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OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Time-resolved microscopy with random lasers Alexandre Mermillod-Blondin,* Heiko Mentzel, and Arkadi Rosenfeld Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Straße, D-12489 Berlin, Germany *Corresponding author: mermillod@mbi‑berlin.de Received July 19, 2013; revised September 3, 2013; accepted September 4, 2013; posted September 10, 2013 (Doc. ID 194255); published October 9, 2013 We demonstrate that random lasers provide an outstanding strobe light source for time-resolved microscopy. Utilizing a random laser to illuminate a commercially available microscope enables single exposure, speckle-free time-resolved imaging. Aside from conventional optical transmission microscopy, we also perform time-resolved investigations in phase contrast mode. We apply this method to the monitoring of fs-laser-induced microdot formation in bulk a-SIO2 . Time-resolved investigations show that microdot formation lasts over several microseconds after laser excitation. © 2013 Optical Society of America OCIS codes: (180.0180) Microscopy; (140.3390) Laser materials processing; (110.2945) Illumination design. http://dx.doi.org/10.1364/OL.38.004112
Time-resolved optical imaging methods are extensively used for monitoring a wide variety of rapid phenomena. Lifetime imaging microscopy, for instance, is a popular time-resolved microscopy method which enables one to follow intracellular biochemical reactions on the nanosecond timescale [1]. Time-resolved optical methods are also a natural technique for monitoring the dynamics of laser processing during laser-based forward transfer [2], laser-induced microjet formation [3], laser-induced ablation [4], laser-induced melt dynamics [5], or laser-induced refractive-index changes in the volume of transparent materials [6]. The illumination source constitutes the most critical component of any time-resolved imaging system. If some characteristics of the ideal illumination source largely depend on the final application, some other features are universal. Typically, an illumination source suitable for time-resolved imaging should be synchronizable (either by optical delay or triggering electronics), it should be able to generate a short illumination flash (nanosecond to femtosecond duration), it should be uniform, and it should possess a low spatial coherence. Up to the present time, spark lamps and lasers have been employed as illumination sources. Spark lamps possess a very low spatial coherence, providing a speckle-free background illumination. Flash lamps have been successfully used for optical transmission microscopy (OTM) [3] or in Schlieren imagery with low numerical aperture optics [7]. When a better time-resolution and/ or more brightness is necessary, pulsed lasers stand as the most popular solution. However, the nonuniformity of the transverse intensity distribution and the high coherence of laser sources are very detrimental to the final image quality. Beam nonuniformity is usually addressed by expanding a Gaussian beam with the help of a telescope [8] and selecting its central part. High coherence implies that light scattered from objects containing high frequency components (including dust particles) forms a high contrast interference pattern in the image plane [9]. High coherence can be reduced either by propagating through a long multimode fiber [7] or by using beam diffusers [6]. However, propagation inside a long multimode fiber inevitably provokes a substantial pulse broadening (a broadening of 6–18 ns was reported in [7]) and beam diffusers produce a speckled background illumination. 0146-9592/13/204112-04$15.00/0
Speckle contrast can be efficiently diminished by averaging several pictures of the same event acquired with different speckled backgrounds [6], but this approach precludes time-resolved investigations in the single exposure regime. Random lasers are currently a very active research topic (see [10] and references therein). Recently, Cao et al. [11] have demonstrated that random lasers are able to deliver intense and spatially incoherent light pulses. The possibility of using random lasers for imaging has been reported in [12], but in the static regime only. In this Letter, we propose using a random laser as an alternative illumination source for performing stroboscopic microscopy. We aim at monitoring the formation of fs-laser-induced microdots in OTM and phase contrast (PC) microscopy. The time-resolved microscope is depicted in Fig. 1. The fused silica sample, a rectangular parallelepiped polished on all sides, is mounted on a three axis motorized station (not represented in Fig. 1). The microdots are produced by focusing a single fs laser pulse (70 fs duration at FWHM) inside the sample. The pulse energy was 2.5 μJ. The focusing optics is a microscope objective with a numerical aperture of 0.45. We set the focal plane at a depth of 200 μm from the air/dielectric interface in order
Fig. 1. Implementation of the time-resolved microscope for PC investigations. Optical microscopy can be performed by replacing the PC objective by a conventional microscope objective. λ, central wavelength; τ, pulse duration at FWHM; f , focal length; EMCCD, electron multiplying charge coupled device. © 2013 Optical Society of America
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
