Three-dimensional laser micromachining system with integrated sub-100†nm resolution in-situ measurement.

In this study, a three-dimensional (3D) laser micromachining system with an integrated sub-100 nm resolution in-situ measurement system was proposed. The system used the same femtosecond laser source for in-situ measurement and machining, avoiding errors between the measurement and the machining positions. It could measure the profile of surfaces with an inclination angle of less than 10°, and the measurement resolution was greater than 100 nm. Consequently, the precise and stable movement of the laser focus could be controlled, enabling highly stable 3D micromachining. The results showed that needed patterns could be machined on continuous surfaces using the proposed system. The proposed machining system is of great significance for broadening the application scenarios of laser machining.

Arbitrary fabrication of complex lithium niobate three-dimensional microstructures for second harmonic generation enhancement.

Lithium niobate (LN) crystal plays important roles in future integrated photonics, but it is still a great challenge to efficiently fabricate three-dimensional micro-/nanostructures on it. Here, a femtosecond laser direct writing-assisted liquid back-etching technology (FsLDW-LBE) is proposed to achieve the three-dimensional (3D) microfabrication of lithium niobate (LN) with high surface quality (Ra = 0.422 nm). Various 3D structures, such as snowflakes, graphic arrays, criss-cross arrays, and helix arrays, have been successfully fabricated on the surface of LN crystals. As an example, a microcone array was fabricated on LN crystals, which showed a strong second harmonic signal enhancement with up to 12 times bigger than the flat lithium niobate. The results indicate that the method provides a new approach for the microfabrication of lithium niobate crystals for nonlinear optics.

Single femtosecond pulse writing of a bifocal lens.

In this Letter, a method for the fabrication of bifocal lenses is presented by combining surface ablation and bulk modification in a single laser exposure followed by the wet etching processing step. The intensity of a single femtosecond laser pulse was modulated axially into two foci with a designed computer-generated hologram (CGH). Such pulse simultaneously induced an ablation region on the surface and a modified volume inside the fused silica. After etching in hydrofluoric acid (HF), the two exposed regions evolved into a bifocal lens. The area ratio (diameter) of the two lenses can be flexibly adjusted via control of the pulse energy distribution through the CGH. Besides, bifocal lenses with a center offset as well as convex lenses were obtained by a replication technique. This method simplifies the fabrication of micro-optical elements and opens a highly efficient and simple pathway for complex optical surfaces and integrated imaging systems.

High-Transmission Biomimetics Structural Surfaces Produced via Ultrafast Laser Manufacturing.

Inspired by periodically aligned micro/nanostructures on biological surfaces, researchers have been fabricating biomimetic structures with superior performance. As a promising and versatile tool, an ultrafast laser combined with other forms of processing technology has been utilized to manufacture functional structures, e.g., the biomimetic subwavelength structures to restrain the surface Fresnel reflectance. In this review paper, we interpret the biomimetic mechanism of antireflective subwavelength structures (ARSSs) for high-transmission windows. Recent advances in the fabrication of ARSSs with an ultrafast laser are summarized and introduced. The limitations and challenges of laser processing technology are discussed, and the future prospects for advancement are outlined, too.

Ultralow birefringent glass waveguide fabricated by femtosecond laser direct writing.

Optical waveguides prepared by femtosecond laser direct writing have birefringent properties, which can affect polarization encoding and entanglement on chips. Here, we first propose a shape-stress dual compensation fabrication scheme to decrease birefringence. Ultralow birefringent waveguides (1 × 10-9) were obtained by controlling the cross sectional shape of the main waveguide and adjusting the position of the auxiliary lines. In addition, we prepared polarization-independent directional coupler and demonstrated the evolution of polarization-independent waveguide array with different polarized light. In the future, ultralow birefringent waveguides will be widely applied in polarization encoding and entangled quantum photonic integrated circuits.

Machine vision-based high-precision and robust focus detection for femtosecond laser machining.

