Optics: measuring and testing – For light transmission or absorption – Of fluent material
Reexamination Certificate
2001-08-09
2003-02-04
Font, Frank G. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
Of fluent material
C536S023100, C536S022100, C536S024300, C359S296000, C435S325000, C422S082050, C422S082070
Reexamination Certificate
active
06515749
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates, in general, chemical sensing and, more particularly, to an improved chemical sensor providing both selectivity and high sensitivity.
BACKGROUND OF THE INVENTION
Chemical sensing is fundamental to economic development, national security, and the quality of life. The demand for better sensing or detection technologies is ever-increasing to address needs in many different areas, such as the detection of concealed explosives in airports, chemical warfare agents that are fatal at extreme trace levels, or chlorine produced by chemical plants. To be effective, a chemical sensing technology must provide sufficient sensitivity and selectivity. Stability, robustness, and portability are also necessary or at least highly desirable characteristics. Therefore, any significant advance in current chemical sensing technology that improves sensitivity, selectivity, or adaptability will have a significant impact on national and global needs.
Although many transduction mechanisms exist for chemical sensing, optical absorption, in particular, is widely used. The ultimate sensitivity of an optical absorption measurement is limited by quantum noise arising from the discrete nature of light, although this limit is rarely achieved in practice. Recently, with the development of cavity ring-down spectroscopy (CRDS), the potential for routine quantum noise limited optical absorption measurements has become apparent. (See R. D. van Zee, J. T. Hodges, and J. P. Looney,
App. Opt.
38, 3951 (1999)).
The principles and applications of CRDS are discussed, e.g., in (See A. O'Keefe and D. A. G. Deacon,
Rev. Sci. Instrum.
59, 2544 (1988);
Cavity
-
Ringdown Spectroscopy
, K. W. Busch and M. A. Busch, eds. Coxford University Press, 1999) and these references, among others, may be consulted for a more complete discussion of CRDS. However, in brief, a typical gas-phase CRDS experiment, a stable optical cavity is formed from a pair of concave, highly reflective mirrors. When light, usually from a pulsed laser source, is injected into the cavity, the intensity of the circulating light decays exponentially with a frequency-dependent “ring-down” time, &tgr;(&ohgr;), given by the ratio of the round-trip time, t
r
, to the sum of the round-trip losses, or
τ
(
ω
)
=
t
r
L
o
(
ω
)
+
L
a
bs
(
ω
)
where L
0
(&ohgr;) is the intrinsic cavity loss and L
abs
(&ohgr;) arises from absorption by gases contained within the cavity. The difference in intensity decay rates for gas-filled and empty cavities, as a function of laser frequency, provides the absolute absorption spectrum of the sample. Since the intensity decay rate (&agr;1/&tgr;) is employed instead of a ratio of intensities, as in conventional absorption spectroscopy, the measurement is essentially immune to noise introduced by light source intensity fluctuations. The minimum detectable absorption in CRDS can be expressed as the product of the relative uncertainty in the ring-down time and the intrinsic cavity loss, or (L
abs
)
min
=L
0
*(&Dgr;T/T)=L
0
*2&sgr;
T
/(TN) where &sgr;
T
is the standard deviation of the ring-down time and N is the number of decay times averaged. (See P. Zalicki and R. N. Zare,
J. Chem. Phys.
102, 2708, (1995); D. Romanini and K. K. Lehmann,
J. Chem. Phys.
99, 6287-6301, (1993).) This expression for (L
abs
)
min
reveals both the simplicity and challenge of CRDS: minimize the intrinsic cavity loss and determine the ring-down time with the highest possible precision.
A variant of CRDS, termed evanescent wave cavity ring-down spectroscopy (EW-CRDS), has recently been developed, which permits application of CRDS to surfaces, films, and liquids. (See A. C. R. Pipino, J. W. Hudgens, R. E. Huie,
Rev. Sci. Instrum.
68 (8), 2978, (1997); A. C. R. Pipino, J. W. Hudgens, R. E. Huie,
Chem. Phys. Lett.