to minimize wavefront distortion. Time-resolved images in phase-contrast and OTM are acquired at different times τ after fs laser excitation by delaying the emission of the random laser pulse. The timing is set by a pulsed delay generator (SRS DG-535). The random laser employed for illumination is pumped by a frequency doubled YAG laser (8 ns duration, 15 mJ per pulse) focused with a plane convex lens (200 mm focal length). Random lasing takes place in a colloidal solution composed of 250 nm microbeads (at a concentration of ≈1.23 × 1013 ppmL) immersed in a laser dye solution (Rhodamine B, 2.1 g:L−1 dissolved in ethanol). In our pumping conditions (150 mW, 10 Hz), the full width at half-maximum (FWHM) of the random laser spectrum is ≈20 nm. When pumping the same colloidal solution with a modest average power of 5 mW, we measured a FWHM of ≈36 nm. This strong spectrum narrowing is an indication that random lasing takes place [10]. We measured the pulse duration with a fast photodiode and found a pulse width of ≈8 ns, suggesting that the pulse duration is limited by the pulse length of the pump laser. We hypothesize that using a shorter pulse for pumping would lead to the generation of significantly shorter random laser pulses, suitable to study rapid mechanisms such as molecular transitions in biological samples taking place on subnanosecond timescales. A bandpass filter placed in front of the camera rejects light that does not carry useful information, mostly plasma luminescence and light scattered from the exciting fs laser pulse. The spectral response of the bandpass filter matches the emission spectrum of the random laser (central wavelength, 575 nm; width, 25 nm). The influence of plasma luminescence can not be totally suppressed as the emission spectrum of the laser generated plasma is continuous. In Fig. 2, we show micrographs of a laser-induced microdot several hours after formation. The microdot was imprinted with a single laser pulse. The laser pulse comes from the left-hand side of the picture. The image plane corresponds to the plane of incidence of the laser. In order to assess the performance of the random laser illumination source, we compare the images obtained with a LED [Figs. 2(a) and 2(b)], a YAG laser [Figs. 2(c)
Fig. 2. Comparison of optical transmission (left column) and PCMs (right column) of a laser-induced microdot obtained with an illumination source consisting of (a) and (b) a LED, (c) and (d) a YAG laser, and (e) and (f) a random laser RL. The background contrast and the microdot contrast are indicated on each picture.
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and 2(d)], and a random laser [Figs. 2(e) and 2(f)]. The YAG laser beam was phase-randomized and homogenized with the help of a scattering plate. We performed investigations in both OTM and positive phase contrast microscopy (PCM). In positive PCM, pixels darker than the background denote a positive refractive index change whereas pixels brighter than the background indicate a refractive index decrease. We quantifiy the visibility of the laser-induced microdot by measuring the contrast to noise ratio CNR (see [12]) CNR
I¯ − I bkg ; 0.5 × σ I σ I bkg
(1)
where •¯ is the average operator and σ • represents the standard deviation of pixel intensity. A feature is considered visible when its CNR exceeds a value of 1. Figures 2(a) and 2(b) are used here as a reference. They were obtained with LED illumination, not suitable for time-resolved investigations because of their relatively low peak power. Figure 2(b) reveals that the laser-induced microdot is composed of a microvoid prolonged by a filament with contrast values of 8.6 and 2.9, respectively. In OTM [Fig. 2(a)], the microvoid is the only detectable feature, with a contrast of 4.0. Figures 2(c) and 2(d) illustrate the difficulty of performing single shot imaging with a highly coherent illumination source. A Nd:YAG laser was employed in this case. The result is a highly speckled background rendering the microdot invisible. Although Fig. 2(c) was acquired in brightfield mode, the image background is remarkably dark. This feature can be explained by examining the probability density function of the speckle intensity. In the hypothesis of a fully developed speckle, the probability density ¯ exp−I∕I. ¯ Thus, the function pI I reads pI I 1∕I most probable intensity for an arbitrary pixel is zero. The results obtained with random laser illumination are shown in Figs. 2(e) and 2(f). In Fig. 2(e), the microvoid appears with a CNR of 3.1 (4.0 on the LED-illuminated micrograph), confirming the visual impression of similarity with Fig. 2(a). The same applies to PCM [Fig. 2(f)], where the microvoid exhibits a high CNR of 8.2, close to the reference value of 8.6 obtained with the LED illumination. However, the CNR drops to a value of 1.0 in the filamentary region. We assign this low contrast to the performance of our EMCCD in low light conditions. In PCM, about 90% of the illumination light is blocked before the condenser. Therefore, the gray level associated with a positive refractive index change is situated on the bottom part of the EMCCD dynamics, where the dark current and the readout noise play an appreciable role. When acquiring with a LED, increasing the exposure time on the camera suffices to offset the gray level of the darkest features away from the noisy detection region. Increasing the exposure time in the case of a random laser illumination would be counterproductive, as the pulse duration is much shorter than the minimum exposure time (8 ns versus 20 μs, respectively). In Fig. 3, we show time-resolved snapshots of the microdot formation presented in Fig. 2. Again, we conducted our measurements in OTM [Fig. 3(a)] and in PCM [Fig. 3(b)]. The background illumination is in both
