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Nonlinear Optics|143 Article(s)
Ultrafast optical response and efficient wavelength conversion in epsilon-near-zero aluminum-doped zinc oxide thin films
Yuanyuan Gou, Yuan He, Tiantian Zhou, Yi Feng, Lili Miao, and Chujun Zhao
We demonstrated that the epsilon-near-zero (ENZ) aluminum-doped zinc oxide (AZO) thin film exhibited ultrafast nonlinear optical response and efficient third-harmonic generation (THG) experimentally. The AZO film showed sub-picosecond response and broadband wavelength-dependent nonlinear absorption and refraction properties. In addition, the AZO thin film can produce efficient THG with an efficiency of 0.63 × 10-6 at the ENZ wavelength. The experimental results revealed the exceptional nonlinear optical behavior in the AZO thin film, and may provide insights for designing all-optical ultrafast optoelectronic devices. We demonstrated that the epsilon-near-zero (ENZ) aluminum-doped zinc oxide (AZO) thin film exhibited ultrafast nonlinear optical response and efficient third-harmonic generation (THG) experimentally. The AZO film showed sub-picosecond response and broadband wavelength-dependent nonlinear absorption and refraction properties. In addition, the AZO thin film can produce efficient THG with an efficiency of 0.63 × 10-6 at the ENZ wavelength. The experimental results revealed the exceptional nonlinear optical behavior in the AZO thin film, and may provide insights for designing all-optical ultrafast optoelectronic devices.
Chinese Optics Letters
- Publication Date: Aug. 29, 2025
- Vol. 23, Issue 9, 091901 (2025)
Fabrication of nonlinear photonic crystals with few-micrometer periodicity via vacuum pyroelectric poling
Dingwei Liu, Xinyu Liu, Dan Wei, Yuntao Mo, Lei Shi, and Dunzhao Wei
Nonlinear photonic crystals (NPCs) with modulated second-order nonlinear coefficients (χ(2)) enable quasi-phase-matching (QPM) for efficient frequency conversion. Traditional electric-field poling is limited to two-dimensional domain engineering and cannot achieve three-dimensional (3D) χ(2) distributions, while femtosecond laser writing (FLW) offers greater control but introduces crystal damage. In this work, we use the pyroelectric-based fabrication process by performing the cooling step in a vacuum after FLW, suppressing thermal fluctuations, and maximizing the pyroelectric field. Vacuum cooling significantly improves domain inversion probability and uniformity compared to air cooling, making the periodicity close to electrical poling. Real-time polarized microscopy reveals improved domain growth, while nonlinear diffraction analysis confirms negligible refractive index changes. We demonstrate domain-inverted NPCs with a periodicity of 4 µm, achieving QPM at near-infrared wavelengths. This method provides a scalable and efficient pathway for advanced nonlinear photonic devices. Nonlinear photonic crystals (NPCs) with modulated second-order nonlinear coefficients (χ(2)) enable quasi-phase-matching (QPM) for efficient frequency conversion. Traditional electric-field poling is limited to two-dimensional domain engineering and cannot achieve three-dimensional (3D) χ(2) distributions, while femtosecond laser writing (FLW) offers greater control but introduces crystal damage. In this work, we use the pyroelectric-based fabrication process by performing the cooling step in a vacuum after FLW, suppressing thermal fluctuations, and maximizing the pyroelectric field. Vacuum cooling significantly improves domain inversion probability and uniformity compared to air cooling, making the periodicity close to electrical poling. Real-time polarized microscopy reveals improved domain growth, while nonlinear diffraction analysis confirms negligible refractive index changes. We demonstrate domain-inverted NPCs with a periodicity of 4 µm, achieving QPM at near-infrared wavelengths. This method provides a scalable and efficient pathway for advanced nonlinear photonic devices.
