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Ultra-Short Pulse laser Resonators IN the Terahertz

Periodic Reporting for period 3 - SPRINT (Ultra-Short Pulse laser Resonators IN the Terahertz)

Reporting period: 2019-09-01 to 2021-02-28

"Ultra-short light pulses with large instantaneous intensities can probe light-matter interaction phenomena, capture snapshots of molecular dynamics and drive high-speed communications. In a semiconductor laser, mode-locking is the primary way to generate ultrafast signals. Despite the intriguing perspectives, operation at Terahertz (THz) frequencies is facing fundamental limitations: engineering ""ultrafast"" THz semiconductor lasers from scratch or finding an integrated technology to shorten THz light pulses are currently two demanding routes.

SPRINT aims to combine the potential offered by the band-gap and lithographic engineering of THz frequency QCLs with the wealth of unique physical properties of graphene (or polaritonic) components to deploy new concepts for pulse generation and sensing to drive quantum cascade lasers in the ""ultrafast"" regime. By devising quasi-crystal and random electrically pumped THz semiconductor lasers, I plan to explore the technological and scientific routes toward a completely novel generation of passively mode-locked, high-power THz QCLs operating on a controllable frequency bandwidth. To address the issue, I plan to innovatively exploit the direct integration of intracavity graphene saturable absorbers, and/or the design of “cavity-coupled” graphene metamaterial saturable mirrors or polaritonic saturable filters. A new detection approach for the measurement of such “ultrafast” high-intense light pulses and for the investigation of the fascinating intracavity dynamics will be also developed: I will exploit the nonlinearity of quantum dot in a nanowire - based transistors, engineered to detect, on ultra-short time scales, the stable THz ultrafast comb generated in the QCL resonator cavity.

Society, economic and technological impact: SPRINT is a cutting-edge multidisciplinary project built on a collaborative research across a broad range of disciplines including nano- and optoelectronics, photonics, material science, quantum engineering, frequency metrology and solid-state physics. The targeted goal is to provide groundbreaking technological steps toward the development of a new ubiquitous ultrafast technology in the underexploited THz-frequency range, aiming to unprecedented compactness, sensitivity and resolution for spectroscopic, imaging, metrological and ICT applications. SPRINT will open up horizons and research opportunities on longer-term topics: taking snapshots of ultrafast dynamics; real-time pulsed imaging and time of flight tomography; time-resolved THz spectroscopy of gases, complex molecules and cold samples; coherent control of quantum systems; quantum optics, where high-power pulses can drive molecular samples out of equilibrium; metrology, where laser excitation can match the energy levels splitting of molecules, and its pulsed nature can down-convert the spectrum to the RF domain; ultra high-speed communications where THz frequency carriers will become increasingly more important in the quest for higher bandwidth data communications. Furthermore, ultra-short pulses and high-power THz micro-lasers combined with highspeed electronic detectors can become a groundbreaking and compact alternative to bulky THz time-domain systems promising extraordinary impacts on the market for biomedical imaging, security and process control."
Research activities developed during the 1st and 2nd reporting periods.

Workpackage 1.

- Task 1.1:

As a first step, we engineered the GaAs/AlGaAs THz QCL active media. The first objective was the design a long-lived upper laser state, exploiting a reduced wavefunction-overlap between the two radiative
levels. To address this issue, spatially diagonal designs and injection strategies, targeting a reduced electron temperature in the upper laser state, were implemented. Consequently, the phonon-assisted gain relaxation will be strongly suppressed and the gain relaxation lifetime increased. The second objective was the design of a wide (beyond 1 THz) gain bandwidth, providing multi-frequency emission that provides inhomogeneous broadening of the gain spectrum. To address this issue, we overlapped the spectral gain of several different subsections with a wider spectral separation. A broad spectral gain is expected to shorten the pulse duration below picoseconds, leading to higher peak optical power.
Typically, to address applications needs, continuous-wave (CW) operation, low-divergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam divergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. In this first reporting period we employed the designed broadband THz QCL active regions to demonstrate wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies within the engineered gain bandwidth, has led to the achievement of low-divergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.

