Due to the epsilon near zero (ENZ) effect, indium tin oxide (ITO) can be used in optical modulators and reduce the modulator’s size dramatically. The tunability of optical properties and the CMOS compatible capability make ITO more attractive. To study the properties of ITO thin films, several works have been done. Firstly, thin ITO thin films were obtained by magnetron sputtering with different oxygen flow rates ranging from 0 to 50sccm. Secondly, EDS was carried out to investigate the elements' content. It can be found that increasing oxygen flow rate increases the percentage of oxygen atom and Sn atom of ITO thin films. Thirdly, surface profiler was used to measure the stress value of the ITO thin films. We find that the tensile stress of ITO thin films tends to transform into compressive stress when the oxygen flow rate rises, which is worth considering in the design of devices. Fourthly, spectrometer and Hall effect measurement were applied to measure the normal incidence transmittance and electrical properties of the ITO thin films. Larger oxygen flow rate leads to the normal incidence transmittance of ITO thin films becoming larger. Hall effect measurement contributes to the conclusion that the carrier concentration of ITO thin films is able to range from 1019 to 1021 cm-3, and that when the oxygen flow rate is not too large, as the environment oxygen increases, the carrier concentration decreases and the mobility increases. This research can contribute to the design of compact ITO based optical modulators so as to achieve a better performance, which can further the integration of optical modulators.
A compact polarization demultiplexer (P-DeMux) is proposed and characterized to enable wavelength-divisionmultiplexing and mode-division-multiplexing simultaneously. The proposed structure is composed of a microring resonator in ultrathin waveguide and two bus channels in the novel silicon nitride silica silicon horizontal slot waveguides. In the slot waveguide, the transverse electric (TE) mode propagates through the silicon layer, while the transverse magnetic (TM) mode is confined in the slot region. In the designed ultra-thin waveguide, the TM mode is cutoff. The effective index of the TE modes for ultrathin and slot waveguides have comparable values. Thanks for these distinguishing features, the input TE mode can be efficiently filtered through the ultra-thin microring at the resonant wavelength, while the TM mode can directly output from the through port. Simulation Results show that the extinction ratio of the proposed P-DEMUX for TE and TM modes are ∼36.5 and 31.27 dB, and the insertion losses are ∼0.22 and 0.249 dB respectively.
Based on the epsilon-near-zero (ENZ) effect of indium tin oxide (ITO), we numerically demonstrate a high efficiency ITO phase/intensity modulator by exploiting ultra-thin silicon strip waveguide configuration. Heavily n-doped indium tin oxide is used as the semiconductor together with p-doped silicon and hafnium oxide (HfO2) to form a MOS waveguide. Due to the special feature of the ultra-thin silicon waveguide structure, the propagating transverse electric (TE) mode is less confined to the silicon core and penetrates deeper into the cladding layer, which will enhance the interaction between the active material and the optical mode. The combination of the ultra-thin silicon strip waveguide and ITO material exhibits high modulation efficiency together with broad optical bandwidth. When the modulator operates as a phase modulator, the effective refractive index change can reach the value 8:95x10-3 for the light wavelength λ = 1550 nm when the applied voltage is 6 V. Thus, the phase shifter length which can induce a π phase shift is supposed to be only about 97 µm, giving a corresponding VπL of 0.58 V∙mm. The effective index change even keeps > 7:32 x 10-3 with the wavelength increasing from 1300 nm to 1800 nm, indicating the broad modulation bandwidth. Meanwhile, the modulator can also operate as a variable optical attenuator or an intensity modulator. The modulation depth (MD) is about 0.074 dB/µm at 9 V when the wavelength is 1550 nm. This device confirms electrical phase shifting in ITO enabling its use in applications such as compact phase shifters, sensing, and phased array applications for LiDAR.
An interpretation of optical unitary transformation is proposed for general non-overlapping-image multimode interference (MMI) couplers with any input and output ports based on the matrix mechanics. The light transformation in the MMI couplers can be considered as the optical field matrix acting on the input light column vector. We investigate the general phase principles of output light images. The complete proof of nonoverlapping-image MMI coupler’s optical unitarity is provided along with the phase analysis of matrix element. Based on a two-dimensional finite-difference time-domain simulation, the unitary transformation is obtained for a 4×4 non-overlapping-image MMI coupler within the deviation of 4×10-2 for orthogonal invariance among the C-band spectral range.
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