pdf file
In a typical CRD experiment a short light pulse is coupled into a stable
optical cavity, formed by two highly reflecting plano-concave mirrors.
The fraction of light entering the cavity on one side `rings' back and
forth many times between the two mirrors. The time behavior of the light
intensity inside the cavity I_{CRD}(t), can be monitored by the small fraction
of light that is transmitted through the other mirror. If the only loss
factor in the cavity is the reflectivity loss of the mirrors, one can show
that the light intensity inside the cavity decays exponentially in time
with a decay constant tau, the `ring down time', given by: tau = d/{c(|ln
R|)}. Here d is the optical path length between the mirrors, c is the speed
of light and R is the reflectivity of the mirrors. If R is close to unity,
the approximation tau = d/c(1-R) can be made.
If there is additional loss inside the cavity due to the presence of absorbing
and light scattering species, the light intensity inside the cavity will
still decay exponentially in time provided the absorption follows Beer's
law. It can be shown that in this case the ring-down transient is proportional
to the integral from 0 to infinity over I(nu) x exp{-t/tau(nu)} where I(nu)
is the spectral distribution of the light that makes it into the cavity
and where tau(nu) is given by: {d}/{c(|ln R(nu)| + sum sigma_i(nu) x int_0^d
N_i(x) dx)} and the sum is over all light scattering and absorbing species
with frequency dependent cross-sections sigma_i(nu) and a line-integrated
number density int_0^d N_i(x) dx. The product of the frequency dependent
absorption cross-section with the number density N_i(x) is commonly expressed
as the absorption coefficient kappa _i(nu, x). In an experiment it is most
convenient to record 1/{c tau(nu)} as a function of frequency, i.e. to
record the total cavity loss as a function of frequency, as this is directly
proportional to the absorption coefficient, apart from an offset which
is mainly determined by the finite reflectivity of the mirrors. To explicitly
demonstrate the use of CRD for trace gas detection (atomic mercury, known
oscillator strength -> density) and for measuring absolute oscillator strengths
(molecular oxygen, known density -> absorption cross section), a CRD absorption
measurement of ambient air around 253 nm is shown (Figure on the right).
A typical experimental set-up is shown in the figure on the left. The
pulsed narrow band laser radiation is usually obtained from a tunable (dye)
laser system, that runs at 5-50 Hz, and delivers ns duration pulses with
approximately 1-10 mJ of energy. The ring down cavity is formed by two
plano-concave mirrors placed at a distance slightly less than twice their
radius of curvature apart. The mirrors are coated for an optimum reflectivity
in the desired wavelength range, and in the visible and near UV range of
the spectrum reflectivities of better than 0.999 up to 0.99999 can be obtained.
The mirrors often act at the same time as windows for the closed cell,
which contains the substance to be studied. In order to ensure that all
transverse modes are detected with equal probability the photo multiplier
tube (PMT) that is used to measure the ring down transients is placed directly
behind the output mirror. The signal of the PMT is amplified and recorded
on a fast and high resolution digitizer. At every wavelength position some
pre-defined number of transients, every transient recorded over a sufficiently
long time interval, are summed and read out by a PC. The characteristic
ring down time is determined by fitting the natural logarithm of the data
to a straight line, using a weighted least squares fitting algorithm. Typically,
the ring-down time can be determined with a relative accuracy of 10-3.
With a typical mirror reflectivity loss of (1-R)=10-4, this
implies that line integrated absorption coefficients of better than 10-7,
i.e. absorption coefficients below 10-9 cm-1 in a
1 meter cavity, can be measured.
In all the CRD experiments reported to date polarized laser radiation has
been used for absorption measurements. By putting a polarization selective
optical element in front of the detector, it should in principle be possible
to measure the optical rotation of the plane of polarization of the incoming
beam upon passage through the ring down cavity. With a polarization analyzer
that selects radiation polarized parallel to that of the incoming beam
placed in front of the light detector, in addition to the `normal' absorption,
rotation of the plane of polarization in the ring down cavity will be detected
as an apparent absorption, i.e. as a shortening of the ring down time.
