Jun Zhu

Surface-plasmon-enhanced GaN-LED based on a multilayered M-shaped nano-grating.

A multilayered metallic M-shaped nano-grating is proposed to enhance the internal quantum efficiency, light extraction efficiency and surface-plasmon (SP) extraction efficiency of the gallium nitride-based light emitting diodes. This structure is fabricated by the low-cost nano-imprint lithography. The suitable grating based on quasi-symmetrical-waveguide structure has a high transmission in the visible region. The properties of SP mode and the Purcell effect in this type of LED is investigated. The experimental results demonstrate that its peak photoluminescence intensity of the proposed LED is over 10 times greater than that from a naked GaN-LED without any nanostructure.

Design method of freeform off-axis reflective imaging systems with a direct construction process

In this paper, a novel method is proposed to design the freeform off-axis reflective imaging systems. A special algorithm is demonstrated to calculate the data points on the unknown freeform surface using the rays from multiple fields and different pupil coordinates. These points are used to construct the multiple three-dimensional freeform surfaces in an imaging system which works for a certain object size and a certain width of light beam. An unobscured design with freeform surfaces can be obtained directly with this method, and it can be taken as a good starting point for further optimization. The benefit of this design method is demonstrated by designing a freeform off-axis three-mirror imaging system with high performance and system specifications. The final system operates at F/1.49 with a 64mm entrance pupil diameter and an 8° × 9° field-of-view (FOV). The performance of the system is diffraction limited at LWIR (long-wavelength-infrared).

Design of a low F-number freeform off-axis three-mirror system with rectangular field-of-view

In this paper, a low F-number freeform off-axis three-mirror system with rectangular field-of-view (FOV) is presented, and the system has an integrated configuration that can be aligned with ordinary methods. The initial structure is solved analytically, and the optimization process is described in detail. This F/1.38 freeform system with a 4° × 5° FOV provides good imaging performance, especially in middle-wave and long-wave infrared bands. The primary mirror and the tertiary mirror are approximately tangent, so these two freeform mirrors can be fabricated on a single substrate. This configuration provides a solution to the practical application of freeform off-axis three-mirror systems. Additionally, a prototype of this system is demonstrated, and the imaging performance of the prototype is tested experimentally. The actual performance of the prototype is discussed and analyzed.

Direct design of freeform surfaces and freeform imaging systems with a point-by-point three-dimensional construction-iteration method.

In this paper, we proposed a general direct design method for three-dimensional freeform surfaces and freeform imaging systems based on a construction-iteration process. In the preliminary surfaces-construction process, the coordinates as well as the surface normals of the data points on the multiple freeform surfaces can be calculated directly considering the rays of multiple fields and different pupil coordinates. Then, an iterative process is employed to significantly improve the image quality or achieve a better mapping relationship of the light rays. Three iteration types which are normal iteration, negative feedback and successive approximation are given. The proposed construction-iteration method is applied in the design of an easy aligned, low F-number off-axis three-mirror system. The primary and tertiary mirrors can be fabricated on a single substrate and form a single element in the final system. The secondary mirror is simply a plane mirror. With this configuration, the alignment difficulty of a freeform system can be greatly reduced. After the preliminary surfaces-construction stage, the freeform surfaces in the optical system can be generated directly from an initial planar system. Then, with the iterative process, the average RMS spot diameter decreased by 75.4% compared with the system before iterations, and the maximum absolute distortion decreased by 94.2%. After further optimization with optical design software, good image quality which is closed to diffraction-limited is achieved.

Theoretical and experimental study of intensity branch phenomena in self-mixing interference in a He–Ne laser

A theoretical analysis and a comparison with the experimental results on the intensity branch of the self-mixing interference are presented. The theoretical model described considers dual optical feedback. Two important parameters ρ and δ are defined as the leaning factor and the intensity branch factor. ρ determines the intensity branch’s size in the feedback curve and whether the curve is leaning and how much it leans. δ determines the intensity branch’s position and the leaning direction in each period of the feedback curve. Experiments are done and the results are in good agreement with the theoretical analysis.

Generating optical freeform surfaces considering both coordinates and normals of discrete data points

Through direct design methods, both coordinates and normals of discrete data points on a freeform surface are usually obtained, but traditionally the freeform surface is generated by only fitting the coordinates. In this paper, a novel generating method that fits both the coordinates and the normals is proposed based on the mathematical multiobjective optimization theory. This new method is suited for generating some freeform polynomial surfaces. Two design examples are introduced, and their optical performance is significantly improved when applying the new method compared with the traditional method. This new method is validated to be effective, and it is widely useful as long as the coordinates and the normals are provided in advance.

Enhanced light extraction from a GaN-based green light-emitting diode with hemicylindrical linear grating structure

Significant enhancement in the light output from GaN-based green light-emitting diodes (LEDs) was achieved with a hemicylindrical grating structure on the top layer of the diodes. The grating structure was first optimized by the finite-difference time-domain (FDTD) method, which showed that the profile of the grating structure was critical for light extraction efficiency. It was found that the transmission efficiency of the 530 nm light emitted from the inside of the GaN LED increased for incidence angles between 23.58° and 60°. Such a structure was fabricated by electron-beam lithography and an etching method. The light output power from the LED was increased approximately 4.7 times compared with that from a conventional LED. The structure optimization is the key to the great increase in transmission efficiency. Furthermore, the light emitted from the edge of the LED units could be collected and extracted by the grating structures in adjacent LED units, thus enhancing the performance of the whole LED chip.

Design method of surface contour for a freeform lens with wide linear field-of-view

In this paper, a design method of surface contour for a freeform imaging lens with a wide linear field-of-view (FOV) is developed. During the calculation of the data points on the unknown freeform surfaces, the aperture size and different field angles of the system are both considered. Meanwhile, two special constraints are employed to find the appropriate points that can generate a smooth and accurate surface contour. The surfaces obtained can be taken as the starting point for further optimization. An f-θ single lens with a ± 60° linear FOV has been designed as an example of the proposed method. After optimization with optical design software, the MTF of the lens is close to the diffraction limit and the scanning error is less than 1 μm. This result proves that good image quality and scanning linearity were achieved.

Polarization control in a He-Ne laser using birefringence feedback

The polarization dynamics of laser subjected to weak optical feedback from birefringence external cavity are studied theoretically and experimentally. It is found that polarization flipping with hysteresis is induced by birefringence feedback, and the intensities of two eigenstates are both modulated by external cavity length. The variations of hysteresis loop and duty ratios of two eigenstates in one period of intensity modulation with phase differences of birefringence element in external cavity are observed. When the phase difference is ?/2, the two eigenstates will equally alternatively oscillate, and the width of hysteresis loop is the smallest.

True-color real-time imaging and spectroscopy of carbon nanotubes on substrates using enhanced Rayleigh scattering

Single-walled carbon nanotubes (SWCNTs) illuminated by white light should appear colored due to resonance Rayleigh scattering. However, true-color imaging of SWCNTs on substrates has not been reported, because of the extremely low scattering intensity of SWCNTs and the strong substrate scattering. Here we show that Rayleigh scattering can be greatly enhanced by the interface dipole enhancement effect. Consequently colorful SWCNTs on substrates can be directly imaged under an optical microscope by wide field supercontinuum laser illumination, which facilitates high throughput chirality assignment of individual SWCNTs. This approach, termed “Rayleigh imaging microscopy”, is not restricted to SWCNTs, but widely applicable to a variety of nanomaterials, which enables the colorful nanoworld to be explored under optical microscopes.