On the crest of the wave: Electronics on a time scale shorter than a cycle of light

An intense lightwave drives ultrafast electronic motion in a bulk crystal. A novel quantum interference creates free electrons and causes the emission of ultrashort high-harmonic light bursts. Credit: B. Baxley /parttowhole.com

 http://phys.org/news/2015-07-crest-electronics-scale-shorter.html#jCp

Physicists from Regensburg and Marburg, Germany have succeeded in taking a slow-motion movie of speeding electrons in a solid driven by a strong light wave. In the process, they have unraveled a novel quantum phenomenon, which is reported in the recent edition of Nature.

The advent of ever faster electronics featuring clock rates up to the multiple-gigahertz range has revolutionized our day-to-day life. Researchers and engineers all over the world have racked their brains about one central question: Is there a fundamental limit for the speed of electronics? Indeed, all electronic circuits rely on charge motion controlled by electric fields. Future high-speed electronics would, therefore, benefit immensely from bias fields that switch faster than state-of-the-art electronic clocks. The solution to this challenge may be surprisingly straightforward: One could try to employ the fastest alternating electric field available in nature – a light wave.

The team of researchers from Germany has now directly observed the electrons' motion in a semiconductor driven by a strong light pulse in the terahertz spectral region. The pioneering experiment carried out in Rupert Huber's group at the University of Regensburg enabled the first simultaneous clocking measurement of extremely broadband radiation sent out by the accelerated electrons, so-called high-order harmonics, and the driving light wave. It turns out that the harmonics are emitted in ultrashort light bursts which have now been characterized with a temporal resolution of approximately one femtosecond – the millionth of a billionth part of a second. In combination with numerical simulations performed in the groups of Mackillo Kira and Stephan W. Koch at the University of Marburg, this study provides unprecedented insights into the quantum world of a solid.

The results shed light onto a surprising behavior of the crystal electrons: During an extremely short timespan after excitation, the strong light field drives an electron simultaneously along multiple paths instead of one only. This strange scenario is possible in the quantum world where particles can behave like waves. As an indisputable quantum wave aspect, the electrons were shown to interfere constructively (destructively) only at the positive (negative) crests of the driving field, massively reshaping the temporal emission of the harmonics. While such quantum effects are often fragile and usually become observable only in extremely gentle fields the newly discovered phenomenon is qualitatively different because it is robust, producing pronounced interference contrast especially for extremely strong driving fields.

The breakthrough reveals the temporal structure of high-harmonics from a solid for the first time and thus helps the development of new sources of ever shorter light pulses. Moreover, this discovery opens new perspectives for modern high-speed electronics and sets an important milestone on the way towards light-wave-driven electronics.

 Explore further: Lightwave electronics at sharp metal tips

More information: M. Hohenleutner, F. Langer, O. Schubert, M. Knorr, U. Huttner, S. W. Koch, M.Kira und R. Huber, Real-time observation of interfering crystal electrons in high-harmonic generation, Nature (2015), DOI: 10.1038/nature14652

Abstract-Interferometric Characterization of a Semiconductor Disk Laser driven Terahertz Source

Matthias Wichmann, 

Markus Stein, 

Arash Rahimi-Iman, 

Stephan W. Koch, 

Martin Koch

http://link.springer.com/article/10.1007%2Fs10762-014-0069-9

Michelson interferometry is used to investigate the pump-power dependent emission bandwidth of a THz emitter based on intra-cavity difference frequency generation in a two-color VECSEL. A multi-mode CW THz signal is found to result from the two simultaneously lasing groups of longitudinal VECSEL modes. Power-dependent experiments reveal a clear correlation between the THz signal bandwidth and the numbers of optical modes involved.

Abstract-Narrow linewidth single-frequency terahertz source based on difference frequency generation of vertical-external-cavity source-emitting lasers in an external resonance cavity

Justin R. Paul, Maik Scheller, Alexandre Laurain, Abram Young, Stephan W. Koch, and Jerome Moloney

http://www.opticsinfobase.org/ol/abstract.cfm?uri=ol-38-18-3654

We demonstrate a continuous wave, single-frequency terahertz (THz) source emitting 1.9 THz. The linewidth is less than 100 kHz and the generated THz output power exceeds 100 μW. The THz source is based on parametric difference frequency generation within a nonlinear crystal located in an optical enhancement cavity. Two single-frequency vertical-external-cavity source-emitting lasers with emission wavelengths spaced by 6.8 nm are phase locked to the external cavity and provide pump photons for the nonlinear downconversion. It is demonstrated that the THz source can be used as a local oscillator to drive a receiver used in astronomy applications.

