When thermal and stress confinement conditions are satisfied, the propagation of laser-induced ultrasound (LIU) is governed by an inhomogeneous wave equation for pressure, and the laser temporal profile can be modeled by a Dirac delta distribution. If these conditions are not fulfilled, the coupled differential equations for temperature and pressure must be solved, considering a laser pulse with finite time-width. Here, an exact solution of the boundary value problem for the 1D-wave equation is obtained in both the frequency domain and the time domain. Since highly absorbent optical materials are out of the validity range of the confinement conditions, these are used as a numerical and experimental model. It is shown that the impulse-response model correctly predicts the time of flight of photoacoustic waves. However, when considering a laser pulse with finite time-width, the resultant theoretical amplitude of the LIU signal decays rapidly to zero, which is not observed in the experiment. To overcome this, we propose a thermal correction on the LIU source, defined typically as the optical penetration length, which imposes a redefinition of stress and thermal confinement in just one statement. Additionally, with the aim of comparing the corrected-theoretical results with the acquired electrical signals, the sensor and the oscilloscope were modeled as an RC circuit in parallel. It was found that the amplitude of the electrical signal was proportional to the difference of the LIU amplitudes at the faces of the sensor. It is demonstrated that even though the sensor impulse response is modeled as a Dirac delta distribution, this difference strongly affects the shape of the LIU electrical signals, hiding relevant information of the acoustic waves.

Our current work exploits direct laser writing (DLW) and low one-photon absorption (LOPA) in a low-cost three-dimensional optical fabrication system designed to print micrometric polymeric structures. Micropedestals were obtained by focusing a laser beam on a photoresist layer deposited on a silica glass substrate. Subsequent coating with rhodamine 6G dye allows these pedestals to function as microlasers upon optical excitation at 532 nm. Our microlasers, with a diameter of ∼ 53 µ m and a height of ∼ 40 µ m , exhibit a broad fluorescence peak in the spectral range 540–600 nm, in addition to narrow lasing peaks, exhibiting quality factors Q exceeding 2000 and a lasing threshold of ∼ 5 µ J c m − 2 . The observed free spectral range associated with the lasing peaks of ∼ 1.3 n m is consistent with simulations, which we include in this paper. In addition, we present simulations for the longitudinal shift of the patterning laser spot, which occurs particularly for relatively thick photoresist layers, coupled with a large index contrast at the photoresist top surface. Such a shift could introduce errors in the resulting microfabricated structures if left unaccounted for. We hope that our work will contribute to the development of microlasers for various photonic applications, particularly if dimensions can be reduced, for on-chip optical communications and data processing.

This paper aims to implement a laser-induced ultrasound imaging reconstruction method based on the delay-and-sum beamforming through the synthetic aperture focusing technique (SAFT) for a circular scanning, performed with a tomograph that had one acoustic sensor and a system that rotates the sample around a fixed axis. The proposed method, called the Single-sensor Scanning Synthetic Aperture Focusing Technique, considers the size of the sensor and the detection procedure inside the SAFT's algebra. This image reconstruction method was evaluated numerically, using the Green function for the laser-induced ultrasound wave equation to generate a forward problem, and experimentally, using a solid object of polylactic acid, and a Sprague–Dawley rat heart located in a tissue-mimicking phantom. The resulting images were compared to those obtained from the time reversal and the conventional delay-and-sum reconstruction algorithms. The presented method removes the sidelobe artifacts and the comet tail sign, which produces a more distinguishable target on the image. In addition, the proposed method has a faster performance and lower computational load. The implementation of this method in photoacoustic microscopy techniques for image reconstruction is discussed.