We demonstrate the deterministic control of femtosecond dual-comb soliton motion in an Er:fiber laser. Introducing fast intra-cavity modulation, we control individual solitons within two interlaced soliton combs in the harmonically-modelocked state. Upon extra-cavity recombination of both combs, we can obtain reconfigurable pulse pairs at arbitrary delays. Based on realtime spectroscopy, we resolve the transient inter-soliton trajectories.The approach employs laser-intrinsic soliton dynamics and provides all-optically swept pulse pairs for i.e. pump-probe spectroscopy at ⪆kHz scanning frequencies.
Thanks to their high tunability and ease of integration into microstructured devices, colloidal quantum dots (CQDs) have become a widespread building block in electro-optic technology. In order to translate this success into the realm of quantum science and technology, spectral stability and coherence, two closely linked topics, need to be addressed. In this talk, I will describe our recent effort in answering one of the fundamental questions in this regard: what is the underlying cause of spectral noise in the emission of CQDs? We embed CdSe/CdS core/shell quantum dots into microcapacitors and perform high-resolution photoluminescence spectroscopy of individual nanoparticles at cryogenic temperatures. Applying an external electric bias, we shift their emission wavelength and its sensitivity to spurious electric fields. Increasing this sensitivity, we observe a clear emhancement in the dynamic variance of the energy of the emitted photons. Fitting this result to a straightforward model, we provide direct evidence that microscopic stochastic electric fields are the root cause of spectral fluctuations in CQDs.
Stable arrangements of temporal solitons are reported for essentially every implementation of femtosecond fiber lasers. Their formation can be tracked with today’s real-time instrumentation. However, the underlying interaction mechanism frequently remained elusive and predictions of soliton separations in actual sources are often missing. Here, we present the experimental analysis of bound-state trajectories in a femtosecond Er:fiber laser and reveal the underlying universal coupling mechanism. We demonstrate all-electronic switching between two stable soliton molecules and the feasibility of tuning bound-state separations. The results are applicable to various ultrafast sources and may readily be adapted for generating femtosecond pulse pairs in spectroscopy and material processing.
Germanium is a nonpolar semiconductor with missing Reststrahlen band. In spite of other promising properties including low bandgap and small effective mass, its long, µs-scale recombination time has been prohibitive for applications as photoconductive THz emitters. Using Au-implantated Ge, with recombination times reduced to sub-ns values, we have demonstrated a broadband photoconductive THz emitter compatible with modelocked fibre lasers operating at wavelengths of 1.1 and 1.55 µm and with pulse repetition rates of 78 MHz. Reaching up to 70 THz bandwidth, this approach points towards the possibility of compact, high-bandwidth THz photonic devices compatible with Si CMOS technology.
Reliable storage and manipulation of spin coherence is at the heart of quantum information processors. Isolated singly-charged semiconductor quantum dots (QDs) are typically characterized by bright ground state emission and hold promise as efficient spin-to-photon interfaces. Ultrafast interactions in condensed phase could enable high-bandwidth information processors, with an additional possibility to engineer quantum properties of light on sub-picosecond timescales. Toward these goals, we employ multi-color femtosecond write and readout pulses to analyze intrinsic spin dynamics in isolated singly-charged epitaxial CdSe/ZnSe QDs. With this we demonstrate control over single-photon-added amplification as well trion quatnum beats, arising from long intraband coherence.
Frequency combs are an enabling technology for metrology and spectroscopic applications in fundamental and life sciences. While frequency combs in the 1 μm regime, produced from Yb-based systems have already exceeded the 100 W – level, high power coverage of the interesting mid-infrared wavelength range remains yet to be demonstrated. Tm- and Ho-doped laser systems have recently shown operation at high average power levels in the 2 μm wavelength regime. However, frequency combs in this wavelength range have not exceeded the 5 W-average power level. In this work, we present a high power frequency comb, delivered by a Tm-doped chirped-pulse amplifier with subsequent nonlinear pulse compression. With an integrated phase noise of <320 mrad, low relative intensity noise of <0.5% and an average power of 60 W at 100 MHz repetition rate (and <30 fs FWHM pulse duration), this system demonstrates high stability and broad spectral coverage at an unrivalled average power level in this wavelength regime. Therefore, this laser will enable metrology and spectroscopy with unprecedented sensitivity and acquisition time. It is our ongoing effort to extend the spectral coverage of this system through the utilization of parametric frequency conversion into the mid-IR, thus ultimately enabling high power fingerprint spectroscopy in the entire molecular fingerprint region (2 – 20 μm).
