Michael Ney

Modeling of reflectometric and ellipsometric spectra from the skin in the terahertz and submillimeter waves region

The human skin is modeled as a stack of homogeneous layers in the terahertz and submillimeter waves regions with some anisotropy due to the helical sweat glands and other elongated entities. A dielectric model for the skin is presented, valid for a wider frequency range (up to the terahertz region) taking into account the dispersive nature of the effective conductivity. Polarized reflectivity and generalized ellipsometric parameters are calculated versus angle and wavelength. Recent studies have claimed that the helical sweat ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the sub-terahertz region. We found that water absorption, dispersion and multiple interference effects play the major role in shaping the spectrum without the need for the assumption of the sweat ducts acting as low-Q helical antennae. High sensitivities to the water content are found particularly in the ellipsometric parameters at large incidence angles. Hence a new methodology is proposed to detect skin cancer using variable angle ellipsometry or polarized reflectometry. The parameter found with the highest sensitivity to water content is cos Δ(pp) with Δ(pp) being the phase of the on-diagonal reflection matrix ratio between p-to-p polarization.

Does human skin truly behave as an array of helical antennae in the millimeter and terahertz wave ranges

The sweat ducts of the human perspiration system are helically shaped tubes, filled with a conductive aqueous solution. Recent studies have claimed that these ducts act as an array of low-Q helical antennae and are dominant in shaping the spectral response in the subterahertz region. Using local homogenization theory for the skin embedded with sweat ducts, we found that multiple interference effects from the skin layers play the major role in determining the skin electromagnetic characteristics in the millimeter and terahertz regions without the need for the assumption of the sweat ducts acting as low-Q helical antennae.

Comprehensive Monte-Carlo simulator for optimization of imaging parameters for high sensitivity detection of skin cancer at the THz

Skin cancer detection at its early stages has been the focus of a large number of experimental and theoretical studies during the past decades. Among these studies two prominent approaches presenting high potential are reflectometric sensing at the THz wavelengths region and polarimetric imaging techniques in the visible wavelengths. While THz radiation contrast agent and source of sensitivity to cancer related tissue alterations was considered to be mainly the elevated water content in the cancerous tissue, the polarimetric approach has been verified to enable cancerous tissue differentiation based on cancer induced structural alterations to the tissue. Combining THz with the polarimetric approach, which is considered in this study, is examined in order to enable higher detection sensitivity than previously pure reflectometric THz measurements. For this, a comprehensive MC simulation of radiative transfer in a complex skin tissue model fitted for the THz domain that considers the skin`s stratified structure, tissue material optical dispersion modeling, surface roughness, scatterers, and substructure organelles has been developed. Additionally, a narrow beam Mueller matrix differential analysis technique is suggested for assessing skin cancer induced changes in the polarimetric image, enabling the tissue model and MC simulation to be utilized for determining the imaging parameters resulting in maximal detection sensitivity.

Parallel phase shift microscopy, vibrometry and focus tracking systems (Conference Presentation)

