2021 Vol. 10, No. 2

News & Views
OCT on a chip aims at high-quality retinal imaging
Dierck Hillmann
Published. 2021, 10(2) : 138-139 doi: 10.1038/s41377-021-00467-z
Optical coherence tomography (OCT) has become one of the most important techniques in ophthalmic diagnostics, as it is the only way to three-dimensionally visualize morphological changes in the layered structure of the retina at a high resolution. In addition, OCT is applied for countless medical and technical purposes. Recent developments pave the way for small-footprint OCT systems at significantly reduced costs, thereby extending possible use cases. Now, it appears increasingly likely that, in the near future, OCT will find its way into many more industrial and medical applications, including disease monitoring at home.
Band-structure-engineered high-gain LWIR photodetector based on a type-II superlattice
Arash Dehzangi, Jiakai Li, Manijeh Razeghi
Published. 2021, 10(2) : 140-146 doi: 10.1038/s41377-020-00453-x
The LWIR and longer wavelength regions are of particular interest for new developments and new approaches to realizing long-wavelength infrared (LWIR) photodetectors with high detectivity and high responsivity. These photodetectors are highly desirable for applications such as infrared earth science and astronomy, remote sensing, optical communication, and thermal and medical imaging. Here, we report the design, growth, and characterization of a high-gain band-structure-engineered LWIR heterojunction phototransistor based on type-II superlattices. The 1/e cut-off wavelength of the device is 8.0 µm. At 77 K, unity optical gain occurs at a 90 mV applied bias with a dark current density of 3.2 × 10-7 A/cm2. The optical gain of the device at 77 K saturates at a value of 276 at an applied bias of 220 mV. This saturation corresponds to a responsivity of 1284 A/W and a specific detectivity of 2.34 × 1013 cm Hz1/2/W at a peak detection wavelength of ~6.8 µm. The type-II superlattice-based high-gain LWIR device shows the possibility of designing the high-performance gain-based LWIR photodetectors by implementing the band structure engineering approach.
Extraordinary evanescent field confinement waveguide sensor for mid-infrared trace gas spectroscopy
Marek Vlk, Anurup Datta, Sebastián Alberti, Henock Demessie Yallew, Vinita Mittal, et al.
Published. 2021, 10(2) : 147-153 doi: 10.1038/s41377-021-00470-4
Nanophotonic waveguides are at the core of a great variety of optical sensors. These structures confine light along defined paths on photonic chips and provide light-matter interaction via an evanescent field. However, waveguides still lag behind free-space optics for sensitivity-critical applications such as trace gas detection. Short optical pathlengths, low interaction strengths, and spurious etalon fringes in spectral transmission are among the main reasons why on-chip gas sensing is still in its infancy. In this work, we report on a mid-infrared integrated waveguide sensor that successfully addresses these drawbacks. This sensor operates with a 107% evanescent field confinement factor in air, which not only matches but also outperforms free-space beams in terms of the per-length optical interaction. Furthermore, negligible facet reflections result in a flat spectral background and record-low absorbance noise that can finally compete with free-space spectroscopy. The sensor performance was validated at 2.566 μm, which showed a 7 ppm detection limit for acetylene with only a 2 cm long waveguide.
Slow light bimodal interferometry in one-dimensional photonic crystal waveguides
Luis Torrijos-Morán, Amadeu Griol, Jaime García-Rupérez
Published. 2021, 10(2) : 154-165 doi: 10.1038/s41377-020-00460-y
Strongly influenced by the advances in the semiconductor industry, the miniaturization and integration of optical circuits into smaller devices has stimulated considerable research efforts in recent decades. Among other structures, integrated interferometers play a prominent role in the development of photonic devices for on-chip applications ranging from optical communication networks to point-of-care analysis instruments. However, it has been a long-standing challenge to design extremely short interferometer schemes, as long interaction lengths are typically required for a complete modulation transition. Several approaches, including novel materials or sophisticated configurations, have been proposed to overcome some of these size limitations but at the expense of increasing fabrication complexity and cost. Here, we demonstrate for the first time slow light bimodal interferometric behaviour in an integrated single-channel one-dimensional photonic crystal. The proposed structure supports two electromagnetic modes of the same polarization that exhibit a large group velocity difference. Specifically, an over 20-fold reduction in the higher-order-mode group velocity is experimentally shown on a straightforward all-dielectric bimodal structure, leading to a remarkable optical path reduction compared to other conventional interferometers. Moreover, we experimentally demonstrate the significant performance improvement provided by the proposed bimodal photonic crystal interferometer in the creation of an ultra-compact optical modulator and a highly sensitive photonic sensor.
