Advances in large viewing angle and achromatic 3D holography

Optical holography is a promising technique to achieve a naked-eye 3D display.However,the narrow viewing angle and chromatic aberration are the two key issues that usually limit the holographic display performance.A recent work proposes a novel way to circumvent these constraints by introducing a color liquid crystal grating into a timesequenced holography system.

A concept for multiterawatt fibre lasers based on coherent pulse stacking in passive cavities

Since the advent of femtosecond lasers,performance improvements have constantly impacted on existing applications and enabled novel applications.However,one performance feature bearing the potential of a quantum leap for high-field applications is still not available:the simultaneous emission of extremely high peak and average powers.Emerging applications such as laser particle acceleration require exactly this performance regime and,therefore,challenge laser technology at large.On the one hand,canonical bulk systems can provide pulse peak powers in the multi-terawatt to petawatt range,while on the other hand,advanced solid-state-laser concepts such as the thin disk,slab or fibre are well known for their high efficiency and their ability to emit high average powers in the kilowatt range with excellent beam quality.In this contribution,a compact laser system capable of simultaneously providing high peak and average powers with high wall-plug efficiency is proposed and analysed.The concept is based on the temporal coherent combination(pulse stacking)of a pulse train emitted from a high-repetition-rate femtosecond laser system in a passive enhancement cavity.Thus,the pulse energy is increased at the cost of the repetition rate while almost preserving the average power.The concept relies on a fast switching element for dumping the enhanced pulse out of the cavity.The switch constitutes the key challenge of our proposal.Addressing this challenge could,for the first time,allow the highly efficient dumping of joule-class pulses at megawatt average power levels and lead to unprecedented laser parameters.

Dielectric metasurfaces for distance measurements and three-dimensional imaging

Ultrathin metasurfaces have shown the capability to influence all aspects of light propagation.This has made them promising options for replacing conventional bulky imaging optics while adding advantageous optical properties or functionalities.We demonstrate that such metasurfaces can also be applied for single-lens three-dimensional(3-D)imaging based on a specifically engineered point-spread function(PSF).Using Huygens’metasurfaces with high transmission,we design and realize a phase mask that implements a rotating PSF for 3-D imaging.We experimentally characterize the properties of the realized double-helix PSF,finding that it can uniquely encode object distances within a wide range.Furthermore,we experimentally demonstrate wide-field depth retrieval within a 3-D scene,showing the suitability of metasurfaces to realize optics for 3-D imaging,using just a single camera and lens system.

GOBO projection for 3D measurements at highest frame rates: a performance analysis

Aperiodic sinusoidal patterns that are cast by a GOBO(GOes Before Optics)projector are a powerful tool for optically measuring the surface topography of moving or deforming objects with very high speed and accuracy.We optimised the first experimental setup that we were able to measure inflating car airbags at frame rates of more than 50 kHz while achieving a 3D point standard deviation of~500μm.Here,we theoretically investigate the method of GOBO projection of aperiodic sinusoidal fringes.In a simulation-based performance analysis,we examine the parameters that influence the accuracy of the measurement result and identify an optimal pattern design that yields the highest measurement accuracy.We compare the results with those that were obtained via GOBO projection of phase-shifted sinusoidal fringes.Finally,we experimentally verify the theoretical findings.We show that the proposed technique has several advantages over conventional fringe projection techniques,as the easy-to-build and cost-effective GOBO projector can provide a high radiant flux,allows high frame rates,and can be used over a wide spectral range.