Preserving the integrity of DNA is fundamental for cell survival. Therefore, DNA repair research has a high demand for methods to induce DNA damage with high spatial and temporal selectivity. To apply and study the effects of fs-laser irradiation in live cells, we have developed a multicolor fs-laser system. This turnkey system is based on single-mode fs-Er: and Yb:fiber laser technology. It synchronously provides pulses at 515 nm, 775 nm and 1035 nm wavelength with 40 MHz repetition rate. All three branches feature closely matched, bandwidth-limited pulse durations between 80 and 95 fs in the focus of a commercial laser-scanning microscope. An average optical output power from 80 to 2000 mW in the corresponding branches is provided. We apply a tandem scanning scheme in order to decouple nonlinear photomanipulation from conventional imaging.
We extensively analyzed the induction of DNA damage upon fs-laser irradiation via immunocytochemistry. A set of irradiation working conditions at 515 nm and 1035 nm has been identified that specifically induce either DNA strand breaks or UV-photoproducts. In subsequent live-cell experiments, we observed the generation of secondary breaks due to the activation of nucleotide-excision-repair at 515 nm wavelength irradiation. Such secondary reactions escape detection by conventional immunocytochemistry, but are revealed by our approach. Furthermore, we identified working conditions of irradiation at 775 nm driving two-photon-photoactivation of fluorescently labelled proteins within the nucleus without simultaneously triggering unwanted DNA lesions. We can therefore study the mobility of e.g. chromatin proteins at sites of DNA damage or perform functional cellular studies of mutant DNA repair proteins
A time-domain approach to quantum electrodynamics is presented, covering the entire mid-infrared and terahertz frequency ranges. Ultrabroadband electro-optic sampling with few-femtosecond laser pulses allows direct detection of the vacuum fluctuations of the electric field in free space [1,2]. Besides the Planck and electric field fundamental constants, the variance of the ground state is determined solely by the inverse of the four-dimensional space-time volume over which a measurement or physical process integrates. Therefore, we can vary the contribution of multi-terahertz vacuum fluctuations and discriminate against the trivial shot noise due to the constant flux of near-infrared probe photons. Subcycle temporal resolution based on a nonlinear phase shift provides signals from purely virtual photons for accessing the ground-state wave function without amplification to finite intensity.
Recently, we have succeeded in generation and analysis of mid-infrared squeezed transients with quantum noise patterns that are time-locked to the intensity envelope of the probe pulses. We find subcycle temporal positions with a noise level distinctly below the bare vacuum which serves as a direct reference. Delay times with increased differential noise indicate generation of highly correlated quantum fields by spontaneous parametric fluorescence. Our time-domain approach offers a generalized understanding of spontaneous emission processes as a consequence of local anomalies in the co-propagating reference frame modulating the quantum vacuum, in combination with the boundary conditions set by Heisenberg’s uncertainty principle.
[1] C. Riek et al., Science 350, 420 (2015)
[2] A. S. Moskalenko et al., Phys. Rev. Lett. 115, 263601 (2015)
Terahertz (THz) electromagnetic radiation is located between the realms of electronics and optics and has successfully been used to probe and even control numerous low-energy excitations including phonons, excitons and Cooper pairs. Here, we show that THz spectroscopy is also a highly useful tool in the field of ultrafast spinbased electronics (spintronics) and consider the resonant manipulation of the magnetization of an antiferromagnet (through the THz Zeeman torque) and the probing of the tailored transport of spin density from a ferromagnetic into a nonmagnetic metal (through the THz inverse spin Hall effect).
