Chapter 1 Introduction
1.3 The organization of this dissertation
reflectivity or transmittance. From the ultrafast dynamical spectroscopy, the microscopic mechanism of the magnetoelectric coupling or magnetoelastic behavior can be revealed.
1.3 The organization of this dissertation
This dissertation consists of five chapters. In chapter 1, we simply introduce the multiferroic materials and magnetoelectric coupling phenomena. The motivation of this study is also included. In chapter 2, we give a brief introduction to the growth of high quality samples. Moreover, the crystal structures and orientations of all samples were examined by x-ray diffraction (XRD). The magnetic properties of all samples were inspected by a Quantum Design® superconducting quantum interference device (SQUID) system. The other focal point in chapter 2 is the system setup of the ultrafast femtosecond pump-probe and described in details. In chapter 3, we are going to discuss the fundamental principle of pump-probe technique. First, we present the fundamental physics in ultrashort pulse laser. Next, we discuss the principle of time-resolved pump-probe spectroscopy. Finally, we focus on the origin of the coherent spike signals induced by two laser beam interference. In chapter 4, we present all of the pump-probe experimental results. The pump induced dynamics of electrons, holes, and phonons are influenced by their interaction with each other. We demonstrate and explain that the ultrafast dynamics of electrons and lattice in hexagonal HoMnO3 (h-HMO) single crystals.
In the pump-probe experiments, we observed the dynamical behavior of electrons coupling with antiferromagnetic (AFM) ordering at Néel temperature (TN) which is magnetoelectric coupling. Moreover, through the ultrafast lattice dynamics we directly observed the giant and anisotropic magnetoelastic coupling at TN on a-b plane and along c-axis. In chapter 5, we summarize the significant results in this dissertation.
References
[1] H. Schmid, Ferroelectrics 162, 317 (1994).
[2] M. Fiebig, J. Phys. D 38, R123 (2005).
[3] C. W. Nan, M. I. Bichurin, S. Dong, and D. Viehland, G. Srinivasan, J. Appl. Phys. 103, 031101 (2008).
[4] D. Khomskii, Physics 2, 20 (2009).
[5] W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature (London) 442, 759 (2006).
[6] T. Lottermoser, T. Lonkai, U. Amann, D. Hohlwein, J. Ihringer, M. Fiebig, Nature 430, 541 (2004).
[7] B. Lorenz, Y. Q. Wang, and C. W. Chu, Phys. Rev. B 76, 104405 (2007).
[8] B. Lorenz, Y. Q. Wang, Y. Y. Sun, and C. W. Chu, Phys. Rev. B 70, 212412 (2004).
[9] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, Nature. 426, 55 (2003).
[10] M. Fiebig, T. Lottermoser, D. Frohlich, A.V. Goitsev, and R.V. Pisarev, Nature 419, 818 (2002).
[11] N. Hur, S. Park, P. A. Sharma, J. S. Ahn, S. Guha, and S.-W. Cheong, Nature 429, 392 (2004).
[12] P. Curie, J. Physique 3, 393 (1894).
[13] L. D. Landau and E. M. Lifshitz, Electrodynamics of continuous media (Fizmatgiz, Moscow) (1959).
[14] I. E. Dzyaloshinskii, Sov. Phys. JETP 10, 628 (1959).
[15] D. N. Astrov, Sov. Phys. JETP 11, 708 (1960).
[16] B. I. Alshin and D. N. Astrov, Sov. Phys. JETP 17, 809 (1963).
[17] E. Ascher, H. Rieder, H. Schmid, and H. Stoessel, J. Appl. Phys. 37, 1404 (1966).
[18] C. Michel, J. M. Moreau, G. D. Achenbach, R. Gerson, and W. J. James, Solid state
Comm. 7, 701 (1969).
[19] G. T. Rado, Phys. Rev. Lett. 13, 335 (1964).
[20] B. B. Krichevtsov, V. V. Pavlov, and R. V. Pisarev, JETP Lett. 49, 535 (1989).
[21] N. A. Hill, Annu. Rev. Mater. Res. 32, 1 (2002).