With the analyzer rotated (almost) 90 degrees relative to the plane of
polarization of the incoming beam, there will still be shortening of the
cavity decay time due to absorption but optical rotation will cause a time-dependent
{\it increase} in the detected signal, i.e. it will appear as an apparent
lengthening of the ring down time (`cavity ring up'). When both signals
are measured simultaneously, one can discriminate between the effects of
absorption and optical rotation. Magnetic rotation spectroscopy \cite{Carroll},
based on the Zeeman effect, is a zero-background absorption technique that
is considerably more sensitive than direct single- or double-beam absorption.
This technique has been applied to a variety of paramagnetic species, and
the resonant magneto-optic spectrum of the $\rm v'=0 \leftarrow v''=0$
transition of the atmospheric band of molecular oxygen has recently been
reported \cite{Takubo}. The sensitivity of the technique can in principle
be increased by multi-passing through the sample (the polarization rotates
further with each traversal). In practice, however, multi-passing is often
found to be limited by the deterioration of the degree of polarization
with each mirror reflection \cite{Adams,Smith}. We have theoretically outlined
and experimentally demonstrated the combination of Polarization Spectroscopy
with Cavity Ring Down spectroscopy. The $\rm b^1\Sigma_g^+ (v'=2) \leftarrow
X^3\Sigma_g^- (v''=0)$ magnetic dipole transition of molecular oxygen around
628 nm is used to demonstrate the possibility to selectively measure either
the polarization dependent absorption or the resonant magneto-optical rotation.
The Polarization Dependent CRD (PD-CRD) detection scheme outlined here
offers a significant improvement in sensitivity and utility over the individual
methods. In contrast to most other schemes for polarization spectroscopy,
the PD-CRD scheme is not completely background-free, and is therefore not
overly sensitive to the above-mentioned deterioration of the degree of
polarization upon multi-passing through the cavity. In the Voigt configuration
(external magnetic field perpendicular to the axis of the cavity) the detection
limit for molecular oxygen is lowered by more than an order of magnitude
in the PD-CRD set-up relative to the `conventional' CRD set-up. In addition,
there is a sign-difference for absorptions polarized parallel and perpendicular
to the magnetic field, which aids considerably in the assignment. In the
Faraday configuration (external magnetic field parallel to the axis of
the cavity) the experimental arrangement can be made such that one is exclusively
sensitive to optical rotation, and no effect of (polarization dependent)
absorption is observed. In the PD-CRD detection scheme, the {\it rate}
of polarization rotation is measured, which enables the rotation to be
quantitatively determined. The experiments show a noise-equivalent polarization
rotation of 10$^{-8}$ rad./cm. Apart from studying electro-optic and magneto-optic
phenomena on gas phase species, the PD-CRD technique is shown to be applicable
to the study of optical rotation of transparent solid samples as well.
(reference).
The experimental details for the CRD setup do not differ substantially from the setup we have been using before, and a scheme of the experimental setup is shown. The ring down cavity is formed by two identical plano-concave mirrors with a diameter of 25.4 mm and a radius of curvature of -1.0 m placed a distance d = 45.5 cm apart. The reflection coatings are optimized for $\lambda$ = 10.50 $\mu$m, and have a specified reflection coefficient $R \approx $ 0.995. The ring down times thus expected for the empty cavity are on the order of 300 ns. To be able to use the FELIX pulse for CRD experiments it is important to have the trailing flank of the IR pulse decaying faster than the typical photon-lifetime in the cavity. To achieve this, the 'semiconductor switching' technique is applied. This technique is based on the dependence of the reflection and transmission properties of a material on the density profile of free carriers within it. In our case, the second-harmonic radiation of a short-pulse Nd:YAG laser (532 nm, 100 ps, 30 mJ) is used to produce a high density of free carriers in a Si-slab, thereby temporarily changing its IR reflection and transmission characteristics. In the experimental setup, the linearly polarized radiation of the FEL is incident on the semi-conductor Si-slab under Brewster angle ($74 ^{\circ}$ @ 1000 cm$^{-1}$), and passes through the Si-slab in the absence of the Nd:YAG laser pulse. During and shortly after the Nd:YAG laser pulse the Si-slab becomes conducting, and a FEL pulse of approximately 20 ns duration (10 - 20 $\mu$J pulse energy) is efficiently reflected from the Si-slab. This shortened IR pulse is used for the CRD experiments. The IR pulse is coupled into the cavity through one of the mirrors with a weakly focusing mirror (f = 1 m). A long pass filter (7.2 $\mu$m cut-off wavelength) is used to prevent the higher harmonics that might be present in the FELIX-beam from entering the cavity. The time dependence of the light intensity inside the cavity is monitored by detection of the light that leaks out of the cavity through the other mirror. This light is either detected directly or after passage through a FT-spectrometer (Bruker IFS 66v, with step-scan option) using a liquid-nitrogen cooled HgCdTe (MCT) detector (HCT-21-C, Braseby, UK), with a response time of about 40 ns. The signal of the detector is amplified and digitized with a fast (100 Ms/s) and high resolution (10 bit) digital oscilloscope (LeCroy 9430). After averaging of the ring down transients over a pre-defined number of shots (20-50) on the 16 bit on-board memory of the oscilloscope the data are transferred to a PC for extraction of the frequency dependent ring down times.
