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Unravelling electronic dynamics within atoms and molecules represents a frontier in modern chemistry and biology, offering profound insights into the fundamental mechanisms driving chemical reactions and biological processes. Ultrafast sciences,harnessing the power of cutting-edge spectroscopy, diffraction, and imaging techniques, have emerged as pivotal in capturing these transient events with unprecedented temporal resolution. Advancements in ultrafast laser technology enable researchers to observe and manipulate electronic states within their natural attosecond, femtosecond, and picosecond timescales, thus providing a deeper understanding of the electronic processes that underpin structural changes, reaction pathways, and energy transfer in complex systems. By bridging the gap between theoretical predictions and empirical evidence, ultrafast sciences are revolutionizing our comprehension of the microscopic world, with significant implications for chemistry, biology, and material science. In this dissertation, we lay a theoretical foundation for light-matter interaction, laser-induced transient effects, and pump-probe interactions. We present the recent development in ultrafast laser technology using solid-state lasers such as Ti:Sapphire and Ytterbium-based lasers. We extend to novel spectral broadening and pulse compression techniques such as self-phase modulation in hollow-core fibers and chirped-mirror dispersion compensation. We showcase the application of such novel ultrafast pulses in probing dark state dynamics with femtosecond temporal resolution in a novel three-color wave mixing spectroscopic scheme. The proposed three-color scheme provides additional advantages in accessing excited dark states through background-free emission by exploiting non-commensurate probe IR fields. Finally, we provide technical instrumental details involved in typical pump-probe absorption and photoelectron spectroscopies. Such elegant techniques require a delegate vacuum setup

Metamaterials, or engineered materials with properties not found in nature, are at the fore-front of an exciting and rapidly developing inter-disciplinary field of material science that encompasses many sub-fields of physics and engineering. The ability to spatially tailor the wide variety of material properties afforded by metamaterials provides scientists and engineers the capability to create meta-devices with unprecedented and novel capabilities. Paradoxically, added flexibility and an expanded catalog of design degrees of freedom can increase the difficulty in converging to a point design. The added degrees of freedom can risk overwhelming a prospective innovator without advanced methods of analysis to complement their raw inspiration. This dissertation leverages Radio Frequency (RF) metamaterials and the methods of mathematical physics to design and analyze quasi-optical RF devices with novel properties. Two case studies are presented whereby methods and device architectures adapted from optics form the inspiration of new RF devices. The first case study concerns the development of a device that launches non-diffracting Bessel beams. The methods of mathematical optics are applied to synthesize a gradient index lens that forms a Bessel beam from an antenna located at the surface of the lens. The spherical symmetry of the lens lends itself well to the implementation of multiple electrically steered beam systems via placement of a multitude of antennas across the surface of the lens. The performance of the Bessel launcher is analyzed via ray tracing and full wave simulations and found to show decent agreement with a Bessel beam synthesized via standard methods. The second case study concerns the development of a phase conjugator implemented via nonlinear RF metamaterials. The strong magnetic third order nonlinear susceptibility provided by varactor split ring resonator (SRR) media provides a medium with nonlinear properties that dwarf the capabilities of natural

Semiconductor microcavities are among the most widely used laser technologies today. They can exhibit various macroscopic quantum phenomena, including Bose-Einstein condensation (BEC) of polaritons, Bardeen-Cooper-Schrieffer (BCS) states of polaritons, and photon lasing (lasing with negligible Coulombic exciton effects). These states have primarily been studied by measurement of the laser emission, while driven fluctuations of the order parameter are also an important tool for characterization. However, optical-probe spectroscopy of microcavities is constrained by the wavelength-selective distributed Bragg reflectors (DBRs), which typically form the end faces of the cavity. Terahertz radiation is little affected by the DBRs, and therefore THz spectroscopy can effectively probe the lasing system. In this dissertation, we construct and explain the microscopic theory of this continuous wave (CW) optical-pump THz-probe spectroscopy of quasi-two-dimensional (2D) semiconductor media, which include gallium arsenide (GaAs) quantum wells (QWs). We analyze the spectrum of spatially uniform fluctuation modes in the linear electromagnetic response of a semiconductor laser. With the condensate formed by composite particles, comprised of electrons, holes, and cavity light field photons, the set of zero-momentum single-frequency fluctuations contain both continua of single-particle electron-hole modes; and discrete-energy collective modes, which constitute coherent oscillations of all electronic states in the two bands. The quasiparticle excitations of the electron-hole system together determine the spectroscopic observables, such as the THz conductivity. The eigenmode spectra confirm that the interactions between the two bands induce their splitting, into an effective set of four bands, with the new intraband gap referred to as the BCS gap. The BCS gap is a signature of a macroscopic quantum state, and in the semiconductor laser is determined by the Coulomb interaction between ch

