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

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.

Cavity-enhanced measurements for determining dielectric-membrane thickness and complex index of refraction

The material properties of silicon nitride (SiN) play an important role in the performance of SiN membranes used in optomechanical applications. An optimum design of a subwavelength high-contrast grating requires accurate knowledge of the membrane thickness and index of refraction, and its performance is ultimately limited by material absorption. Here we describe a cavity-enhanced method to measure the thickness and complex index of refraction of dielectric membranes with small, but nonzero, absorption coefficients. By determining Brewster's angle and an angle at which reflection is minimized by means of destructive interference, both the real part of the index of refraction and the sample thickness can be measured. A comparison of the losses in the empty cavity and the cavity containing the dielectric sample provides a measurement of the absorption.

Q-factor measurements through injection locking of a semiconductor-glass hybrid laser with unknown intracavity losses

The injection locking properties of a newly developed waveguide-based external cavity semiconductor laser have been investigated. Using the injection locking properties to measure the Q-factor of complex optical cavities with unknown internal losses, has been demonstrated for the first time.

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.

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)].

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.

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.