We present an optical slot antenna and its application. By measuring transmission spectra and far-field radiation pattern of metallic slots with nanometer scale, we show that a metallic nanoslot has the properties of an antenna, which are resonance, polarization, and bidirectional far-field radiation pattern, and can be regarded as a magnetic dipole in optical region. Additionally, we also make the unidirectional radiation by adapting the geometry of RF Yagi-Uda antenna and applying slot antenna. By the aid of phase analysis based on 3-dimensional finite-difference time-domain simulation, we can increase the front-to-back ratio of an optical slot Yagi-Uda antenna up to about 5. As the application of a slot antenna, we integrate a metal-insulator-metal plasmonic waveguide with a slot antenna. A surface plasmon waveguide mode propagating in MIM structure is well-coupled to a slot antenna and radiates into free-space in form of dipole radiation. By adding an auxiliary structure that has the role of reflector as like a slot Yagi-Uda antenna, the direction of radiation from a slot antenna integrated with a plasmonic waveguide can be controlled efficiently. Besides the possibility of integration with a waveguide, we expect that a slot antenna can be applied to active devices such as light emitting diodes or lasers for the future.

Electrically driven optical antennas are attracting much attention, in particular, due to necessity to develop integrated electrical source of surface plasmons for future plasmonic nanocircuitries. By default, this term denotes a metal nanostructure, in which electromagnetic oscillations at optical frequencies are excited by electrons, tunneling between metallic parts of the structure when a bias voltage is applied between them. Instead of relying on an inefficient inelastic light emission in a tunnel gap, we are suggesting to use ballistic nanoconstrictions as the feed element of an optical antennas in order to excite electromagnetic plasmonic modes. Similarly to tunneling structures, the voltage applied at the constriction falls over the contact of nanoscale length. Electron passing through the contact ballistically can gain the energy provided by the bias ~1eV and exchange it into an mode of the optical antenna. We discussed the underlying mechanisms responsible for the optical emission, and show that with nanoscale contact, one can reach quantum efficiency orders of magnitude larger than with standard tunneling structures.

We present our latest results for a new class of phenomena in condensed matter nanooptics when a strong optical field ∼1-3 V/Å changes a solid within optical cycle [1-8]. Such a pulse drives ampere-scale currents in dielectrics and adiabatically controls their properties, including optical absorption and reflection, extreme UV absorption, and generation of high harmonics [9] in a non-perturbative manner on a 100-as temporal scale. Applied to a metal, such a pulse causes an instantaneous and, potentially, reversible change from the metallic to semimetallic properties. We will also discuss our latest theoretical results on graphene that in a strong ultrashort pulse field exhibits unique behavior [10-12]. New phenomena are predicted for buckled two-dimensional solids, silicene and germanene [13]. These are fastest phenomena in optics unfolding within half period of light. They offer potential for petahertz-bandwidth signal processing, generation of high harmonics on a nanometer spatial scale, etc. References 1. M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Metallization of Nanofilms in Strong Adiabatic Electric Fields, Phys. Rev. Lett. 105, 086803-1-4 (2010). 2. M. Durach, A. Rusina, M. F. Kling, and M. I. Stockman, Predicted Ultrafast Dynamic Metallization of Dielectric Nanofilms by Strong Single-Cycle Optical Fields, Phys. Rev. Lett. 107, 086602-1-5 (2011). 3. A. Schiffrin, T. Paasch-Colberg, N. Karpowicz, V. Apalkov, D. Gerster, S. Muhlbrandt, M. Korbman, J. Reichert, M. Schultze, S. Holzner, J. V. Barth, R. Kienberger, R. Ernstorfer, V. S. Yakovlev, M. I. Stockman, and F. Krausz, Optical-Field-Induced Current in Dielectrics, Nature 493, 70-74 (2013). 4. M. Schultze, E. M. Bothschafter, A. Sommer, S. Holzner, W. Schweinberger, M. Fiess, M. Hofstetter, R. Kienberger, V. Apalkov, V. S. Yakovlev, M. I. Stockman, and F. Krausz, Controlling Dielectrics with the Electric Field of Light, Nature 493, 75-78 (2013). 5. V. Apalkov and M. I. Stockman, Metal Nanofilm in Strong Ultrafast Optical Fields, Phys. Rev. B 88, 245438-1-7 (2013). 6. V. Apalkov and M. I. Stockman, Theory of Dielectric Nanofilms in Strong Ultrafast Optical Fields, Phys. Rev. B 86, 165118-1-13 (2012). 7. F. Krausz and M. I. Stockman, Attosecond Metrology: From Electron Capture to Future Signal Processing, Nat. Phot. 8, 205-213 (2014). 8. O. Kwon, T. Paasch-Colberg, V. Apalkov, B.-K. Kim, J.-J. Kim, M. I. Stockman, and D. E. Kim, Semimetallization of Dielectrics in Strong Optical Fields, Sci. Rep, 6, 21272-1-9 (2016). 9. T. Higuchi, M. I. Stockman, and P. Hommelhoff, Strong-Field Perspective on High-Harmonic Radiation from Bulk Solids, Phys. Rev. Lett. 113, 213901-1-5 (2014). 10. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Wannier-Stark States of Graphene in Strong Electric Field, Phys. Rev. B 90, 085313-1-11 (2014). 11. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Graphene in Ultrafast and Superstrong Laser Fields, Phys. Rev. B 91, 0454391-8 (2015). 12. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Attosecond Strong-Field Interferometry in Graphene: Chirality, Singularity, and Berry Phase, Phys. Rev. B 93, 155434-1-7 (2016). 13. H. K. Kelardeh, V. Apalkov, and M. I. Stockman, Ultrafast Field Control of Symmetry, Reciprocity, and Reversibility in Buckled Graphene-Like Materials, Phys. Rev. B 92, 045413-1-9 (2015).

