Cavity ring down spectroscopy on solid C60

Absorption coefficients and scattering losses of TGG, TGP, KTF, FS, and CeF3 magneto-optical crystals in the visible via cavity ring-down spectroscopy

We demonstrate a method for determining small absorption coefficients and surface-scattering losses of crystals using cavity ring-down spectroscopy and perform measurements on magneto-optical crystals of terbium gallium garnet (TGG), terbium gallium phosphate (TGP), fused silica (FS), potassium terbium fluoride (KTF), and C e F 3 at 532 and 634 nm. Surface scattering is distinguished from absorption losses by using crystals of different lengths. A figure of merit (FoM) for magneto-optical crystals is defined to evaluate their suitability as intracavity optics in optical cavity applications. It is found that TGP has the highest FoM for crystal lengths up to ∼10m m, whereas C e F 3 and FS potentially outperform TGP for longer crystals. Single-pass applications are also briefly discussed.

Simultaneous determination of optical loss, residual reflectance and transmittance of highly anti-reflective coatings with cavity ring down technique

Cavity ring down (CRD) technique was employed to measure optical losses (absorption and scattering losses), residual reflectance and transmittance of anti-reflectively (AR) coated laser components with transmittance higher than 99.9%. By inserting the AR coated laser component with parallel optical surfaces into the ring-down cavity and measuring the ring-down time versus the angle of incidence with respect to the surface normal, the optical loss and residual reflectance of the laser component were determined respectively at normal and out-of-normal incidences with repeatability of part-per-million level. The transmittance was also determined simultaneously. Experimental results demonstrated that CRD is a simple, inexpensive and fast technique for highly accurate measurements of optical loss, residual reflectance, and transmittance of AR coated laser components widely used in high-power laser systems.

Note: Broadband cavity ring-down spectroscopy of an intra-cavity bulk sample

A cavity ring-down (CRD) setup equipped with a pulsed broadband light source (480 nm ≤ λ ≤ 650 nm) and a multichannel detection system (temporal gate width Δτ = 20 ns) is used to simultaneously record the optical loss spectrum of an intra-cavity CaF2 sample and its changes upon transverse ArF laser irradiation at 193 nm. The CRD setup with mirrors of high reflectivity (R > 99.93%) allows to register loss changes of 5 × 10(-5) with a spectral resolution of 0.3 nm in less than 2 min.

Photon-trap spectroscopy applied to molecules adsorbed on a solid surface: probing with a standing wave versus a propagating wave

We apply photon-trap spectroscopy, a generalized scheme of cavity ringdown spectroscopy, to infrared spectroscopy of molecular adsorbates on a solid substrate. The storage lifetime of light in a high-finesse Fabry-Perot cavity provides a high absorbance sensitivity for the substrate sample, which is placed exactly normal to the light beam in the cavity to minimize optical losses. Infrared spectra of the C-H stretching vibration of alkylsiloxane monolayer films on a silicon substrate are measured in three ways, namely by employing pulsed and continuous-wave lasers as well as by conventional Fourier transform infrared spectroscopy. The magnitude of optical absorption is shown to vary by the character of the interacting light used in the measurement, i.e., a standing wave versus a propagating wave.

Spectrally resolved cavity ring down measurement of high reflectivity mirrors using a supercontinuum laser source

We investigate a cavity ring down setup that offers the possibility to measure the spectrally resolved reflectivities of high reflectivity mirrors. The setup consists of a resonator (ring down cavity) and an intensified CCD camera system combined with a spectrograph for spectral resolution. A commercial supercontinuum laser (350-1750 nm) is applied as a compact excitation source. It is based on a photonic crystal fiber that is pumped by a q-switched microchip laser (1.6 ns pulse duration, 25 kHz repetition rate). This combination allows simultaneously recording the transmittance of the cavity over a wide wavelength range determined by the excitation source and the spectral sensitivity of the detector. The photon lifetimes inside the cavity (ring down times) are measured with high spectral resolution by means of an intensified camera system. Subsequently shifting the "gate" of the image intensifier from short to long delay times after the excitation pulse allows calculation of the reflectivity spectrum of the mirrors. Comparison of these results with measurements using a conventional setup (laser diode 675 nm and photomultiplier tube) clearly shows the high potential of the method due to the multichannel excitation and the detection scheme.

