This PDF file contains the front matter associated with SPIE Proceedings Volume 10916, including the Title Page, Copyright information, Table of Contents, Author and Conference Committee lists.

Traditionally, strong-field physics explores phenomena in laser-driven matter (atoms, molecules, and solids) that cannot be understood by treating the laser field as a small perturbation. Therefore, the presence of an extremely strong external field is usually a prerequisite for observing strong-field phenomena. However, even in the complete absence of an external electromagnetic field, strong-field phenomena can arise when matter strongly couples with the zero-point field of the quantum vacuum state, i.e., fluctuating electromagnetic waves whose expectation value is zero. This can occur in free space where the matter strongly interacts with a continuum of photon modes, but some of the most striking examples of strong-field physics without an external field occur in a cavity setting, in which an ensemble of two-level atoms resonantly interacts with a single photonic mode of vacuum fields, producing vacuum Rabi splitting. In particular, the nature of the matter-vacuum-field coupled system fundamentally changes when the coupling rate (equal to one half of the vacuum Rabi splitting) becomes comparable to, or larger than, the resonance frequency. In this so-called ultrastrong coupling regime, a non-negligible number of photons exist in the ground state of the coupled system. Furthermore, the coupling rate can be cooperatively enhanced (via so-called Dicke cooperativity) when the matter is comprised of a large number of identical two-level particles, and a quantum phase transition is predicted to occur as the coupling rate reaches a critical value. Low-energy electronic or magnetic transitions in many-body condensed matter systems with large dipole moments are ideally suited for searching for these predicted phenomena. Here, we discuss two condensed matter systems that have shown cooperative ultrastrong interactions in the terahertz frequency range: a Landau-quantized two-dimensional electron gas interacting with high-quality-factor cavity photons, and an Er³⁺ spin ensemble interacting with Fe³⁺ magnons in ErFeO₃.

As conventional electronics approaches its ultimate limits, novel concepts of fast quantum control have been sought after. Lightwave electronics – the foundation of attosecond science – has opened a new arena by utilizing the oscillating carrier wave of intense light pulses to control electrons faster than a cycle of light. We employ atomically strong terahertz electromagnetic pulses to accelerate electrons through the entire Brillouin zone of solids, drive quasiparticle collisions, and generate high-harmonic radiation as well as high-order sidebands. The unique band structures of topological insulators allow for all-ballistic and quasi-relativistic acceleration of Dirac quasiparticles over distances as large as 0.5 μm. In monolayers of transition metal dichalcogenides, we switch the electrons’ valley pseudospin, opening the door to subcycle valleytronics. Finally, we show that lightwave electronics can be combined with ultimate atomic spatial resolution in state-selective ultrafast scanning tunneling microscopy.

When a light beam traveling in one direction in a crystal experiences a different absorption and refractive index compared to the beam traveling in the opposite direction, it is called nonreciprocal directional anisotropy, or simply nonreciprocity. This phenomenon is governed by the fundamental symmetries of crystals under spatial inversion and time reversal symmetries. We will discuss the necessary symmetry conditions for the nonreciprocity of light propagation and of other excitations in solids. Among specific examples, we will consider light propagation, in polar magnetic materials along and opposite the toroidal vector. In this case, a crystal can be completely transparent in one direction and completely opaque in the opposite one – an optical diode. We report a giant optical diode effect in the polar material FeZnMo3O8, where we find more than a 100-fold difference in intensity of light transmitted in the two opposite directions. In addition to the high magnitude of the effect, we show that the effect exists at high temperature in the magnetically disordered state. We will also present a study of the nonreciprocal reflectance of magneto-plasma in semiconductor InSb. This material can be used for the construction of high-performance terahertz isolation devices, as no dominant technology has emerged yet for this application. Room-temperature operation, moderate applied magnetic field, and an unmatched simplicity of design make this material a good candidate for practical terahertz isolators.