Excitons are strongly correlated pairs of electrons and holes and dominate the absorption spectrum of semiconductors near the bandgap energy. The intraexcitonic transition energy lies in the terahertz (THz) frequency range with large dipole moments, making exictons useful systems not only for studying the fundamentals of nonlinear optics but also for exploiting THz excitonic interactions in optical communication devices. However, so far, the study of the effect of excitonic dressed states on the optical field has been limited because of the usage of a single optical frequency as a probe. The strong electric field with a stable phase that is associated with optical pulses may make it possible to investigate the role of the dressed state. Here, we probe the transient absorption changes of a near-infrared (NIR) pulse in a GaAs quantum well in the presence of a multi-cycle THz wave. By changing the delay between the NIR probe and the THz wave, the absorption strengths can be modulated on a sub-cycle THz timescale, and the frequency analysis shows the formation of THz-induced dressed states of excitons.

In this work, we summarize recent findings on ultrafast nonlinear and strong-field phenomena in silicon-loaded nanoplasmonic waveguides. Coupling ultrafast λ= 1:55 μm pulses into such structures gives rise to both high- efficiency third harmonic generation (THG) and ponderomotive electron acceleration. We show THG efficiencies of 2.3 ×10 5 in waveguides with an ultracompact footprint of 0.43 μm^-2, resulting in visible green light emission. Remarkably, broadband white light emission is observed as well. This phenomenon is found to originate from an electron avalanche induced by the ponderomotive acceleration of electrons generated via two photon absorption. Thus, this nanoplasmonic device presents a versatile platform for realizing ultrafast nonlinear phenomenon within all-optical circuitry.

Strong light-matter coupling lies at the heart of quantum optics and recently has been successfully explored also in the GHz and THz range. New, intriguing quantum optical phenomena have been predicted in the ultrastrong coupling regime, when the coupling strength Omega becomes comparable to the unperturbed frequency of the system omega_c. We recently proposed a new experimental platform where the physics of the ultrastrong coupling can be investigated at GHz-THz frequencies. We couple the inter-Landau level transition of an high-mobility 2 dimensional electron gas (2DEG) to the subwavelength photonic mode of an LC meta-atom. Our system benefits from the collective enhancement of the light-matter coupling which comes from the scaling of the coupling constant Omega with the square root of the number of electrons in the last Landau level. In our previous experiments and in literature this number varies from 10000-1000 electrons per resonator. Here we present ultrastrong coupling between a high-mobility 2DEG (mu=2.3X 10^6 cm^2/Vs) and an extremely subwavelength hybrid-LC resonator ensemble (11 resonators) with an highly reduced effective mode volume V_eff=4 x 10^-19 m^3=4 x 10^(-10) lambda^3 at a frequency of 300 GHz. The number of optically active electrons is given by the flux quantum multiplied by the effective resonator area and is proportional to the magnetic field. At the anticrossing field of B=0.73 T we measure less than 80 electrons ultrastrongly coupled to the resonator with a normalized coupling ratio Omega/omega_c=0.35. This experiment paves the way towards the study of ultrastrong coupling physics in the regime of quantum non-linearities.

In this paper, we present a polariton description of a semiconductor double microcavity system. The polariton formalism is derived from a microscopic theory for the exciton fields inside the quantum wells coupled to the confined optical cavity fields in a double cavity system. The polariton picture helps simplify theoretical studies of the observed phenomena in the double cavity system, such as nonlinear optical spin Hall effect.

We demonstrate an entirely new method to probe quantum measurement phenomena in semiconductor quantum dot (QD) spin qubits [1]. In addition to providing direct evidence for the quantum nature of solid state qubits, we show that our method has practical importance since it provides a completely alternative route for measuring ensemble and quantum dephasing times, T2* and T2, using only repeated projective measurements and without the need for coherent spin control. Our approach is based on measuring time-correlators of a spin qubit in an optically active QD beyond the second order. We utilize a quantum dot spin-storage structure to initialize a single electron spin in a quantum dot subject to a magnetic field applied in Voigt geometry through tunnel ionization and perform repeated projective measurements of the spin at times t1 and t2. This measurement is repeated, corresponding to ensemble averaging, and the resulting third-order time correlations reveals rich physics: For times t1 or t2 < T2* Larmor precession is observed which reveals the ensemble dephasing time T2*. Importantly, even though the time-correlators were obtained through averaging many measurements for times t1 and t2 > T2* oscillations are observed that decay with the dephasing time T2 and allow its determination even without the need for coherent spin control. Finally, combining the third-order time correlator with the second-order time correlator allows to demonstrate a violation of Leggett-Garg type inequalities for certain times providing clear evidence for the quantum nature of the quantum dot spin. [1] A. Bechtold et al. Phys. Rev. Lett. 117, 027402 (2016)

The availability of single attosecond (as) XUV pulses allows investigating ultrafast electron dynamics on the as time scale by recording slight temporal shifts of the photoelectron streaking in a simultaneously present strong IR field. The physical origin of the observed small delays is not yet understood and controversial theoretical models coexist demonstrating our still limited understanding of the fundamentals of the photoemission process. Here we report our progress to model and interpret photoemission delays measured using as-time-resolved photoemission from the layered crystals WSe2 and the non-centrosymmetric BiTeCl. Quantum mechanical modelling on the single particle level and classical trajectory calculations yield no satisfactory explanation of the observed relative delays between photoemission events from different initial states. Local atomic effects and many body corrections occurring inside the atom from which the electron is emitted yield significant corrections to the total photoemission delay and improve the match between experimental observation and theoretical prediction. This sheds new light on the fundamental mechanism involved in the photoemission process and shows that a refined model of photoemission that accounts for these local effets is needed.

Realizing low-power (few-photon) nonlinear optics in a scalable way is important for both fundamental scientific studies to build strongly correlated “quantum fluids of light” and technological applications, including optical information processing. In recent years, such single photon nonlinearity has been reported using cavity coupled single emitters, including quantum dots, and atoms. However, the macroscopic size of atomic physics cavities, and stochastic spatial and spectral nature of quantum dots pose a serious problem for the scalability. In my talk, I will introduce a new platform with cavity coupled to patterned monolayer materials to accomplish this goal. I will present theoretical analysis of a coupled system of cavity-transition metal dichalcogenides and provide some preliminary experimental data on nonlinear optics with cavity and monolayer materials.