Liquid-phase and evanescent-wave cavity ring-down spectroscopy in analytical chemistry

Due to its simplicity, versatility, and straightforward interpretation into absolute concentrations, molecular absorbance detection is widely used in liquid-phase analytical chemistry. Because this method is inherently less sensitive than zero-background techniques such as fluorescence detection, alternative, more sensitive measurement principles are being explored. This review discusses one of these: cavity ring-down spectroscopy (CRDS). Advantages of this technique include its long measurement pathlength and its insensitivity to light-source-intensity fluctuations. CRDS is already a well-established technique in the gas phase, so we focus on two new modes: liquid-phase CRDS and evanescent-wave (EW)-CRDS. Applications of liquid-phase CRDS in analytical chemistry focus on improving the sensitivity of absorbance detection in liquid chromatography. Currently, EW-CRDS is still in early stages: It is used to study basic interactions between molecules and silica surfaces. However, in the future this method may be used to develop, for instance, biosensors with high specificity.

Infrared spectra of organic monolayer films in a standing wave measured by photon-trap spectroscopy

Photon-trap spectroscopy, a generalized scheme of cavity ringdown spectroscopy, is applied to measure an infrared spectrum of the C-H stretching vibration of alkylsiloxane monolayer films grafted on a silicon substrate. A continuous-wave laser beam is introduced into a high-finesse Fabry-Pérot cavity containing the substrate placed exactly normal to the light beam to minimize optical losses. The lifetime of the light trapped in the cavity is measured to detect optical absorption sensitively. The results show clear dependence of the absorbance on the location of the monolayers with respect to a standing wave formed in the cavity; the absorbance is practically zero when the monolayers on both the surfaces are adjusted at nodes, whereas it is maximized at antinodes. The present experiment is materialized on the basis of the principles established by our previous study [Terasaki et al., J. Opt. Soc. Am. B 22, 675 (2005)].

Dynamics of Clusters Initiated by Photon and Surface Impact

Clusters of atoms/molecules show dynamics characteristic of the method of excitation. Two contrasted processes are discussed: (1) electronic excitation via single-photon absorption and (2) impulsive excitation of nuclear motions by surface impact. Process 1 is exemplified by photodissociation dynamics of size-selected metal cluster ions. The electronic energy is converted most likely to vibrational energy of internal modes; dissociation follows via statistical mechanism to produce energetically favored fragments. Exceptionally, a silver cluster ion, Ag4(+), is shown to undergo nonstatistical dissociation along the potential-energy surface of the excited state. Energy partitioning to translational and vibrational modes of fragments is analyzed as well as bond dissociation energies. Furthermore, the spectrum of the photodissociation yield provides electronic and geometrical structures of a cluster with the aid of ab initio calculations; manganese, Mn(N)(+), and chromium, Cr(N)(+), cluster ions are discussed, where the importance of magnetic interactions is manifested. On the other hand, momentum transfer upon surface impact plays a role in process 2. An impulsive mechanical force triggers extraordinary chemical processes distinct from those initiated by atomic collision as well as photoexcitation. Experiments on aluminum, Al(N)(-), silicon, SiN(-), and solvated, I(2)(-)(CO2)(N), cluster anions provide evidence for reactions proceeding under extremely high temperatures, such as pickup of surface atoms, annealing of products, and mechanical splitting of chemical bonds. In addition, a model experiment to visualize and time-resolve the cluster impact process is performed by using a micrometer-sized liquid droplet. Multiphoton absorption initiates superheating of the droplet surface followed by a shock wave and disintegration into a number of small fragments (shattering). These studies further reveal how the nature of chemical bonds influences the dynamics of clusters.

Cavity ring-down spectroscopy for detection in liquid chromatography at UV wavelengths using standard cuvettes in a normal incidence geometry

Liquid chromatography (LC) with cavity ring-down spectroscopy (CRDS) detection, using flow cuvettes (put under normal incidence inside the ring-down cavity), is demonstrated. Fresnel reflections are maintained within the capture range of a stable cavity of 4 cm length. This method circumvents the need for specific Brewster's angles and possible mirror degradation is avoided. The flow cuvettes are commercially available at low cost. At 355 nm (the frequency-tripled output of a Nd:YAG laser), the system surpasses the performance of conventional absorbance detectors; the baseline noise was 1.3 x 10(-5)AU and detection limits (injected concentrations) were between 40 and 80 nM for nitro-polyaromatic hydrocarbons with an extinction coefficient epsilon of 7.3-10.2 x 10(3)M(-1)cm(-1). The system was also tested at 273 nm, but in the deep UV the reflectivity of the currently best available mirrors (R>or=99.91%) is still too low to show a significant improvement as compared to conventional UV-vis detection.

