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Project supported by the National Natural Science Foundation of China (Grant Nos. 11327806, 11404385, and GZ1123) and the National Key Research and Development Program of China (Grant Nos. 2016YFA0300902 and 2017YFA0302904).
Exploring, manipulating, and understanding new exotic quantum phenomena in condensed-matter systems have generated great interest in the scientific community. Static and time resolved optical spectroscopies after photoexcitations are important experimental tools for probing charge dynamics and quasiparticle excitations in quantum materials. In Synergetic Extreme Condition User Facility (SECUF), we shall construct magneto-infrared and terahertz measurement systems and develop a number of ultrafast femtosecond laser based systems, including intense near to mid-infrared pump terahertz probe. In this article, we shall describe several systems to be constructed and developed in the facilities, then present some examples explaining the application of magneto optics and time resolved spectroscopy techniques.
Optical spectroscopy is a primary experimental tool for probing single particle and collective excitations of solids over a wide range of energies. The physical quantities extracted from optical probes are so-called optical constants, such as complex reflective index, real part and imaginary part of dielectric function, real part and imaginary part of optical conductivity, etc., which in general are frequency and temperature dependent. Those optical constants can be measured either directly by using, for example, ellipsometry spectroscopy or time domain terahertz (THz) spectroscopy, or indirectly by performing reflectance spectrum measurement over a broad energy scale using a Fourier transform infrared spectrometer (FTIR) and subsequent Kramers–Kronig transformation. The low lying excitations in the far-infrared or terahertz region are of particular importance since the electronic properties of quantum materials are determined mainly by the low lying charge and magnetic excitations. Most elementary excitations of solids occur in such energy scales, including free carrier contribution in metallic compounds, the lattice vibrations (phonons), collective spin wave excitations in magnets (magnons), binding energy of electron–hole pairs (excitons), pairing energy gaps and Josephson plasmons in high temperature superconductors, amplitude modes or phase modes in charge density wave compounds, etc. (see Fig.
In the last two decades, much progress has been made in the development of ultrashort and ultrafast femtosecond laser and its applications. The time resolved spectroscopy based on the standard pump probe geometry provides another route to study the cooperative phenomena in condensed matter physics. Since the relaxation time of lattice, spin, and electrons is different after excitation by pumping pulses, one may resolve the different contributions to the exotic ground state through measuring the time evolution of reflectivity, polarization, or optical conductivity. More intriguingly, when the electric field of the pumping pulse is strong enough and comparable to the potential gradients of atoms in investigated systems, one may access to the non-perturbation regime where the intense pulse can induce strong nonlinear response, or even drive a system to certain metastable state or new exotic phase which are not present or cannot be achieved in the absence of excitations.
In Synergetic Extreme Condition User Facility (SECUF), we plan to construct magneto-infrared measurement systems and develop a number of ultrafast femtosecond laser based systems, including time domain terahertz spectroscopy (TDTS) under magnetic field and time resolved THz system with pump excitations ranging from THz to near infrared regime. In this article, we shall describe several systems to be constructed and developed in SECUF, then present several examples explaining the application of magneto optics and time resolved spectroscopy techniques. Finally, we make some perspective remarks.
The magneto-infrared measurement system in SECUF is mainly composed of FTIR spectrometers and superconducting magnets. The bolometer detectors working at liquid helium temperature are used for far- to mid-infrared measurement. This system has two types of superconducting magnets: cylinder-coil magnet and split-coil magnet. Using the cylinder-coil magnet, one can perform magneto-infrared measurements under magnetic fields up to 20 T or even above. There are two frequently used configurations for the magneto-infrared measurements: Faraday and Voigt configurations. In Faraday configuration, the magnetic field is applied along the wave vector of the incident light, while in Voigt configuration, the magnetic field is perpendicular to the wave vector of the incident light. With cylinder-coil magnet, it is easy to realize the Faraday configuration (see Fig.
