We demonstrate optical coherent control of the emission direction of THz radiation. Femtosecond laser pulses are used to excite different types of ultrafast photocurrents along different directions in a bulk GaAs sample. The overall emission pattern can be modified by changing the phase of the optical excitation. With this method, THz beam steering of about 8 degrees is realized. A simple dipole-based model allows us to relate the size of the steering effect to the amplitude ratio between the different photocurrent contributions and to diffraction effects resulting from the excitation spot size.
A full (2$π$) phase modulation is critical for efficient wavefront manipulation. In this article, a metasurface based on graphene long/short-strip resonators is used to implement a dynamic 2$π$ phase modulation by applying different voltages to different graphene resonators. The configuration is found to have high reflection efficiency (minimum 56%) and has a full phase modulation in a wide frequency range. Terahertz (THz) beam steering as large as 120 degrees ($\pm60^\circ$) is demonstrated in a broad frequency range (1.2 to 1.9 THz) by changing the Fermi levels of different graphene resonato…
Antiferromagnetic materials have been proposed as new types of narrowband THz spintronic devices owing to their ultrafast spin dynamics. Manipulating coherently their spin dynamics, however, remains a key challenge that is envisioned to be accomplished by spin-orbit torques or direct optical excitations. Here, we demonstrate the combined generation of broadband THz (incoherent) magnons and narrowband (coherent) magnons at 1 THz in low damping thin films of NiO/Pt. We evidence, experimentally and through modelling, two excitation processes of magnetization dynamics in NiO, an off-resonant insta…
Coherent control is an optical technique to manipulate quantum states of matter. The coherent control of 40-THz optical phonons in diamond was demonstrated by using a pair of sub-10-fs optical pulses. The optical phonons were detected via transient transmittance using a pump and probe protocol. The optical and phonon interferences were observed in the transient transmittance change and its behavior was well reproduced by quantum mechanical calculations with a simple model which consists of two electronic levels and shifted harmonic oscillators.
Generation and characterization of coherent terahertz radiation from 100-TW laser-wakefield acceleration
Institute for Basic Science（基础科学研究院）；Gwangju Institute of Science and Technology（光州科学技术院）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器中心）；University of Maryland, College Park（马里兰大学帕克分校）
Selective enhancement of coherent optical phonons in Bi-Sb mixed crystals has been performed using terahertz (THz) rate pulse trains generated by spatial phase modulation of femtosecond pulse with a liquid-crystal spatial light modulator. The profile of the shaped pulse train has a flat-toped shape. The Bi-Bi, Bi-Sb, and Sb-Sb vibrations can be enhanced and canceled selectively for pulse trains with a suitable repetition rate.
By combining a tilted-pulse-intensity-front scheme using a LiNbO3 crystal and a chirped-pulse-beating method, we generated a narrowband intense terahertz (THz) pulse, which had a maximum electric field of more than 10 kV/cm at around 2 THz, a bandwidth of about 50 GHz,and frequency tunability from 0.5 to 2 THz. By performing THz-pump and near-infrared-probe experiments on GaAs quantum wells, we observed that the resonant excitation of the intraexcitonic 1s-2p transition induces a clear and large Autler-Townes splitting. Our time-resolved measurements show that the splitting energy observed in …
Ultrafast charge transport in strongly biased semiconductors is at the heart of highspeed electronics, electro-optics, and fundamental solid-state physics. Intense light pulses in the terahertz (THz) spectral range have opened fascinating vistas: Since THz photon energies are far below typical electronic interband resonances, a stable electromagnetic waveform may serve as a precisely adjustable bias. Novel quantum phenomena have been anticipated for THz amplitudes reaching atomic field strengths. We exploit controlled THz waveforms with peak fields of 72 MV/cm to drive coherent interband polar…
We present both chip-scale and free space coherent arrays of continuous-wave THz photomixers. By altering the relative phases of the exciting laser signals, the relative THz phase between the array elements can be tuned, allowing for beam steering. The constructive interference of the emission of N elements leads to an increase of the focal intensity by a factor of NxN while reducing the beam width by ~1/N, below the diffraction limit of a single source. Such array architectures strongly improve the THz power distribution for stand-off spectroscopy and imaging systems while providing a huge ba…
The transition frequency between nonthermal coherent magnetic precessions and ultrafast demagnetization is arguably the most sought after answer in magnetism science and technology nowadays. So far, it is believed to be in the terahertz (THz) range. Here, using an ultra-intense low frequency THz bullet, and thin magnetic layers, we report on experimental evidences that fully coherent nonthermal THz magnetic switching may never be reachable in conventional ferromagnetic thin films. At high excitation intensities, while the spins still coherently precess with the THz magnetic field, the deposite…
A portion of the electromagnetic wave spectrum between $\sim 0.1$ and $\sim 10$ terahertz (THz) suffers from the lack of powerful, effective, easy-to-use and inexpensive emitters, detectors and mixers. We propose a multilayer graphene -- boron-nitride heterostructure which is able to emit radiation in the frequency range $\sim 0.1-30$ THz with the power density up to $\sim 0.5$ W/cm$^2$ at room temperature. The proposed device is extremely thin, light, flexible, almost invisible and may completely cover the needs of science and technology in the sources of terahertz radiation.
