Sub-Wavelength THz Imaging of the Domains in Periodically Poled Crystals Through Optical Rectification
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Sub-Wavelength THz Imaging of the Domains in Periodically Poled Crystals Through Optical Rectification Gizem Soylu 1 & Emilie Hérault 1 & Benoît Boulanger 2 & Fredrik Laurell 3 & Jean-Louis Coutaz 1 Received: 30 September 2019 / Accepted: 12 May 2020/ # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
By focusing a femtosecond laser beam onto the lateral face of a periodically poled KTP crystal, we generate a THz pulse through optical rectification of the laser pulse. The THz signal is recorded by means of a common THz time-domain technique, which allows us to obtain both the magnitude and sign of the THz waveform, and then the magnitude and phase of its spectral components thanks to a numerical Fourier-transform. By moving the laser beam along the crystal, we record a THz signal that renders for the alternative orientation of the crystal domains, with a lateral resolution as good as 10 μm, whatever the THz wavelength. This demonstrates the potential of the Optical Rectification TeraHertz Imaging (ORTI) technique to produce sub-wavelength THz images. Keywords THz microscopy . Optical rectification . Periodically poled crystals . THz time-domain spectroscopy
1 Introduction Performing microscopy in the terahertz (THz) range of the electromagnetic spectrum, i.e., recording images with a micron or sub-micron lateral resolution is a difficult task. Classical methods are limited by diffraction to the order of the THz wavelength, i.e., around 100 μm [1, 2]. Today, most of the THz sub-wavelength imaging systems are based on near-field techniques. Basically, images are recorded by either super-transmission or scanning near-field optical microscopy (SNOM). Super-transmission is achieved by placing and scanning a thin
* Jean-Louis Coutaz coutaz@univ–smb.fr
1
IMEP-LAHC, University Savoie Mont Blanc, 73376 Le Bourget du Lac Cedex, France
2
CNRS, Grenoble INP, Institut Néel, Université Grenoble Alpes, 38000 Grenoble, France
3
Department of Applied Physics, Royal Institute of Technology, Roslagstullsbacken 21, 10691 Stockholm, Sweden
Journal of Infrared, Millimeter, and Terahertz Waves
sample over a metal sheet in which a micron-diameter hole has been drilled. The sample is illuminated by a THz beam, and the signal is recorded by a receiver located just after the hole in the near-field region. A lateral resolution of a few micrometers is achieved [3] but the recording time is very long because of the weak transmission. In the SNOM technique, the signal of interest is the impinging THz probe beam that is resonantly diffracted by the edge of a nanometer probe. To get the image, the probe is scanned at a near-field distance over the sample [4]. Such a technique gives an impressive nanometer resolution [5]. Recently, amazing results have been published by the team of Komiyama [6, 7], who produced a THz image with the same technique, but the THz signal was emitted by the sample itself as a part of its thermal radiation: The weakness of such signal was then overcome by using an extremely sensitiv
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