We have furthermore designed and demonstrated a broadband, heterogeneous terahertz frequency quantum cascade laser by exploiting an active region design based on longitudinal optical-phonon-assisted interminiband transitions.
This allowed obtaining continuous wave laser emission with a threshold current density of ∼120 A/cm2, a record dynamic range of ∼3.1 and an emission spectrum spanning from 2.4 to 3.4 THz at 15 K.
This also allowed to demonstrate record performance frequency combs at THz frequencies during the second reporting period

We indeed demonstrate that the devised broadband lasers operate as THz optical frequency comb synthesizers in continuous-wave, with a maximum optical output power of 4 mW (0.73µW in the comb regime). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24) significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of ≈ 1.05 THz and a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB. Analysis of the electrical and thermal beatnote tuning reveals a current-tuning coefficient ranging between 5 MHz/mA and 2.1 MHz/mA and a temperature-tuning coefficient of –4 MHz/K. The ability to tune the THz QCL combs over their full spectral range by temperature and current paves the way for their use as a powerful spectroscopy tool that can provide broad frequency coverage combined with high precision spectral accuracy.

To date, however, stable comb operation is only observed over a small operational current range in which the bias-depended chromatic dispersion is compensated. As most dispersion compensation techniques in the THz range are not tunable, this limits the spectral coverage of the comb and the emitted output power, restricting potential applications in, for example, metrology and ultrashort THz pulse generation. Here, we demonstrate an alternative architecture that provides a tunable, lithographically independent, control of the free-running coherence properties of THz QCL FCs. This is achieved by integrating an on-chip tightly coupled mirror with the QCL cavity, providing an external cavity and hence a tunable Gires Tournois interferometer (GTI). By finely adjusting the gap between the GTI and the back-facet of an ultra-broadband, high dynamic range QCL, we attain wide dispersion compensation regions, where stable and narrow (~3 kHz linewidth) single beatnotes extend over an operation range that is significantly larger than that of dispersion-dominated bare laser cavity counterparts. Significant reduction of the phase noise is registered over the whole QCL spectral bandwidth (1.35 THz). This agile accommodation of a tunable dispersion compensator will help enable uptake of QCL-combs for metrological, spectroscopic and quantum technology−oriented applications.

As an additional approach we device a homogeneous THz QCL emitting, with a total spectral emission of about 0.6 THz, centered around 3.3 THz, a current density dynamic range Jdr = 1.53 and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as an optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ∼200 µW of optical power per optical mode, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of about 500 Hz, is observed over a current density range of 240 A/cm2 and even across the negative differential resistance region. We further prove that the QCL FC can be injection locked with moderate radio frequency power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological applications requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.


- Task 1.2: We have developed a full 3D model, aiming to simulate a set of QCL resonators having defined architectures, set by the choice of the aperiodic crystal patterning. Main objective was defining the quasicrystal tiles and the computer-generated random patterns that provide a reasonably large number of high-Q modes at eigenfrequency values well within the bandwidth of the fabricated THz QCLs (2.5 THz- 3.6 THz).
The resonator structure was represented by an equivalent crystal composed of two materials with different local effective dielectric constants, one for the unpatterned area, one for regions comprising the holes positioned at the vertex of the aperiodic (random) tiling of the simulated quasi-crystals (random patterns). Two configurations will be considered: the first one exploits the post-fabrication insertion of diluted graphene in the resonator holes; the second one includes a graphene membrane on the device surface.

- Task 1.3: Technically, we fabricated quasi crystal and random THz QCLs by embedding the QCL active region between two metallic cladding layers, to create a micro-strip like double-metal waveguide which confines, with an almost unitary confinement factor, the THz radiation in the z-direction and allows its propagation in the x-y plane, therefore making the device a nearly ideal 2D photonic system. To implement the quasi-crystal and the random laser, we etched open holes on the top metal layer of the resonator, at the vertices of the specifically selected tiling. In this way, the waveguide mode will be strongly modified in the hole regions (as the upper metallic cladding layer is missing locally) and the radiation will be forced to extend outside the semiconductor. Each opening then acts not only as a scatterer for the propagating radiation, but also as an aperture through which radiation can be out-coupled. In the case of random resonators, the random patterning was confined on a dry-etched mesa, whose area was varied regularly to allow the coexistence of many optical modes without spatial overlap, and, in same cases to simultaneously allow continuous wave operation. In both cases, absorbing boundary or irregular-shaped borders were lithographically implemented to suppress whispering gallery or Fabry-Perot -like modes. The spectral, electronic, transport and optical operational characteristics have been investigated to evaluate the efficacy of the selected pattern.