[22] W. Prellier, M. P. Singh, and P. Murugavel, J. Phys.: Condens. Matter 17, R803 (2005).
[23] S. -W. Cheong and M. Mostovoy, Nature Mater. 6, 13 (2007).
[24] A. P. Levanyuk and D. G. Sannikov, Sov. Phys. Usp. 17, 199 (1974).
[25] T. Katsufuji, S. Mori, M. Masaki, Y. Moritomo, N. Yamamoto, and H. Takagi, Phys. Rev.
B 64, 104419 (2001).
[26] B. B. Van Aken, T. T. M. Palstra, A. Filippetti, and N. A. Spaldin, Nature Mater. 3, 164 (2004).
[27] C. J. Fennie and K. M. Rabe, Phys. Rev. B 72, 100103 (2005).
[28] C. Kittel, Introduction to Solid State Physics, 8th. Edition (Wiley, 2005), p. 305.
[29] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, Nature 426, 55 (2003).
[30] T. H. Lin, H. C. Shih, C. C. Hsieh, C. W. Luo, J.-Y. Lin, J. L. Her, H. D. Yang, C.-H. Hsu, K. H. Wu, T. M. Uen, and J. Y. Juang, J. Phys.: Condens. Matter 21, 026013 (2009).
[31] T. C. Han, J. G. Lin, C. T. Wu, M. W. Chu, and C. H. Chen, Jpn. J. Appl. Phys. 49, 041501 (2010).
[32] K. H. Wu, I. C. Gou, C. W. Luo, T. M. Uen, J.-Y. Lin, J. Y. Juang, C. K. Chen, J. M. Lee, and J. M. Chen, Thin Solid Films 518, 2275 (2010).
[33] J. M. Chen, T. L. Chou, J. M. Lee, S. A. Chen, T. S. Chan, T. H. Chen, K. T. Lu, W. T.
Chuang, H.-S. Sheu, S. W. Chen, C. M. Lin, N. Hiraoka, H. Ishii, K. D. Tsuei, and T. J.
Yang, Phys. Rev. B 79, 165110 (2009).
[34] T. H. Lin, C. C. Hsieh, H. C. Shih, C. W. Luo, T. M. Uen, K. H. Wu, J. Y. Juang, J.-Y. Lin,
C.-H. Hsu, and S. J. Liu, Appl. Phys. Lett. 92, 132503 (2008).
[35] C. C. Hsieh, T. H. Lin, H. C. Shih, C.-H. Hsu, C. W. Luo, J.-Y. Lin, K. H. Wu, T. M. Uen, and J. Y. Juang, J. Appl. Phys. 104, 103912 (2008).
[36] T. H. Lin, C. C. Hsieh, C. W. Luo, J.-Y. Lin, C. P. Sun, H. D. Yang, C.-H. Hsu, Y. H. Chu, K. H. Wu, T. M. Uen, and J. Y. Juang, J. Appl. Phys. 106, 103923 (2009).
[37] M. N. Iliev, M. V. Abrashev, H. G. Lee, V. N. Popov, Y. Y. Sun, C. Thomsen, R. L. Meng, and C. W. Chu, Phys. Rev. B 57, 2872 (1998).
[38] J. S. Zhou and J. B. Goodenough, Phys. Rev. Lett. 96, 247202 (2006).
[39] W. C. Yi, C. S. Seo, S. I. Kwun, and J. G. Yoon, Appl. Phys. Lett. 77, 1044 (2000).
[40] H. N. Lee, Y. T. Kim, and S. H. Choh, Appl. Phys. Lett. 76, 1066 (2000).
[41] S. Imada, T. Kuraoka, E. Tokumitsu, and H. Ishiwara, Jpn. J. Appl. Phys. 40, 666 (2001).
[42] Y. T. Kim, I. S. Kim, S. I. Kim, D. C. Yoo, and J. Y. Lee, J. Appl. Phys. 94, 4859 (2003) [43] T. Yoshimura, N. Fujimura, and T. Ito, Appl. Phys. Lett. 73, 414 (1998).
[44] B. G. Ueland, J. W. Lynn, M. Laver, Y. J. Choi, and S.-W. Cheong, Phys. Rev. Lett. 104, 147204 (2010).
