Effect of skin color on optical properties and the implications for medical optical technologies: a review.

Significance: Skin color affects light penetration leading to differences in its absorption and scattering properties. COVID-19 highlighted the importance of understanding of the interaction of light with different skin types, e.g., pulse oximetry (PO) unreliably determined oxygen saturation levels in people from Black and ethnic minority backgrounds. Furthermore, with increased use of other medical wearables using light to provide disease information and photodynamic therapies to treat skin cancers, a thorough understanding of the effect skin color has on light is important for reducing healthcare disparities. Aim: The aim of this work is to perform a thorough review on the effect of skin color on optical properties and the implication of variation on optical medical technologies. Approach: Published in vivo optical coefficients associated with different skin colors were collated and their effects on optical penetration depth and transport mean free path (TMFP) assessed. Results: Variation among reported values is significant. We show that absorption coefficients for dark skin are ∼6% to 74% greater than for light skin in the 400 to 1000 nm spectrum. Beyond 600 nm, the TMFP for light skin is greater than for dark skin. Maximum transmission for all skin types was beyond 940 nm in this spectrum. There are significant losses of light with increasing skin depth; in this spectrum, depending upon Fitzpatrick skin type (FST), on average 14% to 18% of light is lost by a depth of 0.1 mm compared with 90% to 97% of the remaining light being lost by a depth of 1.93 mm. Conclusions: Current published data suggest that at wavelengths beyond 940 nm light transmission is greatest for all FSTs. Data beyond 1000 nm are minimal and further study is required. It is possible that the amount of light transmitted through skin for all skin colors will converge with increasing wavelength enabling optical medical technologies to become independent of skin color.

Relevance and utility of the in-vivo and ex-vivo optical properties of the skin reported in the literature: a review [Invited].

Imaging non-invasively into the human body is currently limited by cost (MRI and CT scan), image resolution (ultrasound), exposure to ionising radiation (CT scan and X-ray), and the requirement for exogenous contrast agents (CT scan and PET scan). Optical imaging has the potential to overcome all these issues but is currently limited by imaging depth due to the scattering and absorption properties of human tissue. Skin is the first barrier encountered by light when imaging non-invasively, and therefore a clear understanding of the way that light interacts with skin is required for progress on optical medical imaging to be made. Here we present a thorough review of the optical properties of human skin measured in-vivo and compare these to the previously collated ex-vivo measurements. Both in-vivo and ex-vivo published data show high inter- and intra-publication variability making definitive answers regarding optical properties at given wavelengths challenging. Overall, variability is highest for ex-vivo absorption measurements with differences of up to 77-fold compared with 9.6-fold for the in-vivo absorption case. The impact of this variation on optical penetration depth and transport mean free path is presented and potential causes of these inconsistencies are discussed. We propose a set of experimental controls and reporting requirements for future measurements. We conclude that a robust in-vivo dataset, measured across a broad spectrum of wavelengths, is required for the development of future technologies that significantly increase the depth of optical imaging.

Near-field manipulation of Tamm plasmon polaritons.

Tamm plasmon polaritons (TPPs) arise from electromagnetic resonant phenomena which appear at the interface between a metallic film and a distributed Bragg reflector. They differ from surface plasmon polaritons (SPPs), since TPPs possess both cavity mode properties and surface plasmon characteristics. In this paper, the propagation properties of TPPs are carefully investigated. With the aid of nanoantenna couplers, polarization-controlled TPP waves can propagate directionally. By combining nanoantenna couplers with Fresnel zone plates, asymmetric double focusing of TPP wave is observed. Moreover, radial unidirectional coupling of the TPP wave can be achieved when the nanoantenna couplers are arranged along a circular or a spiral shape, which shows superior focusing ability compared to a single circular or spiral groove since the electric field intensity at the focal point is 4 times larger. In comparison with SPPs, TPPs possess higher excitation efficiency and lower propagation loss. The numerical investigation shows that TPP waves have great potential in integrated photonics and on-chip devices.

