Single Optical Cycle Research Articles

Time-refraction optics with single cycle modulation

Abstract We present an experimental study of optical time-refraction caused by time-interfaces as short as a single optical cycle. Specifically, we study the propagation of a probe pulse through a sample undergoing a large refractive index change induced by an intense modulator pulse. In these systems, increasing the refractive index abruptly leads to time-refraction where the spectrum of all the waves propagating in the medium is red-shifted, and subsequently blue-shifted when the refractive index relaxes back to its original value. We observe these phenomena in the single-cycle regime. Moreover, by shortening the temporal width of the modulator to ∼5–6 fs, we observe that the rise time of the red-shift associated with time-refraction is proportionally shorter. The experiments are carried out in transparent conducting oxides acting as epsilon-near-zero materials. These observations raise multiple questions on the fundamental physics occurring within such ultrashort time frames, and open the way for experimenting with photonic time-crystals, generated by periodic ultrafast changes to the refractive index, in the near future.

Build-up and dephasing of Floquet-Bloch bands on subcycle timescales.

Strong light fields have created opportunities to tailor novel functionalities of solids1-5. Floquet-Bloch states can form under periodic driving of electrons and enable exotic quantum phases6-15. On subcycle timescales, lightwaves can simultaneously drive intraband currents16-29 and interband transitions18,19,30,31, which enable high-harmonic generation16,18,19,21,22,25,28-30 and pave the way towards ultrafast electronics. Yet, the interplay of intraband and interband excitations and their relation to Floquet physics have been key open questions as dynamical aspects of Floquet states have remained elusive. Here we provide this link by visualizing the ultrafast build-up of Floquet-Bloch bands with time-resolved and angle-resolved photoemission spectroscopy. We drive surface states on a topological insulator32,33 with mid-infrared fields-strong enough for high-harmonic generation-and directly monitor the transient band structure with subcycle time resolution. Starting with strong intraband currents, we observe how Floquet sidebands emerge within a single optical cycle; intraband acceleration simultaneously proceeds in multiple sidebands until high-energy electrons scatter into bulk states and dissipation destroys the Floquet bands. Quantum non-equilibrium calculations explain the simultaneous occurrence of Floquet states with intraband and interband dynamics. Our joint experiment and theory study provides a direct time-domain view of Floquet physics and explores the fundamental frontiers of ultrafast band-structure engineering.

Parametric waveform synthesis: a scalable approach to generate sub-cycle optical transients.

The availability of electromagnetic pulses with controllable field waveform and extremely short duration, even below a single optical cycle, is imperative to fully harness strong-field processes and to gain insight into ultrafast light-driven mechanisms occurring in the attosecond time-domain. The recently demonstrated parametric waveform synthesis (PWS) introduces an energy-, power- and spectrum-scalable method to generate non-sinusoidal sub-cycle optical waveforms by coherently combining different phase-stable pulses attained via optical parametric amplifiers. Significant technological developments have been made to overcome the stability issues related to PWS and to obtain an effective and reliable waveform control system. Here we present the main ingredients enabling PWS technology. The design choices concerning the optical, mechanical and electronic setups are justified by analytical/numerical modeling and benchmarked by experimental observations. In its present incarnation, PWS technology enables the generation of field-controllable mJ-level few-femtosecond pulses spanning the visible to infrared range.

Further Development of the Short‐Pulse Petawatt Laser: Trends, Technologies, and Bottlenecks

AbstractThe petawatt (PW) laser has experienced a rapid development in the past two decades, and tens of giant facilities have been constructed worldwide. After realizing 10–100 PW, it seems to be close to some sort of engineering limit but its focused peak intensity still is much lower than the Schwinger limit, and therefore some technology improvements or innovations become indispensable for further increasing the peak power as well as the focused peak intensity. By quick reviewing the development of the PW laser in history, it is shown that reducing the pulse duration to near a single optical cycle is a feasible (easy and cheap) choice for this purpose. Here, technologies for the optical‐cycle pulse generation, ultrabroadband amplification, and capability boosting that aim to provide possible approaches for optical‐cycle PW lasers are briefly reviewed and discussed. Meanwhile, key bottlenecks that challenge the current as well as the future short‐pulse PW lasers and their possible solutions are summarized and discussed. This technology review aims to provide a possible roadmap for the next‐stage development of the short‐pulse PW laser.

