Terahertz time-domain spectroscopy (THz-TDS) is a method used in research and industry for non-invasive characterization of products and materials. Many THz-TDS systems rely on parametric conversion in semiconductor crystals to generate and detect phase-locked THz pulses, providing reliable access to frequencies below 3 THz. Accessing higher frequencies, however, often requires a sophisticated near-infrared (NIR) source delivering sub- 100 fs pulses to access the required spectral bandwidth and thin nonlinear crystals (few hundred micrometers thick) to minimize phase mismatch during both the THz generation and detection processes. As a result, broadband THz- TDS configurations rely on laser systems which are often bulky and costly, resulting in inefficient THz generation and detection processes due to a limited nonlinear interaction length in the crystals. To overcome these limitations, we introduce three modules to a THz-TDS system employing a compact and cost-effective pulsed laser. First, a fiberbased component is used to broaden the output laser spectrum and compress the pulse duration. This module provides the NIR frequency content needed for broadband THz generation through optical rectification and a pulse duration short enough to efficiently resolve high THz frequencies during electro-optic sampling. The other two modules utilize a thick nonlinear crystal with a periodically patterned surface to optimize the efficiencies of the broadband THz generation and detection processes. In this configuration, a long nonlinear interaction length is guaranteed while noncollinear phase matching provides access to a broad spectral range. The combination of these modules extends the THz spectrum from 3 THz to beyond 6 THz with a peak dynamic range >50 dB at 3.5 THz.
The effect of the substrate to the absorption spectra of silver nanoparticles of sphere-like shapes is investigated. Silver nanoparticles of broken spherical symmetry are placed on substrates of different dielectric functions at various contact angles (α). The absorption efficiency of the supported nanoparticles is calculated by using the Discrete Dipole Approximation (DDA) method. Increasing the value of α and, hence the contact area of the supported nanoparticles,
results in an inhomogeneous distribution of the polarization charges over the nanoparticle-substrate system. This leads to the excitation of plasmonic bands of different characters (dipolar and quadrupole modes). The admixture of both dipolar and quadrupole modes is found to be more pronounced when a nanoparticle with highest contact area (α = 90o,
hemispherical shape) is considered. The band position of the Longitudinal Mode (LM) is red-shifted with α, while the resonance wavelength of the Transverse Mode (TM) is blue-shifted.
The optical aspects of plasmon coupling occurring through the near-field interactions among metallic spherical
nanoparticles assembled in close proximity to each other in two-dimensional and three-dimensional arrays have been
examined using the discrete dipole approximation (DDA). Calculations were performed for nano-sized close-packed
spheres of silver, gold or copper, hexagonally arranged in a planar monolayer target and extended gradually to three-dimensional
multilayer targets with a fastened interparticle spacing. Those targets were simulated under the incident ppolarized
light with an energy range of 1.5 - 4.5 eV by executing an open-source code of the DDA. The optical response
of three-dimensional targets was revealed in the absorption spectra calculated at various angles of the polarized incident
light, showing a blue shift of the plasmon resonance (PR) peak for both gold and copper targets. The splitting of the
surface plasmon resonance (SPR) observed in the response of the two-dimensional silver system eventually disappeared
into one well-defined resonance peak as the system grew in the third dimension. Moreover, to shed light on the nature of
the plasmon coupling among close-packed nanospheres of different metals, we simulated a target composed of mono-sized
nanospheres of the three metals placed spatially in three consecutive layers. A combined optical behaviour was
thus observed through the absorption spectrum, where the plasmon peaks attributed to the silver, gold and copper
interacting nanospheres emerged at the original energy values as if it was applied in isolated planar hexagonal arrays.
Discrete Dipole Approximation (DDA) is a computational technique to simulate the optical properties of nanostructures
of different shapes, sizes, and compositions. The influence of the target size on the optical response of 5 nm-diameter
nanoparticles arranged in a monolayer hexagonal array is investigated by using DDA at various incident angles of the
incident light on the target considered for silver (Ag), gold (Au) and copper (Cu) nanoparticles. In our study, the target
size is controlled by the number of the spherical nanoparticles used to generate the two dimensional arrays. The
interparticle distance is kept constant in all the simulations. The anisotropic response of noble-metal nanoparticles is
generally characterized by the excitation of the high-energy (transverse) surface plasmon (SP) mode and the low-energy
(longitudinal) SP mode. Results of the simulations for the three chosen metals show an exponential dependency of the
absorption efficiency of the SP modes with respect to the target size. As the target size is increased, the energy of Ag-longitudinal
SP mode is red-shifted and it displays an exponential decay while the band position of the transverse mode
is blue-shifted. They however overlap when the smallest target size is considered. Although, the optical response of Au
and Cu nanoparticle arrays shows the same dependency on the target size as observed in the case of Ag, the positions of
their respective longitudinal and transverse modes are very close, making these almost indistinguishable. The
dependency of the absorption efficiency of SP modes on the incident angle is fitted linearly for Cu, Au and longitudinal-
Ag modes to the target size, while the transverse-Ag mode shows an exponential fitting. No change in the Ag-SP band
position is observed when the incident angle is changed, but the SP bands for both Au and Cu exhibit exponential
variation behavior.
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