However, due to the limited diameter (∼ 50 mm) of the cylinder-coil magnet, it is quite difficult to perform magneto-infrared measurements in Voigt configuration. The split-coil magnet has four windows. The magnetic field is along the two windows. Therefore, Voigt configuration may be achieved in the split-coil magnet by inserting the samples into the magnet along the other two windows. By using a side-loading helium-flow cryostat, it will be convenient to change the temperature (from ∼ 4 K to 300 K) of the samples. The highest magnetic field of commercially available split-coil may reach 12–14 T, which is lower than the cylinder-coil magnet, however it is more convenient to measure optical spectrum over broad energy scale since the detectors being placed at the end of the optical path (outside of the magnet) can be easily switched or changed. Figure
There are multiple techniques for developing time domain and time resolved optical measurement systems. The time resolved magneto-optics systems contain two separate spectrometers: time domain terahertz spectrometer, and pump probe spectrometer under intermediate and high magnetic fields with multiple terahertz sources. There are different components for the time-resolved systems which we shall describe in detail below.
Ultrafast pulsed lasers play a key role in constructing time domain or time resolved optical systems. We have two goals in mind, one is to use weak laser pulses to construct high signal-to-noise time domain terahertz spectrometer and pump probe spectrometer, both under magnetic field. The other goal is to access strong field regime to manipulate or control the ground state of quantum materials, in which photo-induced phase transition or metastable state can be achieved and investigated. To this end, it is necessary to generate a strong electric and magnetic field of pumping radiation and to tune the pump beam wavelength to be resonant with certain modes of investigated systems. For the equilibrium or weak pump probe systems under magnetic field, in order to obtain high quality data we shall use high repetition rate 100 kHz laser system and a pulse duration of 10 fs, which can provide enough bandwidth and time resolution to resolve the intrinsic time evolution of measured optical constant. For the realization of photo induced phase transition or metastable state, we shall employ 1 kHz amplified laser system, which can generate high intensity and carrier envelope phase (CEP) stable THz and mid infrared radiations.
In the time resolved spectrometer, we shall develop intense terahertz/mid-infrared/near-infrared pump terahertz probe system. The energy of the pump beam could be tuned from near infrared down to terahertz by different techniques, covering a frequency range from 0.4 THz to 60 THz as shown in Fig.
We now present some examples illustrating the application of magneto optics and time resolved terahertz spectrocopy.
From the 1970s, the two-dimensional electron gas (2DEG) from semiconductor hetero-structures was at the focus of the magneto-optical spectroscopy research. The dynamics of a simple two-dimensional metal in the terahertz range can be well understood by considering a system of free electrons in the presence of an out-of-plane applied magnetic field. When the magnetic field is strong enough to drive the free carriers to a quantum limit, the charged particle will undergo cyclotron motion in the plane perpendicular to the magnetic field. The cyclotron moving carriers can resonantly absorb light at the frequency ωc = eB/m*, known as a cyclotron resonance. For the conventional 2DEG, the typical energy scale of cyclotron resonance is located in the terahertz range for a magnetic field ∼ 10 T. In the recorded time trace of terahertz pulses, the resonant absorption manifests as an oscillation in time domain. The damping of oscillation is directly linked to the scattering rate of carriers.[8]
In recent years, new quantum materials such as graphene, topological insulators, Dirac or Weyl semimetals have attracted more attention. One of the most remarkable characteristics of these materials is the presence of linear band dispersion. A striking property associated with the linear dispersion is the unequal spacing between Landau levels (LLs) formed under applied magnetic field. Indeed,
As an example, figure
Besides the resonant absorption arising from optical transition between different Landau levels, the giant Faraday rotation at low frequency is another striking property of materials with the presence of linear dispersion, in which the polarization of a linearly polarized beam rotates when transmitted through a material under magnetic field. The Faraday rotation is related to the fine-structure constant. However, the quantitative relation with fine structure constant is different between 2DEG, graphene, and 3D topological insulator.[14,15] Using time domain magneto-spectroscopy, which allows for a direct determination of the frequency dependent complex Kerr angle (Kerr rotation and ellipticity) in THz spectral range, a number of novel phenomena were revealed and discovered, including a giant (106 rad·T−1·m−1) and quantized THz Faraday rotation in HgTe quantum wells,[16,17] graphene,[18] and intrinsic topological insulator thin film,[19] etc.
