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1. Improved Terahertz Output from Layered Organic Crystal Structures

   https://doi.org/10.1364/CLEO_AT.2023.JW2A.53  

   Ludlow, Daisy J.; Alejandro, Aldair; Petersen, Paige K.; Holland, Kayla M.; N’diaye, Fatoumata; Manwaring, Tanner; Michaelis, David J.; Johnson, Jeremy A. (January 2023, CLEO 2023)

   We have improved the THz output and damage threshold of yellow organic THz generation crystals by fusing them to sapphire and using liquid crystals as index matching fluid to reduce reflective losses. 

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2. Custom terahertz waveforms using complementary organic nonlinear optical crystals

   https://doi.org/10.1364/OL.474343  

   Rader, Claire; Nielson, Megan_F; Knighton, Brittany_E; Zaccardi, Zachary_B; Michaelis, David_J; Johnson, Jeremy_A (November 2022, Optics Letters)

   Organic nonlinear optical (NLO) crystals are among the most efficient (>1%) terahertz (THz) radiation generators. However, one of the limitations of using organic NLO crystals is that the unique THz absorptions in each crystal make it difficult to obtain a strong, smooth, and broad emission spectrum. In this work, we combine THz pulses from two complementary crystals, DAST and PNPA, to effectively fill in spectral gaps, creating a smooth spectrum with frequencies out to 5 THz. The combination of pulses also increases the peak-to-peak field strength from 1 MV/cm to 1.9 MV/cm. 

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3. 3D‐Printed Photonic Crystal Sub‐Terahertz Leaky‐Wave Antenna

   https://doi.org/10.1002/admt.202300698  

   Guerboukha, Hichem; Sakaki, Masoud; Shrestha, Rabi; Li, Jingwen; Kölbel, Johanna; Benson, Niels; Mittleman, Daniel M. (March 2024, Advanced Materials Technologies)

   Abstract Wireless systems are facing increasing pressure due to the growing demand for data transmission. One potential solution to this problem is to shift communication frequencies toward the terahertz (THz) spectrum. However, this requires the development of new components that can efficiently process signals at these high frequencies and transmit them via highly directional beams. In this study, a novel approach is proposed to achieving efficient THz signal processing by combining two existing technologies: photonic crystals and leaky‐wave antennas. Incorporating a 2D photonic crystal inside a leaky‐wave waveguide allows to manipulate the wave vector of the guided wave in unique ways, which in turn impacts the far‐field radiation pattern emitted through the leaky‐wave aperture. The device fabrication uses 3D printing of alumina and allows for convenient and scalable manufacturing. Through numerical simulations and experiments, free‐space data transmission at rates of few hundred Mbps at a carrier frequency of 101.2 GHz is demonstrated. The findings illustrate the feasibility of photonic crystal‐based leaky‐wave antennas and lay the groundwork for the development of compact and high‐performance components for THz wireless communication systems. 

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4. Phonon Polaritonics in Broad Terahertz Frequency Range with Quantum Paraelectric SrTiO ₃

   https://doi.org/10.1002/adma.202302974  

   Xu, Rui; Lin, Tong; Luo, Jiaming; Chen, Xiaotong; Blackert, Elizabeth_R; Moon, Alyssa_R; JeBailey, Khalil_M; Zhu, Hanyu (July 2023, Advanced Materials)

   Abstract Photonics in the frequency range of 5–15 terahertz (THz) potentially open a new realm of quantum materials manipulation and biosensing. This range, sometimes called “the new terahertz gap”, is traditionally difficult to access due to prevalent phonon absorption bands in solids. Low‐loss phonon–polariton materials may realize sub‐wavelength, on‐chip photonic devices, but typically operate in mid‐infrared frequencies with narrow bandwidths and are difficult to manufacture on a large scale. Here, for the first time, quantum paraelectric SrTiO₃enables broadband surface phonon–polaritonic devices in 7–13 THz. As a proof of concept, polarization‐independent field concentrators are designed and fabricated to locally enhance intense, multicycle THz pulses by a factor of 6 and increase the spectral intensity by over 90 times. The time‐resolved electric field inside the concentrators is experimentally measured by THz‐field‐induced second harmonic generation. Illuminated by a table‐top light source, the average field reaches 0.5 GV m⁻¹over a large volume resolvable by far‐field optics. These results potentially enable scalable THz photonics with high breakdown fields made of various commercially available phonon–polariton crystals for studying driven phases in quantum materials and nonlinear molecular spectroscopy. 

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5. Room temperature terahertz semiconductor frequency comb

   https://doi.org/10.1038/s41467-019-10395-7  

   Lu, Quanyong; Wang, Feihu; Wu, Donghai; Slivken, Steven; Razeghi, Manijeh (June 2019, Nature Communications)

   Abstract A terahertz (THz) frequency comb capable of high-resolution measurement will significantly advance THz technology application in spectroscopy, metrology and sensing. The recently developed cryogenic-cooled THz quantum cascade laser (QCL) comb has exhibited great potentials with high power and broadband spectrum. Here, we report a room temperature THz harmonic frequency comb in 2.2 to 3.3 THz based on difference-frequency generation from a mid-IR QCL. The THz comb is intracavity generated via down-converting a mid-IR comb with an integrated mid-IR single mode based on distributed-feedback grating without using external optical elements. The grating Bragg wavelength is largely detuned from the gain peak to suppress the grating dispersion and support the comb operation in the high gain spectral range. Multiheterodyne spectroscopy with multiple equally spaced lines by beating it with a reference Fabry-Pérot comb confirms the THz comb operation. This type of THz comb will find applications to room temperature chip-based THz spectroscopy. 

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