Volume 14, Issue 10 - Special Issue: Emerging phenomena in nanophotonics

Unitarity serves as a fundamental concept for characterizing linear and conservative wave phenomena in both classical and quantum systems. Developing platforms that perform unitary operations on light waves in a universal and programmable manner enables the emulation of complex light–matter interactions and the execution of general-purpose functionalities for wave manipulations, photonic computing, and quantum circuits. Recently, numerous approaches to implementing programmable photonic unitary circuits have been proposed and demonstrated, each employing different design strategies that distinctly impact overall device performance. Here, we review foundational design principles and recent achievements in the implementation of programmable photonic unitary circuits, with a particular focus on integrated photonic platforms. We classify the design strategies based on the dimensionality of nontrivial unit operations in their building blocks: lower-dimensional unitary units, such as SU(2) operations, and higher-dimensional ones, such as Fourier transforms. In each category, recent efforts to leverage alternative physical axes, such as the temporal and frequency domains, to address scalability challenges are also reviewed. We discuss the underlying concepts, design procedures, and trade-offs of each design strategy, especially in relation to light-based computing.

Nonlinear optics, the study of intense light–matter interactions, traditionally uses bulk materials like LiNbO 3 for device fabrication. However, these materials face challenges such as limited nonlinear susceptibility, large dimensions, and phase matching issues, limiting compact and integrated devices. Recent research has illuminated that a variety of low-dimensional materials exhibit markedly stronger nonlinear optical responses than their bulk counterparts. This has made nonlinear optics in low-dimensional materials a dynamic area of study, allowing for rapid light–matter interactions and advancing nonlinear nanophotonic and optoelectronic applications. These applications span diverse areas, from wavelength conversion and the generation of ultrashort laser pulses to advancements in quantum photonics and integrated photonic technologies. This review covers two-dimensional materials such as graphene and transition metal dichalcogenides to one-dimensional forms like carbon nanotubes and nanowires, and further to zero-dimensional structures including nanoparticles and quantum dots. By providing a comprehensive overview of the current state of non-linear optics in the context of low-dimensional materials, this review not only encapsulates the existing knowledge base but also charts a course for future explorations in this rapidly progressing domain.

Over the past two decades, two-dimensional (2D) van der Waals (vdW) semiconductors have garnered significant attention in the field of light sources due to their unique optoelectronic properties, such as high excitonic binding energy, tunable bandgaps, and strong optical anisotropy. These properties make 2D vdW semiconductors highly promising for next-generation light sources, offering advantages like enhanced efficiency, wavelength tunability, and polarization control. In this review, we summarize the development of various 2D vdW material-based light sources and their modulation mechanisms. We first provide an overview of excitonic properties and light-emission principles that aim to develop light sources with low-power, high-efficiency. Next, we discuss advances in 2D semiconductor lasers, including intralayer and interlayer exciton lasers, cavity-free systems, and exciton-polariton sources. We then look into single-photon emission and their integration into on-chip systems, followed by studies on nonlinear optical properties like high-order harmonic generation and P-band emission. Additionally, we cover advancements in electrically pumped light sources. The review concludes with an outlook on future developments of 2D vdW semiconductor light sources.

With the ongoing advancement of micro- and nanofabrication techniques, there has been a notable revival of interest in the field of stimulated Brillouin scattering within micro- and nanoscale waveguide structures in recent years. A variety of micro- and nanophotonic devices with different functions have been designed and fabricated, including lasers, amplifiers, isolators, sensors, filters, delay lines, and memory devices. Here, we provide a comprehensive review of stimulated Brillouin scattering in micro/nanophotonic waveguides and resonators on various promising material platforms, covering several key aspects such as the generation mechanisms of Brillouin nonlinear interactions in different waveguide structures and material platforms, methods for enhancing Brillouin gain, and a range of typical applications. Concluding our review, we offer insights into prospective future directions for this field.