We achieved the following major results in the second and third reporting periods:

1. Quasi-crystal distributed feedback lasers do not require any form of mirror cavity to amplify and extract the radiation. Once implemented on the top surface of a semiconductor laser, a quasi-crystal pattern can be used to tune both the radiation feedback and the extraction of high-radiative, and high-quality factor, optical modes that do not have a defined symmetric or anti-symmetric nature. This methodology therefore offers the possibility to achieve efficient emission, combined with tailored spectra and controlled beam-divergence. Here we apply this concept to a one-dimensional quantum cascade wire laser.
We conceive and demonstrate quasi-crystal THz QCL resonators exploiting a surface grating following the Octonacci design, which are capable of significantly boosting the state-of-the-art performance of surface emitting THz lasers. By tuning the laser width as well as the patterning slit size, the interplay between the grating scattering wavevectors and the photon propagation is optimized to achieve highly efficient surface THz emission via dual lobe beam profiles symmetrically placed at 25° from the surface normal, 240 mW peak optical power, and the highest slope efficiency (570 mW/A at 78K, 700 mW/A at 20K) reported to date in an electrically pumped multimode, surface emitting, disordered THz laser. Switching between multimode and single-mode emission is achieved through adjusting the lithographic pattern, to engineer the resultant photonic pseudo-bandgaps. Furthermore, frequency tuning of 20 GHz is demonstrated by coupling the laser to an external mirror driven by a piezoelectric-actuator.
The possibility to easily switch from single-mode to multimode emission, while preserving high output powers and high slope efficiency, clearly unveils that our photonic quasi-crystal provides a robust performance enhancement for both regimes, very differently from all previously reported architectures which instead operate either on a single laser mode or on a broad bandwidth


2. We demonstrated the first random lasers operating in continuous wave.
Random lasers are a class of devices in which feedback arises from multiple elastic scattering in a highly disordered structure, providing an almost ideal light source for artefact-free imaging due to achievable low spatial coherence.
However, for many applications ranging from sensing and spectroscopy to speckle-free imaging, it is essential to have high-radiance sources operating in continuous-wave (CW).
Miniaturized, electrically pumped, continuously tunable RLs, operating in a continuous wave (CW) regime, are indeed necessary for many spectroscopic and multicolor imaging applications across the THz frequency range. But, CW operation of an RL has yet to be demonstrated at THz frequencies, whether optically or electrically pumped, or indeed in the infrared part of the electromagnetic spectrum.
We have exploited a new resonator geometry in which a 2D random distribution of air holes, patterned into the top metal layer of a double-metal resonator, is combined with irregular borders to confine the active region. Patterning is, however, only implemented in the upper metal and highly doped semiconductor cladding, leaving the active region core unperturbed. We thus overcome the previous technological limitations with THz RLs, enabling us to demonstrate the first multimode CW emission. In our work, we designed and controlled the interference pattern in our RLs by accurately engineering the geometric properties of the photonic structures, and investigated the effects of disorder on power extraction and spectral emission. Highly-collimated vertical emission was achieved in the resonant feedback random lasing regime at ~3 THz, with a maximum bandwidth of ≈ 400 GHz, device operation up to a 115 K heat sink temperature, a peak optical power of ≈ 21 mW in pulsed mode, and up to ≈ 1.7 mW of CW emission. In addition to the ability to tune the laser emission frequency coarsely by varying the surface pattern distribution, we also demonstrated a new route for fine control of the spectral properties of our RLs by placing a movable mirror over the top surface, in an external coupled-cavity configuration. A complex spectral dynamic was unveiled, with multiple modes simultaneously being continuously tuned over an 11 GHz range of frequencies, which was further increased to ≈ 20 GHz with mode hopping.

3. We report on the development of one-dimensional THz-frequency random wire lasers, patterned on the top surface of a double-metal quantum cascade laser with fully randomly arranged apertures, not arising from the perturbation of a regular photonic structure. By performing finite element method simulations, we engineer photonic patterns supporting strongly localized random modes in the 3.05–3.5 THz range. Multimode laser emission over a tunable-by-design band of about 400 GHz and with ∼2 mW of peak power has been achieved, associated with 10° divergent optical beam patterns. The achieved performances were then compared with those of perturbed Fabry–Perot disordered lasers, showing continuous-wave operation in the 3.5–3.8 THz range with an order of magnitude larger average power output than their random counterpart, and an irregular far field emission profile.


Workpackage 2.

- Task 2.1 and Task 2.3 :Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. In the first 18 months of the project we fabricated THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. The achieved results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications, providing unprecedented compactness and resolution.


