Characterization of T-ray binary lenses

Terahertz Fresnel-zone-plate thin-film lens based on a high-transmittance double-layer metamaterial phase shifter

Planar diffractive lenses, with metamaterial artificial structures and subwavelength thickness, provide unique and flexible platforms for optical design in the terahertz (THz) regime. Here, we present a metamaterial-based Rayleigh-Wood Fresnel-zone-plate (FZP) thin-film lens designed to focus a monochromatic THz beam at 1.0 THz with a high transmittance of 80%, short focal length of 24 mm, and subwavelength thickness of 48 µm. Specifically, the FZP lens is composed of 8 alternating concentric zones through a polymer film substrate, where odd zones are patterned with double-layer un-split ring resonators (USRRs) that provide a polarization-independent phase shift of π/2 compared to un-patterned even zones. Both simulation and experiment confirm that our FZP lens creates a focused beam at the designed frequency of 1.0 THz by constructive interference through alternating concentric metamaterial-patterned and un-patterned zones, producing a diffraction-limited resolution of 0.6 mm for imaging applications. In contrast to conventional approaches in which the uniform periodic array of metamaterial unit cells has been treated as an effective material, we newly find that double-layer USRRs can work as an independent meta-atom without degradation of its performances, which benefits the behavior of small arrays of double-layer USRRs located in the outer zones of the FZP lens. Such a planar thin-film lens would enable us to realize compact and lightweight THz systems.

Laser-processed diffractive lenses for the frequency range of 4.7 THz

The development of diffractive lenses for the upper terahertz (THz) frequency range above 1 THz was successfully demonstrated by employing a direct laser ablation (DLA) technology. Two types of samples such as the Soret zone plate lens and the multi-level phase-correcting Fresnel lens were fabricated of a metal foil and crystalline silicon, respectively. The focusing performance along the optical axis of a 4.745 THz quantum cascade laser beam with respect to the positioning angle of the sample was studied by using a real-time microbolometric camera. A binary-phase profile sample demonstrated the values of the focusing gain and focused beam size up to 25 dB and 0.15 mm (2.4λ), respectively. The increase of the phase quantization level to eight led to higher (up to 29 dB) focusing gain values without a measurable increase of optical losses. All the samples were tolerant to misalignment as large as 10 deg of oblique incidence with a focusing power drop no larger than 10%. The results pave the way for new applications of industry-ready DLA technology in the entire THz range.

Field imaging near to the surface of terahertz reflective optics using a vector network analyzer

A vector network analyzer-based quasi-optical measurement system that is suitable for mapping electric field intensity and phase near to the surface of terahertz reflective optics is presented. The system uses a fixed five parabolic mirror and transmitter/receiver head arrangement that has the benefit of requiring only the sample to be swept during measurement. The system has been tested with a micromilled aluminum zone plate reflector used as an exemplar structure. The measured focal point of the zone plate reflector, at its designed frequency of 1 THz, is shown to correspond well to both finite difference time-domain simulations and analytical theory.

3D printed diffractive terahertz lenses

A 3D printer was used to realize custom-made diffractive THz lenses. After testing several materials, phase binary lenses with periodic and aperiodic radial profiles were designed and constructed in polyamide material to work at 0.625 THz. The nonconventional focusing properties of such lenses were assessed by computing and measuring their axial point spread function (PSF). Our results demonstrate that inexpensive 3D printed THz diffractive lenses can be reliably used in focusing and imaging THz systems. Diffractive THz lenses with unprecedented features, such as extended depth of focus or bifocalization, have been demonstrated.

Terahertz Artificial Dielectric Lens

We have designed, fabricated, and experimentally characterized a lens for the THz regime based on artificial dielectrics. These are man-made media that mimic properties of naturally occurring dielectric media, or even manifest properties that cannot generally occur in nature. For example, the well-known dielectric property, the refractive index, which usually has a value greater than unity, can have a value less than unity in an artificial dielectric. For our lens, the artificial-dielectric medium is made up of a parallel stack of 100 μm thick metal plates that form an array of parallel-plate waveguides. The convergent lens has a plano-concave geometry, in contrast to conventional dielectric lenses. Our results demonstrate that this lens is capable of focusing a 2 cm diameter beam to a spot size of 4 mm, at the design frequency of 0.17 THz. The results further demonstrate that the overall power transmission of the lens can be better than certain conventional dielectric lenses commonly used in the THz regime. Intriguingly, we also observe that under certain conditions, the lens boundary demarcated by the discontinuous plate edges actually resembles a smooth continuous surface. These results highlight the importance of this artificial-dielectric technology for the development of future THz-wave devices.