Towards efficient near-infrared fluorescent organic light-emitting diodes
Alessandro Minotto, Ibrahim Bulut, Alexandros G. Rapidis, Giuseppe Carnicella, Maddalena Patrini, et al.
Published. 2021, 10(2) : 166-175 doi: 10.1038/s41377-020-00456-8
The energy gap law (EG-law) and aggregation quenching are the main limitations to overcome in the design of near-infrared (NIR) organic emitters. Here, we achieve unprecedented results by synergistically addressing both of these limitations. First, we propose porphyrin oligomers with increasing length to attenuate the effects of the EG -law by suppressing the non-radiative rate growth, and to increase the radiative rate via enhancement of the oscillator strength. Second, we design side chains to suppress aggregation quenching. We find that the logarithmic rate of variation in the non-radiative rate vs. EG is suppressed by an order of magnitude with respect to previous studies, and we complement this breakthrough by demonstrating organic light-emitting diodes with an average external quantum efficiency of ~1.1%, which is very promising for a heavy-metal-free 850 nm emitter. We also present a novel quantitative model of the internal quantum efficiency for active layers supporting triplet-to-singlet conversion. These results provide a general strategy for designing high-luminance NIR emitters.
Enhanced third-harmonic generation by manipulating the twist angle of bilayer graphene
Seongju Ha, Nam Hun Park, Hyeonkyeong Kim, Jiseon Shin, Jungseok Choi, et al.
Published. 2021, 10(2) : 176-185 doi: 10.1038/s41377-020-00459-5
Twisted bilayer graphene (tBLG) has received substantial attention in various research fields due to its unconventional physical properties originating from Moiré superlattices. The electronic band structure in tBLG modified by interlayer interactions enables the emergence of low-energy van Hove singularities in the density of states, allowing the observation of intriguing features such as increased optical conductivity and photocurrent at visible or near-infrared wavelengths. Here, we show that the third-order optical nonlinearity can be considerably modified depending on the stacking angle in tBLG. The third-harmonic generation (THG) efficiency is found to significantly increase when the energy gap at the van Hove singularity matches the three-photon resonance of incident light. Further study on electrically tuneable optical nonlinearity reveals that the gate-controlled THG enhancement varies with the twist angle in tBLG, resulting in a THG enhanced up to 60 times compared to neutral monolayer graphene. Our results prove that the twist angle opens up a new way to control and increase the optical nonlinearity of tBLG, suggesting rotation-induced tuneable nonlinear optics in stacked two-dimensional material systems.
High-resolution impedance mapping using electrically activated quantitative phase imaging
Cristina Polonschii, Mihaela Gheorghiu, Sorin David, Szilveszter Gáspár, Sorin Melinte, et al.
