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For automated driving, the perception provided by lidar, radar, and camera sensors is safety-critical. Validating sensor perception reliability with standard empirical tests is impractical, owing to the large required test effort and the need for a reference truth to identify sensor errors. To address these challenges, we investigate the possibility of estimating sensor perception reliability without a reference truth. In particular, we propose a framework to learn sensor perception reliability solely by exploiting sensor redundancies. We formulate a likelihood function for redundant binary sensor data without a reference truth and propose a Gaussian copula to model dependent sensor errors. Synthetic numerical experiments show that under an adequate dependence model, correct sensor perception reliabilities can be estimated without a reference truth. Because the selection of an adequate dependence model is challenging without a reference truth, we also investigate how inadequate dependence models influence the estimation. The proposed framework is a step toward the validation of sensor perception reliability because it could enable the learning of reliabilities from a fleet of driver-controlled vehicles equipped with series sensors.

Over the past decade, the performance of generation and detection schemes for short electromagnetic waveforms in the THz spectral range has been improved at a rapid pace and facilitated the exploration of novel quantum phenomena of solid-state systems. Here, we present a selection of recent key advances and discuss their impact on cutting-edge fundamental research. Widely tunable high-field sources providing atomically strong, phase-locked few-cycle lightwaves with field amplitudes of up to 1 V/A at few-kHz repetition rates have driven dynamical Bloch oscillations in bulk semiconductors, quasiparticle collisions, and control of the valley pseudo-spin in monolayer dichalchogenides. Novel, highly phase-stable sources operating at repetition rates of 190 kHz provide comparably strong fields of up to 13 MV/cm, foreshadowing future applications in ultrabroadband frequency-comb metrology or lightwave scanning tunneling microscopy. Similarly, electro-optic sensors have been pushed to the quantum level, detecting THz waveforms which contain only 0.1 photons, on average, enabling strongly subdiffractional and subcycle tomography of interface polaritons black phosphorus in THz near-field scattering experiments. Moreover, customized FPGA technology has enabled synchronous single-shot detection of incoherent THz electric fields with amplitudes of less than 0.5 V/cm. In combination with femtosecond fiber lasers and an acousto-optical delay line, we have implemented fast-scan THz time-domain spectroscopy with a waveform refresh rate of 36 kHz and a bandwidth of 3 THz. The acquisition time of only 28 µs and signal-to-noise ratio of 27 for a single waveform, or 1.7×10^5/√Hz, may power multidimensional spectroscopy or real-time monitoring of non-repeatable processes in technology or biology.

Automotive environment sensors such as LIDARs (light detection and ranging) are the backbone of automated driving. It has to be made sure that the performance of these sensors is sufficient, even under adverse environment influences like accumulated dirt on a sensor cover. This paper describes the collection and analysis of real world road dirt samples, accumulated on the plastic cover of a LIDAR sensor. The effect of the dirt on the sensor performance is experimentally quantified by measuring the transmission and reflection properties of the dirty sensor plastic cover. Moreover, an analysis of the sensor's uncertainty in raw data position measurements is presented. Different alternatives to include these effects in a virtual simulation of the sensor are discussed. While the sample size of the dirty plastic covers is too small to representatively describe the effect of dirt on LIDAR performance, this study shows that an abstract effect on sensor performance such as dirt can be described, quantified and eventually be represented in a virtual simulation.

Machine vision based on automotive environment sensors is the enabling technology for advanced driver assistance systems and automated driving. Due to its important role, the reliability of environment sensing is highly safety relevant and has thus to be assessed and demonstrated during the development of the system. The main challenges associated with this task are low target error rates and the stochastic influence of different uncertain environmental conditions on the sensor performance. As a basis for the reliability assessment of environment sensors we introduce comprehensive performance metrics that allow a formal description of the uncertainties in a digital environmental model. Due to the influence of environmental conditions on the sensor performance, these metrics however are not constant but are random variables themselves. This leads to a hierarchical uncertainty structure including higher order uncertainties. To quantify the influence of the environmental conditions on the sensor reliability, we use a Bayesian hierarchical regression model. The utility of this method is demonstrated with a case study in which the influence of temperature on sensor reliability is examined. The results show that the proposed methodology is capable of identifying and quantifying the influence of the temperature on sensor performance. The introduced metrics and the proposed methodology are an important step towards a formalized reliability assessment of automotive environment sensing. In order to predict if the sensor reliability complies with the target error rates, the presented methodology has to be adapted and extended with additional stochastic methods.

