This review is aimed at presenting the state-of-the-art of time domain (TD) functional near-infrared spectroscopy (fNIRS). We first introduce the physical principles, the basics of modeling and data analysis. Basic instrumentation components (light sources, detection techniques, and delivery and collection systems) of a TD fNIRS system are described. A survey of past, existing and next generation TD fNIRS systems used for research and clinical studies is presented. Performance assessment of TD fNIRS systems and standardization issues are also discussed. Main strengths and weakness of TD fNIRS are highlighted, also in comparison with continuous wave (CW) fNIRS. Issues like quantification of the hemodynamic response, penetration depth, depth selectivity, spatial resolution and contrast-to-noise ratio are critically examined, with the help of experimental results performed on phantoms or in vivo. Finally we give an account on the technological developments that would pave the way for a broader use of TD fNIRS in the neuroimaging community.

The study of photon propagation in highly scattering media (diffuse optics) is a powerful tool to noninvasively investigate biological tissues in depth.^1,2 For most biological media in the 600- to 1100-nm range, the scattering coefficient is much higher than the absorption coefficient, thus hindering information from the human body that in absence of scattering would appear transparent as a feebly colored glass. The attractiveness of probing the human tissues and their functions with light has fostered great efforts to cope with the challenges of “diffuse optics,” leading to a whole range of new physical concepts, theoretical models, instruments, measurement approaches, and applications. The terms near-infrared spectroscopy (NIRS) and diffuse optical imaging (DOI) are used in the scientific community to refer to the techniques used for noninvasive monitoring and imaging with diffuse light. Typically, light is injected into the medium and the signal re-emitted at some distance(s) on the surface is analyzed to retrieve information carried by photons along their paths. In the simplest case, continuous wave (CW) illumination and detection are employed.³ 4. ^{– 5} Conversely, the adoption either of a frequency-domain (FD)⁶ or of a time-domain (TD)⁷ 8. 9. ^{– 10} scheme permits to add a further dimension in the measurement, e.g., disentangling absorption from scattering contributions.

Semiconductor devices are the most appealing systems for information encoding using two-dimensional spatial solitons, recently reported both theoretically and experimentally in model systems. We show that stable solitons can be realized and controlled, both in passive microresonators with excitonic nonlinearity and in below-threshold vertical-cavity lasers. The solitons are robust enough to withstand significant carrier diffusion and self-defocusing, which is very encouraging for device applications.

We consider a semiconductor microcavity driven by a coherent and stationary holding beam, in two distinct configurations. In the first, no carriers are injected in the multiple-quantum-well structure and the optical nonlinearity is governed by an excitonic resonance. The second corresponds to that of a vertical-cavity surface-emitting laser kept slightly below threshold. We describe both configurations using a unified model that includes both field diffraction and carrier diffusion. We calculate numerically both the time evolution and the stationary profile of the solitonic solutions, using a generalization of the radial integration technique introduced by Firth and Scroggie [Phys. Rev. Lett. 76, 1623 (1996)]. We analyze the instability that forms spatial patterns and especially cavity spatial solitons. We predict the existence of these solitons in various parametric domains for both configurations. We demonstrate that these results are independent of the periodic boundary conditions used in the simulations. We show that, introducing a simple phase modulation in the holding beam, one can eliminate the motions of solitons that arise from noise and from amplitude gradients. The solitons are robust with respect to parametric variations, to carrier diffusion, and even to some amount of self-defocusing. This picture points to the possibility of realizing arrays of solitonic pixels using semiconductor microresonators.

We have come a long way since Jöbsis first demonstrated that noninvasive cerebral monitoring was possible in humans within the near-infrared light window (approximately 700 to 1000 nm) in the late 1970s.¹ What started as a tool to quantify blood oxygenation in the brain has exploded into a whole field of study that aims to exploit this window to reveal a battery of information related to human brain physiology, including brain metabolism, blood flow, autoregulation, perfusion pressure, water concentration, light scattering, exogenous contrast agents, and more. In this text, we use the term “diffuse optics” to broadly describe any technique that employs a near-infrared light source and a photodetector to capture multiply scattered photons in the diffusion limit (i.e., in the limit wherein the absorption coefficient, ${\mu }_a$, is much less than the reduced scattering coefficient, ${\mu }_s^{\prime }$, and the source-detector separation is much greater than the photon mean free path). The term “diffuse” has historical connotations in the field and reflects how near-infrared photons propagate through biological tissue. However, it should be noted that this term is in no way associated with how the field/community is organized or conducts its science. On the contrary, the breadth and depth of the progress made in the field over the past decade could only be accomplished by a cohesive, focused, and tight-knit community of researchers!