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Resolved‑Sideband Raman Cooling of an Optical Phonon in Semiconductor Materials This document is downloaded from DR‑NTU (https://dr.ntu.edu.sg) Nanyang Technological University, Singapore. Resolved‑sideband Raman cooling of an optical phonon in semiconductor materials Zhang, Jun; Zhang, Qing; Wang, Xingzhi; Kwek, Leong Chuan; Xiong, Qihua 2016 Zhang, J., Zhang, Q., Wang, X., Kwek, L. C., & Xiong, Q. (2016). Resolved‑sideband Raman cooling of an optical phonon in semiconductor materials. Nature Photonics, 10(9), 600‑605. doi:10.1038/nphoton.2016.122 https://hdl.handle.net/10356/140449 https://doi.org/10.1038/nphoton.2016.122 © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved. This paper was published in Nature Photonics and is made available with permission of Macmillan Publishers Limited, part of Springer Nature. Downloaded on 26 Sep 2021 04:46:14 SGT 1 Resolved Sideband Raman Cooling of an Optical Phonon in 2 Semiconductor Materials 3 Jun Zhang1,2,*, Qing Zhang1, Xingzhi Wang1, Leong Chuan Kwek3,4, and Qihua Xiong1,5,6,* 4 1Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, 5 Nanyang Technological University, 637371, Singapore. 6 2State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese 7 Academy of Sciences, Beijing, 100083, China 8 3Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, 9 117543, Singapore 10 4National Institute of Education and Institute of Advanced Studies, Nanyang Technological 11 University, 1 Nanyang Walk, 637616, Singapore 12 5NOVITAS, Nanoelectronics Center of Excellence, School of Electrical and Electronic 13 Engineering, Nanyang Technological University, 639798, Singapore. 14 6MajuLab, CNRS-UNS-NUS-NTU International Joint Research Unit, UMI 3654, Singapore 15 *Correspondence should be addressed to Email: [email protected], [email protected] 16 17 18 The radiation pressure of light has been widely used to cool trapped atoms or the 19 mechanical vibrational modes of optomechanical systems. Recently, by using 20 electrostrictive forces of light, spontaneous Brillouin cooling and stimulated Brillouin 21 excitation of whispering-gallery type acoustic modes have been demonstrated. The laser 22 cooling of specific lattice vibrations in solids (i.e., phonons) proposed by Dykman in 1980s, 23 however, still remains sparsely investigated. Here we demonstrate the first strong 24 spontaneous Raman cooling and heating of longitudinal optical phonon (LOP) with a 6.23 25 THz frequency in polar semiconductor zinc telluride nanobelts. We use the exciton to 26 resonate and assist photo-elastic Raman scattering from LOPs due to strong exciton-LOP 27 coupling. By detuning the laser pump to lower (higher) energy resolved-sideband to make 28 spontaneous scattering photon resonate with exciton at anti-Stokes (Stokes) frequency, the 29 dipole oscillation of the LOPs is beat and photo-elastically attenuated (enhanced) to a 30 colder (hotter) state. 31 Spontaneous Raman and Brillouin scattering is described in terms of the statistical nature 32 of the fluctuations in the optical properties of the medium1,2. Incident photons are inelastically 33 scattered to lower (Stokes) or higher (anti-Stokes) frequency via electron-phonon interactions, 34 corresponding to the creation or annihilation of phonons respectively. When the scattered photon 35 frequency resonates with an internal state (such as electronic transition), i.e. 36 ω = ω ± Ω internal laser n phonon (n is an integer), and the corresponding frequency of phonon is larger 37 Ω > γ than the damping of internal state, i.e. phonon internal , the corresponding scattering peak of ±Ω 38 n phonon exhibits the highest intensity. Depending on the resonant situation, the energy flow 39 direction of scattering can be controlled either as phonon heating pumped at blue-resolved- 40 sideband (Stokes resonance) or phonon cooling at red-resolved-sideband pumping (anti-Stokes 41 resonance). The highest cooling or heating efficiency is achievable at the first resolved-sideband, 42 i.e. n=1. Such technique is called the resolved-sideband cooling, which has been demonstrated at 43 low frequency oscillators in the trapped atoms3-7 and cavity optomechanical systems8-11. 44 Resolved-sideband cooling plays an important role in cold atom study, quantum ground state 45 preparation and coherent quantum-state manipulation4,5,7,12-14. 