Simulation-based design optimization of the holographic wavefront sensor in closed-loop adaptive optics

Adaptive optics systems are used to compensate for wavefront distortions introduced by atmospheric turbulence.The distortions are corrected by an adaptable device,normally a deformable mirror.The control signal of the mirror is based on the measurement delivered by a wavefront sensor.Relevant characteristics of the wavefront sensor are the measurement accuracy,the achievable measurement speed and the robustness against scintillation.The modal holographic wavefront sensor can theoretically provide the highest bandwidth compared to other state of the art wavefront sensors and it is robust against scintillation effects.However,the measurement accuracy suffers from crosstalk effects between different aberration modes that are present in the wavefront.In this paper we evaluate whether the sensor can be used effectively in a closed-loop AO system under realistic turbulence conditions.We simulate realistic optical turbulence represented by more than 2500 aberration modes and take different signal-to-noise ratios into account.We determine the performance of a closed-loop AO system based on the holographic sensor.To counter the crosstalk effects,careful choice of the key design parameters of the sensor is necessary.Therefore,we apply an optimization method to find the best sensor design for maximizing the measurement accuracy.By modifying this method to take the changing effective turbulence conditions during closed-loop operation into account,we can improve the performance of the system,especially for demanding signal-to-noise-ratios,even more.Finally,we propose to implement multiple holographic wavefront sensors without the use of additional hardware,to perform multiple measurement at the same time.We show that the measurement accuracy of the sensor and with this the wavefront flatness can be increased significantly without reducing the bandwidth of the adaptive optics system.

Spatially engineered nonlinearity in resonant metasurfaces

Spatial engineering of the nonlinear susceptibility χ^((2)) in resonant metasurfaces offers a new degree of freedom in the design of the far-field response of second-harmonic generation(SHG). We demonstrate this by applying electric field poling to lithium niobate(LN) thin films, which inverts the spontaneous polarization and thus the sign of χ^((2)). Metasurfaces fabricated in periodically poled LN films reveal the distinct influence of theχ^((2))-patterning on the spatial distribution of the second harmonic. This work is a first step toward far-field engineering of SHG in metasurfaces with electric field poling.

Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation

Rare earth-doped fibres are a diode-pumped,solid-state laser architecture that is highly scalable in average power.The performance of pulsed fibre laser systems is restricted due to nonlinear effects.Hence,fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest.Ytterbium-doped,rod-type,large-pitch fibres(LPF)enable extreme fibre dimensions,i.e.,effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation,by exploiting the novel concept of delocalisation of higher-order transverse modes.The non-resonant nature of the operating principle makes LPF suitable for high power extraction.This design allows for an unparalleled level of performance in pulsed fibre lasers.

Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources

The process of high harmonic generation(HHG)enables the development of table-top sources of coherent extreme ultraviolet(XUV)light.Although these are now matured sources,they still mostly rely on bulk laser technology that limits the attainable repetition rate to the low kilohertz regime.Moreover,many of the emerging applications of such light sources(e.g.,photoelectron spectroscopy and microscopy,coherent diffractive imaging,or frequency metrology in the XUV spectral region)require an increase in the repetition rate.Ideally,these sources are operated with a multi-MHz repetition rate and deliver a high photon flux simultaneously.So far,this regime has been solely addressed using passive enhancement cavities together with low energy and high repetition rate lasers.Here,a novel route with significantly reduced complexity(omitting the requirement of an external actively stabilized resonator)is demonstrated that achieves the previously mentioned demanding parameters.A krypton-filled Kagome photonic crystal fiber is used for efficient nonlinear compression of 9 μJ,250 fs pulses leading to,7 μJ,31 fs pulses at 10.7 MHz repetition rate.The compressed pulses are used for HHG in a gas jet.Particular attention is devoted to achieving phase-matched(transiently)generation yielding.10^(13) photons s^(-1)(.50 μW)at 27.7 eV.This new spatially coherent XUV source improved the photon flux by four orders of magnitude for direct multi-MHZ experiments,thus demonstrating the considerable potential of this source.