R. Engeln and G. Meijer, A Fourier transform Cavity Ring Down spectrometer.
Rev. Sci. Instrum. 67 (1996) 2708-2713 R. Engeln, G. von Helden, G. Berden
and G. Meijer, Phase shift Cavity Ring Down absorption spectroscopy. Chem. Phys. Lett.
262 (1996) 105-109(also available as postscript file)
Richard Engeln,
Giel Berden, Esther van den Berg, and Gerard Meijer, Polarization Dependent
Cavity Ring Down Spectroscopy. J. Chem. Phys. 107 (1997) 4458
R. Engeln, E. van den Berg, G. Meijer, L. Lin, G. Knippels and A. van der
Meer. Cavity Ring Down Spectroscopy with a Free-Electron Laser. Chem. Phys.
Lett. 269 (1997) 293
T.G. Slanger,
D.L. Huestis, P.C. Cosby, H. Naus and G. Meijer. O2 photoabsorption
in the 40950-41300 cm-1 region; new Herzberg bands, new absorption
lines, and improved spectroscopic data. J. Chem. Phys. 105 (1996)
9393-9402
Giel Berden, Richard Engeln, Peter Christianen, Jan Kees Maan, and Gerard
Meijer. Cavity Ring Down Spectroscopy on the Oxygen A Band in Magnetic
Fields up to 20 Tesla. Phys. Rev. A, 58 (1998), 3114-3123
(more info available on the oxygen page)
Richard Engeln, Giel Berden, Rudy Peeters, and Gerard Meijer. Cavity Enhanced
Absorption and Cavity Enhanced Magnetic Rotation Spectroscopy.
Rev. Sci. Instrum. 69 (1998), 3763-3769 (also available as
postscript file)
Richard Engeln, Gert von Helden, Andre J.A. van Roij, and Gerard Meijer.
Cavity Ring Down Spectroscopy on Solid C60. J. Chem. Phys. 110 (1999), 2732-2733
Giel Berden, Rudy Peeters, and Gerard Meijer. Cavity enhanced absorption spectroscopy of
the 1.5 mum band system of jet cooled ammonia.
Chem. Phys. Lett. 307 (1999), 131-138
Rudy Peeters, Giel Berden, Arnoud Apituley, and Gerard Meijer.
Open-path trace gas detection of ammonia based on
cavity-enhanced absorption spectroscopy.
Appl. Phys. B 71 (2000), 217-223 Giel Berden, Rudy Peeters, and Gerard Meijer.
Cavity ring-down spectroscopy: Experimental schemes and applications.
Int. Rev. Phys. Chem. 19 (2000), 565-607
Rudy Peeters, Giel Berden, and Gerard Meijer.
Sensitive absorption technique for spectroscopy.
American Laboratory 33 (2001), 60.
Rudy Peeters, Giel Berden, Ari Olafsson, Lucas
J.J. Laarhoven, and Gerard Meijer.
Cavity Enhanced Absorption spectroscopy in the 10 mum region using a
waveguide CO2 laser.
Chem. Phys. Lett. 337 (2001), 231-236.Giel Berden, Gerard Meijer, and Wim Ubachs.
Spectroscopic applications using ring-down cavities.
Experimental methods in the physical sciences 40 (2002), 47-82.
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