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 polarization combined with dynamical Bloch oscillations in semiconducting gallium selenide. These dynamics entail the emission of phase-stable high-harmonic transients, covering the entire THz-to-visible spectral domain between 0.1 and 675 THz. Quantum interference of different ionization paths of accelerated charge carriers is controlled via the waveform of the driving field and explained by a quantum theory of inter- and intraband dynamics. Our results pave the way towards all-coherent THz-rate electronics.

A microscopic theory is presented to describe high-harmonic generation in solids with the semiconductor-Bloch equations. The approach includes the relevant interband polarizations and intraband currents. The appearance of even harmonic orders is shown to require at least three electronic bands and a mutual interband coupling between them. In experimental and theoretical time-resolved studies, this also manifests as a unipolar emission signature of the high-harmonic radiation.

The optical response of hybrid metal-semiconductor nanoparticles exhibits different behaviors due to the proximity between the disparate materials. For some hybrid systems, such as CdS-Au matchstick-shaped hybrids, the particles essentially retain the optical properties of their original components, with minor changes. Other systems, such as CdSe-Au dumbbell-shaped nanoparticles, exhibit significant change in the optical properties due to strong coupling between the two materials. Here, we study the absorption of these hybrids by comparing experimental results with simulations using the discrete dipole approximation method (DDA) employing dielectric functions of the bare components as inputs. For CdS-Au nanoparticles, the DDA simulation provides insights on the gold tip shape and its interface with the semiconductor, information that is difficult to acquire by experimental means alone. Furthermore, the qualitative agreement between DDA simulations and experimental data for CdS-Au implies that most effects influencing the absorption of this hybrid system are well described by local dielectric functions obtained separately for bare gold and CdS nanoparticles. For dumbbell shaped CdSe-Au, we find a shortcoming of the electrodynamic model, as it does not predict the "washing out" of the optical features of the semiconductor and the metal observed experimentally. The difference between experiment and theory is ascribed to strong interaction of the metal and semiconductor excitations, which spectrally overlap in the CdSe case. The present study exemplifies the employment of theoretical approaches used to describe the optical properties of semiconductors and metal nanoparticles, to achieve better understanding of the behavior of metal-semiconductor hybrid nanoparticles. © 2011 American Chemical Society.

The crystallization dynamics in wet-chemical synthesis is the key parameter controlling nanoparticle morphology, which determines the frequency of the optical plasmon resonance in metal nanoparticles. Despite several in situ (dark-field microscopy) and ex situ (electron microscopy) studies of metal nanoparticle growth, the anisotropic growth kinetics are not well-understood mainly because the crystallization is a nonequilibrium process. Using simultaneous optical spectroscopy and time-resolved small-angle X-ray scattering at a synchrotron X-ray source, we directly monitor the anisotropic growth kinetics of gold and gold-copper nanorods and extract the growth parameters for both crystal directions (along the rod’s long and short axes) independently. We find a crossover from 1D to 3D growth modes at 8 and 12 min, respectively, where the nanorods attain their maximum aspect ratio. The growth model explains and predicts this crossover point without the need of a switch for the growth mode and allows for the f...

We report a quantitative analysis of the forces acting on optically trapped single gold nanorods. Individual nanorods with diameters between 8 and 44 nm and aspect ratios between 1.7 and 5.6 were stably trapped in three dimensions using a laser wavelength exceeding their plasmon resonance wavelengths. The interaction between the electromagnetic field of an optical trap and a single gold nanorod correlated with particle polarizability, which is a function of both particle volume and aspect ratio.

Silver coating gold nanorods reduces the ensemble plasmon line width by changing the relation connecting particle shape and plasmon resonance wavelength. This change, we term "plasmonic focusing", leads to less variation of resonance wavelengths for the same particle size distribution. We also find smaller single particle linewidth comparing resonances at the same wavelength but show that this does not contribute to the ensemble linewidth narrowing.

High harmonic bursts from a bulk solid are studied in the time domain. The data reveal a non-resonantly driven, non-perturbative quantum interference of crystal electrons from multiple valence bands.

A direct time-domain study of terahertz-driven high-harmonic (HH) generation from a bulk solid is presented. The HH radiation is emitted as a sequence of subcycle bursts which coincide temporally with the field crests of one polarity of the driving terahertz waveform, attesting to a novel non-perturbative quantum interference involving electrons from multiple valence bands. The results identify key mechanisms for future solid-state attosecond sources and next-generation lightwave electronics.