The intriguing electronic properties of two-dimensional materials motivates experiments to resolve their rapid, microscopic interactions and dynamics across momentum space. Essential insight into the electronic momentum-space dynamics can be obtained directly via time- and angle-resolved photoemission spectroscopy (trARPES). We discuss the development of a high-repetition rate trARPES setup that employs a bright source of narrowband, extreme-UV harmonics around 22.3 eV, and its application to sensitive studies of materials dynamics. In the bulk transition-metal dichalcogenide MoSe₂ momentum-space quasiparticle scattering is observed after resonant excitation at the K-point exciton line, resulting in the time-delayed buildup of electrons at the Σ-point conduction band minimum. We will discuss this and other aspects of the non-equilibrium electronic response accessible with the extreme-UV trARPES probe.

The recent integration of silicene in field-effect transistors (FET) opened new challenges in the comprehension of the chemical and physical properties of this elusive two-dimensional allotropic form of silicon. Intense efforts have been devoted to the study of the epitaxial Silicene/Ag(111) system in order to elucidate the presence of Dirac fermion in analogy with graphene; strong hybridization effects in silicene superstructures on silver have been invoked as responsible for the disruption of π and π* bands. In this framework, the measured ambipolar effect in silicene-based FET characterized by a relatively high mobility, points out to a complex physics at the silicene-silver interface, demanding for a deeper comprehension of its details on the atomic scale. Here we elucidate the role of the metallic support in determining the physical properties of the Si/Ag interface, by means of optical techniques combined with theoretical calculations of the optical response of the supported system. The silicene/Ag(111) spectra, which turn out to be strongly non-additive, are analyzed in the framework of theoretical density functional based calculations allowing us to single out contributions arising from different localization. Electronic transitions involving silver states are found to provide a huge contribution to the optical absorption of silicene on silver, compatible with a strong Si-Ag hybridization. The results point to a dimensionality-driven peculiar dielectric response of the two-dimensional-silicon/silver interface, which is confirmed by means of Transient-Reflectance spectroscopy. The latter shows a metallic-like carrier dynamics, (both for silicene and amorphous silicon), hence providing an optical demonstration of the strong hybridization arising in silicene/Ag(111) systems.

The electro-magnetic field generated within and around dissipative nano-structures upon light radiation is intimately associated to the formation of localized heat sources. In turn, this phenomenon determines localized temperature variations, effect which can be exploited for applications such as photocatalysis [1], nanochemistry [2] or sensor devices [3]. Here we show how the geometrical characteristics of plasmonic nano-structures can indeed be used to modulate the temperature response. The idea is that when metallic structures interact with an electromagnetic field they heat up due to Joule effect. The corresponding temperature variation modifies the optical response of the structure [4,5] and thus its heating process. The key finding is that, depending on the structures geometry, absorption efficiency can either increase or decrease with temperature. Since absorption relates to the thermal energy dissipation and thus to temperature increase, the mechanism leads to positive or negative loops. Consequently, not only an error would be made by neglecting the role of temperature, but it would be not even possible to know, a priori, if the error is towards higher or lower absorption values. Our model can be utilized to study opto-thermal phenomena when high temperature or high intensity sources are employed. [1] M. Honda et al., Appl. Phys. Lett. 104, 061108 (2014) [2] G. Baffou et al., Chem. Soc. Rev. 43, 3898 (2014) [3] S. Ozdemir et al., J. Lightwave Tech. 21, 805 (2003) [4] A. Alabastri et al., ACS Photonics 2, 115 (2015) [5] A. Alabastri et al., Materials 6, 4879 (2013)

Tamm plasmon-polariton (TPP) is an optical analogue of Tamm state and appears as spatial localization of the electromagnetic field near the boundary of one-dimensional photonic crystal (PC) (distributed Bragg reflector) and a metal film. TPP can be detected experimentally as a narrow resonance in the reflectance or transmittance spectrum of a PC/metal structure. Contrary to surface plasmon-polariton TPP occurs at any angles of incident light for both TE and TM polarizations, and it excitation does not require sophisticated optical schemes (such as Kretchmann scheme). The peculiarities of TPP optical properties led to considerable interest to the design, fabrication and study of TPP-supported structures in the past several years. In present work, the ultrafast relaxation dynamics of TPP excited in the PC/metal structures is measured using intensity cross-correlation scheme. The TPP lifetime is obtained for different polarizations and incident angles of light, and compared with one obtained from numerical calculations. A femtosecond pulse reflected from such a structure is found to be significantly distorted if its spectrum overlaps with the TPP resonance. The TPP lifetime possesses strong polarization and angular dependence and is shown to vary from 20 fs for p-polarized light to 40 fs for s-polarized light at a 45◦ angle of incidence. The reported lifetime of TPP is several times smaller than the previously reported lifetime of surface plasmons. Short lifetime and sharpness of resonance make TPP a good candidate for use in all-optical switches and modulators.

We propose a novel hybrid ridge-plasmonic waveguide Faraday rotator for high-speed polarization manipulation in nanoplasmonic circuitry. Our design, based on bismuth-substituted yttrium iron garnet (Bi:YIG), provides a unique geometrical mechanism of phase matching both the plasmonic TM and photonic TE waveguide modes, and hence facilitates effective mode conversion via the Faraday effect. This structure yields 99.4% mode conversion within 830 μm, which is easily attainable within the long (>1mm) propagation lengths of the two supported modes. Furthermore, our simulations show that the application of magnetic field transients can alter the magnetization of the Bi:YIG to actively switch the polarization state, or produce a polarization oscillator at frequencies up to 10GHz. This structure is envisioned to play a fundamental role in future integrated nanoplasmonic networks.

In unbiased non-centrosymmetric semiconductors electronic currents can be excited on ultrashort time scales using purely optical excitation. A combined approach of k.p-perturbation theory and the semiconductor Bloch equations can be used to theoretical describe these photocurrents in bulk and quantum well systems. Including the Coulomb interaction and resulting excitonic effects is a very challenging task due to the large numerical requirements. Here, we present a non-standard grid which significantly reduces the numerical requirements while still ensuring converged results. We analyze and compare the convergence behavior of standard Cartesian and geodesic grids for shift current simulations and present results on the enhancement of the shift current by the excitonic resonance in bulk GaAs which are based on an anisotropic three-dimensional k.p band structure.

