Long-term carrier-envelope phase stability from a grating-based, chirped pulse amplifier

Carrier-Envelope-Phase Modulated Currents in Scanning Tunneling Microscopy

Carrier-envelope-phase (CEP) stable optical pulses combined with state-of-the-art scanning tunneling microscopy (STM) can track and control ultrafast electronic tunneling currents. On the basis of nonequilibrium Green's function formalism, we present a time and frequency domain theoretical study of CEP-stable pulse-induced tunneling currents between an STM tip and a metal substrate. It is revealed that the experimentally observed phase shift between the maximum tunneling current and maximum electric field is caused by the third-order response to the electric field. The shift is also found to be sensitive to the duration of pulses. The tunneling process can thus be precisely manipulated by varying the phase and duration of these pulses.

Dual trace inter-pulse interferometer for measurement of phase stability of ultra short laser pulse train

A dual trace intra-pulse and inter-pulse spatio-spectral interferometer has been set up to study the temporal stability of a ∼200 fs duration laser pulse train from a cw mode-locked laser oscillator. Simultaneous recording of twin interferograms helps identify the phase error in inter-pulse interferograms due to the diagnostic setup kept in a standard laboratory environment. Applicability of inter-pulse tilted pulse-front interferograms has been demonstrated to constitute an alternative inexpensive method for visual detection and estimation of phase slippage and pulse repetition frequency of an ultra short pulse train. The effect of pump beam intensity on the repetition rate of pulses due to accumulated intra-cavity non-linear phase shifts is also presented.

Mid-infrared 333 MHz frequency comb continuously tunable from 1.95 to 4.0 μm

We report a 333 MHz femtosecond optical parametric oscillator in which carrier-envelope offset stabilization was implemented by using a versatile locking technique that allowed the idler comb to be tuned continuously over the mid-infrared range from 1.95 to 4.0 μm. A specially designed multi-section, multi-grating, periodically poled KTP crystal provided simultaneously phase-matched parametric down-conversion and pump + idler sum-frequency generation, enabling strong heterodyne signals with the pump supercontinuum (employed for locking) to be obtained across the tuning range of the device. The idler comb offset was stabilized to a 10 MHz reference frequency with a cumulative phase noise from 1 Hz-64 kHz of <1.3  rad maintained across the entire operating range, and average idler output powers up to 50 mW.

Carrier-envelope phase stabilization of a terawatt level chirped pulse amplifier for generation of intense isolated attosecond pulses

We demonstrate the first carrier-envelope phase (CEP)-stabilized chirped pulse amplification system with pulse peak-powers in the terawatt regime. The system, which eventually is intended to be used in the generation of isolated attosecond pulses, consists of two consecutive multipass amplification stages. The first amplification stage is a commercial CEP-stable kHz system including a single 13-pass amplifier reaching a pulse energy of 2.3 mJ. Pulses are picked after the first stage at a repetition rate of 50 Hz and are further amplified in a 5-pass power-amplifier to pulse energies that reach up to 80 mJ before compression. After compression the pulse energy is 35mJ at a pulse duration of 32 fs, signifying a peak power of 1.1 terawatt. Peak-powers exceeding 1.5 TW should easily be achievable by improving the efficiency of the grating compressor. The CEP-stability of the terawatt system is demonstrated by single shot measurements of the residual CEP jitter at the full repetition rate and show an excellent root-mean-square value of 315 mrad.

Carrier-Envelope Phase stabilization of a 20 W, grating based, chirped-pulse amplified laser, using electro-optic effect in a LiNbO₃ crystal

Using an original CEP stabilization technique based on the linear electro-optical effect in a specific crystal, we achieved long term CEP stabilization of a 20 W, 1 kHz laser with residual noise as low as 440 mrad (rms). At 3 W, the CEP shot to shot noise is kept as low as 320 mrad (rms) over half an hour.