Hemoglobin Adsorption to Silica Monitored with Polarization-Dependent Evanescent-Wave Cavity Ring-Down Spectroscopy

Evanescent-wave cavity ring-down spectroscopy was used to monitor the adsorption of human hemoglobin to a fused-silica surface from aqueous solution. An uncoated dove prism was situated in a ring-down cavity such that the beam entered and exited with a normal-incidence geometry. This afforded ring-down times as high as 5 mus and values of sigma(tau)/tau as low as 0.3%. Normal-incidence geometry permits the detection of both S- and P-polarized light, yielding some information of the orientation of adsorbates. The orientation of the adsorbed hemoglobin molecules is found to change as adsorption progresses, but with a different time profile than adsorption itself. The adsorption kinetics from a quiescent solution is consistent with a reaction-diffusion model that includes both reversible and irreversible adsorption operating in parallel. Systems behaving according to this model also seem to display adsorption isotherms, although the increased adsorption from more concentrated solutions is only a consequence of the system being under kinetic control. In some cases, this may be sufficient to explain the paradox of protein adsorption systems which seem to be both irreversible and consistent with isotherm models as well.

Capillary Electrophoresis Absorption Detection Using Fiber-Loop Ring-Down Spectroscopy

The application of phase-shift, fiber-loop, ring-down spectroscopy (PS-FLRDS) as an on-line detector for capillary electrophoresis (CE) of biomolecules is demonstrated. CE was conducted using a custom-designed capillary/fiber interface coupled to an absorption detector, which is based on the ring-down of an optical signal in a closed fiber waveguide loop. The ring-down times were obtained by measuring the phase difference between intensity modulated light entering and exiting the fiber loop. The incorporation of a microlens to enhance transmission through the sample gap led to an improvement of the sensitivity by up to 80% compared to the square-cut fiber and a reduction in the detection limit. The performance of the PS-FLRDS absorption technique as an online detector was characterized by flow injection through a capillary. Good repeatability and linear response were obtained, and the detection limit using the lensed fiber/capillary interface system was determined to be alpha(min) = 1.6 cm(-1) for an absorption path of approximately 30 microm. PS-FLRDS coupled to CE was also applied to the analysis of human serum albumin (HSA) by using a NIR dye as a noncovalent label. The excess free dye and the dye/protein complex were resolved. The labeling coefficient was determined to be approximately 6, and good repeatability of peak areas (RSD = 8.7%) was obtained for the analysis of HSA. Furthermore, an excellent linear response (R2 > 0.99) was obtained between the peak areas and concentrations of HSA. The detection limit of labeled HSA was determined to be 1.67 microM.

Optical absorption spectrum of gold atoms deposited on SiO(2) from cavity ringdown spectroscopy

The optical properties of gold atoms supported on amorphous silica (alpha-SiO2) were studied experimentally and theoretically in the visible range. Samples were prepared in situ by depositing Au atoms at low coverages (5 x 10(12) cm(-2)) in UHV, and the optical absorption spectra were recorded by cavity ringdown spectroscopy. The atomic absorption bands can be attributed to gold atoms trapped at [triple bond] Si-O(.-) and [triple bond]Si-O(-) defect sites. The absence of optical transitions typical for Au(2) shows that the atoms are efficiently anchored at these defect sites, preventing diffusion and aggregation. Furthermore, these experimental results reveal that it is now possible to study optical properties of well-defined nanostructures at surface coverages as low as 5 x 10(11) cm(-2).

Absolute surface coverage measurement using a vibrational overtone

Determination of absolute surface coverage with sub-monolayer sensitivity is demonstrated using evanescent-wave cavity ring-down spectroscopy (EW-CRDS) and conventional CRDS by employing conservation of the absolute integrated absorption intensity between gas and adsorbed phases. The first C-H stretching overtones of trichloroethylene (TCE), cis-dichloroethylene, and trans-dichloroethylene are probed using the idler of a seeded optical parametric amplifier having a 0.075 cm(-1) line width. Polarized absolute adsorbate spectra are obtained by EW-CRDS using a fused-silica monolithic folded resonator having a finesse of 28 500 at 6050 cm(-1), while absolute absorption cross sections for the gas-phase species are determined by conventional CRDS. A measure of the average transition moment orientation on the surface, which is utilized for the coverage determination, is derived from the polarization anisotropy of the surface spectra. Coverage measurement by EW-CRDS is compared to a mass-spectrometer-based surface-uptake technique, which we also employ for coverage measurements of TCE on thermally grown SiO(2) surfaces. To assess the potential for environmental sensing, we also compare EW-CRDS to optical waveguide techniques developed previously for TCE detection.