- Task 2.2: In the third reporting period we report on the first evidence of THz saturable absorption in multilayer graphene films, grown via CVD on Nickel. FTIR spectroscopy was used to determine the linear absorption α0 in the THz frequency range. Spectra are acquired with deuterated triglycine sulfate (DTGS)-polyethylene. In the frequency range comprised between 2.5 THz and 9.0 THz, the linear absorption coefficient is almost flat around an average value of 70%. We unveiled a clear transmission increase around z≈0 μm, corresponding to the region of highest laser intensity impinging on the sample. The absorption bleaching increases
"During this first, second and third reporting period we had a plethora of relevant progresses beyond the state of the art:

- Continuous-wave highly-efficient low-divergence terahertz wire lasers. Terahertz (THz) quantum cascade lasers (QCLs) have undergone rapid development since their demonstration, showing high power, broad-tunability, quantum-limited linewidth, and ultra-broadband gain. Typically, to address applications needs, continuous-wave (CW) operation, low-divergent beam profiles and fine spectral control of the emitted radiation, are required. This, however, is very difficult to achieve in practice. Lithographic patterning has been extensively used to this purpose (via distributed feedback (DFB), photonic crystals or microcavities), to optimize either the beam divergence or the emission frequency, or, both of them simultaneously, in third-order DFBs, via a demanding fabrication procedure that precisely constrains the mode index to 3. We have demonstrated wire DFB THz QCLs, in which feedback is provided by a sinusoidal corrugation of the cavity, defining the frequency, while light extraction is ensured by an array of surface holes. This new architecture, extendable to a broad range of far-infrared frequencies, has led to the achievement of low-divergent beams (10°), single-mode emission, high slope efficiencies (250 mW/A), and stable CW operation.

- The first frequency tunable continuous wave random THz lasers . Random lasing has long been extensively studied theoretically and experimentally reported in a number of different systems, such as optically pumped suspended microparticles in laser dye and fine powders. Quantum cascade lasers (QCLs) represent a promising platform for the integration of aperiodic photonic patterns with the aim of controlling the intra-cavity propagation of light and its extraction into the free space. Here, we conceive and devise random THz QCLs, exploiting a broadband active material and a double-metal waveguide, operating for the first time in continuous wave (CW) with remarkably high optical powers and a rich sequence of optical modes distributed over a 500 GHz bandwidth.

- The first one dimensional quasi crystal THz laser providing single mode or multimode emission over a 530 GHz bandwidth, with maximum peak optical power of 240 mW (190 mW) in multimode (single-mode) lasers with record slope efficiencies up to ≈570 mW/A at 78 K and ≈700 mW/A at 20 K, wall-plug efficiencies of η ≈ 1% and low divergent emission.

- The first one-dimensional THz-frequency random wire lasers supporting strongly localized random modes in the 3.05–3.5 THz range. Multimode laser emission over a tunable-by-design band of about 400 GHz and with ∼2 mW of peak power has been achieved, associated with 10° divergent optical beam patterns.

- THz QCL combs. We demonstrated broadband THz QCLs exploiting a heterogeneous active region scheme and have a current density dynamic range of 3.2 significantly larger than the state of the art, over a 1.3 THz bandwidth. These devices operate as THz optical frequency comb synthesizers in continuous-wave, with a maximum optical output power of 4 mW). Measurement of the intermode beatnote map reveals a clear dispersion-compensated frequency comb regime extending over a continuous 106 mA current range (current density dynamic range of 1.24) significantly larger than the state of the art reported under similar geometries, with a corresponding emission bandwidth of ≈ 1.05 THz and a stable and narrow (4.15 KHz) beatnote detected with a signal-to-noise ratio of 34 dB.

- Tunable, lithographically independent, control of the free-running coherence properties of THz QCL FCs. This is achieved by integrating an on-chip tightly coupled mirror with the QCL cavity, providing an external cavity and hence a tunable Gires Tournois interferometer (GTI). By finely-adjusting the gap between the GTI and the back-facet of an ultra-broadband, high dynamic range QCL, we attain wide dispersion compensation regions, where stable and narrow (~3 kHz linewidth) single beatnotes
extend over an operation range that is significantly larger than that of dispersion dominated bare laser cavity counterparts. Significant reduction of the phase noise is registered over the whole QCL spectral bandwidth (1.35 THz).

- We engineer miniaturized THz FCSs, comprising a heterogeneous THz QCL, integrated with a tightly coupled, on-chip, solution-processed, graphene saturable-absorber reflector that preserves phase-coherence between lasing modes, even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FCS with over 90 optical modes, through 55% of the laser operational range. We also achieve stable injection-locking, paving the way to a number of key applications, including high-precision tunable broadband-spectroscopy and quantum-metrology. This is the state of the art in the field of miniaturized THz frequency combs.