Parallel phase shift interferometric detection systems were developed using polarized interferometry, three detectors and multiple wavelengths. In phase shift interferometry (PSI) several phase shifted interference images are usually acquired in a sequence and are algebraically combined to extract the phase information. However, phase imaging is limited both by the 2π phase modulo limiting the ability to map structures with heights only up to half the sources wavelength i.e. several hundreds of nm, and also by error induced by the movements of the sample between the acquisitions of phase shifted interference images. Several approaches for dealing with these limitations have been developed that provided only a limited solution, e.g. using a beat wavelength interferogram by a two wavelength illumination but that is more sensitive to phase noise and thus less accurate and parallel PSI in which all phase shifted images are acquired simultaneously but that does not resolve the height limitation. We have developed a combined and improved technique for parallel PSI and three wavelength illumination enabling overcoming both limitations without elevating phase noise sensitivity. Several bench prototypes were built: some allowing video rate 3D imaging of moving samples such as biological live samples or high throughput scanning of metrology samples with nm scale resolution, and others allowing single point very high speed axial motion tracking and vibrometry with sub-nm scale resolution and max step height of few tens of µm. Selected References: 1. Michael Ney, Avner Safrani, Ibrahim Abdulhalim, Instantaneous high-resolution focus tracking and a vibrometery system using parallel phase shift interferometry, J. Opt. A (Letters) 18, 09LT05 (5pp) (2016). 2. Michael Ney, Avner Safrani, Ibrahim Abdulhalim, Three wavelengths parallel phase-shift interferometry for real-time focus tracking and vibration measurement, Optics Letter 42, 719-22 (2017). 3. Avner Safrani and Ibrahim Abdulhalim, High speed 3D imaging using two wavelengths parallel phase shift interferometry, Optics Letters 40, 4651-4 (2015). 4. Avner Safrani and Ibrahim Abdulhalim, Full field parallel interferometry coherence probe microscope for high speed optical metrology, Appl. Opt. 54, 5083-87 (2015). 5. Avner Safrani and I. Abdulhalim, Real Time, Phase Shift, Interference Microscopy, Optics Letters 39, 5220-23 (2014).

Parallel spectroscopic ellipsometry system for fast correction of surface variability in metrology-based systems (Conference Presentation)

In interferometry based metrology systems, the sample’s topography is extracted from the interference phase which is determined by a combination of both the topography of the sample and its reflectance\transmission phase governed by the sample‘s structure and material composition. Since the two contributions cannot be distinguished, variations of the samples surface (surface variability) can be falsely interpreted as topography changes and undermine the reliability of interferometric measurement. For this reason, knowledge of the sample’s structure\composition is required to eliminate its effect on the interference phase, which is rarely available a priori. Spectroscopic ellipsometry is a well-known metrology technique for the determination of the optical and thus structural properties of multilayered samples with high accuracy through a measurement of some ellipsometric parameters. These parameters are extracted for each pixel in the sample by a combination of sequential measurements taken with a variation in the orientation of a polarizer or a compensator, or most commonly a rotating analyzer\compensator. Thus the acquisition time and throughput are limited in addition to the constraint of stationary sample during data acquisition (rotation) for noise and errors reduction. These techniques are not suitable for high throughput oriented production lines and dynamic samples where the sample is in constant motion. We present a novel fast spectroscopic ellipsometry system enabling the parallel acquisition of all necessary data enabling high speed and accurate sample characterization. This parallel spectroscopic ellipsometer is integrated with our dynamic focusing probe, allowing high-speed and surface variability immune interferometry based axial position tracking.

Combining three wavelength illumination and parallel phase shift interferometry for high-speed high-resolution and real-time motion tracking and 3D imaging (Conference Presentation)

Phase based imaging and sensing have been for decades effective optical methodologies for high resolution surface profiling. Several techniques for acquiring the phase information encoding the surface topography have been developed; the most prominent is phase shift interferometry (PSI) in which several phase shifted interference images are usually acquired in a sequence and are algebraically combined to extract the phase information. However, phase imaging is limited both by the 2π phase modulo limiting the ability to map structures with heights only up to half the sources wavelength i.e. several hundreds of nm, and also by error induced by the movements of the sample between the acquisitions of phase shifted interference images. Several approached for dealing with these limitations have been developed that provided only a limited solution, e.g. using a beat wavelength interferogram by a two wavelength illumination but that is more sensitive to phase noise and thus less accurate and parallel PSI in which all phase shifted images are acquired simultaneously but that does not resolve the height limitation. We have developed a combined and improved technique for parallel PSI and three wavelength illumination enabling overcoming both limitations without elevating phase noise sensitivity and have set up two prototypes: the first allowing video rate 3D imaging of moving samples such as biological live samples or high throughput scanning of metrology samples with nm scale resolution, and the second allowing single point very high speed axial motion tracking and vibrometry with sub-nm scale resolution and max step height of 30µm.