Angle-based Wavefront Sensor Research Tackles the Near-field Effect in Flat Optics

November 07, 2021 by Abdulwaliy Oyekunle

University of Wisconsin-Madison researchers have introduced a novel angle-based sensor made from exploring the near-filed effect of flat optics, which claims could help create atomic-level measurements.

There’s a need to always look into the fine details—the tiny measurements—of materials on an atomic level, especially as technology continues to shrink. Interferometry, such as white light interferometry and angle-based sensing, are two approaches used to indirectly measure the wavefront of light.

To better understand this need, this article will dive into the concept of white light interferometers and angle-based sensors, their challenges, and how a new angle-based wavefront sensor from the University of Wisconsin-Madison aims to solve these challenges. 


Interferometers and Wavefront Light

White light interferometers and angle-based sensors such as the Shack-Hartmann wavefront sensor offer a spatial resolution to measure sample material features in nanometers. In addition, they are used in wavefront sensing as well as medical imaging. 

An interferometer as an optical device is adopted in a wide range of applications due to its high detection sensitivity. For instance, when a light wave travels through a transparent medium, an interferometer can measure the tiny refractive index changes associated with the wave. 

Interferometry is based on the principles of measuring and compensating the aberrations in the resulting phase of a light beam or the gradient of the phase when interference occurs between a primary incident light wave and a reference light wave.


An illustration of the basics of an inference wavefront sensor. White light interferometry is an example of an interference-based wavefront sensor.

An illustration of the basics of an inference wavefront sensor. White light interferometry is an example of an interference-based wavefront sensor. Image used courtesy of Soongyu Yi

The reference light wave could however be a replica of the primary incident light wave. A broadband light source from a superluminescent diode is a good light source that can be employed in a white light interferometer.


Contrast Between Interferometers and Angle-based Sensor

Interferometry has come into play in several scientific discoveries. One of which is the detection of gravitational waves using a laser interferometer. Interferometer offers high spatial resolution and accuracy compared to an angle-based Shack-Hartmann sensor. Moreover, the Shack-Hartmann wavefront sensor may be used to determine the beam quality of a laser.

One limitation of an interferometer is that measurements may be affected by mechanical noise or noise from the light source.

An angle-based sensor such as the Shack-Hartmann sensor makes use of micro-lenses to sample the incident angles of light waves on a set of grid points. These angles are combined and further evaluated to determine the wavefront of the light waves.


Illustration of an angle-based wavefront sensor.

Illustration of an angle-based wavefront sensor. Image used courtesy of Soongyu Yi


Though angle-based sensors are widely used in adaptive optics such as ocular diagnostics, astronomical telescopes, and so on, their limitations include a low spatial resolution, which can be attributed to the large size of the detector lenses.

Hoping to overcome some of these limitations, researchers have created a new angle-based sensor.


Solving Spatial Resolution in Angle-based Wavefront Sensors

Researchers from the Electrical and Computer Engineering Department at the University of Wisconsin-Madison recently designed an angle-based wavefront sensor that aims to solve this challenge of measuring at microscopic levels. 

The sensor, made from flat optics, has a high spatial resolution and a wide dynamic range. The researchers manipulated the near-field effect of the flat optics to create a sensor that takes tiny measurements of sample materials at 30 frames per second.


Image of the fabricated angle-based wavefront sensor. Image used courtesy of Soongyu Yi


Using a monochrome complementary metal-oxide-semiconductor (CMOS) sensor, the researchers used photolithography to fabricate the angle sensor. This technique formed an array of square patterns on a negative tone photoresist layer of the CMOS sensor, and each square pattern, together with its four pixels, makes up one angle sensor. 

Consequently, a large array of angle sensors eventually resulted in a wavefront sensor with a very high spatial resolution.

This novel sensor finds application in quantitative phase imaging. Additionally, the sensor claimed to improve sampling density and angular dynamic range compared to a conventional Shack-Hartmann wavefront sensor.

Though measuring at microscopic levels quantitatively still proves difficult, this new device is hopeful that users can reveal the fine details in atomic levels of sample materials.



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