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Fig. 3. (a) Optical transmission and (b) PC time-resolved micrographs of the laser-induced microdot shown in Fig. 2. A random laser provided the illumination source.
cases very uniform, and the object of interest stands out with a very satisfying contrast. Time-resolved OTM investigations demonstrate that the absorption region persists until τ 100 ns at least. At first sight, a decay time of the laser-induced plasma spanning over several tens of nanosecond seems in contradiction with the 150 fs attributed to the trapping time of free electrons [13]. However, we emphasize that in our irradiation conditions, the free electron density is close to the critical density ncr 1.74 × 1021 , two orders of magnitude higher than in [13]. At such high densities, the total number of available trapping sites (intrinsic and extrinsic) is probably not sufficient to neutralize the whole free electron population. We can also hypothesize that after multiple collisions, the average kinetic energy of the free electrons becomes eventually low enough to consider efficient small polaron formation with energy levels close to the bandgap as suggested in [13]. Figure 3(b) reveals that the irradiated material did not reach equilibrium yet after the absorption transients vanished. Further changes are visible until at least τ 10 μs. For τ < 1 μs, a direct interpretation of time-resolved PC pictures is not possible without further processing, because of the strong absorption in the focal region [14]. Starting at τ > 1 μs, the laser irradiated region can be considered as a pure phase object. The time-resolved PCM picture acquired 1 μs after laser excitation shows the onset of a zone of lower refractive index region, labeled I in Fig. 3(b). This lower refractive index shell is consistent with a local density decrease presumably due to the thermomechanical reaction of the material. Based on an analysis developed in [15], we propose the following scenario to explain material depression following fs laser excitation. At the earliest stages of laser–matter interaction, the fs laser pulse acts as a localized heat source inducing a hot pressurized
region. Thermal expansion creates a shell of elastically compressed matter around the hot region and hoop stress builds up. Approaching the microsecond timescale, thermal dissipation starts. The emission of a heat wave from the focal region is accompanied by a pressure drop. Hoop stress release commences. Because of the lower pressure in the cooling region, hoop stress relaxation is favored inward and a highly pressured volume builds up on the optical axis. As a consequence, the region located outside the elastically compressed shell undergoes a transient depression. At τ 10 μs, the high pressure volume starts to relax. A pressure wave is launched. The outward flow of mechanical stress translates into a region of higher density with fuzzy contours, labeled II in Fig. 3(b). The presence of a high density region in the vicinity of the optical axis 10 μs after excitation has important consequences for laser material processing at repetition rates of 100 kHz and higher, as E 0 centers formation is more efficient in densified fused silica [16]. We mentioned earlier that random lasers enable single exposure time–resolved microscopy. We exploit this property to demonstrate time-resolved imaging of laserinduced structural modifications in the multipulse regime. The results are shown in Fig. 4. This regime is particularly interesting because laser direct writing of photonic structures usually involves multipulse accumulation (see for instance [17] and references therein). Although we kept all experimental parameters as before (laser pulse energy of 2.5 μJ, numerical aperture of 0.45, same focusing depth), a comparison between Figs. 3(a) and 4(a) reveals that the morphology of the transient absorption at τ 100 ns is greatly altered in the case of multipulse irradiation. In the single shot mode, the absorption appeared to be quite uniform over all the focal volume. In the multipulse regime, the absorption is limited to a finite number of spots, see for instance region III in Fig. 4. Whether the strong localization of the transient absorption is due to a modified energy deposition or to a faster energy relaxation can not be established solely on the results presented here. Further investigations are necessary to clarify this point. On the time-resolved OTM snapshot acquired in permanent regime, those spots of high transient absorption are still visible. We attribute this feature to the scattering properties of those regions. Intense scattering was confirmed by illuminating the structure with a He–Ne laser propagating collinear with respect to the fs laser. Region IV is also particularly worthy of interest, as several authors observed the onset of
Fig. 4. (a) Optical transmission and (b) PC time-resolved micrographs of the laser-induced objects in the multipulse regime 100 ns after laser excitation (upper image) and after relaxation (lower image). N 100 laser pulses were accumulated.