Diffractive microlens integrated into Nb(5)N(6) microbolometers for THz detection

We fabricated square diffractive microlens array with five staircases in the THz wave band for Nb₅N₆ microbolometers. With each microlens intergrated with an Nb₅N₆ microbolometer on the same substrate, an array chip was fabricated in the 4 inches silicon wafer. The lens exhibits good focusing and improves the coupling efficiency. The voltage response of the microbolometer integrated with diffractive microlens is 16 times higher than that of the microbolometer fabricated on silicon substrate. The microbolometers used as room-temperature detectors yield a good responsivity of 71 V/W and a noise equivalent power of 1.0 × 10^-10 W/Hz. The diffractive microlens array features light weight, low absorption loss, and high resolution and can be mass produced using standard micro-fabrication techniques.

Metasurfaces based dual wavelength diffractive lenses

We demonstrate experimentally and by simulations a method for using thin nanostructured plasmonic metasurfaces to design diffractive Fresnel zone plate lenses that focus pairs of wavelengths to a single focal point. The metasurfaces are made of tightly packed cross and rod shaped optical nanoantennas with strong polarization and wavelength selectivity. This selectivity allows multiplexing two different lenses with low spectral crosstalk on the same substrate and to address any superposition of the two colors at the focus of the lenses by controlling the polarization of light. This concept can open the door to use ultrathin diffractive lenses in fluorescence microscopy and in stimulated emission depletion microscopy.

T-ray Imaging and Tomography

We demonstrate two algorithms used forreconstructing the target's structure basedon the diffracted pulses and additionallyshow that a three-dimensional target can bereconstructed using the broadband pulsesand a Fresnel lens by virtue of itsfrequency dependent focal length. Oneadvantage of T-ray imaging is the abilityto measure the far-infrared spectralresponse of the target. To highlight theimportance of this spectral information, wedemonstrate T-ray classification imagingwith different biological samples using asimple classification algorithm and twodimensional T-ray spectroscopic images.

Three-dimensional terahertz wave imaging

Pulsed terahertz (THz) wave sensing and imaging is a coherent measurement technology. Like radar, based on the phase and amplitude of the THz pulse at each frequency, THz waves provide temporal and spectroscopic information that allows us to develop various three-dimensional (3D) terahertz tomographic imaging modalities. The 3D THz tomographic imaging methods we investigated include THz time-of-flight tomography, THz computed tomography (CT) and THz binary lens tomography. THz time-of-flight uses the THz pulses as a probe beam to temporally mark the target, and then constructs a 3D image of the target using the THz waves scattered by the target. THz CT is based on geometrical optics and inspired from X-ray CT. THz binary lens tomography uses the frequency-dependent focal-length property of binary lenses to obtain tomographic images of an object. Three-dimensional THz imaging has potential in such applications as non-destructive inspection. The interaction between a coherent THz pulse and an object provides rich information about the object under study; therefore, 3D THz imaging can be used to inspect or characterize dielectric and semiconductor objects. For example, 3D THz imaging has been used to detect and identify the defects inside a Space Shuttle insulation tile.

Defining the Fresnel zone for broadband radiation

The concept of the Fresnel zone is central to many areas of imaging. In tomographic imaging, the transverse spatial resolution can be limited by the size of the first Fresnel zone, usually defined only for monochromatic radiation. With the increasing prevalence of broadband tomographic imaging systems, a generalization of this concept is required. Here, a proposed generalization is described in the context of femtosecond optics, and experimentally verified using terahertz time-domain spectroscopy. Based on this definition, a simple zone plate design is demonstrated.