The advancement of the ultrafast pulse shaping and waveform synthesis allowedto coherently control the atomic and electronic motions in matter. In this work, we demonstrate the light field synthesizer of high-power waveforms spanning two optical octaves, from near-infrared to deep-ultraviolet with attosecond resolution. Moreover, we introduced and utilized a new alloptical light-field-sampling methodology—based on reflectivity modulation of the fused silica dielectric system in a strong light field—with attosecond resolution to complement the light field synthesis scheme. This synthesis scheme is exploited to demonstrate the on-demand control of electron motion in a dielectric using synthesized light waveforms, which paves the way for establishing long-anticipated ultrafast electronic switches. Furthermore, we exploited the reflectivity modulation to demonstrate the optical switching with attosecond resolution and the capability of controlling the optical switching signal with complex synthesized fields of ultrashort laser pulses for data binary encoding. Lastly, we introduce the all-optical attosecond metrology to study the light-field induced electron dynamics in different dielectric systems and measure their relative electronic delay responses (in the order of a few hundred attoseconds) triggered by a strong field of few-cycle pulses.

Magnetic tunneling junctions (MTJs), promising for the development of next-generation information processing and storage devices, consist of two magnetic layers separated by an insulating barrier. Lower resistance, read as 0, occurs when the magnetizations of two layers are parallel, while higher resistance, read as 1, results from antiparallel alignment. Three key requirements of memory devices are: writing data, storing data, and reading data. While reading data, the voltage is applied, and the resistance state of the devices is read. To get the lower read error rate, the magnetoresistance of the devices must be higher. We achieved a magnetoresistance of 257%, the highest value reported in voltage controllable perpendicular MTJ with conventional MgO barrier. We explored MgAl2O4 as an alternative tunneling barrier and demonstrated its superior performance in high voltage operation. For longer data retention, the thermal stability factor of the devices must be higher. The thermal stability factor is directly proportional to the thickness of the free layer. We successfully demonstrated a perpendicular MTJ with multi-interface free layer, showcasing a reasonably high magnetoresistance, 212%. The multi-interface free layer allows a thicker free layer, addressing the thermal stability issue of the memory devices. Writing speed is associated with the switching speed of the recording or free layer. We designed a unique multilayer, [Gd/Co2Fe6B2]N ferrimagnetic free layer structure, achieving the magnetoresistance of above 45% competent of doing spin transfer torque (STT) switching in ferrimagnetic structure. The switching speed of 0.35 ns was demonstrated, at least 5 times faster than that in similar ferromagnetic structures fabricated by us.

The discovery of graphene led to an eruption of research into the expansive collection of two-dimensional materials. The ability to fabricate stacked heterostructures with van der Waals materials layer-by-layer has allowed the production of unique devices and has rapidly advanced research in electronics and optics. Understanding the dynamics of carrier and phonon interactions within these systems is crucial for the development of optoelectronic devices. This thesis explores the dynamics of photo-excited carriers in two-dimensional systems: the effects of substrate choice on carrier relaxation in graphene and phonon induced bandgap renormalization in monolayer tungsten disulfide at high carrier densities. Graphene-hBN (hexagonal boron nitride) heterostructures show promising use in electronics applications due to high carrier mobility. We first explore the effect of the hBN substrate on the relaxation rates of photo-excited carriers in these heterostructures using femtosecond pump-probe spectroscopy. Time dynamics of photo-excited carriers in graphene-hBN heterostructures show a cooling rate approximately four times faster on hBN substrates compared to silicon oxide substrates. We next study the effect of variation in isotopic concentration in hBN substrates on the relaxation rates of photo-excited carriers. We measure and compare the time dynamics of photo-excited carriers in graphene-hBN heterostructures using naturally occurring hBN (containing 20% 10B and 80% 11B) and isotopically pure hBN (containing 100% 10B or 100% 11B). We observed a carrier relaxation rate ~1.7 times faster for isotopically pure hBN substrate. Isotopically pure hBN substrates samples allow more efficient decay of optical phonons from graphene into acoustic phonons in the substrate, while the isotopic disorder in naturally occurring hBN causes isotope-phonon scattering. Monolayer transition metal dichalcogenides are another van der Waals material which have garnered a lot of interest due to t

