Fourier-transform Limit Research Articles

Octave-spanning supercontinuum coherent soft x-ray for producing a single-cycle soft x-ray pulse.

This study demonstrates the potential to generate a soft x-ray single-cycle attosecond pulse using a single-cycle mid-infrared pulse from advanced dual-chirped optical parametric amplification (DC-OPA). A super continuum high harmonic (HH) spectrum was generated in argon (80-160 eV) and neon (150-270 eV). The experimental spectra reasonably agree with those calculated by the strong-field approximation model and Maxwell's equations. In addition, simulation results indicate that the dispersion of HHs in argon can be compensated using a 207-nm Zr filter to obtain 40 as (Fourier transform limited (FTL)) pulses (1.1 cycles at 118 eV). For neon, a 278-nm Sn filter can compensate for the dispersion of HH and create 23 as FTL pulses (1.1 cycles at 206 eV). This soft x-ray single-cycle attosecond pulse is expected to be highly valuable for ultrafast science and applications in quantum information science.

Pushing photon-pair generation rate in microresonators by Q factor manipulation.

Photon pairs generated by employing spontaneous nonlinear effects in microresonators are critically essential for integrated optical quantum information technologies, such as quantum computation and quantum cryptography. Microresonators featuring high-quality (Q) factors can offer simple yet power-efficient means to generate photon pairs, thanks to the intracavity field enhancement. In microresonators, it is known that the photon-pair generation rate (PGR) is roughly proportional to the cubic power of the Q factor. However, the upper limit on PGR is also set by the Q factor: a higher Q factor brings a longer photon lifetime, which in turn leads to a lower repetition rate allowing for photon flow emitted from the microresonator, constrained by the Fourier-transform limit. Exceeding this limit will result in the overlap of photon wave packets in the time domain, thus degrading the quantum character of single-photon light beams. To push the limit of PGR in a single resonator, we propose a method by harnessing the resonance linewidth-manipulated microresonators to improve the maximum achievable photon repetition rate while keeping the power efficiency. The maximum achievable PGR and power efficiency are thus balanced by leveraging the combination of low and high-Q resonances.

A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources.

Triggered, indistinguishable single photons are crucial in various quantum photonic implementations. Here, we realize a novel n+-i-n++ diode structure embedding semiconductor quantum dots: the gated device enables spectral tuning of the transitions and deterministic control of the charged states. Blinking-free single-photon emission and high two-photon indistinguishability are observed. The line width's temporal evolution is investigated across over 6 orders of magnitude time scales, combining photon-correlation Fourier spectroscopy, high-resolution photoluminescence spectroscopy, and two-photon interference (visibility of VTPI,2ns = (85.8 ± 2.2)% and VTPI,9ns = (78.3 ± 3.0)%). Most of the dots show no spectral broadening beyond ∼9 ns time scales, and the photons' line width ((420 ± 30) MHz) deviates from the Fourier-transform limit by a factor of 1.68. The combined techniques verify that most dephasing mechanisms occur at time scales ≤2 ns, despite their modest impact. The presence of n-doping implies higher carrier mobility, enhancing the device's appeal for high-speed tunable, high-performance quantum light sources.

Time-reversal focusing of ultrashort pulses through thin scattering media.

When ultrashort pulses propagate through a disordered medium, scattering occurs and the intensity of the ballistic component decreases drastically. This limits the applicability of time-resolved nonlinear optical spectroscopy and microscopy. The wavefront shaping technique makes it possible to focus light through the scattering medium; however, complete time-reversal of the ultrashort pulses (as short as 10 fs) is still a very challenging problem. This is due to the in-depth characterization and precise control needed for such pulses in the time domain in order to compress down the Fourier-transform limit. In this work, we develop new spatiotemporal wavefront shaping techniques to focus ultrashort pulses at the target position through a thin scattering medium. Compared to other studies, one significant advantage of this method is that most of the characterization of the spectrally-resolved transmission matrix and temporal profile of the ultrashort pulses can be done using single-beam geometry. An interferometer with external reference is necessary to measure the difference of the phase profile between the focused and reference pulses. Furthermore, the number of controllable phase components in the spectral domain is not limited by the spectral correlations of the speckle patterns because we used a pulse shaper in the time domain to optimize the temporal properties of the ultrashort focused pulse. Our new method provides increased flexibility and precise control for manipulating extremely ultrashort pulses through thin scattering media in order to achieve time-reversal focusing at the target position.