Surface-plasmon-resonance-enhanced cavity ring-down detection

The cavity ring-down technique is used to probe the absolute optical response of the localized surface plasmon resonance (SPR) of a gold nanoparticle distribution to adsorption of trichloroethylene (TCE) and perchloroethylene (PCE) from the gas phase. Extended Mie theory for a coated sphere with a particle-size-dependent dielectric function is used to elucidate size-dispersion effects, the size-dependence of the SPR sensitivity to adsorption, and the kinetics of adsorption. An approximate Gaussian distribution of nanospheres with a mean diameter of 4.5 nm and a standard deviation of 1.1 nm, as determined by atomic force microscopy, is provided by the intrinsic granularity of an ultrathin, gold film, having a nominal thickness of approximately 0.18 nm. The cavity ring-down measurements employ a linear resonator with an intracavity flow cell, which is formed by a pair of ultrasmooth, fused-silica optical flats at Brewster's angle, where the Au film is present on a single flat. The total system intrinsic loss is dominated by the film extinction, while the angled flats alone contribute only approximately 5 x 10(-5)/flat to the total loss. Based on a relative ring-down time precision of 0.1% for ensembles averages of 25 laser shots from a pulsed optical parametric oscillator, the minimum detectable concentrations of PCE and TCE obtained by probing the SPR response are found to be 2 and 7 x 10(-8) mol/L, respectively, based on a 30 s integration time.

Characterization of thin-film losses with a synchronously pumped ringdown cavity

We describe the use of a synchronously pumped ringdown cavity for measuring total optical losses, absorption and scattering, in thin optical films of arbitrary thickness on transparent substrates. This technique is compared with a single-pulse ringdown cavity regime and is shown to have a superior signal-to-noise ratio and resolution. We also provide an analysis of the factors affecting the resolution of the technique. Using this ringdown cavity pumped by a conventional mode-locked Ti:sapphire laser, we experimentally detect losses of only 58 +/- 9 and 112 +/- 9 parts per million in Ta2O5 and SiO2 films, respectively. To our knowledge, these are so far the lowest losses measured in thin films on stand-alone transparent substrates.

Cavity ring-down spectroscopy as a detector for liquid chromatography

We have demonstrated the use of cavity ring-down spectroscopy (CRDS) as a detector for high performance liquid chromatography (HPLC). For this use, we have designed and implemented a Brewster's angle flow cell such that cavity ring-down spectroscopy can be performed on microliter volumes of liquids. The system exhibits a linear dynamic range of 3 orders of magnitude (30 nM to 30 microM quinalizarin at 470 nm) for static measurements and 2 orders of magnitude (0.5 microM to 50 microM) for HPLC measurements. For the static measurements, the baseline noise is 2.8 x 10(-6) AU rms and 1.0 x 10(-5) AU peak-to-peak, and for the HPLC separations, it is 3.2 x 10(-6) AU rms and 1.3 x 10(-5) AU peak-to-peak. The baseline noise is determined after the data are smoothed by an 11-point boxcar average. The peak areas detected from HPLC separations are reproducible to within 2-3%. The HPLC mass detection limit for a molecule with epsilon = 9 x 10(3) M(-1) cm(-1) in a 300-microm path length cell (illuminated volume, 0.5 microL) is reported as 2.5 x 10(-8) g/mL. These results were obtained using a simple pulsed CRDS system and are comparable to, if not better than, a high-quality commercial UV-vis absorption detector for the same path length.

Cavity ringdown spectroscopy of thin films in the mid-infrared

To demonstrate the potential of the cavity ringdown technique in mid-infrared spectroscopy of thin film samples, we measured absorption losses in a C60 film on a BaF2 substrate using a tunable optical parametric amplifier source. With a Brewster angle sample geometry, we achieved a fractional loss sensitivity as small as 1.3 x 10(-7) with 1.5 cm(-1) resolution, an improvement in sensitivity of 2 orders of magnitude compared to standard Fourier transform infrared methods. At an absorption sensitivity of 5 x 10(-7), spectra of several C60 overtone lines were recorded.

Cavity ringdown detection of losses in thin films in the telecommunication wavelength window

The method of cavity ringdown spectroscopy (when a tunable pulsed optical parametric oscillator was used) was extended for the loss evaluation in thin films (2-20-microm thickness). The technique was applied in two key telecommunication wavelength ranges of 1260-1330 and 1480-1650 nm. The measurement sensitivity was determined to be 50 ppm (5 x 10(-5)). The results for polymer films are in close correlation with conventional spectrophotometric data and propagation loss for planar waveguides. Films of greater thickness and better optical quality are expected to provide an even higher loss resolution.

Monolithic folded resonator for evanescent wave cavity ringdown spectroscopy

An optical resonator is characterized that employs both ultrahigh-reflective coated surfaces and total internal reflection to enable cavity ringdown spectroscopy of surfaces, films, and liquids. The monolithic folded design possesses a polarization-independent finesse that allows polarization-dependent phenomena, such as molecular orientation, to be probed. Although a restricted bandwidth (~15% of the design wavelength) results from use of reflective coatings, the resonator provides high sensitivity and facile operation. A minimum detectable absorption of 2.2 x 10(-6) was obtained for single laser shots by use of multimode excitation at 530 nm with an excimer-pumped, pulsed dye laser.