Photonic quantum information processing is a type of information processing based on the principles of quantum optics. Continuous-variable (CV) quantum information of photons underpins a variety of quantum sensing and communication applications. Various of these quantum applications involve the generation and distribution of the two-mode squeezed vacuum (TMSV) state photons. My research topics cover these important subjects and can be divided into three parts. First, Itheoretically proposed an integrated photonic platform for the generation of TMSV photons and experimentally fabricated the structure on the chip to successfully carry out these TMSV photons on the chip. This part is compatible with complementarymetal-oxide-semiconductor (CMOS) technology and paves the road of mass production of quantum integrated photonic platforms. Second, I theoretically proposed a CV quantum repeater architecture with the assistance of quantum error correction (QEC) and optical Gottesman-Kitaev-Perskill state to realize long-haul entanglement establishment with high fidelity. To prove its usefulness, I applied these protocols on three representative use cases for quantum communication and sensing. Once optical GKP states with sufficient squeezing become available, the proposed QR architecture will enable CV quantum states to be faithfully transmitted over unprecedented distances, thereby making a large stride forward in the development of quantum technology. Finally, I theoretically proposed a quantum-radar scheme by transmitting the pulse-compression microwave field for investigating the direction of a distant object with higher sensitivity than the classical counterpart. This work generalizes the previous results in quantum radar ranging in [Phys. Rev. Lett. 128, 010501 (2022)] towards a general quantum radar detection system capable of detecting various properties of targets.

This dissertation presents experiments studying electron dynamics in atoms and molecules using the tools of ultrafast photoelectron spectroscopy. The first experiment studies extreme-ultraviolet excitation and relaxation dynamics in molecular oxygen and identifies a previously undiscovered excitation and dissociation pathway, filling a hole in the spectroscopic library of oxygen that had existed for almost twenty years. This is in fact a multielectron excitation and would therefore be expected to have a very low excitation cross-section. Fresh analysis of the Fano approach to autoionizing states revealed that this can be explained as a sort of backwards autoionization– “excitation” initially proceeds to the continuum and population is transferred back to the discrete state via the same coupling mechanism as autoionization. The second and third experiments stem serendipitously from the same few sets of exploratory data sets taken in argon when testing a new optical technique. In one an argon atom is excited into a superposition of autoionizing states and this wavepacket is probed with a delayed IR pulse. This has the unintended but beneficial effect of inducing Raman transitions among the wavepacket constituent states and allowing study of its dynamics with unprecedented time and energy resolution simultaneously. Finally, a similar Raman process is used to probe a superposition of bound states, this time leading to rich and unanticipated angular structure in the electron emission. Categorically, the method used across all these experiments is known as “time-resolved photoelectron spectroscopy” and consists of an initial excitation, or “pump,” of an atom or molecule followed by a time-delayed “probe” inducing some measurable effect. Most simply, the probe photoionizes the system, thereby producing a photoelectron with measurable momentum and energy, but more subtle probes can be designed where, for example, it effects the rate of autoionization. The experimental appar

The advent of ultrafast nonlinear spectroscopy with weak extreme-ultraviolet (XUV) and strong-field near-infrared (NIR) pulses has enabled investigations of electronic processes in atomic and molecular systems with femtosecond and attosecond resolution. Conventional XUV-NIR attosecond transient absorption (ATA) and multi-wave mixing (MWM) spectroscopies have successfully probed coherent electron dynamics in a wide range of quantum systems. However, these optical setups are often limited by the commensurate nature of the harmonic XUV pump and the driving NIR probe. In this work, we enhance the versatility of these techniques by using optical parametric amplification to generate tunable infrared (IR) pulses. This approach allows us to adjust the frequencies of the XUV and IR pulses independently of each other. To demonstrate the power of tunable ATA and MWM schemes, we apply these techniques to study and manipulate laser-induced couplings and autoionizing resonances embedded in the continuum of polyelectronic systems. We begin by investigating the degeneracy between autoionizing bright and light-induced states (LIS) in argon, which leads to the formation of entangled light-matter states known as polaritons. By tuning the IR pulse parameters we demonstrate control over polaritonic evolution and their decay pathways. In the next study, we use the incommensurate IR pulse to drive resonant background-free wave-mixing processes to probe quantum beats that arise from coherent electronic superpositions of singly- and doubly-excited states in krypton. Finally, we discuss the extension of these techniques to molecular systems and macroscopic propagation regimes. Our results further the understanding of autoionization (AI) dynamics in multielectron systems and demonstrate novel methods for the optical measurement and control of excited-state dynamics in the continuum.