Single-cycle-pulse generation in a coherently mode-locked laser with an ultrashort cavity

We theoretically investigate the possibility of a passive mode-locking regime in a two-section laser with an ultrashort cavity of the length around ten wavelengths of the medium resonant transition or even less. It is shown that such a regime is enabled through the coherent mode-locking phenomena arising due to the self-induced transparency when the gain and absorption bandwidths do not impose any restrictions on the duration of the generated pulse. Under such conditions, a laser with an ultrashort cavity could produce output pulses with the terahertz (THz) repetition rate and the duration up to a single optical cycle. Moreover, several scaling rules were derived, which allow for further shortening of the output pulse duration with the respective scaling of the media parameters.

Field-resolved detection of the temporal response of a single plasmonic antenna in the mid-infrared

Unveiling the spatial and temporal dynamics of a light pulse interacting with nanosized objects is of extreme importance to widen our understanding of how photons interact with matter at the nanoscale and trigger physical and photochemical phenomena. An ideal platform to study light–matter interactions with an unprecedented spatial resolution is represented by plasmonics, which enables an extreme confinement of optical energy into sub-wavelength volumes. The ability to resolve and control the dynamics of this energy confinement on the time scale of a single optical cycle is at the ultimate frontier towards a full control of nanoscale phenomena. Here, we resolve in the time domain the linear behavior of a single germanium plasmonic antenna in the mid-infrared by measuring the complex optical field response in amplitude and phase with sub-optical-cycle precision, with the promise to extend the observation of light–matter interactions in the time domain to single quantum objects. Accessing this fundamental information opens a plethora of opportunities in a variety of research areas based on plasmon-mediated photonic processes and their coherent control, such as plasmon-enhanced chemical reactions and energy harvesting.

Giant Isolated Attosecond Pulses from Two-Color Laser-Plasma Interactions.

A new regime in the interaction of a two-color (ω,2ω) laser with a nanometer-scale foil is identified, resulting in the emission of extremely intense, isolated attosecond pulses-even in the case of multicycle lasers. For foils irradiated by lasers exceeding the blow-out field strength (i.e., capable of fully separating electrons from the ion background), the addition of a second harmonic field results in the stabilization of the foil up to the blow-out intensity. This is then followed by a sharp transition to transparency that essentially occurs in a single optical cycle. During the transition cycle, a dense, nanometer-scale electron bunch is accelerated to relativistic velocities and emits a single, strong attosecond pulse with a peak intensity approaching that of the laser field.

A review of recent progress on laser-plasma acceleration at kHz repetition rate

We report on recent progress on laser-plasma acceleration using a low energy and high-repetition rate laser system. Using only few milliJoule laser energy, in conjunction with extremely short pulses composed of a single optical cycle, we demonstrate that the laser-plasma accelerator (LPA) can be operated close to the resonant blowout regime. This results in the production of high charge electron beams (>10 pC) with peaked energy distributions in the few MeV range and relatively narrow divergence angles. We highlight the importance of the plasma density profile and gas jet design for the performance of the LPA. In this extreme regime of relativistic laser-plasma interaction with near-single-cycle laser pulses, we find that the effect of group velocity dispersion and carrier envelope phase can no longer be neglected. These advances bring LPAs closer to real scientific applications in ultrafast probing.

High average power and single-cycle pulses from a mid-IR optical parametric chirped pulse amplifier

In attosecond and strong-field physics, the acquisition of data in an acceptable time demands the combination of high peak power with high average power. We report a 21 W mid-IR optical parametric chirped pulse amplifier (OPCPA) that generates 131 μJ and 97 fs (sub-9-cycle) pulses at a 160 kHz repetition rate and at a center wavelength of 3.25 μm. Pulse-to-pulse stability of the carrier envelope phase (CEP)-stable output is excellent with a 0.33% rms over 288 million pulses (30 min) and compression close to a single optical cycle was achieved through soliton self-compression inside a gas-filled mid-IR antiresonant-guiding photonic crystal fiber. Without any additional compression device, stable generation of 14.5 fs (1.35-optical-cycle) pulses was achieved at an average power of 9.6 W. The resulting peak power of 3.9 GW in combination with the near-single-cycle duration and intrinsic CEP stability makes our OPCPA a key-enabling technology for the next generation of extreme photonics, strong-field attosecond research, and coherent x-ray science.

