An on-chip wavefront image sensor capable of measuring phase gradients would benefit a wide variety of applications, including phase contrast microscopy, adaptive optics and machine vision. Phase measurements are usually performed using Shack–Hartmann sensors, but the cost and size of these devices precludes their use in many applications.

Xiquan Cui and co-workers from Caltech and Harvard Medical School in the USA have now realized a cost-effective phase sensor on a semiconductor chip that may potentially suit mass-production (Opt. Express 18, 16685–16701; 2010). The integrated device is made from a 280 x 350 array of circular apertures (each 6 μm in diameter) on top of a metal-coated CMOS sensor chip and has a sensing area of 3.08 mm × 3.85 mm. The metal coating is a 150-nm-thick layer of aluminium and the chip is comprised of 1,944 × 2,592 pixels, each 2.2 μm long.

A transparent spacer made from 10-μm-thick SU8 resin separates the apertures from the sensor. A spot forms under each aperture when a planar light wave is incident on the sensor. However, if the incoming wave is phase-tilted or distorted, the position of the spots changes according to the local phase gradient. The phase profile of a beam can then be determined by analysing the change in position.

Credit: © 2010 OSA

Tests suggest that the sensor has a phase gradient sensitivity of 0.1 mrad and a measurement range of ±15 mrad. The researchers used the sensor in a microscope to generate high-quality phase gradient images of potato starch and starfish embryos. They say that the sensor can be easily adapted to fit most standard microscope systems without any major modification, and that it should be possible to mass-produce cheaply in large quantities, similar to how commercial image sensor chips are fabricated for use in digital cameras. They also comment that the phase images produced using their technique rival those captured by traditional differential interference contrast microscopes and are not prone to artefacts generated by birefringence.