Si nanoparticles are new prospective optoelectronic materials. Unlike bulk Si cry-stals, Si nanoparticles display intriguing room-temperature photoluminescence. A major challenge in the fabrication of Si nanoparticles is the control of their size distribution. The rare-earth element Er has unique photo emission properties, including low pumping power, and a temperature independent, sharp spectrum. The emission wavelength matches the transmission window of optical fibers used in the telecommunications industry. Therefore, the study of Er-doped Si nanoparticles may have practical significance. The goals of the research described in this dissertation are to investigate vapor phase pyrolysis methods and to characterize the microstructure and associated defects, particles size distributions and photoluminescence efficiencies of doped and undoped Si nanoparticles using analytical transmission electron microscopy, high resolution electron microscopy, and optical spectroscopy. Er-doped and undoped Si nanoparticles were synthesized via vapor-phase pyrolysis of disilane at Texas Christian University. To achieve monodisperse size distributions, a process with fast nucleation and slow growth was employed. Disilane was diluted to 0.48% with helium. A horizontal pyrolysis oven was maintained at a temperature of 1000 °C. The oven length was varied from 1.5 cm to 6.0 cm to investigate the influence of oven length on the properties of the nanoparticles. The Si nanoparticles were collected in ethylene-glycol. The doped and undoped Si nanoparticles have a Si diamond cubic crystal structure. Neither Er precipitation, Er oxides or Er silicides were detected in any of the samples. The Er dopant concentration was about 2 atom% for doped samples from the 3.0 and 6.0 cm ovens as determined by quantitative analysis using X-ray energy dispersive spectroscopy. The average Si nanoparticle size increases from 11.3 to 15.2 nm in the doped samples and from 11.1 to 15.7 nm in the undoped samples as the oven length increases from 1.5 to 6.0 cm. HREM data show that average Si nanocrystallite size varies from 6.4 to 3.3 to 5.9 nm in the doped samples, and from 7.5 to 12.2 nm in the undoped samples as the oven length increases. Room-temperature Er photoluminescence has been detected near 1.54 :m from all doped samples. Saturation of the Er photoluminescence intensity at large emission power and the monotonic decrease of the intensity as a function of the emission wavelength in the doped sample from the 3.0 cm oven suggest that a carrier-mediated energy transfer process occurs in the Er-doped Si nanoparticles. It is the first time to successfully fabricate and investigate Er-doped Si nanoparticles.

Orientador: Newton Cesário Frateschi / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-09-02T20:57:13Z (GMT). No. of bitstreams: 1 Souza_MarioCesarMendesMachadoDe_D.pdf: 15167961 bytes, checksum: b18ee3c5c89a54254813d054e000f1d7 (MD5) Previous issue date: 2017 / Resumo: Após duas décadas de progresso contínuo, a fotônica integrada apresenta-se como uma tecnologia indispensável, exibindo soluções para importantes demandas tecnológicas atuais como o tráfego e processamento de sinais ópticos ultra-rápidos. Ao mesmo tempo, ela permite avanços substanciais em áreas emergentes como o "laboratório-no-chip" (lab-on-a-chip). No entanto, enquanto funcionalidades básicas necessárias para a maioria das aplicações (fontes de luz, moduladores, filtros, linhas de atraso, detectores, etc.) já estão disponíveis em uma variedade de dispositivos e plataformas, alguns desafios ainda permanecem. Nos últimos quatro anos, estivemos interessados em identificar alguns desses desafios e fornecer abordagens interessantes para enfrentá-los. Esta tese, que engloba uma parcela importante dessas investigações, pode ser dividida em dois tópicos. No primeiro, apresentamos microresonadores acoplados como dispositivos que permitem um controle espectral flexível e reconfigurável. Explorando as características desses dispositivos, demonstramos novas funcionalidades como o controle reconfigurável do "splitting" entre ressonâncias, fornecemos novas ferramentas de modelagem como uma teoria de modos acoplados modificada e propomos um modulador que emprega anéis acoplados, capaz de superar a limitação entre eficiência de modulação e largura de banda enfrentada por moduladores baseados em um único anel. No segundo tópico apresentamos o desenvolvimento de um espectrômetro a transformada de Fourier integrado em um chip, utilizando fotônica de silício. Os desafios para obter esse dispositivo, como a não-idealidade inerente à plataforma de silício (dispersão e não-linearidade termo-ótica) são discutidos em detalhe, além da demonstração experimental que indica como tal dispositivo pode abrir caminho para espectrômetros portáteis robustos e econômicos / Abstract: After two decades of continuous progress, integrated photonics has proven its indisputable role as an enabling technology. It addresses important technological demands of our time such as ultrafast optical data transfer and processing while allowing substantial progress in emerging areas, including lab-on-a-chip. Although the basic functionalities