Tianli Feng

Electric-field induced second-harmonic generation of femtosecond pulses in atmospheric air

Electric-field induced second harmonic generation with a femtosecond Ti:sapphire pump laser in atmospheric air is investigated. Phase matching between fundamental and second harmonic waves is achieved via quasi-phase matching, employing periodic electrode arrangements on printed circuit boards with a total length of about 0.5 m. The weak chromatic dispersion of the atmospheric air enables broadband phase matching conditions and makes this method tailorable for frequency doubling broadband femtosecond laser pulses. A maximum energy of 0.13 nJ is observed in a 20 nm bandwidth around 395 nm.Electric-field induced second harmonic generation with a femtosecond Ti:sapphire pump laser in atmospheric air is investigated. Phase matching between fundamental and second harmonic waves is achieved via quasi-phase matching, employing periodic electrode arrangements on printed circuit boards with a total length of about 0.5 m. The weak chromatic dispersion of the atmospheric air enables broadband phase matching conditions and makes this method tailorable for frequency doubling broadband femtosecond laser pulses. A maximum energy of 0.13 nJ is observed in a 20 nm bandwidth around 395 nm.

Hidden correlation in the CEP noise of mode-locked lasers

Stabilizing the carrier-envelope phase (CEP) of mode-locked laser sources has made tremendous progress in recent years, enabling the generation of isolated attosecond pulses and precision frequency metrology at the 10−18 uncertainty level. While some laser types can be very well stabilized, other interesting laser materials have proven notoriously difficult or even impossible to be CEP stabilized. The exact mechanisms behind these problems are poorly understood. Here, we present a novel diagnostic method that provides unprecedented insight into the inner workings of noise generating mechanisms in laser oscillators.

Electro-optic Kerr effect measurement based on carrier-envelope phase demodulation

A precise control of the electric field nodes relative to the maximum of the intensity envelope is a prerequisite for attosecond pulse generation and experiments in high-field physics [1]. Many carrier-envelope phase (CEP) stabilization schemes start with stabilization of an oscillator, which is nevertheless often difficult to obtain as the CEP has proven highly sensitive to environmental parameters, including temperature, air pressure, and humidity. This very high susceptibility to environmental changes is considered more of a nuisance than anywhere useful. Here, we show that the extreme sensitivity of the CEP can be exploited for quantitative measurements of the electro-optic Kerr effect in atmospheric air, i.e., an extremely weak effect that has previously only been measurable by combining effective path lengths of 100m with tens of kilovolt voltages. In contrast, we can obtain a measurable effect by placing an 8 cm long high voltage capacitor into the intracavity beam path of a mode-locked few-cycle Ti: sapphire oscillator, and we only require rather moderate driver voltages of less than 5 kV.

Hidden amplitude-phase correlations in the carrier-envelope noise of mode-locked lasers

Precision frequency metrology and attosecond pulse generation critically rely on stabilization of the carrier-envelope phase (CEP) of mode-locked lasers. So far, only a relatively small class of lasers has been successfully stabilized to warrant phase jitters of a few hundred milliradians as they are required for the generation of an isolated attosecond pulse. For stabilizing certain laser types, the exact reasons for the observed difficulties (or the lack thereof) is only poorly understood. Here we compare the free-running CEP noise of four different lasers, including a femtosecond Ti:sapphire laser and three mode-locked fiber lasers. This study indicates a correlation between amplitude and frequency fluctuations at low Fourier frequencies for essentially all lasers investigated. This finding is well explained with technical noise sources and thermal coupling mechanisms below the upperstate lifetime of the laser gain material. However, for one of the lasers under test, we observe a broadband amplitude-to-phase coupling mechanism well above the upperstate lifetime. This coupling mechanism is related to a dynamical loss modulation. We verify our explanation by numerical simulations, which identify resonances of the saturable absorber mirror as a possible explanation for the coupling mechanism. In case of high modulation depth and resonantly enhanced saturation characteristics, such a saturable absorber can give rise to broadband conversion of spontaneous emission amplitude noise into phase noise, which may cause, in turn, extremely broadband noise signatures, exceeding a megahertz bandwidth.

Intracavity hyperpolarizability measurement from carrier envelope phase demodulation

Using an intracavity high-voltage capacitor, we experimentally show a CEP modulation arising from the quadratic electro-optic effect in atmospheric air, offering a novel approach towards precise measurements of the third-order nonlinear susceptibility of gases.

Intracavity measurement of the electro-optic Kerr effect via carrier-envelope phase demodulation

Placing a sinusoidally driven air capacitor in the intracavity beam path of a mode-locked few-cycle Ti:sapphire oscillator, we measure the influence of the electro-optical Kerr effect on the carrier-envelope phase of the laser pulses. Using a capacitor length of only 8 cm at atmospheric pressure, we observe a Kerr-induced frequency modulation of the carrier-envelope beat note. From the measured frequency excursion, we determine a Kerr constant of the order of 10-27  m2/V2, which is found to agree with theoretically computed hyperpolarizabilities of the nitrogen and oxygen molecules. The carrier-envelope phase only depends on the dispersion of the hyperpolarizability, which has been previously found very challenging to measure in the gas phase. Our substantially more sensitive measurement method for the electro-optic Kerr effect in air may prove a valuable tool for non-contact measurements of high voltages in power grids and even for monitoring atmospheric electric fields during thunderstorms.