![]() ![]() 1(a)) such that between L1 and L2, there is a vertical separation of the pump and first probing paths. The transmitted leg of this probe is focused coincident with the pump source onto the phase mask at an angle θ 1 (out of the plane of the page in Fig. Prior to incidence onto the phase mask, the probe path is split using a non-polarizing 50:50 beam splitter ( BS). All of the probing beams are generated from the same continuous wave laser source. To record two heterodyne phases concurrently, two additional laser paths are added to the standard TGS implementation as a probing beam and reference oscillator pair. For the case considered here, fixing ϕ to 0° and 180° allows for the collection of a complete TGS measurement of the excitation of interest without the constraint of mechanical actuation. 2 In light of this complexity, the performance of the DH-TGS methodology is characterized in terms of the measured acoustic response alone. 13,14 However, the surface displacement response cannot be interrogated independently, causing the thermal transport dynamics of measurements sensitive to acoustic propagation to be a combination of both displacement and reflectivity responses. 2 Of these two excitations, it is the displacement dynamics that contains information about both acoustic and thermal properties, while the reflectivity dynamics contain information about thermal transport properties alone. For materials that exhibit strong excitations in both displacement and reflectivity under an imposed transient grating, different dynamics can be probed by choosing the values of ϕ other than ϕ = 0 ° and ϕ = 180 °. In this letter, a modification to the optical arrangement for heterodyne amplified TGS experiments is presented which allows for the simultaneous collection of two signals with differing heterodyne phases, hereafter referred to as dual heterodyne phase collection TGS (DH-TGS). Such systems of interest may include irradiation-induced material evolution, the thermal transport properties of surfaces as oxide layer growth occurs, and the kinetics of low-temperature phase change materials. This limitation must be lifted if material systems undergoing dynamic changes are to be studied using TGS. 2 Measurements relying on this type of manual phase control are time-limited by the actuation time between collections at different values of ϕ, typically in the range of tens of seconds to single minutes. A phase difference between probing and reference beams is generated as a function of the tilt angle of this flat due to small changes in the path length. In practice, the heterodyne phase is controlled by a manually adjustable, highly parallel optical flat in the path of the probe laser beam. Such noise could include the impulse response of the photodetector or other electrical noise present near the experiment. In addition to further amplifying the recorded signal intensity, taking a set of measurements in this manner allows for the removal of any systematic noise from signals used for quantitative analysis. A temporal resolution of between 1 and 10 s, demonstrated here on single crystal metallic samples, will allow TGS experiments to be used as an in-situ, time-resolved monitoring technique for many material processing applications. Measurements are instead constrained only by the desired signal-to-noise ratio. This arrangement allows for complete, heterodyne amplified TGS measurements to be made in a manner not constrained by a mechanical actuation time. ![]() In this letter, an optical configuration is presented which allows for collection of TGS measurements at two heterodyne phases concurrently through the use of two independently phase controlled interrogation paths. To date, this has been accomplished by manually controlling the heterodyne phase between measurements with an optical flat. The application of optical heterodyne detection for transient grating spectroscopy (TGS) using a fixed, binary phase mask often relies on taking the difference between signals captured at multiple heterodyne phases. ![]()
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