required for most applications (light sources, modulators, filters, delay lines, detectors, etc.) are now available in a variety of designs and platforms, a few challenges remain and room for improvement can still be found. During the last four years, we have been interested in identifying some of these challenges and in providing interesting approaches to tackle a handful. This thesis, encompassing an important share of such investigations, can be divided into two topics. First, we present coupled microresonators as devices allowing for flexible and reconfigurable spectral control. Exploiting these devices, we demonstrate novel functionalities like the reconfigurable resonance-splitting control, we provide novel modeling tools such as a modified coupled mode theory, and we propose a coupled-ring modulator that overcomes the trade-off between modulation efficiency and bandwidth faced by single microrings modulators. The second topic addresses the realization of an on-chip Fourier transform spectrometer using silicon photonics. We discuss the challenges of realizing such device due to non-idealities inherent to the silicon platform (dispersion and thermo-optic non-linearity) and we provide an experimental demonstration indicating how this device can pave the way for robust and cost-effective portable spectrometers / Doutorado / Física / Doutor em Ciências / 156281/2013-9 / 2014/04748-2, 2015/20525-6 / CNPQ / FAPESP

Kinetic inductance detectors (KIDs) are superconducting thin-film microresonators that are sensitive photon detectors. These detectors are a candidate for the next generation of experiments designed to measure the polarization of the cosmic microwave background (CMB). I discuss the basic theory needed to understand the response of a KID to light, focusing on the dynamics of the quasiparticle system. I derive an equation that describes the dynamics of the quasiparticle number, solve it in a simplified form not previously published, and show that it can describe the dynamic response of a detector. Magnetic flux vortices in a superconducting thin film can be a significant source of dissipation, and I demonstrate some techniques to prevent their formation. Based on the presented theory, I derive a corrected version of a widely-used equation for the quasiparticle recombination noise in a KID. I show that a KID consisting of a lumped-element resonator can be sensitive enough to be limited by photon noise, which is the fundamental limit for photometry, at a level of optical loading below levels in ground-based CMB experiments. Finally, I describe an ongoing project to develop multichroic KID pixels that are each sensitive to two linear polarization states in two spectral bands, intended for the next generation of CMB experiments. I show that a prototype 23-pixel array can detect millimeter-wave light, and present characterization measurements of the detectors.

Two dimensional (2D) materials have unique properties that make them exciting candidates for various optical and electronic applications. Materials such as graphene and transition metal dichalcogenides (TMDCs) have been intensively studied recently with researchers racing to show advances in 2D device performance while developing a better understanding of the material properties. Despite recent advances,there are still significant roadblocks facing the use of 2D materials for real-world applications. The ability to make reliable, low-resistance electrical contact to TMDCs such as molybdeum disulfide (MoS22) has been a challenge that many researchers have sought to overcome with novel solutions. The work laid out in this dissertation uses novel techniques for addressing these issues through the use of improved device fabrication and with a clean, and potentially scalable doping method to tune 2D material properties.A high-performance field-effect transistor (FET) was fabricated using a new device platform that combined graphene leads with dielectric encapsulation leading to the highest reported value for electron mobility in MoS2. Device fabrication techniques were also investigated and a new, commercially available lithography tool (NanoFrazor) was used to pattern contacts directly onto monolayer MoS2. Through a series of control experiments with conventional lithography, a clear improvement in contact resistance was observed with the use of the NanoFrazor. Plasma-doping, a dry and clean process, was investigated as an alternative to traditional wet-chemistry doping techniques. In addition to developing doping parameters with a chlorine plasma treatment of graphene, a series of experiments on doped graphene were conducted to study its effect on optical properties. Whereas previous studies used electrostatic gating to modify graphene’s optical properties, this work with plasma-doped graphene showed the ability to tune absorbence and plasmon wavelength without the need for an applied bias opening the door to the potential for low-power applications. This work is a just small contribution to the larger body of research in this field but hopefully represents a meaningful step towards a greater understanding of 2D materials and the realization of functional applications.