We propose a machine vision-based focus detection method (MVFD) for femtosecond laser machining. By analyzing the laser focus pattern, the defocus direction and distance are obtained simultaneously. The proposed technique presents high precision with an average error of 0.047 µm and a root mean square error (RMSE) of 0.055 µm. Moreover, the method is robust and is less affected by the tilted sample. For the curved surface sample, the average error and RMSE are 0.093 and 0.145 µm, respectively. Thus, the proposed focus detection method can be easily combined with laser processing equipment, which is widely used in large-range and high-precision femtosecond laser processing.

Ultrafast laser-inscribed nanogratings in sapphire for geometric phase elements.

We systematically studied femtosecond laser-inscribed self-organized nanogratings and geometric phase elements such as a polarization diffraction focusing lens and Q-plate in sapphire crystal. Besides the void structures observed in the focus, nanogratings with periods of 150~300 nm were observed, depending on a nanoslit that took the role of a seeding effect by localized light field enhancement. The non-polarized refractive index change and birefringence were measured with values around 1∼2×10-3 and 6×10-4, respectively. Based on the laser-inscribed form birefringence, a geometric phase lens and Q-plate were successfully demonstrated in sapphire with high imaging and a focusing effect. We expect that our findings may promote the understanding of laser-induced nanogratings in bulk and potential applications in geometric phase elements.

O-FIB: far-field-induced near-field breakdown for direct nanowriting in an atmospheric environment.

Nanoscale surface texturing, drilling, cutting, and spatial sculpturing, which are essential for applications, including thin-film solar cells, photonic chips, antireflection, wettability, and friction drag reduction, require not only high accuracy in material processing, but also the capability of manufacturing in an atmospheric environment. Widely used focused ion beam (FIB) technology offers nanoscale precision, but is limited by the vacuum-working conditions; therefore, it is not applicable to industrial-scale samples such as ship hulls or biomaterials, e.g., cells and tissues. Here, we report an optical far-field-induced near-field breakdown (O-FIB) approach as an optical version of the conventional FIB technique, which allows direct nanowriting in air. The writing is initiated from nanoholes created by femtosecond-laser-induced multiphoton absorption, and its cutting "knife edge" is sharpened by the far-field-regulated enhancement of the optical near field. A spatial resolution of less than 20 nm (λ/40, with λ being the light wavelength) is readily achieved. O-FIB is empowered by the utilization of simple polarization control of the incident light to steer the nanogroove writing along the designed pattern. The universality of near-field enhancement and localization makes O-FIB applicable to various materials, and enables a large-area printing mode that is superior to conventional FIB processing.

Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass.

Polarization and geometric phase shaping via a space-variant anisotropy has attracted considerable interest for fabrication of flat optical elements and generation of vector beams with applications in various areas of science and technology. Among the methods for anisotropy patterning, imprinting of self-assembled nanograting structures in silica glass by femtosecond laser writing is promising for the fabrication of space-variant birefringent optics with high thermal and chemical durability and high optical damage threshold. However, a drawback is the optical loss due to the light scattering by nanograting structures, which has limited the application. Here, we report a new type of ultrafast laser-induced modification in silica glass, which consists of randomly distributed nanopores elongated in the direction perpendicular to the polarization, providing controllable birefringent structures with transmittance as high as 99% in the visible and near-infrared ranges and >90% in the UV range down to 330 nm. The observed anisotropic nanoporous silica structures are fundamentally different from the femtosecond laser-induced nanogratings and conventional nanoporous silica. A mechanism of nanocavitation via interstitial oxygen generation mediated by multiphoton and avanlanche defect ionization is proposed. We demonstrate ultralow-loss geometrical phase optical elements, including geometrical phase prism and lens, and a vector beam convertor in silica glass.

Mirror-rotation-symmetrical single-focus spiral zone plates.

In this Letter, we report mirror-rotation-symmetrical single-focus spiral zone plates (MS-SZPs) fabricated by femtosecond laser direct writing. The novel optical element can generate a single-focus vortex beam, owing to the element's complicated continuous surface. The MS-SZP surface possesses reverse mirror-rotation symmetry, which ensures that the transfer element has the same surface morphology as the original element. Both the transfer element and original element have good optical properties. The single-focus behavior was investigated by a microscopic imaging system and found to be in good agreement with theoretical simulation results. The innovative optical component is expected to be widely used in optical communication, quantum computation, optical manipulation, and other fields.

