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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. Intense THz Generation with New Organic NLO Crystal NMBA

   https://doi.org/10.1002/adom.202302402  

   Ludlow, Daisy JH; Palmer, Bruce_Wayne H; Green, Natalie K; Ho, Sin‐Hang Enoch; Wayment, Coriantumr Z; Kelleher, Brenan M; Barlow, Connor D; Rollans, Olivia N; Hunter, Brittan P; Rader, Claire; et al (January 2024, Advanced Optical Materials)

   Abstract Large crystal growth and characterization of the optical and terahertz (THz) properties of organic nonlinear optical (NLO) crystal NMBA (4‐Nitro‐4′‐methylbenzylidene aniline) is presented. The reported method of crystal growth consistently produces high‐quality single crystals. The THz generation efficiency and optical properties of NMBA to other THz generation crystals including BNA and MNA are compared. The low THz absorptions that make NMBA an ideal source for THz time‐domain spectroscopy are highlighted. 

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3. Efficient Broadband THz Generation from New Standards in Optical Rectification

   https://doi.org/10.1364/CLEO_AT.2023.JTh2A.115  

   Green, Natalie K.; Palmer, Bruce Wayne; Rader, Claire; Ho, Sin-Hang Enoch; Zaccardi, Zachary B.; Ludlow, Daisy J.; Lutz, Matthew J.; Alejandro, Aldair; Nielson, Megan F.; Valdivia-Berroeta, Gabriel A.; et al (January 2023, CLEO 2023)

   We present the optical and THz properties of highly efficient organic nonlinear optical crystals PNPA and MNA. These crystals outperform industry standards in broadband THz generation through optical rectification of IR light pulses. 

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4. Improved Generation of Terahertz Light through Heterogeneous Multi‐Layered Organic Crystal Structures

   https://doi.org/10.1002/adom.202502694  

   Alejandro, Aldair; Petersen, Paige K; Ludlow, Daisy_J H; Holland, Kayla M; N'diaye, Fatoumata; Manwaring, Tanner W; Biggs, Megan F; Michaelis, David J; Johnson, Jeremy A (October 2025, Advanced Optical Materials)

   Abstract Yellow organic crystals, like BNA, MNA, and NMBA, can be used to generate terahertz (THz) pulses of light through optical rectification of infrared ultrafast laser pulses. When producing THz with these organic crystals, one needs to consider that 1) their damage thresholds are low due to having low melting points and 2) Fresnel reflection losses due to multiple interfaces reduce the efficiency of the generated THz output. In this work, new heterogeneous multi‐layer “sandwich” structures are developed with these yellow organic crystals by 1) fusing them to sapphire plates to permit the crystal to withstand higher laser fluences and 2) using an index‐matching fluid (liquid crystal MBBA) to decrease Fresnel reflection losses and improve the THz output. It is shown that the sapphire plates increase the damage threshold of these yellow organic crystals by a factor of two or more, thus allowing the crystals to generate higher THz electric fields. Furthermore, it is shown that the THz light output efficiency increases by assembling the yellow crystals into multi‐layered sandwich structures. For some yellow organic crystals, the sandwich structures increase the THz intensity by more than a factor of two. 

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5. Cavity-Assisted Coherent Phonon Generation and Control in a WSe ₂ /Au Structure

   https://doi.org/10.1021/acs.jpclett.5c01190  

   Brennan, Christian; Jo, Kiyoung; Wang, Chih-Feng; Joly, Alan G; O’Callahan, Brian T; Farias, William Edward; Bielicki, Alexander Donald; Teklu, Alem; Shi, Leilei; Zhang, Qian; et al (June 2025, The Journal of Physical Chemistry Letters)

   Coherent phonons in the Terahertz (THz) regime have gained attention as potential candidates for next-generation high-speed, low-energy information carriers in atomically thin phononic or phonon-integrated on-chip devices. Nevertheless, achieving efficient control of the phonon generation dynamics over THz coherent phonons continues to pose a considerable challenge. In this work, we explore THz coherent phonon generation in exfoliated van der Waals (vdW) flakes of WSe2 on Au (WSe2/Au) and Si (WSe2/Si) by using time-resolved pump–probe spectroscopy. The generation of THz coherent phonons was studied as a function of the WSe2 layer thickness and laser wavelength. Notably, a significant enhancement in THz coherent phonon generation was observed in the WSe2/Au structure, but only within a specific range of WSe2 thicknesses and laser wavelengths. The results from numerical simulations, which consider a self-hybridized optical cavity depending on WSe2 thickness and optical reflectance and Raman spectroscopy measurements, all align well with the time-domain observations of THz coherent phonon generation. We propose that the observed enhancement in THz coherent phonon generation is strongly influenced by light–matter interactions in the WSe2 cavity, a mechanism that may be applicable to a broader range of vdW materials. These findings offer promising insights for the development of THz phononic or phonon-integrated devices. 

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