Quantum emitters (QEs) are essential building blocks for quantum applications, such as quantum communication, quantum computing and metrology. Two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs) and hexagonal boron nitride (hBN), are promising platforms for scalable QE generation due to their unique properties, including their compatibility with external photonic structures. Advances in defect engineering and strain manipulation enable precise localization of emission sites within these materials, while integration with nanophotonic structures, including cavities and waveguides, enhances photon emission through the Purcell effect. This integration supports quantum functionalities like single-photon routing and spin-photon interactions. Challenges include achieving precise QE placement and emission control, as environmental factors can affect QE purity and indistinguishability. Nonetheless, electrically driven QEs, strain-tunable emission, and the integration of van der Waals magnets present opportunities for compact, scalable quantum devices with on-demand single-photon sources and spin-based quantum memory, positioning 2D QEs as foundational for next-generation quantum devices.

In this work, we explore how the optical properties of isotropic materials can be modulated by adjacent anisotropic materials, providing new insights into anisotropic light-matter interactions in van der Waals heterostructures. Using a WS 2 /ReS 2 heterostructure, we systematically investigated the excitation angle-dependent photoluminescence (PL), differential reflectance, time-resolved PL, and power-dependent PL anisotropy of WS 2 . Our findings reveal that the anisotropic optical response of WS 2 , influenced by the crystallographically low symmetry and unique dielectric environment of ReS 2 , significantly impacts both the optical and temporal behavior of WS 2 . We observed that the emission anisotropy increases with optical power density, highlighting that anisotropic localization of photo-generated carriers and subsequent charge transfer dynamics are key contributors to the polarization-sensitive optical response. These findings provide a framework for leveraging optical density-sensitive anisotropy mirroring to design advanced anisotropic optoelectronic and photonic devices.

As Moore’s law approaches its physical limits, the semiconductor industry has begun to focus on improving I/O density and power efficiency through 2.5D/3D packaging. Heterogeneous integration, which combines integrated circuit blocks from different linewidth processes into a single package, is central to these developments. To ensure stable connections with high yield in the back-end processes, high precision and high speed 3D surface measurement is the prerequisite. Existing methods such as white-light interferometry and confocal microscopy face challenges in balancing resolution, speed, and accuracy in 3D measurements. Here, we report a frequency-comb-referenced multiwavelength interferometry for the measurement of 3D sample profiles without 2π phase ambiguity for advanced packaging. Using four frequency-comb-referenced wavelengths with a fractional stability of 4.77 × 10 −12 , the measurement range was extended from ∼400 nm ( λ /2) to 1 mm, with the measurement repeatability of 0.258 nm for 32 measurements. The standard step-height samples with 500-µm and 4.5-µm steps, as well as real industrial microbumps in heterogeneous integration packaging, were all successfully measured. Therein, we devised a sequential phase detection method, which enables 5,000 times faster solution determination than the traditional recursive excess fraction method, while maintaining its reliability under noisy conditions. As 2.5D/3D packaging architectures become increasingly complex, our approach will readily meet the critical industrial demands for high-precision and high-speed measurement of multiscale features in advanced semiconductor packaging.

We develop a nanoheater utilizing a single Si nanowire with a porous segment that produces localized heat. The 19-fold higher resistivity of the porous segment compared to the solid segment in the nanowire facilitates the substantial confinement of heat to the porous segment by Joule heating. The heat profiles of the nanowire are examined using scanning thermal microscopy, a direct thermal imaging technique. The profiles recorded along the longitudinal and cross-sectional axes of the nanowire reveal that heat is concentrated in the sub-micrometer region of the porous segment, whereas it is uniformly distributed along the whole axis of the homogeneous solid Si nanowire. Moreover, the HfO 2 -passivated nanowire device exhibits a temperature increase above 10 °C within a 0.4 × 1 μm 2 area, which is advantageous compared to the 3.3 °C increase observed in the hBN-passivated device. These point heaters demonstrate considerable potential for future applications in biomedical engineering and optoelectronics.

The increasing global temperatures have escalated the demand for indoor cooling, thus requiring energy-saving solutions. Traditional approaches often integrate metal layers in cooling windows to block near-infrared (NIR) sunlight, which, albeit effective, lack the broad modulation of visible transmission and lead to heat accumulation due to sunlight absorption. Here, we address these limitations by developing cooling windows using ZnS/MgF 2 multilayers, optimized through a binary optimization-based active learning process. We demonstrated that these multilayers, with a total thickness below 1 µm, effectively reduced indoor temperatures by blocking NIR sunlight while achieving desired visible transmittance. The designed multilayers exhibited visible transmittance ranging from 0.41 to 0.89 while retaining decent NIR reflectance between 0.37 and 0.52. These spectral characteristics remained consistent up to incident angles of >60°, ensuring their practical applicability for vertically oriented windows. Outdoor experiments showed substantial temperature reductions of up to 8.8 °C on floors compared to uncoated glass windows. The active learning-based multilayers exhibited superior performance compared to analytical ZnS/MgF 2 distributed Bragg reflectors with equivalent thicknesses by improving NIR reflectance and modulating visible transmittance. In addition, multilayers with a greater number of bits extensively tuned transmission color, enabling customization for aesthetic purposes. These findings suggest that all-dielectric multilayers can provide a scalable, cost-effective alternative for reducing energy consumption in buildings and vehicles with large glass surfaces, supporting efforts to mitigate climate change through enhanced energy efficiency.