Interest in atomically-thin transition metal dichalcogenide (TMD) semiconductors such as MoS2 and WSe2 has exploded in the last few years, driven by the new physics of coupled spin/valley degrees of freedom and their potential for new spintronic and ‘valleytronic’ devices. Although robust spin and valley degrees of freedom have been inferred from polarized photoluminescence (PL) studies of excitons, PL timescales are necessarily constrained by short-lived (1–30 ps) recombination timescales of excitons. Direct probes of spin and valley dynamics of the resident electrons and holes in n-type or p-type doped TMD monolayers, which may persist long after recombination ceases, are still at a relatively early stage. In this work, we directly measure the coupled spin-valley dynamics of resident electrons and resident holes in n-type and p-type monolayer TMD semiconductors using time-resolved Kerr rotation. Very long relaxation timescales in the nanosecond to microsecond range are observed at low temperatures – orders of magnitude longer than typical exciton lifetimes. In contrast with III-V or II-VI semiconductors, electron spin relaxation in monolayer MoS2 is found to accelerate rapidly in small transverse magnetic fields. This indicates a novel mechanism of electron spin dephasing in monolayer TMDs that is driven by rapidly-fluctuating internal spin-orbit fields that, in turn, are due to fast electron scattering between the K and K’ conduction bands [1]. More recent studies of gated TMD monolayers also allow observation of very long spin/valley relaxation of resident holes, a consequence of spin-valley locking [2]. These studies provide direct insight into the physics underpinning the spin and valley dynamics of resident electrons and holes in 2D TMD semiconductors. [1] L. Yang et al., Nature Physics 11, 830 (2015). [2] P. Dey et al., Phys. Rev. Lett. 119, 137401 (2017).

The strong Coulomb interaction in two-dimensional transition metal dichlacogenides gives rise to tightly bound excitons, which dominate their optical properties. Because of their complex quasi-particle band structure, TMDs possess a variety of optically bright and dark excitonic states. Here, we present a theoretical framework to efficiently describe excitonic intervalley physics. Within this model, we first calculate the coherence lifetime and optical line shape under influence of phonons. Secondly, we present the time-resolved formation and thermalization of bright and dark intra- and intervalley excitons. We find that momentum-forbidden dark excitonic states play a crucial role in tungsten-based TMDs.

Chirality characterizes an object that is not identical to its mirror image. In condensed matter physics, Fermions have been demonstrated to obtain chirality through structural and time-reversal symmetry breaking. These systems display unconventional electronic transport phenomena such as the quantum Hall effect and Weyl semimetals. However, for bosonic collective excitations in atomic lattices, chirality was only theoretically predicted and has never been observed. We experimentally show that phonons can exhibit intrinsic chirality in monolayer tungsten diselenide, whose lattice breaks the inversion symmetry and enables inequivalent electronic K and -K valley states. The time-reversal symmetry is also broken when we selectively excite the valley polarized holes by circularly polarized light. Brillouin-zone-boundary phonons are then optically created by the indirect infrared absorption through the hole-phonon interactions. The unidirectional intervalley transfer of holes ensures that only the phonon modes in one valley are excited. We found that such photons are chiral through the transient infrared circular dichroism, which proves the valley phonons responsible to the indirect absorption has non-zero pseudo-angular momentum. From the spectrum we further deduce the energy transferred to the phonons that agrees with both the first principle calculation and the double-resonance Raman spectroscopy. The chiral phonons have significant implications for electron-phonon coupling in solids, lattice-driven topological states, and energy efficient information processing.

Color centers in diamond are point defects within the diamond host lattice that absorb and emit light at optical frequencies. Besides contributing to the striking visual characteristics of "fancy colored" diamonds as gemstones, the centers--particularly the negatively charged nitrogen-vacancy (NV) center and silicon-vacancy (SiV-) center--offer a number of possibilities for quantum computation and quantum information processing. In this talk, I will summarize recent progress made in characterizing negatively charged silicon-vacancy centers in diamond using the technique of optical multidimensional coherent spectroscopy (MDCS). By comparing photoluminescence-based and heterodyne-detection based signal collection schemes in a high-density SiV- center sample, we have selectively identified a population of long-lived and nonradiative silicon-vacancy centers in diamond with more than 40 times as much inhomogeneous spectral broadening as the radiative silicon-vacancy center states that are more commonly observed using photoluminescence. Estimates of the degree of inhomogeneity and overall sample characteristics indicate that strain is likely to play a large role in the formation of these nonradiative states. The findings open possibilities for being able to actively tune the degree of radiative coupling in silicon-vacancy center systems, opening possibilities for new types of quantum-optical devices.

