On the occasion of the 40th anniversary of the Institute of Modern Optics (IMO), Nankai University (19th October 2024), Chinese Optics Letters (COL) is publishing this special issue to highlight the most exciting research on optics and photonics performed recently at the IMO or by the alumni in the world widely, with all manuscripts being subject to the normal standards and peer-review processes of COL.

The IMO was founded in 1984 by the late academician Guoguang Mu, who was the former President of Nankai University, Chairman of the China Optical Society, and Vice Chairman of the International Optical Committee. Since then, the IMO has conducted top-level research and provided education in optics and photonics. The IMO has been honored as the first-level national key discipline since 2002. In 2003, in order to broaden the scope of the academic curriculum and to provide a unique learning experience for undergraduate students, the Department of Optoelectronic Technology and Science was established for joint education with Tianjin University in Optoelectronic Information Science and Engineering. During the past 40 years, more than 1800 degrees have been awarded by the IMO. Its alumni spread around the globe, and many have become internationally renowned scientists. To recognize its educational achievement, the IMO has received the Society of Photo-Optical Instrumentation Engineers (SPIE) Excellence in Education Award. The IMO will continue to carry forward the character of Nankai, develop its spirit, persist in giving top priority to cultivating the integrity of students, strengthen its quality, and develop towards the goal of building a world-leading institute.

Here, we thank Chinese Laser Press for celebrating the 40th anniversary of the IMO with us in the form of a special issue. In this special issue, we organize relevant experts to report their latest research achievements and look forward to future trends in the fields of micro-nano optics, ultrafast laser optics, THz photonics, fiber optics, and so on. The successful publication of this special issue will help to expand our communication with colleagues all over the world to better serve the optics and photonics community.

This special issue includes nine research articles^[1–9] and one invited review article^[10] whose research discuss the generation of multi-freedom controllable helical beam using high-order Bessel beams^[1]; a nanotip design for high-resolution terahertz scattering-type scanning near-field optical microscopy (s-SNOM)^[2]; a high-modulation-efficiency thin-film lithium niobate (TFLN) modulator using a three-mode folded phase shifter^[3]; an eight-channel microwave-photonics transceiver photonic integrated circuit (PIC)^[4]; the remote sensing of the trace element Na in sea salt aerosol particles with a sensitivity level of 10pg/m3^[5]; the generation and reconfiguration of interference-pattern helico-conical beams (HCBs)^[6]; a TFLN dual parallel Mach–Zehnder modulator (DPMZM) for a simple photonic system for Doppler frequency shift (DFS) measurement^[7]; compact 1×3 waveguide beam splitters with variable splitting ratios in a LiNbO3 crystal fabricated by femtosecond laser direct writing^[8]; the rapid diagnosis of a femtosecond laser filament by a single laser shot-induced acoustic pulse^[9]; and the integrated solid-state light detection and ranging (LiDAR) employing orthogonal polarizations and counter propagation^[10].

Jiang et al.^[1] propose a scheme to generate helical optical fields with multi-freedom controllable features, including the generation of high-quality helical lobes with adjustable radii, chirality, and lobe numbers, and control of the pitch of the helical beam. The scheme is demonstrated in both simulations and experiments and is promising for rapid fabrications of helical structures with diverse parameters.

Zhang et al.^[2] report a feasible model to design optimal parameters of a THz near-field scattering nanotip with 0.5 nm (1/1200000 wavelength) spatial resolution while maintaining an excellent near-field coupling efficiency. This model describes the nanotip’s electromagnetic response in the near field and provides a feasible method for designing the nanotip of the THz s-SNOM system.

Liu et al.^[3] propose and experimentally demonstrate an electro-optic modulator with a small footprint and high modulation efficiency, which is achieved through the utilization of a mode-folded phase shifter with lumped electrodes. A 3.7-times improvement of the modulation efficiency is obtained experimentally. A low VπL for TFLN Mach–Zehnder modulators of 1 V·cm is realized with a device footprint of 2.7mm×0.6mm (0.5 mm for phase shifter).

Li et al.^[4] propose a multi-channel PIC that integrates lasers, modulators, amplifiers, and detectors in the module, and an eight-channel array transceiver module is manufactured successfully. The cascaded bandwidth of the eight-channel transceiver module is greater than 40 GHz, and the spurious-free dynamic range of the broadband array receiver module is greater than 94 dBm·Hz^2/3. The noise figure is less than −35dB, and the link gain is greater than −26dB.

Wang et al.^[5] develop a remote sea salt aerosol fluorescence spectroscopy system integrating a high-power industrial-grade femtosecond laser to enhance detection sensitivity and precision in complex environments. This system successfully detects sea salt aerosol particles, achieving a detection limit of 0.015ng/m3 for the neutral Na element (Na I) at 589 nm wavelength, with a detection range of 30 m.

Xu et al.^[6] propose an approach for the generation of interference-pattern HCBs both theoretically and experimentally. The HCBs exhibiting intricate fringe structures are obtained by exploiting amplitude modulation and interference techniques. By manipulating the azimuthal term within the helico-conical phase expression, several illustrative cases are presented.

Tao et al.^[7] design and fabricate a DPMZM based on TFLN, achieving a 3 dB electro–electro bandwidth of 29 GHz and a low drive voltage (Vπ=6V). A single-channel DFS measurement system is proposed based on this device for verification experiments. By measuring the beat frequency of the reference signal and the echo signal, a DFS value is obtained without directional ambiguity.

Yu et al.^[8] fabricate waveguide beam splitters (1×3) with type I modifications in a LiNbO3 crystal using femtosecond laser direct writing. A waveguide beam splitter with a 1:1:1 splitting ratio is fabricated by changing the relative widths of the three branch-waveguides. Guiding performances at 532 nm wavelength are measured and analyzed by a typical end-face coupling system. The simulation and experimental results demonstrate that the beam splitting ratio of the waveguide splitter can be precisely regulated by the positions and widths of the sub-waveguides.

Zhang et al.^[9] propose a novel filament diagnosing method that can measure the longitudinal spatial distribution of the filament by a single laser shot-induced acoustic pulse. Using the measured relation among the acoustic emission and the detection distance and angle, a single measurement of the acoustic emission generated by a single laser pulse can diagnose the spatial distribution of the laser filament through the Wiener filter deconvolution algorithm. The results obtained by this method are in good agreement with those of traditional point-by-point acoustic diagnosis methods.

He et al.^[10] present a review that introduces the fundamental principle of an optical phased array for beam steering, and provides an overview of the recent advancements in integrated solid-state LiDAR utilizing orthogonal polarization and counter propagation to enhance beam steering range and angular resolution.

Finally, we express our sincere gratitude to COL for providing the opportunity to publish this special issue, and we thank the editorial team of COL for their hard and outstanding work. We also appreciate all the authors for contributing their high-quality papers to this special issue.