A total-internal-reflection-based Fabry-Pérot resonator for ultra-sensitive wideband ultrasound and photoacoustic applications.

In photoacoustic and ultrasound imaging, optical transducers offer a unique potential to provide higher responsivity, wider bandwidths, and greatly reduced electrical and acoustic impedance mismatch when compared with piezoelectric transducers. In this paper, we propose a total-internal-reflection-based Fabry-Pérot resonator composed of a 12-nm-thick gold layer and a dielectric resonant cavity. The resonator uses the same Kretschmann configuration as surface plasmon resonators (SPR). The resonators were analyzed both theoretically and experimentally. The experimental results were compared with those for an SPR for benchmarking. The 1.9-µm-thick-PMMA- and 3.4-µm-thick-PDMS-based resonators demonstrated responsivities of 3.6- and 30-fold improvements compared with the SPR, respectively. The measured bandwidths for the PMMA, PDMS devices are 110 MHz and 75 MHz, respectively. Single-shot sensitivity of 160 Pa is obtained for the PDMS device. The results indicate that, with the proposed resonator in imaging applications, sensitivity and the signal-to-noise ratio can be improved significantly without compromising the bandwidth.

Design of a dynamic multi-topological charge graphene orbital angular momentum metalens.

Traditional OAM generation devices are bulky and can generally only create OAM with one specific topological charge. Although metasurface-based devices have overcome the volume limitations, no tunable metasurface-based OAM generators have been demonstrated to date. Here, a dynamically tunable multi-topological charge OAM generator based on an ultrathin integrable graphene metalens is demonstrated by simulation using the detour phase technique and spatial multiplexing. Different topological charges can be designed on different focal planes. Stretching the OAM graphene metalens allows the focal plane and the topological values to be changed dynamically. This design method paves an innovative route toward miniaturization and integrating OAM beam-type photonic devices for practical applications.

Data transmission with up to 100 orbital angular momentum modes via commercial multi-mode fiber and parallel neural networks.

This work presents an artificial intelligence enhanced orbital angular momentum (OAM) data transmission system. This system enables encoded data retrieval from speckle patterns generated by an incident beam carrying different topological charges (TCs) at the distal end of a multi-mode fiber. An appropriately trained network is shown to support up to 100 different fractional TCs in parallel with TC intervals as small as 0.01, thus overcoming the problems with previous methods that only supported a few modes and could not use small TC intervals. Additionally, an approach using multiple parallel neural networks is proposed that can increase the system's channel capacity without increasing individual network complexity. When compared with a single network, multiple parallel networks can achieve the better performance with reduced training data requirements, which is beneficial in saving computational capacity while also expanding the network bandwidth. Finally, we demonstrate high-fidelity image transmission using a 16-bit system and four parallel 14-bit systems via OAM mode multiplexing through a 1-km-long commercial multi-mode fiber (MMF).

Bloch surface waves assisted active modulation of graphene electro-absorption in a wide near-infrared region.

Light modulation has been recognized as one of the most fundamental operations in photonics. In this paper, we theoretically designed a Bloch surface wave assisted modulator for the active modulation of graphene electro-absorption. Simulations show that the strong localized electrical field generated by Bloch surface waves can significantly enhance the graphene electro-absorption up to 99.64%. Then by gate-tuning the graphene Fermi energy to transform graphene between a lossy and a lossless material, electrically switched absorption of graphene with maximum modulation depth of 97.91% can be achieved. Meanwhile, by further adjusting the incident angle to tune the resonant wavelength of Bloch surface waves, the center wavelength of the modulator can be actively controlled. This allows us to realize the active modulation of graphene electro-absorption within a wide near-infrared region, including the commercially important telecommunication wavelength of 1550 nm, indicating the excellent performance of the designed modulator via such mechanism. Such Bloch surface waves assisted wavelength-tunable graphene electro-absorption modulation strategy opens up a new avenue to design graphene-based selective multichannel modulators, which is unavailable in previous reported strategies that can be only realized by passively changing the structural parameters.