In quantum information technology, it is necessary to develop a light-matter quantum interface that transfers and stores quantum information. As a bandwidth of quantum entangled photon pairs used for quantum information increases, a quantum interface with broad bandwidth will be required. The combination of quantum dot (QD) ensemble and photon echo (PE) method is one of promising methods for broadband quantum interface. Since the bandwidth of the quantum interface using this method is limited only by the inhomogeneous width of the QDs, it is possible to implement a quantum interface with the bandwidth of 10 THz at telecommunication wavelength. However, in the PE method, the spatial inhomogeneousity of the laser intensity and the inhomogeneousity of the resonance frequency of the QDs result in the uniform quantum control of excitons in QDs. As a result, the regeneration efficiency of the PE light is significantly deteriorated. To solve this problem, it is effective to introduce a quantum control method using chirped pulses (Adaptive Rapid Passages; ARPs) which is robust to inhomogeneousities. In this study, we demonstrate that the regeneration efficiency of PE in inhomogeneous QDs can be improved by ARPs using femtosecond pulses. By performing numerical simulation and optical experiments, it was found that the regeneration efficiency improves as the chirp amount and the pulse area increase, and saturates at a certain condition.

Due to their excellent optical properties, quantum dots are promising for applications in photonic quantum technologies. For on-demand single-photon generation, a two-level system given by an excitonic transition is typically excited with a resonant laser pulse of area π. This prepares the two-level system in its excited state from where it spontaneously emits a single photon. However, emission that occurs already during the presence of the laser pulse allows for re-excitation and, thus, multi-photon emission which limits the single-photon purity [1]. In contrast, when exciting the system with a pulse of area 2π, the system is expected to be returned to the ground state. However, in this case emission during the presence of the pulse is most likely to occur when the system is in its excited state – exactly after an area of π has been absorbed. This restarts the Rabi oscillation with a pulse area of π remaining in the pulse which leads to re-excitation with near-unity probability and the emission of a second photon within the excited state lifetime [2,3]. Finally, we present the generation of single photons with ultra-low multi-photon probability [4]. Using two-photon excitation of the bi-exciton suppresses re-excitation and improves the single photon purity by several orders of magnitude for short pulses. [1] K. A. Fischer, et al., New J. Phys. 18, 113053 (2016) [2] K. A. Fischer, et al., Nature Physics 13, 649-654 (2017) [3] K. A. Fischer, et al., Quantum Sci. Technol. 3, 014006 (2017) [4] L. Hanschke et al., arxiv: 1801.01672 (2018)

Micro-resonators with small mode volume and high quality factor are widely used to enhance the interaction between light and matter. On the other hand, simple semiconductor thin films with high crystallinity, which provide long coherent length of the wave functions of center-of-mass motion of excitons, bring about size dependent enhancement of the interaction caused by nonlocal wave-wave coupling between light and exciton beyond long wavelength approximation. Actually, we found peculiar optical responses, e.g. large energy shift and sub-picosecond radiative decay of the weakly confined excitons in CuCl thin films with thickness of hundreds of nm [Phys. Rev. Lett. 103, 257401 (2009)]. Recently, thickness dependence of radiative decay rate was clearly observed in a single crystal Cu2O film with continuously varying thickness (16-1000 nm) [Phys. Rev. B 97, 205305 (2018)]. In the case of ZnO, where A and B excitons closely located, the radiation-induced interaction between the two excitons, which further enhances the energy shift and radiative decay rate, is theoretically predicted [Phys. Rev. B 94, 045441 (2016)]. Therefore, we investigated ZnO thin films by transient grating spectroscopy. The samples were fabricated by pulsed laser deposition on Al2O3 substrates. We confirmed that the measured spectra reflect the shapes of the calculated spectra based on the theory. Moreover, we found the temporal profile of the signal shows ultrafast decay faster than 100 fs at 5 K in the film with a thickness of 289 nm. The results show that strong spatial interaction between light and excitons is realized in multicomponent excitonic systems.

The dynamic increase in terahertz photoconductivity resulting from energetic intraband relaxation was used to track the formation of highly mobile charges in thin films of the tin iodide perovskite Cs_1-xRb_xSnI₃ and compared to the lead based Cs_0:05(FA_0:83MA_0:17)_0:95Pb(I_0:83Br_0:17)₃. Energy relaxation times were found to be around 500 fs, comparable to those in GaAs and longer than the ones of the lead-based perovskite (around 300 fs). At low excess energies the efficient intraband relaxation can be understood within the context of the Frohlich electron-phonon interaction. For higher excess energies the photoconductivity rise time lengthens in accordance with carrier injection higher in the bands, or into multiple bands. The findings contribute to the development of design rules for photovoltaic devices capable of extracting hot carriers from perovskite semiconductors.