Enhancing precision in fs-laser material processing by simultaneous spatial and temporal focusing

In recent years,femtosecond(fs)-lasers have evolved into a versatile tool for high precision micromachining of transparent materials because nonlinear absorption in the focus can result in refractive index modifications or material disruptions.However,when high pulse energies or low numerical apertures are required,nonlinear side effects such as self-focusing,filamentation or white light generation can decrease the modification quality.In this paper,we apply simultaneous spatial and temporal focusing(SSTF)to overcome these limitations.The main advantage of SSTF is that the ultrashort pulse is only formed at the focal plane,thereby confining the intensity distribution strongly to the focal volume and suppressing detrimental nonlinear side effects.Thus,we investigate the optical breakdown within a water cell by pump-probe shadowgraphy,comparing conventional focusing and SSTF under equivalent focusing conditions.The plasma formation is well confined for low pulse energies,2 mJ,but higher pulse energies lead to the filamentation and break-up of the disruptions for conventional focusing,thereby decreasing the modification quality.In contrast,plasma induced by SSTF stays well confined to the focal plane,even for high pulse energies up to 8 mJ,preventing extended filaments,side branches or break-up of the disruptions.Furthermore,while conventional focusing leads to broadband supercontinuum generation,only marginal spectral broadening is observed using SSTF.These experimental findings are in excellent agreement with numerical simulations of the nonlinear pulse propagation and interaction processes.Therefore,SSTF appears to be a powerful tool to control the processing of transparent materials,e.g.,for precise ophthalmic fs-surgery.

Watt-scale super-octave mid-infrared intrapulse difference frequency generation

The development of high-power,broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics,spectroscopy,microscopy,and fundamental science.One of the major,long-standing challenges in improving the performance of these applications has been the construction of compact,broadband mid-infrared radiation sources,which unify the properties of high brightness and spatial and temporal coherence.Due to the lack of such radiation sources,several emerging applications can be addressed only with infrared(IR)-beamlines in largescale synchrotron facilities,which are limited regarding user access and only partially fulfill these properties.Here,we present a table-top,broadband,coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18μm by several orders of magnitude.This result is enabled by a high-power,few-cycle Tm-doped fiber laser system,which is employed as a pump at 1.9μm wavelength for intrapulse difference frequency generation(IPDFG).IPDFG intrinsically ensures the formation of carrierenvelope-phase stable pulses,which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy.

Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers

Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability(TMI)above an average power threshold,which represents a severe problem for many applications.To obtain a deeper understanding of TMI,the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time.This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern,which is obtained by applying a pump power variation to the fiber amplifier.It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large.The experiments indicate that at higher powers,the refractive index grating becomes more sensitive to such phase shifts,which will ultimately trigger TMI.Furthermore,the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift.This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.

Bright,high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre

Bright,coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences.To date,a high photon flux in this spectral region can only be delivered by synchrotrons,free-electron lasers or high-order harmonic generation sources,which are driven by kHz-class repetition rate lasers with very high peak powers.Here,we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experimentin which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-orderharmonic generation in a single,helium-illed antiresonant hollow-core fibre.This enables the first 100 kHz-classrepetition rate,table-top soft X-ray source that delivers an application-relevant flux of 2.8×10^6 photon s^-1 eV^-1 around 300 ev.The fibre integration of temporal pulse self-compression(leading to the formation of the necessarystrong-field waveforms)and pressure-controlled phase matching will allow compact,high-repetition-rate lasertechnology,including commercially available systems,to drive simple and cost-effective,coherent high-flux softX-ray sources.

Petawatt and exawatt class lasers worldwide

In the 2015 review paper‘Petawatt Class Lasers Worldwide’a comprehensive overview of the current status of highpower facilities of>200 TW was presented.This was largely based on facility specifications,with some description of their uses,for instance in fundamental ultra-high-intensity interactions,secondary source generation,and inertial confinement fusion(ICF).With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification(CPA),which made these lasers possible,we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed.We are now in the era of multi-petawatt facilities coming online,with 100 PW lasers being proposed and even under construction.In addition to this there is a pull towards development of industrial and multi-disciplinary applications,which demands much higher repetition rates,delivering high-average powers with higher efficiencies and the use of alternative wavelengths:mid-IR facilities.So apart from a comprehensive update of the current global status,we want to look at what technologies are to be deployed to get to these new regimes,and some of the critical issues facing their development.

