Ultrafast terahertz spectroscopy and control of collective modes in semiconductors

In this dissertation we applied methods of ultrafast terahertz (THz) spectroscopy to study several aspects of semiconductor physics and in particular of collective mode excitations in semiconductors. We detect and analyze THz radiation emitted by these collective modes to reveal the underlying physics of many-body interactions. We review a design, implementation and characterization of our ultrafast terahertz (THz) time-domain spectroscopy setup, with additional features of mid-infrared tunability and coherent as well as incoherent detection capabilities. Temperature characterization of the collective plasmon excitation in indium antimonide (InSb) is presented to reveal the importance of non-parabolicity corrections in quantitative description. We also obtain electronic mobility from the radiation signals, which, once corrected for ultrafast scattering mechanisms, is in good agreement with DC Hall mobility measurements. Exhibited sensitivity to non-parabolicity and electronic mobility is applicable to non-contact characterization of electronic transport in nanostructures. As a first goal of this work, we have addressed the possibility of an all-optical control of the electronic properties of condensed matter systems on an ultrafast time scale. Using femtosecond pulses we have demonstrated an ability to impose a nearly 20% blue-shift of the plasma frequency in InSb. Preliminary investigations of control of the electron dynamics using third-order nonlinearity were also carried out in solid state and gaseous media. In particular, we have experimentally verified the THz coherent control in air-breakdown plasmas and have demonstrated the ability to induce quantum-interference current control in indium arsenide crystals. As a second focus of this dissertation, we have addressed manipulation of the plasmon modes in condensed matter systems. After development of the analytical model of radiation from spatially extended longitudinal modes, we have applied it to analysis of two experiments. In first, we established the ability to control plasmon modes in InSb by means of a plasmonic one dimensional cavity. By control of the cavity geometry, we shifted the plasmon mode into the regime where non-local electron-electron interaction is enforced. We observed the consequential Landau damping of the collective mode, in good agreement with the predictions made within the random-phase approximation. In the second experiment we have invoked plasmon confinement in all three dimensions via a nanowire geometry. We observed enhancement of terahertz emission which we attributed to leaky modes of the waveguide. We attributed this emission to the low-energy acoustic surface plasmon mode of the nanowire, which was also supported by our numerical modeling results and independent DC electronic measurements

Paraxial Theory of Direct Electro-Optic Sampling of the Quantum Vacuum

Direct detection of vacuum fluctuations and analysis of sub-cycle quantum properties of the electric field are explored by a paraxial quantum theory of ultrafast electro-optic sampling. The feasibility of such experiments is demonstrated by realistic calculations adopting a thin ZnTe electro-optic crystal and stable few-femtosecond laser pulses. We show that nonlinear mixing of a short near-infrared probe pulse with multi-terahertz vacuum field modes leads to an increase of the signal variance with respect to the shot noise level. The vacuum contribution increases significantly for appropriate length of the nonlinear crystal, short probe pulse durations, tight focusing, and sufficiently large number of photons per probe pulse. If the vacuum input is squeezed, the signal variance depends on the probe delay. Temporal positions with noise level below the pure vacuum may be traced with a sub-cycle accuracy.Comment: 10 pages, 6 figure

Direct measurement of the Husimi-Q function of the electric-field in the time-domain

We develop the theoretical tools necessary to promote electro-optic sampling to a time-domain quantum tomography technique. Our proposed framework implements detection of the time evolution of both the electric-field of a propagating electromagnetic wave and its Hilbert transform (quadrature). Direct detection of either quadrature is not strictly possible in the time-domain, detection efficiency approaching zero when an exact mode-matching to either quadrature is reached. As all real signals have a limited bandwidth, we can trace out the irrelevant sampling bandwidth to optimize the detection efficiency while preserving quantum information of the relevant signal. Through the developed understanding of the mode structure of the amplitude and Hilbert transform quadratures, we propose multiplexing and mode-matching operations on the gating function to extract full quantum information on both quantities, simultaneously. The proposed methology is poised to open a novel path toward quantum state tomography and quantum spectroscopy directly in the time domain.Comment: 9 pages, 7 figure

