Organic Crystals for THz Photonics

Abstract

Organic crystals with second-order optical nonlinearity feature very high and ultra-fast optical nonlinearities and are therefore attractive for various photonics applications. During the last decade, they have been found particularly attractive for terahertz (THz) photonics. This is mainly due to the very intense and ultra-broadband THz-wave generation possible with these crystals. We review recent progress and challenges in the development of organic crystalline materials for THz-wave generation and detection applications. We discuss their structure, intrinsic properties, and advantages compared to inorganic alternatives. The characteristic properties of the most widely employed organic crystals at present, such as DAST, DSTMS, OH1, HMQ-TMS, and BNA are analyzed and compared. We summarize the most important principles for THz-wave generation and detection, as well as organic THz-system configurations based on either difference-frequency generation or optical rectification. In addition, we give state-of-the-art examples of very intense and ultra-broadband THz systems that rely on organic crystals. Finally, we present some recent breakthrough demonstrations in nonlinear THz photonics enabled by very intense organic crystalline THz sources, as well as examples of THz spectroscopy and THz imaging using organic crystals as THz sources for various scientific and technological applications.

Efficient Gap‐Free Broadband Terahertz Generators Based on New Organic Quinolinium Single Crystals

“…This is evidence that such strong dimples originate mainly from intrinsic phonon vibrational modes and are only marginally influenced by phase matching condition. A similar behavior was observed in other benchmark organic ionic electro‐optic crystals having a high absorption coefficient in the THz region 3,7c. In contrast, OHQ‐TFO crystals exhibit a very flat THz spectrum without strong absorption dimples across the entire pump wavelength range tested (Figure e).…”

“…In THz photonics, gap‐free broadband THz wave generation with a very flat spectral shape and frequency‐independent detection are of great interest for diverse applications . Most of the existing organic and inorganic THz wave generators and detectors are hindered by fundamental limitations in spectral generation and detection as well as by bandwidth limits due to intrinsic phonon and molecular vibrational modes . In nonlinear optical frequency conversion processes, such as optical rectification and difference frequency generation, organic electro‐optic crystals are more beneficial for broadband THz wave generation and detection as compared to their inorganic counterparts .…”

“…For example, organic 2‐(4‐hydroxystyryl)‐1‐methylquinolinium (OHQ) and 2‐(4‐hydroxy‐3‐methoxystyryl)‐1‐methylquinolinium (HMQ) crystals showing good phase matching ability between optical and THz frequencies with near‐infrared (NIR) pumping exhibit broadband THz wave generation characteristics with an upper cut‐off frequency over 8–10 THz in a relatively simple collinear experimental scheme using a standard pump configuration . Note that although inorganic LiNbO 3 crystals can generate intense THz waves, the generated THz bandwidth is practically lower than 3 THz and a sophisticated tilted‐pulse‐fronts configuration is required . For coherent THz wave detection, a THz‐induced lensing (TIL) technique was realized using state‐of‐the‐art organic electro‐optic crystals, which can be used for sensitive broadband detection even at high THz frequencies …”

X‐Shaped Alignment of Chromophores: Potential Alternative for Efficient Organic Terahertz Generators

“…Organic nonlinear optical crystals offer high potential for various electro‐optic and nonlinear optical applications, including intense broadband or narrowband THz wave generation 1–7. In optical‐to‐THz conversion processes,8,9 benchmark organic nonlinear optical crystals simultaneously achieve high optical‐to‐THz conversion efficiency and broad bandwidth by using a diagonal optical nonlinearity from the nearly parallel alignment of nonlinear optical chromophores 10–16. For example, compared to widely used inorganic electro‐optic ZnTe crystals, organic ionic nonlinear optical crystals exhibit one order of magnitude higher THz generation efficiency, and over two times broader bandwidth, with an upper cut‐off frequency of >5 THz 17–20.…”

“…For example, compared to widely used inorganic electro‐optic ZnTe crystals, organic ionic nonlinear optical crystals exhibit one order of magnitude higher THz generation efficiency, and over two times broader bandwidth, with an upper cut‐off frequency of >5 THz 17–20. This is possible in organic crystals due to their large diagonal component of macroscopic second‐order optical nonlinearity, and good phase matching between optical pump and generated THz waves 10…”