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 LiNbO₃ crystals 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.

Figure 3.2: (a) Collective coherent control over material structure. A single-cycle terahertz-frequency electric field moves all the ions it encounters toward their positions in a new crystalline phase. In strontium titanate, the initial high-symmetry configuration around each Ti4+ ion has no dipole moment and the crystal is paraelectric. The incident field drives the “soft” lattice vibrational mode, moving the ions along the directions indicated into a lower- symmetry geometry with a dipole moment. Long-range ordering of dipole moments in the same direction yields a ferroelectric crystalline phase. (B) Experimental setup. THz field-induced lowering of the SrTiO3 crystal symmetry is observed using 800-nm probe pulses that are partially depolarized (terahertz Kerr effect, or TKE) and which are partially converted to the second harmonic frequency (THz field-induced second harmonic, or TFISH). DM: dichroic mirror; PMT: photomultiplier tube.

Further reading – [20] "Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO₃," X. Li, T. Qiu, J. Zhang, E. Baldini, J. Lu, A. M. Rappe, and K. A. Nelson, Science 364, 1079-1082 (2019) 

(3) THz modulation of optical properties in quantum dots

In addition to the ability of driving phase transitions of solids, THz pulses can modulate the optical properties of solids. In quantum dots, THz pulse can act as a quasi-dc field to induce quantum-confined stark effect that would significantly modulate the electronic states of quantum dots. This result is directly observed through the emission of intense electroluminescence of quantum dots under strong THz field as well as the revival of quenched fluorescence of quantum dots on metal surface. Recently, we have also demonstrated the ability to control the blinking behaviors in quantum dots using mid-IR pulses.

Figure 3.3.1: Schematic of THz field-enhancing microslits and THz pump white light probe spectroscopy on CdSe and CdSe-CdS quantum dots.

Figure 3.3.2: Schematic of THz field-induced reemergence of quenched fluoresence in quantum dots.

Further reading – [19] "Terahertz-driven luminescence and colossal Stark effect in CdSe-CdS colloidal quantum dots," B. C. Pein, W. Chang, H. Y. Hwang, J. Scherer, I. Coropceanu, X. Zhao, X. Zhang, V. Bulović, M. Bawendi, and K. A. Nelson, Nano Lett. 17, 5375-5380 (2017) http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.7b01837">[url]

[21] "Terahertz-Driven Stark Spectroscopy of CdSe and CdSe-CdS Core-Shell Quantum Dots," B. C. Pein, C. K. Lee, L. Shi, J. Shi, W. Chang, H. Y. Hwang, J. Scherer, I. Coropceanu, X. Zhao, X. Zhang, V. Bulovic, M. G. Bawendi, A. P. Willard, K. A. Nelson, Nano Lett. 19, 8125-8131 (2019). 

[24]. "All-optical fluorescence blinking control in quantum dots with ultrafast mid-infrared pulses," Shi, J., Sun, W., Utzat, H., Farahvash, A., Gao, F., Zhang, Z., Barotov, U., Willard, A. P., Nelson, K. A., Bawendi, M. G. Nat. Nanotechnol. 16, 1355-1361 (2021). 

[25]. "Terahertz Field-Induced Reemergence of Quenched Photoluminesence in Quantum Dots. Shi, J., Gao, F., Zhang, Z., Utzat, H., Barotov, U., Farahvash, A., Han, J., Deschamps, J., Baik, C., Cho, K. S., Bulovic, V., Willard, A. P., Baldini, E., Gedik, N., Bawendi, M. G., Nelson, K. A. Nano Lett. 22, 4, 1718-1725 (2022). 