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
self-organized nanoplanes upon multipulse irradiation at comparable intensities [18]. Time-resolved PCM results [Fig. 4(b)] reveal that at τ 100 ns, the optical properties of region IV are very different from the permanent regime. Some structures of negative refractive index start to be visible, but region IV is still mostly of positive refractive index. Again, a single time-resolved picture does not provide enough information to be conclusive about the underlying physical phenomena leading to the features of region IV as they appear after complete relaxation. Nevertheless, Fig. 4 demonstrates that the possibility to perform single exposure acquisition provides powerful insights into the dynamics of laserinduced material modification, even in the multipulse regime. In conclusion, we have demonstrated that random lasers exhibit simultaneously all the properties of an ideal illumination source for time-resolved microscopy. They are easily triggerable, produce short light pulses (≈8 ns in our case) with a low coherence and a high brightness. The unique combination of these characteristics enabled us to perform time-resolved PCM, a very demanding microscopy technique in terms of source luminance. We studied the formation dynamics of a fslaser-induced microdot in a fused silica substrate at its early formation stages. We established that the transient absorption lasts for at least 100 ns. Transient changes of the refractive index visible in PCM persist at least 10 μs after laser excitation, in good agreement with the thermoelastic response of the substrate. Furthermore, we also showed that the study of laser material interaction can also be carried out in the multipulse regime, thanks to the possibility to perform single exposure acquisition. The authors would like to thank Dr. T. Schultz for providing the pump laser and Dr. N. Zhavoronkov for his support with the fs laser source. This study was funded through the project 16891 BG microdots, financed by the Forschungsvereinigung Feinmechanik Optik und Medizintechnik e.V. (FOM) through the Arbeitsgemeinschaft
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industrieller Forschung (AiF) in the framework of the program “Förderung der industriellen Gemeinschaftsforschung (IGF)” from the ministry for economy and technology (BMWi), in application of a decision from the German parliament, and by the DFG project RO2074/5-3, 5-1. References 1. P. I. Bastiaens and A. Squire, Trends Cell Biol. 9, 48 (1999). 2. D. Young, R. C. Y. Auyeung, A. pique, D. B. Chrisey, and D. D. Dlott, Appl. Phys. Lett. 78, 3169 (2001). 3. M. Brown, N. Kattamis, and C. Arnold, Microfluid. Nanofluid. 11, 199 (2011). 4. J. Bonse, G. Bachelier, J. Siegel, and J. Solis, Phys. Rev. B 74, 134106 (2006). 5. C. Unger, J. Koch, L. Overmeyer, and B. N. Chichkov, Opt. Express 20, 24864 (2012). 6. A. Mermillod-Blondin, C. Mauclair, J. Bonse, R. Stoian, E. Audouard, A. Rosenfeld, and I. V. Hertel, Rev. Sci. Instrum. 82, 033703 (2011). 7. A. Vogel, I. Apitz, S. Freidank, and R. Dijkink, Opt. Lett. 31, 1812 (2006). 8. C. Unger, M. Gruene, L. Koch, J. Koch, and B. Chichkov, Appl. Phys. A 103, 271 (2011). 9. R. Hard, R. Zeh, and R. Allen, J. Cell Sci. 23, 335 (1977). 10. D. S. Wiersma, Nat. Phys. 4, 359 (2008). 11. B. Redding, M. A. Choma, and H. Cao, Opt. Lett. 36, 3404 (2011). 12. B. Redding, M. A. Choma, and H. Cao, Nat. Photonics 6, 497 (2012). 13. P. Audebert, P. Daguzan, A. Dos Santos, J. C. Gauthier, J. P. Geindre, S. Guizard, G. Hamoniaux, K. Krastev, P. Martin, G. Petite, and A. Antonetti, Phys. Rev. Lett. 73, 1990 (1994). 14. T. Noda and S. Kawata, J. Opt. Soc. Am. A 9, 924 (1992). 15. A. Mermillod-Blondin, I. M. Burakov, Y. P. Meshcheryakov, N. M. Bulgakova, E. Audouard, A. Rosenfeld, A. Husakou, I. V. Hertel, and R. Stoian, Phys. Rev. B 77, 104205 2008). 16. C. Itoh, T. Suzuki, and N. Itoh, Phys. Rev. B 41, 3794 (1990). 17. M. Beresna and P. G. Kazansky, Opt. Lett. 35, 1662 (2010). 18. Y. Shimotsuma, P. Kazansky, J. Qiu, and K. Hirao, Phys. Rev. Lett. 91, 247405 2003).