The nonlinear optical response of direct-gap semiconductors is investigated with a focus on non-degenerate multiphoton absorption processes. The theoretical approach is based on the semiconductor Bloch equations and yields the absorption rate either perturbatively or non-perturbatively in the incident light intensities. We describe the semiconductor by a two-band model and consider a pump-probe scheme where the weak probe pulse provides one of the simultaneously absorbed photons. The perturbative response can be described analytically within some approximations and we give simple expressions for two, three, and four-photon absorption coefficients. These are compared with numerical results for the absorption of pulses with a finite duration, where the influence of dephasing and relaxation as well as higher-order corrections are also investigated. For strong pump fields that are treated non-perturbatively we demonstrate non-trivial dependencies of the absorption on the time delay between the pulses. In the non-perturbative response of a single light pulse characteristic modulations appear in the absorption dependence on the field strength that may be interpreted as multi-photon Rabi oscillations. Finally, we present measurements of the non-degenerate two-photon absorption coefficient of bulk GaAs via time-delay and polarization-dependent transmissivity changes in a pump probe setup. The observed strong increase of the absorption coefficient with frequency ratios deviating from unity qualitatively agrees with theoretical expectations.

Ultrafast transmission electron microscopy (UTEM) is a promising experimental approach for the investigation of ultrafast dynamics on nanometer length scales [1]. In UTEM, optically triggered dynamics are imaged by femtosecond electron pulses, utilizing the versatile imaging and diffraction capabilities of state-of-the-art transmission electron microscopy. In the Göttingen UTEM project, we developed nanoscale laser-driven photocathodes, delivering ultrashort high-coherence electron pulses. With this approach, we achieve, at the sample position, electron focal spot sizes down to below one nanometer and pulse durations of about 200 fs [2], which now enables the detailed real-space investigation of fast processes in nanostructured systems. In this contribution, first applications of the Göttingen UTEM instrument are presented, including the coherent phase modulation of electron pulses in optical near-fields [3] and its use for the formation of attosecond electron pulse trains [4], the nanoscale mapping of optically induced ultrafast structural dynamics [5], and our current progress towards ultrafast magnetic imaging using phase sensitive imaging [6]. [1] A. H. Zewail, Science 328, 187–93 (2010). [2] A. Feist et al., Ultramicroscopy 176, 63-73 (2017). [3] A. Feist et al., Nature 521, 200–203 (2015). [4] K. E. Priebe et al., Nature Photonics 11, 793 (2017). [5] A. Feist et al., Structural Dynamics 5, 014302 (2018). [6] N. Rubiano da Silva et al., Phys. Rev X, accepted (2018).

We numerically compute the effective diffraction index and attenuation of coplanar stripline circuits with microscale lateral dimensions on various substrates including sapphire, GaN, silica glass, and diamond grown by chemical vapor deposition. We show how to include dielectric, radiative and ohmic losses to describe the pulse propagation in the striplines to allow femtosecond on-chip electronics with frequency components up to 10 THz.

Absorption of energetic photons in a semiconductor leads to hot electrons and holes that usually cool to the band edge by thermal relaxation. In nanomaterials this cooling can be intercepted by excitation of additional electrons across the band gap. In this way, one photon generates multiple electron-hole pairs via Carrier Multiplication (CM), which is of interest for highly efficient photovoltaics. We studied cooling and CM in: a) films of PbSe nanocrystals coupled by organic ligands, and b) 2D superlattices of directly coupled PbSe nanocrystals. The studies were performed using pump-probe spectroscopy with optical or terahertz/microwave conductivity detection. Reducing the size of ligands between nanocrystals strongly increases the charge mobility. Removal of ligands and direct coupling of nanocrystals in a square or honeycomb superlattice allowed us to further tune the mobility and to realize values as high as 150 cm^2/Vs. We found that a high mobility is essential for multiple electron-hole pairs formed via CM to escape from recombination. The coupling between the nanocrystals was found to strongly affect the competition between cooling of hot charges by phonon emission and CM. In square 2D superlattices of nanocrystals CM is much more efficient than in films with ligands between the nanocrystals. In square superlattices CM occurs in a step-like fashion with threshold near the minimum energy of twice the band gap. The factors governing the charge mobility and the efficiency of CM, as well as the impact on the efficiency of photovoltaic devices, will be discussed on the basis of theoretical modeling.