Generation of monocycle squeezed light in chirped quasi-phase-matched nonlinear crystals

We present a quantum theory of parametric down-conversion of light in chirped quasi-phase-matched second-order nonlinear crystals with undepleted quasi-monochromatic pump. This theory allows us to consider generation of ultrabroadband squeezed states of light and is valid for arbitrary, sufficiently slowly-varying nonlinear poling profiles. Using a first-order approximate quantum solution for the down-converted light field, we calculate the squeezing spectra and the characteristic squeezing angles. We compare the approximate solutions with the exact and numerical ones and find a very good agreement. This comparison validates our approximate solution in the regime of moderate gain, where the existing approaches are not applicable. Our results demonstrate that aperiodically poled crystals are very good candidates for generating ultrabroadband squeezed light with the squeezing bandwidth covering almost all the optical spectrum and the correlation time approaching a single optical cycle.

Improved retrieval of complex supercontinuum pulses from XFROG traces using a ptychographic algorithm.

We demonstrate that time-domain ptychography, a recently introduced iterative ultrafast pulse retrieval algorithm, has properties well suited for the reconstruction of complex light pulses with large time-bandwidth products from a cross-correlation frequency-resolved optical gating (XFROG) measurement. It achieves temporal resolution on the scale of a single optical cycle using long probe pulses and low sampling rates. In comparison to existing algorithms, ptychography minimizes the data to be recorded and processed, and significantly reduces the computational time of the reconstruction. Experimentally, we measure the temporal waveform of an octave-spanning, 3.5 ps long, supercontinuum pulse generated in photonic crystal fiber, resolving features as short as 5.7 fs with sub-fs resolution and 30 dB dynamic range using 100 fs probe pulses and similarly large delay steps.

Self-phase modulation in various regimes of intense laser–plasma interactions

Creation of plasmas by intense laser pulses and consequent laser–plasma interactions involve highly non-linear processes. In particular, a variation of the refractive index induced by the laser action turns into a self-phase modulation (SPM) of the laser field. This effect, already observed with nanosecond laser pulses, achieves striking evidence with ultra-short pulses whose intensity can produce index changes in a time as short as a single optical cycle. In this condition, spectral modifications of the laser pulse produced by SPM may strongly modify the laser–plasma interaction. At the same time, the spectral analysis of the transmitted and scattered laser radiation gives valuable information on a variety of processes occurring in the plasma. In this paper we simply consider a few results, which can be attributed to SPM, with the aim of comparing nanosecond and sub-picosecond laser interaction regimes at moderate laser intensities.

Noncollinear Polarization Gating of Attosecond Pulse Trains in the Relativistic Regime.

High order harmonics generated at relativistic intensities have long been recognized as a route to the most powerful extreme ultraviolet pulses. Reliably generating isolated attosecond pulses requires gating to only a single dominant optical cycle, but techniques developed for lower power lasers have not been readily transferable. We present a novel method to temporally gate attosecond pulse trains by combining noncollinear and polarization gating. This scheme uses a split beam configuration which allows pulse gating to be implemented at the high beam fluence typical of multi-TW to PW class laser systems. Scalings for the gate width demonstrate that isolated attosecond pulses are possible even for modest pulse durations achievable for existing and planned future ultrashort high-power laser systems. Experimental results demonstrating the spectral effects of temporal gating on harmonic spectra generated by a relativistic laser plasma interaction are shown.

Ultrafast Infrared Nanoscopy with Sub-Cycle Temporal Resolution

Ultrafast microscopy of surfaces with simultaneous nanometer spatial resolution and femtosecond temporal resolution can be achieved by coupling ultrafast optical pulses to sharp metal tips [1-10]. In the case of scattering-type near-field scanning optical microscopy (s-NSOM), evanescent fields scattered from the tip apex provide a window into a drastically subwavelength world. Moreover, since these scattered fields can be measured using established far-field technologies, ultrafast infrared pulses emerging from a nanoscale volume can be detected with the best possible time resolution: faster than a single optical oscillation cycle [1].