[45] M. Fiebig, D. Fröhlich, K. Kohn, St. Leute, Th. Lottermoser, V. V. Pavlov, and R. V.
Pisarev, Phys. Rev. Lett. 84, 5620 (2000).
[46] M. Fiebig, C. Degenhardt, and R. V. Pisarev, Phys. Rev. B 88, 027203 (2001).
[47] O. P. Vajk, M. Kenzelmann, J. W. Lynn, S. B. Kim, and S.-W. Cheong, Phys. Rev. Lett.
94, 087601 (2005).
[48] H. Lueken, Angew. Chem. Int. Ed. 47, 8562 (2008).
[49] Daniel Khomskii, Physics 2, 20 (2009).
[50] A. Muñoz, J. A. Alonso, M. J. Martínez-Lope, M. T. Casáis, J. L. Martínez, and M. T.
Fernández-Díaz, Phys. Rev. B 62, 9498 (2000).
[51] T. Tohei, H. Moriwake, H. Murata, A. Kuwabara, R. Hashimoto, T. Yamamoto, and I.
Tanaka, Phys. Rev. B 79, 144125 (2009).
[52] Th. Lonkai, D. G. Tomuta, U. Amann, J. Ihringer, R. W. A. Hendrikx, D. M. Többens, and J. A. Mydosh, Phys. Rev. B 69, 134108 (2004).
[53] A. Muñoz, J. A. Alonso, M. J. Martínez-Lope, M. T. Casáis, J. L. Martínez, and M. T.
Fernández-Díaz, Chem. Mater. 13, 1497 (2001).
[54] M. Fiebig, C. Degenhardt, and R.V. Pisarev, J. Appl. Phys. 91, 8867 (2002).
[55] Th. Lonkai, D. G. Tomuta, and J.-U. Hoffmann, J. Appl. Phys. 93, 8191 (2003).
[56] Z. J. Huang, Y. Cao, Y. Y. Sun, Y. Y. Xue, and C. W. Chu, Phys. Rev. B 56, 2623 (1997).
[57] B. Lorenz, A. P. Litvinchuk, M. M. Gospodinov, and C. W. Chu, Phys. Rev. Lett. 92, 087204 (2004).
[58] B. Lorenz, F. Yen, M. M. Gospodinov, and C. W. Chu, Phys. Rev. B 71, 014438 (2005).
[59] F. Yen, C. R. dela Cruz, B. Lorenz, Y. Y. Sun, Y. Q. Wang, M. M. Gospodinov, and C. W.
Chu, Phys. Rev. B 71, 180407(R) (2005).
[60] D. G. Tomuta, S. Ramakrishnan, G. J. Nieuwenhuys, and J. A. Mydosh, J. Phys.: Condens.
Matter 13, 4543 (2001).
[61] C. dela Cruz, F. Yen, B. Lorenz, Y. Q. Wang, Y. Y. Sun, M. M. Gospodinov, and C. W.
Chu, Phys. Rev. B 71, 060407(R) (2005).
[62] Seongsu Lee, A. Pirogov, Misun Kang, Kwang-Hyun Jang, M. Yonemura, T. Kamiyama, S.-W. Cheong, F. Gozzo, Namsoo Shin, H. Kimura, Y. Noda, and J.-G. Park, Nature 451,809 (2008).
[63] N. A. Spaldin, and M. Fiebig, Science 309, 391 (2005).
Chapter 2
Experimental tools and procedures
Time-resolved spectroscopy is a very important and direct tool for investigating the problems of carrier dynamics. In this chapter, we will briefly discuss and describe the experimental systems and some other significant experiments. The basic properties of the h-HMO crystals will be shown first. Then, the optical measurement systems will be described in detailed.
2.1 Charaterization and preparation of h-HMO single crystals
The fabrication of single crystals is very important for both fundamental researches and industrial purposes. Figure 2-1 shows the hexagonal HoMnO3 single crystals used in this study. The pure polycrystalline hexagonal HoMnO3 single crystals were synthesized by a solid-state reaction of stoichiometric amount of Ho2O3 (99.99 %) and MnO2 (99.99 %). The single crystals of hexagonal HoMnO3 have been grown via the high temperature flux method [1-3] and in a floating zone furnace. The samples were synthesized for 15 hours at 1290 ℃, then annealed for 5 hours at 1150 ℃ in a platinum crucible and in an oxygen atmosphere.