Multilevel phase-type diffractive lens embedded in sapphire.

Herein, we report a kinoform phase-type lens (KPL), which is fabricated by femtosecond (fs)-laser-induced refractive index change inside sapphire crystal. By fabricating volume phase gratings in sapphire and measuring the energy ratio of the grating's first and second diffraction orders, the refractive index change in sapphire induced by fs-laser modification was obtained. Then a four-level KPL was designed and fabricated inside sapphire following the experimentally established scaling of the refractive index change and fs-laser power. Importantly, the KPL has unique UV focusing and imaging capability as well as a stable optical performance in different refractive index environments. The KPL embedded in sapphire has the same optical performance after a high-temperature (1050°C) annealing for 30 min. The KPLs in sapphire have great potential to increase light extraction efficiency in GaN blue-UV light-emitting diodes and can be used in micro-optical sensor applications in chemically harsh and high-temperature environments.

Single-mode unidirectional microcavity laser.

In this Letter, we report a suspended whispering gallery mode microdisk with a hole pierced through its surface. The novel disk is made up of Rhodamine B-doped resin, which is fabricated by femtosecond laser direct writing technology. The pierced microcavity achieves highly directional emission of single-mode lasing with a far field divergence angle of about 10 deg, and its high Q factor exceeds 2.6×103. The excellent properties are confirmed by numerical simulation based on the finite-difference time-domain method. The effect of the pierced hole on the microcavity performance is discussed in detail. The method is easy to implement and has a guiding significance for improving the characteristics of an existing microcavity by simple modification.

Fabrication of an anti-reflective microstructure on sapphire by femtosecond laser direct writing.

Herein, we report a facile approach for the maskless production of subwavelength-structured antireflective surfaces on sapphire with high and broadband transmittance in the mid-IR: femtosecond laser direct writing assist with wet etching. With this method, inverted pyramid and cone arrays with a pitch of about 2 µm and a total height of near 900 nm on the sapphire were produced. The resulting subwavelength structures greatly suppress specular reflection at normal incidence. The transmission measurements between 3 and 5 µm are in agreement with the simulations performed using VirtualLab, and the transmittance reached a maximum value of 92.5% at 4 µm. The sapphire with subwavelength structures also exhibits angle-independent transmittance characteristics up to a high θ=60°. Therefore, these subwavelength structures on sapphire are of great technological importance in mid-IR optics, especially for the harsh-condition-applicable windows of military mid-IR devices.

Focal varying microlens array.

We report a novel microlens array with different curvature unit lenses (MLADC) fabricated with femtosecond laser direct writing technology. The MLADC consisted of hexagonal hyperboloid unit microlenses, which have different heights and curvatures from others. The unique optical performance of imaging and focusing capability were demonstrated. An object was imaged at different positions from the MLADC by unit lenses, as the ability of adjusting the curvature of the image plane for overall MLADC. In addition, the experiment had a good agreement with simulation results, which was based on the analysis of the finite element method. The novel MLADC will have important applications in improving the performance of optical systems, especially in field curvature correction and real-time three-dimensional imaging.

Electro-optical detection based on large Kerr effect in polymer-stabilized liquid crystals.

In this Letter, polymer-stabilized liquid crystals with experimentally observed large electro-optic effect are introduced to the electro-optical detection to improve the voltage sensitivity. The Kerr constant of materials prepared in this study reached as high as 7.2×10(-9) m/V(2), increasing by 1000 times the sensitivity of the conventional electro-optical materials. The noncontact detection configuration, using a laser beam as a probe, enables quick two-dimensional scanning measurements.

Ultrahigh sensitivity electric field detection with a liquid electro-optical film.

In this Letter, an electro-optical probe configuration with polar molecule liquids as the sensing film is proposed to improve the voltage sensitivity. This method exhibited increases in intrinsic sensitivities better than 0.1 mV/√Hz, 2 orders of magnitude larger than the normal method using a GaAs probe in the same measurement system. Based on the mechanism of orientation polarization, the electro-optic coefficient was measured to be 250 pm/V by the Teng-Man method at a modulation field of 100 Hz. This technology will be promising in applications of low-frequency field detection.