Ultrashort pulses in the terahertz (THz) spectral range allow us to study and control spin dynamics on time scales faster than a single oscillation cycle of light. In a first set of experiments, we harness an optically triggered coherent lattice vibration to induce a transient spin-density wave in BaFe2As2, the parent compound of pnictide superconductors. The time-dependent multi-THz response of the non-equilibrium phases shows that the ordering quasi-adiabatically follows a coherent lattice oscillation at a frequency as high as 5.5 THz. The results suggest important implications for unconventional superconductivity. In a second step, we utilize the magnetic field component of intense THz transients to directly switch on and off coherent spin waves in the antiferromagnetic nickel oxide NiO. A femtosecond optical probe traces the magnetic dynamics in the time domain and verifies that the THz field addresses spins selectively via Zeeman interaction. This concept provides a universal ultrafast handle on magnetic excitations in the electronic ground state.
We present a minimal-cost Cr:LiSAF laser that is pumped by one single-spatial-mode diode. The pumping system
(diode, diode driver, and the diode holder) has a total cost of about $500 and provided 130 mW of diffraction-limited
pump power around 660 nm. The entire Cr:LiSAF laser system has an estimated total material cost below $5k, a
footprint of about 20 cm × 35 cm, does not require cooling and can be driven by batteries, making the system ideal for
applications that require portability. In continuous wave (cw) laser experiments, we have demonstrated lasing thresholds
as low as 2 mW, slope efficiencies as high as 52%, output powers up to 58 mW, and a record tuning range extending
from 780 nm to 1110 nm. In cw mode-locked operation, using a 0.5% output coupler, 100-fs pulses with an average
power of 38 mW, and with an optical spectrum centered around 865 nm have been obtained at a repetition rate of 235
MHz. With a more compact cavity and using a 0.1% output coupler, 70-fs pulses with an average power of 20 mW have
been obtained at a repetition rate of 509 MHz. We believe that this portable, minimal cost Cr:LiSAF laser system might
be an attractive source for applications like amplifier seeding that do not require high average output power levels.
Ti:Sapphire lasers could provide tunable femtosecond pulses in the 680-1180 nm region; however, due to the
requirement of expensive green pump sources, its current cost sets a barrier to its widespread adoption. As an alternative,
Cr :Colquiriites (Cr:LiCAF, Cr:LiSAF, Cr:LiSGaF) also possess broad gain bandwidths and their total cw tuning range
cover the 720-1110 nm region. Moreover, their broad absorption bands around 650 nm enable direct diode pumping by
low-cost red laser diodes. However, so far the limited brightness of red diodes required combination of four to six pump
diodes to reach reasonable output power levels from Cr :Colquiriites. This complex pumping geometry increases cost
and causes stability issues in long-term operation. In this study, we report compact, low-cost and efficient Cr:Colquiriite
lasers pumped by a single 1.2 W tapered laser diode at 675 nm. In continuous wave laser operation, output powers of 500
mW and 410 mW together with slope efficiencies of 47% and 41% were demonstrated from Cr:LiSAF and Cr:LiCAF,
respectively. In cw mode-locked operation, sub-100-fs pulse trains with average power between 200 mW and 250 mW
were obtained at repetitions rates around 100 MHz. These results indicate that tapered diodes in the red spectral region
are likely to become the standard pump source for Cr:Colquiriite lasers in the near future. Moreover, the simplified
pumping scheme might facilitate efficient commercialization of these low-cost systems, bearing the potential to
significantly boost applications of cw and femtosecond lasers in this spectral region.
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.
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.
