Alexander Sell

Phase-locked generation and field-resolved detection of widely tunable terahertz pulses with amplitudes exceeding 100 MV/cm

Phase-locked terahertz transients with peak electric fields of 108 MV/cm and center frequencies continuously tunable from 10 to 72 THz are generated via difference-frequency mixing of two parametrically amplified pulse trains from a single white-light seed. Free space electro-optic sampling with 8 fs gating pulses from a two-branch Er:fiber laser allows us to monitor all transients directly in the time domain. We identify extreme terahertz nonlinearities in the detector crystal with subcycle resolution.

Sub-cycle switch-on of ultrastrong light-matter interaction

Controlling the way light interacts with material excitations is at the heart of cavity quantum electrodynamics (QED). In the strong-coupling regime, quantum emitters in a microresonator absorb and spontaneously re-emit a photon many times before dissipation becomes effective, giving rise to mixed light–matter eigenmodes. Recent experiments in semiconductor microcavities reached a new limit of ultrastrong coupling, where photon exchange occurs on timescales comparable to the oscillation period of light. In this limit, ultrafast modulation of the coupling strength has been suggested to lead to unconventional QED phenomena. Although sophisticated light–matter coupling has been achieved in all three spatial dimensions, control in the fourth dimension, time, is little developed. Here we use a quantum-well waveguide structure to optically tune light–matter interaction from weak to ultrastrong and turn on maximum coupling within less than one cycle of light. In this regime, a class of extremely non-adiabatic phenomena becomes observable. In particular, we directly monitor how a coherent photon population converts to cavity polaritons during abrupt switching. This system forms a promising laboratory in which to study novel sub-cycle QED effects and represents an efficient room-temperature switching device operating at unprecedented speed.

8-fs pulses from a compact Er:fiber system : quantitative modeling and experimental implementation

We demonstrate an all-fiber turnkey source of extremely stable 2-cycle pulses at a center wavelength of 1.17 microm. Taylor-cut highly nonlinear germano-silica bulk fibers (HNFs) provide smooth supercontinua with a bandwidth of 560 nm and a spectral shape precisely controlled by the dispersion of the fiber and the phase of the 1.55 microm pump pulses. Alternatively these fibers are capable of generating pulses with central wavelengths continuously tunable from 0.9 microm up to 1.4 microm. These results are based on parameter-free simulations of nonlinear pulse propagation including higher-order dispersion as well as instantaneous Kerr and retarded Raman contributions.

Compact coherent anti-Stokes Raman scattering microscope based on a picosecond two-color Er:fiber laser system

We present a compact coherent anti-Stokes Raman scattering microscope based on a widely tunable picosecond Er:fiber laser. Intense and bandwidth-limited 1 ps pump pulses at a center wavelength of 775 nm are generated via frequency mixing within the broadband fundamental at 1.55 microm. Narrowband Stokes pulses are obtained by frequency shifting of solitons in a highly nonlinear bulk fiber and subsequent second-harmonic generation. The tuning range from 850 nm to 1100 nm gives access to vibrational resonances between 1150 cm(-1) and 3800 cm(-1). A first imaging application in the spectral region of CH stretch vibrations is demonstrated.

Single-cycle multiterahertz transients with peak fields above 10 MV/cm

Phase-locked single-cycle transients with frequency components between 1 and 60THz and peak fields of up to 12MV/cm are generated as the idler wave of a parametric amplifier. To achieve broadband conversion in GaSe nonlinear crystals, we match the group velocities of signal and idler components. The influence of group-velocity dispersion is minimized by long-wavelength pumping at 1.18mum. Free-space electro-optic sampling monitors the multiterahertz waveforms with direct field resolution.

Attosecond relative timing jitter and 13 fs tunable pulses from a two-branch Er:fiber laser

We present what is believed to be the first direct measurement of the relative timing jitter between the two parallel pulse trains of a two-branch femtosecond erbium-doped fiber laser, operated without active stabilization. The system provides independently tunable pulses in the near infrared with durations down to 13 fs. Using an interferometric optical cross-correlator, the phase-noise spectral density is measured with high sensitivity in a range from 1 Hz up to the Nyquist frequency of 24.5 MHz. We find an integrated jitter of 11 attoseconds directly after the amplifier stages and 43 as after propagation through free-space optics and nonlinear fibers for frequency conversion.

Field-resolved detection of phase-locked infrared transients from a compact Er:fiber system tunable between 55 and 107 THz

We demonstrate phase-stable generation and ultrabroadband electro-optic detection of few-cycle multi-terahertz transients tunable from 55 to 107 THz based on a versatile telecom compatible three-branch Er:fiber laser system. Difference frequency mixing of two laser pulse trains at the full repetition rate of 49 MHz provides passively phase-locked multi-terahertz transients with peak fields of 69 kV/cm and average powers of up to 1.4 mW. 8 fs gating pulses from the third laser port allow for field-sensitive sampling throughout the midinfrared with a signal to noise ratio of up to 104 Hz1/2. A leadoff application in trace gas detection is discussed.

Ultrabroadband background-free coherent anti-Stokes Raman scattering microscopy based on a compact Er:fiber laser system

We demonstrate a scheme for efficient coherent anti-Stokes Raman scattering (CARS) microscopy free of nonresonant background. Our method is based on a compact Er:fiber laser source. Impulsive excitation of molecular resonances is achieved by 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).

High-energy, phase-stable, ultrabroadband kHz OPCPA at 2.1 μm pumped by a picosecond cryogenic Yb:YAG laser

We report on a kHz, mJ-level, carrier-envelope phase (CEP)-stable ultrabroadband optical parametric chirped-pulse amplifier (OPCPA) at 2.1-μm wavelength, pumped by a high-energy, 14 ps, cryogenic Yb:YAG pump laser, and its application to high-order harmonic generation (HHG) with Xe. The pre-amplifier chain is pumped by a 12-ps Nd:YLF pump laser and both pump lasers are optically synchronized to the signal pulse of the OPCPA. An amplified pulse energy of 0.85 mJ was obtained at the final OPCPA stage with good beam profile. The pulse is compressed to 4.5 optical cycles (<32 fs) with a spectral bandwidth of 474 nm supporting 3.5 optical cycles. The CEP stability was measured to be 194 mrad and the super-fluorescence noise is estimated to be ~9%. First HHG results are demonstrated with Xe showing significant cutoff extension to >85 eV with an efficiency of ~10-10 per harmonic, limited by the maximum gas pressure and flow into the chamber. This demonstrates the potential of this 2.1-μm source for scaling of photon energy and flux in the water-window range when applied to Ne and He at kHz repetition rate.

Pulse synthesis in the single-cycle regime from independent mode-locked lasers using attosecond-precision feedback

We report the synthesis of a nearly single-cycle (3.7 fs), ultrafast optical pulse train at 78 MHz from the coherent combination of a passively mode-locked Ti:sapphire laser (6 fs pulses) and a fiber supercontinuum (1-1.4 μm, with 8 fs pulses). The coherent combination is achieved via orthogonal, attosecond-precision synchronization of both pulse envelope timing and carrier envelope phase using balanced optical cross-correlation and balanced homodyne detection, respectively. The resulting pulse envelope, which is only 1.1 optical cycles in duration, is retrieved with two-dimensional spectral shearing interferometry (2DSI). To our knowledge, this work represents the first stable synthesis of few-cycle pulses from independent laser sources.