Narrow band pass filter pdf

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Narrow band pass filter Apr. Student at Koneru Lakshmaiah College of Engineering. Tripping and control of impulse generators. Clippers AND Clampers.

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What to Upload to SlideShare. Related Books Free with a 30 day trial from Scribd. Dry: A Memoir Augusten Burroughs. The proposed hyperbolic metamaterial filter in this project is designed with the metal wire mesh perpendicular to the alternative layers of dielectric materials, keeps TM center wavelength unchanged for the different angle of incident light in MDIR regime.

The geometric size of this nanostructure is smaller than the working wavelength and supports big wavevectors due to hyperbolic dispersion. In contrast with conventional Bragg stack, the copper fakir bed makes the transmission properties of the filter the same. For this purpose, the state- of-the-art fabrication methods are required to make such small dimensions in alternative layers of amorphous silicon and silicon dioxide.

In this work, first we demonstrate the simulation of Bragg stack with RCWA and finite element methods. Finally, we experimentally verify the optical characteristic of the fabricated filter using Fourier-transform infrared spectroscopy. The experimental and spectrometry data shows that transmission properties of the hyperbolic metamaterial filter remain the same for oblique TM polarized incident light. Keywords: nanofabrication, hyperspectral metamaterial, lithography, angle independency, negative photoresist 1.

Among various types of the metamaterials, hyperbolic metamaterials HMM have gained more attentions in sensing and imaging applications [1, 2]. The dielectric tensor of this media has positive and negative values, makes the non-magnetic HMM anisotropic. In our work, the proposed HMM Brag stack is composed of metal wire mesh as hyperbolic metamaterial and conventional Bragg stack, makes the angle shift negligible and therefore, remains the TM center wavelength the same.

This is in contrast with angle-dependent conventional notch filter that center wavelength of light changes for different angles of incident light. This type of filter is applied in many hyperspectral and imaging systems that are sensitive to the shift of the wavelength of light. The unit cell of the homogenous HMM is much smaller than the wavelength of light. The shrinkage of feature size of these structures leads to investigate on new techniques of lithography, as the old fashion fabrication methods have the resolution limits [3].

Therefore, novel fabrication methods have been introduced to guarantee the high resolution and mass production of nanostructures. The main goal of this article is to introduce the new fabrication methods to build the proposed metal-dielectric HMMS and then measuring the optical properties of the filter, in addition to show the simulation results with RCWA and finite element methods. In first section, the background and related work to HMM is explained.

Section 2 investigates on 1Send correspondence to Golsa Mirbagheri E-mail: gm duke. Next section discusses about the optical result of the fabricated HMM and the last section, concludes the project. However, recently they have been investigated intensively in opto-electronics area for their extraordinary phenomena including the high-k modes support, phase transition and density of photonic states [4, 5, 6].

In this part, some recent works relative to hyperbolic metamaterials and surface plasmon are revied. In [7] , the surface plasmon dispersion is adjusted by tuning the permittivity and electric filed of the metal-like multilayers structures, this is applicable by adding several dielectric layers or doped semiconductor conductive layers.

These SP structures are similar to antenna, in such a way that their concentric periodic circles couple the light cone with SP modes and make a rise to the field around the aperture, which leads to extraordinary transmission. In [9] showed that the engineered structure with dimensions smaller than working wavelength can have the negative refraction, like negative index metamaterial NIM.

The dispersion relation of the proposed indefinite hyperbolic metamaterial media is composed of perpendicular negative and parallel positive permittivities.

The pointing vector in this structure is normal to the anisotropic surface, but in different direction in isotropic media, which is in contrast with NIM with anti-parallel directions.

High index prism or periodic structure were used to excite surface plasmon modes with light cone, showed in [10]. The narrowband hyperspectral GMRF acts as a Fabry-Perot microcavity, mirrors the pixels of the filter to the sensor pixels. In [12], the angle-independent filter composed of high refractive index material is described. The Fabry-Perot cavity-based filter has the a-Si dielectric layer sandwiched between the silver layer and chrome layer, makes the considerable reflection phase shift that reduce the angle sensitivity [13, 14].

