Ultrafast spectra of layered materials
The fundamental scattering processes in condensed matter span the femtosecond to picosecond time scale and can be monitored in real time by means of ultrafast spectroscopy. This kind of techniques are employed in condensed matter to investigate the carrier’s photoexcitation and relaxation dynamics, the excitonic effects, the electron-electron interaction and the coupling between electron and phonon modes of the nuclear lattice [1-3]. To these purposes, several pump-probe techniques have been developed, such as Pump-Probe Transient Absorption Spectroscopy, Pump-Probe Reflection Spectroscopy, Time Resolved Fluorescence Spectroscopy and Time- and Angle-Resolved Photoelectron Spectroscopy. Besides the common ultrafast sources based on visible light, also ultraviolet, infrared and Terahertz (THz) pulses are today available, extending the scope of optical pump-probe spectroscopy to the control of collective excitations lying in the meV energy range or, conversely, to the direct observation of electronic motion on the attosecond timescale. The wealth of these time resolved techniques has been used to study a large variety of physical phenomena, complex many-body states such as Cooper pair and vortex dynamics in superconductors , CDW motion , metal-insulator phase transitions . Most recently, some notable efforts have been even made in order to explore metamaterials that may lead to novel functionalities.
Optical pump-probe transient absorption spectroscopy
The optical pump-probe transient absorption spectroscopy is the most common technique to follow the temporal evolution of photoexcited electrons and nuclear vibrations in molecules or condensed matter systems . An illustration of the optical pump-probe transient absorption measurement is shown in Fig. 2.1. Several review articles have explained in detail the working principles of this technique [8-10]. For example, in a transient absorption measurement, a fraction of the electrons in the valence band is promoted to an electronically excited state by means of an excitation (or pump) pulse . Depending on the type of experiment, this fraction typically ranges from 0.1% to tens of percents. In order to avoid multiphoton/multistep processes during probing, a weak probe is sent through the sample with a delay τ with respect to the pump pulse (Fig.2.1). A difference absorption spectrum is then extracted, i.e., the absorption spectrum of the excited sample minus the absorption spectrum of the sample in the ground state (ΔA) without pumping. By changing the time delay τ between the pump and the probe and recording a ΔA spectrum at each time delay, a ΔA profile as a function of τ and wavelength λ, i.e., a ΔA(λ,τ) is obtained. ΔA(λ,τ) contains information on the dynamic processes that occur in the excited state, such as thermalization among electrons, emission of phonons, plasmonic modes and possible photoinduced transition.
Figure 2.1: The diagram of the optical pump-probe transient absorption measurement .
By means of optical pump-probe transient absorption spectroscopy in the VIS-NIR region, Qianqian Ding and collaborators experimentally investigated the femtosecond-resolved plasmon-exciton interaction of graphene-Ag nanowire hybrids . As another example, Petr A. Obraztsov and his co-workers reported the ultrafast light-induced absorbance change in CVD-grown multilayer graphene . Using femtosecond pump−probe measurements in 1100−1800 nm spectral range, they revealed broadband absorbance change when the probe photon energy was higher than that of the pump photon. The observed phenomenon is interpreted in terms of the Auger recombination and impact ionization playing a significant role in the dynamics of photoexcited carriers in graphene.
Optical pump-probe reflection spectroscopy
Alternatively, the temporal evolution of optical properties can be also retrieved by detecting the transient changes in the reflectivity of the probe beam. A typical setup to measure transient reflection is shown in Fig. 2.2 . In the specific case the experiment allows for the measurement of the reflectivity (R, blue curve) and and an extraction of optical conductivity σ (red curve) of in Nd2CuO4. The schematic of the experimental setup is illustrated in Fig. 2.2b. Pump and probe pulses are generated from a non-collinear optical parametric amplifier. The cross-correlation profile (10 fs) of the pump and probe pulses is shown in Fig. 2.2c. The spectrum of the pulse is shown by the colored area in Fig. 2.2a, which is indeed located within the optical transition corresponding to the Mott-gap of Nd2CuO4. In some several cases, the technique has been employed to observe photoinduced change of the electronic states at high pumping fluence. S. Pagliara and his co-workers demonstrated that a renormalization of the π−π* band gap takes place when a UV-laser pulse excites a carrier density larger than 10% of the π* density of state in graphite, which has been achieved by detecting the transient reflectivity and the associated decay time of an infrared probe following the excitation of a UV pump pulse tuned across the π−π* absorption resonance . The pump photon energy at which both the transient reflectivity and the decay time are maximum is downshifted by 500 meV with respect to the relative absorption maximum at equilibrium. This finding is interpreted as a transient π−π* band gap shrinking of similar magnitude, near the M point of the Brillouin zone.