After that, the temperature was decrease to room temperature with a rate of 1 ℃/h. The flux was decanted and well-shaped hexagonal platelike crystals with typical size of 2 × 3 × 0.25 mm3 were removed from the bottom of the crucible.
2.1.1 Orientation and structure of HoMnO
3single crystals
After growing the HoMnO3 single crystals, the most important thing is to characterize their physical properties. Fig. 2-2 shows the X-ray diffraction (XRD) θ-2θ pattern (Cu Kα, λ = 1.5406 Å) for the h-HMO single crystals grew by the floating zone furnace method. The XRD data evidently confirm the formation of the pure hexagonal HoMnO3 with the c-axis (space group: P63cm) oriented normal to the largest crystal surface. The crystal structure of HoMnO3
has been shown in section 1.1 of chapter 1. The full width at half maximum (FWHM) ( 0.25°) of X-ray data indicates the good crystalline quality and grain alignment of the h-HMO crystals. To further examine the in-plane texture of the crystals, we also measured the -scan around the h-HMO reflection. The -scans display an evenly behaved six-fold symmetry,
Figure 2-1: The HoMnO3 single crystals used in this study. The largest surface is around 3×3 mm2 and the thickness of this crystal is about 0.5 mm
indicating that the in-plane grain alignment on the a-b plane well. The fitted lattice constant of HoMnO3 single crystals for a- and c-axis were 6.142 Å and 11.408 Å, respectively [4-7].
The other useful parameters, e.g. lattice parameters, atomic positions, and discrepancy factors in Table. 2-1, from high-resolution neutron diffraction experiments were reported by A.
Muñoz et al. [8].
Figure 2-2: The XRD results of the h-HMO single crystals grew by the floating zone furnace method. The θ-2θ scans (plotted in semi-logarithmic scale) reveal that HMO crystals indeed hexagonal with c-axis orientation.
10 20 30 40 50 60
HMO(006)
HMO(004)
Intensity (arb. units)
2 (degree)
HMO(002)
Table 2-1: The lattice constant, atomic positions, and discrepancy factors corresponding to the crystal structure of HoMnO3 [8].
2.1.2 Temperature–dependent susceptibility measurements
The magnetic properties were measured in a Quantum Design superconducting quantum interference device (SQUID) system. Fig. 2-3(a) and (b) show the characteristics of magnetization in the platelet samples examined by SQUID. The Mn-spin rotation transition (TSR ~ 33 K) and the magnetic order of the Ho3+ ions (THO ~ 5 K) could be clearly observed in the magnetization measurements (the arrows in Fig. 2.2(a) and the enlarge scale in Fig.
2-3(b)). However, the AFM transition of h-HoMnO3 is difficult recognized in magnetization measurements due to the huge paramagnetic signal from rare-earth ion. These results consist with those reported by other researchers [9].
The AFM exchange coupling in a triangular lattice gives rise to spin frustration effects and, at TN the Mn3+ moments order in a way so that neighboring Mn-moments form a 1200 angle [10]. In addition, most rare-earth ions carry their own magnetic moment oriented along
0 20 40 60 80 100
Figure 2-3: (a) The temperature-dependent susceptibility (χ(T)) of h-HMO with a magnetic field of 100 Oe applied along c-axis. The inset shows the inverse susceptibility. The dashed line: the Curie-Weiss high temperature extrapolation. (b) To enlarge the figure in order to show the behavior of spin rotation.
