Tran Trung Luu

Synthesized Light Transients

Light spanning the near infrared to the ultraviolet has been confined in pulses shorter than a single optical cycle. Manipulation of electron dynamics calls for electromagnetic forces that can be confined to and controlled over sub-femtosecond time intervals. Tailored transients of light fields can provide these forces. We report on the generation of subcycle field transients spanning the infrared, visible, and ultraviolet frequency regimes with a 1.5-octave three-channel optical field synthesizer and their attosecond sampling. To demonstrate applicability, we field-ionized krypton atoms within a single wave crest and launched a valence-shell electron wavepacket with a well-defined initial phase. Half-cycle field excitation and attosecond probing revealed fine details of atomic-scale electron motion, such as the instantaneous rate of tunneling, the initial charge distribution of a valence-shell wavepacket, the attosecond dynamic shift (instantaneous ac Stark shift) of its energy levels, and its few-femtosecond coherent oscillations.

Extreme ultraviolet high-harmonic spectroscopy of solids

Extreme ultraviolet (EUV) high-harmonic radiation emerging from laser-driven atoms, molecules or plasmas underlies powerful attosecond spectroscopy techniques and provides insight into fundamental structural and dynamic properties of matter. The advancement of these spectroscopy techniques to study strong-field electron dynamics in condensed matter calls for the generation and manipulation of EUV radiation in bulk solids, but this capability has remained beyond the reach of optical sciences. Recent experiments and theoretical predictions paved the way to strong-field physics in solids by demonstrating the generation and optical control of deep ultraviolet radiation in bulk semiconductors, driven by femtosecond mid-infrared fields or the coherent up-conversion of terahertz fields to multi-octave spectra in the mid-infrared and optical frequencies. Here we demonstrate that thin films of SiO2 exposed to intense, few-cycle to sub-cycle pulses give rise to wideband coherent EUV radiation extending in energy to about 40 electronvolts. Our study indicates the association of the emitted EUV radiation with intraband currents of multi-petahertz frequency, induced in the lowest conduction band of SiO2. To demonstrate the applicability of high-harmonic spectroscopy to solids, we exploit the EUV spectra to gain access to fine details of the energy dispersion profile of the conduction band that are as yet inaccessible by photoemission spectroscopy in wide-bandgap dielectrics. In addition, we use the EUV spectra to trace the attosecond control of the intraband electron motion induced by synthesized optical transients. Our work advances lightwave electronics in condensed matter into the realm of multi-petahertz frequencies and their attosecond control, and marks the advent of solid-state EUV photonics.

Optical attosecond pulses and tracking the nonlinear response of bound electrons.

The time it takes a bound electron to respond to the electromagnetic force of light sets a fundamental speed limit on the dynamic control of matter and electromagnetic signal processing. Time-integrated measurements of the nonlinear refractive index of matter indicate that the nonlinear response of bound electrons to optical fields is not instantaneous; however, a complete spectral characterization of the nonlinear susceptibility tensors—which is essential to deduce the temporal response of a medium to arbitrary driving forces using spectral measurements—has not yet been achieved. With the establishment of attosecond chronoscopy, the impulsive response of positive-energy electrons to electromagnetic fields has been explored through ionization of atoms and solids by an extreme-ultraviolet attosecond pulse or by strong near-infrared fields. However, none of the attosecond studies carried out so far have provided direct access to the nonlinear response of bound electrons. Here we demonstrate that intense optical attosecond pulses synthesized in the visible and nearby spectral ranges allow sub-femtosecond control and metrology of bound-electron dynamics. Vacuum ultraviolet spectra emanating from krypton atoms, exposed to intense waveform-controlled optical attosecond pulses, reveal a finite nonlinear response time of bound electrons of up to 115 attoseconds, which is sensitive to and controllable by the super-octave optical field. Our study could enable new spectroscopies of bound electrons in atomic, molecular or lattice potentials of solids, as well as light-based electronics operating on sub-femtosecond timescales and at petahertz rates.

Invited Article: Attosecond photonics: Synthesis and control of light transients

Ultimate control over light entails the capability of crafting its field waveform. Here, we detail the technological advances that have recently permitted the synthesis of light transients confinable to less than a single oscillation of its carrier wave and the precise attosecond tailoring of their fields. Our work opens the door to light field based control of electrons on the atomic, molecular, and mesoscopic scales.