Utilizing the temporal superresolution approach in an optical parametric synthesizer to generate multi-TW sub-4-fs light pulses.

The Fourier-transform limit achieved by a linear spectral phase is the typical optimum by the generation of ultrashort light pulses. It provides the highest possible intensity, however, not the shortest full width at half maximum of the pulse duration, which is relevant for many experiments. The approach for achieving shorter pulses than the original Fourier limit is termed temporal superresolution. We demonstrate this approach by shaping the spectral phase of light from an optical parametric chirped pulse amplifier and generate sub-Fourier limited pulses. We also realize it in a simpler way by controlling only the amplitude of the spectrum, producing a shorter Fourier-limited duration. Furthermore, we apply this technique to an optical parametric synthesizer and generate multi-TW sub-4-fs light pulses. This light source is a promising tool for generating intense and isolated attosecond light and electron pulses.

High-Damage-Threshold Chirped Mirrors for Next-Generation Ultrafast, High-Power Laser Systems

This study presents the concept of high-damage-threshold chirped mirrors (HDTCMs) for ultrafast high-power laser systems. We describe a novel design method for the fabrication of HDTCMs to simultaneously achieve high reflectivity and high laser-induced damage threshold (LIDT) over a broad bandwidth. In the proposed novel structure, the squared electric field distribution oscillations of the laser pulse stagger each other, leading to a low average squared E-field distribution and effectively enhancing the LIDT. As a proof-of-concept study, an HDTCM is designed based on our simulation. The LIDT of the HDTCM increases by a factor of ~2.3. Moreover, a HDTCM pair is successfully applied in a cross-polarized-wave (XPW) filter, supporting a near Fourier transform limit (FLT) 17.7-fs pulse generation. This concept can be generalized to virtually any optical coating design, thus advancing high damage resistant mirror fabrication to a level well suited to high-power laser applications.

Dispersion management for a 100 PW level laser using a mismatched-grating compressor

Abstract We report dispersion management based on a mismatched-grating compressor for a 100 PW level laser, which utilizes optical parametric chirped pulse amplification and also features large chirped pulse duration and an ultra-broadband spectrum. The numerical calculation indicates that amplified pulses with 4 ns chirped pulse duration and 210 nm spectral bandwidth can be directly compressed to sub-13 fs, which is close to the Fourier-transform limit (FTL). More importantly, the tolerances of the mismatched-grating compressor to the misalignment of the stretcher, the error of the desired grating groove density and the variation of material dispersion are comprehensively analyzed, which is crucially important for its practical application. The results demonstrate that good tolerances and near-FTL compressed pulses can be achieved simultaneously, just by keeping a balance between the residual second-, third- and fourth-order dispersions in the laser system. This work can offer a meaningful guideline for the design and construction of 100 PW level lasers.

Use of double-grating Offner stretcher for dispersion control in petawatt level optical parametric chirped pulse amplification systems

The aberration-free characteristic endows the double-grating Offner stretcher with the advantages of good beam quality and perfect-dispersion-match with conjugated Treacy compressor, and therefore makes it an ideal pulse stretcher for optical parametric chirped pulse amplification (OPCPA) systems featuring with low material dispersion. We numerically and experimentally demonstrate the feasibility of dispersion control in OPCPA systems by the use of double-grating Offner stretcher. The numerical results indicate that near Fourier-transform-limited (FTL) pulses can be realized in the double-grating Offner stretcher based petawatt (PW) level broadband OPCPA systems. Moreover, in a proof-of-principle experiment, amplified pulses with ∼210 nm spectral bandwidth and ∼3 ns chirped pulse duration are also compressed to near FTL pulse duration of 15.4 fs just by cooperating a double-grating Offner stretcher with a Treacy compressor. To our knowledge, it is the first time to realize near FTL compression of laser pulses with such a broadband spectrum and such a large chirped pulse duration simultaneously, while without using any extra dispersion compensation methods.