Compact, high-power, room-temperature, narrow-line terahertz source

http://spie.org/x47313.xml?highlight=x2416&ArticleID=x47313

Jerome V. Moloney, Joe M. Yarborough, Mahmoud Fallahi, Maik Scheller, Stephan W. Koch and Martin Koch

A terahertz external-cavity surface-emitting-laser source delivering milliwatt powers at any targeted frequency across the tHz gap could address a broad range of applications.

15 March 2011, SPIE Newsroom. DOI: 10.1117/2.1201102.003523

Continuous-wave (CW) terahertz (THz) technology has long interested astronomers because approximately half of the total luminosity and 98% of the photons emitted since the Big Bang fall into the submillimeter (THz) and far-IR regimes. Submillimeter astronomy is the prime technique to unveil the birth and early evolution of a broad range of astrophysical objects. It is a relatively new branch of observational astrophysics that focuses on studies of the cold universe, i.e., objects radiating a significant (if not dominant) fraction of their energy at wavelengths from 100μm to 1mm. THz-continuum observations are particularly powerful to measure the luminosities, temperatures, and masses of cold, dusty objects. Examples of the latter include star-forming clouds in the Milky Way, prestellar cores and deeply embedded protostars, and protoplanetary disks around young stars, as well as nearby starburst galaxies and dust-enshrouded high-redshift galaxies in the early universe. In THz astronomy, high-power CW local oscillators (LOs) driving multipixel heterodyne receivers at key frequencies (e.g., at 1.4, 1.9, and 2.7THz) could be deployed on space-based and suborbital platforms.¹

THz waves have been notoriously difficult to generate and, consequently, this remains a relatively unexplored part of the electromagnetic spectrum. They also do not propagate far through the atmosphere, except in narrow transmission bands at selected frequencies. CW THz sources are highly desirable for a wide range of applications, from THz astronomy to spectroscopy, biosensing, and security and quality inspection in various industries, as well as for medical and pharmaceutical purposes. Many attempts have been made to fill the elusive THz gap (see Figure 1). On the low-frequency side, up to around 0.8THz, electronics-based sources have been successful, although their output power falls off rapidly with increasing frequency (see Figure 1). On the other side of the gap, fundamental physics limitations forces cryogenic operation of quantum-cascade-laser THz sources, since they have to overcome kT (∼26meV at room temperature, where k is Boltzmann's constant and T denotes temperature) to achieve population inversion. They typically emit poor beams (without addition of plasmonic-waveguide structures) with broad spectral lines, while performance drops off significantly below 3THz.

 

Figure 1. Terahertz (THz)-power performance of different sources around the THz gap. RTD: Resonant tunnel diode. IMPATT: Impact ionization avalanche transit-time diode. Gunn: Gunn laser. QCL: Quantum-cascade laser. III–V: Denotes groups III and V in the periodic table of elements. The yellow ellipse illustrates how the THz external-cavity surface-emitting-laser (TECSEL) source can span the gap.

Our THz external-cavity surface-emitting-laser (TECSEL) source, operating at room temperature, is based on intracavity difference-frequency generation using a periodically poled lithium niobate crystal.² Different nonlinear crystals can be employed in the cavity and designed for either collinear or noncollinear phase matching. The IR dual-wavelength, high circulating power is generated using an intracavity etalon placed in a high-finesse vertical-external-cavity surface-emitting laser (VECSEL) cavity. Dual-wavelength emission is facilitated by spectral-hole burning in the very broad gain of the host semiconductor medium, enabling generation of THz frequencies from 0.1 to 5THz and beyond. The choice of circulating IR wavelength is arbitrary, and the VECSEL chip can be designed either for IR, visible, or mid-IR emission to minimize intracavity losses and optimize THz extraction. Figure 2 shows the TECSEL cavity and breadboard setup for a noncollinear phase-matching geometry.

 

Figure 2. Illustration of the setup. The pump laser is directed by mirror M3 to the vertical-external-cavity surface-emitting laser (VECSEL)'s surface. The curved and flat mirrors M1 and M2, and the VECSEL chip, form the laser cavity. A Brewster window (BW) and an etalon are placed within the resonator to ensure generation of linearly polarized emission at two wavelengths. The two laser modes mix inside the nonlinear crystal (NLC) and generate THz radiation. THz waves are emitted from the crystal surface and collimated by a cylindrical THz lens. (b) Photograph of our setup. 1: Semiconductor VECSEL device. 2: Etalon. 3: BW. 4: M1. 5: NLC. 6: M2. 7: Cylindrical THz lens. 8: Head of the pump laser. 9: Circulating IR laser mode.

Our measured intrinsic linewidth is less than 1MHz, making it ideal as a local oscillator (LO) and/or spectroscopic source. We have already demonstrated that this source can generate milliwatt power levels at 0.825 (remote detection of explosives), 1, and 1.9THz (LO for an astronomical THz receiver). Figure 3 shows a direct comparison of the power output for the 1 and 1.9THz sources, showing quadratic power scaling with increasing pump power and frequency.