Chinese Optics Letters
- Publication Date: Jul. 31, 2025
- Vol. 23, Issue 8, 081902 (2025)
A comparison study on optical diffraction of various white light sources by a dual linear–nonlinear grating
Lihong Hong, Yu Zou, Junming Liu, and Zhi-Yuan Li
We make a comparison study of linear and nonlinear diffraction by a periodically poled lithium niobate (PPLN) dual linear–nonlinear thin-plate grating with weak surface corrugation against four types of white light sources. They are the ordinary halogen lamp thermal radiation source (WL1), the silica photonic crystal fiber supercontinuum nanosecond laser source (WL2), and two femtosecond white lasers made by a Ti:sapphire pump laser beam passing through a fused silica plate with significant spectrum broadening (WL3) and through a cascaded silica plate and a chirped PPLN crystal with ultrabroadband second-harmonic generation (WL4). The experiments show that the coherence, peak power, spectral bandwidth, profile, and flatness of the pump white light all contribute to shaping the characteristics of linear and nonlinear optical diffraction patterns. In particular, for WL1, WL2, and WL4, the peak power is not sufficiently large; thus, only linear diffraction occurs. For incoherent WL1, the interference is absent. Remarkably, when a femtosecond white laser with sufficiently large peak power (WL3) shines upon the PPLN grating, bright and sharp linear Bragg scattering spots, dispersive colored linear diffraction patterns, and nonlinear Cherenkov radiation nonlinear diffraction patterns are observed simultaneously by naked eyes. The experiments would enrich the basic physical and optical understanding of linear and nonlinear optical diffraction characteristics of ultrabroadband white laser sources. We make a comparison study of linear and nonlinear diffraction by a periodically poled lithium niobate (PPLN) dual linear–nonlinear thin-plate grating with weak surface corrugation against four types of white light sources. They are the ordinary halogen lamp thermal radiation source (WL1), the silica photonic crystal fiber supercontinuum nanosecond laser source (WL2), and two femtosecond white lasers made by a Ti:sapphire pump laser beam passing through a fused silica plate with significant spectrum broadening (WL3) and through a cascaded silica plate and a chirped PPLN crystal with ultrabroadband second-harmonic generation (WL4). The experiments show that the coherence, peak power, spectral bandwidth, profile, and flatness of the pump white light all contribute to shaping the characteristics of linear and nonlinear optical diffraction patterns. In particular, for WL1, WL2, and WL4, the peak power is not sufficiently large; thus, only linear diffraction occurs. For incoherent WL1, the interference is absent. Remarkably, when a femtosecond white laser with sufficiently large peak power (WL3) shines upon the PPLN grating, bright and sharp linear Bragg scattering spots, dispersive colored linear diffraction patterns, and nonlinear Cherenkov radiation nonlinear diffraction patterns are observed simultaneously by naked eyes. The experiments would enrich the basic physical and optical understanding of linear and nonlinear optical diffraction characteristics of ultrabroadband white laser sources.
Chinese Optics Letters
- Publication Date: Jul. 22, 2025
- Vol. 23, Issue 8, 081901 (2025)
Conical Raman–Nath nonlinear optical diffraction upon PPLN nonlinear gratings
Yu Zou, Lihong Hong, Jiacheng Li, Jianluo Chen, and Zhi-Yuan Li
In classical linear optics, when light shines upon a grating in the normal configuration where the incident plane of light is perpendicular to the optical axis of the grating, ordinary Bragg diffraction will occur. However, when light is incident in the general conical configuration where the incident plane of light is oblique to the optical axis of the grating, the Bragg diffraction becomes much more complicated. What happens to the nonlinear diffraction of a laser beam by a nonlinear grating? In this Letter, we wish to answer this interesting and fundamental question in the realm of nonlinear optics. We shine a Ti:sapphire femtosecond pulse laser beam (with a central wavelength at 800 nm) upon a periodically poled lithium niobate (PPLN) thin plate nonlinear grating and systematically investigate the Raman–Nath nonlinear diffraction (NRND) pattern of a second-harmonic generation (SHG) laser beam in various “conical” (or off-plane) incidence configurations characterized by both the polar angle α and azimuthal angle φ. By analyzing the diffraction characteristic and uncovering the underlying mechanisms of conical NRND nonlinear diffraction, we have provided a comprehensive understanding of its spatial behavior. The study of conical nonlinear diffraction enriches the understanding of the complicated interaction between the pump laser beam and the structured nonlinear medium and broadens the arena of nonlinear optics. In classical linear optics, when light shines upon a grating in the normal configuration where the incident plane of light is perpendicular to the optical axis of the grating, ordinary Bragg diffraction will occur. However, when light is incident in the general conical configuration where the incident plane of light is oblique to the optical axis of the grating, the Bragg diffraction becomes much more complicated. What happens to the nonlinear diffraction of a laser beam by a nonlinear grating? In this Letter, we wish to answer this interesting and fundamental question in the realm of nonlinear optics. We shine a Ti:sapphire femtosecond pulse laser beam (with a central wavelength at 800 nm) upon a periodically poled lithium niobate (PPLN) thin plate nonlinear grating and systematically investigate the Raman–Nath nonlinear diffraction (NRND) pattern of a second-harmonic generation (SHG) laser beam in various “conical” (or off-plane) incidence configurations characterized by both the polar angle α and azimuthal angle φ. By analyzing the diffraction characteristic and uncovering the underlying mechanisms of conical NRND nonlinear diffraction, we have provided a comprehensive understanding of its spatial behavior. The study of conical nonlinear diffraction enriches the understanding of the complicated interaction between the pump laser beam and the structured nonlinear medium and broadens the arena of nonlinear optics.
