Ultrafast, broadband and tunable terahertz reflector and neutral density filter based on high resistivity silicon

Abstract

We report THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. Amplified Ti:Sapphire femtosecond laser pulses at 800 nm were used to create fluence-dependent carrier density on the front surface of the wafer which modifies the dielectric properties at the THz frequencies. Time-resolved experiments in the optical pump-THz probe configuration were conducted in which THz pulses reflected off from the surface at 0° and 45° angles of incidence make it possible to measure the pump-fluence dependent ultrafast evolution of the reflection and transmission coefficients in 0.5-6 THz range. An analytical model, where both the Drude contributions from the photo-excited electrons and holes account for the change of the dielectric constant of the photo-excited silicon, has been used to evaluate the THz reflection and transmission coefficients at steady state. Thus obtained results match well with the experimental results and demonstrate an all-optical means to convert a silicon wafer into an ultrafast, tunable and broadband neutral density filter or reflector in the THz frequency range.

Large interfacial contribution to ultrafast THz emission by inverse spin Hall effect in CoFeB/Ta heterostructure

“…The collimated excitation beam diameter was ∼3 mm, while the fluence at the sample point was fixed at ∼1 mJ/cm 2 . At this fluence value, the optical pump induced changes in the reflection and transmission properties of the substrate are nearly unaffected ( Degert et al., 2022 ). The emitted THz radiation is then collected by gold-coated 90° off-axis parabolic mirrors and focused onto a 500 mm thick (110)-oriented ZnTe crystal for detection.…”

Optical Filter Design for Daylight Outdoor Electroluminescence Imaging of PV Modules

“…The neutral density filter [19,20], on the other hand, works by reducing the overall amount of light that reaches the sensor, helping to eliminate unwanted background light that can interfere with the EL signal. By reducing the amount of background light, the signal-to-noise ratio of the EL signal is significantly improved, leading to clearer and more accurate imaging of the PV cells.…”

A bidirectional broadband multifunctional terahertz device based on vanadium dioxide

“…Terahertz (THz) waves, a type of electromagnetic radiation, occupy a unique frequency range spanning from 0.1 to 10 THz, have significant advantages, including excellent permeability, water absorption, and unique spectral fingerprint properties [1][2][3][4]. These excellent properties have prompted the emergence of THz wave functional devices including absorbers, detectors, modulators, and optical converters [5][6][7]. THz absorbers have garnered immense attention lately due to their pivotal role in diverse applications ranging from sensing and imaging to wireless communications [8,9].…”