Background: As an important optical element, concave microlens arrays are utilized in many applications. How to fabricate a mass of concave microlens arrays efficiently at a low cost is a key problem to be solved.
Aim: We propose a method of fabricating a concave microlens array based on single mask ultraviolet (UV)-photolithography and dual-step potassium hydroxide (KOH) etching, which has proven to be efficient.
Approach: An arrayed silicon-based concave microlens utilized in the infrared wavelength range was designed and fabricated based on single mask UV-photolithography and dual-step KOH etching. Combining the computation simulation and the evolving microstructural mechanism based on the silicon anisotropic corrosion characteristics in a common KOH solution with several control factors such as the solution concentration, temperature, and corrosion period, an arrayed concave microlens with a spherical profile over a silicon wafer with the required crystal orientation was simulated, designed, and fabricated effectively.
Results: Both the scanning electron microscopy and the surface profile measurements indicate that the fabricated concave microlens arrays present a high filling-factor of more than 80% and a small surface roughness with a root mean square value in several tens of nanometer scale. The common optical measurements demonstrate that the fabricated silicon-based concave microlens presents a good infrared beam divergence performance.
Conclusions: The method highlights the prospect of the industrial production of large-area silicon-based concave microlens arrays for infrared beam shaping and control light applications.
Currently, optical antenna has already become a research hotspot because of its remarkable local field enhancement effect and resonance propagation characteristics. Optical antennas are usually designed as a type of sub-wavelength-scaled metal structures. By transmitting the field enhancement signal of the optical antenna to the infrared detector, the weak signal detection ability of the infrared detector can be improved. As demonstrated, the resonant wavelength of the local surface plasmons is determined by the structure and material of the antenna and also the material properties of the surrounding medium. By changing the geometry of the antenna or the dielectric characteristics of the circumstance medium, the response frequency of the optical antenna can be regulated. As a two-dimensional material with unique electrical and optical properties, the dielectric properties of graphene can be regulated by applied bias voltage. By selecting the geometry of the antenna and applying bias voltage, the optical antenna with unique characteristics can be obtained. In this paper, an optical antenna with a graphene-silica-silicon trilayer structure is designed and a planar-tip array is fabricated over the graphene layer. The influence of the geometry of the planar-apex array and the thickness of the silica dielectric layer corresponding to the optical properties of the graphene antenna are analyzed. Simulation results show that by changing the shape of the planar-tip and the thickness of the silica dielectric layer, the position and intensity of the absorption peak of the graphene optical antenna can be controlled effectively. At the same time, under the control of external bias voltage, the resonance peak also appears an obvious movement of a maximum range of about three microns.
As demonstrated, surface plasmons (SPs) stimulated by incident lightwaves are one of the most popular research fields, currently. The researches about the remarkable generation and efficient transmission and effective manipulation of relatively strong SPs are generally limited by a range constraint of wavelength or sub-wavelength-scaled structures. So far, the interaction between the electromagnetic field and the free electrons over the metal and medium interface or special metal micro-nano-structure has been mainly studied. In reality, a type of adjustable ionic exciter device is needed, which lead to a new focus about the adjustable ionic exciter materials. At present, two-dimensional graphene materials already demonstrate several excellent optical and electrical properties, and their conductivity and dielectric constant can be easily affected by external bias electric field, so as to exhibit a prospect as a kind of basic materials for adjustable and other excitation components. In this paper, the adjustable properties of single crystal graphene are studied. The effects based on the factors including the temperature and the scattering rate and the chemical potential corresponding to some parameters such as the conductivity and dielectric constant of graphene are analyzed carefully. In addition, the composite structure of the graphene grating nano-apexes is designed, which is characterized based on the multi-frequency points resonance according to incident light at the waveband of 4~11μm. The key graphene-based structure is modeled and simulated by the FDTD solution based on a finite difference time domain method under the different chemical potential. Then, the transmission and reflection and absorption behaviors of the graphene-based structure were analyzed according to the near electric field intensity distribution curves.