Chinese Optics Letters
- Publication Date: Jun. 17, 2025
- Vol. 23, Issue 7, 071902 (2025)
Optical orbital angular momentum transformation based on nonlinear fork-shaped gratings
Yilin Li, Tianxiang Xu, Ruwei Zhao, Shan Liu, and Yan Sheng
We fabricated a three-dimensional nonlinear photonic crystal in a Sr0.61Ba0.39Nb2O6 (SBN) crystal using femtosecond laser direct writing of ferroelectric domain structures. The crystal features three layers of fork-shaped gratings, each oriented differently. These gratings convert an incident vortex beam into a second-harmonic Gaussian beam in specific directions. By altering the vortex beam’s topological charge, we can control the emission direction of the second-harmonic Gaussian beam, enabling flexible all-optical switching and manipulation. This work provides a foundation for controlling photon angular momentum in nonlinear optical frequency-conversion processes. We fabricated a three-dimensional nonlinear photonic crystal in a Sr0.61Ba0.39Nb2O6 (SBN) crystal using femtosecond laser direct writing of ferroelectric domain structures. The crystal features three layers of fork-shaped gratings, each oriented differently. These gratings convert an incident vortex beam into a second-harmonic Gaussian beam in specific directions. By altering the vortex beam’s topological charge, we can control the emission direction of the second-harmonic Gaussian beam, enabling flexible all-optical switching and manipulation. This work provides a foundation for controlling photon angular momentum in nonlinear optical frequency-conversion processes.
Chinese Optics Letters
- Publication Date: Jun. 06, 2025
- Vol. 23, Issue 7, 071901 (2025)
Giant enhancement of second-harmonic generation from microcavity-integrated monolayer WS2|Editors' Pick
Boyu Xu, Xiao Xiong, Rui Niu, Guangyuan Qu, Chunhua Dong, Guang-Can Guo, and Xifeng Ren
Transition metal dichalcogenide two-dimensional (2D) materials, exhibiting extraordinary properties absent in their bulk forms, have garnered significant attention in the field of nonlinear optical devices. However, the atomic-level thickness limits the light absorption, which makes the intensity of the nonlinear signal extremely weak. Through transferring monolayer WS2 onto a silica microsphere, we report a giant second-harmonic generation (SHG) enhancement for approximately 1.46 × 107 times. The second-harmonic (SH) signal reaches 2.56 MHz pumped by a continuous wave laser of 2.5 mW. It is attributed to an enhancement of the pump laser due to the whispering gallery mode of the microsphere cavity. This work demonstrates the potential of microcavity-integrated monolayer 2D materials for nonlinear optics in integrated photonics. Transition metal dichalcogenide two-dimensional (2D) materials, exhibiting extraordinary properties absent in their bulk forms, have garnered significant attention in the field of nonlinear optical devices. However, the atomic-level thickness limits the light absorption, which makes the intensity of the nonlinear signal extremely weak. Through transferring monolayer WS2 onto a silica microsphere, we report a giant second-harmonic generation (SHG) enhancement for approximately 1.46 × 107 times. The second-harmonic (SH) signal reaches 2.56 MHz pumped by a continuous wave laser of 2.5 mW. It is attributed to an enhancement of the pump laser due to the whispering gallery mode of the microsphere cavity. This work demonstrates the potential of microcavity-integrated monolayer 2D materials for nonlinear optics in integrated photonics.