The W-band is essential for applications like high-resolution imaging and advanced monitoring systems, but high-frequency signal attenuation leads to poor signal-to-noise ratios, posing challenges for compact and multi-channel systems. This necessitates distinct frequency selective surfaces (FSS) on a single substrate, a complex task due to inherent substrate resonance modes. In this study, we use a digital metasurface platform to design W-band FSS on a glass substrate, optimized through binary optimization assisted by active learning. The digital metasurface is composed of a periodic array of sub-wavelength unit cells, each containing hundreds of metal or dielectric pixels that act as binary states. By utilizing a machine learning model, we apply active learning-aided binary optimization to determine the optimal binary state configurations for a given target FSS profile. Specifically, we identify optimal designs for distinct FSS on a conventional glass substrate, with transmittance peaks at 79.3 GHz and Q-factors of 32.7.

Understanding and controlling the antiferromagnetic order in multiferroic materials on an ultrafast time scale is a long standing area of interest, due to their potential applications in spintronics and ultrafast magnetoelectric switching. We present an optical pump-terahertz (THz) probe study on multiferroic Eu 0.75 Y 0.25 MnO 3 . The optical pump predominantly excites the d-d transitions of the Mn 3+ ions, and the temporal evolution of the pump-induced transient conductivity is measured with a subsequent THz pulse. Two distinct, temperature-dependent decay times are revealed. The shorter relaxation time corresponds to spin-lattice thermalization, while the longer one is ascribed to electron-hole recombination. A spin-selection rule in the relaxation process is proposed in the magnetic phase. Slight suppression of the electromagnons was observed after the optical pump pulse within the spin-lattice thermalization time scale. These observed fundamental magnetic processes can shed light on ultrafast control of magnetism and photoinduced phase transitions in multiferroics.

Photodetectors are crucial for modern technologies such as optical communications, imaging, autonomous vehicles, and machine vision. However, conventional semiconductor-based photodetectors require additional filtering systems due to their broad spectral response, leading to increased costs and complexity. Here, we present a narrow spectral response photodetector using hexagonally arranged plasmonic Au nanohole structures, eliminating the need for optical filters. The device achieves a full-width at half maximum (FWHM) bandwidth of ∼40 nm with a response peak at 760 nm and a linear photocurrent responsivity of 0.95 μA/W. The photothermoelectric effect, induced by the nonradiative decay of plasmonic resonance, converts optical radiation into an electric potential on the Au surface. The hexagonal nanohole design generates polarization-independent photocurrents and allows spectral tuning beyond the cutoff region of silicon photodetectors. This versatile approach enables customizable response characteristics across a broad wavelength range through geometric design, enhancing its potential for diverse applications.

We propose and experimentally demonstrate a small-mode volume bowtie cavity design for all-optical switching applications using a waveguide-cavity structure that exploits asymmetric Fano resonance lineshapes. The bowtie cavity has a mode volume that is five times smaller than conventional (H0-type) photonic crystal point-defect cavities enabling higher nonlinearity and faster switching. Blue and red-detuned Fano resonant devices based on bowtie cavity designs have been fabricated and characterized. Measured linear transmission spectra have been compared to coupled-mode theory models to extract key parameters such as Q-factors. Furthermore, all-optical switching at 2.5 Gbps have been demonstrated in a wavelength-conversion experiment.

