Time-resolved photoacoustic calorimetry (PAC) gives access to lifetimes and energy fractions of reaction intermediates by deconvolution of the photoacoustic wave of a sample (E-wave) with that of the instrumental response (T-wave). The ability to discriminate between short lifetimes increases with transducer frequencies employed to detect the PAC waves. We investigate the lifetime resolution limits of PAC as a function of the transducer frequencies using the instrumental response obtained with the photoacoustic reference 2-hydroxybenzophenone in toluene or acetonitrile. The instrumental response was obtained for a set of transducers with central frequencies ranging from 0.5 MHz up to 225 MHz. The simulated dependence of the lifetime resolution with the transducer frequencies was anchored on experimental data obtained for the singlet state of naphthalene with a 2.25 MHz transducer. The shortest lifetime resolved with the 2.25 MHz transducer was 19 ns and our modelling of the transducer responses indicates that sub-nanosecond lifetimes of photoacoustic transients can be resolved with transducers of central frequencies above 100 MHz.
Adiabatic frequency conversion has some key advantages over nonlinear frequency conversion. No threshold and no phase-matching conditions need to be fulfilled. Moreover, it exhibits a conversion efficiency of 100 % down to the single-photon level. Adiabatic frequency conversion schemes in microresonators demonstrated so far suffer either from low quality factors of the employed resonators resulting in short photon lifetimes or small frequency shifts. Here, we present an adiabatic frequency conversion (AFC) scheme by employing the Pockels effect. We use a non-centrosymmetric ultrahigh-Q microresonator made out of lithium niobate. Frequency shifts of more than 5 GHz are achieved by applying just 20 V to 70-µm-thick crystal. Furthermore, we demonstrate that already with the same setup positive and a negative frequency chirps can be generated. With this method, by controlling the voltage applied to the crystal, almost arbitrary frequency shifts can be realized. The general advances in on-chip fabrication of lithium-niobate-based devices make it feasible to transfer the current apparatus onto a chip suitable for mass production.Optical frequency conversion based on nonlinear optics in microresonators has been advanced over the last decades. Nonlinear frequency conversion is based on the nonlinear response of the material to light . 1 For example, frequency combs in microresonators made out of centrosymmetric materials 2 and tunable optical OPOs in non-centrosymmetric microresonators have been demonstrated. 3, 4 High conversion efficiencies require high intensities, as well as phase matching conditions need to be fulfilled . 5 Moreover, a pump threshold must be overcome for the most versatile conversion mechanism, optical parametric oscillation. An alternative optical conversion technique is the so-called adiabatic frequency conversion (AFC). Here, the frequency of light traveling in a resonator is shifted due to a change of the optical round-path length. One implementation is to change the refractive index of the material and to keep the geometrical path length constant. The frequency of light changes then accordingly to 6 ∆ν ν ≈ − ∆n n .(1)The change of the refractive index must happen in a time ∆t shorter than the propagation time t of the light in the material, i.e. before it gets lost by absorption or scattering. Since microresonators act as light traps, that can store light for many nanoseconds or even milliseconds, depending on their quality factor. Thus they are supposed to be well suited to realize AFC . 7 For AFC, no threshold has to be overcome. This frequency conversion scheme has been realized experimentally in several implementations, even down to a single photon level. 8,9 For example AFC was shown in photonic crystals , 10-12 in waveguides 9, 13-15 and in fiber grating cavities . 16 So far, two different attempts involve microresonators: Conduction-band electrons generated by laser pulses, inducing a change of the refractive index allow to shift a few hundred GHz to shorter wavelengths. 17 However, the ...
The transfer of photon momentum at the water/air interface is important for optical manipulation of minute particles and is at the heart of the Minkowski-Abraham controversy. We use photoacoustic (PA) detection of ultrasound waves generated when pulsed laser light meets the water/air interface at 3.9°C (zero thermal expansion), to distinguish momentum transfer from thermoelastic effects. The PA waves dependence on the angle of incidence reveals that momentum transfer maximizes at the critical angle. Momentum transfer is most efficient when the photons travel in water and remain in water after total reflection at the interface, rather than when they cross the interface between dielectric media.
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