Chinese Optics Letters
- Publication Date: May. 23, 2025
- Vol. 23, Issue 6, 061901 (2025)
Cascaded optical parameter oscillator within lithium tantalate microdisk based on two periodically poled structures|Editors' Pick
Kun Zhang, Yifan Chen, Chongyang Xu, Hongquan Yao, Jian Ning, Xinjie Lü, Gang Zhao, Peng Zhan, Zhenda Xie, and Shining Zhu
The whispering gallery resonator (WGR) represents a promising avenue for the miniaturization of optical devices, while cascaded optical parameter oscillator (OPO) processes have not been realized in the WGR, to the best of our knowledge. We present a microdisk with quality factors up to 3.2 × 107, then embed two quasi-phase-matching structures inside it to demonstrate cascaded OPO. The cascaded OPO exhibits the same idler light output with the threshold of 32.7 mW at 36°C (1063.8 nm → 1566.6 nm + 3314.6 nm/1566.6 nm → 2970.4 nm + 3314.6 nm), while the operating threshold of OPO without cascade process is 4.32 mW. Moreover, diverse cascaded processes are observed, with the longest output wavelength reaching 4802.9 nm. Our results suggest the potential for a low-threshold cascade OPO based on WGR. The whispering gallery resonator (WGR) represents a promising avenue for the miniaturization of optical devices, while cascaded optical parameter oscillator (OPO) processes have not been realized in the WGR, to the best of our knowledge. We present a microdisk with quality factors up to 3.2 × 107, then embed two quasi-phase-matching structures inside it to demonstrate cascaded OPO. The cascaded OPO exhibits the same idler light output with the threshold of 32.7 mW at 36°C (1063.8 nm → 1566.6 nm + 3314.6 nm/1566.6 nm → 2970.4 nm + 3314.6 nm), while the operating threshold of OPO without cascade process is 4.32 mW. Moreover, diverse cascaded processes are observed, with the longest output wavelength reaching 4802.9 nm. Our results suggest the potential for a low-threshold cascade OPO based on WGR.
Chinese Optics Letters
- Publication Date: Apr. 10, 2025
- Vol. 23, Issue 4, 041902 (2025)
Nonlinear Cherenkov radiation in rotatory nonlinear optics
Zhongmian Zhang, Dazhi Lu, Haohai Yu, Huaijin Zhang, and Yicheng Wu
Nonlinear Cherenkov radiation is a phenomenon of light first observed in 1970 that can be manipulated by phase matching conditions. However, under a rotatory symmetry, the nonlinear Cherenkov radiation was still untouched, where the rotation parameters in optics would introduce an additional phase to the beam, change the phase velocity of the electromagnetic wave, and lead to novel optical phenomena. Here, we introduce rotation as a new freedom and study the nonlinear Cherenkov radiation in optically rotatory crystals in theory. With a quartz crystal as the representative, we derive theoretical variations, which show that the phase velocity of the crystal-coupled wave is found to be accelerated or decelerated by the rotational angular velocity, corresponding to the change of the Cherenkov radiation angle. In addition, the variation on the effective nonlinear coefficient of quartz crystals with rotational polarization direction is analyzed theoretically and used to simulate the Cherenkov ring distribution in rotatory nonlinear optics. This work introduces the rotation parameter into the non-collinear phase matching process and may inspire the development of modern photonics and physics in rotatory frames. Nonlinear Cherenkov radiation is a phenomenon of light first observed in 1970 that can be manipulated by phase matching conditions. However, under a rotatory symmetry, the nonlinear Cherenkov radiation was still untouched, where the rotation parameters in optics would introduce an additional phase to the beam, change the phase velocity of the electromagnetic wave, and lead to novel optical phenomena. Here, we introduce rotation as a new freedom and study the nonlinear Cherenkov radiation in optically rotatory crystals in theory. With a quartz crystal as the representative, we derive theoretical variations, which show that the phase velocity of the crystal-coupled wave is found to be accelerated or decelerated by the rotational angular velocity, corresponding to the change of the Cherenkov radiation angle. In addition, the variation on the effective nonlinear coefficient of quartz crystals with rotational polarization direction is analyzed theoretically and used to simulate the Cherenkov ring distribution in rotatory nonlinear optics. This work introduces the rotation parameter into the non-collinear phase matching process and may inspire the development of modern photonics and physics in rotatory frames.