Deep Learning on Synthesized Sensor Characteristics and Transmission Spectra Enabling MEMS-Based Spectroscopic Gas Analysis beyond the Fourier Transform Limit

Miniaturized Fourier transform infrared spectrometers serve emerging market needs in many applications such as gas analysis. The miniaturization comes at the cost of lower performance than bench-top instrumentation, especially for the spectral resolution. However, higher spectral resolution is needed for better identification of the composition of materials. This article presents a convolutional neural network (CNN) for 3X resolution enhancement of the measured infrared gas spectra using a Fourier transform infrared (FTIR) spectrometer beyond the transform limit. The proposed network extracts a set of high-dimensional features from the input spectra and constructs high-resolution outputs by nonlinear mapping. The network is trained using synthetic transmission spectra of complex gas mixtures and simulated sensor non-idealities such as baseline drifts and non-uniform signal-to-noise ratio. Ten gases that are relevant to the natural and bio gas industry are considered whose mixtures suffer from overlapped features in the mid-infrared spectral range of 2000–4000 cm−1. The network results are presented for both synthetic and experimentally measured spectra using both bench-top and miniaturized MEMS spectrometers, improving the resolution from 60 cm−1 to 20 cm−1 with a mean square error down to 2.4×10−3 in the transmission spectra. The technique supports selective spectral analysis based on miniaturized MEMS spectrometers.

Additional Cover

The cover image is based on the Special Issue – Research Article Development of a highly stable picosecond time-resolved Raman spectrometer near Fourier-transform limit with a femtosecond light source by Tsukasa Tokita et al., https://doi.org/10.1002/jrs.6232.

A 515-nm laser-pumped idler-resonant femtosecond BiB3O6 optical parametric oscillator

We report on an idler-resonant femtosecond optical parametrical oscillator (OPO) based on BiB3O6 (BiBO) crystal, synchronously pumped by a frequency-doubled, mode-locked Yb:KGW laser at 515 nm. The idler wavelengths of OPO can be tuned from 1100 nm to 1540 nm. At a repetition rate of 75.5 MHz, the OPO generates as much as 400 mW of idler power with 3.1 W of pump power, the corresponding pulse duration is 80 fs, which is 1.04 times of Fourier transform-limited (FTL) pulse duration at 1305 nm. In addition, the OPO exhibits excellent beam quality with M 2 < 1.8 at 1150 nm. To the best of our knowledge, this is the first idler-resonant femtosecond OPO pumped by 515 nm.

Extremely regular periodic surface structures in a large area efficiently induced on silicon by temporally shaped femtosecond laser

Femtosecond laser-induced periodic surface structures (LIPSS) have several applications in surface structuring and functionalization. Three major challenges exist in the fabrication of regular and uniform LIPSS: enhancing the periodic energy deposition, reducing the residual heat, and avoiding the deposited debris. Herein, we fabricate an extremely regular low-spatial-frequency LIPSS (LSFL) on a silicon surface by a temporally shaped femtosecond laser. Based on a 4 f configuration zero-dispersion pulse shaping system, a Fourier transform limit (FTL) pulse is shaped into a pulse train with varying intervals in the range of 0.25–16.2 ps using periodic π -phase step modulation. Under the irradiation of the shaped pulse with an interval of 16.2 ps, extremely regular LSFLs are efficiently fabricated on silicon. The scan velocity for fabricating regular LSFL is 2.3 times faster, while the LSFL depth is 2 times deeper, and the diffraction efficiency is 3 times higher than those of LSFL using the FTL pulse. The formation mechanisms of regular LSFL have been studied experimentally and theoretically. The results show that the temporally shaped pulse enhances the excitation of surface plasmon polaritons and the periodic energy deposition while reducing the residual thermal effects and avoiding the deposition of the ejected debris, eventually resulting in regular and deeper LSFL on the silicon surface.

