One important area of application (and the original motivation for its development) is in resolution enhancement for atomic-scale imaging using X-rays or electrons where the manufacture of good quality lenses is difficult or very expensive.  The technique is now widely employed at X-ray wavelengths where it is close to becoming a standard imaging tool. Electron ptychography has been shown to work with proof-of-principle experiments, but its development is as yet relatively immature.

Although visible light optics has near-perfect focussing lenses, it turns that there are very important areas of application at these wavelengths.  The retrieved ptychographic image has very high phase sensitivity (of the order of 0.01 rad): this signal is quantitative, direct and absolute (not differential) over an unlimited field of view. It therefore has wide applications in biological imaging, especially of unstained, live cells which would otherwise have very low contrast.  It can also image objects that introduce strong phase variations, for example for the characterisation of ophthalmic contact lenses. Other capabilities, such as post-experiment focussing, some degree of three-dimensional imaging, and the ability to have very large working distances are also beneficial.  Visible light ptychographic microscopes are now available commercially.

Research into ptychography is very active, especially in the X-ray community where related diffractive imaging methods have been established for over ten years.  The focus is on algorithm improvements for the reconstruction process and in the development of experimental formats using, for example, poor quality lenses to assist in the data acquisition, the use of beam diffusers to enhance the expression of high-resolution information and the development of super-resolution techniques.

Optical


Figure 1: Phase image of 549 adenocarcinomic human alveolar basal epithelial cells.  Field of view 350mm. Unlike with other methods, such as Zernike phase contrast, the edges of the features are well defined and have high contrast on a substantially flat background.

Given the maturity of visible light optical imaging, it may at first sight seem like ptychography is a very complicated way of doing something that can be done very easily by a lens.  However it turns that ptychography has some unique and important advantages.  Perhaps the most important of these is that the method produces two images.  One corresponds to a map of how much the object has absorbed or scattered the light passing through it.  The second represents the phase that has been introduced into the light.  It is the quality, contrast and quantitative nature of the second signal which is unique to ptychography (see above).  Many specimens of interest – say biological cells – are essentially transparent so that they produce no contrast at all in the conventional bright field image.  Biologists have developed ways of approximately imaging this phase information; say by using a Zernike phase plate or differential phase contrast.  But these methods, although referred to as ‘phase imaging,’ do not produce a quantitative phase map over the entire field of view.  One of the most extreme examples of the power of ptychography is shown below, which is the phase profile of a toric contact lens.

Figure 2:  Phase plot of the whole of a 14mm diameter Menicon eye contact lens.  Colour denotes phase.  The thickest part of this lens is 250mm deep, which, given the refractive index, corresponds to 33 complete phase wraps: a very strong object.  The phase has been unwrapped in the image on the right.

The phase signal is directly proportional to the optical thickness of the lens.  The field of view in this picture is 14mm in diameter – enormous by the standards of conventional imaging – but the phase image is accurately quantitative with optical thickness sensitivity of better than 0.1 µm and does not suffer from any artefacts.

In reflection mode, we have measured phase changes of a fraction of 0.1% of p, corresponding to a height variation of less than one nanometre. The figure below is an image of a semiconductor wafer in reflection mode.

Figure 3:  Phase(colour) and amplitude (hue) image of a semiconductor device seen in reflection mode.  The phase is affected by both the height of the features and their dielectric constant.

It would be wrong to suggest that there are not other interferometric techniques that can deliver surface profile and/or wave phase information.  However, these methods (for example holography or white light profilometry) require exacting specifications on the physical stability of the measuring kit.  In contrast, because ptychography uses diffraction alone – i.e. the specimen itself is the interferometer – it is very insensitive to vibration. 

Because ptychography uses a computer-generated lens, it has some further distinct advantages.  The method can operate at extremely long working distances (5cm or more).  As long as the detector subtends a suitably large effective numerical aperture at the specimen plane, it is possible to obtain high-resolution images through thick sealed bottles, Petri dishes, etc.  Furthermore, once the data has been collected, the user can retrospectively refocus the computational lens, just like turning the focus knob on a conventional microscope.

Figure 4: (A) Modulus of the reconstructed image after 150 iterations: dark features correspond to absorbtion.
(B) Phase of the object corresponding to its optical thickness.
(C) Image obtained by scanning the aperture across the object to the same positions used in the reconstruction, this represents the intrinsic unprocessed resolution of the optical configuration.
(D) Conventional bright field image obtained on a x100 microscope.

This is the first published experimental demonstration of this technique, which used visible wavelength laser light to collect 100 diffraction patterns, in a 10x10 grid, scattered by a red ant mounted on a glass slide. The object is reconstructed in both phase and amplitude, and has a resolution 47 times better than the intrinsic optical resolution of the system, dictated by the diameter of the illuminating spot.

Electron

Figure 5: FeNi particles imaged with the TEM bright field (left) and a Ptychographic reconstruction: Amplitude (centre) and Phase (right)

Example Images

The following images show the amplitude and phase of various objects, with amplitude represented by brightness and phase represented by hue:

Lily pollen	Butterfly wing	Teardrop crystals

Contact lens	iii-v wafer in 3D