Chinese Optics Letters
- Publication Date: Apr. 16, 2025
- Vol. 23, Issue 4, 041901 (2025)
THz radiation coherent accumulation along a two-color laser filament in air
Zeliang Zhang, Qiang Su, Lu Sun, Pengfei Qi, Zhiqiang Yu, Olga Kosareva, and Weiwei Liu
Terahertz (THz) radiation generation by two-color femtosecond laser filamentation is a promising path for high-intensity THz source development. The intrinsic characteristics of the filament, especially its length, play a crucial role in determining the THz radiation strength. However, a detailed analysis of the underlying physical mechanism and the quantitative correlation between the laser filament length and the THz radiation intensity under a high-peak-power driving laser is still lacking. In this paper, the effect of filament length on the THz radiation is investigated by modulating the basic characteristics of the two-color laser field and changing the focal length. Experimental results show that the long filament length is advantageous for improving THz radiation intensity. The theoretical simulation indicates that enhancement of THz radiation arises from coherent accumulation of THz wave produced at each cross-section along the filament. These insights suggest that extending the filament length is an effective scheme to enhance the intensity of THz radiation generated by the two-color femtosecond laser filament. Terahertz (THz) radiation generation by two-color femtosecond laser filamentation is a promising path for high-intensity THz source development. The intrinsic characteristics of the filament, especially its length, play a crucial role in determining the THz radiation strength. However, a detailed analysis of the underlying physical mechanism and the quantitative correlation between the laser filament length and the THz radiation intensity under a high-peak-power driving laser is still lacking. In this paper, the effect of filament length on the THz radiation is investigated by modulating the basic characteristics of the two-color laser field and changing the focal length. Experimental results show that the long filament length is advantageous for improving THz radiation intensity. The theoretical simulation indicates that enhancement of THz radiation arises from coherent accumulation of THz wave produced at each cross-section along the filament. These insights suggest that extending the filament length is an effective scheme to enhance the intensity of THz radiation generated by the two-color femtosecond laser filament.
Chinese Optics Letters
- Publication Date: Feb. 28, 2025
- Vol. 23, Issue 2, 021902 (2025)
Wave interference under self-phase modulation and triple frequency generation owing to few-cycle terahertz pulses propagating in a cubic nonlinear medium
Ilia Artser, Maksim Melnik, Anton Tcypkin, Igor Gurov, and Sergei Kozlov
Recently, in the field of nonlinear optics of the terahertz frequency range, numerous unique features have been discovered that distinguish it advantageously from nonlinear optics of the optical frequency range. This study demonstrates that the interference of radiations generated at triple frequencies and those due to self-phase modulation in a cubic nonlinear medium can be either constructive or destructive, depending on the parameters of the pulse at the input of the medium. As a result, for a single-cycle pulse, mutual attenuation of these effects is observed by a factor of 20, while for a single and a half-cycle pulse, mutual enhancement occurs by a factor of 1.7. The obtained features are in good agreement with existing experimental data. Thus, by varying the parameters of few-cycle terahertz waves, it is possible to control the nonlinear processes observed in optical media. This will allow for the future development of light-to-light control devices based on these principles. Recently, in the field of nonlinear optics of the terahertz frequency range, numerous unique features have been discovered that distinguish it advantageously from nonlinear optics of the optical frequency range. This study demonstrates that the interference of radiations generated at triple frequencies and those due to self-phase modulation in a cubic nonlinear medium can be either constructive or destructive, depending on the parameters of the pulse at the input of the medium. As a result, for a single-cycle pulse, mutual attenuation of these effects is observed by a factor of 20, while for a single and a half-cycle pulse, mutual enhancement occurs by a factor of 1.7. The obtained features are in good agreement with existing experimental data. Thus, by varying the parameters of few-cycle terahertz waves, it is possible to control the nonlinear processes observed in optical media. This will allow for the future development of light-to-light control devices based on these principles.
Chinese Optics Letters
- Publication Date: Mar. 05, 2025
- Vol. 23, Issue 2, 021901 (2025)
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