Terahertz Microjets as Realizations of Subwavelength Focusing in the Terahertz Spectrum

Mitchell, Michael E.; Bergen, Mark H.; Reich, Jason; Holzman, Jonathan F.

doi:10.1109/tthz.2022.3220598

Cited by 4 publications

(8 citation statements)

References 32 publications

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“…Future studies could reoptimize the dielectric spheres for higher frequencies to produce even smaller THz microjets. 10 These THz microjets could then be coupled to smaller apertures (to achieve finer spatial resolutions) without losing spectral bandwidth (as f c would lie below the desired spectrum). Such efforts would have implications on the long-standing challenge of focusing sufficient power to subwavelength scales and would aid in biological applications of THz spectroscopy.…”

Section: Discussionmentioning

confidence: 99%

“…The dielectric spheres have a diameter of 20 mm, and are formed from microcomposite material. 10,11 The large capture cross-section of the spheres gives them the ability to focus appreciable powers to the subwavelength scale, which is of particular importance given the challenge of limited signal strength at such a scale. After analyzing its performance in comparison to other elements, we couple THz microjets and apertures to drive this power down to even smaller scales.…”

Section: Methodsmentioning

confidence: 99%

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Governing limits of spatial resolution and spectral bandwidth for implementations of terahertz spectroscopy on the subwavelength scale

Guidi,

Mitchell,

Holzman

2024

Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XVII

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In this work, we consider the governing limits of spatial resolution and spectral bandwidth in pursuing implementations of terahertz (THz) spectroscopy on the subwavelength scale. We discuss the need for effective focusing elements in driving sufficient levels of power from the (macroscopic) incident THz beam down to the (microscopic) focal spot. Such elements, when effective, enable large signal strengths and wide bandwidths, but this has proven to be challenging in contemporary implementations of near-field THz imaging and spectroscopy. To this end, we show theoretical and experimental results for focusing via parabolic mirrors, high-resolution lenses, and engineered dielectric spheres, with the latter yielding THz microjets with especially intense and small focal spots. We then discuss the need for near-field spatial constriction, to drive the spatial resolution down to an even smaller scale, and show that this constriction can lead to dispersive (i.e., frequencydependent) characteristics. In this work, we demonstrate spatial constriction via simple circular apertures, which function as high-pass filters. Ultimately, our theoretical and experimental results reveal that implementations of THz spectroscopy on the subwavelength scale are governed by a spatial-spectral product-whereby reductions in the aperture's diameter (to improve the spatial resolution) raise the aperture's cutoff frequency (at the expense of spectral bandwidth).

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Section: Discussionmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Governing limits of spatial resolution and spectral bandwidth for implementations of terahertz spectroscopy on the subwavelength scale

Guidi,

Mitchell,

Holzman

2024

Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XVII

Self Cite

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“…These THz microjets take the form of intense foci at the rear of engineered dielectric (microcomposite) spheres. Following our prior work 36 , 37 , the 20-mm-diameter spheres are fabricated out of microcomposite material having SiO 2 microparticles embedded in an ultra-high-molecular-weight polyethylene host. The microcomposite and its refractive index are optimized to yield intense focusing of the THz radiation with a small focal spot size (480 µm at 0.5 THz) and broad bandwidth (0.3–0.7 THz).…”

Section: Methodsmentioning

confidence: 99%

Spatial and spectral characteristics in realizations of broadband terahertz spectroscopy on a subwavelength scale

Guidi

Mitchell

Holzman

2023

Sci Rep

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In this work, we take aim at the fundamental challenge for realizations of broadband terahertz (THz) spectroscopy on a subwavelength scale. We introduce apertured THz microjets in this effort to resolve the fundamental limits of spatial resolution and spectral bandwidth. The THz microjets are formed as intense foci at the rear of engineered (microcomposite) spheres and are coupled through subwavelength (circular) apertures. Such coupling enables effective transmission of THz power through samples with broad spectral bandwidths and fine spatial resolutions. We show that the apertures function as high-pass filters, with their diameter d enabling strong transmission above a cutoff frequency fc. Our theoretical and experimental results reveal that the values for d and fc are prescribed by a fixed spatial-spectral product dfc, whereby reductions in d (to improve the spatial resolution) can raise fc into the targeted spectrum (at the expense of spectral bandwidth). We use this understanding to demonstrate broadband (0.3–0.7 THz) THz spectroscopy of lactose at the subwavelength (365 µm) scale. These results for apertured THz microjets represent a 20-fold improvement in spatial resolution over analogous apertured THz plane waves. Overall, our findings show promise for studies of carcinogenesis, pathogenesis, and the like.

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“…To create a THz microjet, we pass THz radiation through engineered dielectric spheres that have been optimized for our target spectrum. As the THz radiation passes through, it is focused down to an intense, subwavelength-sized focal spot 7,8 . Fig.…”

Section: Apertured Terahertz Microjetsmentioning

confidence: 99%

“…A common solution for the aforementioned loss of bandwidth and power is to focus the THz beam; here, we use THz microjets, which produce subwavelength focal spots in the entire 0.3-0.7 THz range 7,8 . However, THz microjets must be used alongside apertures to allow for spectroscopy on the deeply subwavelength scale.…”

Section: Spatial Constriction and Power Intensification Via Apertured...mentioning

confidence: 99%

Technologies for focusing and aperturing terahertz radiation in the realization of terahertz spectroscopy on the subwavelength scale

Guidi

Mitchell

MacGillivray

et al. 2023

Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XVI

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This work demonstrates the realization of terahertz (THz) spectroscopy on a subwavelength scale. We do so by using dielectric spheres as focusing elements and apertures as spatially constricting elements for THz radiation. Such a configuration forms intense, subwavelength-sized THz microjets. Two implementations are used to demonstrate the effectiveness of THz microjets, as follows: apertured THz plane waves and apertured THz microjets. Seven aperture diameters were chosen for each implementation to discern their capabilities at the subwavelength scale. We investigated the effectiveness of each implementation in mapping the material characteristics of the sample onto the THz beam. Such analyses show that apertured THz microjets were able to map material characteristics (via refractive index and extinction coefficient) onto the beam (via phase and amplitude) effectively and reliably. This is expected as the beam produced by apertured THz microjets has a small cross-sectional area (apertures) and high intensity (THz microjets). Here, we illustrate the capabilities of apertured THz microjets for a biological specimen, being lactose, to show the potential for biological applications. Overall, this work demonstrates the ability of apertured THz microjets to perform THz spectroscopy at a subwavelength scale. Such findings could bring about biological characterization with cellular-scale resolution.

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Terahertz Microjets as Realizations of Subwavelength Focusing in the Terahertz Spectrum

Cited by 4 publications

References 32 publications

Governing limits of spatial resolution and spectral bandwidth for implementations of terahertz spectroscopy on the subwavelength scale

Governing limits of spatial resolution and spectral bandwidth for implementations of terahertz spectroscopy on the subwavelength scale

Spatial and spectral characteristics in realizations of broadband terahertz spectroscopy on a subwavelength scale

Technologies for focusing and aperturing terahertz radiation in the realization of terahertz spectroscopy on the subwavelength scale

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