Interlayer excitons in 2D semiconductors have been the subject of intensive study due to their long lifetimes and spin-valley physics, with long standing goals including single IX trapping and diffusion control for valleytronic applications. In this work, we use nano-patterned graphene gates to create a quasi-zero and one-dimensional electrostatic IX traps. This approach uses the IX’s permanent dipole moment in a sharply spatially varying electric field to create a nanoscale custom potential energy landscape for IXs, with features as small as 17 nm. In zero dimensional structures, etched holes in the graphene gate create a quantum dot-like potential for IXs. The trapped IXs show the predicted electric field dependent energy, saturation at low excitation power, and increased lifetime, all signatures of strong spatial confinement. Additionally, photoluminescence from these trapped IXs exhibits a unique power dependent blue-shift, where we observe narrow linewidth IX emission (<0.9 meV) and discrete jumps in the emission energy with increasing power. These jumps can be attributed to quantized increases of the number occupancy of IXs within the electrostatically defined trap. These traps are advantageous over other trapping methods involving strain or moiré traps, due to their deterministic placement in the lithographically defined process, 100 meV energy tunability by applied gate voltage, and scalability to create large arrays of single quantum emitters with controllable placement and spacing. Next, we explore slanted one-dimensional traps for excitons, defined by a triangular etch in the graphene gate to create a water slide-like potential, where IXs created at the top of the water slide will flow unidirectionally to the bottom. By performing spatially and temporally resolved photoluminescence measurements, smoothly varying IX energy along the structure and high speed exciton flow are measured. Furthermore, IX current can be controlled by saturating exciton populatio

In recent years, femtosecond and attosecond laser pulses have enabled scientists to probe ultrafast electron dynamics through various pump-probe techniques. One technique which has emerged in the past decade is extreme ultraviolet transient absorption spectroscopy (XTAS). In XTAS, extreme ultraviolet (XUV) pulses are produced through high-harmonic generation (HHG) of a femtosecond IR pulse. A portion of the same IR pulse is used to probe the sample with a variable time delay, modifying the absorption of the XUV spectrum through the sample. So far, XTAS experiments have been driven entirely by fixed-frequency IR pulses. Because of this, the odd harmonics which appear in the XUV spectrum cannot be shifted in energy, making the areas in between the harmonics inaccessible to experiments. Also, the fixed IR frequency means that one does not have control over IR-driven couplings between excited states. These couplings, which have profound effects on the XUV absorption spectrum, have never been studied as a function of the IR frequency in XTAS. In this work, I introduce new innovations to the XTAS technique. First, I send the IR probe pulse through an optical parametric amplifier (OPA) which allows me to tune the IR pulse energy without changing the XUV spectrum. This pulse is used to resonantly couple two excited states of Helium, leading to Autler-Townes splitting. By changing the detuning of the probe pulse, I was able to observe a smooth transition from resonant Autler-Townes splitting to transient light-induced states. I also observed spatially isolated four-wave mixing emissions which are not visible using the traditional XTAS pulse sequence. Second, I use high-order frequency mixing of two IR pulses to generate XUV pulses with tunable spectra. These tunable XUV pulses were applied to XTAS experiments, allowing me to probe XUV transitions in between the odd harmonics of a single IR pulse. Lastly, I used the tunable IR pulse to probe couplings between autoionizing sta

