Frequency-resolved optical gating for characterization of VUV pulses using ultrafast plasma mirror switching

Phase-matching-free ultrashort laser pulse characterization from a transient plasma lens

A phase-matching-free ultrashort pulse retrieval based on the defocusing of a laser-induced plasma is presented. In this technique, a pump pulse ionizes a rare gas providing a plasma lens whose creation time is ultrafast. A probe pulse propagating through this gas lens experiences a switch of its divergence. The spectrum of the diverging part, isolated by a coronograph, is measured as a function of the pump-probe delay, providing a spectrogram that allows for a comprehensive characterization of the temporal properties of the probe pulse. The method, called PI-FROSt for "plasma-induced frequency-resolved optical switching," is simple, is free of phase-matching constraints, and can operate in both self- and cross-referenced configurations at ultrahigh repetition rate in the whole transparency range of the gas. The assessment of the method demonstrates laser pulse reconstructions of high reliability in both near-infrared (NIR) and ultraviolet (UV) spectral ranges.

Temporal characterization of two-octave infrared pulses by frequency resolved optical switching

We present the temporal characterization of infrared pulses with spectra extending from 0.55 to 2.5 μm by using the frequency resolved optical switching (FROSt) technique. The pulses are obtained by broadening femtosecond pulses at 1.75 μm central wavelength in a two-stage hollow core fiber setup. This work demonstrates the capability of the FROSt technique to temporally characterize pulses with ultra-broadband spectra. Being free of phase-matching constraints, it enables the characterization of pulses with very low energy at the limit of the detection threshold and with arbitrary long pulse duration. This strength of the FROSt technique is illustrated by the characterization of supercontinua pulses whose spectra span over two octaves and with only 150 nJ energy that is spread temporally over almost 40 ps. The FROSt capabilities provide a versatile tool for the characterization of sub-cycle pulses and to study nonlinear processes such as supercontinuum generation.

Measuring an ultrashort, ultraviolet pulse in a slowly responding, absorbing medium

Frequency-resolved optical gating (FROG) is a common technique for measuring ultrashort laser pulses using an instantaneous, nonlinear-optical interaction as a fast time-gate to measure the pulse intensity and phase. But at high frequencies, materials are often absorbing and it is not always possible to find a medium with a fast nonlinear-optical response. Here we show that an ultrashort, ultraviolet (UV) pulse can be measured in a strongly absorbing medium, using the absorption as the nonlinear-optical time-gate. To do this, we build on our recent implementation of FROG, known as induced-grating cross-correlation FROG (IG XFROG), where an unknown, higher-frequency pulse creates a transient grating that is probed with a lower-frequency, more easily detectable reference pulse. We demonstrate this with an 800 nm reference pulse to characterize 400 nm or 267 nm pulses using ZnS as the nonlinear-optical medium, which is absorptive at and below 400 nm. By scanning the delay between the two UV pulses which create the transient grating, we show that the phase-sensitive instantaneous four-wave-mixing contribution to the nonlinear signal field can be detected and separated from the slower, incoherent part of the response. Measuring a spectrally-resolved cross-correlation in this way and then applying a simple model for the response of the medium, we show that a modified generalized projections (GP) phase-retrieval algorithm can be used to extract the pulse amplitude and phase. We test this approach by measuring chirped UV pulses centered at 400 nm and 267 nm. Since interband absorption (or even photoionization) is not strongly wavelength-dependent, we expect IG XFROG to be applicable deeper into the UV.

Encoding the complete electric field of an ultraviolet ultrashort laser pulse in a near-infrared nonlinear-optical signal

We introduce a variation on the cross-correlation frequency-resolved optical gating (XFROG) technique that uses a near-infrared (NIR) nonlinear-optical signal to characterize pulses in the ultraviolet (UV). Using a transient-grating XFROG beam geometry, we create a grating using two copies of the unknown UV pulse and diffract a NIR reference pulse from it. We show that, by varying the delay between the UV pulses creating the grating, the UV pulse intensity-and-phase information can be encoded into a NIR signal. We also implemented a modified generalized-projections phase-retrieval algorithm for retrieving the UV pulses from these spectrograms. We performed proof-of-principle measurements of chirped pulses and double pulses, all at 400 nm. This approach should be extendable deeper into the UV and potentially even into the extreme UV or x-ray range.