For the magnetic insulators, it is often assumed that the charge degree of freedom is frozen by the strong Coulomb interaction below the charge gap. Only phonon and collective spin excitations contribute to the low energy absorption. In general, external magnetic field can easily couple to spin moments through Zeeman effect and lift the degeneracy of spin excitation or modify the ground state. Magnto-THz spectrometer is an standard method to probe magnon excitations at the Brillouin zone center.[20] The technique is particularly useful in the study of multiferroic materials, since the electromagnons and magnons arising from the electric and magnetic active spin wave excitations are rather rich and different configurations between propagating vector and magnetic direction result in different optical responses, so-called optical nonreciprocal directional dichroism. Those excitations locate in the terahertz range and have been widely probed by the magneto-THz measurement techniques.[21–24]
Magneto-THz and infrared have also been used to study magnetic excitations in other quantum magnets, for example, the one-dimensional (1D) Ising-like antiferromagnetic chain compound SrCo2V2O8.[25] Theoretically, spin one half chain under transverse magnetic field is a prominent example for studying quantum phase transition. At zero field, the THz spectroscopy measurement revealed two series of excitations corresponding to acoustic and optical branches of confined spinons below the Neel temperature of 5 K in SrCo2V2O8, which could be described by a one-dimensional Schrodinger equation with linear confining potential. By applying a small transverse field, the confinement of the optical and of the acoustic spinons is suppressed. At high transverse field (over 7 T), a quantum disordered phase is induced, in which three emergent fermionic excitations with different transverse-field dependencies were revealed by the terahertz spectroscopy.[25,26]
Ultrashort intense laser pulses, usually at 1 kHz repetition rate, have been proven as a powerful tool for light control of different orders in complex electronic materials. An intense laser pumping can drive a system to certain metastable state or induce a phase transition. In particular, by tuning the excitation energy to be resonant with some mode energies, e.g., phonons, magnons, many novel phenomena were induced and observed. Among others, the light-induced superconductivity in cuprates is perhaps the most intriguing and exciting observation. The first experimental realization was done on a specially doped cuprate La1.675Eu0.2Sr0.125CuO4, in which the 3D superconducting coherence is suppressed below 2 K due to the charge stripe order. However, by exciting the sample with a 16 μm mid infrared pulse at 10 K in the normal state, which is assumed to be resonant with the vibration of apical oxygen modes, Fausti et al. found that a new Josephson plasmon edge like shape emerged at the time delay of 5 ps after excitation, which was identified as a signature of light induced transient superconductivity,[27] as displayed in Fig.
There are many other prominent progresses and development in the THz or mid-infrared pump THz probe spectroscopy. For example, the Higgs mode in a superconductor can be excited by taking the superconductor out of equilibrium by an ultrafast intense THz pump pulse and then detected by the THz probe spectroscopy as an oscillation in the time domain.[31] Over the past two decades, ultrafast magnetism has become increasingly popular and emerged as one of the most significant research branches in magnetism.[32] Magnetization dynamics has been studied intensively with pump probe magneto-optical spectroscopy under intermediate magnetic field, in which an ultrafast infrared laser pulse was usually used to excite the electrons in materials and subsequently the magnetization dynamics was traced by probe pulse.
We would like to make some remarks and perspectives finally. As we mentioned above, although optical spectroscopy covers a broad range of energy, the terahertz and far IR regions are of particular importance since the physical properties of a quantum material are determined by its low lying excitations. Furthermore, most of important single-particle and collective excitations in quantum matters locate in these regions. With the rapid advance of the ultrafast techniques, several of the world renowned high magnetic field user facilities, for example, the Dresden and the Nijmegen High magnetic field laboratory of the European Magnetic Field Laboratory (EMFL) have integrated low energy free electron lasers (FEL) to their magnetic facilities. FEL provides very high intensity, quasi-monochromatic pulsed radiation. On the other hand, table-top laser based high intensity terahertz and mid infrared radiation sources have their own advantage. In Fig.
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