Nonlinear Optical Phenomena in Emerging Low-Dimensional Materials

Yongchang Dong, Clemson University


As digital information technologies continue to evolve at much faster rates than the growth of Si-based processors, the encroachment of light-based technologies into computing seems inevitable. With the advent of lasers, photonic crystals, and optical diodes, photonic computing has made significant strides in information technology over the past 30 years. This continuing integration of light into all-optical computing, optoelectronic components, and emerging optogenetic technologies demands the ability to control and manipulate light in a predictable fashion, or by design. Of particular interest, is the passive control and manipulation of light in all-optical switches, photonic diodes, and optical limiting which can be achieved by leveraging intrinsic non-linear optical properties of low dimensional materials.

The reverse saturable absorption in fullerenes has been widely used to realize excellent passive optical limiters for the visible region up to 650 nm. However, there is still a need for passive optical switches and limiters with a low limiting threshold (<0.5 J/cm2) and higher damage limits. The electronic structure of fullerenes can be modified either through doping or by the encapsulation of endohedral clusters to achieve exotic quantum states of matter such as superconductivity. Building on this ability, we discuss in Chapter 2 that the encapsulation of Sc3N, Lu3N or Y3N in C80 alters the HOMO-LUMO gap and leads to passive optical switches with a significantly low limiting threshold (0.3 J/cm2) and a wider operation window (average pulse energy >0.3 mJ in the ns regime).

In addition to extraordinary and strongly anisotropic electronic properties, two dimensional (2D) materials such as graphene and boron nitride, exhibit strong light-matter interactions despite their atomic thickness. The nonlinear light-matter interactions in 2D materials are well suited for several applications in photonics and optoelectronics, such as ultrafast optical switching and optical diodes. Unlike most 2D materials that display nonlinear saturable absorption or increased light transmission at higher fluences, hexagonal boron nitride nanoplatelets (BNNPs) exhibit enhanced opaqueness with increasing light fluence. A two-photon absorption (2PA) process was previously proposed to explain the intrinsic non-linear absorption in BNNPs at 1064 nm or 1.16 eV (Kumbhakar et al., Advanced Optical Materials, vol. 3, pp. 828, 2015); which is counter-intuitive because a 2PA process at 1.16 eV cannot excite electrons across the wide band gap of BNNPs (~5.75 eV). Here, through a systematic study of the non-linear properties of BNNPs we uncover a notoriously rare non-linear phenomenon, viz., five-photon absorption (5PA) at 1064 nm for low laser input fluences (below 0.6 J/cm2) that irreversibly transforms to a 2PA for higher laser input fluences (above 0.6 J/cm2). Our detailed experimental and theoretical findings delineated in Chapter 3 provide compelling evidence that the high laser fluence generates defects in BNNPs (e.g., oxygen/carbon doping), which support a 2PA process by inducing new electronic states within the wide band gap of BNNPs.

MXenes comprise a new class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides that exhibit unique light-matter interactions. Recently, 2D Ti3C2Tx (Tx represents functional groups such as –OH and –F) was found to exhibit nonlinear saturable absorption (SA) or increased transmittance at higher light fluences that is useful for mode locking in fiber-based femtosecond lasers. However, the fundamental origin and thickness-dependence of SA behavior in MXenes remains to be understood. We fabricated 2D Ti3C2Tx thin films of different thicknesses using an interfacial film formation technique to systematically study their nonlinear optical properties. Using the open aperture Z-scan method, we find that the SA behavior in Ti3C2Tx MXene arises from plasmon-induced increase in the ground state absorption at photon energies above the threshold for free carrier oscillations. The saturation fluence and modulation depth of Ti3C2Tx MXene was observed to be dependent on the film thickness. Unlike other 2D materials, Ti3C2Tx was found to show higher threshold for light-induced damage with up to 50% increase in nonlinear transmittance. Lastly, building on the SA behavior of Ti3C2Tx MXenes, we demonstrate in Chapter 4 a Ti3C2Tx MXene-based photonic diode that breaks time-reversal symmetry to achieve non-reciprocal transmission of nanosecond laser pulses. Finally, in Chapter 5, we discuss the equilibrium and non-equilibrium free carrier dynamics in a 16 nm thick Ti3C2Tx film. High (~2 x 1021 cm-3) intrinsic charge carrier density and relatively high (~34 cm2/Vs) mobility of carriers within individual nanoplates (that comprise the Ti3C2Tx film) result in an exceptionally large (~ 46 000 cm-1) absorption in the THz range, implying the potential use of Ti3C2Tx for THz detection. We also demonstrate that Ti3C2Tx conductivity and THz transmission can be manipulated by photoexcitation, as absorption of near-infrared 800 nm pulses is found to cause transient suppression of the conductivity that recovers over hundreds of picoseconds. The possibility of controlling THz transmission and conductivity via photoexcitation makes 2D MXenes suggests a promising material for application in THz modulation devices and variable electromagnetic shielding.