The Dirac-cone surface states of topological insulators are characterized by a chiral spin texture in k-space with the electron spin locked to its parallel momentum. Mid-infrared pump pulses can induce spin-polarized photocurrents in such a topological surface state by optical transitions between the occupied and unoccupied part of the Dirac cone. We monitor the ultrafast dynamics of the corresponding asymmetric electron population in momentum space directly by time- and angle-resolved two-photon photoemission (2PPE). The elastic scattering times of 2.5 ps deduced for Sb₂Te₃ corresponds to a mean-fee path of 0.75 μm in real space.

Presently, there exists no reliable in-situ time-resolved method that selectively isolates both the recombination and escape times relevant to photocurrent generation in the ultrafast regime. Transport based measurements lack the required time resolution, while purely optical measurement give a convoluted weighted-average of all electronic dynamics, offering no selectivity for photocurrent generating pathways. Recently, the ultrafast photocurrent (U-PC) autocorrelation method has successfully measured the rate limiting electronic relaxation processes in materials such as graphene, carbon nanotubes, and transition metal dichalcogenide (TMD) materials. Here, we unambiguously derive and experimentally confirm a generic U-PC response function by simultaneously resolving the transient absorption (TA) and U-PC response for highly-efficient (48% IQE at 0 bias) WSe2 devices and twisted bilayer graphene. Surprisingly, both optical TA and electrical U-PC responses give the same E-field-dependent electronic escape and recombination rates. These rates further accurately quantify a material’s intrinsic PC generation efficiency. We demonstrate that the chirality of the incident light impacts the U-PC kinetics, suggesting such measurements directly access the ultrafast dynamics need to complex electronic physics such as the valley-Hall effect. By combining E-field dependent ultrafast photocurrent with transient absorption microscopy, we have selectively imaged the dominant kinetic bottlenecks that inhibit photocurrent production in devices made from stacked few-layer TMD materials. This provides a new methodology to intelligently select materials that intrinsically avoid recombination bottlenecks and maximize photocurrent yield.

We show the reduction of the nonlinear electron emission order of an eCarbon/gold bilayer driven by a surface plasmon wave. The eCarbon layer allows for higher confined electric fields and increased electric field enhancement which increases lower order electron emission compared to an Au film. This bilayer represents a unique platform for the next generation of ultrafast electron sources operating at low laser intensities.

We performed optical pump-THz probe spectroscopy on bulk GaAs to investigate the nature of exciton Mott transition. The behavior of excitonic correlation in the proximity of the Mott transition density is elucidated through the resonant excitation of 1s excitons with using a nonlinear terahertz spectroscopy technique. We discuss the anomalous charge carrier dynamics of the metallic phase on the verge of Mott transition that appears only at low temperatures.

Antiferromagnets are an important class of ordered spin systems, common in spintronic applications and providing a testbed for studying magnetism. Recently, the injection of magnons – coherent spin waves – has been explored by broadband terahertz pulses in antoferromagnets, such as MnO. Here, terahertz time-domain spectroscopy is used to detect magnon resonances in MnF2, which is a model antiferromagnet with uniaxial anisotropy and a Néel temperature of 67 K. Temperature dependence of a one-magnon resonances is examined from 5 K to 70 K. The center frequency of the one-magnon is recorded below the Néel temperature and fit to a Brillouin function. It is found that the degree of correlation between neighboring spins is j = 1.1. Namely, a weak correlation and appropriately modeled by mean-field theory befitting this simple system. From low temperature to room temperature, a two-magnon resonance is observed to broaden and strengthen as the temperature increases. Two-magnon modes arise due to zone-edge magnons being stimulated with -k and +k momenta and do not require magnetic ordering. Over this same temperature range, THz transients are used to monitor the time-of flight through the crystal, the refractive index, the internal energy and the heat capacity. Overall these quantities decrease with decreasing temperature, with behavior that falls into three regimes: a thermal dominated region above the Néel temperature, a magnetic regime below the Néel temperature; and a hyperfine interaction region at temperatures below 6 K. The latter is the first direct observation of the hyperfine interaction using terahertz time-domain spectroscopy.

In the past few years, perovskite film has been considered as a promising materials for solar cell devices due to its outstanding performance. To maximize the perovskite solar cell performance, it is necessary to understand the crystallization mechanism of perovskite film. In this study, we monitored the crystallization and decrystallization of the lead halide perovskite (MAPbI3-xClx) film under thermal annealing and UV-laser exposure processes by using in-situ terahertz time-domain spectroscopy. The strength of vibrational resonances in THz frequency range is found to be a good indicator of perovskite crystallinity. We measured the THz spectra as we annealed the perovskite film at various temperatures in order to achieve the degree of crystallization, i.e., the transition of perovskite structure from the intermediate phase to the tetragonal phase. In addition, we investigated the UV-laser-induced phase transition of the perovskite film. Because it is widely known that UV light illumination on perovskite film tends degrade the perovskite cell efficiency, its influence on the crystallization is our primary concern. Surprisingly, the crystallization phase increases for 10 min, until it starts to degrade over a couple of hours. We also studied the transient transport properties of the films under UV illumination. The correlation between the degree of crystallization (obtained from THz transmission) and the transport parameters exhibited the electric percolation threshold behaviors in the perovskite films. These information are expected to be crucial for optimizing the fabrication method of perovskite solar cell.

