Nonlinear Terahertz Spectroscopy

Coherent Spectroscopy and Control of Various Material Systems Using THz Fields

This lab is located at 2-063.

1. Introduction

In recent years there has been great progress in the exploration of the electromagnetic spectrum between 0.1 and 10 THz, also known as the THz gap. This region gives access to many interesting physical properties of semiconductors, molecular crystals, ferroelectrics, gas molecules, superconductors and biological objects, whose spectroscopic signatures are largely to be discovered in the THz range. In our lab, we are focused on the development of generation of broadband/tunable narrow band THz radiation with high field strengths and characterization of various material systems' nonlinear and collective behaviors induced by the intense THz fields.


2. High field THz generation

Nonlinear THz spectroscopy requires broad bandwidth/tunable narrow band, and high electric field strengths. We generate single- and multi-cycle THz pulses using optical rectification in LiNbOcrystals via noncollinear phase matching with tilted pulse front excitation from a Ti:Sapphire amplifier system. THz pulses are characterized by electro-optic (EO) sampling in EO crystals such ZnTe. The single-cycle THz pulses we generated typically have electric field strengths exceeding 800kV/cm and spectra ranging through 0.1-3 THz. We also developed the chirp and delay method to generate multiple-cycle THz pulses with flexible tunability and high field strength. The THz electric fields can be further enhanced by an order of magnitude using near field enhancement structures. These generation methods open up new possibilities of observation of nonlinear collective materials responses.

(a)                                                                                     (b)

Figure 2.1 (a) Schematic setup of THz generation using tilted-pulse-front pump. (b) Single cycle THz waveform and its FFT spectrum (inset), with the noise level shown in the corner.


3. Nonlinear THz Spectroscopy

(1) Nonlinear THz Rotational Spectroscopy on Gas Molecules

Multiple interactions between the field and the molecule invoke a broad range of coherences between the rotational states. For example, two field-molecule interactions result in alignment of the molecules, and are described by the coherences induced between rotational states |J,m> and |J±2,m>. Due to the restriction of selection rules, the two-quantum coherence must be brought to a one-quantum coherence, |J±1,m> by a THz field interaction before it can radiate back to the population state. Such coherence excitation pathway cannot be distinguished from normal one-quantum coherence signal in the 1D spectroscopy. We study these responses in a multiple-level system using 2D nonlinear spectroscopy. For example, the measurements shown in figure 3.1.3a of acetonitrile at ambient temperature includes ~40 thermally populated rotational levels. Distinct sidebands in the two-quantum coherence signals in figure 3.1.3b for water signals the transient formation of a metastable water complex.

   (a)                                                                  (b)


Figure 3.1: (a) The 2D THz rotational spectrum of acetonitrile obtained by taking the absolute value of the 2D Fourier transformation of the time domain signal. The light dashed lines are alongν=0, ±f and 2f, respectively. Theobserved third-order spectral peaks include NR, R, PP, and 2-Q (magnified 8×inside the red dashed area) signals. The spectrum is normalized and plottedaccording to the color map shown. (b) Many-body interactions in water vapor. 2Q spectra nearfprobe=0.558THz, fpump=1.115THz (Bottom Left) andfprobe=0.753 THz, fpump=1.506THz (Bottom Right) from water vapor at 60 °C. a1 and a2 are 2Q diagonal peaks. b1, c1, andb2 are 2Q off-diagonal features. d1, e1, c2, and d2 are side peaks that arise from distinct coherence pathways. Correspondences between peaks in the 1Dspectrum and the 2Q peaks (indicated by vertical lines) are shown (Top).

Further reading – [17]. “Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase,” Lu, J., Zhang, Y., Hwang, H. Y., Ofori-Okai, B. K., Fleischer, S., Nelson, K. A. PNAS 113 (42) 11800-11805 (2016).

[23]. “Nonlinear rotational spectroscopy reveals many-body interactions in water molecules,” Zhang, Y., Shi, J., Li, X., Coy, S. L., Field, R. W., Nelson, K. A. PNAS 118 (40) e2020941118 (2021).


(2) THz driven ferroelectric phase transition

Besides being to orient molecules in the gas and liquid phase, THz pulses can also access metastable phases of materials. Strontium titanite, SrTiO3 (STO), is a widely used dielectric material with cubic perovskite structure at room temperature. Many of its family members such as lead titanite, PbTiO3, undergoes ferroelectric phase transitions at low temperature. However, STO is known to be a quantum paraelectric, where its quantum fluctuation at low temperature overcomes the long-range ferroelectric orderings, resulting in no net macroscopic electric polarization. Using THz pulse excitation, we can excite STO’s soft mode, which leads to a transient ferroelectric phase indicates by the dramatic growth in the THz field induced second harmonic signal.