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Spotlight on Optics


  • May 2014

Optics InfoBase > Spotlight on Optics > Achromatic prism-type wave plate for broadband terahertz pulses

Achromatic prism-type wave plate for broadband terahertz pulses

Published in Optics Letters, Vol. 39 Issue 9, pp.2794-2797 (2014)
by Yoichi Kawada, Takashi Yasuda, Atsushi Nakanishi, Koichiro Akiyama, Kento Hakamata, and Hironori Takahashi

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Spotlight summary: Terahertz (THz) optics and technologies are rapidly evolving areas of research now that reliable methods to generate broadband pulses are available. Among the many unusual characteristics of terahertz optics is the characteristics wavelength λ=300μm, the vast range of wavelengths in a unique pulse (30μm-3mm), and the ability to directly measure temporally evolving electric fields with synchronous detection. Yet, designing basic optical setups for the complete pulse spectrum still requires new developments. In particular, polarization control is required to achieve functional imaging/optics/telecommunication setups and applications.

The authors of this Optics Letters article propose new designs for terahertz achromatic quarter- and half-wave plates based on phase retardation by total internal reflection. Exploiting well-known Fresnel phase difference between s- and p-polarized light due to total internal reflection, the authors use rhomb prisms to achieve the required polarization control. The phase difference between the components depends only on the angle of incidence and the refractive index of the prism, so as long as the index does not vary significantly with frequency the effect is achromatic. Thus, by adjusting the number of internal reflections in the prism, the authors can control the plate to be either a half-wave plate or a quarter-wave plate. As the prism does not rely on birefringence to achieve quarter/half wave retardation, there is no time delay, hence leading to identical waves for ±λ/4 polarization except for the rotation direction.

In terahertz optics, large aperture beams are often required for propagation or imaging, the consequence being the need of using very large prisms. The authors resolve this issue by using Multi Stacked Prisms (MSP) and adjusting them to produce the desired plate. Finally, the authors apply their setup to a toy experiment by imaging two bundles of hair overlaid in the shape of a cross, proving the efficiency of a MSP to image selectively the two directions of the hair bundles.

--Jean-Baptiste Masson

Technical Division: Light–Matter Interactions
ToC Category: Physical Optics
OCIS Codes: (230.5440) Optical devices : Polarization-selective devices
(260.3090) Physical optics : Infrared, far
(260.5430) Physical optics : Polarization
(300.6495) Spectroscopy : Spectroscopy, teraherz
(110.6795) Imaging systems : Terahertz imaging

Posted on May 19, 2014

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