Octave-spanning spectral phase control for single-cycle bi-photons

The quantum correlation of octave-spanning time–energy entangled bi-photons can be as short as a single optical cycle. Many experiments designed to explore and exploit this correlation require a uniform spectral phase (transform-limited) with very low loss. So far, transform-limited single-cycle bi-photons have not been demonstrated, primarily due to the lack of precise, broadband control of their spectral phase. Here, we demonstrate the correction of the spectral-phase of near-octave spanning bi-photons to over an octave in frequency –2600 nm). Using a prism-pair with an effectively negative separation for shaping the bi-photons’ spectral phase, we obtain a tuned, very low-loss compensation of both the second and fourth dispersion orders. An essential requisite for precise tuning over such a broad bandwidth is a measure of the spectral phase that provides feedback for the tuning even when the overall dispersion is far from compensated. This is achieved by a non-classical bi-photon interference, which enables direct verification of the corrected bi-photon spectral phase.

Subcycle transient structures in time-dependent multiphoton-ionization rates

We perform an ab initio and accurate exploration of the subcycle transient multiphoton-ionization dynamics of atomic and molecular systems subject to intense near-infrared laser fields on the subfemtosecond time scale. Multiple-ionization bursts within a single optical cycle are found in the time-dependent ionization rates not only for diatomic molecules ${{\mathrm{H}}_{2}}^{+}$ and ${\mathrm{HHe}}^{2+}$, but also for the hydrogen atom. The analysis of the electron density reveals that several distinct density portions can be shaped and detached from the target within a half cycle of the laser field.

Nanostructured ultrafast silicon-tip optical field-emitter arrays.

Femtosecond ultrabright electron sources with spatially structured emission are an enabling technology for free-electron lasers, compact coherent X-ray sources, electron diffractive imaging, and attosecond science. In this work, we report the design, modeling, fabrication, and experimental characterization of a novel ultrafast optical field emission cathode comprised of a large (>100,000 tips), dense (4.6 million tips·cm(-2)), and highly uniform (<1 nm tip radius deviation) array of nanosharp high-aspect-ratio silicon columns. Such field emitters offer an attractive alternative to UV photocathodes while providing a direct means of structuring the emitted electron beam. Detailed measurements and simulations show pC electron bunches can be generated in the multiphoton and tunneling regime within a single optical cycle, enabling significant advances in electron diffractive imaging and coherent X-ray sources on a subfemtosecond time scale, not possible before. At high charge emission yields, a slow rollover in charge is explained as a combination of the onset of tunneling emission and the formation of a virtual cathode.

Sub-cycle slicing of phase-locked and intense mid-infrared transients

We demonstrate sub-cycle manipulation of mid-infrared optical waveforms in the time domain. This goal is accomplished via efficient reflection at a semiconductor surface induced by femtosecond interband excitation. The ultrafast response of this process allows slicing of high-field multi-terahertz transients down to the single optical cycle. Ultrabroadband and phase-stable transients with peak amplitudes beyond 10 MV cm−1 are obtained, paving the way for efficient coherent control of light–matter interaction in the non-perturbative regime. The microscopic analysis of electron–hole plasma generation in germanium reveals a decisive role of two-photon absorption allowing efficient slicing up to mid-infrared frequencies.

The generation, characterization and applications of broadband isolated attosecond pulses

The generation of extremely short isolated attosecond pulses requires both a broad spectral bandwidth and control of the spectral phase. Rapid progress has been made in both aspects, leading to the generation of light pulses as short as 67 as in 2012, and broadband attosecond continua covering a wide range of extreme ultraviolet and soft X-ray wavelengths. Such pulses have been successfully applied in photoelectron and photoion spectroscopy and recently developed attosecond transient absorption spectroscopy to study electron dynamics in matter. In this Review, we discuss significant recent advances in the generation, characterization and applications of ultrabroadband, isolated attosecond pulses with spectral bandwidths comparable to the central frequency. These pulses can in principle be compressed to a single optical cycle. This review discusses significant recent advances in the generation, characterization and application of ultrabroadband isolated attosecond pulses with a spectral bandwidth comparable to the central frequency, which can in principle be compressed to a single optical cycle.

An electrical insulator turns metallic within a femtosecond

When silica is driven by an ultraintense and ultrashort light pulse, its electrical conductivity can rise and fall by 18 orders of magnitude during a single optical cycle.