All-dielectric metasurface designs for spin-tunable beam splitting via simultaneous manipulation of propagation and geometric phases.

Metasurfaces offer diverse wavefront control by manipulating amplitude, phase, and polarization of light which is beneficial to design subwavelength scaled integrated photonic devices. Metasurfaces based tunable circular polarization (CP) beam splitting is one functionality of interest in polarization control. Here, we propose and numerically realize metasurface based spin tunable beam splitter which splits the incoming CP beam into two different directions and tune the splitting angles by switching the handedness of incident light polarization. The proposed design approach has potential in applications such as optical communication, multiplexing, and imaging.

Plasmonic and Graphene-Functionalized High-Performance Broadband Quasi-Two-Dimensional Perovskite Hybrid Photodetectors.

Quasi-two-dimensional (2D) layered organic-inorganic hybrid perovskites have attracted extensive attention, owing to their excellent optoelectronic tunability and moisture stability compared with three-dimensional perovskite counterparts and show great potential for application in photodetectors (PDs). However, owing to the unavoidable grain boundary defects of perovskite polycrystalline films, the photocurrent is limited by poor light absorption and charge mobility. Therefore, the preparation of quasi-2D perovskite films with strong light trapping and high charge mobility has been challenging. In this study, novel broadband quasi-2D perovskite (BA)2(FA)n-1PbnI3n+1 hybrid-structure PDs with good stability were fabricated by combining both monolayer graphene and Au square nanoarrays. The hybrid system using both graphene and Au square nanoarrays effectively improved the carrier mobility and light absorption and simultaneously maximized light trapping and light-induced carrier extraction, which resulted in PDs with greatly enhanced photocurrent in the visible and near-infrared range. The graphene-Au array-perovskite-based PDs had a low dark current of 10-10 A, large on/off ratio of 104, high responsivity of 18.71 A W-1, and detectivity of 2.21 × 1013 Jones. The responsivity and detectivity were two orders of magnitude higher than those of PDs based only on perovskites. This work demonstrates a promising and feasible device based on the coupling of a gold array, layered graphene, and quasi-2D perovskites, which shows great potential for the development of high-performance broadband perovskite PDs.

Active tuning of longitudinal strong coupling between anisotropic borophene plasmons and Bloch surface waves.

Strong coupling between the resonant modes can give rise to many resonant states, enabling the manipulation of light-matter interactions with more flexibility. Here, we theoretically propose a coupled resonant system where an anisotropic borophene localized plasmonic (BLP) and Bloch surface wave (BSW) can be simultaneously excited. This allows us to manipulate the spectral response of the strong BLP-BSW coupling with exceptional flexibility in the near infrared region. Specifically, the strong longitudinal BLP-BSW coupling occurs when the system is driven into the strong coupling regime, which produces two hybrid modes with a large Rabi splitting up to 124 meV for borophene along both x- and y-directions. A coupled oscillator model is employed to quantitatively describe the observed BSW-BLP coupling by calculating the dispersion of the hybrid modes, which shows excellent agreement with the simulation results. Furthermore, benefited from the angle-dependent BSW mode, the BSW-BLP coupling can be flexibly tuned by actively adjusting the incident angle. Such active tunable BLP-SBW coupling with extreme flexibility offered by this simple layered system makes it promising for the development of diverse borophene-based active photonic and optoelectronic devices in the near infrared region.

Highly In-Plane Anisotropic Two-Dimensional Ternary Ta2NiSe5 for Polarization-Sensitive Photodetectors.

Intriguing anisotropic electrical and optoelectrical properties in two-dimensional (2D) materials are currently gaining increasing interest both for fundamental research and emerging optoelectronic devices. Identifying promising new 2D materials with low-symmetry structures will be rewarding in the development of polarization-integrated nanodevices. In this work, the anisotropic electron transport and optoelectrical properties of multilayer 2D ternary Ta2NiSe5 were systematically researched. The polarization-sensitive Ta2NiSe5 photodetector shows a linearly anisotropy ratio of ≈3.24 with 1064 nm illumination. The multilayer Ta2NiSe5-based field-effective transistors exhibit an excellent field-effective mobility of 161.25 cm2·V-1·s-1 along the a axis (armchair direction) as well as a great current saturation characteristic at room temperature. These results will promote a better understanding of the optoelectrical properties and applications in new categories of the in-plane anisotropic 2D materials.