OPTICS LETTERS / Vol. 38, No. 20 / October 15, 2013
Time-resolved microscopy with random lasers Alexandre Mermillod-Blondin,* Heiko Mentzel, and Arkadi Rosenfeld Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, Max-Born-Straße, D-12489 Berlin, Germany *Corresponding author: mermillod@mbi‑berlin.de Received July 19, 2013; revised September 3, 2013; accepted September 4, 2013; posted September 10, 2013 (Doc. ID 194255); published October 9, 2013 We demonstrate that random lasers provide an outstanding strobe light source for time-resolved microscopy. Utilizing a random laser to illuminate a commercially available microscope enables single exposure, speckle-free time-resolved imaging. Aside from conventional optical transmission microscopy, we also perform time-resolved investigations in phase contrast mode. We apply this method to the monitoring of fs-laser-induced microdot formation in bulk a-SIO2 . Time-resolved investigations show that microdot formation lasts over several microseconds after laser excitation. © 2013 Optical Society of America OCIS codes: (180.0180) Microscopy; (140.3390) Laser materials processing; (110.2945) Illumination design. http://dx.doi.org/10.1364/OL.38.004112
Time-resolved optical imaging methods are extensively used for monitoring a wide variety of rapid phenomena. Lifetime imaging microscopy, for instance, is a popular time-resolved microscopy method which enables one to follow intracellular biochemical reactions on the nanosecond timescale [1]. Time-resolved optical methods are also a natural technique for monitoring the dynamics of laser processing during laser-based forward transfer [2], laser-induced microjet formation [3], laser-induced ablation [4], laser-induced melt dynamics [5], or laser-induced refractive-index changes in the volume of transparent materials [6]. The illumination source constitutes the most critical component of any time-resolved imaging system. If some characteristics of the ideal illumination source largely depend on the final application, some other features are universal. Typically, an illumination source suitable for time-resolved imaging should be synchronizable (either by optical delay or triggering electronics), it should be able to generate a short illumination flash (nanosecond to femtosecond duration), it should be uniform, and it should possess a low spatial coherence. Up to the present time, spark lamps and lasers have been employed as illumination sources. Spark lamps possess a very low spatial coherence, providing a speckle-free background illumination. Flash lamps have been successfully used for optical transmission microscopy (OTM) [3] or in Schlieren imagery with low numerical aperture optics [7]. When a better time-resolution and/ or more brightness is necessary, pulsed lasers stand as the most popular solution. However, the nonuniformity of the transverse intensity distribution and the high coherence of laser sources are very detrimental to the final image quality. Beam nonuniformity is usually addressed by expanding a Gaussian beam with the help of a telescope [8] and selecting its central part. High coherence implies that light scattered from objects containing high frequency components (including dust particles) forms a high contrast interference pattern in the image plane [9]. High coherence can be reduced either by propagating through a long multimode fiber [7] or by using beam diffusers [6]. However, propagation inside a long multimode fiber inevitably provokes a substantial pulse broadening (a broadening of 6–18 ns was reported in [7]) and beam diffusers produce a speckled background illumination. 0146-9592/13/204112-04$15.00/0
