Preferential coupling of an incident wave to reflection eigenchannels of disordered media

Wavefront Shaping Concepts for Application in Optical Coherence Tomography-A Review

Optical coherence tomography (OCT) enables three-dimensional imaging with resolution on the micrometer scale. The technique relies on the time-of-flight gated detection of light scattered from a sample and has received enormous interest in applications as versatile as non-destructive testing, metrology and non-invasive medical diagnostics. However, in strongly scattering media such as biological tissue, the penetration depth and imaging resolution are limited. Combining OCT imaging with wavefront shaping approaches significantly leverages the capabilities of the technique by controlling the scattered light field through manipulation of the field incident on the sample. This article reviews the main concepts developed so far in the field and discusses the latest results achieved with a focus on signal enhancement and imaging.

Time-gated iterative phase conjugation for efficient light energy delivery in scattering media

Light waves propagating through complex biological tissues are spatially spread by multiple light scattering, and this spread limits the working depth in optical bioimaging, phototherapy, and optogenetics. Here, we propose the iterative phase conjugation of time-gated backscattered waves for enhancing the light energy delivered to a target object embedded in a scattering medium. We demonstrate the enhancement of light energy delivered to a target object hidden behind a 200-µm-thick mouse skull by more than ten times in comparison with the initial random input. The maximum enhancement was reached in only 10 iterations, more than a hundred times smaller than existing methods based on either a time-gated reflection matrix or iterative feedback optimization of the time-gated reflection intensity. Consequently, the proposed method is less sensitive to sample perturbations. Furthermore, the number of images required for optimization remained almost unchanged with an increase in the illumination area, unlike existing methods, where the convergence time scales with the illumination area. The proposed method provides high operation speed over a wide illumination area, which can facilitate the use of wavefront shaping in practical applications.

A New handheld singlet oxygen detection system (SODS) and NIR light source based phantom environment for photodynamic therapy applications

Photodynamic therapy (PDT) is an emerging treatment modality in various areas such as cancer treatment and disinfection. The photosensitizer and oxygen have crucial roles for effective PDT treatment. The quantitative evaluation of singlet oxygen, which is a gold standard for monitoring effective treatment, remains as an important problem for PDT. However, low quantum yield and low life span of the singlet oxygen make the system expensive, unnecessarily large and unadaptable for clinical usage. In our study, a new mobile singlet oxygen detection system (SODS) was designed to detect singlet oxygen illumination during PDT and a new singlet oxygen phantom environment was constituted to test the designed SODS system. The singlet oxygen phantom environment composed of fast switching led driver & microcontroller and led light source (1200-1300 nm radiation). The elements of the singlet oxygen detection system are optic filter and collimation, avalanche photodiode transimpedance amplifier, differential amplifier and a signal processing block. According to the performance evaluation of the system on the phantom environment, the presented SODS can measure the illuminations at 1270 nm wavelength between 10 ns and 15 µs timespans. The results showed that the proposed system might be a good candidate for clinical PDT applications.

Iterative optimization of time-gated reflectance for the efficient light energy delivery within scattering media

In complex media, light waves are diffused both in space and time due to multiple light scattering, and its intensity is attenuated with the increase of propagation depth. In this paper, we propose an iterative wavefront shaping method for enhancing time-gated reflection intensity, which leads to efficient light energy delivery to a target object embedded in a highly scattering medium. We achieved an over 10 times enhancement of reflectance at the specific flight time and demonstrated the focusing of light energy to the target object. Since the proposed method does not require reflection matrix measurement, it will be highly suited to samples in mechanically dynamic conditions.

Enhanced deep detection of Raman scattered light by wavefront shaping

Light scattering limits the penetration depth of non-invasive Raman spectroscopy in biological media. While safe levels of irradiation may be adequate to analyze superficial tissue, scattering of the pump beam reduces the Raman signal to undetectable levels deeper within the tissue. Here we demonstrate how wavefront shaping techniques can significantly increase the Raman signal at depth, while keeping the total irradiance constant, thus increasing the amount of Raman signal available for detection.