Cavity-enhanced resonant absorption

Seletskiy, D., Hasselbeck, M.P., Sheik-Bahae, M., Epstein, R.I., Bigotta, S., Tonelli, M., 2008. Cooling of Yb YLF using cavity enhanced resonant absorption. Proc. SPIE 6907, 69070B. 

Hu J., Carlie N., Petit L., Agarwal A., Richardson K., and Kimerling L. C., Cavity-enhanced IR absorption in planar chalcogenide glass microdisk resonators Experiment and analysis, RID B-9534-2008, RID A-6012-2011,/. Lightwave TechnoL, 27, 5240-5245 (2009). 

Freely suspended liquid droplets are characterized by their shape determined by surface tension leading to ideally spherical shape and smooth surface at the subnanometer scale. These properties suggest liquid droplets as optical resonators with extremely high quality factors, limited by material absorption. Liquid microdroplets have found a wide range of applications for cavity-enhanced spectroscopy and in analytical chemistry, where small volumes and a container-free environment is required for example for protein crystallization investigations. This chapter reviews the basic physics and technical implementations of light-matter interactions in liquid-droplet optical cavities. 

Nitkowski A., Chen L., and Lipson M., Cavity-enhanced on-chip absorption spectroscopy using microring resonators. Opt Express, 16, 11930-11936(2008). 

S. Murtaza, K. Anselm, C. Hu, H. Me, B. Streetman, J. Campbell, Resonant-cavity enhanced (RCE) separate absorption and multiplication (SAM) avalanche photodetector (APD). IEEE Photonics Technol. Lett 7(12), 1486-1488 (1995)... 

Sensitivity can be improved by factors of 10 using intracavity absorption, placing an absorber inside a laser resonator cavity and detecting dips in the laser emission spectmm. The enhancement results from both the increased effective path length, and selective quenching of laser modes that suffer losses by being in resonance with an absorption feature. 

The setup for ESR spectroscopy is a cross between NMR and micro-wave techniques (Section 5.8). The source is a frequency-stabilized klystron, whose frequency is measured as in microwave spectroscopy. The microwave radiation is transmitted down a waveguide to a resonant cavity (a hollow metal enclosure), which contains the sample. The cavity is between the poles of an electromagnet, whose field is varied until resonance is achieved. Absorption of microwave power at resonance is observed using the same kind of crystal detector as in microwave spectroscopy. Sensitivity is enhanced, as in microwave spectroscopy, by the use of modulation The magnetic field applied to the sample is modulated at, say, 100 kHz, thus producing a 100-kHz signal at the crystal when an absorption is reached. The spectrum is recorded on chart paper. 

An extremely sensitive technique able to detect the nature of radical pairs in a photochemical reaction is called chemically induced dynamic nuclear polarization (CIDNP), which depends on the observation of an enhanced absorption in a nuclear magnetic resonance (NMR) spectrum of the sample, irradiated in situ, in the cavity of a NMR spectrometer. The background to and interpretation of CIDNP are discussed by Gilbert and Baggott (28). 

LIF (Ezekiel and Weiss, 1968 Cruse, et al., 1973 Zare and Dagdigian, 1974 Kinsey, 1977) is an example of an indirect technique for the detection of a one-photon resonant upward transition. There are many other indirect detection techniques (optogalvanic, optothermal, photoacoustic, cavity ringdown), but Multi-Photon Ionization (MPI) is a special type of indirect technique uniquely well suited for combining absorption detection with other useful functionalities (see Section 1.2.1.1). In MPI, photo-ion detection replaces photon detection. The one-color, singly-resonant-enhanced (n + m) REMPI f process consists of an n-photon resonant e, v, J <— e",v",J" excitation, followed by a further nonresonant m-photon excitation into the ionization continuum... 

The novel feature of the operation occurs if the source scan-range includes a fixed frequency spectral absorption. This absorption will be enhanced as the cavity resonance is tracked through it, by an increase in the effective path length. At this point the absorption is detected by the second phase-coherent detector that sees it as a variation of the second harmonic of the modulation frequency superposed on the fixed cavity signal. Thus the spectrometer will respond to all spectral features in the range through which the source is tracked rather than to a single one located at whatever frequency the cavity happens to be resonant. 

This sensitivity enhancement in detecting small absorptions has no direct correlation with the gain medium and can be also realized in external passive resonators. If the laser output is mode matched (Vol. 1, Sect. 5.2.3) by lenses or mirrors into the fundamental mode of the passive cavity containing the absorbing sample (Fig. 1.11), the radiation power inside this cavity is q times larger. The enhancement factor q may become larger if the internal losses of the cavity can be kept low. 

With an iodine cell inside the resonator of a cw multimode dye laser, an enhancement factor of g = 10 could be achieved, allowing the detection of I2 molecules at concentrations down to n < 10 /cm [20]. This corresponds to a sensitivity limit of aL < 10 . Instead of the laser output power, the laser-induced fluorescence from a second iodine cell outside the laser resonator was monitored as a function of wavelength. This experimental arrangement (Fig. 1.16) allows demonstration of the isotope-specific absorption. When the laser beam passes through two external iodine cells filled with the isotopes l2 and l2, tiny traces of l2 inside the laser cavity are sufficient to completely quench the laser-induced fluorescence from the external l2 cell, while the l2 fluorescence is not affected [21]. This demonstrates that those modes of the broadband dye laser that are absorbed by the internal l2 are completely suppressed. 

The CRDS technique uses the same principle as intra-cavity spectroscopy, namely increasing the effective absorption path length. The difference is that in CRDS the absorption coefficient is determined from a time measurement, i.e., the decay time of the ringing cavity , while in intra-cavity spectroscopy the gain competition between different resonator modes is used as the enhancement factor. 

The sensitivity can be further enhanced by frequency modulation of the laser (Sect. 1.2.2) and by intracavity absorption techniques. With the spectraphone inside the laser cavity, the photoacoustic signal due to nonsaturating transitions is increased by a factor as a result of a -fold increase of the laser intensity inside the resonator (Sect. 1.2.3). The optoacoustic cell can be placed inside a multipath optical cell (Fig. 1.28) where an effective absorption pathlength of about 50 m can be readily realized [74]. 

All these techniques may be combined with intracavity absorption when the sample molecules are placed inside the laser resonator to enhance the sensitivity. Cavity ring-down spectroscopy yields absorption spectra with a detection sensitivity that is comparable to the most advanced modulation techniques in multipass absorption spectroscopy. 

For high-speed applications, both the junction area and the intrinsic layer thickness need to be small. An external resonant microcavity approach has been proposed to enhance p in such a situation (Dentai et al, 1987). In this approach the absorption region is placed inside a high Q cavity so that a large portion of the photons can be absorbed even with a small detection volume. An integral lens can be incorporated in the backside of the photodiode for focusing more photons to the detection region. 

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