Speckle contrast can be efficiently diminished by averaging several pictures of the same event acquired with different speckled backgrounds [6], but this approach precludes time-resolved investigations in the single exposure regime. Random lasers are currently a very active research topic (see [10] and references therein). Recently, Cao et al. [11] have demonstrated that random lasers are able to deliver intense and spatially incoherent light pulses. The possibility of using random lasers for imaging has been reported in [12], but in the static regime only. In this Letter, we propose using a random laser as an alternative illumination source for performing stroboscopic microscopy. We aim at monitoring the formation of fs-laser-induced microdots in OTM and phase contrast (PC) microscopy. The time-resolved microscope is depicted in Fig. 1. The fused silica sample, a rectangular parallelepiped polished on all sides, is mounted on a three axis motorized station (not represented in Fig. 1). The microdots are produced by focusing a single fs laser pulse (70 fs duration at FWHM) inside the sample. The pulse energy was 2.5 μJ. The focusing optics is a microscope objective with a numerical aperture of 0.45. We set the focal plane at a depth of 200 μm from the air/dielectric interface in order
Fig. 1. Implementation of the time-resolved microscope for PC investigations. Optical microscopy can be performed by replacing the PC objective by a conventional microscope objective. λ, central wavelength; τ, pulse duration at FWHM; f , focal length; EMCCD, electron multiplying charge coupled device. © 2013 Optical Society of America
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
to minimize wavefront distortion. Time-resolved images in phase-contrast and OTM are acquired at different times τ after fs laser excitation by delaying the emission of the random laser pulse. The timing is set by a pulsed delay generator (SRS DG-535). The random laser employed for illumination is pumped by a frequency doubled YAG laser (8 ns duration, 15 mJ per pulse) focused with a plane convex lens (200 mm focal length). Random lasing takes place in a colloidal solution composed of 250 nm microbeads (at a concentration of ≈1.23 × 1013 ppmL) immersed in a laser dye solution (Rhodamine B, 2.1 g:L−1 dissolved in ethanol). In our pumping conditions (150 mW, 10 Hz), the full width at half-maximum (FWHM) of the random laser spectrum is ≈20 nm. When pumping the same colloidal solution with a modest average power of 5 mW, we measured a FWHM of ≈36 nm. This strong spectrum narrowing is an indication that random lasing takes place [10]. We measured the pulse duration with a fast photodiode and found a pulse width of ≈8 ns, suggesting that the pulse duration is limited by the pulse length of the pump laser. We hypothesize that using a shorter pulse for pumping would lead to the generation of significantly shorter random laser pulses, suitable to study rapid mechanisms such as molecular transitions in biological samples taking place on subnanosecond timescales. A bandpass filter placed in front of the camera rejects light that does not carry useful information, mostly plasma luminescence and light scattered from the exciting fs laser pulse. The spectral response of the bandpass filter matches the emission spectrum of the random laser (central wavelength, 575 nm; width, 25 nm). The influence of plasma luminescence can not be totally suppressed as the emission spectrum of the laser generated plasma is continuous. In Fig. 2, we show micrographs of a laser-induced microdot several hours after formation. The microdot was imprinted with a single laser pulse. The laser pulse comes from the left-hand side of the picture. The image plane corresponds to the plane of incidence of the laser. In order to assess the performance of the random laser illumination source, we compare the images obtained with a LED [Figs. 2(a) and 2(b)], a YAG laser [Figs. 2(c)
Fig. 2. Comparison of optical transmission (left column) and PCMs (right column) of a laser-induced microdot obtained with an illumination source consisting of (a) and (b) a LED, (c) and (d) a YAG laser, and (e) and (f) a random laser RL. The background contrast and the microdot contrast are indicated on each picture.