With strongly bound and stable excitons at room temperature, single-layer, two-dimensional organic-inorganic hybrid perovskites are viable semiconductors for light-emitting quantum optoelectronics applications. In such a technological context, it is imperative to comprehensively explore all the factors—chemical, electronic, and structural—that govern strong multiexciton correlations. Here, by means of two-dimensional coherent spectroscopy, we examine excitonic many-body effects in pure, single-layer (PEA)2PbI4 (PEA= phenylethylam- monium). Using coherent two-dimensional excitation spectroscopy, we determine the binding energy of biexcitons—correlated two-electron, two-hole quasiparticles—to be 44 ± 5meV at room temperature. The extraordinarily high values are similar to those reported in other strongly excitonic two-dimensional materials such as transition-metal dichalcogenides. Importantly, we show that this binding energy increases by ∼25% upon cooling to 5 K. Our work highlights the importance of multiexciton correlations in this class of technologically promising, solution-processablematerials, in spite ofthe strong effects of lattice fluctuations and dynamic disorder. We demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic countercation. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensionalWannier formalism. We include four distinct excitonic states separated by 35 ± 5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. We demonstrate, using high-resolution impulsive stimulated Raman spectroscopy, that the coupling of distinct excitonic resonanes to the lattice is unique, with some excitonic transition displaying lattice reorganization akin to photocarriers. Our work highlights the complexity of excitonic properties in this class of nanostructured materials.

We present measurements of nonlinear refraction and absorption in transparent conductive oxides (TCO), which are essentially highly-doped semiconductors, at Epsilon-Near-Zero (ENZ). In this spectral region, where the real part of the permittivity crosses zero, materials demonstrate interesting nonlinear properties such as enhanced harmonic generation, nonlinear absorption (NLA), and nonlinear refraction (NLR). We find that induced changes in the refractive index of Indium Tin Oxide, can be very large with respect to the initial index. This means that the even the Fresnel coefficients are highly irradiance-dependent. Therefore, the nonlinear transmission, reflection, and absorption of the material will be significantly different from conventional materials which means that it is challenging to use conventional methods such as Z-Scan and time-resolved transmission and reflection techniques to accurately determine the underlying nonlinear optical coefficients. We have studied optical nonlinearities of TCOs using the Beam-Deflection (BD) method to independently characterize the temporal dynamics and polarization dependence of the NLR. This method enables us to resolve NLR in the presence of large NLA backgrounds. In addition, we can also study the dependence on relative polarization and incidence angle of excitation and probe waves to characterize the enhancement mechanism and the physical origin of the nonlinear response. We conduct these BD measurements in conjunction with Z-Scan and transient reflection and transmission at different wavelengths, incidence angle, and polarization. Our measurements reveal that there is a strong wavelength dependence of nonlinearities around the ENZ point. We find that the wavelength dependence is quite different for excitation and probe waves.

We present the first observation of nonlinear optical response mediated by ultrafast magneto-electric (ME) rectification. The control of magnetic properties of materials by ultrafast optical field enable novel sensing technology, energy conversion, terahertz emission, and ultrafast data storage. However, the interaction of the magnetic field of light with materials is normally ignored due to low magnetic susceptibilities at high frequencies. Optical nonlinearities driven jointly by electric and magnetic field components of light provide a new route in controlling magnetic properties of bulk media. Several novel physical phenomena arise from curved motion of bound electrons driven jointly by electric and magnetic fields such as longitudinally polarized second harmonic radiation, induced transverse magnetization at the optical frequency, and charge separation along the propagation direction. We investigate an ME charge separation in pentacene semiconductors using a time-resolved second harmonic generation technique. A femtosecond laser beam acted as an optical pump with photon energy well below the bandgap of the material to induce ME charge separation. The DC electric field from the ME charge separation interacted with the optical field from a second laser beam, the probe, in a four-wave-mixing interaction that induced second harmonic (M-EFISH) generation. We also sought evidence of ME charge separation by searching for THz emission. By monitoring time-resolved M-EFISH and THz emission, we were able to study the ME charge separation dynamics for the first time.