In this thesis, we investigate how the optical and electronic properties of graphene may be modified in proximity to various other materials. We present several examples of how modification in this way can help make graphene better suited for specific device applications. We develop a method of up-scaling the fabrication of FeCl3-intercalated few-layer graphene from micron-sized flakes to macroscopic films so that it may be used as a transparent electrode in flexible light-emitting devices. We also find that photo-responsive junctions can be arbitrarily written into FeCl3-intercalated few-layer graphene by means of optical lithography. These junctions produce photocurrent signals that are directly proportional to incident optical power over an extended range compared to other graphene photodetectors. Through theoretical analysis of these junctions, we conclude that the enhanced cooling of hot carriers with lattice phonons is responsible for this behaviour. Finally, we trial rubrene single crystals as the light-absorbing layer in a graphene phototransistor. We find that rubrene single crystal-graphene interfaces exhibit enhanced charge transfer efficiencies under illumination with extremely weak light signals. Through a comparative study with similar devices, we conclude that the wide variation in sensitivity amongst graphene phototransistors is largely due to extraneous factors relating to device geometry and measurement conditions.

A metal-oxide-silicon (MOS) tunneling diode is utilized to embed beta-FeSi 2 precipitates and give strong 1.5 tam electroluminescence at 80 K. And this simple MOS structure with beta-FeSi2 was fabricated by Fe ion implantation and rapid thermal oxidation (RTO) at 900°C, which is fully compatible with ultra-large scale integration (ULSI) processes. / beta-FeSi2 precipitates are also incorporated into a silicon-on-insulator (SOI) rib waveguide and a p+-i-n+ photodetector is monolithically integrated with this SOI rib waveguide. The photoresponse to 1550 nm laser of beta-FeSi2 precipitates was observed and compared to intrinsic silicon. / Beta-phase iron disilicide (beta-FeSi2) is a semiconductor that can act as a light emitting material at the wavelength of 1.55 mum and can also be grown epitaxially on Si substrates. In this thesis, Fe ion implantation into silicon using a metal vapor vacuum arc (MEVVA) ion source was performed to synthesize nano-scale beta-FeSi2 precipitates in silicon matrix. The implantation was performed at ∼-120°C and the effects of silicon substrate and conditions for the following thermal annealing on luminescence properties were studied. The samples were characterized by employing various analytical techniques including Rutherford backscattering spectrometry (RBS), transmission electron microscopy (TEM), atomic force microscope (AFM), photoluminescence (PL), and electroluminescence (EL). / It is found that the PL intensity is optimized in p-100 silicon substrates (with the resistivity of 15-25 O·cm) using Fe ion implantation at a voltage of 80 kV and dosage of 5x1015 cm -2. Formation of beta-FeSi2 can be completed after rapid thermal annealing (RTA) and strong photoluminescence is present. We also found that RTA could maintain the strain in beta-FeSi2 precipitates and there exists an epitaxial relationship between beta-FeSi2 and silicon. Additional furnace annealing at 850°C can relax the strain in beta-FeSi2 precipitates. / The development of both modern microelectronics and lightwave technologies has enabled the establishment of the Internet which has introduced a profound change in our everyday lives. Because of Moore's law, computing today is limited less by the computation ability of microprocessors than by the rate at which the processor can communicate with the outside world. Lightwave technology has had many successes in the long-haul communication field over the past decade. The advantages of lightwave technology over conventional electronics are becoming apparent for shorter and shorter reach applications and lightwave communications may eventually replace copper-based interconnects in microelectronics. To make possible optical interconnects, optical components, especially light emitters may be needed to be integrated on conventional silicon microchips. However, to date, no efficient on-chip silicon-based light emitter is fabricated in silicon photonics. / Sun, Caiming. / Adviser: Hon K. Tsang. