Successful Generation of 1.7-Cycle Intense Pulses at High Repetition Rate of 1 MHz～towards the next-generation bright attosecond pulse sources～

https://doi.org/10.1364/OL.477372[Japanese]
T. Okamoto1, Y. Kunihashi1, Y. Shinohara1, H. Sanada1, M-C. Chen2, K. Oguri1
1 NTT Basic Research Laboratories, Japan
2 Institute of Photonics Technologies, National Tsing Hua University, Taiwan
Fig. 1. Multiple-plate compressor developed in this study 

Intense pulse sources with a pulse duration of only 1.7 optical cycles has been successfully generated at very high repetition rate*1 (1 MHz = 100,000 times per second, several hundred times faster than conventional rates). It has been realized by applying a technique called "multiple-plate compression*2" to ytterbium (Yb)-based laser pulses, and possesses top-class performance despite its simple setup without large-scale laser systems and somplicated cooling systems. Since it is expected to enable the more convenient and low-cost use of high-repetition, few-cycleintense pulse sources at the MHz level previously only generated in limited large facilities is available for large number of users. Therefore, our work brings impact to various research fields including ultrafast phenomena and high-field physics.

Intense laser pulses have been usable after the emergence of the chirped pulse amplification *3, which was awarded the 2018 Nobel Prize in Physics. Today, such intense pulses are sometimes compressed for further increaing the power, and applied to numerous applications in strong-field physics, with particular emphasis placed on the generation of ultimately short pulses through high harmonic generation*4. These ultimately short pulses are known as attosecond pulses, which flickers for an extremely brief time of attoseconds (one quintillionth of a second, 10-18 seconds) and corresponds to the "flash of a camera" with the shortest exposure time ever possessed by humankind. The Quantum Optical Device Research Group at NTT Basic Research Laboratories has been dedicated to the observation of unexplored ultrafast phenomena using attosecond pulses and possesses world-class techniques in this field. In 2014, we successfully observed dynamic of inner-shell electrons. In 2016 and 2018, they found the highest-frequency response in solid materials.

As described above, few-cycle intense pulses generate attosecond pulses through high-order harmonic generation. However, the conversion efficiency of attosecond pulse generation is very low, ranging from 0.01 to 0.0001%, resulting in a very weak output of attosecond pulses. This low intensity of the attosecond pulses sometimes necessitates measurements lasting more than a day or even a week to obtain a sufficient signal. Conducting measurements under various conditions becomes impractical due to the substantial amount of time required. If this issue can be addressed, it would not only unveil previously hidden ultrafast phenomena obscured by noise, but also pave the way for new optical technologies such as groundbreaking video-rate imaging with the highest frame rate of attosecond.

In this study, we successfully generated intense pulses with a duration of only 1.7 cycles (optical periods) at a MHz repetition rate by compressing pulses from ytterbium (Yb) amplifiers*6. Conventionally, intense pulses are generated using titanium-sapphire amplifiers*7 operating at kHz repetition rates (1,000 pulses per second, 103 Hz). However, in this study, we utilized Yb amplifiers capable of MHz operation. While it offers the advantage of achieving high average power (~100 W), there is a drawback of longer pulse duration compared to titanium-sapphire amplifiers. By compressing the pulse duration, the power within a shorter time can be comcentrated, which opens possibilities for strong-field applications such as attosecond pulse generation. Therefore, we compressed the Yb laser pulses by a solid-based compression technique called "multiple-plate compression" composed of thin glass plates. By effectively using the strong response of the solid materials, we developed a very compact and simple setup compared to the conventional gas-based hollow fiber compressor*8 (see Fig. 2). With this system, we successfully generated intense pulses with a duration of only 1.7 cycles (optical periods), as depicted in Fig. 3. These 1.7-cycle intense pulses can reach an intensity of 710 TW/cm2, exceeding the required intensity (100 TW/cm2) for attosecond pulse generation, thereby demonstrating the applicability of attosecond pulse generation. Our achievement paves the way for the realization of high-repetition-rate attosecond pulse light sources.