We propose an all-optical anti-waveguide mechanism for steering, splitting, and cloning of an optical beam beyond the diffraction-limit. We use a spatially inhomogeneous pump beam to create an anti-waveguide structure in a Doppler broadened N -type four-level Raman gain medium for a co-propagating weak probe beam. We show that a transverse modulated index of refraction and gain due to the spatially dependent pump beam hold the keys to steering, splitting and cloning of an optical beam. We have also shown that an additional control field permits the propagation of an optical beam through an oth…
Perfect absorption of radiation in a graphene sheet may play a pivotal role in the realization of technologically relevant optoelectronic devices. In particular, perfect absorption of radiation in the terahertz (THz) spectral range would tremendously boost the utility of graphene in this difficult range of photon energies, which still lacks cheap and robust devices operating at room temperature. In this work we show that unpatterned graphene flakes deposited on appropriate substrates can display gate-tunable coherent perfect absorption (CPA) in the THz spectral range. We present theoretical es…
High-order nonlinear light-matter interactions in gases enable generation of x-ray and attosecond light pulses, metrology, and spectroscopy. Optical nonlinearities in solid-state materials are particularly interesting for combining optical and electronic functions for high-bandwidth information processing. Third-order nonlinear optical processes in silicon have been used to process optical signals with greater than 1 GHz bandwidths. Fundamental physical processes for a Si-based optical modulator in the THz bandwidth range, however, have not yet been explored. Here we demonstrate ultrafast phon…
We report the coherent generation and detection of terahertz (THz) pulses covering the bandwidth of 0.1-9 THz in a high repetition rate, low pulse energy laser system. In this work we demonstrate the application and evaluation of solid-state biased coherent detection in combination with a spintronic emitter. This combination was used to generate and detect THz pulses in a time-domain spectroscopy (TDS) setup and tested on bulk nonlinear crystals. These results establish a new promising candidate to extend the possibilities for compact, broadband THz TDS systems driven by high repetition rate l…
In nonlinear microresonators driven by continuous-wave (cw) lasers, Turing patterns have been studied in the formalism of Lugiato-Lefever equation with emphasis on its high coherence and exceptional robustness against perturbations. Destabilization of Turing pattern and transition to spatio-temporal chaos, however, limits the available energy carried in the Turing rolls and prevents further harvest of their high coherence and robustness to noise. Here we report a novel scheme to circumvent such destabilization, by incorporating the effect of local mode hybridizations, and attain globally stabl…
A new type of the THz laser is proposed. A coherent tera-hertz light is emitted through the backward Raman scattering between a visible light laser and a relativistic electron beam. The threshold conditions for the laser intensity and the electron beam density are identified. The scheme may lead to one of the most intense tera-hertz coherent light sources.
We report a THz reflectarray metasurface which uses graphene as active element to achieve beam steering, shaping and broadband phase modulation. This is based on the creation of a voltage controlled reconfigurable phase hologram, which can impart different reflection angles and phases to an incident beam, replacing bulky and fragile rotating mirrors used for terahertz imaging. This can also find applications in other regions of the electromagnetic spectrum, paving the way to versatile optical devices including light radars, adaptive optics, electro-optical modulators and screens.