Non-Hermitian photonic wave modulators utilizing exceptional points (EPs) was previously proposed as a potential approach to realize high-performance and compact optical modulators. However, their practical implementation has been restricted by their substantial footprint size limit due to stringent adiabatic conditions near EPs. Here, we demonstrate a principle for efficient wave modulation through optimized parametric trajectories around an EP. By employing rapid encirclement or parametric jump over the EP, we achieve an extinction ratio of 43 dB within a total device length of 15 coupling-length unit. Importantly, we show that adiabatic processes around the EP in the initial stage of the entire evolution can be replaced by an instantaneous parametric jump without compromising the switching performance in stark contrast to the conventional wisdom from the standard adiabaticity.

Solution-based optical gain materials offer a cost-effective path to coherent light sources. Further, bound states in the continuum (BICs) have garnered great interest owing to their diverging quality ( Q ) factors. Therefore, a hybrid of these – a solution-based material for optical gain and a BIC structure for the lasing mode – should constitute an ideal form factor for low-cost and low-threshold nanolasers. However, the nonuniform surface topography induced during the thin-film formation of a solution-based material, especially on top of a prepatterned substrate, can easily disrupt the structural symmetry required for a high- Q BIC, resulting in a degradation of Q . Thus, in this study, a simple surface-flattening technique utilizing a soft and flexible squeegee was applied, which realized the planar surface topography crucial for preserving the high Q promised by the BIC and achieving low-threshold lasing. We fabricated BIC nanolasers by incorporating colloidal quantum dots (CQDs) for optical gain into a two-dimensional photonic crystal backbone layer composed of Si 3 N 4 . By leveraging the unique properties of the BIC mode with a well-ordered surface, our CQD-based BIC laser exhibited a lasing threshold as low as 10.5 kW/cm 2 , which is significantly lower than those reported in previous studies.

Excitons, which are bound states of electrons and holes, in transition metal dichalcogenides (TMDCs) have been studied as an information carrier for realizing new types of optoelectronic devices. However, the charge neutrality of excitons inhibits the electric control of their motion, as seen in conventional electronic devices, except when utilizing a heterostructure. Here, we investigated the drift motion of trions, quasiparticles composed of an exciton bound to an excess charge, at room temperature in a suspended WS 2 monolayer by applying a gate-tunable electric field. Using a simple bottom-gate device, we can tune the electric field intensity and exciton-to-trion conversion ratio by increasing the charge density in the monolayer. Consequently, we experimentally observed that locally excited trions drift toward the center of the suspended monolayer. To understand the underlying mechanisms, we numerically simulated the trion drift using the drift-diffusion equation, accounting for the contributions from both the electric field and strain. The results confirmed that the electric field plays the dominant role in the drift phenomena. Our work offers a useful platform for realizing trion-based optoelectronic devices that are capable of operating even at room temperature.

We present a framework for the efficient and accurate computation of resonance modes in photonic waveguides. The framework is based on AAA rational approximation with the application of special light sources. It allows one to calculate only relevant modes, such as the fundamental resonance modes localized in the central core of the waveguides. We demonstrate the framework using an example from the literature, a hollow-core photonic crystal fiber. This waveguide supports many other modes, such as cladding modes and higher-order modes. These nonrelevant modes are not calculated, so that challenging post-processing with mode filtering is not required.

The nanoscale optical properties of high-quality MoS 2 nanoribbons are investigated using THz nanoscopy based on a scattering-type scanning probe. The nanoribbons comprise a multilayer core, surrounded by monolayer edges. A featureless complex permittivity spectrum covering the range 0.6–1.6 THz is extracted from experimental time-domain measurements through a minimization procedure, adopting an extended finite-dipole model of the probe–sample interaction. Real-space mapping of the nanoribbon reveals variations in the local permittivity down to the instrument-limited resolution, on the order of 30 nm. Clustering analysis statistically identifies regions of lower apparent permittivity that we attribute to a high curvature at the edges of the nanoribbon causing an increase in local material strain or cross-talk in the measured signal with topography-induced measurement artifacts. The core of the nanoribbon contains two regions that follow tightly distributed, but slightly shifted Gaussian statistics in complex permittivity space, with the real part mean of both distributions lying around 5.4 and compatible with literature values of the static permittivity of thin-film MoS 2 reported previously. Our results show that the nanoribbons exhibit a modest degree of dielectric variation at the nanoscale that could be explained by heterogeneous doping or variations in the local defect density. We believe that our approach could be useful for the direct real-space measurement of dielectric disorder in other low-dimensional semiconducting material systems.