Proton beam quality enhancement by spectral phase control of a PW-class laser system

We report on experimental investigations of proton acceleration from solid foils irradiated with PW-class laser-pulses, where highest proton cut-off energies were achieved for temporal pulse parameters that varied significantly from those of an ideally Fourier transform limited (FTL) pulse. Controlled spectral phase modulation of the driver laser by means of an acousto-optic programmable dispersive filter enabled us to manipulate the temporal shape of the last picoseconds around the main pulse and to study the effect on proton acceleration from thin foil targets. The results show that applying positive third order dispersion values to short pulses is favourable for proton acceleration and can lead to maximum energies of 70 MeV in target normal direction at 18 J laser energy for thin plastic foils, significantly enhancing the maximum energy compared to ideally compressed FTL pulses. The paper further proves the robustness and applicability of this enhancement effect for the use of different target materials and thicknesses as well as laser energy and temporal intensity contrast settings. We demonstrate that application relevant proton beam quality was reliably achieved over many months of operation with appropriate control of spectral phase and temporal contrast conditions using a state-of-the-art high-repetition rate PW laser system.

Widely wavelength-tunable mode locked Er-doped fiber oscillator from 1532 nm to 1594 nm with high signal-to-noise ratio

A widely wavelength tunable mode locked Er-doped fiber laser oscillator based on nonlinear polarization rotation (NPR) effect was demonstrated. By intentionally introducing a position-and-width adjustable slit and a pair of reflective gratings into the cavity, which worked as a wavelength and bandwidth tunable spectral filter, continuous wavelength tunable mode locked operation from 1532 nm to 1594 nm was achieved, corresponding to a tuning range of 62 nm, which is a new record for mode locked continuously wavelength-tunable single mode Er-doped fiber oscillators based on nonlinear polarization rotation, to the best of our knowledge. During the wavelength tuning, the pulse durations for different wavelengths changed slightly and closed to the Fourier-transform limit. The signal-to-noise ratio (SNR) of the radio frequency (RF) spectrum of the output pulses is higher than 89 dB over the entire wavelength tuning range, superior to the typical SNR of 50–70 dB reported in most papers. In addition, the pulse duration could be tuned from 904 fs to 1339 fs by adjusting the width of the slit, for which the corresponding spectral bandwidth changed from 3.4 nm to 1.7 nm.

Performance improvement of a 200TW/1Hz Ti:sapphire laser for laser wakefield electron accelerator

For investigations on laser wakefield electron acceleration, there is a strong demand for driving laser exhibiting high stability and good spatial-temporal quality. Here we present an upgraded 200TW/1Hz Ti:sapphire laser with following key characteristics. The fluctuation of pulse energy and beam pointing for consecutive 5400 pulses in 90 min are as low as 0.55% and 1.5μrad respectively. The compressed pulse duration is 23.7 fs, which is about 1.1 times of the Fourier-transform limit. The focal spot obtained by an f/30 off-axis parabolic mirror is 54 μm × 52 μm at 1/e2, which is very close to the diffraction limit. Moreover, in the laser wakefield electron acceleration experiments driven by this upgraded 200TW/1Hz laser, quasi-mono-energetic electron beams are reproduced in consecutive shots. For successive 300 shots, the average peak energy of produced electron beams is 667 MeV, and the peak energy fluctuation is only 3.4%. The progresses on both driving laser and laser wakefield electron acceleration make the realization of compact X-ray free electron laser possible.

Optical frequency comb-based cavity-enhanced Fourier-transform spectroscopy: Application to collisional line-shape study

Direct-comb spectroscopy techniques uses optical frequency combs (OFCs) as spectroscopic light source. They deliver high sensitivity, high frequency resolution and precision in a broad spectral range. Due to these features, the field has burgeoned in recent years. In this work we constructed an OFC-based cavity-enhanced Fourier-transform spectrometer in the near-infrared region and used it for a line-shape study of rovibrational transitions of CO perturbed by Ar. The highly sensitive measurements spanned the wavenumber range from 6270 cm−1 to 6410 cm−1, which covered both P and R branch of the second overtone band of CO. The spectrometer delivers high-resolution surpassing the Fourier-transform resolution limit determined by interferogram length, successfully removing ringing and broadening effects caused by instrumental line shape function. The instrumental-line-shape-free method and high signal-to-noise ratio in the measurement allowed us to observe collisional effects beyond those described by the Voigt profile. We retrieved collisional line-shape parameters by fitting the speed-dependent Voigt profile and found good agreement with the values given by precise cavity ring-down spectroscopy measurements that used a continuous-wave laser referenced to a stabilized OFC. The results demonstrate that OFC-based cavity-enhanced Fourier-transform spectroscopy is a strong tool for accurate line-shape studies that will be crucial for future spectral databases.

