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1. Second Harmonic Generation and Optical Rectification Using Intense Terahertz Pulses

   https://doi.org/10.1364/FIO.2022.JTu4B.23  

   Ludlow, Daisy J.; Rader, Claire; Green, Natalie K.; Johnson, Jeremy A. (January 2022, Frontiers in Optics + Laser Science 2022)

   Nonlinear optical processes with THz frequency light have rarely been measured and characterized. Using 2D THz spectroscopy, we clearly observe THz-optical rectification and THz- second harmonic generation in a variety of materials. 

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2. 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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3. Ultrafast modulation of terahertz waves using on-chip dual-layer near-field coupling

   https://doi.org/10.1364/OPTICA.469461  

   Zhang, Yaxin; Ding, Kesen; Zeng, Hongxin; Kou, Wei; Zhou, Tianchi; Zhou, Hongji; Gong, Sen; Zhang, Ting; Wang, Lan; Liang, Shixiong; et al (November 2022, Optica)

   As a key potential component of future sixth-generation (6G) communication systems, terahertz (THz) technology has received much attention in recent years. However, a lack of effective high-speed direct modulation of THz waves has limited the development of THz communication technology. Currently, most high-speed modulators are based on photonic systems that can modulate electromagnetic waves with high speed using sophisticated optoelectronic conversion techniques. Yet, they usually suffer from low conversion efficiency of light to the THz range, resulting in low output power of the modulated THz waves. Here, we describe a guided-wave modulator for THz signals whose performance nearly matches that of existing in-line fiber-optic modulators. Our results demonstrate a maximum modulation depth greater than 20 dB (99%) and a maximum sinusoidal modulation speed of more than 30 GHz, with an insertion loss around 7 dB. We demonstrate the capabilities of this modulator in a point-to-point communication link with a 25 Gbit/s modulation speed. Our modulator design, based on near-field coupling of a THz transmission line to a single resonant meta-element, represents a powerful improvement for on-chip integrated high-performance THz devices. 

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4. 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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5. 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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