- The first THz graphene saturable absorber obtained by ink-jet printing of graphene. Saturable absorbers (SA) operating at terahertz (THz) frequencies can open new frontiers in the development of passively mode-locked THz micro-sources. We reported the fabrication of THz SAs by transfer coating and inkjet printing single and few-layer graphene films prepared by liquid phase exfoliation of graphite. Open-aperture z-scan measurements with a 3.5 THz quantum cascade laser show a transparency modulation ∼80%, almost one order of magnitude larger than that reported to date at THz frequencies. Fourier-transform infrared spectroscopy provides evidence of intraband-controlled absorption bleaching. These results pave the way to the integration of graphene-based SA with electrically pumped THz semiconductor micro-sources, with prospects for applications where excitation of specific transitions on short time scales is essential, such as time-of-flight tomography, coherent manipulation of quantum systems, time-resolved spectroscopy of gases, complex molecules and cold samples and ultra-high speed communications, providing unprecedented compactness and resolution.

- The first electrically switchable graphene terahertz (THz) modulator with a tunable‐by‐design optical bandwidth, exploited to compensate the cavity dispersion of a quantum cascade laser (QCL). we achieve 90% modulation depth of the intensity, combined with a 20 kHz electrical bandwidth in the 1.9–2.7 THz range. The modulator is then integrated with a multimode THz QCL. By adjusting the modulator operational bandwidth, the authors demonstrate that the graphene modulator can partially compensate the QCL cavity dispersion, resulting in an integrated laser behaving as a stable frequency comb over 35% of the operational range, with 98 equidistant optical modes and a spectral coverage ~1.2 THz.

- We ultrastrongly couple intersubband transitions of semiconductor quantum wells to the photonic mode of a metallic cavity in order to custom-tailor the population and polarization dynamics of intersubband cavity polaritons in the saturation regime. Two-dimensional THz spectroscopy reveals strong subcycle nonlinearities including six-wave mixing and a collapse of light-matter coupling within 900 fs. This collapse bleaches the absorption, at a peak intensity one order of magnitude lower than previous all-integrated approaches and well achievable by state-of-the-art QCLs, as demonstrated by a saturation of the structure under cw-excitation.

- The first near-field terahertz probes with room-temperature nanodetectors for subwavelength resolution imaging. Near-field imaging with terahertz (THz) waves is emerging as a powerful technique for fundamental research in photonics and across physical and life sciences. Spatial resolution beyond the diffraction limit can be achieved by collecting THz waves from an object through a small aperture placed in the near-field. However, light transmission through a sub-wavelength size aperture is fundamentally limited by the wave nature of light. We conceived a novel architecture that exploits inherently strong evanescent THz field arising within the aperture to mitigate the problem of vanishing transmission. The sub-wavelength aperture is originally coupled to asymmetric electrodes, which activate the thermo-electric THz detection mechanism in a transistor channel made of flakes of black-phosphorus or InAs nanowires. The proposed novel THz near-field probes enable room-temperature sub-wavelength resolution coherent imaging with a 3.4 THz quantum cascade laser, paving the way to compact and versatile THz imaging systems and promising to bridge the gap in spatial resolution from the nanoscale to the diffraction limit.

- Ultrafast and sensitive terahertz detection using an antenna-integrated graphene pn-junction. We demonstrate that this novel detector has excellent sensitivity, with a noise-equivalent power of 80 pW/Hz1/2 at room temperature, a response time below 30 ns (setup-limited), a high dynamic range (linear power dependence over more than 3 orders of magnitude) and broadband operation (measured range 1.8-4.2 THz, antenna-limited), a combination that is currently missing in the state of the art.

- uncooled terahertz PDs combining the low (∼2000 kB μm–2) electronic specific heat of high mobility (>50202f000 cm2 V–1 s–1) hexagonal boron nitride-encapsulated graphene, with asymmetric field enhancement produced by a bow-tie antenna, resonating at 3 THz. This produces a strong photo-thermoelectric conversion, which simultaneously leads to a combination of high sensitivity (NEP ≤ 160 pW Hz–1/2), fast response time (≤3.3 ns), and a 4 orders of magnitude dynamic range,

- Room-temperature THz nano-receivers exploiting antenna-coupled graphene field effect transistors integrated with lithographically-patterned high-bandwidth (∼100 GHz) chips, operating with a combination of high speed (hundreds ps response time) and high sensitivity (noise equivalent power ≤120 pW Hz−1/2) at 3.4 THz. These are the faster and more sensitive receivers developed so far.