A two branch Er:fiber laser was developed for coherent anti-Stokes Raman scattering (CARS) microspectroscopy. The
compact and highly stable light source allows for fast single-frequency CARS microspectroscopy with a wide tuning
range from 1150 cm-1 up to 3800 cm-1. Single-pass frequency conversion enables easy tunability. The spectral selectivity
of the system is shown using polymer beads. Imaging of biological samples is demonstrated on C. elegans and yeast
cells. Modification of the light source allows for broadband background-free CARS microspectroscopy. Impulsive
excitation of molecular resonances is achieved using an 11 fs pulse at 1210 nm. Broadband excitation gives access to
molecular resonances from 0 cm-1 up to 4000 cm-1. Time-delayed narrowband probing at 775 nm enables sensitive and
high-speed spectral detection of the CARS signal, free of nonresonant background with a resolution of 10 cm-1.
Phase-locked electromagnetic transients in the terahertz (THz) spectral domain have become a unique contact-free probe
of the femtosecond dynamics of low-energy excitations in semiconductors. Access to their nonlinear response, however,
has been limited by a shortage of sufficiently intense THz emitters. Here we introduce a novel high-field source for THz
transients featuring peak amplitudes of up to 108 MV/cm. This facility allows us to explore the non-perturbative
response of semiconductors to intense fields tailored with sub-cycle precision. In a first experiment intense transients
drive Rabi-oscillations between excitonic states in Cu2O, implying exciting perspectives for future THz quantum optics.
At electric fields beyond 10 MV/cm, we observe the breakdown of the power expansion of the nonlinear polarization in
bulk semiconductors. Furthermore, we employ the intense magnetic field components of our transients to coherently
control spin waves in antiferromagnetically ordered solids. Finally, intersubband cavity polaritons in semiconductor
microcavities are exploited to push light-matter coupling to an unprecedented ultrastrong and sub-cycle regime.
Recent advances of femtosecond semiconductor physics at the limits of single electrons and photons down to sub-cycle
time scales are presented. The first part deals with ultrafast measurements on single-electron systems: The transient
quantum dynamics in a single CdSe/ZnSe quantum dot is investigated via femtosecond transmission spectroscopy. A
two-color Er:fiber laser with excellent noise performance is key to these first resonant pump-probe measurements on a
single-electron system. We have observed ultrafast bleaching of an electronic transition in a single quantum dot due to
instantaneous Coulomb renormalization. Since we were also able to invert the two-level system, optical gain due to a
single electron has been detected. By using π-pulses for probing, we could deterministically add or remove a single
photon to or from a femtosecond light pulse, leading to non-classical states of the light field. In order to optimize
electron-photon coupling, nanophotonic concepts like dielectric microresonators and metal optical antennas are explored.
In the second part of the paper, we present multi-terahertz measurements on low-energy excitations in semiconductors.
These studies lead towards a future time-domain quantum optics on a time scale of single cycles of light: Intense multiterahertz
fields of order MV/cm are used to coherently promote optically dark and dense para excitons in Cu2O from the
1s into the 2p state. The nonlinear field response of the intra-excitonic degrees of freedom is directly monitored in the
time domain via ultrabroadband electro-optic sampling. The experimental results are analyzed with a microscopic many-body
theory, identifying up to two internal Rabi cycles. Subsequently, intersubband cavity polaritons in a quantum well
waveguide structure are optically generated within less than one cycle of light by a femtosecond near-infrared pulse.
Mid-infrared probe transients trace the non-adiabatic switch-on of ultrastrong light-matter coupling and the conversion of
bare photons into cavity polaritons directly in the time domain.
We study the ultrafast transition of a pure longitudinal optical phonon resonance to a coupled phonon-plasmon system. Following 10-fs photoexcitation of intrinsic indium phosphide, ultrabroadband THz opto-electronics monitors the buildup of coherent beats of the emerging hybrid modes directly in the time domain with sub-cycle resolution. Mutual repulsion and redistribution of the oscillator strength of the interacting phonons and plasmons are seen to emerge on a delayed femtosecond time scale. Both branches of the mixed modes are monitored for various excitation densities N. We observe a pronounced anticrossing of the coupled resonances as a function of N. The characteristic formation time for phonon-plasmon coupling exhibits density dependence. The time is approximately set by one oscillation cycle of the upper branch of the mixed modes.
