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Highly Efficient Beamsplitter for THz Uses Artificial Dielectrics

Photonics.com
Jul 2017
PROVIDENCE, R.I., July 24, 2017 — A novel technique for manipulating the polarization of light at terahertz (THz) frequencies uses stacks of closely spaced steel plates to make a polarizing beamsplitter (PBS). Each pair of plates forms a parallel-plate waveguide. When THz light is shined on the stack at a 45-degree angle, it splits the beam by exciting two waveguide modes. One beam of vertically polarized light passes straight through the device, while another beam of horizontally polarized light is reflected in a 90-degree angle from the original beam axis.

PBS for THz region, Brown and Osaka Universities.
Researchers have used a new technique to create a polarizing beamsplitter for terahertz waves. Courtesy of Mittleman Lab/Brown University.

The approach is based on an artificial dielectric medium — a uniformly spaced stack of metal plates that is the electromagnetic equivalent to a stacked array of parallel-plate waveguides — that is scalable to a range of frequencies. The operation of the device relies on the lowest-order, transverse-electric and transverse-magnetic modes of the parallel-plate waveguide. This is in contrast to existing methods of manipulating polarization in the THz range, which rely primarily on the transverse electric mode.

The PBS demonstrated extinction ratios as high as 42 decibels (dB) in transmission and 28 dB in reflection with overall insertion loss of 0.18 dB.

Building on their experimental PBS, the research team, from Brown and Osaka Universities, added a second similar artificial dielectric structure to demonstrate a THz isolator with nonreciprocal transmission, providing an isolation of 52 dB and an insertion loss less than one dB, at a frequency of 0.46 THz. The performance of the team’s isolator exceeded that of other experimentally demonstrated THz isolators, and came close to the performance of commercially available isolators for optical wavelengths.

PBS for THz region, Brown and Osaka Universities.
Researchers have developed a new device that can split a beam of terahertz radiation by polarization state. Courtesy of Mittleman Lab/Brown University.

The waveguide-based artificial dielectrics are low loss, inexpensive and easy to fabricate, making this approach a potentially promising route for polarization control of free-space Thz beams.

“This stack-of-plates idea has advantages over traditional methods of manipulating polarization in the terahertz region,” said professor Dan Mittleman. “It’s cheaper and physically more robust than other methods.”

The plates are also much less fragile than traditional wire filters. Polarizing filters for THz are typically an array of metal wires a few microns in diameter and spaced several microns apart.

“The artificial dielectric concept also makes the device more versatile,” professor Rajind Mendis said. "The device can be easily tuned for use at different terahertz frequencies simply by changing the size of the spacers separating the plates or by changing the illuminating angle."

Such a beamsplitter could be useful in a wide variety of systems that use THz radiation, from microscopy to future communications networks. Use of THz waves in technological applications such as spectroscopy, sensing, imaging and ultrahigh-bandwidth communications is growing, and researchers are working to develop the hardware components necessary to build these advanced THz systems.

The researchers are in the process of patenting the new artificial dielectric devices, and are hopeful that these devices will enable the development of new THz systems with far better capabilities.

“In anything you might want to do with an optical system, it's useful to be able to manipulate polarization,” Mittleman said. “This is a simple, efficient, effective and versatile way to do that.”

The research was published in Scientific Reports (doi:10.1038/s41598-017-06297-7)


GLOSSARY
beamsplitter
An optical device for dividing a beam into two or more separate beams. A simple beamsplitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A more sophisticated type consists of two right-angle prisms cemented together at their hypotenuse faces. The cemented face of one prism is coated, before cementing, with a metallic or dielectric layer having the desired reflecting properties, both in the percentage of reflection...
optoelectronics
A sub-field of photonics that pertains to an electronic device that responds to optical power, emits or modifies optical radiation, or utilizes optical radiation for its internal operation. Any device that functions as an electrical-to-optical or optical-to-electrical transducer. Electro-optic often is used erroneously as a synonym.
polarization
With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. In a beam of electromagnetic radiation, the polarization direction is the direction of the electric field vector (with no distinction between positive and negative as the field oscillates back and forth). The polarization vector is always in the plane at right angles to the beam direction. Near some given stationary point in space the polarization direction in the beam...
Research & TechnologyAmericaseducationAsia-PacificCommunicationsterahertzbeamsplitterterahertz opticsoptoelectronicsPbSdielectricspolarizationfree space optics

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