We study the ultra-strong coupling (USC) of Landau level transitions in strained Germanium quantum wells (sGe QW) to THz metasurfaces. The spin-splitting of the heavy-hole cyclotron resonance in sGe QWs due to the Rashba spin-orbit interaction in magnetic field offers an excellent platform to investigate ultra-strong coupling to a non-parabolic system. THz split ring resonators can be tuned to coincide with the single cyclotron transition (around 0.4 THz and a magnetic field of 1.5 T) or the spin-resolved transitions of the sGe QWs (at 1.3 THz and 4.5 T). Coupling to the single cyclotron yields a normalized USC rate of 25%, resulting from fitting with a Hopfield-like Hamiltonian model. Coupling to two or three cyclotron resonances in sGe QWs lead to the observation of multiple polaritons branches, one polariton branch for each oscillator involved in the system. An adaption of the theory allows to also describe this multiple-oscillator system and to determine the coupling strengths. The different Rabi-splittings for the multiple cyclotrons coupling to the same resonator mode relate to the underlying differences in the material. Furthermore, the visibility of an additional transition, possibly a light hole transition with very low carrier density, is strongly enhanced due to the coupling to the LC-resonance with a normalized strong coupling ratio of 4.7%. Future perspectives include controlling spin-flip transitions in USC and studying the impact of non-parabolicity on the ultra-strong coupling physics.

Metallic antenna arrays fabricated on high resistivity silicon are used to localize and enhance the incident THz field resulting in high electric field pulses with peak electric field strength reaching several MV/cm on the silicon surface near the antenna tips. In such high electric field strengths high density of carriers are generated in silicon through impact ionization process. The high density of generated carriers induces a change of refractive index in silicon. By measuring the change of reflectivity of tightly focused 800 nm light, the local density of free carriers near the antenna tips is measured. Using the NIR probing technique, we observed that the density of carriers increases by over 8 orders of magnitude in a time duration of approximately 500 fs with an incident THz pulse of peak electric field strength 700 kV/cm. This shows that a single impact ionization process is happening in a time duration of less than 20 fs. The measurement is repeated by exciting the sample with an optical pump beam at a wavelength of 400 nm. The optical pump sets the initial free carrier density before the THz-induced impact ionization. The measurements show that the carrier generation mechanism depends on the initial free carrier density which confirms that the carrier generation mechanism is impact ionization, rather than the alternative carrier generation mechanism in high electric field, i.e. Zener tunneling. Finally this technique can be extended to investigate carrier dynamics in other semiconductors.

Two-dimensional electron gases have been the subject of research for decades. Modulation doped GaAs quantum wells in the absence of magnetic fields exhibit interesting many-body physics such as the Fermi edge singularity or Mahan exciton and can be regarded as a collective excitation of the system. Under high magnetic fields Landau levels form which have been studied using transport and optical measurements. Nonlinear coherent two-dimensional Fourier transform (2DFT) spectroscopy however provides new insights into these systems. We present the 2DFT spectra of Mahan Excitons associated with the heavy-hole and light-hole resonances observed in a modulation doped GaAs/AlGaAs single quantum well [1]. These resonances are observed to be strongly coupled through many-body interactions. The 2DFT spectra were measured using co-linear, cross-linear, and co-circular polarizations and reveal striking differences. Furthermore, 2DFT spectra at high magnetic fields performed at the National High Magnetic Field Lab (NHMFL) in Tallahassee, Florida will be discussed. The spectra exhibit new features and peculiar line shapes suggesting interesting underlying physics. [1] J. Paul, C. E. Stevens, C. Liu, P. Dey, C. McIntyre, V. Turkowski, J. L. Reno, D. J. Hilton, and D. Karaiskaj, Phys. Rev. Lett.116, 157401 (2016).

The interest in perovskite-based solar cell absorber materials has skyrocketed in recent years due to the rapid rise in solar cell efficiency and the potential for cost reductions tied to solution-processed device fabrication. Due to complications associated with the presence of strong static and dynamic disorder in these organic-inorganic materials, the fundamental photophysical behavior of photo-excited charge carriers remains unclear. We apply four-wave mixing spectroscopy to study the charge carrier dynamics in CH₃NH₃PbI₃ thin films. Our experiments reveal two discrete optical transitions below the band gap of the semiconductor with binding energies of 13 meV and 29 meV, attributed to free and defect-bound excitons respectively.

In this work we study the ultrafast exciton dynamics in CdTe nanorods by using two-dimensional electronic spectroscopy (2DES). By simultaneously exciting the lowest three excitonic transitions (i.e. S1, S2 and S3) we extract the electron and hole relaxation pathways, owing to the combined temporal and spectral resolution of 2DES. In particular, we directly observe hot hole relaxation from the second to the first exciton state in about 30 fs by excitation of the S2 transition. Additionally, we extract a direct charge relaxation to S1 by disentangling the overlapping bleach and excited state induced energy level shifts after excitation of S3.

Recently, two-dimensional materials have gained much interest for various applications in nanophotonics and quantum optics, as they possess a strong luminescence and are able to host single quantum emitters. Excitation of quantum emitters via a two-photon process can be employed for high resolution imaging and has applications in quantum optics. Here, we present one- and two-photon excitation of single defects in hexagonal boron nitride (hBN) and analyse the properties of the emitted light [1]. We find clear antibunching signals that prove the single emitter character in both excitation cases. To gain further knowledge, we also obtain saturation curves. From a comparison of one- and two-photon case insights about the level structure of the defects can be obtained. These results will not only help the fundamental understanding of defects in hBN, but also help to introduce this class of emitters in optical imaging, as the defects in hBN are of small spatial extend, photostable and emit their fluorescence well in the wavelength region of the biological optical window. [1] A. W. Schell et al. arXiv:1606.09364 (2016)

The reported work describes different regimes of exciton-polariton oscillatory dynamics in a microcavity, in the conservative case as well as in the presence of continuous-wave pumping from the high-energy excitonic reservoir. Accounting for exciton-photon energy detuning, linear and non-linear decay, gain, and interactions, we discuss the influence of different ingredients of the system on the dynamics in conservative and non-conservative cases, and show the existence of non-trivial regimes reminiscent of internal Josephson effect, van der Pol oscillations, and the inverted Kapitza pendulum. Conditions of experimental observation of the predicted effects are considered.