Figure 2.2: Schema of a pump-probe reflection setup and optical spectra of CuO2 planes in Nd2CuO4 .
Other works investigates the response of topological materials. By using the time-resolved pump-probe reflectance (ΔR/R) measurement at room temperature under a strong optical pumping, Guohao Zhai and his cooperators explored the photoexcited hot carrier dynamics of a bulk single crystal Cd3As2 in the mid-IR region . They analyzed the transient hot carrier redistribution upon a strong photoexcitation and the subsequent interband transitions combined the experimental ΔR/R results and theoretical model. They showed that the ΔR/R response of Cd3As2 has a complex behavior, due to the interplay of transitions between Dirac bands and transitions involving both the Dirac and non-Dirac bands. Throughout the mid-IR region measured, they found that ΔR/R is contributed primarily by changes in the refractive index, rather than by changes in the extinction coefficient.
Time-resolved fluorescence spectroscopy
The time-resolved fluorescence spectroscopy is widely employed to understand photo-induced carrier’s dynamics and recombination mechanisms in layered semiconductors. The most advanced setups make use of ultrafast streak cameras to digital count photons that are time-correlated in relation to a short excitation light pulse. An overview of the streaking technique to measure time resolved fluorescence is shown in Fig. 2.3 . A pulse beam photoexcite the sample and the subsequent fluorescence is monitored as a function of time after excitation. Fluorescence lifetimes across the semiconductor bandgap and or from trapped states are typically occurring in the time spectral region ranging from picoseconds to nanoseconds. By making use of the time-resolved fluorescence spectroscopy, H. Fang and his cooperators  demonstrated that the exciton radiative rate in some van der Waals heterostructures such as MoSe2 in hexagonal boron nitride (hBN) can be tailored by a simple change of the hBN encapsulation layer thickness, as a consequence of the Purcell effect. The time- resolved photoluminescence measurements showed that spontaneous emission time of neutral excitons can be tuned by one order of magnitude depending on the thickness of the surrounding hBN layers. Understanding the role of these electrodynamical effects is important to predict the complex dynamics of relaxation and recombination for both neutral and charged excitons. On a practical level, monochromatic TMD-based emitters would be beneficial for low-dimensional devices, but this challenge is yet to be resolved. Indeed, the photoluminescence spectra of TMD monolayers display a large number of features that are particularly challenging to decipher. In their work Lorchat’s and colleagues showed that graphene, directly stacked onto TMD monolayers, enables single and narrow-line photoluminescence arising solely from TMD neutral excitons . The authors thought this filtering effect stems from complete charge neutralization of the TMD by graphene, combined to selective non-radiative transfer of some long-lived excitonic species to graphene. These measurements proved that monolayer electroluminescent systems could emit visible and near-infrared photons with linewidths approaching the homogeneous limit.