30 32 34 36 38 40
Temperature (K) TSR =34.1 K
(b)
the c-axis of the P63cm structure. The rare-earth moment can interact with the Mn3+ spins and the dielectric polarization and thus increase the complexity of the phase diagram and the physical phenomena that can be observed. For example, the complex magnetic phase transition and different spin arrangements have been canvassed by the second harmonic generation or the neutron scattering measurements [8,11-12] which show two additional phase transitions below TN indicating subtle changes in the magnetic order of the Mn3+ and Ho3+
ions at zero external magnetic field. At TSR ~ 33 K, a sharp Mn-spin reorientation transition takes place at which all Mn-moments rotate in-plane with an angle of 900 and changes the magnetic symmetry from P63cm (T > TSR) to P63cm (T < TSR) (illustrated in Fig. 2-4 [10]). At lower temperatures, THo ~ 5 K, another change of the magnetic structure has been reported but the magnetic order in this phase is still a matter of discussion. The transitions at TSR and THo
are accompanied by partial or complete magnetic ordering of the Ho3+ moments, but the detail of the Ho-spin order has not been resolved yet. All magnetic transitions are well below the FE Curie temperature of TC = 875 K.
Figure 2-4: The three Mn3+ spin configuration in hexagonal HoMnO3 [10].
The open circles indicate Mn ions at z=0, filled circles indicate Mn ions at z=c/2.
T ~ TN TSR < T < TN T < TSR
2.1.3 Transmittance spectrum
Fig. 2-5 shows the transmittance spectrum in the HoMnO3 single crystals which was measured by a grating-type spectrophotometer (HITACHI high-technologies Corporation) in the photon energy range of 0.7-5.0 eV. According to the literatures [13-16], there are three common features in the absorption spectrum of hexagonal ReMnO3 which performed by Fourier-transform infrared spectrometer. First, the optical excitation causes the absorption peak near ~ 1.7 eV at low temperature which is attributed to the charge transfer from e2g
orbitals (d andxy dx2y2) to a1g orbitals (d3z2r2). Second, the relatively weak peak near ~ 2.2 eV comes from the charge transfer from e1g orbitals (d andxz d ) to ayz 1g orbitals (d3z2r2) between the Mn 3d levels. Finally, a much stronger absorption peak at higher energy region above 3 eV caused by the continuous charge transfer from O 2p to Mn 3d states as shown in Fig. 2-5. In virtue of the transition metal Mn3+ ion sits at the center of a triangular bipyramid
Figure 2-5: The transmittance spectrum of hexagonal HoMnO3 single crystals. The arrows show the three features. The inset shows the local environment MnO5 for photon energy above and below Edd.
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
of five O2- ions at each corner, the d orbitals of the Mn3+ ion are split into three parts (e1g 、 e2g、a1g) due to the ligand field effects which arises mainly from the strong electrostatic Coulomb repulsion of the negatively charged electrons in the oxygen orbitals (show in Fig.
2-6) [17-20].
The electric structures of h-HoMnO3 can be simulated by the first-principles calculations since its structural parameters have been sufficiently studied. To deal with the effects of strong Coulomb interactions among 3d electrons, one can use the local density approximation (LDA) +U methods based on the density functional theory, as implemented in a linear combination of localized pseudo atomic orbital (LCPAO) code. Fig. 2-7 shows the density of states for hexagonal YMnO3 calculated by Choi et al. [16].
Figure 2-6: The electric structures of MnO5 and MnO6 [20].
2.2 Femtosecond time-resolved systems
In this section, we are going to introduce the optical systems used in this study which were built by ourselves. This measuring system has three parts which can be operated individually. The first part (subsection 2.2.1) is a pump-probe system. The second part (subsection 2.2.2) is a terahertz time-domain spectroscopy system. The final part (subsection 2.2.3) is a whole system combining the first and second parts, so-called optical pump terahertz probe system. In the following subsections in this dissertation will exhibit various systems respectively. In this section we only discuss the optical apparatus among these systems. On the contrary, the principles of operation will be discussed in chapter 3.
Figure 2-7: The orbital-resolved densities of states of Mn 3d orbitals and the in-plane O 2p orbital for YMnO3 [16].