The next objective is to develop attosecond pulse generation at MHz repetition rates using this light source, aiming to pioneer groundbreaking attosecond imaging techniques. Additionally, we aim to complement large-scale facilities such as synchrotrons and free-electron lasers by harnessing such bright attosecond pulse sources.

Fig. 2. Pictures of holow-core fiber and multiple-plate compressor. This comparison shows that the multiple-plate compressor has a very compact and simple configuration.

Fig. 3. a) Pulse shape of a 1.7-cycle pulses. (b) The beam profile at a focus. It looks beautifully round, corresponding the high focusing ability.

Terminology

  1. repetition rate
    2. The number of pulse occurrences per second at a fixed interval. For example, if 10 pulses occur in one second, the repetition rate is 10 Hz.
  2. multiple-plate compression
    3. A pulse-compression method achieved by optimally arranging thin transparent solid plates (in this study, glass was used). While the conventional compression method utilizes gases, solid-based compression offers the advantage of a stronger response due to the higher density of atoms in solids, enabling a compact setup.
  3. chirped pulse amplification
    4. A scheme for amplifying pulses. The weak seed pulses are temporarily stretched in time, then their intensity is amplified. Finally, the stretched and amplified pulses are compressed, resulting in intense pulses.
  4. high-harmonic generation
    5. When intense pulses are focused on a medium, such as a noble gas, they generate light with shorter wavelengths that have frequency components that are integer multiples of the fundamental pulses. This phenomenon, which can reach high orders such as several tens of times the frequency of the fundamental pulses, is called "high-harmonic generation."
  5. attosecond pulses
    6. Pulsed light with a duration in the attosecond range (100 quintillionths of a second, 10^-18 seconds), which is generated through high-harmonic generation. These pulses typically have wavelengths ranging from extreme ultraviolet to soft X-ray. By utilizing attosecond lasers as "flash of a camera", we can observe ultrafast phenomena on the attosecond timescale. Attosecond pulses represent the fastest technologies available to us and are extensively studied worldwide.
  6. ytterbium (Yb)-based amplifier
    7. A laser amplifier that uses laser crystals doped with ytterbium (Yb). In this study, an Yb:KGW amplifier was used, which added Yb to potassium gadolinium tungstate crystals (KGW). Its characteristic is efficient generation with reduced heat generation, allowing it to operate at high outputs exceeding 100 W.
  7. titanium-sapphire amplifier
    8. A laser amplifier that has been widely used for a long time to obtain intense pulses. It employs a laser medium consisting of sapphire crystals doped with titanium. The advantage is very short pulse duration, while the limitation of operation at only up to kHz repetition rates.
  8. hollow-core fiber compression
    9. A gas-based pulse compression method that has been extensively utilized. Since gases have relatively weak responses to light, pulses must be introduced into a long hollow fiber enclosing gas to achieve long-distance interaction.
  9. TW/cm2 (Terawatt per square centimeter)
    10. A unit that indicates how much power is concentrated on an area of 1 cm × 1 cm. TW represents terawatts (10^12 or 100 billion) as an SI prefix for power (W, watts). Therefore, 1 TW/cm2 indicates that 100 billion watts of light is concentrated on an area of 1 cm × 1 cm. When irradiating matter with power exceeding 100 TW/cm2, electrons orbiting around atomic nuclei are stripped off. Attosecond light is generated by these stripped-off electrons, making a laser with high intensity exceeding 100 TW/cm2 essential for attosecond pulse generation.

Flying electron spin control gates

https://doi.org/10.1038/s41467-022-32807-x

Paul L. J. Helgers1,3, J. A. H. Stotz1,2, H. Sanada3, Y. Kunihashi3, K. Biermann1, P. V. Santos1

Fig. 1. Flying control gate for electron spins. A surface acoustic wave (SAW) excited by an interdigital acoustic transducer (IDT) aligned to a narrow semiconductor channel creates a train of dot-like moving potentials. Within the channel, a circularly polarized laser beam excites spin-polarized electrons and holes, which are captured by the moving dots and transport them with the SAW velocity, vSAW. Simultaneously, the SAW fields induce an effective magnetic field BSO that acts on the moving spins with a magnitude proportional to the SAW amplitude. By adjusting the SAW amplitude, the spin precession rate and be controlled yielding an electrically driven flying spin gate.