Mode-locked Er-doped fiber laser systems built on single-mode fiber technology continue to see a remarkable improvement in their performance characteristics. In this contribution, we present an extremely compact and powerful version of such a laser source, delivering elevated peak powers well in excess of 10 kW in combination with ultrashort pulse durations below 100 fs. Eliminating the need for costly pump sources, external cooling as well as daily re-alignment routines, this laser system opens possibilities for an entirely new class of experiments and applications to a much larger group of users than only dedicated laser institutes. The accessible wavelength range is greatly enhanced by generation of a supercontinuum inside an integrated highly nonlinear fiber. We report output spectra with a bandwidth exceeding one full octave which we utilize for phase stabilization of the laser source. As a first proof of principle, a precise frequency measurement is carried out on a cavity-stabilized diode laser over a time interval of 88 hours without interruption. With regard to the time domain pulse structure, the user can select to re-compress defined parts of the continuum to achieve pulse durations below 30 fs. At the same time, the central wavelength of these pulses is easily shifted over a wavelength interval from 1130 nm to 1400 nm. Based on these findings, we demonstrate the generation of widely tunable light pulses in the visible spectral range by efficient frequency doubling. Potential applications for this novel light source are discussed.
Ultrabroadband THz spectroscopy is employed to observe how many-particle interactions build up in an extreme non-equilibrium electron-hole plasma. The plasma is photogenerated in bulk GaAs via resonant interband absorption of a 10 fs laser pulse. Subsequently, the dynamics of the complex dielectric function throughout the mid-infrared is directly monitored with uncertainty-limited temporal resolution with a single-cycle THz pulse. Field sensitive detection allows us to measure simultaneously real and imaginary part of the complex dielectric function of the plasma in the multi-THz regime. We show that collective phenomena such as Coulomb screening and plasmon scattering exhibit a delayed onset. This observation is explained in terms of the ultrafast formation of dressed quasiparticles. The time scale for this transient behavior is of the order of the inverse plasma frequency. Our findings support recent quantum kinetic calculations of the temporal evolution of the Coulomb interaction after ultrafast excitation of a dense electron-hole plasma.
We report on the first observation of wave mechanical features in free carrier relaxation. The emission of LO phonons by non-equilibrium electrons is studied via ultrasensitive transmission spectroscopy. Electrons are excited in a bulk GaAs sample by energetically narrow 120 fs pump pulses. The time evolution of the energy distribution is probed measuring the induced transmission changes with broadband 25 fs pulses. It is demonstrated that energy is not conserved in scattering events on ultrafast time scales. A phonon replica of the initial electron distribution starts energetically broadened. Within the phonon oscillation cycle of 115 fs, memory effects resulting from quantum interference become operative. The phonon satellite narrows towards the line width given by the photoexcited distribution. This process is repeated for every step in the LO cascade. These phenomena dominate hot carrier dynamics whenever scattering rates are comparable to or even faster than the frequency of the energy quanta exchanged. The influence of the electron-phonon coupling strength is investigated via non-degenerate four-wave-mixing experiments: LO phonon quantum beats are found in the decay of the coherent interband polarization in GaAs. In contrast, the oscillations are overdamped in InP as a result of the strong Frohlich interaction in this more polar compound.
Synchronized femtosecond pulses of different wavelength and bandwidth are used to investigate intraband and interband coherent effects in polar semiconductors. At low excitation densities the relaxation of nonthermal electron distributions by spontaneous emission of LO phonons in GaAs is monitored. Quantum kinetic behavior of the dynamics beyond the semiclassical Boltzmann equation is demonstrated for the first time. Violation of the energy conservation as well as memory effects are shown to occur on an ultrashort time scale of 100 fs. For strong excitation we present the first observation of Rabi-oscillations of the occupation density in the absorption continuum of InP. Despite high carrier densities exceeding 1018 cm-3 dephasing time are long enough to allow for the appearance of two strongly damped Rabi-cycles at a peak intensity of the pump pulses of 17 GW/cm2.
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