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3703. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Finally in combination with c-AFM and EFM, the high-k materials breakdown behaviors are also interrogated. The breakdown processes are classified into three stages: pre-breakdown (pre-BD), soft breakdown (SBD) and hard breakdown (HBD). And a HfOx nano-pattern is fabricated with the aid of AFM. The dot growth characteristics on the pulse amplitude, duration and humidity are scrutinized. / In this thesis, the single CdS nanobelt devices are fabricated successfully. The photosensitivity at 1V is up to 8 x 103 A/W and the electron mobility reaches to tens of cm/V·s. Based on these excellent optoelectronic properties, the CdS nanobelt becomes a good choice for interrogation on the charge transport characteristics on a nanometer scale. The transistor measurements show that the performance of CdS nanobelt device can be influenced by illuminations and ambient conditions, which result from the metal/CdS nanobelt contact and nanobelt surface redox reactions. / The intrinsic carrier transport characteristics in CdS nanobelt can be investigated by reconstructing of the local surface band diagram with the aid of SSPM. A ∼0.50 eV upward band bending can be obtained in the dark. The surface depletion length induced by the negative surface oxygen adsorbates is estimated to ∼66nm if a concentration of 1017 cm -3 shallow donors is assumed in the CdS nanobelt. This depletion length is close to the height of the ultra-thin CdS nanobelt. These adsorbates result in the surface depletion region expansion and the conduction channel reduction, which is responsible for the CdS conductance drop. Above the band-gap illumination or to the oxygen-deficient environment can effectively reduce the surface band bending and the depletion region, finally increase the conduction channel, which is one of the main reasons for the large photosensitivity and highly oxygen sensitivity for the single CdS nanobelt device. / To sustain Moore's law scaling trend beyond COMS, one-dimensional (1D) nanostructures, e.g. carbon nanotubes and semiconductor nanowires, are proposed to act as fundamental nanoscale blocks in the future electronic and optoelectronic devices. Therefore it is very crucial to understand the unique nature of electronic properties for 1D nanostructures in designing novel nanoelectronic devices and optimizing the device performance. In this thesis, the charge transport properties of nanostructure devices are studied. A method called photo-assisted scanning surface potential microscopy (SSPM) is developed, which yields a direct measurement of the electrostatic potential distributions across the 'biased' nanostructured device under different illumination conditions. Our efforts provide significant understanding of the nature of charge transport in nanoelectronics. / We can simply fabricate the MSM device using single CdS nanobelt. A positive Schottky barrier is found at the electrode/CdS nanobelt junction because of the unequal work function or the Fermi level pinning by the surface states. The barrier height is estimated to be 0.38 eV by fitting the temperature dependent I-V curves. A big potential drop at the junction can be visualized by SSPM. The calculated contact resistance for the electron injection is much larger than that for the CdS nanobelt, which illustrates that the transport properties of CdS nanobelt device are dominated by the charge injection process. The change in contact resistance and nanobelt resistance under the above bandgap illumination are measured by photo-assisted SSPM. The experimental results show that in the dark, the charge transport for the CdS nanobelt device is dominated by electron injection, while under high light intensity, the charge transport is governed by the intrinsic nature of CdS nanobelt. / With the aid of SPM, the charge injection and carrier transport characteristics of the individual CdS nanobelt device are systematically interrogated and comprehensively demonstrated, which are useful for designing and fabricating the nanostructured electronic and optoelectronic devices. / An, Jin. / Adviser: Jiambin Xu. / Source: Dissertation Abstracts International, Volume: 70-06, Section: B, page: 3674. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [200-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstracts in English and Chinese. / School code: 1307.

