A time-resolved holographic polarization microscopy, based on angular multiplexing holographic technique, is proposed for imaging ultrafast phenomena in polarization-sensitive transparent materials. This method can retrieve and image the complex amplitude distributions of two orthogonal polarization components of two sequential vector wavefronts with ultrashort time interval by a single short recording. Some experimental results for imaging the pulse laser induced ultrafast events based on the method are given. It is demonstrated that this technique may provide a potential tool for characterizing ultrafast processes in polarization-sensitive materials, especially in the non-reproducible experiment conditions.
© 2017 Optical Society of America
Ultrafast laser–material interaction has been extensively studied over the past decades, and it is widely applied in laser micromachining [1–6] in transparent materials. To understand the physical processes of ultrafast laser interaction with materials, the in situ charactering the transmission change and the refractive index change of materials with spatial and temporal resolution are valuable. Up to now, several approaches such as time-resolved shadowgraphy [7,8], phase-contrast microscopy [9,10], spectral interferometry [11,12], wavefront sensing technique [13,14], and digital holography (DH) [15–22] have been proposed. The time-resolved shadowgraphy can only record the diffraction intensity pattern of the specimen, so it is not suitable for pure phase object imaging. Phase contrast microscopy reveals the refractive index change in ultrafast process, but it is impossible to give access to the change in transmittance. Spectral interferometry allows quantitative phase measurement, and the drawback is that only 1D spatial resolution is provided. Although wavefront sensing technique could recover the 2D intensity and phase of object by using a wavefront sensor, the price for this is a reduction of the image resolution .
It was demonstrated that the DH is a powerful approach for the investigation of ultrafast phenomena in transparent materials. Using DH to study the time evolution of the plasma, generated by a focusing femtosecond pulse in air, has been reported by Centurion et al. . Papazoglou et al.  demonstrated the application of in-line DH for characterization of the refractive index perturbations induced by ultrafast laser pulses in transparent media. Balciunas et al.  developed a time-resolved off-axis DH technique, using a Mach–Zehnder interferometer, for the investigation of femtosecond laser-induced plasma filaments in water. These methods can only record a snapshot of the ultrafast events in one-shot measurement. Furthermore, Wang et al.  presented a spatial angular multiplexing DH technique, which can record three pairs of amplitude and phase images of ultrafast dynamic process using one hologram, with a frame interval of 300 fs. However, these previous holographic techniques only studied the ultrafast phenomena in isotropic materials, such as water and air. In the fact, most of transparent materials are polarization-sensitive, such as functional crystals and biological specimens [24,25]. Furthermore, focused femtosecond laser irradiation in isotropic materials, e.g. fused silica [26,27], results in the artificial birefringence modification. Colomb et al.  proposed a polarization microscopy based on off-axis digital holograpy that permits one to image polarization states, this technique was applied in birefringence measurements on fused silica exposed to femtosecond laser pulses by Bellouard et al. . However, this technique can only allow state-of-polarization (SOP) imaging for a static object. And thus, a time-resolved polarization imaging technique is imperatively required for investigating the ultrafast phenomena in polarization-sensitive materials.
In this work we propose a time-resolved holographic polarization microscopy (THPM) based on angular multiplexing holographic technique, for studying ultrafast phenomena in polarization-sensitive transparent materials. This new method can retrieve and image the amplitude and the phase distributions of two orthogonal polarization states of an ultrafast event at two different time in one-shot measurement. It is a valuable technique for charactering ultrafast processes in non-reproducible experiment conditions.