(4) Nonlinear 2D THz Magnetic Resonance Spectroscopy of Magnons

Magnons are quantized low-energy excitations of electron spins. In many ferromagnetic (FM) and antiferromagnetic (AFM) materials, intrinsic magnetic fields in the same range put collective spin waves (magnons) in the THz range. Current ESR spectroscopy remains limited at THz frequencies because the weak sources used only permit measurements of free-induction decay (FID) signals that are linearly proportional to the excitation magnetic field strength. Here, we explore the nonlinearity of magnons using time-delayed intense THz pulse pairs. We develop 2D THz magnetic resonance spectroscopy, which can be understood in terms of multiple field-spin interactions that generate the nonlinear signal fields. Magnons are resonantly excited without promoting electrons to excited states (as in most spintronics excitation) and hence the observed nonlinearities are of purely magnetic origin. The material under study is single-crystal YFeO3 (YFO). The ground state has canted AFM order with Two THz-active magnon modes, the quasi-AFM (AF) and quasi-FM (F) modes, can be constructed based on different cooperative motions of sublattice spins.

Figure 3.4. (a) The canted AFM order in YFO leads to a net magnetization M along the crystal c axis and the AF mode is the amplitude oscillation of M. (b) AF mode magnon signals induced by THz pulse A (blue) and B (red) individually. (c) Magnon signal with the presence of both THz pulses with interpulse delay time 3.7 ps (black) and the nonlinear signal (magnified 50×, magenta). (d) FT magnitude spectrum of the oscillatory signal of the nonlinear signal reveals third and second order peaks. (e) 2D THz magnetic resonance spectra of the AF mode magnons in YFO.

4. References

[1]. ''Velocity matching by pulse front tilting for large area THz-pulse generation." J. Hebling, G. Almasi, et al. Optics Express 10, 1161-1166 (2002).

[2]. "Terahertz polaritonics: High power THz signal generation in ferroelectric crystals," J. Hebling, K.-L. Yeh, M. C. Hoffmann, and K. A. Nelson, Integrated Ferroelectrics 92, 87-94 (2007).

[3]. "High Power THz Generation, THz Nonlinear Optics, and THz Nonlinear Spectroscopy," J. Hebling, K.-L. Yeh, M. C. Hoffmann and Keith A. Nelson, IEEE J. Selected Topics in Quantum Electronics 14, 345-353 (2008). 

[4]. "Generation of high power THz pulses by tilted pulse front excitation and their application possibilities," J. Hebling, K.-L. Yeh, M. C. Hoffmann, B. Bartal, and K. A. Nelson, J. Opt. Soc. Amer. B 25, B6-B19 (2008). 

[5]. "Generation of high average power 1 kHz shaped THz pulses via optical rectification," K.-L. Yeh, J. Hebling, M. C. Hoffmann, and K. A. Nelson, Opt. Comm. 13, 3567-3570 (2008).

[6]. "Impact ionization in InSb probed by terahertz pump-terahertz probe spectroscopy,"  M. C. Hoffmann, J. Hebling, H. Y. Hwang, K. L.Yeh, and K. A. Nelson, Phys. Rev. B 79, 161201(R) (2009).

[7]. "THz-pump/THz-probe spectroscopy of semiconductors at high field strengths [Invited]," M. C. Hoffman, J. Hebling, H. Y. Hwang, K.-L. Yeh, and K. A. Nelson, J. Opt. Soc. Amer. B, 26, No. 9, A29 (2009).

[8]. "Collective coherent control: Synchronization of polarization in ferroelectric PbTiO3 by shaped THz fields," T. Qi, Y.-H. Shin, K.-L. Yeh, K.A. Nelson, and A.M. Rappe, Phys. Rev. Lett. 102, 247603 (2009).

[9]. "Terahertz Kerr effect," M.C. Hoffmann, N.C. Brandt, H.Y. Hwang, K.-L Yeh, and K.A. Nelson, Appl. Phys. Lett. 95, No. 23, 231105 (2009).

[10]. "Impact Ionization in InSb studied by THz-Pump-THz-probe spectroscopy," M.C. Hoffmann, J. Hebling, H.Y. Hwang, K.-L Yeh, and K.A. Nelson, in Ultrafast Phenomena XVI, P. Corkum, S. DeSilvestri, K. Nelson, Phys. Rev. B 79 121201 (2009).