- The first Se-doped black phosphorus THz sensors operating at room temperature with state-of-the-art sensitivity and noise equivalent power and ultrastable time behaviour.

- Prove of ultrafast photo-switching of interface polaritons in black phosphorus heterostructures - The characteristic shine of metals is created by electrons which can freely move in the interior of the material and reflect incoming radiation. Similar to water waves on a pond, waves can form on the surface of this electron sea – so called “surface plasmons”. Instead of a stone that is thrown into water, light is used to generate surface plasmons in the laboratory. When light is focused onto a nanometer-sharp metallic tip, miniature waves propagate on the material’s surface in a circular fashion starting from the tip apex. A nanometer is only approximately ten times the size of the diameter of a single atom. The miniature waves could be used in future compact electronic devices for lightning-fast information transport. So far, however, there has been no means of switching such surface waves on and off on ultrafast timescales, which is essential. Conversely, in conventional electronics the analogous mechanism is realized by transistors. Now, for the first time, we have demonstrated the experimental on/off switching of waves on the electron sea, laying the foundation for future plasma-electronics.
The key was to use a much more sophisticated heterostructure based on a semiconductor. Semiconductors like, for example, silicon are the materials from which computer chips are made. The semiconductor in this heterostructure is an especially modern material: so-called “black phosphorus”. Upon irradiation by intense light pulses, freely moving electrons are generated inside the material. Without these electrons, no surface waves are present and the structure is switched “off”. However, as soon as the first laser pulse generates the free electrons, a subsequent pulse can start the propagation of surface plasmons from the tip. To test how fast this switching process can take place we activated surface plasmons with ultrafast laser pulses that were as short as a few femtoseconds. One femtosecond is the unimaginably short time span of the millionth part of a billionth of a second, i.e. 0, 000 000 000 000 001 seconds. Employing their worldwide unique microscope featuring nanometer spatial resolution in addition to ultrafast temporal resolution, the scientists subsequently traced the expansion of the plasmon waves in extreme slow motion snapshots. In this process it was clearly visible that the switching times where on the femtosecond scale, and thus many orders of magnitude faster than the fastest existing transistors. As a pleasant surprise it was also found, that the wavelength of the surface waves is almost independent of the power of the laser which switches the structure. These results are highly encouraging for future ultrafast electronics based on surface plasmons.

- Phase-resolved THz self-detection near-field microscopy. We devised a THz s-SNOM system that provides both amplitude and phase contrast and achieves nanoscale (60-70nm) in-plane spatial resolution. It features a QCL that simultaneously emits THz frequency light and senses the backscattered optical field through a voltage modulation induced inherently through the self-mixing technique.

- Fully phase-stabilized quantum cascade laser frequency comb. We demonstrate full-phase-stabilization of a QCL-comb against the atomic clock standard, proving the independent and simultaneous control of the two comb degrees of freedom at a metrological level. Each emitted mode exhibits a sub-Hz relative frequency stability, while a correlation analysis on the modal phases confirms the coherence of the emission. These highly miniaturized, though fully-controlled, comb emitters promise to pervade an increasing number of scientific fields, including quantum technologies.

-- A novel, all-electrical tuning, compact THz spectrometer, based on ultrafast passively mode-locked lasers and coherent nano-detectors. Such a system will combine the advantage of high-power pulses, chosen central frequency and bandwidth with small footprint and low costs, and will have a disruptive effect on the uptake of pulsed THz technology, promising concrete impacts on spectroscopy, metrology, imaging and high-speed THz communication systems.

Expected results until the end of the project:

- develop self-mode locked lasers via direct integration in the already demonstrated THz QCLs, ""intracavity"", of the developed graphene saturable absorbers, and/or by designing ""cavity-coupled"" graphene-metamaterial saturable absorber mirrors in a convenient architecture, to allow propagation of ultra-short, high-intensity light pulses. Alternatively, such cavity-coupled approach will be translated, in an elegant architecture, to polaritonic THz saturable filters.

- develop a new detection approach for the measurement of such ""ultrafast"" high-power light pulses and for the investigation of the related unexplored intracavity dynamics. The core idea is to exploit the non-linearity of a quantum dot in a nanowire - based transistor or, alternative 2D nanomaterial or nanowire based transistors"