55 Years of Holographic Non-Destructive Testing and Experimental Stress Analysis:Is there still Progress to be expected?

Holographic methods for non-destructive testing,shape measurement,and experimental stress analysis have shown to be versatile tools for the solution of many inspection problems.Their main advantages are the non-contact nature,the non-destructive and areal working principle,the fast response,high sensitivity,resolution and precision.In contrast to conventional optical techniques such as classical interferometry,the holographic principle of wavefront storage and reconstruction makes it possible to investigate objects with rough surfaces.Consequently,the response of various classes of products on operational or artificial load can be examined very elegantly.The paper looks back to the history of holographic metrology,honors the inventors of the main principles,discusses criteria for the selection of a proper inspection method,and shows exemplary applications.However,the main focus is on modern developments that are inspired by the rapid technological process in sensing technology and digitization,on current applications and future challenges.

Material-specific high-resolution table-top extreme ultraviolet microscopy

Microscopy with extreme ultraviolet(EUV)radiation holds promise for high-resolution imaging with excellent material contrast,due to the short wavelength and numerous element-specific absorption edges available in this spectral range.At the same time,EUV radiation has significantly larger penetration depths than electrons.It thus enables a nano-scale view into complex three-dimensional structures that are important for material science,semiconductor metrology,and next-generation nano-devices.Here,we present high-resolution and material-specific microscopy at 13.5 nm wavelength.We combine a highly stable,high photon-flux,table-top EUV source with an interferometrically stabilized ptychography setup.By utilizing structured EUV illumination,we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm.Moreover,we propose mixed-state orthogonal probe relaxation ptychography,enabling robust phase-contrast imaging over wide fields of view and long acquisition times.In this way,the complex transmission of an integrated circuit is precisely reconstructed,allowing for the classification of the material composition of mesoscopic semiconductor systems.

Multi-positional image-based vibration measurement by holographic image replication

In this study we present a novel and flexibly applicable method to measure absolute and relative vibrations accurately in a field of 148 mm×110 mm at multiple positions simultaneously.The method is based on imaging in combination with holographic image replication of single light sources onto an image sensor,and requires no calibration for small amplitudes.We experimentally show that oscillation amplitudes of 100 nm and oscillation frequencies up to 1000 Hz can be detected clearly using standard image sensors.The presented experiments include oscillations of variable amplitude and a chirp signal generated with an inertial shaker.All experiments were verified using state-of-the-art vibrometers.In contrast to conventional vibration measurement approaches,the proposed method offers the possibility of measuring relative movements between several light sources simultaneously.We show that classical band-pass filtering can be omitted,and the relative oscillations between several object points can be monitored.

Nonlinear quantum spectroscopy with parity-time-symmetric integrated circuits

We propose a novel quantum nonlinear interferometer design that incorporates a passive parity-time(PT)-symmetric coupler sandwiched between two nonlinear sections where signal-idler photon pairs are generated.The PT symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates the sensing of losses in the second waveguide exposed to analyte under investigation,whose absorption can be inferred by measuring only the signal intensity at a shorter wavelength where efficient detectors are readily available.Remarkably,we identify a new phenomenon of sharp signal intensity fringe shift at critical idler loss values,which is distinct from the previously studied PT symmetry breaking.We discuss how such unconventional properties arising from quantum interference can provide a route to enhancing the sensing of analytes and facilitate broadband spectroscopy applications in integrated photonic platforms.

Pulsed DC magnetron sputtering of transparent conductive oxide layers

A comprehensive material study of different transparent conductive oxides （TCOs） is presented. The layers are deposited by pulsed direct current （DC） magnetron sputtering in an inline sputtering system. Indium tin oxide （ITO） films are studied in detail. The optimum pressure of 0.33 Pa （15Ar：202） produces a 300- nm thin film with a specific resistivity p of 2.2 × 10-6 Ωm and a visual transmittance of 81%. Alternatively, ZnO：A1 and ZnO：Ga layers with thicknesses of 200 and 250 nm are deposited with a minimum resistivity of 5.5× 10-6 and 6.8× 10-6Ωm, respectively. To compare the optical properties in the ultraviolet （UV） range, the optical spectra are modeled and the band gap is determined.