Nonlinear acousto-magneto-plasmonics

We review the recent progress in experimental and theoretical research of interactions between the acoustic, magnetic and plasmonic transients in hybrid metal-ferromagnet multilayer structures excited by ultrashort laser pulses. The main focus is on understanding the nonlinear aspects of the acoustic dynamics in materials as well as the peculiarities in the nonlinear optical and magneto-optical response. For example, the nonlinear optical detection is illustrated in details by probing the static magneto-optical second harmonic generation in gold-cobalt-silver trilayer structures in Kretschmann geometry. Furthermore, we show experimentally how the nonlinear reshaping of giant ultrashort acoustic pulses propagating in gold can be quantified by time-resolved plasmonic interferometry and how these ultrashort optical pulses dynamically modulate the optical nonlinearities. The effective medium approximation for the optical properties of hybrid multilayers facilitates the understanding of novel optical detection techniques. In the discussion we highlight recent works on the nonlinear magneto-elastic interactions, and strain-induced effects in semiconductor quantum dots.Comment: 30 pages, 12 figures, to be published as a Topical Review in the Journal of Optic

Precise Determination of Minimum Achievable Temperature for Solid-State Optical Refrigeration

We measure the minimum achievable temperature (MAT) as a function of excitation wavelength in anti-Stokes fluorescence cooling of high purity Yb3+-doped LiYF4 (Yb:YLF) crystal. Such measurements were obtained by developing a sensitive noncontact thermometry that is based on a two-band differential luminescence spectroscopy using balanced photo-detectors. These measurements are in excellent agreement with the prediction of the laser cooling model and identify MAT of 110 K at 1020 nm, corresponding to E4-E5 Stark manifold transition in Yb:YLF crystal.Comment: 10 pages, 6 figure

Subcycle squeezing of light from a time flow perspective

Light as a carrier of information and energy plays a fundamental role in both general relativity and quantum physics, linking these areas that are still not fully compliant with each other. Its quantum nature and spatio-temporal structure are exploited in many intriguing applications ranging from novel spectroscopy methods of complex many-body phenomena to quantum information processing and subwavelength lithography. Recent access to subcycle quantum features of electromagnetic radiation promises a new class of time-dependent quantum states of light. Paralleled with the developments in attosecond science, these advances motivate an urgent need for a theoretical framework that treats arbitrary wave packets of quantum light intrinsically in the time domain. Here, we formulate a consistent time domain theory of the generation and sampling of few-cycle and subcycle pulsed squeezed states, allowing for a relativistic interpretation in terms of induced changes in the local flow of time. Our theory enables the use of such states as a resource for novel ultrafast applications in quantum optics and quantum information.Comment: 24 pages, 7 figures (including supplementary information

Observation of the spectral bifurcation in the Fractional Nonlinear Schr\"{o}dinger Equation

We report a comprehensive investigation and experimental realization of spectral bifurcations of ultrafast soliton pulses. These bifurcations are induced by the interplay between fractional group-velocity dispersion and Kerr nonlinearity (self-phase modulation) within the framework of the fractional nonlinear Schr\"{o}dinger equation. To capture the dynamics of the pulses under the action of the fractional dispersion and nonlinearity, we propose an effective `force' model based on the frequency chirp, which characterizes their interactions as either `repulsion', `attraction', or `equilibration'. By leveraging the `force' model, we design segmented fractional dispersion profiles that directly generate spectral bifurcations \{1\}$\rightarrow$ \{N\} at relevant nonlinearity levels. These results extend beyond the traditional sequence of bifurcations \{1\}$\rightarrow$ \{2\}$\rightarrow$ \{3\} ... $\rightarrow$ \{N\} associated with the growth of the nonlinearity. The experimental validation involves a precisely tailored hologram within a pulse shaper setup, coupled to an alterable nonlinear medium. Notably, we achieve up to N=5 in \{1\}$\rightarrow$ \{N\} bifurcations at a significantly lower strength of nonlinearity than otherwise would be required in a sequential cascade. The proposal for engineering spectral bifurcation patterns holds significant potential for ultrafast signal processing applications. As a practical illustration, we employ these bifurcation modes to optical data squeezing and transmitting it across a 100-km-long single-mode fiber.Comment: 16 pages and 5 figures; welcome comment