The generation of visible light from an Au-LiNbO₃-Au plasmonic waveguide and a CdSiP2 photonic waveguide is investigated for on-chip applications. Frequency-conversion is achieved through the second-order nonlinear process of second harmonic generation. Broadband electric field pulses having an amplitude of 20 kV/cm are produced by the AuLiNbO₃-Au plasmonic waveguide at a conversion efficiency of 11×10^-5 . Electric field pulses of 30 kV/cm are observed from the CdSiP₂ photonic waveguide, where these fields are generated at the conversion efficiency of 80×10^-5 .

Semiconductors have long been known to exhibit large two photon absorption (2PA) coefficients. It has also been shown that for extremely nondegenerate pairs of photons (energy ratios of approximately 10:1) the 2PA in direct gap semiconductors (e.g. GaAs, CdTe, ZnSe, ZnO, GaN) is enhanced over the degenerate value by up to 3 orders of magnitude. Silicon has always been a material of interest for photonics applications due to its low cost and possibility for integration with electronics, but its indirect bandgap provides a challenge since 2PA requires a phonon scattering process. Despite this, the degenerate 2PA in Silicon has been shown to be comparable to that of direct gap semiconductors with similar band gap energy. We present a model for nondegenerate 2PA (ND-2PA) in indirect semiconductors that is nearly identical to the direct gap theory with a phonon transition added as a perturbation step. We also experimentally investigate the enhancement of ND-2PA in Silicon for the extremely nondegenerate case using a femtosecond pump-probe arrangement. In these experiments, the transmittance of a tunable near infrared probe pulse is monitored in the presence of a mid-IR pump pulse generated by difference frequency generation while varying the relative delay between pump and probe pulses. Measurement results reveal that the 2PA enhancements due to nondegeneracy are similar to those seen in direct-gap semiconductors. As with direct gap semiconductors, these enhancements indicate the applicability of using nondegenerate 2PA as possible method for ultrafast, gated mid-IR detection using off-the-shelf Silicon photodiodes.

The implementation of solid-state spin qubits for quantum information processing requires a detailed understanding of the decoherence mechanisms. At low magnetic fields, when the magnitude of the Zeeman energy becomes comparable to intrinsic couplings, electron spins confined to quantum dots (QDs) have been shown to feature a characteristic decoherence signature with several distinct stages: The electron spin undergoes fast ensemble dephasing due to the coherent precession of spin qubits around nearly static but randomly distributed hyperfine fields (∼2ns). At intermediate timescales (∼750ns) we identify an additional stage which corresponds to the effect of coherent dephasing processes that occur in the nuclear spin bath itself induced by quadrupolar coupling of nuclear spins to strain-driven electric field gradients. Finally, a slower process (>1μs) of irreversible relaxation of the spin polarization due to nuclear spin co-flips with the central spin causes the complete loss of coherence [1]. For hole spins, the mainly p-type Bloch function reduces the contact term of the hyperfine interaction due to the reduced wave function overlap with the nuclei. Consequently, at low magnetic fields we observe a more than two orders of magnitude slower dephasing compared to electrons. Time domain measurements of T2* show faster dephasing rates with increasing external magnetic field. We attribute this to electrical noise, which broadens the distribution of Zeeman frequencies. Strategies to counteract this noise source and measurements of T2 (via spin-echo) are discussed [2]. [1] A. Bechtold et al., Nature Physics 11, 1005–1008 (2015) [2] T. Simmet et al., in preparation

We determine the propagation loss of GaAs photonic crystal waveguides by spectral imaging of the spontaneous emission from the embedded InAs/GaAs quantum dots. The results are compared with the loss obtained by imaging the near field of the out-of-plane radiation of the waveguide mode propagating within the light cone. From the corresponding far field, we furthermore measure the mode wavevector, from which we determine the waveguide dispersion. Additionally, we show that spectral imaging allows to determine the relative efficiencies of the couplers. Using the same experiment, and detailed photonic simulations, we have determined the beta factor and the directionality of the emission of the QDs, finding beta factors up to 99% and high directionalities.

Terahertz (THz) imaging has progressed tremendously due the continuous development of new THz emitters and detectors. However, highly integrated array devices are desired for fast THz imaging. Advanced features such as beam steering and phase contrast imaging may be realized using more complex systems that require tight integration. Silicon photonics is an enabler for CW THz applications such as imaging and high-speed communication because of low cost and high level of integration. We present results of our research on continuous-wave THz generation using antennacoupled silicon-germanium photodiodes. THz emission up to 2.2 THz has been demonstrated.

