1. Fourier Ptychographic Microscopy at Telecommunication Wavelength Using a Femtosecond Laser. Ishtiaque Ahmeda,c ,Maged Alotaibia,c, Sueli Skinner-Ramosa,c, Daniel Domingueza,c,Ayrton A. Bernussi b,c, Luis Grave de Peraltaa,c a Department of Physics and Astronomy, Texas Tech University, Lubbock, TX 79409, USA bDepartment of Electrical and Computer Engineering, Texas Tech University, Lubbock, TX 79409, USA cNano Tech Center, Texas Tech University, Lubbock, TX 79409, USA
Abstract
We report the successful implementation of Fourier Ptychographic Microscopy (FPM) in the telecommunication wavelength using an ultrafast, low coherence laser. High resolution with reduced near speckle-free images were obtained using the proposed approach. We also demonstrate that FPM can be used to image periodic structures through a silicon wafer.
Results
A clear real plane (RP) image of the sample with period 𝑝 = 10 μm is shown in Fig. 2(a). In excellent correspondence, several diffraction spots separated by 𝜆∕𝑝 = is observed in the Fourier plane (FP) image shown in Fig. 2(b). The speckle contrast ratio (SC) in the experimental RP were determined as 𝑆𝐶 = 0.28 and 0.95 for Figs. 2(a) and 2(c) confirming the reduced speckle noise. The periodic lines of the Ronchi-Ruling are clearly observed in the obtained intensity and phase distributions shown in Figs. 3(a) and 3(b) using the FPM technique. This is in excellent correspondence with the determined separation 𝜆∕𝑝=0.156 between consecutive diffraction spots observed in the synthetic FP image with 𝑁𝐴𝑠 = 1.18 shown in Fig. 3(c). The line periodicity of the Ronchi-Ruling glass slide is also clearly observed in the intensity, phase distribution, and obtained synthetic FP images shown in Figs. 4(a), 4(b) and 4(c) through a ~375 μm thick silicon wafer using the FPM technique [6].
Introduction
The Fourier Ptychographic Microscopy (FPM) technique was originally demonstrated using planar [1–3] and hemispherical arrays of light emitting diodes (LED) emitting visible light. FPM permits to obtain high-resolution images from a set of low-resolution images captured by illuminating the sample from a different direction each time [1–5]. The images of the sample obtained in a regular microscope are maps of the intensity distribution. However FPM can also recover the unmeasured phase[1- 5]. Prospective applications of the FPM technique involve illumination sources at the telecommunication wavelengths (1.3–1.55 μm range), particularly for use in inspection and metrology in the semiconductor industry. It should be noted that at these wavelengths silicon and other semiconductor substrates are transparent. This makes it possible to image features through the substrate at this spectral range.
Fig. 1. (a) Schematic illustration of the experimental setup used to implement the FPM technique at telecommunication wavelengths. (b) Photograph of the HLG collimator ensemble.
Conclusions
We reported the first implementation of the FPM technique at telecommunication wavelengths using an ultrafast pulsed laser. The low coherence of the illumination source allowed for obtaining high quality and near speckle-free images of a transparent sample composed of periodic features. As a proof-of-concept, we demonstrate the use of FPM for imaging and phase-recovery of the same periodic sample but placed underneath a silicon wafer.
Fig. 2. Experimental (a) RP and (b) FP images obtained illuminating the sample from a direction with 𝑁𝐴𝑐 = The arrow in (b) indicates the zero-order diffraction spot. (c) Experimental RP image of a Ronchi-Ruling with 𝑝 = 1.7 μm obtained using a 570 nm CW laser for comparison purposes.
Acknoledgement
This work was partially supported by the National Science Foundation (NSF) Award (ECCS ).
Methods
The schematic of the FPM setup is shown in Fig. 1(a). We have used a Nikon Eclipse Ti inverted microscope with a 0.65 numerical aperture (𝑁𝐴𝑜) objective lens, an infrared (IR) camera to acquire the images, and a 200 fs fiber-laser centered at 𝜆 =1560 nm wavelength which output was coupled to a fiber-optic collimator. The collimator was inserted manually into the multiple holes of the hemispherical laser guider (HLG) (see Fig. 1(b)) to illuminate the sample at different directions. A commercial Ronchi-Ruling with period 𝑝 = 10 μm was used as the sample to be imaged in our experiments.
Fig. 3. (a) Intensity and (b) phase corresponding to the synthetic RP image, and (c) synthetic FP image with 𝑁𝐴𝑠 = 1.18 that were obtained by processing the experimental RP images(Fig. 2) using the FPM algorithm.
References
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Fig. 4. (a) Intensity and (b) phase corresponding to the synthetic RP image, and (c) synthetic FP image with 𝑁𝐴𝑠 = 1.18 that were obtained using the FPM algorithm and the experimental RP images (Fig. 4) that were obtained through the Si wafer.