Spectral Fresnel filter for pulsed broadband terahertz radiation

Abstract

We proposed a simple and cost-effective method to manipulate the temporal and spectral properties of pulsed terahertz waves. A deep modulation of a pulse spectrum was both numerically and experimentally verified using Fresnel apertures with a radius ranging from several to several tens of the central wavelength of the broadband terahertz radiation. N-fold frequency minima were formed in the spectrum at a specific axial position behind the filter. Non-paraxial properties of this filter were also analyzed. A significant value (35%) of the ratio of the longitudinal to the transverse field component at the filter frequency was obtained. The measured results agree well with the simulation and theoretical predictions. The property of such a diffractive Fresnel notch filter can benefit the generation of longitudinal terahertz fields and relevant applications.

Roadmap of Terahertz Imaging 2021

“…One can mention, for instance, THz QCLs by design of two back-to-back mounted elliptical silicon lenses and an opening aperture defined on a thin gold layer between the lenses [232] which enables filtering of the non-Gaussian beam from the QCL to a nearly fundamental Gaussian beam. As for multispectral THz imaging utilizing broadband THz pulses, an important innovation can stem from Fresnel apertures and filters that provide possibilities not only for the pulse shaping but also generation of longitudinal THz fields [233], hence exciting relevant applications.…”

Design of broadband terahertz vector and vortex beams: II. Holographic assessment

“…Therefore, when designing a beam shaper, before embedding any component, it is necessary to take into account all the specifics of its operation. The optimal tool for this task is THz pulse time-domain holography (PTDH) [18,19,20], since this technique provides the ultimate opportunity to track the dynamics of all characteristics of the broadband THz field in the spatial, temporal, spectral and angular coordinates during its propagation, both in free space and in optical systems with known characteristics [21,22,6,23].…”

Single-scan multiplane phase retrieval with a radiation of terahertz quantum cascade laser

“…In this field, THz imaging [1][2][3] represents one of the most promising approaches, since it provides spatially resolved information about the investigated objects. THz phase imaging (THz PI) [4,5], which includes pulse time-domain holography (PTDH) [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] as well as continuous wave (CW) holography [23][24][25][26][27][28][29][30], Hartmann wavefront sensing [31][32][33][34][35], shearorgaphy [36], ptychography [37][38][39][40][41], and other phase retrieval techniques [42][43][44][45][46][47][48][49], ...…”

“…A lot of technical solutions, experimental layouts, and reconstruction algorithms for THz PI have been proposed, and this field is reaching a certain level of maturity [5]. THz PTDH provides ultimate opportunities to track the dynamics of all the characteristics of a broadband THz field in spatial, temporal, spectral, and angular coordinates [50] during its interference [18,19,21] and propagation, both in free space [9,12,17,22], through designed optical elements [51], obstacles [13,20], and investigated objects [8,10,11,16]. THz wavefront characterization with subsequent compensation through adaptive optics also serves as a compelling use case [52].…”

“…In addition, it should be noted that the THz frequency range differs greatly from the visible wavelength range through the loss of the paraxial nature of the diffraction processes, which is usually caused by the small transverse dimension of a typical THz beam, with respect to the operating wavelength [20]. Thus, the choice of the optimal number and arrangement of measurements planes presents certain complexity and often have to be empirically assessed, and carried out through several series of experimental measurements or preliminary numerical simulations, hence remaining a time-consuming anticipation process.…”