Published. 2021, 10(2) : 186-196 doi: 10.1038/s41377-020-00461-x
Retrieving electrical impedance maps at the nanoscale rapidly via nondestructive inspection with a high signal-to-noise ratio is an unmet need, likely to impact various applications from biomedicine to energy conversion. In this study, we develop a multimodal functional imaging instrument that is characterized by the dual capability of impedance mapping and phase quantitation, high spatial resolution, and low temporal noise. To achieve this, we advance a quantitative phase imaging system, referred to as epi-magnified image spatial spectrum microscopy combined with electrical actuation, to provide complementary maps of the optical path and electrical impedance. We demonstrate our system with high-resolution maps of optical path differences and electrical impedance variations that can distinguish nanosized, semi-transparent, structured coatings involving two materials with relatively similar electrical properties. We map heterogeneous interfaces corresponding to an indium tin oxide layer exposed by holes with diameters as small as ~550 nm in a titanium (dioxide) over-layer deposited on a glass support. We show that electrical modulation during the phase imaging of a macro-electrode is decisive for retrieving electrical impedance distributions with submicron spatial resolution and beyond the limitations of electrode-based technologies (surface or scanning technologies). The findings, which are substantiated by a theoretical model that fits the experimental data very well enable achieving electro-optical maps with high spatial and temporal resolutions. The virtues and limitations of the novel optoelectrochemical method that provides grounds for a wider range of electrically modulated optical methods for measuring the electric field locally are critically discussed.
Laser particles with omnidirectional emission for cell tracking
Shui-Jing Tang, Paul H. Dannenberg, Andreas C. Liapis, Nicola Martino, Yue Zhuo, et al.
Published. 2021, 10(2) : 197-207 doi: 10.1038/s41377-021-00466-0
The ability to track individual cells in space over time is crucial to analyzing heterogeneous cell populations. Recently, microlaser particles have emerged as unique optical probes for massively multiplexed single-cell tagging. However, the microlaser far-field emission is inherently direction-dependent, which causes strong intensity fluctuations when the orientation of the particle varies randomly inside cells. Here, we demonstrate a general solution based on the incorporation of nanoscale light scatterers into microlasers. Two schemes are developed by introducing either boundary defects or a scattering layer into microdisk lasers. The resulting laser output is omnidirectional, with the minimum-to-maximum ratio of the angle-dependent intensity improving from 0.007 (−24 dB) to > 0.23 (−6 dB). After transfer into live cells in vitro, the omnidirectional laser particles within moving cells could be tracked continuously with high signal-to-noise ratios for 2 h, while conventional microlasers exhibited frequent signal loss causing tracking failure.
Arbitrary polarization conversion dichroism metasurfaces for all-in-one full Poincaré sphere polarizers
Shuai Wang, Zi-Lan Deng, Yujie Wang, Qingbin Zhou, Xiaolei Wang, et al.
Published. 2021, 10(2) : 208-216 doi: 10.1038/s41377-021-00468-y
The control of polarization, an essential property of light, is of broad scientific and technological interest. Polarizers are indispensable optical elements for direct polarization generation. However, arbitrary polarization generation, except that of common linear and circular polarization, relies heavily on bulky optical components such as cascading linear polarizers and waveplates. Here, we present an effective strategy for designing all-in-one full Poincaré sphere polarizers based on perfect arbitrary polarization conversion dichroism and implement it in a monolayer all-dielectric metasurface. This strategy allows preferential transmission and conversion of one polarization state located at an arbitrary position on the Poincaré sphere to its handedness-flipped state while completely blocking its orthogonal state. In contrast to previous methods that were limited to only linear or circular polarization, our method manifests perfect dichroism of nearly 100% in theory and greater than 90% experimentally for arbitrary polarization states. By leveraging this attractive dichroism, our demonstration of the generation of polarization beams located at an arbitrary position on a Poincaré sphere directly from unpolarized light can substantially extend the scope of meta-optics and dramatically promote state-of-the-art nanophotonic devices.
All-optical information-processing capacity of diffractive surfaces
Onur Kulce, Deniz Mengu, Yair Rivenson, Aydogan Ozcan
Published. 2021, 10(2) : 217-233 doi: 10.1038/s41377-020-00439-9
The precise engineering of materials and surfaces has been at the heart of some of the recent advances in optics and photonics. These advances related to the engineering of materials with new functionalities have also opened up exciting avenues for designing trainable surfaces that can perform computation and machine-learning tasks through light–matter interactions and diffraction. Here, we analyze the information-processing capacity of coherent optical networks formed by diffractive surfaces that are trained to perform an all-optical computational task between a given input and output field-of-view. We show that the dimensionality of the all-optical solution space covering the complex-valued transformations between the input and output fields-of-view is linearly proportional to the number of diffractive surfaces within the optical network, up to a limit that is dictated by the extent of the input and output fields-of-view. Deeper diffractive networks that are composed of larger numbers of trainable surfaces can cover a higher-dimensional subspace of the complex-valued linear transformations between a larger input field-of-view and a larger output field-of-view and exhibit depth advantages in terms of their statistical inference, learning, and generalization capabilities for different image classification tasks when compared with a single trainable diffractive surface. These analyses and conclusions are broadly applicable to various forms of diffractive surfaces, including, e.g., plasmonic and/or dielectric-based metasurfaces and flat optics, which can be used to form all-optical processors.
