Abstract-Multicycle terahertz pulse generation by optical rectification in LiNbO3, LiTaO3, and BBO crystals

Dogeun Jang and Ki-Yong Kim

 (a) Experimental setup for multicycle THz generation and detection. (b) Input laser spectrum measured (black line) before entering the experimental setup with a Gaussian fit (red line). (c) Measured (black dotted line) and estimated (red solid line) laser pulse duration as a function of input GDD (top 𝑥$x$-axis), practically controlled by varying the distance (bottom 𝑥$x$-axis) between the grating pair in the laser compressor. (d) Focused THz beam profile captured by a microbolometer focal plane array.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-28-14-21220

We report multicycle, narrowband, terahertz radiation at 14.8 THz produced by phase-matched optical rectification of femtosecond laser pulses in bulk lithium niobate (LiNbO3) crystals. Our experiment and simulation show that the output terahertz energy greatly enhances when the input laser pulse is highly chirped, contrary to a common optical rectification process. We find this abnormal behavior is attributed to a linear electro-optic (EO) effect, in which the laser pulse propagating in LiNbO3 is modulated by the terahertz field it produces, and this in turn drives optical rectification more effectively to produce the terahertz field. This resonant cascading effect can greatly increase terahertz conversion efficiencies when the input laser pulse is properly pre-chirped with additional third order dispersion. We also observe similar multicycle terahertz emission from lithium tantalate (LiTaO3) at 14 THz and barium borate (BBO) at 7 THz, 10.6 THz, and 14.6 THz, all produced by narrowband phase-matched optical rectification.

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Single-shot terahertz spectrometer using a microbolometer camera

Dogeun Jang, Hanran Jin, Ki-Yong Kim

https://arxiv.org/abs/2006.03220

We demonstrate a single-shot terahertz spectrometer consisting of a modified Mach-Zehnder interferometer and a microbolometer focal plane array. The spectrometer is simple to use and can measure terahertz field autocorrelations and spectral power with no moving parts and no ultrashort-pulsed laser. It can effectively detect radiation at 10∼40 THz when tested with a thermal source. It can be also used to measure the complex refractive index of a sample material. In principle, it can characterize both laser-based and non-laser-based terahertz sources and potentially cover 1∼10 THz with specially-designed terahertz microbolometers.

Abstract-Scalable terahertz generation by large-area optical rectification at 80 TW laser power

Dogeun Jang, Chul Kang, Seong Ku Lee, Jae Hee Sung, Chul-Sik Kee, Seung Woo Kang, and Ki-Yong Kim

(a) Experimental setup for high-energy terahertz generation from a large-aperture LN wafer. The inset shows the incident laser beam profile. (b) Laser spectral power (black line) and spectral phase (blue line). (c) Estimated laser pulse duration as a function of laser GDD. (d) Sample pyroelectric detector signal.

https://www.osapublishing.org/ol/abstract.cfm?uri=ol-44-22-5634

We demonstrate high-energy terahertz generation from a large-aperture (75-mm diameter) lithium niobate wafer by using a femtosecond laser with energy up to 2 J. This scheme utilizes optical rectification in a bulk lithium niobate crystal, where most terahertz energy is emitted from a thin layer of the rear surface. Despite its simple setup, this scheme can yield 0.19 mJ of terahertz energy with laser-to-terahertz conversion efficiencies of ∼10−4$\sim {10}^{-4}$, about 3 times better than ZnTe when pumped at 800 nm. The experimental setup is upscalable for multimillijoule terahertz generation with petawatt laser pumping.

© 2019 Optical Society of America

Abstract-Highly enhanced terahertz conversion by two-color laser filamentation at low gas pressures

Yung-Jun Yoo, Dogeun Jang, and Ki-Yong Kim

Fig. 1 Experimental setup for THz generation from two-color laser filamentation inside a long gas tube. The emitted THz radiation is refocused by an off-axis parabolic mirror onto a pyroelectric detector for energy measurement and an uncooled microbolometer focal plane array for imaging. Synchronized probe pulses at variable delays are used to measure THz waveforms via electro-optic sampling with a thin GaP crystal. The inset shows a long plasma filament in argon captured by optical side imaging.

https://www.osapublishing.org/oe/abstract.cfm?uri=oe-27-16-22663

We present an experimental study on pressure-dependent terahertz generation from two-color femtosecond laser filamentation in various gases. Contrary to short-focusing geometry, we find that long filamentation yields higher terahertz energy at lower gas pressures in most gases. This counter-intuitive phenomenon occurs due to multiple peculiar properties associated with filamentation. In practice, filamentation in low-pressure argon provides a maximum laser-to-terahertz conversion efficiency of 0.1%, about 10 times higher than in atmospheric air. In addition, our pressure-dependent study reveals an anticorrelation between terahertz output energy and local plasma fluorescence brightness. This determines the absolute phase difference between two-color laser fields for maximal terahertz generation, as well as verifies the microscopic mechanism of terahertz generation in two-color laser mixing.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Abstract-Spectral Characterization of a Microbolometer Focal Plane Array at Terahertz Frequencies

Dogeun Jang,  Malik Kimbrue, Yung-Jun Yoo, Ki-Yong Kim

https://ieeexplore.ieee.org/document/8613916

We have developed a method to characterize the spectral response of an uncooled microbolometer focal plane array at a broad range of terahertz (THz) frequencies (4–50 THz). This is achieved by using a spectrum-shaped blackbody radiator as a broadband THz source and measuring its spectral power with a Fourier transform infrared interferometer. With an additional measurement with a pyroelectric detector as a reference, the spectral response of the microbolometer relative to the pyroelectric reference is obtained with a signal-to-noise ratio of 100 over a > 50-THz bandwidth.