280, 104 (1997); A. C. R. Pipino in Proceedings of SPIE, Vol. 3535, Boston, Mass. (1998); A. C. R. Pipino,
Phys. Rev. Lett.
83 (15), 3093-3096, (1999); A. C. R. Pipino in Proceedings of SPIE, 3858, Boston, Mass., (1999); A. C. R. Pipino,
Appl. Opt.
39 (9), 1449 (2000); U.S. Pat. Nos. 5,835,231; 5,835,231; 5,986,768.) This technology is described in some detail in these references but in brief, EW-CRDS employs intracavity total internal reflection (TIR) to generate an evanescent wave at a resonator surface that allows optical absorption of condensed matter to be probed in a manner similar to attenuated total reflection (ATR) spectroscopy (see N. J. Harrick, Internal Reflection Spectroscopy, (Interscience Publishers, New York, (1967)), but with much higher sensitivity. In particular, a minuscule fraction (<10
−4
) of a molecular layer of molecules can be detected at the TIR surface with EW-CRDS. Several resonator designs have been demonstrated for EW-CRDS, including variations that permit a miniature, robust optical absorption sensor to be achieved, thereby facilitating portability.
In many chemical sensing applications, detection of the analyte at a surface by direct absorption has major advantages. However, the analyte must have a significant absorption cross-section (or molar absorptivity) at the probe wavelength, which limits the minimum analyte concentration that can be detected. Typically, absorption cross-sections are largest for electronic transitions occurring in the visible region of the spectrum. Operation in the visible region also benefits from the availability of inexpensive sources including diode lasers, low-noise high-quantum efficiency detectors, and high transmission optical materials. However, many chemical species of interest do not have a significant visible absorption, and show instead significant absorption in the ultraviolet or infrared spectral regions. As discussed below, one aspect of the invention concerns circumventing this limitation.
One chemical sensing strategy that employs visible absorption, but permits detection of analytes that do not absorb at the probe wavelength, involves the use of surface plasmon polariton resonance (SPPR). This technology is described, for example, in J. Homola, S. S. Yee and G. Gauglitz,
Sens. Act. B,
54, 3, (1999). In brief, SPPR is a surface electromagnetic wave that arises from the collective excitation of free electrons. A typical, conventional apparatus for making a SPPR measurement is shown at
10
in FIG.
1
. In apparatus
10
, a metal film
11
is deposited on the base of a prism
12
, forming a three layer system consisting of the prism
12
, the metal film
11
, and the ambient medium indicated at
13
. A visible laser beam, or a light beam from another visible light source, is denoted
14
and is incident on the metal film
11
at an angle of incidence &thgr;
i
that exceeds the critical angle, defined by &thgr;
c
=sin
−1
(n
o
i
) where n
i
and n
o
are the refractive indexes of the material of the prism
12
and the ambient medium
13
, respectively. Since &thgr;
i
>&thgr;
c
, total internal reflection occurs, giving rise to an evanescent wave
15
. (See also N. J. Harrick, Internal Reflection Spectroscopy, (Interscience Publishers, New York, (1967).) For a certain angle &thgr;
r
, with &thgr;
i
=&thgr;
r
>&thgr;
c
, the evanescent wave
15
generated at the prism-metal interface excites the SPPR at the metal/ambient medium interface. The SPPR efficiently absorbs the incident light, trapping the electromagnetic energy in the form of a surface wave with a locally enhanced electric field.
The SPPR apparatus
10
is highly sensitive to environmental conditions at the metal/ambient medium interface. Hence, the angle of resonance, &thgr;
r
, or the absorbance magnitude at a given &thgr;
i
near &thgr;
r
, are very sensitive to chemical and physical interactions at the interface. In some cases, a reaction of the analyte occurs directly with the metal of the metal film
11
. In other cases a thin film is applied to the metal that responds selectively to the analyte, changing in the local environment sensed by th
Font Frank G.
Larson & Taylor PLC
Nguyen Sang H.
The United States of America as represented by the Secretary of
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