Abnormal spectral bands in broadband sum frequency generation induced by bulk absorption and refraction

In this paper, the time-resolved broadband sum frequency generation (BB-SFG) spectra from a bare Au surface with a distorted infrared (introduced with a 10 µm polyethylene film in the IR light path) and principal component generalized projection (PCGP) algorithm were used to investigate the bulk distortion on the measured BB-SFG spectra. Besides the SFG intensity reduction from the bulk absorption, the frequency dependent refraction of the bulk layer led to misleading SFG features at the positive delay times beyond the Au dephasing time. These results suggest that SFG spectroscopy is not entirely 'bulk-free' for the buried interfaces because of the bulk absorption and refraction of the incident pulses.

Plasma-mirror frequency-resolved optical gating using a liquid-sheet jet in ultraviolet region

We demonstrate plasma-mirror frequency-resolved optical gating (PM-FROG) using a liquid-sheet jet of water and characterize a waveform of a high-repetition laser pulse (1 kHz) in the ultraviolet (UV) region. The measured PM-FROG trace is reconstructed by the least-squares generalized projections algorithm. The retrieved UV spectral phase is consistent with that by self-diffraction FROG. The complex reflection coefficient of a plasma mirror is discussed in terms of the free electron density dependence. The PM-FROG technique is applicable to not only the pulse characterization, but also the investigation of ionization dynamics of a liquid.

Characterization of 20-fs VUV pulses by plasma-mirror frequency-resolved optical gating

We demonstrate that vacuum ultraviolet (VUV) pulses (λ∼160  nm) with 20-fs duration can be fully characterized by plasma-mirror frequency-resolved optical gating. Plasma is formed as an ultrafast optical switch on a fused silica surface by irradiation of an intense near-infrared (NIR) femtosecond laser pulse. We measure the spectra of the VUV pulses reflected off plasma as a function of the delay with respect to the NIR pulse and reconstruct it to simultaneously extract a nearly Fourier-transform limited VUV pulse shape and a time-dependent reflectivity of plasma using the least-squares generalized projections algorithm. We show that there is almost no spatial dependence of the VUV pulse shape, whereas the plasma mirror reflectivity strongly depends on the spatial region.

Full-field ultrafast oscilloscope based on temporal imaging

As the ultrafast laser has been developed, the measurement of ultra-speed transient and dynamic processes has gained much attention. An ultrafast oscilloscope based on the optical stretch method is promising to address this problem, but the measurement is restricted to the temporal profile. This means that the temporal phase is lost. In this work, we propose a full-field ultrafast oscilloscope using two temporal phase retrieval methods: the temporal annealing Gerchberg-Saxton (TAGS) algorithm and temporal ptychography. These could provide complete information, including temporal profile and phase, of high-rate repetitive transient pulses. The functions of an ultrafast oscilloscope with 230 GHz bandwidth and the two phase retrieval methods are verified by simulation and experimental results. This full-field ultrafast oscilloscope promises more applications in phase encoding, phase-contrast imaging, and sensing in the time domain.

Frequency-domain nonlinear regression algorithm for spectral analysis of broadband SFG spectroscopy

The resonant spectral bands of the broadband sum frequency generation (BB-SFG) spectra are often distorted by the nonresonant portion and the lineshapes of the laser pulses. Frequency domain nonlinear regression (FDNLR) algorithm was proposed to retrieve the first-order polarization induced by the infrared pulse and to improve the analysis of SFG spectra through simultaneous fitting of a series of time-resolved BB-SFG spectra. The principle of FDNLR was presented, and the validity and reliability were tested by the analysis of the virtual and measured SFG spectra. The relative phase, dephasing time, and lineshapes of the resonant vibrational SFG bands can be retrieved without any preset assumptions about the SFG bands and the incident laser pulses.