Optical coherence tomography (OCT) has developed rapidly since its first realisation in medicine and is currently an emerging technology in the diagnosis of skin disease. OCT is an interferometric technique that detects reflected and backscattered light from tissue and is often described as the optical analogue to ultrasound. The inherent safety of the technology allows for in vivo use of OCT in patients. The main strength of OCT is the depth resolution. In dermatology, most OCT research has turned on non-melanoma skin cancer (NMSC) and non-invasive monitoring of morphological changes in a number of skin diseases based on pattern recognition, and studies have found good agreement between OCT images and histopathological architecture. OCT has shown high accuracy in distinguishing lesions from normal skin, which is of great importance in identifying tumour borders or residual neoplastic tissue after therapy. The OCT images provide an advantageous combination of resolution and penetration depth, but specific studies of diagnostic sensitivity and specificity in dermatology are sparse. In order to improve OCT image quality and expand the potential of OCT, technical developments are necessary. It is suggested that the technology will be of particular interest to the routine follow-up of patients undergoing non-invasive therapy of malignant or premalignant keratinocyte tumours. It is speculated that the continued technological development can propel the method to a greater level of dermatological use.

https://link.springer.com/content/pdf/bfm%3A978-3-319-06419-2%2F1.pdf

Optical Coherence Tomography

We summarize the problem of measuring an ultrashort laser pulse and describe in detail a technique that completely characterizes a pulse in time: frequency-resolved optical gating. Emphasis is placed on the choice of experimental beam geometry and the implementation of the iterative phase-retrieval algorithm that together yield an accurate measurement of the pulse time-dependent intensity and phase over a wide range of circumstances. We compare several commonly used beam geometries, displaying sample traces for each and showing where each is appropriate, and we give a detailed description of the pulse-retrieval algorithm for each of these cases.

Measuring ultrashort laser pulses in the time-frequency domain using frequency-resolved optical gating

http://nathan.instras.com/documentDB/paper-69.pdf

Porous silicon: a quantum sponge structure for silicon based optoelectronics

Current experimental and theoretical progress toward the goal of controlling quantum dynamics is summarized. Two key developments have now revitalized the field. First, appropriate ultrafast laser pulse shaping capabilities have only recently become practical. Second, the introduction of engineering control concepts has put the required theoretical framework on a rigorous foundation. Extrapolations to determine what is realistically possible are presented.

https://science.sciencemag.org/content/sci/259/5101/1581.full.pdf

Coherent control of quantum dynamics: the dream is alive.

An optical coherence tomography system is described which can image up to video rate. The system utilizes a high power broadband source and real time image acquisition hardware and features a high speed scanning delay line in the reference arm based on Fourier-transform pulse shaping technology. The theory of low coherence interferometry with a dispersive delay line, and the operation of the delay line are detailed and the design equations of the system are presented. Real time imaging is demonstrated in vivo in tissues relevant to early human disease diagnosis (skin, eye) and in an important model in developmental biology (Xenopus laevis).

In vivo video rate optical coherence tomography.

The dynamical properties of a third-order nonlinear optical process, in which two incident light pulses with wave vectors k 1 and k 2 generate a light wave with 2 k 2 - k 1 in resonant media, are discussed with particular attention to the effect of transverse relaxation time T 2 . It is shown that the correlation trace (output light energy vs incident pulse separation) for this process provides a general means of obtaining information on ultrashort T 2 for both the homogeneously and inhomogeneously broadened transitions, and for both t p (pulse width) T 2 . An experimental demonstration for t p > T 2 is given by using a dye molecule and a picosecond light source.

Spatial Optical Parametric Coupling of Picosecond Light Pulses and Transverse Relaxation Effect in Resonant Media

A new method of picosecond and femtosecond transient spectroscopy yielding information about ultrafast relaxation processes without requiring ultrashort light pulses is presented. It is based on the present theoretical analysis of the resonant degenerate four-wave-mixing process excited by two temporally incoherent light beams with wave vectors ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{1}$ and ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{2}$ which are originated from a single beam at frequency $\ensuremath{\omega}$ but have mutual time delay $\ensuremath{\tau}$. Under the assumption that the incoherent light field has Gaussian random complex amplitude and that the resonant material consists of the usual two-level atoms, the statistically averaged intensity of the output light field with ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{3}=2{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{2}\ensuremath{-}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}}_{1}$ at $\ensuremath{\omega}$ is calculated as a function of $\ensuremath{\tau}$. Even with the light having a much longer duration than both ${T}_{1}$ (the longitudinal relaxation time) and ${T}_{2}$ (the transverse relaxation time), the correlation trace, i.e., output intensity versus $\ensuremath{\tau}$, represents a decay profile determined mainly by ${T}_{2}$ for both homogeneously and inhomogeneously broadened transitions, as long as the light correlation time ${\ensuremath{\tau}}_{c}$ is much shorter than the relaxation times. The correlation trace does not always represent a single-exponential decay but is sometimes slightly deformed by the ${T}_{1}$ effect. However, it does not cause a significant error in the determination of ${T}_{2}$. Moreover, as $\frac{{T}_{2}}{{T}_{1}}\ensuremath{\rightarrow}0$, the trace becomes a single-exponential decay curve determined only by ${T}_{2}$. The feature of the results obtained by the present method is similar to that obtained by the conventional coherent transient spectroscopy with short pulses, such as the photon echo. The time resolution in the present method, however, is limited only by ${\ensuremath{\tau}}_{c}$ much shorter than the light duration. By regarding the incoherent light as a series of random ultrashort pulses, the present four-wave-mixing process is also interpreted as the ensemble of numerous transient four-wave-mixing processes caused by various combinations of these pulses.