Ultrafast transient sub-bandgap absorption of monolayer MoS2
Susobhan Das, Yadong Wang, Yunyun Dai, Shisheng Li, Zhipei Sun
Published. 2021, 10(2) : 234-242 doi: 10.1038/s41377-021-00462-4
The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~ < 1 µm, and exciton states in the long wavelength region of ~ > 1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.
Thermally stable and highly efficient red-emitting Eu3+-doped Cs3GdGe3O9 phosphors for WLEDs: non-concentration quenching and negative thermal expansion
Peipei Dang, Guogang Li, Xiaohan Yun, Qianqian Zhang, Dongjie Liu, et al.
Published. 2021, 10(2) : 243-255 doi: 10.1038/s41377-021-00469-x
Red phosphor materials play a key role in improving the lighting and backlit display quality of phosphor-converted white light-emitting diodes (pc-WLEDs). However, the development of a red phosphor with simultaneous high efficiency, excellent thermal stability and high colour purity is still a challenge. In this work, unique non-concentration quenching in solid-solution Cs3Gd1 − xGe3O9: xEu3+ (CGGO: xEu3+) (x = 0.1–1.0) phosphors is successfully developed to achieve a highly efficient red-emitting Cs3EuGe3O9 (CEGO) phosphor. Under the optimal 464 nm blue light excitation, CEGO shows a strong red emission at 611 nm with a high colour purity of 95.07% and a high internal quantum efficiency of 94%. Impressively, this red-emitting CEGO phosphor exhibits a better thermal stability at higher temperatures (175–250 ℃, > 90%) than typical red K2SiF6: Mn4+ and Y2O3: Eu3+ phosphors, and has a remarkable volumetric negative thermal expansion (coefficient of thermal expansion, α = −5.06 × 10−5/℃, 25–250 ℃). By employing this red CEGO phosphor, a fabricated pc-WLED emits warm white light with colour coordinates (0.364, 0.383), a high colour rendering index (CRI = 89.7), and a low colour coordinate temperature (CCT = 4508 K). These results indicate that this highly efficient red-emitting phosphor has great potential as a red component for pc-WLEDs, opening a new perspective for developing new phosphor materials.
Loss-induced nonreciprocity
Xinyao Huang, Cuicui Lu, Chao Liang, Honggeng Tao, Yong-Chun Liu
Published. 2021, 10(2) : 256-263 doi: 10.1038/s41377-021-00464-2
Nonreciprocity is important in both optical information processing and topological photonics studies. Conventional principles for realizing nonreciprocity rely on magnetic fields, spatiotemporal modulation, or nonlinearity. Here we propose a generic principle for generating nonreciprocity by taking advantage of energy loss, which is usually regarded as harmful. The loss in a resonance mode induces a phase lag, which is independent of the energy transmission direction. When multichannel lossy resonance modes are combined, the resulting interference gives rise to nonreciprocity, with different coupling strengths for the forward and backward directions, and unidirectional energy transmission. This study opens a new avenue for the design of nonreciprocal devices without stringent requirements.
