Shubin Li

Polarization-Independent Absorber Based on a Cascaded Metal–Dielectric Grating Structure

The spectrum selective absorption effect of a vertically cascaded metal-dielectric grating structure is studied. A polarization-independent spectrum selective absorber based on two pairs of stacked metal-dielectric grating structures is presented for the infrared frequencies. The near-unity absorption with polarization independence is observed under normal incidence. The understanding of such a perfect selective absorption mechanism is illustrated by investigating the electric field distributions and power loss density at the resonant wavelength. The angle independence is also studied for this absorber and it is found that the absorber can maintain high absorbance around large incident angle range (0° -33° ), especially for the TM polarization.

Mode conversion and coupling in a slanted grating

We have proposed a novel transmission slanted grating at the central wavelength of 1550 nm, which can be used in optical communication. We have presented an approximate analytical expression that provides an insightful physical description of the simplified modal method for the slanted grating. The odd grating mode, which only exists in the asymmetric structure under normal incidence, plays the positive role of enhancing the -1st order diffraction efficiency. The analytic expressions of mode conversion and coupling can be obtained to explain the asymmetric field distribution, which cannot occur in the rectangular grating region. Numerical results achieved by the rigorous wave analysis verify the validity of the simplified modal method. We expect that the theoretical modal method set forth in this work will be helpful for the tremendous potential application of the slanted grating.

Generation of controllable rotating petal-like modes using composited Dammann vortex gratings

A new type of diffractive optical element, called a composited Dammann vortex grating (CDVG), is proposed for generation of multiple equal-energy controllable rotating petal-like modes extra cavity. As an example, it is shown that a petal-like mode is well generated for each nonzero diffraction order by a binary pure-phase 1×7 CDVG. Mode decomposition is digitally implemented by a programmable spatial light modulator (SLM), and the experimental results show that those generated petal-like patterns are in high mode purity (∼90%) for all six different nonzero orders. Also, controllable rotating petal-like modes are demonstrated when the CDVG is digitally implemented by the programmable SLM, which provides the possibility to quantitatively control the rotation rate of this type of optical tweezers. Furthermore, tunable petal-like modes are also demonstrated experimentally by introducing a vortex incident field with different topological charges.

Simple design of slanted grating with simplified modal method

A simplified modal method (SMM) is presented that offers a clear physical image for subwavelength slanted grating. The diffraction characteristic of the slanted grating under Littrow configuration is revealed by the SMM as an equivalent rectangular grating, which is in good agreement with rigorous coupled-wave analysis. Based on the equivalence, we obtained an effective analytic solution for simplifying the design and optimization of a slanted grating. It offers a new approach for design of the slanted grating, e.g., a 1×2 beam splitter can be easily designed. This method should be helpful for designing various new slanted grating devices.

High-efficiency fused-silica reflection grism

A fused-silica reflection grism (combination of grating and prism) based on the phenomenon of total internal reflection (TIR), and used in the -1st order, is designed and fabricated. The grism is etched directly into the fused-silica prism, which greatly facilitates the use of the TIR grating as no other angle coupling devices are involved. The grating profile is optimized by the use of the rigorous coupled-wave analysis method. Diffraction efficiency of larger than 99% at a wavelength of 980 nm for TM-polarized waves can be theoretically obtained. Two-beam interference lithography and inductively coupled plasma etching techniques are used to manufacture such grism. Diffraction efficiencies of larger than 95% are experimentally demonstrated.

Pitch evaluation of high-precision gratings

Optical encoders and laser interferometers are two primary solutions in nanometer metrology. As the precision of encoders depends on the uniformity of grating pitches, it is essential to evaluate pitches accurately. We use a CCD image sensor to acquire grating image for evaluating the pitches with high precision. Digital image correlation technique is applied to filter out the noises. We propose three methods for determining the pitches of grating with peak positions of correlation coefficients. Numerical simulation indicated the average of pitch deviations from the true pitch and the pitch variations are less than 0.02 pixel and 0.1 pixel for these three methods when the ideal grating image is added with salt and pepper noise, speckle noise, and Gaussian noise. Experimental results demonstrated that our method can measure the pitch of the grating accurately, for example, our home-made grating with 20μm period has 475nm peak-to-valley uniformity with 40nm standard deviation during 35mm range. Another measurement illustrated that our home-made grating has 40nm peak-to-valley uniformity with 10nm standard deviation. This work verified that our lab can fabricate high-accuracy gratings which should be interesting for practical application in optical encoders.

Study of a grating interferometer with high optical subdivision technique

Displacement laser interferometers and grating interferometers are two main apparatus for the micron-nanometer displacement measurement over a long range. However, the laser interferometers, whose measuring scale is based on the wavelength, are very sensitive to the environment. On the contrast, the grating interferometers change the measuring scale from wavelength to grating period, which is much stable for the measurement results. But the resolution of grating interferometer is usually lower than that of laser interferometer. Therefore, further investigation is needed to improve the performance of grating interferometer. As we known, the optical subdivision is a main factor that affects the measurement resolution. In this paper, a grating interferometer with high optical subdivision is presented based on the Littrow configuration. We mainly use right angle prisms accompanied with plane mirrors to make the measuring lights diffracted by the grating scale for many times. An optical subdivision factor of 1/24 can be obtained by this technique. A main difficulty of this technique is that the grating scale should be with high diffraction efficiency. Fortunately, the measuring light is incident on the grating scale at the Littrow angle, the grating scale can be designed with very high efficiency easily in this condition. Compared with traditional grating interferometers, this kind of grating interferometer can greatly increase the measuring resolution and accuracy, which could be widely used in nanometer-scale fabrications and measurements.

High-density grating pair for displacement measurement

A novel method of displacement measurement based on a high density grating pair is proposed. When a laser beam is incident normal to a closely placed high density grating pair, efficiencies of transmission diffraction orders will change periodically along with the relative displacement of the two gratings in the grating period direction. The period of efficiency changing is equal to the grating period, thus measurement of displacement in the grating period direction can be achieved by detecting the power of diffraction orders.

Polarization-independent Talbot effect

We report the first observation of polarization-independent Talbot effect with a high-density grating for TE and TM polarizations, which is attributed to the identical phases and diffraction efficiencies of the diffraction orders for both polarizations. We introduce the simplified modal method that provides an insightful physical description for explanation of the diffraction efficiency and phase of the polarization-independent Talbot effect. Only two even grating modes can be excited, which determines the diffraction properties of the near-field image. We expect that this theoretical work will be helpful for the tremendous potential applications of the Talbot effect.

Simplified modal method for holographic grating

we present the simplified modal method for the holographic grating, which is a physical insight method. The difficult diffraction process can be vividly interpreted by this simple method.