A time-domain approach to quantum electrodynamics is presented, covering the entire mid-infrared and terahertz frequency ranges. Ultrabroadband electro-optic sampling with few-femtosecond laser pulses allows direct detection of the vacuum fluctuations of the electric field in free space [1,2]. Besides the Planck and electric field fundamental constants, the variance of the ground state is determined solely by the inverse of the four-dimensional space-time volume over which a measurement or physical process integrates. Therefore, we can vary the contribution of multi-terahertz vacuum fluctuations and discriminate against the trivial shot noise due to the constant flux of near-infrared probe photons. Subcycle temporal resolution based on a nonlinear phase shift provides signals from purely virtual photons for accessing the ground-state wave function without amplification to finite intensity. Recently, we have succeeded in generation and analysis of mid-infrared squeezed transients with quantum noise patterns that are time-locked to the intensity envelope of the probe pulses. We find subcycle temporal positions with a noise level distinctly below the bare vacuum which serves as a direct reference. Delay times with increased differential noise indicate generation of highly correlated quantum fields by spontaneous parametric fluorescence. Our time-domain approach offers a generalized understanding of spontaneous emission processes as a consequence of local anomalies in the co-propagating reference frame modulating the quantum vacuum, in combination with the boundary conditions set by Heisenberg’s uncertainty principle. [1] C. Riek et al., Science 350, 420 (2015) [2] A. S. Moskalenko et al., Phys. Rev. Lett. 115, 263601 (2015)

Nonlinear optics revolutionized the ability to create directed, laser-like light particularly in the regions where lasers based on conventional population inversion are not practical. New breakthroughs in attosecond extreme nonlinear optics promise a similar revolution in the X-ray regime. In this talk, I will discuss the fundamental quantum physics and the phase matching limits of high order harmonic generation in the context of creating coherent X-ray waveforms in the soft X-ray region that can be tailored at the moment of generation. Such a versatile designer light is ideal for 4D studies of various bio- and nano-materials systems with attosecond temporal and nanometer spatial resolution, as well as with element specificity. I will also discuss the path forward for generating bright coherent X-ray beams from a laboratory-scale apparatus at photon energies of 10 keV and greater with unprecedented attosecond-to-zeptosecond pulse durations, and with arbitrary spectral, temporal shapes, and polarization states. A fully spatially and temporally coherent version of the Roentgen X-ray tube with exquisite quantum control of the properties of the soft and hard X-ray light may be possible. 1. T. Popmintchev, et al., Nature Photonics 4, 822 (2010); Science 336, 1287 (2012). 2. D. Popmintchev, et al., Science 350, 1225 (2015). 3. T. Fan, et al., PNAS 112, 14206 (2015).

In condensed matter physics, ultrafast photoexcitation has been shown to result in modification of macroscopic material properties, sometimes involving phase changes, on a subpicosecond time scale. In semiconductors, irreversible non-thermal solid-to-liquid structural transitions have been demonstrated at high laser fluences. In the pump-probe experiments reported here, we observe a striking continuously varying low-fluence pump-induced time-dependent structural symmetry modification in intrinsic gallium arsenide (GaAs) using a probe that produces femtosecond polarization-resolved second harmonic generation (f-PRSHG) data. SHG spectroscopy is particularly suited to monitor symmetry changes since its magnitude is governed by the nonlinear optical susceptibility tensor whose elements are determined by the underlying material symmetry. Conceptually, these experiments seek to provide insight into the details of the time evolution of symmetry arising from laser induced transient states of matter in GaAs. Overall, the basic explanation of these experimental observations is that as a result of the photoinduced electronic excitation, many electrons, including bond electrons are excited to higher states. This results in subpicosecond changes in the local anharmonic potential and produces a changing nonlinear polarization response thus accounting for the nonthermal time dependent symmetry changes. Clearly, our approach may be easily extended to many different crystalline materials with different levels of defects, dopants and stresses to fully characterize the time dependent behavior of laser induced transient states in material systems.

THG is used nowadays in many practical applications such as a substance diagnostics, and biological objects imaging, and etс. Therefore, THG features understanding are urgent problem and this problem attracts an attention of many researchers. In this paper we analyze THG efficiency of a femtosecond laser pulse. Consideration is based on computer simulation of the laser pulse propagation with taking into account a selfand cross- modulation of the interacting waves, and their SOD, and phase mismatching. Moreover, we analyze an influence of the non-homogeneous phase mismatching along laser pulse propagation coordinate. In this case, a phase matching occurs only in narrow area of longitudinal coordinate. Due to strong self- and crossmodulation of interacting waves it is possible to manage effective THG. Using the frame-work of long pulse duration approximation and plane wave approximation as well as an original approach we write the explicit solution of Schrödinger equations describing the frequency tripling of femtosecond pulse. It should be stressed, that the main feature of our approach consists in conservation laws using corresponding to wave interaction process.

Extreme-ultraviolet (EUV) attosecond vortices carrying orbital angular momentum (OAM) are produced through high-order harmonic generation (HHG) from the nonlinear conversion of infrared twisted beams. While previous works demonstrated a linear scaling law of the vortex OAM content with the harmonic order, an unexpectedly rich scenario for the OAM buildup appears when HHG is driven by a vortex combination. The non-perturbative nature of HHG increases the OAM content of the attosecond vortices when the driving field presents an azimuthally varying intensity profile. We theoretically explore the underlying mechanisms for this diversity and disentangle the perturbative and non-perturbative nature in the generation of EUV attosecond twisted through numerical simulations.

Atomically thin semiconductors, such as monolayer MoSe₂ and WSe₂, have emerged as novel optoelectronic materials, with coupled spin-valley electronic physics and excitons that are strongly bound at room temperature. It has recently been shown that these materials can form the basis for atomically thin p-n junctions, transistors, light emitting diodes, and low threshold nanolasers. In this presentation, I will discuss optoelectronics and spin effects in heterostructures of MoSe₂ and WSe₂, with type-II band alignment, with the lowest conduction band in the MoSe₂ layer, and the highest valence band in the WSe₂ layer. Upon optical excitation, electrons transfer to the MoSe₂ layer and holes transfer to the WSe₂ layer. Due to the strong attractive Coulomb interaction between these spatially separated layers can form interlayer excitons which have many similarities to the spatially indirect excitons of coupled GaAs quantum wells. However, unlike the coupled quantum well system, here the constituent electrons and holes are located in momentum space valleys on the edge of the Brillouin zone. The conduction and valence band valley alignment can be tuned by twist angle between layers to realize optically bright interlayer excitons with an optical selection rule allowing for optical control of the valley degree of freedom. I will discuss the dynamics and spin-valley effects of these bright interlayer excitons.