Irradiating metallic nanostructures by an ultrafast laser beam produces highly localised processes on the nanoscale in the surrounding medium. This particular process is mainly attributed to the surface plasmon resonance of the nanostructures. When these nanomaterials are colloidal nanoparticles, their irradiation by an ultrafast laser could results in a highly localized plasma, heat production, pressure wave in the liquid and finally nanocavitation around the nanoparticles (1-5) . In this invited talk, I will cover recent developments of fundamentals aspects describing multiscale processes on the nanoscale as well as on ultrashort time (femtosecond to nanosecond). Multiphysics models based on electromagnetism, two-temperature model, thermodynamics and fluid dynamics have been developed to describe theoretically the productions of nanoplasma, pressure waves and nanocavitation. Models were successfully compared with experiments including shadowgraphy showing nanobubble dynamics and an all-optical pump-probe technique enabling the detection of plasmonic enhanced nanocavitation and pressure wave generation. Applications are in nanomedicine (6-7) in which ultrafast laser irradiation of these nanoparticles located close to a living cell, can produce a nanosurgery, can stimulate neurons or induce drug delivery. (1) E. Boulais et al, Nanoletters, 12, 4763-4769 (2012); (2) E. Boulais et al, Journal of Photochemistry and Photobiology C, 17, 26-49 (2013); (3) R. Lachaine et al ACS Photonics, 1, 331-336 (2014); (4) A. Dagallier, et al Nanoscale 9 (9), 3023-3032, (2017); (5) V. T. De Lille et al, to be submitted (2018); (6) E. Bergeron et al, Nanoscale, 7, 17836-17847 (2015); (7) F. Lavoie-Cardinal et al Scientific Reports, 6, 20619 (2016).

Organic semiconductors in different shapes and composition can be interfaced with living cells. This provides a new, exciting route towards optical control of physiological functions or the restoring of natural functions. In this talk I will present a number of experiments that show the effective abiotic-biotic coupling of organic semiconductors with cells and small animals, suggesting the potential of organic light actuators for geneless opto stimulation. Investigated systems are all based on polythiophene as photoactive layer, in planar films, nanostructured layers or nanoparticles. Spectroscopy, photo-electrochemistry and photo-electrophysiology are exploited to carry out the experimental investigations. While the mechanism explaining such coupling is still unknown, it is appering that thermal, capacitive, faradaic or chemical coupling are all options to be carefully evaluated. To conclude the succesful use of an organic retina implant for restoring visual acuity in blind animals will be reported.

Since the invention of femtosecond pulsed lasers, the field of ultrafast optical science and technology has seen significant progress in the generation and characterization of ultrashort optical pulses. Complimentary to development in generation and characterization techniques, arbitrary temporal shaping of optical pulses has become an integral part of the field. Fourier-transform pulse shaping is the most widely adopted approach that entails parallel modulation of spatially separated frequency components to achieve the desired pulse shape. Recently, dielectric metasurfaces have emerged as a powerful technology for arbitrary control over the amplitude, phase, or polarization of light in a single, compact optical element. Here, we experimentally demonstrate shaping of sub-10 fsec ultrafast optical pulses using a centimeter-scale silicon metasurface acting as both amplitude and phase modulation mask. The deep-subwavelength silicon nanostructures, positioned with nanometer precision, are individual optimized to provide accurate amplitude and phase modulations to each frequency component. Masks of this type offer a lower cost, larger size, higher resolution, high diffraction efficiency, high damage threshold method for controlling ultrafast pulses. The high precision with which metasurfaces can control polarization, amplitude, and phase point toward new, previously unrealizable applications in optical pulse shaping.

We report the giant modulation of PL from cesium lead halid personalities by atmosphere. The crystal tends to exhibit Br deficiency defects which can be passivated by surrounding atmosphere. PL measurement,atmospheric XPS analysis and theoretical simulation offer the evidence.

