Co-editors in chief Anatoly Zayats and Xiao-Cong (Larry) Yuan look back on Advanced Photonics in 2020 and welcome the new year.

Professor Yuri Kivshar relates the course of his scientific research career.

Organic photoresponsive materials can undergo various reversible variations in certain physical and chemical properties, such as optical properties, electrochemical properties, conformation, and conductivity, upon photoirradiation. They have been widely applied in various optoelectronic fields, especially in information storage. We summarize research progress on organic photoresponsive materials for information storage. First, the design strategies and photoswitching mechanisms for various kinds of organic photoresponsive materials, including small organic molecules, metal complexes, polymers, supramolecules, and cholesteric liquid crystals, are systematically summarized. These materials exhibit reversible changes of absorption and/or emission properties in response to different wavelengths of light. Subsequently, the applications of these organic materials in information storage, such as data (re)writing and erasing, encryption and decryption, and anticounterfeiting, are introduced in detail. Finally, the current challenges and future directions in this rapidly growing research field are discussed. The review will provide important guidance on the future works about the design of excellent organic photoresponsive materials for optoelectronic applications.

Cylindrical vector beams and vortex beams, two types of typical singular optical beams characterized by axially symmetric polarization and helical phase front, possess the unique focusing property and the ability of carrying orbital angular momentum. We discuss the formation mechanisms of such singular beams in few-mode fibers under the vortex basis and show recent advances in generating techniques that are mainly based on long-period fiber gratings, mode-selective couplers, offset-spliced fibers, and tapered fibers. The performances of cylindrical vector beams and vortex beams generated in fibers and fiber lasers are summarized and compared to give a comprehensive understanding of singular beams and to promote their practical applications.

Nonlinear holography has been identified as a vital platform for optical multiplexing holography because of the appearance of new optical frequencies. However, due to nonlinear wave coupling in nonlinear optical processes, the nonlinear harmonic field is coupled with the input field, laying a fundamental barrier to independent control of the interacting fields for holography. We propose and experimentally demonstrate high-dimensional orbital angular momentum (OAM) multiplexing nonlinear holography to overcome this problem. By dividing the wavefront of the fundamental wave into different orthogonal OAM channels, multiple OAM and polarization-dependent holographic images in both the fundamental wave and second-harmonic wave have been reconstructed independently in the spatial frequency domain through a type-II second harmonic generation process. Moreover, this method can be easily extended to cascaded χ² nonlinear optical processes for multiplexing in more wavelength channels, leading to potential applications in multicasting in optical communications, multiwavelength display, multidimensional optical storage, anticounterfeiting, and optical encryption.

The fundamental properties of laser-induced plasma in liquid water, such as the ultrafast electron migration and solvation, have not yet been clarified. We use 1650-nm femtosecond laser pulses to induce the plasma in a stable free-flowing water film under the strong field ionization mechanism. Moreover, we adopt intense terahertz (THz) pulses to probe the ultrafast temporal evolution of quasifree electrons of the laser-induced plasma in water on the subpicosecond scale. For the first time, the THz wave absorption signal with a unique two-step decay characteristic in time domain is demonstrated, indicating the significance of electron solvation in water. We employ the Drude model combined with the multilevel intermediate model and particle-in-a-box model to simulate and analyze the key information of quasifree electrons, such as the frequency-domain absorption characteristics and solvation ratio. In particular, we observe that the solvation capacity of liquid water decreases with the increase of pumping energy. Up to ∼50  %   of quasifree electrons cannot be captured by traps associated with the bound states as the pumping energy increases to 90  μJ  /  pulse. The ultrafast electron evolution in liquid water revealed by the optical-pump/THz-probe experiment provides further insights into the formation and evolution mechanisms of liquid plasma.