Ultrahigh-time-resolution coherent transient spectroscopy with incoherent light

A comprehensive theory of a new type of three-wave optical mixing (${\ensuremath{\omega}}_{3}=2{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$) is given for the spectroscopic purpose of determining ultra-short relaxation times (picoseconds or less) associated with excited states of condensed matter. Two incident light frequencies ${\ensuremath{\omega}}_{1}$ and ${\ensuremath{\omega}}_{2}$ are chosen so that they are both resonant with an inhomogeneously broadened optical transition and $|{\ensuremath{\omega}}_{2}\ensuremath{-}{\ensuremath{\omega}}_{1}|$ is in the vicinity of inverse relaxation times. The theory is developed for its major application to broad-band electronic transitions. The nonlinear susceptibility ${\ensuremath{\chi}}_{R}^{(3)}$ for this process (named resonant Rayleigh-type mixing) is calculated by a density-matrix formalism with the model of a two-level atomic system incorporating the distribution of resonance frequencies and longitudinal (${T}_{1}$) and transverse (${T}_{2}$) relaxation times. The result shows a frequency characteristic depending only on ${T}_{1}$, ${T}_{2}$, and ${\ensuremath{\omega}}_{2}\ensuremath{-}{\ensuremath{\omega}}_{1}$ in a broad-band limit, which serves for the determination of ${T}_{1}$ and ${T}_{2}$ in the frequency domain. The analysis is further extended to include the various effects as follows. The calculation of the saturation effect in the lowest order reveals that it modifies the shape of the frequency characteristic of the nonlinear susceptibility but does not affect seriously the determination of relaxation times. The effect of spectral cross relaxation within the inhomogeneous broadening is incorporated by a generalized density-matrix formalism. The resultant ${\ensuremath{\chi}}_{R}^{(3)}$ consists of two terms. The dominant term is the same as before except that ${T}_{1}$ is replaced by a combined relaxation time ${T}_{1}^{\ensuremath{'}}={({T}_{1}^{\ensuremath{-}1}+{T}_{3}^{\ensuremath{-}1})}^{\ensuremath{-}1}$, where ${T}_{3}$ is the cross-relaxation time. The other term originates from the inverse spectral-diffusion process. The effect of other energy levels located between the two levels under study is analyzed with a simple three-level model. The resultant ${\ensuremath{\chi}}_{R}^{(3)}$ also consists of two terms, the dominant one being the same as that for the two-level model except that ${T}_{1}^{\ensuremath{-}1}$ should be interpreted as total population decay rate of the upper level. For accurate derivation of ${\ensuremath{\chi}}_{R}^{(3)}$ from experiment, we discuss the interference between the resonant and nonresonant susceptibility terms and the light-wave propagation effect. Finally, the close analogy between this type of optical mixing and the photon echo is discussed.

Study of ultra-fast relaxation processes by resonant Rayleigh-type optical mixing. I. Theory

The three-wave optical mixing (${\ensuremath{\omega}}_{3}=2{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$) of the resonant Rayleigh type ($|{\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}|\ensuremath{\simeq}\mathrm{inverse}\mathrm{relaxation}\mathrm{time}$, ${\ensuremath{\omega}}_{1} \mathrm{and} {\ensuremath{\omega}}_{2}\ensuremath{\simeq}\mathrm{optical}\mathrm{transition}\mathrm{frequency}$) is applied to measure the subpicosecond relaxation times associated with the ${S}_{0}\ensuremath{\rightarrow}{S}_{1}$ electronic transition of 1,1'-diethyl-4,4'-quinocyanine iodide in ethanol and other dye solutions. Two independently tunable dye lasers excited by a nitrogen laser were used as light sources. The measured frequency responses of the nonlinear susceptibility $|{\ensuremath{\chi}}^{(3)}|$ as a function of ${\ensuremath{\omega}}_{1}\ensuremath{-}{\ensuremath{\omega}}_{2}$ were in good agreement with the theoretical formula based on an inhomogeneously broadened two-level model incorporating the phenomenological relaxation times. We also observed the interference effect between the resonant and nonresonant terms of the nonlinear susceptibility, which agree well with the theory. Thus we were able to determine the transverse relaxation time ${T}_{2}$ in the range of 0.02 \ensuremath{\sim} 2 psec and the effective relaxation time ${T}_{1}^{\ensuremath{'}}$ (a combination of longitudinal- and cross-relaxation times) in the range of 0.1 \ensuremath{\sim} 5 psec. These values were found to be largely dependent on the spectral position inside the broad absorption band and the sample preparation and deterioration, as well as on the kinds of dye materials and solvents. Details of the experimental procedure and a discussion of the interpretation of the experimental results are given.

Study of ultra-fast relaxation processes by resonant Rayleigh-type optical mixing. II. Experiment on dye solutions

Excess noise has been studied on narrow alloyed p - n junctions fabricated on heavily doped p -type and n -type germanium with less than 200 A junction width, which exhibit inverted rectification and negative resistance in the forward direction. Considerable 1/ f noise has been observed in a forward range, the character of which is stable and surface insensitive. Moreover, the noise was found to be mainly associated with the excess forward current that is peculiar to narrow p - n junctions. Measurements were made also at low temperatures down to 77°k on both noise and static characteristics. The origins of the excess forward current and the associated excess noise were discussed in connection with the crystal imperfection.

Tatsuo Yajima

Papers

Spatial Optical Parametric Coupling of Picosecond Light Pulses and Transverse Relaxation Effect in Resonant Media

Ultrahigh-time-resolution coherent transient spectroscopy with incoherent light

Study of ultra-fast relaxation processes by resonant Rayleigh-type optical mixing. I. Theory

Study of ultra-fast relaxation processes by resonant Rayleigh-type optical mixing. II. Experiment on dye solutions

Excess Noise in Narrow Germanium p-n Junctions