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and 2(d)], and a random laser [Figs. 2(e) and 2(f)]. The YAG laser beam was phase-randomized and homogenized with the help of a scattering plate. We performed investigations in both OTM and positive phase contrast microscopy (PCM). In positive PCM, pixels darker than the background denote a positive refractive index change whereas pixels brighter than the background indicate a refractive index decrease. We quantifiy the visibility of the laser-induced microdot by measuring the contrast to noise ratio CNR (see [12]) CNR
I¯ − I bkg ; 0.5 × σ I σ I bkg
(1)
where •¯ is the average operator and σ • represents the standard deviation of pixel intensity. A feature is considered visible when its CNR exceeds a value of 1. Figures 2(a) and 2(b) are used here as a reference. They were obtained with LED illumination, not suitable for time-resolved investigations because of their relatively low peak power. Figure 2(b) reveals that the laser-induced microdot is composed of a microvoid prolonged by a filament with contrast values of 8.6 and 2.9, respectively. In OTM [Fig. 2(a)], the microvoid is the only detectable feature, with a contrast of 4.0. Figures 2(c) and 2(d) illustrate the difficulty of performing single shot imaging with a highly coherent illumination source. A Nd:YAG laser was employed in this case. The result is a highly speckled background rendering the microdot invisible. Although Fig. 2(c) was acquired in brightfield mode, the image background is remarkably dark. This feature can be explained by examining the probability density function of the speckle intensity. In the hypothesis of a fully developed speckle, the probability density ¯ exp−I∕I. ¯ Thus, the function pI I reads pI I 1∕I most probable intensity for an arbitrary pixel is zero. The results obtained with random laser illumination are shown in Figs. 2(e) and 2(f). In Fig. 2(e), the microvoid appears with a CNR of 3.1 (4.0 on the LED-illuminated micrograph), confirming the visual impression of similarity with Fig. 2(a). The same applies to PCM [Fig. 2(f)], where the microvoid exhibits a high CNR of 8.2, close to the reference value of 8.6 obtained with the LED illumination. However, the CNR drops to a value of 1.0 in the filamentary region. We assign this low contrast to the performance of our EMCCD in low light conditions. In PCM, about 90% of the illumination light is blocked before the condenser. Therefore, the gray level associated with a positive refractive index change is situated on the bottom part of the EMCCD dynamics, where the dark current and the readout noise play an appreciable role. When acquiring with a LED, increasing the exposure time on the camera suffices to offset the gray level of the darkest features away from the noisy detection region. Increasing the exposure time in the case of a random laser illumination would be counterproductive, as the pulse duration is much shorter than the minimum exposure time (8 ns versus 20 μs, respectively). In Fig. 3, we show time-resolved snapshots of the microdot formation presented in Fig. 2. Again, we conducted our measurements in OTM [Fig. 3(a)] and in PCM [Fig. 3(b)]. The background illumination is in both
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Fig. 3. (a) Optical transmission and (b) PC time-resolved micrographs of the laser-induced microdot shown in Fig. 2. A random laser provided the illumination source.
cases very uniform, and the object of interest stands out with a very satisfying contrast. Time-resolved OTM investigations demonstrate that the absorption region persists until τ 100 ns at least. At first sight, a decay time of the laser-induced plasma spanning over several tens of nanosecond seems in contradiction with the 150 fs attributed to the trapping time of free electrons [13]. However, we emphasize that in our irradiation conditions, the free electron density is close to the critical density ncr 1.74 × 1021 , two orders of magnitude higher than in [13]. At such high densities, the total number of available trapping sites (intrinsic and extrinsic) is probably not sufficient to neutralize the whole free electron population. We can also hypothesize that after multiple collisions, the average kinetic energy of the free electrons becomes eventually low enough to consider efficient small polaron formation with energy levels close to the bandgap as suggested in [13]. Figure 3(b) reveals that the irradiated material did not reach equilibrium yet after the absorption transients vanished. Further changes are visible until at least τ 10 μs. For τ < 1 μs, a direct interpretation of time-resolved PC pictures is not possible without further processing, because of the strong absorption in the focal region [14]. Starting at τ > 1 μs, the laser irradiated region can be considered as a pure phase object. The time-resolved PCM picture acquired 1 μs after laser excitation shows the onset of a zone of lower refractive index region, labeled I in Fig. 3(b). This lower refractive index shell is consistent with a local density decrease presumably due to the thermomechanical reaction of the material. Based on an analysis developed in [15], we propose the following scenario to explain material depression following fs laser excitation. At the earliest stages of laser–matter interaction, the fs laser pulse acts as a localized heat source inducing a hot pressurized
