light absorption in a sample having a first index of refraction, the method comprising:

• providing a light beam having a selected wavelength width that includes at least one selected wavelength λ;
• receiving the light beam in a ring-down optical cavity formed by at least first and second spaced apart light beam reflectors to receive and reflect the light beam and to establish a resonant mode for the light beam, the optical cavity comprising a medium having a second index of refraction;
• positioning a container in the optical cavity for containing the sample, the container having opposing first and second outer faces that are oriented to receive and transmit the light beam, and opposing first and second inner faces forming a void for containing the sample which inner faces are oriented to receive and transmit the light beam, the first and second outer faces forming first and second cavity/container interfaces and the first and second inner faces forming first and second container/sample interfaces, wherein positioning the container in the optical cavity comprises:
• orienting the first and second outer faces of the container at first and second angles, respectively, with respect to the light path of the light beam such that the first and second cavity/container interfaces transmit and receive the light beam at an incidence angle approximately equal to the Brewster's angle of the respective first and second cavity/container interfaces;
• and
• orienting the first and second inner faces of the container at third and fourth angles, respectively, with respect to the first and second outer faces such that the first and second container/sample interfaces transmit and receive the light beam at an incidence angle approximately equal to the Brewster's angle of the respective first and second container/sample interfaces;
• and
• receiving the light beam after the light beam has been reflected at least once from each reflector to determine the absorption of the light beam by the sample.

A method for measuring light absorption in a sample having a first refractive index, the method comprising:

• providing a light beam having a selected wavelength width that includes at least one selected wavelength λ;
• receiving the light beam in a ring-down optical cavity formed by at least first and second spaced apart light beam reflectors to receive and reflect the light beam and to establish a resonant mode for the light beam, the optical cavity comprising a medium having a second index of refraction;
• positioning a container in the optical cavity for containing the sample, the container having first and second outer faces that are oriented to receive and transmit the light beam, and first and second inner faces forming a void for containing the sample which inner faces are oriented to receive and transmit the light beam, the first and second outer faces forming first and second cavity/container interfaces and the first and second inner faces forming first and second container/sample interfaces;
• transmitting and receiving the light beam at the first cavity/container interface at an incidence angle approximately equal to the Brewster's angle of the first cavity/container interface;
• transmitting and receiving the light beam at the first container/sample interface at an incidence angle approximately equal to the Brewster's angle of the first container/sample interface;
• transmitting and receiving the light beam at the second container/sample interface at an incidence angle approximately equal to the Brewster's angle of the second container/sample interface;
• transmitting and receiving the light beam at the second cavity/container interface at an incidence angle approximately equal to the Brewster's angle of the second cavity/container interface;
• and
• receiving the light beam after the light beam has been reflected at least once from each reflector to determine the absorption of the light beam by the sample.