Chromatic aberration-free meta-devices (e.g., achromatic meta-devices and abnormal chromatic meta-devices) play an essential role in modern science and technology. However, current efforts suffer the issues of low efficiency, narrow operating band, and limited wavefront manipulation capability. We propose a general strategy to design chromatic aberration-free meta-devices with high-efficiency and ultrabroadband properties, which is realized by satisfying the key criteria of desirable phase dispersion and high reflection amplitudes at the target frequency interval. The phase dispersion is tuned successfully based on a multiresonant Lorentz model, and high reflection is guaranteed by the presence of the metallic ground. As proof of the concept, two microwave meta-devices are designed, fabricated, and experimentally characterized. An achromatic meta-mirror is proposed within 8 to 12 GHz, and another abnormal chromatic meta-mirror can tune the reflection angle as a linear function. Both meta-mirrors exhibit very high efficiencies (85% to 94% in the frequency band). Our findings open a door to realize chromatic aberration-free meta-devices with high efficiency and wideband properties and stimulate the realizations of chromatic aberration-free meta-devices with other functionalities or working at higher frequency.

Optical-resolution photoacoustic microscopy (OR-PAM) has been developed for anatomical, functional, and molecular imaging but usually requires multiple scanning for different contrasts. We present five-wavelength OR-PAM for simultaneous imaging of hemoglobin concentration, oxygen saturation, blood flow speed, and lymphatic vessels in single raster scanning. We develop a five-wavelength pulsed laser via stimulated Raman scattering. The five pulsed wavelengths, i.e., 532, 545, 558, 570, and 620  /  640  nm, are temporally separated by several hundreds of nanoseconds via different optical delays in fiber. Five photoacoustic images at these wavelengths are simultaneously acquired in a single scanning. The 532- and 620  /  640-nm wavelengths are used to image the blood vessels and dye-labeled lymphatic vessels. The blood flow speed is measured by a dual-pulse method. The oxygen saturation is calculated and compensated for by the Grüneisen-relaxation effect. In vivo imaging of hemoglobin concentration, oxygen saturation, blood flow speed, and lymphatic vessels is demonstrated in preclinical applications of cancer detection, lymphatic clearance monitoring, and functional brain imaging.

Liquid droplets offer a great number of opportunities in biochemical and physical research studies in which droplet-based microlasers have come into play over the past decade. While the recent emergence of droplet lasers has demonstrated their powerful capabilities in amplifying subtle molecular changes inside the cavity, the optical interactions between droplet resonators and an interface remain unclear. We revealed the underlying mechanism of droplet lasers when interacting with a droplet–solid interface and explored its correlation with intermolecular forces. A vertically oriented oscillation mode—arc-like mode—was discovered, where the number of lasing modes and their Q-factors increase with the strength of interfacial hydrophobicity. Both experimental and theoretical results demonstrated that hydrophobicity characterized by contact angle and interfacial tension plays a significant role in the geometry of droplet cavity and laser mode characteristics. Finally, we demonstrated how tiny forces induced by proteins and peptides could strongly modulate the lasing output in droplet resonators. Our findings illustrate the potential of exploiting optical resonators to amplify intermolecular force changes, providing comprehensive insights into lasing actions modulated by interfaces and applications in biophysics.

High-index dielectric resonators support different types of resonant modes. However, it is challenging to achieve a high-Q factor in a single dielectric nanocavity due to the non-Hermitian property of the open system. We present a universal approach of finding out a series of high-Q resonant modes in a single nonspherical dielectric cavity with a rectangular cross section by exploring the quasi bound-state-in-the-continuum (QBIC). Unlike conventional methods relying on heavy brutal force computations (i.e., frequency scanning by the finite difference time domain method), our approach is built upon Mie mode engineering, through which many high-Q modes can be easily achieved by constructing avoid-crossing (or crossing) of the eigenvalue for pair-leaky modes. The calculated Q-factor of mode TE(5,7) can be up to Q_theory  =  2.3  ×  10⁴ for a freestanding square nanowire (NW) (n  =  4), which is 64 times larger than the highest Q-factor (Q_theory  ≈  360) reported so far in a single Si disk. Such high-Q modes can be attributed to suppressed radiation in the corresponding eigenchannels and simultaneously quenched electric (magnetic) field at momentum space. As a proof of concept, we experimentally demonstrate the emergence of the high-Q resonant modes [Q  ≈  211 for mode TE(3,4), Q  ≈  380 for mode TE(3,5), and Q  ≈  294 for mode TM(3,5)] in the scattering spectrum of a single silicon NW.

The article provides information about the image on the cover of Advanced Photonics, Volume 3, Issue 1.