region. Thermal expansion creates a shell of elastically compressed matter around the hot region and hoop stress builds up. Approaching the microsecond timescale, thermal dissipation starts. The emission of a heat wave from the focal region is accompanied by a pressure drop. Hoop stress release commences. Because of the lower pressure in the cooling region, hoop stress relaxation is favored inward and a highly pressured volume builds up on the optical axis. As a consequence, the region located outside the elastically compressed shell undergoes a transient depression. At τ 10 μs, the high pressure volume starts to relax. A pressure wave is launched. The outward flow of mechanical stress translates into a region of higher density with fuzzy contours, labeled II in Fig. 3(b). The presence of a high density region in the vicinity of the optical axis 10 μs after excitation has important consequences for laser material processing at repetition rates of 100 kHz and higher, as E 0 centers formation is more efficient in densified fused silica [16]. We mentioned earlier that random lasers enable single exposure time–resolved microscopy. We exploit this property to demonstrate time-resolved imaging of laserinduced structural modifications in the multipulse regime. The results are shown in Fig. 4. This regime is particularly interesting because laser direct writing of photonic structures usually involves multipulse accumulation (see for instance [17] and references therein). Although we kept all experimental parameters as before (laser pulse energy of 2.5 μJ, numerical aperture of 0.45, same focusing depth), a comparison between Figs. 3(a) and 4(a) reveals that the morphology of the transient absorption at τ 100 ns is greatly altered in the case of multipulse irradiation. In the single shot mode, the absorption appeared to be quite uniform over all the focal volume. In the multipulse regime, the absorption is limited to a finite number of spots, see for instance region III in Fig. 4. Whether the strong localization of the transient absorption is due to a modified energy deposition or to a faster energy relaxation can not be established solely on the results presented here. Further investigations are necessary to clarify this point. On the time-resolved OTM snapshot acquired in permanent regime, those spots of high transient absorption are still visible. We attribute this feature to the scattering properties of those regions. Intense scattering was confirmed by illuminating the structure with a He–Ne laser propagating collinear with respect to the fs laser. Region IV is also particularly worthy of interest, as several authors observed the onset of
Fig. 4. (a) Optical transmission and (b) PC time-resolved micrographs of the laser-induced objects in the multipulse regime 100 ns after laser excitation (upper image) and after relaxation (lower image). N 100 laser pulses were accumulated.
October 15, 2013 / Vol. 38, No. 20 / OPTICS LETTERS
self-organized nanoplanes upon multipulse irradiation at comparable intensities [18]. Time-resolved PCM results [Fig. 4(b)] reveal that at τ 100 ns, the optical properties of region IV are very different from the permanent regime. Some structures of negative refractive index start to be visible, but region IV is still mostly of positive refractive index. Again, a single time-resolved picture does not provide enough information to be conclusive about the underlying physical phenomena leading to the features of region IV as they appear after complete relaxation. Nevertheless, Fig. 4 demonstrates that the possibility to perform single exposure acquisition provides powerful insights into the dynamics of laserinduced material modification, even in the multipulse regime. In conclusion, we have demonstrated that random lasers exhibit simultaneously all the properties of an ideal illumination source for time-resolved microscopy. They are easily triggerable, produce short light pulses (≈8 ns in our case) with a low coherence and a high brightness. The unique combination of these characteristics enabled us to perform time-resolved PCM, a very demanding microscopy technique in terms of source luminance. We studied the formation dynamics of a fslaser-induced microdot in a fused silica substrate at its early formation stages. We established that the transient absorption lasts for at least 100 ns. Transient changes of the refractive index visible in PCM persist at least 10 μs after laser excitation, in good agreement with the thermoelastic response of the substrate. Furthermore, we also showed that the study of laser material interaction can also be carried out in the multipulse regime, thanks to the possibility to perform single exposure acquisition. The authors would like to thank Dr. T. Schultz for providing the pump laser and Dr. N. Zhavoronkov for his support with the fs laser source. This study was funded through the project 16891 BG microdots, financed by the Forschungsvereinigung Feinmechanik Optik und Medizintechnik e.V. (FOM) through the Arbeitsgemeinschaft
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