Monolayers of transition metal dichalcogenides are direct gap semiconductors, which have attracted much attention in the recent past. Due to a strong Coulomb interaction, they possess strongly bound electron-hole pairs, with binding energies of hundreds of meV which is an order of magnitude larger than in conventional materials. Here, we investigate the microscopic origin of the homogeneous linewidth and coherence lifetime of excitonic resonances in monolayer molybdenum disulfide, taking exciton phonon scattering and radiative recombination into account. We find a superlinear increasing homogeneous linewidth from 2 meV at 5K to 14 meV at room temperature corresponding to a coherence lifetime of 160 fs and 25 fs.

In the past decade, sound amplification by the stimulated emission of (acoustic phonon) radiation (saser) devices for generating coherent terahertz (THz) acoustic waves have been demonstrated [1 – 3]. The devices exploit the electron-phonon interactions in periodic semiconductor nanostructures known as superlattices (SLs) to amplify acoustic phonons. In addition, the particular acoustic properties of SLs can be exploited to make mirrors and cavities for THz phonons. Thus SLs can provide the two essential elements of a saser: the acoustic gain medium and the acoustic cavity. In this presentation I will describe experimental studies of the THz phonon dynamics in a weakly-coupled GaAs/AlAs saser SL, which is DC electrically biased into the Wannier-Stark regime. Picoseconds-duration pulses of coherent THz acoustic phonons were generated using pump light pulses from a femtosecond laser and injected into the SL device. These phonon pulses seeded the saser cavity modes at about 220 and 440 GHz, which were amplified within the device. The phonons were detected using two methods: reflection of femtosecond probe light pulses, in a conventional pump-probe arrangement, and through the transient electrical response of the device itself. When the DC bias conditions for saser were achieved in the device, the amplitude and lifetime of the seeded modes were both increased, analogous to the threshold and spectral line narrowing effects seen in laser devices. [1] R P Beardsley et al., Phys. Rev. Lett. 104, 085501 (2010). [2] W Maryam et al., Nature Communications 4:2184 (2013). [3] K Shinokita et al., Phys. Rev. Lett. 116, 075504 (2016).

The existence of both optical and sub-THz nanomechanical resonances in the same laser microcavity results in strong photon-phonon interaction, and may be explored for the ultrafast control of vertical lasers. In the talk the experiments involving the injection of picosecond strain pulses into optically and electrically pumped vertical lasers, and monitoring of the modulated output laser intensity will be discussed. The results of three recent experiments will be presented: • In the experiments with an optically pumped quantum dot laser, an increase of the lasing output induced by strain pulses by two orders of magnitude has been observed on a picosecond time scale. Such strong and ultrafast increase is due to the inhomogeneous quantum dot ensemble with a spectral broadening much larger than the optical cavity mode width. Thus, the optical resonance required for lasing is achieved for a tiny dot fraction only while non-resonant dots store optical excitation for long time. The strain pulse brings “non-resonant” quantum dots into the resonance with the cavity mode and the stored energy releases almost simultaneously in a form of the intense laser pulses. • Experiments with electrically pumped micropillar lasers show the modulation of the emission wavelength on the frequencies equal to the resonant GHz nanomechanical modes of the micropillar. • Experiments with a quantum well vertical laser showed intensity modulation with the mechanical resonance frequencies (20-40 GHz) of the optomechanical nanoresonator. Prospective application for nanophotonics are discussed.

Amorphous materials, such as glasses, polymers, gels, or even bio-tissues, are an indispensable part of our lives. Unlike crystalline solids, amorphous materials exhibit some anomalous thermal properties that are still under debate. The reduced density of vibrational states versus sound frequency near 1THz disobeys the Debye model and shows a peak, usually termed the boson peak. This excess density of states is often related to a plateau in thermal conductivity and a maximum in the reduced heat capacity around 1-10 Kelvin. This boson peak is expected to provide extra acoustic attenuation for propagating acoustic waves with a frequency around 1THz. In this presentation, we discuss the optimal thickness of a vitreous silica layer in which the THz acoustic waves will propagate through to render the acoustic attenuation constant measurement. In this potential experiment, the thickness of the vitreous silica layer becomes a critical issue. It can’t be too thick because the attenuation will be so high that the THz acoustic wave may be completely depleted; while it can’t be too thin because the wavelength (several nanometers) of the THz acoustic wave can be much longer than the layer thickness and the resulted measurement accuracy will be compromised. In this study, by using femtosecond acoustics with a bandwidth over 1THz, we explore the sample thickness issue of this much-needed experiment. Results with different layer thickness will be presented and will be compared with the current direct or indirect measurement results.

Semiconductor-metal hybrid nanostructures are interesting materials for photocatalysis. Their tunable properties offer a highly controllable platform to design light-induced charge separation, a key to their function in photocatalytic water splitting. Hydrogen evolution quantum yields are influenced by factors as size, shape, material and morphology of the system, additionally the surface coating or the metal domain size play a dominant role.

In this paper we present a study on a well-defined model system of Au-tipped CdS nanorods. We use transient absorption spectroscopy to get insights into the charge carrier dynamics after photoexcitation of the bandgap of CdS nanorods. The study of charge transfer processes combined with the hydrogen evolution efficiency unravels the effects of surface coating and the gold tip size on the photocatalytic efficiency. Differences in efficiency with various surface ligands are primarily ascribed to the effects of surface passivation. Surface trapping of charge carriers is competing with effective charge separation, a prerequisite for photocatalysis, leading to the observed lower hydrogen production quantum yields. Interestingly, non-monotonic hydrogen evolution efficiency with size of the gold tip is observed, resulting in an optimal metal domain size for the most efficient photocatalysis. These results are explained by the sizedependent interplay of the metal domain charging and the relative band-alignments. Taken together our findings are of major importance for the potential application of hybrid nanoparticles as photocatalysts.