Correlated states can be existing in van der Waals materials in various ways, materialslike NbSe2 can hold both superconducting and charge density wave states intrinsically at low temperatures. Through doping or proximation effects, correlated states can be induced in normal materials like graphene. The advances in the van der Waals heterostructure fabrication techniques that realized high-accuracy rotation angle control between the van der Waals layers. When the two layers are twisted from each other at certain angles, the electronic bands will be flattened, and correlates states will appear when the bands are partially filled. This thesis will focus on the characterization of correlated states and flat bands in three different platforms. The first topic is to probe the proximitized superconductivity and charge density waves in graphene/NbSe2 heterostructure. At a temperature of 4.6 K, we have shown that both superconductivity and charge density waves can be induced in graphene from NbSe2 by proximity effects. By applying a vertical magnetic field, we imaged the Abrikosov vortex lattice and extracted the coherence length for the proximitized superconducting graphene. We further show that the induced correlated states can be completely blocked by adding a monolayer hBN between the graphene and the NbSe2, which demonstrates the importance of the tunnel barrier and surface conditions between the normal metal and superconductor for the proximity effect. The second topic is to characterize the flat bands and correlated states in twisted bilayer graphene at the “magic angle” of 1.1 degrees. This small twist angle leads to a long wavelength moiré unit cell on the order of 13 nm and the appearance of two flat bands. We have found that the wavefunctions at the charge neutrality point show reduced symmetry due to the emergence of a charge-ordered state. The wavefunctions of the lower band are delocalized when the top band is partially filled, meanwhile, the wavefunctions are

Spintronics, different from traditional electronics, explores additional routes to manipulate electrons through its spin degree of freedom. During its development, magnetic tunnel junctions (MTJs) have been playing a significant role. This is partially due to their fantastic properties such as symmetry-conserving tunneling, which leads to very large magnetoresistance (TMR) and sensitive detection of spin configuration. The relatively simple core structure of the ferromagnet/tunneling barrier/ferromagnet makes it possible to incorporate MTJs into the well-developed fabrication process and a large variety of material systems. Moreover, in recent years, the voltage-controlled effects discovered at FM/Oxide interfaces enables researchers to explore magneto-electric properties of MTJ and more functionalities have been proposed and verified especially in magnetic switching. In this work, we focus our attention on the voltage-controlled effects based on the conventional CoFeB/MgO magnetic tunnel junctions. After a brief introduction to the fundamentals of spin-dependent transport and perpendicular magnetic anisotropy (PMA) in the CoFeB/MgO MTJs. We divert our efforts to various voltage-controlled mechanism discovered in recent years. Chapter 2 briefly touches on the basic experimental technique used in this work. In Chapter 3, we will introduce a revised MTJ nanopillar fabrication procedure which dramatically lowers the requirement for fabricating high-quality MTJ and increases the yield significantly. Chapter 4 will involve interfacial engineering at CoFeB/MgO interface in MTJs. By dusting a thin layer of heavy metal (Ir), in addition to controlling the annealing condition, the VCMA coefficient can be significantly enhanced and the voltage induced switching energy can be greatly reduced, which demonstrated the promises to incorporate MTJs into next-generation memory applications. In Chapter 5, a novel voltage-controlled exchange bias (VCEB) effect is reported and its impl

Singlet fission (SF), in which a photoexcted spin singlet exciton dissociates into a pair of low lying triplet excitons has the potential to overcome the Shockley-Quessier limit of conventional solar cells. It is well understood that a key intermediate - the multiexcitonic, quantum entangled $^1$(TT)$_1$ state plays a vital role in making the process spin-allowed. Early experiments based on transient absorption spectroscopy (TA) conducted on molecular crystals and dimers of organic molecules reported highly efficient SF. However, with the emergence of optical probes in the infra-red (IR) regime, the ``bound'' triplet pair state has been shown to be stable against decoherence thereby raising questions on SF itself. In this thesis, within a correlated electron model, we report calculations of excited state absorptions (ESAs) from the singlet and triplet excitons and from the triplet-triplet biexciton for pentacene crystal and dimers. We formulate a broad theory of quantum entanglement of triplet-triplet state in these molecular systems and show that spectroscopic differences in the ESA signals between free and bound triplets exist beyond the visible. The triplet exciton primarily absorbs in the visible and any near IR (NIR) absorption depends on the strength of the intermolecular coupling. In contrast, the $^1$(TT)$_1$ state has additional weak ESA in the IR. In the dimers, we show that entanglement between triplets is a function of the proximity as well as the steric hindrance between the molecular chromophores. A weak signal from the singlet exciton is found in the IR and attributed to a transition to a different kind of triplet-triplet state. In summary, our theory of ESA can be extended to other weakly coupled bimolecular systems without any loss of generality.