Preparing single SiV− center in nanodiamonds for external, optical coupling with access to all degrees of freedom

Optical coupling enables intermediate- and long-range interactions between distant quantum emitters. Such interaction may be the basic element in bottom-up approaches of coupled spin systems or for integrated quantum photonics and quantum plasmonics. Here, we prepare nanodiamonds carrying single, negatively-charged silicon-vacancy centers for evanescent optical coupling with access to all degrees of freedom by means of atomic force nanomanipulation. The color centers feature excellent optical properties, comparable to silicon-vacancy centers in bulk diamond, resulting in a resolvable fine structure splitting, a linewidth close to the Fourier-Transform limit under resonant excitation and a good polarization contrast. We determine the orbital relaxation time T1 of the orbitally split ground states and show that all optical properties are conserved during translational nanomanipulation. Furthermore, we demonstrate the rotation of the nanodiamonds. In contrast to the translational operation, the rotation leads to a change in polarization contrast. We utilize the change in polarization contrast before and after nanomanipulation to determine the rotation angle. Finally, we evaluate the likelihood for indistinguishable, single photon emission of silicon-vacancy centers located in different nanodiamonds. Our work enables ideal evanescent, optical coupling of distant nanodiamonds containing silicon-vacancy centers with applications in the realization of quantum networks, quantum repeaters or complex quantum systems.

All-solid-state multipass spectral broadening to sub-20 fs.

In this work, we present a nonlinear spectral broadening and compression scheme based on self-phase modulation in bulk media inside a Herriott-type multipass cell. With this reliable approach, we achieved a spectral broadening factor of 22 while maintaining an efficiency of over 60% at an average input power of 100W, and an excellent output beam quality with M2=1.2. The output pulses were compressed to 18fs, with the broadest spectrum supporting a Fourier-transform limit of 10fs. The high efficiency and approximately four-optical-cycle pulse duration mark an important milestone towards the realization of a compact, high power oscillator-based driver for XUV frequency combs and other nonlinear processes.

Observation of Fourier transform limited lines in hexagonal boron nitride

Single defect centers in layered hexagonal boron nitride (hBN) are promising candidates as single photon sources for quantum optics and nanophotonics applications. However, until today spectral instability hinders many applications. Here, we perform resonant excitation measurements and observe Fourier-Transform limited (FL) linewidths down to $\approx 50$ MHz. We investigate optical properties of more than 600 quantum emitters (QE) in hBN. The QEs exhibit narrow zero-phonon lines (ZPL) distributed over a spectral range from 580 nm to 800 nm and with dipole-like emission with high polarization contrast. The emission frequencies can be divided into four main regions indicating distinct families or crystallographic structures of the QEs, in accord with ab-initio calculations. Finally, the emitters withstand transfer to a foreign photonic platform - namely a silver mirror, which makes them compatible with photonic devices such as optical resonators and paves the way to quantum photonics applications including quantum commmunications and quantum repeaters.

Investigations of gain redshift in high peak power Ti:sapphire laser systems

Gain redshift in high peak power Ti:sapphire laser systems can result in narrowband spectral output and hence lengthen the compressed pulse duration. In order to realize broadband spectral output in 10 PW-class Ti:sapphire lasers, the influence on gain redshift induced by spectral pre-shaping, gain distribution of cascaded amplifiers and Extraction During Pumping (EDP) technique have been investigated. The theoretical and experimental results show that the redshift of output spectrum is sensitive to the spectral pre-shaping and the gain distribution of cascaded amplifiers, while insensitive to the pumping scheme with or without EDP. Moreover, the output spectrum from our future 10 PW Ti:sapphire laser is theoretically analyzed based on the investigations above, which indicates that a Fourier-transform limited (FTL) pulse duration of 21 fs can be achieved just by optimizing the spectral pre-shaping and gain distribution in 10 PW-class Ti:sapphire lasers.