2.2.1 The polarized femtosecond pump-probe system
Figure 2-8 shows the details of the optical pump-probe system used in this study. The experiments were performed with a femtosecond Ti:sapphire laser pumped by an Nd:YAG laser with 532 nm. Fig. 2-9 shows the laser system which is a Ti:sapphire oscillator (Model:
Micra-10) fabricated by “Coherent in USA”. The output laser is wavelength tunable from 815 nm to 740 nm via controlling the interval of the slit. The output spectrum can be measured by a spectrometer (Ocean optics, Model: USB4000-UV-VIS) as shown in Fig. 2-10. The spectral width of the output pulses was adjusted to ~25 nm (FWHM) for our measurements. Following, the output laser beam went through a beamsplitter (BS1) and was reflected 50 % of light as a pump beam (B1) used to generate the terahertz beam, whereas the remnant (B2) was transmitted and served as a probe which was used to probe the terahertz pulse. Moreover, the pump and probe beams used to do pump-probe experiments were taken from B2 and divided
Figure 2-8: The experimental setup for near-IR pump-probe spectroscopy.
Code : BS: beamsplitter, L: lens, A: acousto-optic modulator (AOM), M:
mirror, WP: wave plate, P: polarizer, TS: time-delay stage, D: photodiode.
into B3 via a beamsplitter (BS2). The laser beam B3 passed through a prism pair in order to compensate the dispersion due to the optical components in the system. Both pump and probe beams passed through two acousto-optic modulators (AOM, A1-A2) respectively. However, only one in the pump beam was driven by the RF driver and modulated the pump beam at 1 MHz. After travelling through a delay stage (TS1), a half-wave (λ/2) plate (WP1-WP2), and a polarizer (P1-P2), the pump beam was focused by a 200-mm lens on the surface of a sample with ~ 200 μm in diameter. The λ/2 plate and polarizer allowed us to adjust the intensity and polarization (electric field, E) of pump beam (both needed for intensity control). A mechanical delay stage was used to vary the arrival time of pump pulses related to probe pulses at the position of samples. On the other hand, the probe beam only passed through the λ/2 plate and the polarizer after the AOM and focus on the surface of the sample with 150 μm in diameter by a 150-mm lens.
The powers of pump and probe beams were 50 mW and 2 mW, respectively. The best spatial overlap of pump and probe beams on the samples was realized by monitoring with a CCD
Figure 2-9: The Ti:sapphire laser cavity. The green line represents the pumping source. The red line represents the path of ultrashort pulses.
camera. The reflectivity changes of a probe beam were detected by using a photodiode detector and a lock-in amplifier [21-23].
2.2.2 The terahertz time-domain spectroscopy
In past 20 years, the femtosecond excitation on photoconducting switches, unbiased and biased semiconductor surface, and strain layer heterostructures have been used to generate pulsed THz electromagnetic waves [24-29]. The THz waves can be collimated and transmitted over reasonable distances, and can be detected by using optically-gated photoconductive antennas. By adjusting the delay between the THz signal and the gating pulse, the amplitude and phase of the THz signals can be obtained.
Figure 2-10: The tunable wavelengths from 815 nm (1.52 eV) to 740 nm (1.68 eV) in the Ti:sapphire oscillator used in this study.
700 720 740 760 780 800 820 840 860 880
Norm alize Intentsity (arb. units)
Wavelength (nm)
740nm 755nm 770nm 785nm 800nm 815nm
Figure 2-11 shows the optical setup of the THz time-domain system. In this system, the light source divided into two parts via beamsplitter (BS1), the first one was used to generate THz radiation (B1) called THz pump beam, and another was used to be the gating pulse called THz probe beam (B2). The optical pulses (B1) were normally incident to the terahertz emitter (TE) which was manufactured by low-temperature growth GaAs or InP. The THz pump beam was modulated by a mechanical chopper (CH1) operated at 1.3 kHz. The electric field of a terahertz pulse was sampled by scanning the delay (TS4) between the pump and probe pulses.
A semiconductor GaAs photoconductive emitter was triggered by femtosecond laser pulses and radiated the THz pulses. The emitted THz pulses was collimated by two pairs of off-axis parabolical mirrors (PM1-PM4) and focused onto a nonlinear ZnTe electroptical crystal. A pellicle beamsplitter (PSP2) which is transparent for the terahertz beam was used to reflect
Figure 2-11: The experimental setup for the terahertz time-domain spectroscopy. Code: BS: beamsplitter, L: lens, CH: chopper, M: mirror, TS:
time-delay stage, TE: THz emitter, PM: parabolical mirror, WP: wave plate, PB: polarized beamsplitter, D: photodiode.