[11]. "Observation of nonequilibrium carrier distribution in Ge, Si, and GaAs by terahertz pump-terahertz probe measurements," J. Hebling, M.C. Hoffmann, H.Y. Hwang, K.-L. Yeh, and K.A. Nelson, Phys. Rev. B 81, No. 3, 035201 (2010).

[12]. " Molecular orientation and alignment by single-cycle THz pulses," S. Fleischer, Y. Zhou, R.W. Field, and K.A. Nelson, Phys. Rev. Lett. 107, 163603 (2011).

[13]. "Generation of high power tunable multicycle terahertz pulses." Z. Chen, X. Zhou, C.A. Werly and K.A. Nelson, Appl. Phys. Lett. 99, 071102 (2011).

[14]. ''Nonlinear THz conductivity dynamics in CVD-grown graphene." H. Y. Hwang, N.C. Brandt, H. Farhat, A.L. Hsu, J. Kong and K.A. Nelson, arXiv:1101.4985 [cond-mat.mtrl-sci] (2011). 

[15]. "Terahertz-field-induced insulator-to-metal transition in vanadium dioxide metamaterial," M. Liu, H.Y. Hwang et al, Nature 487, 345-348 (2012).

[16]. "Commensurate two-quantum coherences induced by time-delayed THz fields", S. Fleischer, R.W. Field and K.A. Nelson, Phys. Rev. Lett. 109, 123603 (2012).

[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).

[18]. "Coherent Two-Dimensional Terahertz Magnetic Resonance Spectroscopy of Collective Spin Waves," J. Lu, X. Li, H. Y. Hwang, B. K. Ofori-Okai, T. Kurihara, T. Suemoto, and K. A. Nelson, Phys. Rev. Lett. 118, 207204 (2017)

[19]. "Terahertz-driven luminescence and colossal Stark effect in CdSe-CdS colloidal quantum dots," B. C. Pein, W. Chang, H. Y. Hwang, J. Scherer, I. Coropceanu, X. Zhao, X. Zhang, V. Bulović, M. Bawendi, and K. A. Nelson, Nano Lett. 17, 5375-5380 (2017) 

[20]. "Terahertz field-induced ferroelectricity in quantum paraelectric SrTiO₃," X. Li, T. Qiu, J. Zhang, E. Baldini, J. Lu, A. M. Rappe, and K. A. Nelson, Science 364, 1079-1082 (2019).

[21]. "Terahertz-Driven Stark Spectroscopy of CdSe and CdSe-CdS Core-Shell Quantum Dots," B. C. Pein, C. K. Lee, L. Shi, J. Shi, W. Chang, H. Y. Hwang, J. Scherer, I. Coropceanu, X. Zhao, X. Zhang, V. Bulovic, M. G. Bawendi, A. P. Willard, K. A. Nelson, Nano Lett. 19, 8125-8131 (2019).

[22].  “Room Temperature Terahertz Electroabsorption Modulation by Excitons in Monolayer Transition Metal Dichalcogenides," J. Shi, E. Baldini, S. Latini, S. A. Sato, Y. Zhang, B. C. Pein, P.-C. Shen, J. Kong, A. Rubio, N. Gedik, K. A. Nelson, Nano Lett. 20, 5214-5220 (2020). 

[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).

[24]. "All-optical fluorescence blinking control in quantum dots with ultrafast mid-infrared pulses," Shi, J., Sun, W., Utzat, H., Farahvash, A., Gao, F., Zhang, Z., Barotov, U., Willard, A. P., Nelson, K. A., Bawendi, M. G. Nat. Nanotechnol. 16, 1355-1361 (2021). 

[25]. "Terahertz Field-Induced Reemergence of Quenched Photoluminescence in Quantum Dots," J. Shi*, F. Y. Gao*, Z. Zhang*, H. Utzat, U. Barotov, A. Farahvash, J. Han, J. Deschamps, C.-W. Baik, K. S. Cho, V. Bulovic, A. P. Willard, E. Baldini, N. Gedik, M. G. Bawendi, K. A. Nelson, Nano Lett. 22, 1718-1725 (2022).