Size- and shape-dependent optical properties of gold nanorods allow monitoring their growth using a novel fast single-particle spectroscopy (fastSPS) method. FastSPS uses a spatially addressable electronic shutter based on a liquid crystal device to investigate particles randomly deposited on a substrate, orders of magnitude faster than other techniques. We use fastSPS to observe nanoparticle growth in situ on a single-particle level and extract quantitative data on nanoparticle growth.

Phase-locked few-cycle pulses in the ultrabroadband terahertz (THz) spectral window1–4 have become a powerful tool to access low-energy elementary dynamics in solids, on time scales shorter than a single oscillation cycle of light5–14. Here, we tackle the role of excitons and their dynamics in modern dichalcogenide monolayers and exciton-polariton condensates7–9. Furthermore, intense THz pulses are used as atomically strong bias fields to explore high-harmonic generation by dynamical Bloch oscillations in bulk solids11–13. Finally, we develop a novel microscope that introduces sub-cycle resolution on the few-nm length scale14. The experiments reviewed here challenge modern quantum theories and spark hope for electronics and magnetic storage at optical clock rates.

The strong-field interaction of crystal electrons with a low-frequency light wave leads to the emission of ultrabroadband high-order harmonics. The underlying dynamics are investigated in the frequency and in the time domain. An intricate quantum interference involving multiple electronic states in the solid is unraveled.

Ultrafast transport of electrons in semiconductors lies at the heart of high-speed electronics, electro-optics and fundamental solid-state physics. Intense phase-locked terahertz (THz) pulses at photon energies far below electronic interband resonances may serve as a precisely adjustable alternating bias, strongly exceeding d.c. breakdown voltages. Here, we exploit the near-field enhancement in gold metamaterial structures on undoped bulk GaAs, driven by few-cycle THz transients centered at 1 THz, to bias the semiconductor substrate with field amplitudes exceeding 12 MV/cm. Such fields correspond to a potential drop of the bandgap energy over a distance of only two unit cells. In this extremely off-resonant scenario characterized by a Keldysh parameter of γK ≈ 0.02, massive interband Zener tunneling injects a sizeable carrier density exceeding 1019 cm-3, and strong photoluminescence results. At a center frequency of 30 THz, THz transients with peak fields of 72 MV/cm analogously excite carriers in a bulk, semiconducting GaSe crystal, without metamaterial. Here, in contrast, we are able to drive coherent interband polarization and furthermore dynamical Bloch oscillations of electrons in the conduction band, on femtosecond time scales. The dynamics entail the generation of absolutely phase-stable high-harmonic transients containing spectral components up to the 22nd order of the fundamental frequency, spanning 12.7 optical octaves throughout the entire terahertz-to-visible domain between 0.1 and 675 THz. Our experiments establish a new field of light-wave electronics exploring coherent charge transport at optical clock rates and bring picosecond-scale electric circuitry at the interface of THz optics and electronics into reach.

Acceleration and collision of particles has been a key strategy for exploring the texture of matter. Strong light waves can control and recollide electronic wavepackets, generating high-harmonic radiation that encodes the structure and dynamics of atoms and molecules and lays the foundations of attosecond science. The recent discovery of high-harmonic generation in bulk solids combines the idea of ultrafast acceleration with complex condensed matter systems, and provides hope for compact solid-state attosecond sources and electronics at optical frequencies. Yet the underlying quantum motion has not so far been observable in real time. Here we study high-harmonic generation in a bulk solid directly in the time domain, and reveal a new kind of strong-field excitation in the crystal. Unlike established atomic sources, our solid emits high-harmonic radiation as a sequence of subcycle bursts that coincide temporally with the field crests of one polarity of the driving terahertz waveform. We show that these features are characteristic of a non-perturbative quantum interference process that involves electrons from multiple valence bands. These results identify key mechanisms for future solid-state attosecond sources and next-generation light-wave electronics. The new quantum interference process justifies the hope for all-optical band-structure reconstruction and lays the foundation for possible quantum logic operations at optical clock rates.

Terahertz near fields of gold metamaterials resonant at a frequency of $0.88\,\rm THz$ allow us to enter an extreme limit of non-perturbative ultrafast THz electronics: Fields reaching a ponderomotive energy in the keV range are exploited to drive nondestructive, quasi-static interband tunneling and impact ionization in undoped bulk GaAs, injecting electron-hole plasmas with densities in excess of $10^{19}\,\rm cm^{-3}$. This process causes bright luminescence at energies up to $0.5\,\rm eV$ above the band gap and induces a complete switch-off of the metamaterial resonance accompanied by self-amplitude modulation of transmitted few-cycle THz transients. Our results pave the way towards highly nonlinear THz optics and optoelectronic nanocircuitry with sub-picosecond switching times., Comment: Published in Physical Review Letters

The carrier wave of high-intensity phase-locked multi-THz pulses controls dynamical Bloch oscillations and interband polarization in bulk semiconductors, leading to the emission of all-coherent high-order harmonics covering 12.7 optical octaves from THz to VIS regimes.