2. Principle and experimental setup
A schematic of the experimental setup for THPM is illustrated in Fig. 1. In this setup, a pulse laser is used to generate an ultrashort laser pulse. Then the pulse is divided by a beam splitter BS1 into two parts: the pump beam and the probe beam. The pump beam with higher amount of energy is focused by Lens L1 on the sample with a tilt angle from the illumination light. The probe beam is frequency doubled in a KDP crystal and adjusted into left circularly polarized light by the quarter-wave plate QWP. The probe beam is further split by the beam splitter BS2 into two subpulses, probe pulse 1 and probe pulse 2, with a time delay adjusted by the delay line 1. The delay line 2 is used for achieving the specific time delays between the pump pulse and the first probe subpulse. These two subpulses pass through the 2D orthogonal gratings CG1 and CG2, respectively; the orientation of CG2 is turned 45° relative to the orientation of CG1, and then they are split by the beam splitter BS3 into two paths: the object beam path and the reference beam path. In the object beam path, only zero diffraction orders of the gratings can pass through the filter aperture PF1 (in order to avoid the stray light caused by the high diffraction orders incident on the subsequent elements) and the pinhole filter PF2 as the object beams. The polarization direction of the object beam is orientated at 45° with respect to the horizontal direction by changing the orientation of the polarizer P1. A microscope objectives MO placed behind the laser irradiated area of sample with a 50 times magnification is used for imaging the sample. In the reference path, only four of the first diffraction orders from probe pulse 1 and probe pulse 2 keep passing through the four-pinhole spatial filter PF3, the distributions of these holes is shown in zoom-in picture of Fig. 1, as the reference beams to record a four-channel angular multiplexing hologram, while the zero diffraction orders and other first diffraction orders are blocked by the filter PF3. Two orthogonal linear polarizers attached behind the filter PF3 are used to change polarization states of reference beams. The orientation of linearly polarizer P2 is horizontal, corresponding to x axis of the coordinate system adopted in the following mathematic analysis, while the polarization direction of polarizer P3 is vertical, as illustrated in zoom-in picture of Fig. 1. The polarized reference beams and come from the first diffraction orders of the CG1, and the other two reference beams and come from the corresponding diffraction orders of the CG2. A CCD camera with outer-trigger interface is employed to record the hologram; thus we can use the synchronizing trigger signal generated by the pulse laser system to control the exposure of the CCD camera. And a bandpass filter together with some neutral density filters is used to block scattering pump beam and the fluorescence generated during the laser irradiation from entering the CCD camera. In the Fig. 1, L2 and L4 are the focusing lenses for spatial filtering adopted in the reference and object paths respectively; L3 and L5 are the corresponding collimating lenses used in the two paths.
In the THPM arrangement, owning to the delay time between these two probe pulses is larger than the pulses duration, the object wave can only interfere with the reference wave with the same time delay. And thus, the total intensity I at the recording plane can be expressed asEq. (1) will be named as the time-resolved polarization hologram (TRPH). Equation (3) can be further decomposed as28,30–32]. The retrieving procedures are as follows. At first, we transform the TRPH function into the spatial frequency domain using a fast Fourier transform (FFT) algorithm. Then the spatial spectra of the four items given in Eq. (5) will be separated from all other items in spatial frequency domain and so can be respectively extracted by spatial filtering. Thus the four wanted complex amplitudes and can be respectively retrieved from the extracted spectra components using an inverse FFT algorithm.
3. Experimental results and discussions
3.1 Characterization of the spatial resolution and demonstration of the consistency
In order to characterize the spatial resolution of THPM system, a portion of USAF 1951 resolution test target (group 5 elements 4-6) was used as a test object (sample 1) for the first experimentally demonstration. In the experiments, a commercial picosecond laser oscillator (Ekspla PL2250, the centerl wavelength is, the duration of the pulses (FWHM) is about), is used as the laser source. The diameter of the pump beam is about 2 mm, which is focused on the sample surface by a focusing lens L1 with the focal length of . The pump laser is blocked and the polarization direction of the probing pulse is oriented at 45° with respect to the vertical direction. Figure 2(a) shows an example of the four-channel angular multiplexing holograms recorded by a CCD image sensor with pixel size of and pixel number of . The detailed interferometric pattern of the hologram can be seen from the zoom-in picture shown in Fig. 2(a). Figure 2(b) is just the Fourier transform of the hologram shown in Fig. 2(a). Figures 2(c)-2(j) are the amplitude and phase of the object wave retrieved, respectively, from the four first diffraction orders and as shown in Fig. 2(b), which correspond to the complex amplitudes of two orthogonal polarization components of the object waves at two different probing times with time delay of 500 ps. It can be seen that the smallest objects on the test target (group 5 element6) are clearly distinguishable. Because the tested object is polarization-insensitive and static in this experiment, all the complex amplitudes retrieved from the different orders are nearly same, which also reveal the consistency of the measurements using different diffraction orders.