The Fabry-Perot interferometer (FP) can be used as a kind of filter for obtaining spectral information of targets in several wavelength ranges such as in the visible or infrared regions. So far, the spectral imaging devices based on FP effect mainly include the electrically controlled liquid-crystal filtering structures and the micro-electro-mechanical filtering architectures (MEMS). MEMS are generally micro-structures that integrate micro-sensors for converting incident microbeams into arrayed electronic signals and micro-actuators. The MEMS-FP filter constructed by combining the MEMS and FP functions, can be further integrated into a chip-level imaging spectrometer to achieve spectral imaging operation. The design of distributed Bragg reflectors (DBRs) is an important part to obtain a high transmittance for MEMS-FP structure. Different number of layers of optical film is calculated and compared in this paper and the transmittance can reach 82% and the FWHM is ~ 1nm in the infrared region of 3-5um. Angle of incidence is also considered and the simulation result shows poor robustness. We propose that two liquid-crystal microlens arrays can be mounted on FP arrays to get a high filling-factor and a more flexible range of incident angles.
So far, how to acquire an effective method of integration detection covering a relatively wide wavelength range has become a hot topic in the field of high performance radiation detection. In this work, the microstructure patterned Schottky-typed optical antenna is designed and then fabricated on Gallium Arsenide substrate and further used to sense near-infrared lights and terahertz signals, respectively. The wide frequency terahertz waves generated by InAs crystal are measured through patterned optical antenna device, and then the characteristics of transmitted waves are analyzed, it should be noted that the time delay characteristics of transmitted terahertz signals between 5ps and 8ps are different from microstructure patterned optical antenna. Under the conditions of using near-infrared lasers and also adjusting main parameters such as the exposure time, for example, 0.04ms、0.4ms、0.6ms、0.8ms、1.0ms and 1.5ms, in the experiments, the transmitted image characters acquired using functioned optical antennas with different electrode patterns, are analyzed. In the near-infrared transmission experiments, the transmitted bright light points or spots with relatively large distribution density and high intensity and very small structural size (~1μm), are discovered, which distribute over the top layer of electrode zone without metal structures of optical antenna device. The developed detection architecture based on Schottky-typed functioned optical antennas to sense infrared light and terahertz radiation, is expected to integrated sense electromagnetic signals in wide spectrum regime.
Surface plasmons, as a local electromagnetic field mode generated or stimulated at the interface between common metal and dielectric, can be used to greatly break through the optical diffraction limit and also localize the electric-field and then light energy in a sub-wavelength scale. It is already a research hotspot in recent years. As shown, several patterned metal nano-array can be utilized to produce relatively strong surface plasmon resonance, so as to achieve a nano-scaled localized light field on the surface of the functioned metal structure. In this paper, silicon dioxide materials are used as the substrate, and the common gold materials are fabricated into a metal film, and then the sub-wavelength metal nano-tip arrays with several morphology such as the cone-shaped, the triangular-pyramid-shaped, and the quadrangular-pyramid-shaped, are designed respectively. The functioned metal nano-structures are symmetric and asymmetric coating mode. The electric field distribution characteristics of the structure under the internal excitation mode of the incident light with vertical incidence are analyzed. The simulations show that the local field enhancement can be clearly observed at the nano-tip of the cone-shaped in the asymmetric case, but the symmetry is not. Analysis shows that the destructive interference occurs when the surface plasmons are excited by a linearly polarized light on both sides of a conical structure propagate to its top, so failure to produce focusing effect. Therefore, to the case of symmetrical film through adjusting the incident angle of light, different incident angles will affect the enhancement of the local field at the tip of the cone.