Dynamical Bloch oscillations and coherent interband polarization in bulk GaSe are controlled by CEP-stable, ultra-intense multi-THz waveforms and result in the emission of phase-stable high-order harmonics covering 12.7 optical octaves from THz to VIS regimes.

Here we introduce a novel concept of an ultimate fastscan delay with sub-20 as precision and scan rates of tens of kHz. It is based on an acousto-optic programmable dispersive filter and exploits the propagation of the acoustic wave between successive optical pulses.

High-field multi-THz pulses are employed to analyze the coherent nonlinear response of the narrow-gap semiconductor InSb which is driven off-resonantly. Field-resolved four-wave mixing signals manifest the onset of a non-perturbative regime of Rabi flopping at external amplitudes above 5 MV/cm per pulse. Simulations based on a two-level quantum system confirm these experimental results. published

We demonstrate a fastscan-delay with a precision of 20 as and scan rates exceeding 30 kHz. The fiber-compatible device is based on an acousto-optic programmable dispersive filter and is ideally suited for pump-probe experiments.

Intense multiterahertz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5 MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a model of a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field few-cycle pulses drive the interband resonances into a nonperturbative regime of Rabi flopping. The rotating wave approximation breaks down in this case and the system reaches a complete population inversion.

We present a review of our recent nonlinear spectroscopy experiments on bulk semiconductors performed using a novel source of ultra-intense multi-THz transients. The field-induced interband optical absorption in InP is studied on subcycle timescales. Our simulations corroborate the Franz-Keldysh effect as the main reason for the observed optical anomalies. The time-resolved four-wave mixing signals generated in InSb demonstrate clear signatures of a nonperturbative excitation regime and can be qualitatively reproduced by a simplified model of a two-level system driven far from the resonance.

Cast Ni–Cr–Al base alloys with two Cr ranges, 7–9.5 at.% and 3–4 at.%, respectively, and with Re + Ta refractory element contents of up to 7.5 at.% were subjected to oxidation experiments in air. The oxidation conditions were 100 h continuously at 1,000 °C and discontinuously for 500 h at temperature by cycling between room temperature and 1,000 °C. The times at temperature to the onset of scale spallation were monitored. Scale formation was investigated by SEM, EDS and XRD. The thermal expansion of the alloys and the lattice parameter of their γ-Ni phases were determined to assess their effect on the oxidation performance. Scale adhesion appears to be affected by mechanical interlocking at the alloy/scale interface.

We present recent advances in the generation of single- and few-cycle THz transients with peak fields up to 25 MV/cm and their application for study of the Franz-Keldysh effect and four-wave mixing in bulk semiconductors.

A phase-locked terahertz transient is exploited as an ultrafast phase gate for femtosecond optical pulses. We directly map out the group delay dispersion of a low-power near-infrared pulse by measuring the electro-optically induced polarization rotation as a function of wavelength. Our experiment covers the spectral window from 1.0 to 1.4 μm and reaches a temporal precision better than 1 fs. A quantitative analysis of the detector response confirms that this streaking technique requires no reconstruction algorithm and is also well suited for the characterization of pulses spanning more than one optical octave.

Intense THz electric and magnetic fields are exploited to drive extreme nonlinearities in semiconductors, magnetically ordered solids, and molecular systems. Electro-optic sampling allows us to perform field-sensitive two-dimensional spectroscopy, including four- and six-wave mixing in indium antimonide, as well as THz bias — optical probe experiments beyond static dielectric breakdown fields. Finally, we introduce a novel scheme for coherent spin control in antiferromagnetic nickel oxide mediated by the THz magnetic field component.

Precise characterization techniques for ultrashort optical pulses have become indispensible with the emergence of ever shorter waveforms. Typically the pulse under study is overlapped with an optical gate and analyzed after a nonlinear interaction with the latter. If the duration of the pulse to be characterized is much shorter or longer than available gate sources, streak cameras [1, 2] or electro-optic sampling [3] are used, respectively. If the duration of both pulses is similar, auto- and cross-correlation techniques [4] are required, which need complex deconvolution algorithms and are often limited to bandwidths below one optical octave.

A passive terahertz (THz) emitter based on the lateral photo-Dember effect enables the efficient generation of THz radiation by an Er:fiber laser at 1.55 µm. The substrate material is provided by a thin layer of conventional In 0.53 Ga 0.47 As grown on InP in which unidirectional lateral photo-Dember currents are excited [1].