2 46 In solids, laser cooling research mainly focuses on the anti-Stokes photoluminescence 47 cooling, in which the temperature of the whole solid thus the entire distribution of internal 48 vibrational modes of solids is cooled down15-17. As an extended work of resolved-sideband 49 cooling in atoms, laser cooling of one specific phonon mode in solid is important in the study of 50 solid-based quantum technique. In theory, Dykman discussed the possibility of cooling, heating 51 and amplification of specific phonon in solids by similar physics of resolved-sideband cooling in 52 atoms18,19. In experiment, resolved-sideband cooling of whispering-gallery type acoustic modes 53 in Brillouin scattering regime has been demonstrated20. These works have raised the question of 54 whether it is possible to cool optical phonon modes in solid by means of a Raman process. In 55 order to achieve Raman anti-Stokes cooling, the energetically favorable Stokes process leading 56 to heating must be suppressed20. However, as the high frequency optical phonons have higher 57 dissipation rate than acoustic phonons, eliminating the Stokes process is not readily practical in 58 solid state materials20,21. Herein, we demonstrate the reverse of the energy flow direction in 59 spontaneous Raman scattering in zinc telluride (ZnTe) nanobelts by detuning the pump laser 60 relative to the exciton resonance with an energy of the LOP. Being different from the Brillouin 61 cooling of acoustic modes which is dominated by electrostrictive forces, the “radiation forces” in 62 Raman cooling of LOP is due to photoelastically induced Fröhlich forces2 (Supporting 63 Information), where long-wavelength longitudinal optical (LO) phonon can lead to the separation 64 of the oppositely charged atoms and thus generate a macroscopic electric field inside the crystals. 65 The electric field of the optical excitation interacts with this macroscopic electric field in ways 66 similar to the electrostrictive force of acoustic phonons2 (Supporting Information). The force 67 acted on the optical phonon will make work on the phonon oscillation. Depending on the positive 68 or negative work acted on the phonon, the laser will enhance (heat) or attenuate (cool) the 69 phonon oscillation. 70 The reverse of the energy flow direction in spontaneous Raman scattering 71 Crystalline ZnTe nanoribbon exhibits a zincblende structure belonging to the space group 72 2 Td (F 43M ) . Each unit cell of ZnTe contains two atoms. Consequently, there are 6 lattice 73 dynamical modes: one degenerated transverse acoustic modes and one non-degenerated 74 longitudinal acoustic modes, one degenerated transverse optical modes and one non-degenerated 75 longitudinal optical modes2,22. The dispersion curves of all phonons along L symmetry direction 3 76 are schematically shown in Fig. 1a22. Due to the momentum conservation, only one specific LOP 77 close to Brillouin zone center is involved in Raman scattering. In our case, this specific phonon × −2 -1 πλ 78 is longitudinal optical phonon branch with q ~2.0 10 nm (q ~2 laser , see Supporting 79 Information part 1). Compared to the size of first Brillion zone π a ≈ 5.0 nm-1 (a = 0.61 nm is 80 the lattice constant), we can conclude that the LOP involved in Raman scattering is the zone 81 center phonon. Fig. 1b is the atomic displacement of LOP, a bulk vibration consisting of two 82 counter-oscillating sub-lattices of Zn atoms and Te atoms. The LOP is a macroscopic excitation 83 distributed over ~1016 atoms within the crystal and has a very high oscillating frequency owing 84 to the strong interactions between neighboring atoms2,23. At the bath temperature of 225 K we 85 conducted the experiments with a spectrometer broadening of ~39 GHz (see method and Fig. S1 Ω π = 86 in supporting information), the LOP’s frequency is LO 2 6.23 THz with a linewidth Γ=π = 87 LO 238 GHz, corresponding a quality factor QLO 164 . The effective mass of LOP is − 88 = + = × 28 2,22 mLO mZnmTe (mZn mTe ) 3.16 10 kg . For the Fock states of LOP with well-defined 89 quantized vibrational level nLO , the energy is , and the occupied probability 90 of ground state n = 0 is P = = 1(1+ n ) . The displacement of LOP is LO nLOP 0 LOP 2 = Ω2 4-6,11 = 91 x kBTmLO LO . At the quantum ground state nLO 0 , the zero-point fluctuation = Ω = 4,6,10,11 92 amplitude of LOP is xzpf 2mLO LO 0.16 nm . Fig. 1c shows the Raman spectra of 93 120 nm ZnTe nanoribbon excited by a 0.1 mW 532 nm laser with the sample temperature varied 94 from 340 to 77 K. As the temperature decreases, the exciton frequency gradually blue shifts. 95 ω = ω ± Ω When the scattered photon frequency resonates with the exciton energy, i.e. ex laser n LO 96 Ω (n is an integer), the corresponding Raman peak of n LO shows the highest intensity. Therefore, 97 the energy flow direction of Raman scattering can be reversed from phonon heating pumped at 98 blue-resolved-sideband to phonon cooling at red-resolved-sideband pumping. As shown in Fig. 99 1c, the whole spectra are dominated by sharp Raman peaks with a weak broadband 100 photoluminescence emission background. This dominance by Raman process is due to strong 101 exciton-LOP coupling in our ZnTe nanobelt. Therefore, we can exclude the anti-Stokes 102 fluorescence cooling process from the case of ZnTe nanobelts. Besides the electron-LOP 103 coupling, the much narrower exciton peak in ZnTe compared to CdS15 allows us to push the 4 104 sideband cooling into resolved regime, in which LOP Raman scattering is strongly enhanced.
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