Acoustic phonons in the GHz range have emerged as a suitable platform to study complex wave physics phenomena. This has spurred the development of a large bandwidth of versatile nanophononic devices for full control and manipulation of phonons on the few-nm length scale. Furthermore, the strong interactions between acoustic phonons and other excitations in solids extend the range of applications for nanophononic devices into other areas of research such as electronics and optomechanics. For example, recent advances in material science and fabrication techniques enabled the fabrication of nanometric devices in which photons (VIS-NIR) and phonons (GHz-THz frequencies) are simultaneously confined in a single resonant cavity giving rise to unprecedented large optomechanical coupling factors. In addition, the engineering of acoustic waves with GHz-THz frequencies is also at the base of the study of mechanical quantum phenomena and non-classical states of mechanical motion. In this work I will first introduce and compare strategies to generate, manipulate and detect ultra-high frequency acoustic phonons using ultrashort laser pulses and high resolution Raman scattering. Second, I will describe the acoustic behavior of a series of nanomechanical devices based on nanometric semiconductor multilayers able to control the interactions between light, sound and charge. The presented results open a new playground in the control of acoustic vibrations in solids, providing not only new tools to confine and control the dynamics of ultra-high frequency phonons but also a new platform to study topological and general localization effects.

Achieving room-temperature quantum-mechanical strong coupling, or vacuum Rabi splitting, between a single emitter and a plasmon resonance has been a longstanding goal. Recently, two peaks have been observed in the scattering spectra of plasmonic metal nanostructures coupled to single molecules and single quantum dots, and this was taken as evidence of strong coupling. However, a two-peak scattering structure can also arise at intermediate coupling strengths, below the strong-coupling threshold, due to Fano interference between the plasmon and emitter dipole. We unambiguously distinguish between intermediate and strong coupling by measuring both the scattering spectra and the photoluminescence spectra of coupled plasmon-emitter structures. Specifically, we couple single colloidal quantum dots to a plasmon resonator by placing them in the gap between a gold nanoparticle and a silver film. We observe weak, intermediate, and strong coupling in these hybrid metal-semiconductor structures at room temperature, depending on the detailed nanoscale structure of the metal nanoparticle. These structures have the potential to serve as ultrafast, low-power plasmonic modulators on the nanoscale. Both induced transparency and strong coupling can be canceled by absorbing a photon in the quantum dot, leading to a strong change in extinction at the quantum dot transition frequency. Since only a single photon must be absorbed by the QD for this to happen, the energy needed for modulation has the potential to be extremely low, and the structure has the potential to enable all-optical quantum information processing.

Lead halide perovskites have been under intense research focus due to their potential for efficient solar cells and other devices. Among their outstanding optoelectronic properties is a long carrier lifetime which is difficult to reconcile within a simple direct band gap semiconductor picture. Recent studies point to Rashba band splitting, charge separation and the formation of a large polaron as possible explanations. Understanding band edge excitations and dynamics is therefore critical to developing a working model of these materials. Time-resolved THz spectroscopy (TRTS) is an ideal tool to interrogate these meV level excitations on a sub-100 fs time scale. Here we use an air-plasma based TRTS covering the 4 – 120 meV range to measure the THz differential reflectivity from a facet of a methylammonium lead triiodide single crystal following femtosecond optical excitation at the band edge. Full two–dimensional time/energy reflectivity maps reveal a beat between the THz probe light frequency and the crystal photoconductivity, which coherently oscillates at the LO phonon frequency and vanishes with a sub-picosecond lifetime. This indicates some population of carriers are impulsively formed upon excitation and are strong coupling to the polar lattice, in good agreement with the polaron picture. To our knowledge, this is the first observation of a coherent oscillation in the transport parameters in the lead halide perovskites.

Significant advance has been made over the last decade in the development of broadband optoelectronic devices based on novel technologies such as 2D materials, metamaterials, plasmonics, negative electron affinity photoemission, etc. Understanding carrier dynamics in such devices, especially carrier relaxation and transportation near device surfaces, requires time-resolved, broadband reflective spectroscopy with femtosecond temporal resolutions. Femtosecond pump-probe reflectivity measurement (PPRM) has long been used to study carrier dynamics in semiconductor devices. However, conventional PPRM lacks the necessary bandwidth and the ability to make spectroscopic measurement. In this presentation, we report the demonstration of wavelength-resolved transient reflectivity measurement using a ultrabroad-band few-cycle pump-probe system. The system allows device transient reflectivity to be mapped onto a two-dimensional space formed by time and wavelength, providing a comprehensive characterization of ultrafast carrier dynamics. Preliminary results based on a GaAs substrate and GaAs/AlGaAs layered structures have offered interesting insights into device dynamics that otherwise would not be clear. These results demonstrate the feasibility of performing wavelength-resolved transient reflectivity measurement and the effectiveness of this technique in characterizing broadband optoelectronic devices.