The unusual features of quantum mechanics are enabling the development of technologies not possible with classical physics. These devices utilize nonclassical phenomena in the states of atoms, ions, and solid-state media as the basis for many prototypes. Here we investigate molecular states as a distinct alternative. We demonstrate a memory for light based on storing photons in the vibrations of hydrogen molecules. The THz-bandwidth molecular memory is used to store 100-fs pulses for durations up to 1ns, enabling 10,000 operational time bins. The results demonstrate the promise of molecules fo…
I derive simple non-local dynamic boundary conditions, suitable for modelling of radiation emission from stacked Josephson junctions, and employ them for analysis of flux-flow emission from intrinsic Josephson junctions in high-$T_c$ superconductors. It is shown that due to the lack of Lorenz contraction of fluxons in stacked junctions, high quality geometrical resonances are prerequisite for high power emission from the stack. This leads to a dual role of the radiative impedance: on the one hand, small impedance increases the efficiency of emission from the stack, on the other hand, enhanced …
Coherent continuous wave (CW) terahertz spectroscopy is an extremely valuable technique that allows for the interrogation of systems that exhibit narrow resonances in the terahertz (THz) frequency range, such as high-quality (high-Q) THz whispering-gallery mode resonators. Unfortunately, common implementations are dramatically impaired by deficiencies in the used data analysis schemes. Here, we show that the physics of the problem presents an elegant solution whose full potential has remained overlooked until now. We argue that, thanks to the causality of physical systems, Hilbert transformati…
Coherent terahertz field tomographic imaging in warm Rydberg vapors
Faculty of Physics, University of Warsaw（华沙大学物理学院）；Centre for Quantum Optical Technologies, Centre of New Technologies, University of Warsaw（华沙大学新技术中心量子光学技术研究中心）
We study coherent and incoherent interactions between discrete vortex and fundamental solitons in two-dimensional photonic lattices, presenting a new scheme for all-optical routings and topological transformations of vorticities. Due to the multi-lobe intensity and step-phase structure of the discrete vortex soliton, the coherent soliton-interactions allow both solitons to be steered into multiple different possible destination ports depending on the initial phase of the discrete fundamental soliton. We show that charge-flipping of phase singularities and orbital angular momentum transfer can …
Future information technology demands ultimately fast, low-loss quantum control. Intense light fields have facilitated important milestones, such as inducing novel states of matter, accelerating electrons ballistically, or coherently flipping the valley pseudospin. These dynamics leave unique signatures, such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute - the spin - between two states separated by a potential barrier is to trigger an all-coherent precession. Pioneering experi…
Coherent radiation with frequencies ranging from 0.3 to 30 THz has recently become accessible by femtosecond laser technology. Terahertz (THz) waves have already found many applications in spectroscopy and imaging, and they can be manipulated using static optical elements such as lenses, polarizers, and filters. However, ultrafast modulation of THz radiation is required as well, for instance in short-range wireless communication or for preparing shaped THz transients for coherent control of numerous material excitations. Here, we demonstrate an all-optically created transient metamaterial that…
Using a microscopic theory based on the Maxwell-semiconductor Bloch equations, we investigate the possibility of an optically-assisted electrically-driven THz quantum cascade laser. Whereas in optical conversion schemes the power conversion efficiency is limited by the Manley-Rowe relation, the proposed optically-assisted scheme can achieve higher efficiency by coherently recovering the optical pump energy. Furthermore, due to quantum coherence effects the detrimental effects of scattering are mitigated.
We demonstrate a coherent imaging system based on a terahertz (THz) frequency quantum cascade laser (QCL) phase-locked to a near-infrared fs-laser comb. The phase locking enables coherent electro-optic sampling of the continuous-wave radiation emitted by the QCL through the generation of a heterodyne beat-note signal. We use this beat-note signal to demonstrate raster scan coherent imaging using a QCL emitting at 2.5 THz. At this frequency the detection noise floor of our system is of 3 pW/Hz and the long-term phase stability is <3 degrees/h, limited by the mechanical stability of the apparatu…
The dream of physico-chemists to control molecular reactions with light beyond electronic excitations pushes the development of laser pulse shaping capabilities in the mid-infrared (MIR) spectral range. Here, we present a compact optical parametric amplifier platform for the generation and shaping of MIR laser pulses in the wavelength range between $8\,μ$m and $15\,μ$m. Opportunities for judiciously tailoring the electromagnetic waveform are investigated, demonstrating light field control with a spectral resolution of 60 GHz at a total spectral bandwidth of 5 THz. In experiments focusing on sp…
The ability to dynamically steer fs pulses from a monolithically integrated source is a critical milestone for the fields of nanophotonics and ultrafast optics. The nascent field of reconfigurable metasurfaces -- made of optically resonant meta-atoms -- has shown great promise in manipulating light-matter interactions through subwavelength control of the phase, amplitude, and polarization of light. These active metasurfaces arbitrarily transform an incident wavefront using a reconfigurable spatial phase profile and thus have been limited to manipulating coherent external sources. Light emittin…
