Common-path conoscopic interferometry for enhanced picosecond ultrasound detection

Abstract

We report on a common-path implementation of conoscopic interferometry in picosecond pump-probe reflectometry for simple and efficient detection of picosecond ultrasounds. The interferometric configuration proposed here is greatly simplified, involving only the insertion of a birefringent crystal in a standard reflectometry setup. Our approach is demonstrated by the optical detection of coherent acoustic phonons propagating through thin metal films under two representative geometries, one a particular case where the crystal slab is part of a sample as substrate of a metal film, and the other a more general case where the crystal slab is independent of the sample as part of the detection system. We first illustrate the former with a 300 nm thin film of polycrystalline titanium, deposited by physical vapor deposition on top of a 1 mm-thick uniaxial (0001) sapphire crystal. A signal-to-noise ratio (SNR) enhancement of more than 15 dB is achieved compared to conventional reflectometry. Next, the general case is demonstrated with a 900 nm-tungsten film sputtered on a silicon wafer substrate. More echoes can be discriminated by using the reported approach compared to standard reflectometry, which confirms the improvement in SNR and suggests broad applications for the reported method.

Conoscopic interferometry for optimal acoustic pulse detection in ultrafast acoustics

“…The analytical model we present here combines the influence of the diaphragm aperture on the acoustic wave detection sensitivity in BDD [10] with the Jones calculus formulation for the CIPs [11] [14] [15]. BDD is thus a particular case of the model, where the probe beam is spatially Gaussian.…”

“…Hence, the power of the probe beam reflected from the material surface has a component directly proportional to the elastic strain. The resulting relative variation in laser power is usually in the range of 10 −6 − 10 −4 [2] [10] [11]. The photoelastic constants of the material at the probe laser wavelength set the detection sensitivity.…”

“…The strong dependence of the photoacoustic signal on the photoelastic constants limits the applicability of re-flectometry and thus inspired the development of Beam Distortion Detection (BDD) [10] [12] and Conoscopic Interferometry (CI) [11]. In BDD, the Gaussian spatial profile of the acoustic wave incident on the sample surface causes slight fluctuations in the divergence angle of the reflected probe beam.…”

“…CI makes use of Conoscopic Interference Patterns (CIPs), which are well-known for the characterization of birefringent crystals [11] [14] [15]. By focusing the probe beam with a given polarization through a birefringent crystal and then collimating it before passing through a polarizer, one can obtain a succession of bright (isochromates) and dark (isogyres) fringes.…”

“…The fringes form a pattern characteristic of the birefringence properties of the crystal and the input and output polarizations. Liu et al [11] implemented this phenomenon in BDD by adding a birefringent crystal (sapphire plate) between the objective and the sample and by using a Polarizing Beam Splitter (PBS) as a polarizer to change the spatial profile of the probe beam to a CIP. The resulting CIP is controlled by rotating a quarter-wave plate placed between the PBS and the objective.…”