Spin interactions play a key role in the function of molecular materials from naturally occurring biological complexes to synthetic materials for light-harvesting and light-emission. This thesis investigates the spin interactions of spin-1 triplet excitons formed by singlet fission. Singlet fission produces two triplet excitons from one light-induced singlet state and holds promise to enable solar energy generation beyond traditional efficiency limits. As the lifetime of triplet pairs depends sensitively on their spin degree of freedom, in this thesis we deploy spin-sensitive techniques to understand the interactions and evolution of triplet pairs. After introducing the relevant theoretical and experimental background underlying singlet fission and the role of spin, we describe the first observation of strongly exchange coupled, high-spin triplet-pair states ($S=2$) in a solid-state organic semiconductor and show that the singlet fission process allows for the formation of long-lived, strongly coupled spin-two states. We then describe the development and use of photoluminescence-detected avoided level-crossings in applied magnetic fields to quantify the strength of exchange coupling and identify specific optical signatures of exchange-coupled triplet pairs. Using high magnetic fields ($\leq\mbox{60 T}$) we isolate and measure the exchange coupling and optical signatures of multiple distinct triplet pairs in the same material. Finally, we probe the mechanisms of formation and decay of spin polarization from triplet pair states using pulsed spin resonance. The measured dynamics are consistent with polarization driven by fluctuations in exchange coupling between pairs and spin-orbit mediated decay of triplet excitons to the ground state. The combined measurements of the spin parameters and polarization dynamics of triplet pairs from ns to ms timescales provides a quantitative picture of the spin states generated by singlet fission.

Qi, Huijie = 用于脫氧核糖核酸分子精确识别的Förster共振能量转移与光电压效应相结合的DNA-量子点分子光电开关 / 齐慧杰. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2008. / Includes bibliographical references. / Abstracts in English and Chinese. / Qi, Huijie = Yong yu tuo yang he tang he suan fen zi jing que shi bie de Förster gong zhen neng liang zhuan yi yu guang dian ya xiao ying xiang jie he de DNA-liang zi dian fen zi guang dian kai guan / Qi Huijie. / Abstract --- p.i / Acknowledgements --- p.iv / Table of contents --- p.v / List of Figures --- p.ix / List of Tables --- p.xiii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Background --- p.1 / Chapter 1.1.1 --- Nanotechnology and nanomaterials --- p.1 / Chapter 1.1.2 --- "Semiconductor quantum dots: optical properties, preparation and applications" --- p.3 / Chapter 1.1.2.1 --- Preparation --- p.3 / Chapter 1.1.2.2 --- Applications --- p.5 / Chapter 1.1.3 --- Quantum dot-based DNA recognition --- p.10 / Chapter 1.1.3.1 --- Forster resonance energy transfer --- p.11 / Chapter 1.1.3.2 --- Bioimmobilization technique --- p.13 / Chapter 1.1.3.3 --- Highlights on Quantum dot-based DNA recognition --- p.14 / Chapter 1.2 --- Objective and methodology --- p.16 / Chapter 1.2.1 --- Objective --- p.16 / Chapter 1.2.2 --- General methodology --- p.17 / References --- p.19 / Chapter Chapter 2 --- Instrumentation --- p.23 / Chapter 2.1 --- Introduction --- p.23 / Chapter 2.2 --- Atomic Force Microscopy --- p.23 / Chapter 2.2.1 --- Principle of Atomic Force Microscopy --- p.24 / Chapter 2.2.2 --- Instrumentation --- p.29 / Chapter 2.3 --- Absorption and Fluorescence spectroscopy --- p.30 / Chapter 2.3.1 --- Basic Principle --- p.30 / Chapter 2.3.2 --- Instrumentation --- p.33 / Chapter 2.4 --- I-V characteristic --- p.36 / Chapter 2.4.1 --- Basic principle --- p.36 / Chapter 2.4.2 --- Instrumentation --- p.36 / Chapter 2.5 --- Other instrumentations --- p.37 / References --- p.38 / Chapter Chapter 3 --- Preparation of DNA/QD network systems --- p.39 / Chapter 