Simultaneous displacement and angular drift measurement based on defocus grating

We propose and demonstrate a displacement and angular drift simultaneous measurement technique based on a defocus grating. The displacement and angular drift of the incident beam can be detected by monitoring the movements of +/-1 diffraction order spots of the defocus grating. The relationship between drift of the incident beam and movements of +/-1 diffraction order spots is studied in detail. Compared with other methods, this technique eliminates the requirement of two or more detecting systems for measuring displacement and angular drift simultaneously. The proof-of-principle experiment shows that the root-mean-square errors of displacement and angular drift measurements are less than 0.5 microm and 0.84 microrad, respectively.

Carrier-envelope phase stabilization and control using a transmission grating compressor and an AOPDF

Carrier-envelope phase (CEP) stabilization of a femtosecond chirped-pulse amplification system featuring a compact transmission grating compressor is demonstrated. The system includes two amplification stages and routinely generates phase-stable (approximately 250 mrad rms) 2 mJ, 25 fs pulses at 1 kHz. Minimizing the optical pathway in the compressor enables phase stabilization without feedback control of the grating separation or beam pointing. We also demonstrate for the first time to the best of our knowledge, out-of-loop control of the CEP using an acousto-optic programmable dispersive filter inside the laser chain.

Circularly polarized light in the single-cycle limit: The nature of highly polychromatic radiation of defined polarization

We have developed a general analytic description of polarized light pulses and explored the properties of circularly polarized single-cycle pulses. The temporal evolution of the electric-field vector of such spectrally broad pulses, which may be described in terms of a Hilbert transform relationship, differs significantly from the well-known behavior of quasi-monochromatic radiation. Single-cycle circularly polarized pulses are produced and characterized experimentally in the terahertz spectral region.

Long-term carrier-envelope-phase stabilization of a femtosecond laser by the direct locking method

We have developed a practical solution to implement the direct locking method for the carrier-envelope phase (CEP) stabilization of femtosecond laser pulses and achieved 24-hour CEP stabilization without realignment of any optical components. The direct locking method realizes the CEP stabilization in the time domain by directly quenching the beat signal from an f-to-2f interferometer and, thereby, locking every pulse to a same CEP. We have accomplished the long-term CEP stabilization using commercially available standard feedback electronics, and maintained the CEP stabilization with low jitter without using any frequency-analyzing components, greatly facilitating the accessibility of the CEP stabilization.

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.

Composite frequency comb spanning 0.4-2.4 microm from a phase-controlled femtosecond Ti:sapphire laser and synchronously pumped optical parametric oscillator

A repetition-rate-stabilized frequency comb ranging from the violet to the mid-infrared (0.4-2.4 microm) is obtained by phase locking a femtosecond Ti:sapphire laser and a synchronously pumped optical parametric oscillator to a common supercontinuum reference. The locking results have bandwidths lower than 3 kHz. By changing the locking frequencies, different relative and absolute offsets of the constituent frequency combs are achievable.

Carrier-envelope phase shift caused by variation of grating separation

The effects of variation of the grating separation in a stretcher on the carrier-envelope (CE) phase of amplified pulses are investigated. By translating one of the telescope mirrors in the stretcher with a piezoelectric transducer, it is found that a 1 mum change of the distance causes a 3.7+/-1.2 rad shift of the CE phase, which is consistent with theoretical estimations. The results indicate that optical mounts used for gratings and telescope mirrors must be interferometrically stable; otherwise their vibration and thermal drift will cause significant phase error. The CE phase drift was corrected by feedback controlling the grating separation.

Isolated Single-Cycle Attosecond Pulses

We generated single-cycle isolated attosecond pulses around approximately 36 electron volts using phase-stabilized 5-femtosecond driving pulses with a modulated polarization state. Using a complete temporal characterization technique, we demonstrated the compression of the generated pulses for as low as 130 attoseconds, corresponding to less than 1.2 optical cycles. Numerical simulations of the generation process show that the carrier-envelope phase of the attosecond pulses is stable. The availability of single-cycle isolated attosecond pulses opens the way to a new regime in ultrafast physics, in which the strong-field electron dynamics in atoms and molecules is driven by the electric field of the attosecond pulses rather than by their intensity profile.