Biomedical Engineering Reference
In-Depth Information
and digital sensors technology will develop, holographic SHG imaging is expected to become of even
greater interest in the scientific community.
9.1.2 chapter overview
In this chapter, we aboard holographic SHG imaging by first introducing the reader to the basic prin-
ciples of holography. We make the distinction between classical and digital holography and discuss the
opportunities and related drawbacks the digital era has brought to the field. This section is very impor-
tant since we address holographic SHG imaging only through digital and not classical media.
Then, we explain how a holographic SHG setup can be implemented and SHG holograms be digitally
recorded. Discussing the possible setup implementations, we insist on some key components, most espe-
cially light sources and detectors, to explain both why holographic SHG has only recently been made
possible and why it appears more and more appealing. Addressing the core of the technique, we describe
the numerical reconstruction process of holograms that yield both amplitude and phase of the object
wavefront, before commenting further on the types of image contrast accessible with holographic SHG
imaging.
At last, we review a few fields of application where holographic SHG imaging has been reported.
Notably, we describe its application to imaging of biological structures. Also, we explain how retrieval
of 3D images from a single hologram is made possible by the extended depth of field peculiar to digital
holography and, more particularly, how this is of interest for real-time tracking of nanoprobes. Finally,
we also comment on phase conjugation imaging based on holographic SHG characterization of turbid
media.
9.2 Principle of Holography
9.2.1 introduction
Most light detectors, human eye included, are directly sensitive only to the intensity; they neither per-
ceive nor record the phase or the polarization state. While this is of minor importance when working
with incoherent, unpolarized light sources, it is of greater consequences with polarized and/or coherent
light sources, since any phase- or polarization-related information goes missing when light is recorded
by a detector sensitive to intensity only.
Holography is an interferometric method by which the amplitude and phase of light can be recorded
by an intensity-sensitive detector and successively retrieved. The name holography originates from the
Greek
holos
, meaning whole, and
grafe
, meaning writing or drawing. Holography differs from stan-
dard interferometry by being an imaging technique. Its invention is attributed to the Hungarian-British
physicist Dennis Gabor in 1948 (Gabor, 1948) and earned him the Nobel Prize in Physics in 1971.
9.2.2 coherence
As holography requires spatially and temporally coherent light sources, we will introduce the reader
to the notion of coherence. Coherence describes the correlation properties of the physical quantities of
waves. An electromagnetic wave is said to be coherent when there is a fixed phase relationship between
its electric field values at different locations or at different times.
The temporal coherence is a measure of the degree of monochromaticity of a wave. The narrower
the spectral bandwidth of a wave, the higher its temporal coherence will be. Temporal coherence is
measured in time units, and the coherence time is the time interval within which the phase of the
wave is, on average, predictable. Sometimes, however, it is more convenient to quantify the tempo-
ral coherence in distance units. After all, temporal coherence is often measured with a Michelson
interferometer in which the length of one arm is varied. In such cases, the coherence length simply