Metallic micro-nano-structure arrays can be used to induce a collective oscillation of free electrons on the surface of metal films, so as to generate relatively strong surface plasmons (SPs) at the metal and medium interface and further localized light field under the excitation of incident lightwaves. As the oscillating light field propagating along the interface, the field strength can be increased reasonably at the functioned metal surface such as the incident light energy being localized in the sub-wavelength region defined by the functioned micro-nano-structures. The common beam diffraction limit formed during lightwave transmission or process can be broken effectively. Through constructing SPs over the special micro-nano-structures, the infrared reflection characteristics can be changed and then the local light field originated from incident infrared radiation also be enhanced significantly so as to efficiently perform infrared detection. Generally, the reflectivity and light field distribution behaviors of the functioned metal surface can be modulated by changing featured parameters of the metallic micro-nano-structural arrays. In this paper, a metal micro-nano-patterned structures with an arrayed tip is established for compressing the incident light field and then reducing the reflectivity of the metal surface and thus sensing incident light energy. A finite integral method for simulating and analyzing the structural characters such as the distance between tips, the tip sharpness, the thickness of the metal film, is utilized to acquire the reflectivity and field enhancement characteristics. The infrared reflection spectrum and the near-field intensity distribution of the metallic micro-nano-structure are compared and analyzed. The results show that the response frequency and excitation intensity of SPs over the nano-tip array, the intensity and distribution region of the strong light field, can be controlled by matching the structural parameters and layout. The optimization of the metallic micro-nanostructure arrays is conducted so as to lay a solid foundation for further development of the similar technologies.
The Fabry-Perot interferometer (FP) can be used as a kind of filter for obtaining spectral information of targets in several wavelength ranges such as in the visible or infrared regions. Micro-electro-mechanical systems (MEMS) are generally micro-structures that integrate micro-sensors for converting incident microbeams into arrayed electronic signals and micro-actuators. The MEMS-FP filter constructed by combining the MEMS and FP functions, can be further integrated into a chip-level imaging spectrometer to achieve spectral imaging operation. In our design, the MEMS-FP filter is also mounted a liquid-crystal microlens array with a high filling-factor. The key micro-bridges of each MEMS-FP unit are modeled and simulated in this paper. We have designed two types of supporting structures and simulated them with the simulation software COMSOL Multiphysics 5.2. The key factors include tuning range, filling-factor, and parallelism of the bridges. After calculating and analyzing, we found that the tuning range can be optimized by changing the thickness of the micro-bridge and the arm width of the cantilever beam. The filling-factor is already increased by geometry design. The parallelism of the bridge in the two micro-structures differs greatly, which is related to the shape of the bridge itself. According to the simulations, a tuning range of 160 nm has been achieved in the visible and near-infrared wavelength range, with a maximum filling-factor of more than ~80%.
Generally optical micro-nano-antenna can be used to modulate lightwaves in the sub-wavelength scale, which is a hot and difficult research issue. Patterned metal nano-antenna array can be utilized to stimulate intense surface plasmon polaritons (SPPs), so as to realize sub-wavelength focusing by breaking through diffractive limit, and thus remarkably improving THz imaging efficiency. In this article, firstly, based on SPPs, the Drude dispersion model for metallic film is analyzed, and the dispersion relations and excitation modes of the SPPs are discussed, and the numerical analysis methods of the metallic micro-nano-antenna are also presented including a time-domain finite integral method and a frequency-domain finite element method. According to related literature, the key optical micro-nano-antenna unit is modeled, and a metasurface formed by etching a gold thin film on a silicon substrate is designed. Through regulating parameters including the number and size of the openings and the line width, the SPP excitation in THz band is studied. Using finite element and adaptive mesh division method, the common electromagnetic properties such as transmission intensity and electric field distribution are simulated and analyzed. The simulations show that the optical micro-nano-antenna element can resonantly induct terahertz wave, and demonstrate a resonant electric-field at the aperture gap, which will move towards high frequencies end as increasing the gap size or line width, so as to lay a concrete foundation for continuously fabrication THz-SPPs devices.
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