In [15], the bandpass Fabry-Perot resonator filter composed of two high index Bragg reflectors is separated by a low loss dielectric subwavelength layer. By changing the geometry properties of the middle meta-surface layer, the center wavelength is controlled as phasing shift function.

Conventional Bragg Stack Filters The conventional Bragg stack consists of alternative layers of materials with high and low dielectric constant. The resonant condition, which happens in this middle layer, is written as Eq 1 [17, 18, 19].

However, the notch filter performance restricts to the angle of incidence light, such that the center wavelength of light changes for off-normal incidence light [20, 21, 22, 23]. In middle, each pixel of the HMBS filter with different geometry, transmits different wavelength of light.

In right, center wavelength changes by modifying the unit cell geometry. The HMBS is composed of alternative dielectric layers of silicon oxide and amorphous silicon, with subwavelength-sized copper wires etched in the three middle layers of the stack.

The thickness of SiO2 and a-Si dielectric layers are nm and nm sequentially, while the middle resonant layer thickness is 1. Due to fabrication issues, it was not possible to etch the whole 7 layers down; also, new methods were devised to electroplate the copper wires in the middle layers. The HMBS can transmit the different wavelength of light by adjusting the size of each unit cell, as depicted in Figure 1 middle.

The transmitted light then is broken apart and detected by relative pixel of detector array in spectrometer. In hyperspectral imaging applications, the notch filters need collimated light to keep the dispersion properties unchanged. This adds extra optic equipment, cost and wight to the imaging systems. In this work, the subwavelength-sized wire mesh, perpendicular to the layers of HMBS, removes the dependency of angle of light and therefore, the filter is applicable for both focused and collimated incident MIDIR light [39, 40, 41, 42].

Full-Wave Finite Element Modeling Simulation The Maxwell-Garnett theory applies effective media approximation approach to calculate the permittivity, as discussed before [43, 44, 45].

However, in finite element method, the frequency-dependent epsilon of the materials is computed which resulted in more reliable result. Therefore, the optical results of these two methods are different. Figure 2 shows different results between Maxwell-Garnett and full-wave electromagnetic modeling. Furthermore, the electric filed is close to the metal wires in Maxwell-Garnett theory, while uniform filed is distributed around copper wire in FEM modeling [19]. Figure 2. In fabrication process, there were lithography challenges to etch through the whole layers of the filter, in addition to the electroplating the copper in the etched holes.

In this section, first all the steps to build up the chip are outlined, then we probe the novel techniques to overcome the fabrication issues. Deposit alternative layers The first step of building the stack was depositing the dielectric layers of a-Si and SiO2 on 4inch silicon wafer. Depending on the fabrication plan, the whole layers could be deposited in one step or two with plasma-enhanced chemical vapor deposition PECVD method.

Due to aspect ratio, etching the whole 3 middle layers in one step was not possible, so first two layers of a-Si and SiO2 were deposited on top the first SiO2 layer to be etched before depositing the fourth layer. After lithography process, the two layers etched, and the remaining photoresist removed.

However, depositing the second a-Si on top of the two middle layers filled the holes, as shown in Figure 3. Different scenarios were devised to etch down the three dielectric layers which are discusses in next section [19]. The next steps were using hard mask and backside alignment for the purpose of multiple steps lithography. Figure 3. The top a-Si deposited on the etched two middle layers, left some a-Si in the holes.

Photolithography Nanolithography technology brought significant advance to fabrication of nanometer-scale structures [46, 47]. We started with positive photoresist which is spin-coated and then exposed on the wafer with ultraviolet light through a dark-field mask. The exposed photoresist resolved and disappeared by the solution in developing process, left the designed pattern on the wafer, as depicted in Figure 4 left.

This is different for negative photoresist, where unexposed area is resolved with the solution. Figure 4 right shows the lithography pattern on the wafer with negative resist and bright field mask.