Atomic and organic semiconductors interfaces with inorganic semiconductors are promising materials for use in next-generation electronic and optoelectronic devices. A predictive-level understanding of the interfaces is lacking which hinders rational design of such devices. This is attributed to the key processes, e.g. charge generation, separation, delocalization and extraction, which are heavily interfacial in nature and difficult to simulate. Thus, it is imperative to understand the fundamental processes at individual interfaces to enable design and optimization of next-generation technologies. This work aims to understand these interfaces by studying the interfacial processes using optical photoemission spectroscopy techniques. In this dissertation, I focus on alkali metal and molecular interfaces with layered materials to understand and control the dynamics and electronic and optical properties of the interface. Initially, I aim to characterize the nature of layered materials (SnS2 and MoS2) that exhibit unusual thickness-dependent properties, which already affords tunable materials properties. I demonstrate the anisotropy of the many-layered crystal at ultrafast timescales not yet realized. Pairing the layered material with thin films (K, CuPc and PTCDA) demonstrates layer decoupling and quantum confinement effects that show an enhancement of carrier delocalization at the interface. Furthermore, studies show a systematic tuning of the dielectric environment of the layered material dependent upon adsorbate thin film thickness. Overall, this work elucidates key interfacial interactions of select atoms and molecules on layered materials that can be used to understand this class of materials and their implementation in nanodevices.

The ability to isolate single atomic layers of van der Waals materials has led to renewed interest in the electronic and optical properties of these materials as they can be fundamentally different at the monolayer limit. Moreover, these 2D crystals can be assembled together layer by layer, with controllable sequence and orientation, to form artificial materials that exhibit new features that are not found in monolayers nor bulk. Twisted bilayer graphene is one such prototype system formed by two monolayer graphene layers placed on top of each other with a twist angle between their lattices, whose electronic band structure depends on the twist angle. This thesis presents the efforts to explore the electronic and optical properties of twisted bilayer graphene by Raman spectroscopy and scanning tunneling microscopy measurements. We first synthesize twisted bilayer graphene with various twist angles via chemical vapor deposition. Using a combination of scanning tunneling microscopy and Raman spectroscopy, the twist angles are determined. The strength of the Raman G peak is sensitive to the electronic band structure of twisted bilayer graphene and therefore we use this peak to monitor changes upon doping. Our results demonstrate the ability to modify the electronic and optical properties of twisted bilayer graphene with doping. We also fabricate twisted bilayer graphene by controllable stacking of two graphene monolayers with a dry transfer technique. For twist angles smaller than one degree, many body interactions play an important role. It requires eight electrons per moire unit cell to fill up each band instead of four electrons in the case of a larger twist angle. For twist angles smaller than 0.4 degree, a network of domain walls separating AB and BA stacking regions forms, which are predicted to host topologically protected helical states. Using scanning tunneling microscopy and spectroscopy, these states are confirmed to appear on the domain walls when inversion

Two dimensional (2D) materials are poised to revolutionize the future of optics and electronics. The past decade saw intense research centered around graphene. More recently, the tide has shifted to a bigger class of two-dimensional materials including graphene but more expansive in their capabilities. The so called ‘2D material zoo’ includes metals, semi-metals, semiconductors, superconductors and insulators. The possibility of mixing and matching 2D materials to fabricate heterostructures with desirable properties is very exciting. To make devices with superior electronic, optical and thermal properties, we need to understand how the electrons, phonons and other quasi particles interact with each other and exchange energy in the femtosecond and nanosecond timescales. To measure the timescales of energy distribution and dissipation, I used ultrafast pump-probe spectroscopy to perform time-domain measurements of optical absorption. This approach allows us to understand the impact of manybody interactions on the bandstructure and carrier dynamics of 2D materials. After a brief introduction to femtosecond laser spectroscopy, I will explore the transient absorption dynamics of three classes of 2D materials: intrinsic graphene, graphene-hBN heterostructures and Transition Metal Dichalcogenides (TMDs). We will see that using pumpprobe measurements around the high energy M-point of intrinsicgraphene, we can extract the value of the acoustic deformation potential which is vital in characterizing the electron-acoustic phonon interactions. In the next part of the thesis, I will delineate the role of the substrate in the cooling dynamics in graphene devices. We will see that excited carriers in graphene on hBN substrates cool much faster that on SiO2 substrates due to faster decay of the optical phonons in graphenehBN heterostructures. These results show that graphene-hBN heterostructures can solve the hot phonon bottleneck that plagues graphene devices at high power densitie