Efficiency limits of concentrating spectral-splitting hybrid photovoltaic-thermal (PV-T) solar collectors and systems
Gan Huang, Kai Wang, Christos N. Markides
Published. 2021, 10(2) : 264-277 doi: 10.1038/s41377-021-00465-1
Spectral splitting is an approach to the design of hybrid photovoltaic-thermal (PVT) collectors that promises significant performance benefits. However, the ultimate efficiency limits, optimal PV cell materials and optical filters of spectral-splitting PVT (SSPVT) collectors remain unclear, with a lack of consensus in the literature. We develop an idealized model of SSPVT collectors and use this to determine their electrical and thermal efficiency limits, and to uncover how these limits can be approached through the selection of optimal PV cell materials and spectral-splitting filters. Assuming that thermal losses can be minimized, the efficiency limit, optimal PV material and optimal filter all depend strongly on a coefficient w, which quantifies the value of the delivered thermal energy relative to that of the generated electricity. The total (electrical plus thermal) efficiency limit of SSPVT collectors increases at higher w and at higher optical concentrations. The optimal spectral-splitting filter is defined by sharp lower- and upper-bound energies; the former always coincides with the bandgap of the cell, whereas the latter decreases at higher w. The total effective efficiency limit of SSPVT collectors is over 20% higher than those of either standalone PV modules or standalone ST collectors when w is in the range from 0.35 to 0.50 and up to 30% higher at w ≈ 0.4. This study provides a method for identifying the efficiency limits of ideal SSPVT collectors and reports these limits, along with guidance for selecting optimal PV materials and spectral-splitting filters under different conditions and in different applications.
Optical whispering-gallery mode barcodes for high-precision and wide-range temperature measurements
Jie Liao, Lan Yang
Published. 2021, 10(2) : 278-288 doi: 10.1038/s41377-021-00472-2
Temperature is one of the most fundamental physical properties to characterize various physical, chemical, and biological processes. Even a slight change in temperature could have an impact on the status or dynamics of a system. Thus, there is a great need for high-precision and large-dynamic-range temperature measurements. Conventional temperature sensors encounter difficulties in high-precision thermal sensing on the submicron scale. Recently, optical whispering-gallery mode (WGM) sensors have shown promise for many sensing applications, such as thermal sensing, magnetic detection, and biosensing. However, despite their superior sensitivity, the conventional sensing method for WGM resonators relies on tracking the changes in a single mode, which limits the dynamic range constrained by the laser source that has to be fine-tuned in a timely manner to follow the selected mode during the measurement. Moreover, we cannot derive the actual temperature from the spectrum directly but rather derive a relative temperature change. Here, we demonstrate an optical WGM barcode technique involving simultaneous monitoring of the patterns of multiple modes that can provide a direct temperature readout from the spectrum. The measurement relies on the patterns of multiple modes in the WGM spectrum instead of the changes of a particular mode. It can provide us with more information than the single-mode spectrum, such as the precise measurement of actual temperatures. Leveraging the high sensitivity of WGMs and eliminating the need to monitor particular modes, this work lays the foundation for developing a high-performance temperature sensor with not only superior sensitivity but also a broad dynamic range.
Confocal-based fluorescence fluctuation spectroscopy with a SPAD array detector
Eli Slenders, Marco Castello, Mauro Buttafava, Federica Villa, Alberto Tosi, et al.
Published. 2021, 10(2) : 289-300 doi: 10.1038/s41377-021-00475-z
The combination of confocal laser-scanning microscopy (CLSM) and fluorescence fluctuation spectroscopy (FFS) is a powerful tool in studying fast, sub-resolution biomolecular processes in living cells. A detector array can further enhance CLSM-based FFS techniques, as it allows the simultaneous acquisition of several samples–essentially images—of the CLSM detection volume. However, the detector arrays that have previously been proposed for this purpose require tedious data corrections and preclude the combination of FFS with single-photon techniques, such as fluorescence lifetime imaging. Here, we solve these limitations by integrating a novel single-photon-avalanche-diode (SPAD) array detector in a CLSM system. We validate this new implementation on a series of FFS analyses: spot-variation fluorescence correlation spectroscopy, pair-correlation function analysis, and image-derived mean squared displacement analysis. We predict that the unique combination of spatial and temporal information provided by our detector will make the proposed architecture the method of choice for CLSM-based FFS.