80% of the synchronized optical probe beam. The polarized THz wave and probe beam were collinearly aligned to a <110>-oriented ZnTe crystal. Then, we used a quarter-wave plate (WP5) to add a /2 optical bias in the probe beam, which allows the system to operate in the linear range. A Wollaston polarizer beamsplitter (PBS) was used to convert the terahertz-field-induced phase retardation of the probe beam into an intensity modulation between the two orthogonal linear-polarized beams. The optical intensity modulation was detected by using two balanced photodiodes (D3, D4) and a lock-in amplifier (SR830).
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Figure 2-12: (a) The transient terahertz waveforms generation from the semiconductor InP photoconductive antenna. Black line represents the THz radiation through the free space. Red line shows the THz wave through the quartz window. (b) The terahertz emitter was changed from a photoconductive antenna to a ZnTe crystal. The terahertz wave was generated from nonlinear mechanism which is called the optical rectification.
2.2.3 The optical pump-terahertz probe system setup
The optical pump-THz probe system was performed by combined the optical pump-probe system and THz time domain system. In order to generate THz by nonlinear effect, we change the laser source from the Ti:sapphire oscillator to amplifier (Model:
Coherent, Legend). The high peak power of Ti:sapphire femtosecond amplifier allows for the nonlinear optical conversion of the fundamental wavelength of 800 nm to the wavelengths of ultraviolet or infrared (THz). In fact, the combination of an optical pump with THz probe has been used to investigate the relaxation dynamics of photoexcited carriers in a lot of materials, such as superconductors [30], semiconductors [31-34], dielectrics [35], and liquids [36-37]. A diagram of the optical pump-THz probe experimental setup is shown in Fig. 2-13. The primary light source is a Ti:sapphire regenerative amplifier, which typically produces sub-100 fs (FWHM) and 800 nm pulses at a 5 kHz repetition rate. The pulses from amplifier laser pass through all kinds of optical components as mentioned in above subsections. It should be
Figure 2-13: Schematic of optical pump-THz probe experiment.
emphasized some differences between pump-probe system and THz time domain system. One is the amplifier laser pulses without the dispersion compensation because the low frequency modulation by a mechanical chopper (CH2) instead of an AMO. Another one is the generation of THz radiation from an emitter (TE) of a ZnTe crystal instead of a semiconductor antenna in order to get higher power THz radiation.
The Fig. 2-14 shows the concepts of optical pump-THz probe measurements. The black circles represent the arrival time of a pump pulse at the sample surface. The curve shown in Fig. 2-14 represents the reflectivity changes induced by a pump pulse as increasing the delay time. We can measure the THz waveform with changing the delay time of a pump pulse. Thus, the variation of the dielectric function after pump excitation can be obtained. In general, we can obtain the transient information about the carrier dynamics induced by pump laser. The time-resolved transient dielectric function allows someone to know the behavior of electrons
Delay time
Figure 2-14: The concepts of for the optical pump-THz probe measurements.
or lattice in one material. Fig. 2-15 shows an example, the results of optical pump-THz probe experiments in semiconductor InP. The peak amplitude of THz pulses plotted as a function of delay time of pump pulses. The excitation of pump laser causes the changes of refractive index and thus causes the changes in THz transmittance [38-41].
-50 0 50 100 150 200 250 300 350 400 -10.5
-9.0 -7.5 -6.0 -4.5 -3.0 -1.5 0.0 1.5
Amplitude of THz peak (V) [x10-6 ]
Pump beam delay (ps)
1 mW 1.5 mW 2 mW 2.5 mW
Figure 2-15: The peak amplitude of THz radiation as a function of the delay time of a optical pump pulseat various optical pump power.
Figure 2-16 shows the experimental setup. The experimental apparatus includes the optical cryostat for measuring temperature-dependent optical spectra.
Figure 2-16: The experimental apparatus including the optical cryostat for
Figure 2-16: The experimental apparatus including the optical cryostat for