Two-dimensional-spectroscopy with near-infrared (NIR) or mid-infrared pulses has given insight into ultrafast dynamics and spectral correlations in many systems [1]. Terahertz (THz) four-wave-mixing (FWM) has attracted growing interest as a potential way to access important low-energy excitations such as inter- and intra-molecular vibrations or energy gaps in superconductors. However, lead-off experiments [2] have been limited to model systems due to moderate THz fields. Here we employ a recently developed ultraintense multi-THz source [3,4] to pursue THz four-wave mixing in a bulk semiconductor even under two-photon resonant conditions. The temporal profile of the strong FWM signal shows first indications of a THz photon echo. A non-collinear geometry allows for spatial selection of nonlinear signals and background-free measurements.

We present a table-top source of extremely intense multi-THz transients covering the spectral region between 0.1 and 140 THz. Electric field amplitudes of up to 108 MV/cm and pulse durations as short as a single cycle are demonstrated with our hybrid Er:fiber-Ti:sapphire laser system. All THz waveforms are electro-optically detected. This source opens the door to a regime of non-perturbative THz nonlinearities in condensed matter. First examples range from coherent control of excitons, via a breakdown of the power expansion of the nonlinear polarization in bulk semiconductors to twodimensional multi-wave mixing and direct femtosecond spin control by magnetic field excitation.

Phase-locked multi-THz transients bias semiconductors far above the usual threshold for dielectric breakdown. Few-cycle NIR pulses synchronized to the THz transients on an as-timescale, probe interband transitions under electric fields of 4 MV/cm.

High-intensity single- and few-cycle transients covering the mid and far infrared are generated and electro-optically monitored by a passively CEP-locked laser. These pulses drive strong nonlinearities via electric and magnetic field coupling.

We demonstrate a technique for the characterization of ultrabroadband and low-intensity laser pulses using spectrally resolved THz polarization streaking. Our method yields the spectral amplitude and phase without resorting to complex algorithms.

We study the wavelength and polarization dependent plasmon resonances of single silver and gold nanorods, triangles, cubes, and dimers with a novel single particle spectroscopy method (RotPOL). In RotPOL, a rotating wedge-shaped polarizer encodes the full polarization information of each particle within one image. This reveals the symmetry of the particles and their plasmon modes, allows analyzing inhomogeneous samples and the monitoring of particle shape changes during growth in situ.

An optical fast-scan delay exploiting the near-collinear interaction between a train of ultrashort optical pulses and an acoustic wave propagating in a birefringent crystal is introduced. In combination with a femtosecond Er:fiber laser, the scheme is shown to delay few-fs pulses by up to 6 ps with a precision of 15 as. A resolution of 5 fs is obtained for a single sweep at a repetition rate of 34 kHz. This value can be improved to 39 as for multiple scans at a total rate of 0.3 kHz., 4 pages, 4 figures $\copyright$ 2013 Optical Society of America. Users may use, reuse, and build upon the article, or use the article for text or data mining, so long as such uses are for non-commercial purposes and appropriate attribution is maintained. All other rights are reserved. https://www.osapublishing.org/ol/fulltext.cfm?uri=ol-38-15-2907&id=259829

We present recent advances in the generation of highly intense multiterahertz transients and their application to nonlinear spectroscopy of bulk semiconductors. An optimized scheme of parametric amplification results in broadband single- or few-cycle terahertz transients with peak electric fields up to 10 or 25 MV/cm, respectively. Time-resolved four-wave mixing terahertz spectroscopy of InSb far away from the interband resonance demonstrates clear signatures of a nonperturbative regime of Rabi flopping. We qualitatively explain the observed behavior within a model of a quantum two-level system. In addition, we demonstrate the dynamical Franz-Keldysh effect in InP resolved on a subcycle timescale. The field-induced modulation of the interband optical absorption at the second harmonic of the driving terahertz field is observed in full agreement with theoretical predictions.

A terahertz emitter based on the lateral photo-Dember effect is shown to efficiently generate terahertz radiation with a peak frequency of 0.7 THz and an electric field amplitude up to 5 V/cm when excited by 90 fs pulses centered at 1.55 μm. A thin layer of In_0.53 Ga_0.47 As grown on InP provides the substrate material in which unidirectional lateral photo-Dember currents are excited. Since photo-Dember terahertz emitters do not require an external bias, they do not suffer from high dark currents limiting the application of biased InGaAs photoconductive terahertz emitters.