Plasmonic tweezers: for nanoscale optical trapping and beyond.

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science.

A Varifocal Graphene Metalens for Broadband Zoom Imaging Covering the Entire Visible Region.

The ever-increasing demand for miniaturized optical systems has placed stringent requirements on the core element: lenses. Developing ultrathin flat lenses with a varifocal capability and broadband spectral response is critical for diverse applications, but remains challenging and has been the focus of intensive research. The recent demonstration of tunable focal length for a single wavelength with metalenses marked an important milestone for transforming the complex and bulky tunable lens kit into a single flat lens. However, achieving color imaging with desired tunability over the entire visible spectrum essential for practical applications still remains elusive. Here we propose and demonstrate experimentally a broadband varifocal graphene metalens (250 nm in thickness) covering the entire visible spectrum. It is able to simultaneously tune the focal lengths for different wavelengths continuously. By laterally stretching the lens, an over 20% focal length tuning range can be achieved for red (650 nm), green (550 nm), and blue (450 nm) light as three example wavelengths. Zoom imaging of different objects located along the axial direction has been demonstrated at these wavelengths by simply controlling the stretch ratio of the graphene metalens. This broadband graphene zoom lens enables enormous applications in miniaturized imaging devices such as cell phones, wearable displays, and compact optical or communication systems with multi-color-channel functionalities.

Controllable hybridization between localized and delocalized anisotropic borophene plasmons in the near-infrared region.

In this Letter, we theoretically propose a coupled borophene plasmonic system, where an anisotropic localized plasmonic (LP) mode and a delocalized guided plasmonic (DGP) mode can be simultaneously excited. This allows us to manipulate the optical response of the strong LP-DGP coupling with exceptional flexibility in the near-infrared region, which is not possible with the conventional metallic plasmonic structures, and overcomes some shortcomings of coupled structures based on the other 2D materials. Specifically, the spatially LP-DGP coupling can arise when the system is driven into the strong coupling regime; this gives rise to a transparency window which can be well described by a coupled oscillation model. The bandwidth of the window is governed by the coupling strength which can be passively adjusted by the spacer thickness, while the center wavelength and the number of windows can be actively modulated by tuning the borophene electron density and the incident angle.

Virtual optics and sensing of the retrieved complex field in the back focal plane using a constrained defocus algorithm.

The reflected back focal plane from a microscope objective is known to provide excellent information of material properties and can be used to analyze the generation of surface plasmons and surface waves in a localized region. Most analysis has concentrated on direct measurement of the reflected intensity in the back focal plane. By accessing the phase information, we show that examination in the back focal plane becomes considerably more powerful allowing the reconstructed field to be filtered, propagated and analyzed in different domains. Moreover, the phase often gives a superior measurement that is far easier to use in the assessment of the sample, an example of such cases is examined in the present paper. We discuss how the modified defocus phase retrieval algorithm has the potential for real time measurements with parallel image acquisition since only three images are needed for reliable retrieval of arbitrary distributions.

On-chip plasmonic spin-Hall nanograting for simultaneously detecting phase and polarization singularities.

Phase and polarization singularities are important degrees of freedom for electromagnetic field manipulation. Detecting these singularities is essential for modern optics, but it is still a challenge, especially in integrated optical systems. In this paper, we propose an on-chip plasmonic spin-Hall nanograting structure that simultaneously detects both the polarization and phase singularities of the incident cylindrical vortex vector beam (CVVB). The nanograting is symmetry-breaking with different periods for the upper and lower parts, which enables the unidirectional excitation of the surface plasmon polariton depending on the topological charge of the incident optical vortex beam. Additionally, spin-Hall meta-slits are integrated onto the grating so that the structure has a chiral response for polarization detection. We demonstrate theoretically and experimentally that the designed structure fully discriminates both the topological charges and polarization states of the incident beam simultaneously. The proposed structure has great potential in compact integrated photonic circuits.

