3D imaging by low one-photon absorption technique

Summary form only given. A new method for 3D imaging based on low one-photon absorption is theoretically and experimentally demonstrated. As compared to the two-photon-absorption (TPA) technique, this method is suitable using a continuous laser or an incoherent light. Nano-imaging and fabrication based on far-field techniques have been studied over long decades. Thanks to a confocal system using high numerical aperture objective lens, light can be tightly focused into a focal spot, whose size approaches the so-called of incident light and NA is the numerical aperture of the objective lens. Two excitation processes, namely onephoton (OPA) and two-photon absorption (TPA), based on different photonic mechanisms, can be used, depending of the desired application. For applications in which a thin film is used, it is very convenient to use the OPA excitation method by employing a simple and low cost laser in UV range. For three-dimensional imaging or fabrication, TPA technique is usually utilized [1, 2]. To realize such a TPA process, two conditions are required, such as a pulsed femtosecond or nanosecond laser, providing a high peak intensity and a tightly focused laser beam. Indeed, due to the quadratic dependence of excitation intensity, only the central part of the focusing spot, where light intensity is high enough, could induce TPA phenomena. B y moving this focusing spot, free 3D sampling, imaging or fabrication, can be realized. However, since this technique requires the use of imaging in living cell and the optical system is also quite expensive. simple but very efficient way, which allows to combine the respective diffraction limit, which is defined by 0.61λ/NA, where λ is the wavelength a pulsed laser, it is not adequate for In this work, we demonstrate a advantages of these two techniques . Namely, we exploit the OPA technique, but in our case the wavelength is located in the edge part of the material absorption. Thank to a very low absorption- effect, this technique, called LOPA (low one-photon absorption), presents a great advantage. Indeed, it requires only a simple continuous laser, as used in the case of OPA, and it allows 3D addressing, similar to the case of TPA. To demonstrate this idea, we employ Rhodamine 6G dissolved in ethanol as a target sample. The absorption of this sample is shown in Fig. 1(a). Figure 1(b) shows the simulation of the focusing beam inside the solution taking into account the absorption effect of the Rh6G material. It is clear that due to the strong absorption, the incoming light at 532 nmwavelength (or at 550 nm wavelength) cannot be deeply focused inside the solution. In contrast, by using a light beam whose wavelength is far from a high absorption regime, for example at 633 nm-wavelength, the excitation beam can be focused with almost the same form at any position inside the solution (Fig . 1(b4).). This focusing spot therefore allows to address freely 3D structures without any attenuation effect, exactly same as for the results obtained by TPA (1064 nm-wavelength), shown in Fig. 1(b3). This theoretical prediction has been compared with experimental results using a high NA and a long working distance objective lens, showing an excellent agreement. The great advantage of this technique is that it can be realized with very low intensity (only few microWatt) light sources, such as a continuous laser or even an incoherent light source.