3.1 --- Introduction --- p.39 / Chapter 3.2 --- Preparation of DNA/QD network conjugates in solution --- p.39 / Chapter 3.2.1 --- Experimental --- p.39 / Chapter 3.2.2 --- Characterization --- p.41 / Chapter 3.3 --- Preparation of DNA/QD network on substrates --- p.44 / Chapter 3.3.1 --- Experimental --- p.44 / Chapter 3.3.2 --- Characterization --- p.46 / Chapter 3.4 --- Summary --- p.49 / References --- p.50 / Chapter Chapter 4 --- Fluorescence and I-V characteristics for DNA/QD systems --- p.51 / Chapter 4.1 --- Introduction --- p.51 / Chapter 4.2 --- Experimental --- p.52 / Chapter 4.2.1 --- Patterned Au electrodes --- p.52 / Chapter 4.2.2 --- Electric field induced assembly --- p.52 / Chapter 4.3 --- Results and Discussion --- p.55 / Chapter 4.3.1 --- Optical studies of DNA/QD systems --- p.55 / Chapter 4.3.1.1 --- Optical characteristics of QDs used --- p.55 / Chapter 4.3.1.2 --- Optical studies of DNA/QD network systems in solution --- p.56 / Chapter 4.3.1.3 --- Optical studies of DNA cross-linked QD monolayer on substrates --- p.58 / Chapter 4.3.2 --- PV characteristics --- p.62 / Chapter 4.4 --- Summary --- p.64 / References --- p.66 / Chapter Chapter 5 --- DNA-quantum dot molecular opto-electronic switch with combined Forster resonance energy transfer and photovoltaic effect --- p.68 / Chapter 5.1 --- Introduction --- p.68 / Chapter 5.2 --- Experimental --- p.69 / Chapter 5.2.1 --- Preparation of DNA/QD molecular switch in solution --- p.69 / Chapter 5.2.2 --- Preparation of platform of molecular optoelectronic switch --- p.72 / Chapter 5.2.3 --- EFIA --- p.73 / Chapter 5.3 --- Results and Discussion --- p.73 / Chapter 5.3.1 --- Optical studies of molecular switch constructed of DNA/QD/Cy5 system --- p.73 / Chapter 5.3.1.1 --- Studies on optical properties of molecular switch as well as sole FRET effect between QD and Cy5in solution --- p.73 / Chapter 5.3.1.2 --- Optical studies of dsDNAs cross-linked QD monolayer on substrates with presence of Cy5 --- p.79 / Chapter 5.3.2 --- Studies on FRET mediated PV effect of DNA/QD/Cy5 molecular optoelectronic switch --- p.80 / Chapter 5.4 --- Summary --- p.82 / References --- p.83 / Chapter Chapter 6 --- Conclusions and Future Work --- p.84 / Chapter 6.1 --- Conclusions --- p.84 / Chapter 6.2 --- Future work --- p.85

Lead halide perovskites (LHPs) have forcefully emerged as a promising materials class for next-generation solar cells. The high efficiencies of LHP-based photovoltaics are underpinned by their outstanding optoelectronic properties, including long carrier lifetimes, long carrier diffusion lengths, high radiative efficiencies, and long-lived hot carriers. In conventional semiconductors, high efficiencies are achieved by stringent control over defect densities; higher purity diminishes the number of carrier scattering events and yields better optoelectronic properties. Given the high defect densities of LHPs, these observed behaviors indicate that LHPs are defect-tolerant and disobey this paradigm via dynamic screening of charge carriers. In order to expand the library of defect-tolerant semiconductors, we must elucidate the carrier-lattice interactions that lead to dynamic screening. LHP lattices are highly anharmonic and dynamically disordered, which must play a role in this screening mechanism. This anharmonicity demands a departure from the conventional Fröhlich interaction, which considers the harmonic coupling of a carrier to one phonon, to a picture that incorporates anharmonic phonon-phonon couplings. The objective of this thesis is to investigate the ultrafast anharmonic lattice response associated with dynamic screening of charge carriers. We probe the formation of large polarons in CH3NH3PbBr3 and CsPbBr3 using time-resolved optical Kerr effect spectroscopy. We further investigate the coupling of phonon modes in a model system, CsPbBr3, in the presence of charge carriers using ultrafast coherent phonon spectroscopy.