 

Figure 3. Measured THz output power as a function of the total intracavity power of both IR modes. f: Frequency.

For astronomical applications in particular, our TECSEL source could potentially serve as the LO to drive mixer arrays of ∼100 or more pixels, dramatically increasing the scientific return from suborbital and space-based observing platforms. Recent advances in device fabrication, micromachining, low-noise amplifiers, and digital-signal processing have significantly driven down the cost per pixel of heterodyne arrays. LO power currently determines the possible array size. The number of pixels a heterodyne receiver can have and, consequently, how much of the sky can be observed within a given period of time scales linearly with LO power. Therefore, continued development of VECSEL-based THz sources could significantly reduce required mission lifetimes and cost. We are currently collaborating with both the Steward Observatory Radio-frequency Laboratory (SORAL) at the University of Arizona and TeraVision LLC (Tucson, Arizona) to test our TECSEL source as both an LO at 1.9THz and a remote spectroscopic source for detection of explosives.³ Many common explosives have unique spectral signatures within the relatively inaccessible spectral window between 0.8 and 3THz. A CW narrow-line source can reveal high-resolution spectroscopic features that are not observable with time-domain THz spectroscopy.

In summary, our room-temperature TECSEL source delivers milliwatt-level powers with sub-MHz linewidth in a clean Gaussian beam at any targeted frequency within and outside the THz gap. Further development is needed to optimize the source, minimize thermal effects in the VECSEL chip and nonlinear crystal, and further scale the emitted THz power. For example, we know that at least 50% of the THz power emitted into the NLC is currently absorbed. Our patented technology covers a broad landscape of TECSEL configurations, including collinear difference-frequency generation in transparent NLCs. Moreover, the TECSEL can be operated in pulsed mode to generate extremely high peak THz powers for time-domain-spectroscopy applications.

Our TECSEL 1.9THz source development has been funded by NASA Phase 1 and 2 Small Business Innovation Research (SBIR) contracts to Desert Beam Technologies, while 0.825THz source development was funded by a National Science Foundation Phase 1 SBIR. The authors acknowledge discussions with and support from Christopher Walker of SORAL, and Christian d'Aubigny and Abraham Young of Teravision LLC (Tucson, Arizona).

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Jerome V. Moloney, Joe M. Yarborough, Mahmoud Fallahi

Desert Beam Technologies LLC

Tucson, AZ 

Jerome Moloney is a professor of optical sciences and mathematics at the University of Arizona and a partner in Desert Beam Technologies LLC. His research interests include theory and experimental study of semiconductor lasers, ultrafast extreme nonlinear optics, and computational photonics.

Joe Yarborough is a senior scientist. His research includes tunable lasers, nonlinear optics, and THz sources.

Mahmoud Fallahi is a professor and a partner in Desert Beam Technologies LLC. His research interests are in semiconductor lasers, integrated optics, micro/nanofabrication, and polymer/hybrid components.

Maik Scheller, Stephan W. Koch, Martin Koch

Department of Physics
Philipps University Marburg

Marburg, Germany

Maik Scheller received his Dipl-Ing degree in electrical engineering from the Technical University of Braunschweig (Germany) in 2008. He is currently pursuing a PhD degree. His research interests include nonlinear optics, THz systems, and signal processing.

Stephan W. Koch has been a professor of physics at Philipps University Marburg and a research professor at the Optical Sciences Center of the University of Arizona since 1993. He is also a partner in Desert Beam Technologies LLC. His fields of major current interests include condensed-matter theory, optical and electronic properties of semiconductors, many-body interactions, disorder effects, quantum confinement in solids, coherent and ultrafast phenomena, semiconductor-laser theory, microcavity effects, and optical instabilities and nonlinearities.

Martin Koch is a professor of physics and a partner in Desert Beam Technologies LLC. His research interests include ultrafast spectroscopy of semiconductors, semiconductor physics, and THz-systems technology.

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References:

1. P. Siegel, Terahertz technology, IEEE Trans. Microw. Theory Techn. 50, no. 3, pp. 910, 2002.

2. M. Scheller, J. M. Yarborough, J. V. Moloney, M. Fallahi, M. Koch, S. W. Koch, Room temperature continuous wave milliwatt terahertz source, Opt. Express 18, pp. 27112-27117, 2010.

3. J. F Federici, B. Schulkin, F. Huang, D. Gary, R. Barat, F. Oliveira, D. Zimdars, THz imaging and sensing for security applications: explosives, weapons and drugs,Semicond. Sci. Technol. 20, pp S266, 2005.