Zhen-Nan Tian

Beam shaping of edge-emitting diode lasers using a single double-axial hyperboloidal micro-lens

We report an innovative approach for beam shaping of edge-emitting semiconductor diode lasers using a single double-axial hyperboloidal micro-lens fabricated with femtosecond laser direct writing technology. The two hyperboloids of different axial lengths focus the light from fast and slow axes to an identical focal spot. The divergence angles were shaped from 60° and 9° to 6.9 and 32.3 mrad, respectively, and the single-end fiber coupling efficiency is measured higher than 80%. The device is simple in fabrication, robust in structure, and easy for operation, by which multiple reflections and absorptions at interfaces are reduced, and assembly errors are minimized.

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.

Integrated optofluidic-microfluidic twin channels: toward diverse application of lab-on-a-chip systems

Optofluidics, which integrates microfluidics and micro-optical components, is crucial for optical sensing, fluorescence analysis, and cell detection. However, the realization of an integrated system from optofluidic manipulation and a microfluidic channel is often hampered by the lack of a universal substrate for achieving monolithic integration. In this study, we report on an integrated optofluidic-microfluidic twin channels chip fabricated by one-time exposure photolithography, in which the twin microchannels on both surfaces of the substrate were exactly aligned in the vertical direction. The twin microchannels can be controlled independently, meaning that fluids could flow through both microchannels simultaneously without interfering with each other. As representative examples, a tunable hydrogel microlens was integrated into the optofluidic channel by femtosecond laser direct writing, which responds to the salt solution concentration and could be used to detect the microstructure at different depths. The integration of such optofluidic and microfluidic channels provides an opportunity to apply optofluidic detection practically and may lead to great promise for the integration and miniaturization of Lab-on-a-Chip systems.

High Curvature Concave–Convex Microlens

We report in this letter a concave-convex microlens (CCML) consisting of two different high curvature surfaces. Compared with the conventional plano-convex microlenses, the CCML not only allows for more design freedom, but also offers significantly improved optical performance, particularly minimization of aberration, as is critical when the size of optical components is small. Experimentally, focusing capability at different wavelengths was demonstrated, and the axial chromatic aberration was found significantly reduced to ~4.6 % of the focal length shift under wavelength 450-660 nm.

As-grown graphene/copper nanoparticles hybrid nanostructures for enhanced intensity and stability of surface plasmon resonance

The transfer-free fabrication of the high quality graphene on the metallic nanostructures, which is highly desirable for device applications, remains a challenge. Here, we develop the transfer-free method by direct chemical vapor deposition of the graphene layers on copper (Cu) nanoparticles (NPs) to realize the hybrid nanostructures. The graphene as-grown on the Cu NPs permits full electric contact and strong interactions, which results in a strong localization of the field at the graphene/copper interface. An enhanced intensity of the localized surface plasmon resonances (LSPRs) supported by the hybrid nanostructures can be obtained, which induces a much enhanced fluorescent intensity from the dye coated hybrid nanostructures. Moreover, the graphene sheets covering completely and uniformly on the Cu NPs act as a passivation layer to protect the underlying metal surface from air oxidation. As a result, the stability of the LSPRs for the hybrid nanostructures is much enhanced compared to that of the bare Cu NPs. The transfer-free hybrid nanostructures with enhanced intensity and stability of the LSPRs will enable their much broader applications in photonics and optoelectronics.

Micro-buried spiral zone plate in a lithium niobate crystal

We present a micro-buried spiral zone plate (MBSZP) in the lithium niobate crystal fabricated with femtosecond laser direct writing technology. The microstructures of the MBSZP are buried under the surface of the crystal, which ensures the stability of the optical performance in various refractive index environments. The optical performances of imaging and focusing capabilities were demonstrated. In addition, the experiment showed good agreement with simulation results based on the optical wave propagation method. This novel optical element will have important applications in multistate information encoding, optical manipulation, quantum communication, and computation, especially in high integration, contact coupling, and variable refractive index environments.

Mode-Selecting Micrograting Cavity Laser

We demonstrate a novel mode-selecting microcavity laser composed of gratings and a microdisk and fabricated by femtosecond-laser direct-writing technology. The microcavity laser shows good directivity and a single longitudinal mode, resulting from the mode coupling between the microdisk and gratings. The lasing wavelength can be regulated by adjusting the grating period. The properties of the laser were investigated and show good agreement with theoretical calculation and simulation. Based on the behavior and properties, this novel microcavity laser demonstrates good application prospects in integrated microoptics devices.

Hybrid Refractive–Diffractive Optical Vortex Microlens

We report a novel hybrid refractive-diffractive microlens combined with spiral phase for the generation of optical vortex, which is fabricated via femtosecond laser direct writing technology. The unique optical performance of focusing capability is demonstrated. At the focus position, the hollow focus with different integer topological charges is investigated. Moreover, experimental results are supported by finite-element calculation. The novel microlens generating an optical vortex will fulfill important applications in optical manipulation, multistate information encoding, quantum communication, and computation, particularly in the compaction, integration, and simplification of optical vortex generation system.

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.