Free space delay lines provide pulses of variable time spacing for optical experiments such as pump-probe spectroscopy and coherent quantum control, including spin and photon echo techniques. However, in the terahertz (THz) region of the spectrum, beam divergence due to diffraction limits the useful length of traditional free space delay lines. We present a novel double-folded variable delay line for light in the frequency range 0.24-1.2 THz, which incorporates a symmetric arrangement of lenses whose spacing can be adjusted to compensate for diffraction at each delay. Scalable for use in other…
Optoelectronic signal processing offers great potential for generation and detection of ultra-broadband waveforms in the THz range, so-called T-waves. However, fabrication of the underlying high-speed photodiodes and photoconductors still relies on complex processes using dedicated III-V semiconductor substrates. This severely limits the application potential of current T-wave transmitters and receivers, in particular when it comes to highly integrated systems that combine photonic signal processing with optoelectronic conversion to THz frequencies. In this paper, we demonstrate that these lim…
Graphene-enabled coherent (sub-)terahertz wave detection and thickness determination
Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST（加泰罗尼亚纳米科学和纳米技术研究所（ICN2））；ICFO - Institut de Ciències Fotòniques, The Barcelona Institute of Science and Technology（光子科学研究所（ICFO），巴塞罗那科学技术研究院）；Department of Materials Science and Engineering, University of Ioannina（约阿尼纳大学材料科学与工程系）；Research Center for Electronic and Optical Materials, National Institute for Materials Science（国家物质科学研究所电子与光学材料研究中心）；Research Center for Materials Nanoarchitectonics, National Institute for Materials Science（国家物质科学研究所材料纳米构筑学研究中心）；University Research Center of Ioannina (URCI), Institute of Materials Science and Computing（约阿尼纳大学研究中心（URCI），材料科学与计算研究所）
We propose a method to generate isolated relativistic terahertz (THz) pulses using a high-power laser irradiating a mirco-plasma-waveguide (MPW). When the laser pulse enters the MPW, high-charge electron bunches are produced and accelerated to ~ 100 MeV by the transverse magnetic modes. A substantial part of the electron energy is transferred to THz emission through coherent diffraction radiation as the electron bunches exit the MPW. We demonstrate this process with three-dimensional particle-in-cell simulations. The frequency of the radiation is determined by the incident laser duration, and …
We report the first coherent excitation of intramolecular vibrational modes via the nonlinear interaction of a TeraHertz (THz) light field with molecular liquids. A terahertz-terahertz-Raman pulse sequence prepares the coherences with a broadband, high-energy, (sub)picosecond terahertz pulse, that are then measured in a terahertz Kerr effect spectrometer via phase-sensitive, heterodyne detection with an optical pulse. The spectrometer reported here has broader terahertz frequency coverage, and an increased sensitivity relative to previously reported terahertz Kerr effect experiments. Vibration…
As a key component for next-generation wireless communications (6G and beyond), terahertz (THz) electronic oscillators are being actively developed. Precise and dynamic phase control of ultrafast THz waveforms is essential for high-speed beam steering and high-capacity data transmission. However, measurement of such ultrafast dynamic process is beyond the scope of electronics due to the limited bandwidth of the electrical measuring instruments. Here we surpass this limit by applying photonic technology. Using a femtosecond laser, we generate offset-free THz pulses to phase-lock the electronic …
We report line-by-line control of a coherent discrete spectrum (Raman sidebands) with a frequency spacing of 10.6 THz that is produced by an adiabatic Raman process. We show that the spectral phase of the Raman sidebands is finely controlled to the target (flat relative-spectral-phase). This is achieved by employing a combination of a spatial phase controller and a spectral interferometer, which are specifically designed for a high-power discrete spectrum. We also show that such spectral-phase control produces a train of Fourier transform limited pulses with an ultrahigh repetition rate of 10.…
The growth of 3D imaging across a range of sectors has driven a demand for high performance beam steering techniques. Fields as diverse as autonomous vehicles and medical imaging can benefit from a high speed, adaptable method of beam steering. We present a monolithic, sub-microsecond electro-optic switch as a solution satisfying the need for reliability, speed, dynamic addressability and compactness. Here we demonstrate a laboratory-scale, solid-state lidar pointing system, using the electro-optic switch to launch modulated coherent light into free space, and then to collect the reflected sig…
Excitation of coherent optical phonons in solids provides a pathway for ultrafast modulation of light on a sub-ps timescale. Here, we report on efficient 3.6 THz modulation of light reflected from hybrid metal/semiconductor plasmonic crystals caused by lattice vibrations in a few nm thick layer of elemental tellurium. We observe that surface plasmon polaritons contribute significantly to photoinduced formation of this thin layer at the interface between a telluride-based II-VI semiconductor, such as (Cd,Mg)Te or (Cd,Mn)Te, and a one-dimensional gold grating. The change in interface composition…
We present an extended study concerning the dynamics of dense and harmonic coherent regimes in quantum cascade lasers (QCLs) in a Fabry--Perot (FP) configuration emitting in the terahertz (THz) spectral region. Firstly, we study the device in free running operation, reproducing the main features of the self-generated of optical frequency combs (OFCs) and harmonic frequency combs (HFCs) in this spectral range, commenting on the points of difference from the mid-infrared region, and finding excellent agreement with the most recent experimental evidences. Then, we analyze the THz-QCL dynamics und…