Spin-orbit coupling controlled near-field propagation and focusing of Bloch surface wave.

Bloch surface wave (BSW) can be considered as the dielectric analogue of surface plasmon polariton (SPP) with less loss since it is sustained at the surface of a truncated dielectric multilayer. As dielectric materials show nearly no ohmic loss, BSW can propagates much farther compared to SPP, and thus is beneficial for planar optical devices. In this paper, we study the spin-orbital interaction between incident beam and BSW. We demonstrate that due to the spin-orbital coupling, the near-field properties of generated BSW can be controlled with a meta-antenna structure. The meta-antenna is composed of two gold nano-antennas oriented at 45° and 135° as a near-field coupler. By careful design of the meta-antenna, the generated BSW can be guided and focused depending on the chirality of the incident beam. Three examples of meta-antennas are demonstrated for chiral sensitive focusing, directional switching and asymmetric focusing. The proposed method can be applied as a design method for low-loss on-chip photonic devices.

Identification of waveguide mode and surface plasmon resonance mode using Fourier cross-correlation analysis.

The light reflected into the back focal plane of a microscope objective allows one to gather a great deal of information about the resonant modes excited on a sample. These dips represent modes excited on the sample, which are related to both the material properties and the structure. Automatic identification of these resonances is a vital stage in developing automated machine-learning techniques for high-throughput sample characterization. In previous work, identification of a single isolated mode was demonstrated; here we show how multiple modes can be separately identified using an automated centering procedure in a process we call radial thresholding. Once the center was determined, the radial thresholding process was modified and combined with interpolation to locate the precise modal positions. We show that this method is capable of resolving very closely spaced modes and is sensitive to nanometric changes in sample dimensions. The processing time for the method is sufficiently fast to ensure that it is suited for rapid sample identification.

Spatially resolved random-access pump-probe microscopy based on binary holography.

In this Letter, we present a spatially resolved pump-probe microscope based on a digital micromirror device (DMD). The microscope system enables the measurements of ultrafast transient processes at arbitrarily selected regions in a 3-D specimen. To achieve random-access scanning, the wavefront of the probe beam is modulated by the DMD via binary holography. By switching the holograms stored in the DMD memory, the laser focus can be rapidly moved in space in a discrete fashion. The microscope system has a field of view of 65×130×155 µm3 in the x, y, and z axes, respectively; and a scanning speed of 8 kHz which is limited by the response time of the lock-in amplifier. To demonstrate the pump-probe system, we measured the ultrafast transient reflectivity of 2-D gold patterns on a silicon substrate and on silicon nitride cantilever beams. The results show an excellent signal-to-noise ratio and spatial-temporal resolution, as well as the 3-D random scanning capability. The new pump-probe microscope is a versatile instrument to characterize ultrafast 3-D phenomena with high spatial and temporal resolution, e.g., the propagation of localized surface plasmon resonance on curved surfaces.

Optical detection of ultrasound by lateral shearing interference of a transparent PDMS thin film.

A lateral shearing interferometric technique combined with an 11.6 µm polydimethylsiloxane (PDMS) transparent thin film is proposed and demonstrated for optical detection of ultrasound. We experimentally report the device change of reflectivity with pressure of 5.1×10-7 Pa-1, 9.5 times more sensitive than the critical-angle-based sensor, 31 times more sensitive than the surface-plasmon-based sensor, and comparable to the Fabry-Perot sensor. The objective-lens-based angle scanning characterization setup inspired from a laser scanning system allows direct comparison between the PDMS sensor and critical-angle-based sensor by adjusting the incident angle with a scanning mirror, thereby eliminating optical and electronics system dependence. The sensing element is easily fabricated through spin coating and the detection element incorporated into an existing optical system with minimum modification.