Multi-petahertz electronic metrology

The frequency of electric currents associated with charge carriers moving in the electronic bands of solids determines the speed limit of electronics and thereby that of information and signal processing. The use of light fields to drive electrons promises access to vastly higher frequencies than conventionally used, as electric currents can be induced and manipulated on timescales faster than that of the quantum dephasing of charge carriers in solids. This forms the basis of terahertz (1012 hertz) electronics in artificial superlattices, and has enabled light-based switches and sampling of currents extending in frequency up to a few hundred terahertz. Here we demonstrate the extension of electronic metrology to the multi-petahertz (1015 hertz) frequency range. We use single-cycle intense optical fields (about one volt per ångström) to drive electron motion in the bulk of silicon dioxide, and then probe its dynamics by using attosecond (10−18 seconds) streaking to map the time structure of emerging isolated attosecond extreme ultraviolet transients and their optical driver. The data establish a firm link between the emission of the extreme ultraviolet radiation and the light-induced intraband, phase-coherent electric currents that extend in frequency up to about eight petahertz, and enable access to the dynamic nonlinear conductivity of silicon dioxide. Direct probing, confinement and control of the waveform of intraband currents inside solids on attosecond timescales establish a method of realizing multi-petahertz coherent electronics. We expect this technique to enable new ways of exploring the interplay between electron dynamics and the structure of condensed matter on the atomic scale.

Nonlinear absorbance in dielectric multilayers

Within the last two decades dispersive dielectric multilayer mirrors (DMs), also known as chirped mirrors (CMs), have played a significant role in the progress of ultrafast science. Their ability to manipulate the phase of a light pulse has advanced the synthesis of intense femtosecond optical pulses followed by remarkable progress in the disciplines of nonlinear optics. Meanwhile, the performance of the mirrors themselves has been strictly limited to the linear regime, as essential mirror characteristics such as reflectance, transmittance, and dispersion are evaluated with only intensity-independent values of refractive indices and extinction coefficients taken into the design formalism. Here, we report, to the best of our knowledge, the first observation of a strong nonlinear response of the DMs. We have found that the DM’s multilayer stack causes very significant enhancement of the internal electric field that becomes sufficient to enable third-order nonlinearity. Remarkably, in our particular case, the response is solely emerging in the form of nonlinear absorbance. By modifying the multilayer structure of the mirror, we gained control over observed nonlinearity and were able to predict and to some extent to tune the magnitude of the response, without perturbing the dispersive properties of the DMs. This demonstration not only expands the functionality of DMs into the nonlinear domain, but also marks a new approach to the development of multilayer coatings for applications in ultrafast science.

Nonlinear behavior and damage of dispersive multilayer optical coatings induced by two-photon absorption

We have observed and studied a nonlinear response of dispersive dielectric multilayer mirrors (DM). It was found that the structure of the mirror itself causes strong enhancement of the electric field inside the multilayer stack consequently triggering strong two-photon absorption (2PA). We have developed a mathematical model, that allows estimation of the coefficient of the 2PA, β, subsequent prediction and to some extent tuning of the strength of the nonlinear response of any multilayer coating.

Isolated optical attosecond pulses

We report on the precise synthesis and control of intense isolated attosecond pulses in optical frequencies and the use of them for unprecedented controls of electrons in atoms and molecules.

Efficient broadband highly dispersive HfO_2/SiO_2 multilayer mirror for pulse compression in near ultraviolet

We report on design, production and implementation of a highly dispersive broadband dielectric multilayer mirror covering near ultraviolet range from 290 nm to 350 nm. The described mirrors, having 92% spectrally averaged reflectance in the ultraviolet range and ∼ 85 fs of group delay difference, that allow compression to ∼ 7 fs, provide a strong foundation for generation of few-fs pulses in the near ultraviolet.

Synthesis of isolated optical attosecond pulses

We report on the synthesis and precise control of isolated, intense attosecond pulses in optical frequencies. We use them to explore new control strategies of electrons in atoms and materials.