THz emission spectroscopy
Terahertz (THz) electromagnetic radiation have experienced a large development in recent years [18-22]. It is not exaggeration to say that THz waves can be applied to survey numerous scientific areas, covering the structure analysis in solid state, the electron dynamics in atomic and molecular physics, the discovery of the fundamental building blocks of life in medicine and life sciences, the composition distinguishing in chemistry and the security test in military matters. The frequency range of terahertz radiation, lies between the microwave and infrared regions of the electromagnetic spectrum. Some scientists refer to the range of 0.1–10 THz in frequency as terahertz radiation while others extend this range to frequency as long as 30 THz . Sources of THz radiation can roughly be divided into two categories; broadband sources which are typically based on the conversion of ultrashort optical pulses into few cycle THz pulses and continuous wave (CW) sources which are spectrally very narrow. Herein, we just discuss broadband sources of THz radiation, in which an emitter is excited by a femtosecond driving pulse. The emitter can be a photoconductive antenna, a non-linear crystal for optical rectification or a multilayer with ferromagnetic materials and heavy metals [24, 25]. The THz detection is usually based on an electro-optic (EO) device. The birefringence induced by the THz pulse in the crystal is proportional to the THz electric field. Hence, the THz electric field information is encoded into the polarization state of the THz detection beam after going through the EO crystal. The polarization of the probe beam is further analyzed by a combination of a Wollaston prism and a balanced photodetectors. By scanning the delay of the THz detection beam, we are able to reconstruct the entire THz waveform. The main experimental mechanism is described in Fig. 2.4. The impulsive and broadband THz pulse can be combined with an excitation beam in a pump probe configuration in order to obtain information about the dynamics of excited carriers and collective modes [26, 27].
Figure 2.4: The diagram of THz emission spectroscopy.
Ultrafast photoelectrons spectroscopy
Finally, a complementary and powerful method to investigate photoexcited materials is ultrafast photoelectrons spectroscopy. The working principle of ultrafast photoelectrons spectroscopy is depicted in Fig. 2.5. In ARPES measurements, a photon beam induces the emission of electrons from a solid surface. The band structure of a crystalline solid is reconstructed by analyzing the emission angle and the kinetic energy of the photoemitted electrons. In the case of time resolved photoelectron spectroscopy, a pump pulse is absorbed by the surface, therefore creating a non-equilibrium state. A second pulse in the ultraviolet spectral range probes the photoexicted state via the photoemission process. This technique provides instantaneous photographs of the electronic states with temporal resolution better than 100 fs. The dynamics of the electronic states is obtained by changing the temporal delay between the pump and probe pulse. In recent years, such technique as notably improved, either accessing shorter pulse duration, tighter focusing or high energy resolution. Most important, time resolved photoelectron spectroscopy is capable of provide multi-dimensional information [28, 29], by detecting the electronic states as a function of their wavevector and therefore leading to a complete mapping of the band structure and of the occupation factor in a photoexcited state. Being this technique the core of the present work, its detailed description will be presented in chapter 3.
Table of contents :
Chapter 1 The introduction of layered materials
1.1 Layered semiconductors
1.2 Layered crystals with Charge Density Waves (CDW) and Mott insulator
Chapter 2 Ultrafast spectra of layered materials
2.1 Optical pump-probe transient absorption spectroscopy
2.2 Optical pump-probe reflection spectroscopy
2.3 Time-resolved fluorescence spectroscopy
2.4 THz emission spectroscopy
2.5 Ultrafast photoelectrons spectroscopy
Chapter 3 Time and Angle-resolved Photoemission Spectroscopy
3.1 Angle Resolved Photoemission Spectroscopy (ARPES)
3.2 Time-resolved ARPES
3.3 Experimental setup
3.3.2 Ultrafast laser system for TrARPES
3.3.3 Optical beamlines for TrARPES
3.3.4 The ultra-high-vacuum (UHV) chambers and the analyzer for TrARPES
3.3.5 Sample preparation, sample loading and sample cleaving
Chapter 4 Bandgap Renormalization, Carrier Multiplication and Stark Broadening in Photoexcited Black Phosphorous
4.2 Concurrent work
4.3 Results and Discussion
Chapter 5 Spectroscopy of buried states in black phosphorous with surface doping
5.2 Concurrent work
5.3 Results and Discussion
Chapter 6 Tunability of hot-carrier dynamics in black phosphorus
6.1 Concurrent work
6.2 Results and Discussion
Chapter 7 Ultrafast dynamics of hot carriers in a quasi-two-dimensional electron gas on InSe
7.2 Concurrent work
7.4 Results and discussion
7.5 Theoretical calculations
Chapter 8 Transition from Band Insulator to Mott Insulator in 1T-TaS2 based on photoinjection
8.2 Experimental details
8.3 Results and discussion
Conclusions and Future work
List of Publications