3.2 Imaging of the damage process in a linear polarizer
Next, we demonstrate the feasibility of THPM system for monitoring the laser-induced ultrafast phenomena in polarization-sensitive materials. A film linear polarizer with the thickness of was chosen as the sample 2, and its’ polarization direction is orientated at at 60° with respect to horizontal direction in the next experiment. The amplitude and phase contrasts of the sample corresponding to two orthogonal polarization states are obtained by the THPM system with blocking the pump pulse, as shown in Figs. 3(a)-3(d). Only larger contrast was observed in amplitude distributions, because the phase changes of two orthogonal polarization light transmitted trough a linear polarizer are nearly same. The SOP of the wave is often characterized by the parameters of the azimuth and phase difference , which can be calculated by Figures 4(a) and 4(d) give a calculated result based on the measured complex amplitude shown in Figs. 3(a)-3(d). From Figs. 4(a) and 4(d), it can be seen that the azimuth almost keep a constant 60° and the phase difference is nearly equal to 0. These results demonstrate qualitatively the characteristics of a linear polarizer . The estimated energy fluence of the focused pump pulse at the sample surface is about , which is about 2 times of the damage threshold, measured at a 1064nm single-pulse irradiation. The fluence of the probe pulse is only about 0.5% compared with the pump pulse, so the influence of probe pulse on the sample is negligible. Now, the delay between the pump pulse and the two probe pulses are set as 0.2ns and 1.8ns, respectively. The amplitude and the phase distributions of two orthogonal polarization states of the object waves associated with two different time delays, are simultaneously obtained by the THPM system in one-shot recording, as shown in Figs. 3(e)-3(l). A gradual darker region corresponding to a drop in transmission can be seen in Figs. 3(e) and 3(f), for the time delay of 0.2ns. This is subject to the fast growth of internal stress  caused by rapid temperature rise in the laser focal zone and generation of shock wave  in the sample. A strip type dark region with a length of about is observed, as a consequence of the pump beam illuminates the sample with an incident angle of 45°. And the similar change in phase contrast is also clearly observed in Figs. 3(g) and 3(h). For larger time delays of 1.8ns, the dark region on amplitude contrast images becomes larger indicating the shock wave propagation, as illustrated in Figs. 3(i) and 3(j). And the phase expansion is also clearly observed in Figs. 3(k) and 3(l), simultaneously. The dark region is caused by the stronger absorption of illuminating wave, so it has an undefined phase. Figure 4 present the azimuth and the phase difference images of the ultrafast event. Only small variations appear in the azimuth distributions shown in Figs. 4(b) and 4(c). The similar phenomenon is also observed in phase difference images, as illustrated in Figs. 4(e) and 4(f). This indicates that the relative index change between two orthogonal polarization states is nearly same in the process of laser irradiation for a film linear polarizer.
3.3 Imaging of the damage process in a mica lamina
A mica lamina with the thickness of about is obtained by mechanical exfoliation and used as third sample to carry out the experiment in the same conditions as described previously, except for the delay times. The damage threshold of sample 3 is about , which is measured at a 1064nm single-pulse irradiation. The optical axis of the sample is oriented about at 10° with respect to the horizontal direction. From top row of Fig. 5, it can be seen that both the amplitude and phase distributions corresponding to the two orthogonal components are clearly different. The shock wave generation and propagation in the sample are also observed in Figs. 5(e)-5(l). The shape of dark region is nearly circular not a strip, because of the thickness of the sample is only about. It is interesting to notice that the changes in amplitude and phase contrast associated with the two orthogonal states are clearly different, especially for the time delays of 1.8ns, as shown in Fig. 5(i)-5(l). This phenomenon can be explained by that the sample is anisotropic. Figure 6 present SOP images reconstructed from the images of Fig. 5. A phase difference of about is observed in Fig. 6(a), and an azimuth of about 36° is also seen in Fig. 6(d). Higher variations appear in the azimuth and phase difference images for the time delays of 1.7ns, as illustrated in Figs. 6(c) and 6(f). The phenomena indicate that transmission change and the refractive index change of the mica lamina is anisotropic in the process of laser irradiation.
In summary, we have presented a THPM for studying ultrafast phenomena in polarization-sensitive transparent materials. In comparison with other time-resolved imaging techniques for ultrafast phenomena characterization, an attractive feature of this method is that it can retrieve and image the amplitude and the phase distributions of two orthogonal polarization states of an ultrafast event at two different time in one-shot measurement. A portion of USAF 1951 resolution test target was used as a test object to characterize the spatial resolution and demonstrate the consistency of the THPM system. Furthermore, the dynamic processes of picosecond laser irradiation in two other typical polarization-sensitive samples, a linear polarizer and a mica lamina, were characterized by using the THPM system. The generation and propagation of shock wave was observed in the two samples after several hundreds of picoseconds. Simultaneously, the SOP parameters of object waves transmitted through the samples were presented, which shows that the complex index modulation is anisotropic in an ultrafast event. Absolutely, the THPM system can be applied in studying some other much earlier phenomena in sub-picosecond time-scale of an ultrafast event, such as free-electron generation and lattice heating, by using a femtosecond laser . And thus, we firmly believe that the THPM technique is a good candidate for monitoring laser-induced ultrafast phenomena in polarization-sensitive materials, especially in non-reproducible experiment conditions.
National Natural Science Foundation of China (No. 11474186).
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