Electrons spins are attractive qubits for the implementation of quantum functionalities in semiconductor devices. In this context, the spin transitor proposed by Datta and Das [Datta and Das, Appl. Phys. Lett., 56, 665 (1990)] has been a guiding concept towards the implementation of spin-based functionalities in III-V semiconductor structures. The concept relies on the control of the spin vector of a moving (or flying) electron spin by an electrostatic field applied perpendicular to the motion. While eliminating the need for magnetic fields from spin devices is desirable, the most appealing feature of electrostatic spin control is compatibility with the field effect transitors widely used in semiconductor chips. Electrostatic spin control also requires, however, a precise control of the spin motion on the microscopic scale to avoid fluctuations in the spin orientation (spin decoherence). Consequently, Datta and Das spin transistors have, so far, only been demonstrated for ballistic spin transport along short (< 2 μm) channels [Koo et al., Science 325, 1515 (2009), Chuang et al., Nat. Nanotehnol. 10, 35 (2015)]. A major challenge for efficient spin control is to devise suitable approaches to both drive spin motion and, simultaneously, control the spin orientation while avoiding decoherence.

In a recent publication, Helgers et al. [Nat. Commun. 13, 5384 (2022)] have introduced an elegant solution for this challenge based on spin transport using moving potentials produced by a surface acoustic wave (SAW). SAWs are acoustic vibrations that travel along a surface in a manner analogous to earthquake waves. SAWs with micrometer-sized wavelengths can be electrically generated using interdigitated transducers (IDTs) fabricated on a semiconductor chip, as illustrated in Fig. 1. When applied along a narrow semiconductor channel, the SAW fields create moving potential dots, which capture the electrons and transport them with the acoustic velocity. The limited motion within the dots suppresses spin decoherence and enables electron spin transfer over distances up to 100 μm. [Stotz et al., Nat. Mat. 4, 585 (2005)]. Concomitantly with the transport, the SAW fields forming the dots and moving congruently with them can also manipulate the electron spin vector through spin precession. By changing the strength of the SAW, the precession rate will also be changed. As a result, the moving dots act as contactless, flying spin gates, which simultaneously drive the motion of the spins and dynamically control the rate of spin precession during transport.

Helgers et al. have now experimentally demonstrated the feasibility of this approach by controlling the spin precession rate using the SAW amplitude during transport. In the experiment, spin polarized electrons and holes were optically excited within the dots using a circularly polarized laser beam (cf. red beam in Fig. 1). These carriers were then acoustically transported over distances of up to 100 μm before they are forced to recombine emitting photons (green beam). From the analysis of the photon polarization, which reflects the spin state of the carriers prior to recombination, the authors determined the dependence of the spin vector on the transport distance, and the spins were observed to precess over many cycles during transport. More important, by electrically changing amplitude of the SAW field, the authors show that the precession rate can be change by over 200%, thus demonstrating an efficient flying gate for spin control.

The acoustically driven flying spin gates enable a high degree of dynamic spin control as well as on-chip spin transfer over several tens of micrometers by simply changing the amplitude of the carrier acoustic wave. The approach is compatible with planar technology and also offers a convenient interface for the interconversion between electron spins and polarized phonons for long-distance quantum information transfer. Furthermore, it can be extended for the control of single spin qubits by reducing the dot sizes to enclose single electron spins and, in this way, enable the generation on-demand of single photons with controlled polarization. The flying spin gates can thus act as a single spin qubit control gate, a key element for on-chip quantum information processing, with a photonic interface. We acknowledge the financial support by the Natural Science and Engineering Research Council of Canada and the Alexander von Humboldt Foundation, Germany. This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No 642688.

1 Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V., Hausvogteiplatz 5-7, 10117 Berlin, Germany
2 Department of Physics, Engineering Physics & Astronomy, Queen’s University, Kingston, ON, K7L3N6 Canada
3 NTT Basic Research Laboratories, NTT Corporation, 3-1 Morinosato-Wakamiya, Atsugi, Kanagawa 243-0198, Japan