Scalability of optical devices is a major challenge for quantum optics and quantum cryptography fields. However, non-linear optical processes such as second harmonic generation (SHG) and parametric-down conversion become very inefficient when the active medium is reduced to the nanoscale. Enhancement strategies are therefore mandatory. Here, we first investigate the role of plasmonic resonances in single aluminum nanostructures allowing doubly resonant and mode-matched conditions. We show that the SHG rate can be 36-fold enhanced compared to non-resonant structures. We further infer the origin of the nonlinearity by quantitatively comparing simulated and measured SHG maps obtained by scanning the antennas under a tightly focused beam. The SHG response of a KTP nano-crystal and its modification by the proximity of a plasmonics antenna can then be confidently modeled. We show that the harmonic photon production yield is comparable for a bare nano-crystal and a doubly resonant aluminum antenna, despite the centro-symmetric nature of the latter. Combining the nonlinearity of the KTP crystal and the field enhancements provided by the plasmonic structure at both fundamental and harmonic frequency, we demonstrate that the SHG signal can be magnified by more than two orders of magnitude. The anticipated efficiency of the hybrid nonlinear plasmonic structures is compared to experiments performed at the single structure level, emphasizing the crucial role of the nanocrystal orientation.

Spintronics and spin-based technology rely on the ultra-fast unbalance of the electronic spin population in quite localized spatial regions. However, as a matter of fact, the low susceptibility of conventional materials at high frequencies strongly limits these phenomena, rendering the efficiency of magnetically active devices insufficient for application purposes. Among the possible strategies which can be envisaged, plasmonics offers a direct approach to increase the effect of local electronic unbalancing processes. By confining and enhancing free radiation in nm-size spatial regions, plasmonic nano-assemblies have demonstrated to support very intense electric and magnetic hot-spots. In particular, very recent studies have proven the fine control of magnetic fields in Fano resonance condition. The near-field-induced out-of-phase oscillation of localized surface plasmons has manifested itself with the arising of magnetic sub-diffractive hot-spots. Here, we show how this effect can be further boosted in the mid-infrared regime via the introduction of higher order plasmonic modes. The investigated system, namely Moon Trimer Resonator (MTR), combines the high efficiency of a strongly coupled nano-assembly in Fano interferential condition with the elevated tunability of the quadrupolar resonance supported by a moon-like geometry. The fine control of the apical gap in this unique nanostructure, characterizes a plasmonic device able to tune its resonance without any consequence on the magnetic hot-spot size, thus enabling an efficient squeezing in the infrared.

Monolayer transition-metal dichalcogenides such as MoS2 and WS2 are prime examples of atomically thin semiconducting crystals that exhibit remarkable electronic and optical properties. They have a pair of valleys that can serve as a new electronic degree of freedom, and these valleys obey optical selection rules with circularly polarized light. Here, we discuss how ultrafast laser pulses can be used to tune their energy levels in a controllable valley-selective manner. The energy tunability is extremely large, comparable to what would be obtained using a hundred Tesla of magnetic field. We will also show that such valley tunability can be performed while we effectively manipulate the valley selection rules. Finally, we will explore the prospect of using this technique through photoemission spectroscopy to create a new phase of matter called a valley Floquet topological insulator.

Ultrafast electrically driven light emitter is a critical component in the development of the high bandwidth free-space and on-chip optical communications. Traditional semiconductor based light sources for integration to photonic platform have therefore been heavily studied over the past decades. However, there are still challenges such as absence of monolithic on-chip light sources with high bandwidth density, large-scale integration, low-cost, small foot print, and complementary metal-oxide-semiconductor (CMOS) technology compatibility. Here, we demonstrate the first electrically driven ultrafast graphene light emitter that operate up to 10 GHz bandwidth and broadband range (400 ~ 1600 nm), which are possible due to the strong coupling of charge carriers in graphene and surface optical phonons in hBN allow the ultrafast energy and heat transfer. In addition, incorporation of atomically thin hexagonal boron nitride (hBN) encapsulation layers enable the stable and practical high performance even under the ambient condition. Therefore, electrically driven ultrafast graphene light emitters paves the way towards the realization of ultrahigh bandwidth density photonic integrated circuits and efficient optical communications networks.

Graphene has been recently reported to have a damage threshold high enough to allow for the interaction with ultrashort laser pulses of intensities above 10¹³ W/cm². It is natural to explore if this situation to what extend the laser pulse is able to induce the highly non-linear dynamics that gives rise to high harmonic generation. We perform the exact numerical integration of the set of coupled two-level equations that describe the valence-to-conduction band transitions by a laser pulse, at any point in the reciprocal space. We analyze the dynamics of the excitation to the conduction band, and the spectra of the harmonics produced. We show that harmonic radiation is produced by interband as well as intraband transitions, these later resulting from parametric oscillation. We also analyze the temporal characteristics of the harmonic emission.

Efficient and cost effective measurement of the refractive index profile in an optical fiber is a significant technical job to design and manufacture in-fiber photonic devices and communication systems. For instance, to design fiber gratings, it is required to estimate the refractive index modulation to be inscribed by the fabrication apparatus such as ultraviolet or infrared lasers. Mach-Zehnder interferometer (MZI) based quantification of refractive index change written in single mode microfiber by femtosecond laser radiation is presented in this study. The MZI is constructed by splicing a microfiber (core diameter: 3.75 μm, cladding diameter: 40 μm) between standard single mode fibers. To measure the RI inscribed by infrared femtosecond laser, 200 μm length of the core within the MZI was scanned with laser radiation. As the higher index was written within 200 μm length of the core, the transmission spectrum of the interferometer displayed a corresponding red shift. The observed spectral shift was used to calculate the amount of refractive index change inscribed by the femtosecond irradiation. For the MZI length of 3.25 mm, and spectral shift of 0.8 nm, the calculated refractive index was found to be 0.00022. The reported results display excellent agreement between theory and experimental findings. Demonstrated method provides simple yet very effective on-site measurement of index change in optical fibers. Since the MZI can be constructed in diverse fiber types, this technique offers flexibility to quantify index change in various optical fibers.