Ultrafast nonlinear optical technology 37

It is necessary to accurately control the time evolution of the electric field in several periodic scales of the electromagnetic field oscillation for the generation of the few-period pulse. Once this control is realized, the short-period pulse can repeatedly generate isolated attosecond pulse in gas and solid. In the near-infrared band, the precise dispersion control of more than one octave can adjust the waveform of the short-period pulse and make it play an important role in the field of extreme nonlinear optics.
In the mid-infrared band, the short-period pulse has important applications in the development of semiconductor optoelectronic devices and the ultrafast dynamics of atoms and molecules. Due to the lack of suitable gain medium, the generation of mid-infrared ultrashort pulse mainly depends on the frequency down conversion of near-infrared femtosecond pulse. Due to the lack of a phase controller with a very large bandwidth for the mid-infrared band, it is still a major difficulty to generate a short-period mid-infrared pulse. An alternative is to divide the spectrum into multiple channels, control the amplitude and phase of each channel independently, and then coherently synthesize them to obtain a controllable waveform of short-period pulses. However, this scheme greatly increases the complexity of the system.
Philipp Steinleitner et al. proposed a new control method of mid-infrared short-period pulse waveform [1]. In this method, the mid-infrared pulse generation is based on the technology of intra-pulse auto-difference. By adjusting the carrier envelope phase shift frequency (CEP) of the driving pulse, the waveform of the mid-infrared short-period pulse can be effectively controlled. Philipp Steinleitner et al. established a simplified model to explain this new method, as shown in Figure 1.
Figure 1 Waveform control in cascade nonlinear process [1]
The gray dotted line in Figure 1 represents the driving optical spectrum of the frequency difference process within the pulse, and the time domain corresponds to a few period pulse. In order to simplify the complex nonlinear interaction in the auto-difference process, the author selects two narrowband spectra with frequencies of 130THz and 160THz as the initial spectra. The spectral component with the frequency of 30THz is obtained by the frequency difference between the two initial spectra, and then the spectral component is obtained by the frequency difference with the 130THz part, and the newly generated 100THz part will interact with the 30THz and 160THz parts to generate more new spectral components. The interaction between frequencies is constantly cross, resulting in a series of discrete spectral components.
In the simplified model, the discrete spectral components can be divided into two categories: the odd-order spectrum (blue spectral peak) with the driving light CEP signal and the even-order spectrum (orange spectral peak) not affected by the driving light CEP signal. When the driving light CEP signal changes, the phase of the blue spectrum changes, while the phase of the orange spectrum remains unchanged, and the interference state between the spectra changes, thus affecting the spectrum and the shape of the pulse, and achieving the effect of pulse shaping.
Fig. 2 The diagram of the short-period mid-infrared experimental device [1]
The diagram of the experimental device for realizing the short-period mid-infrared pulse is shown in Figure 2. The front end is a Cr: ZnS mode-locked oscillator, and the output center wavelength is 2.3 μ M. 28fs pulse with pulse energy of 24nJ and repetition frequency of 23MHz. The pulse is first broadened and compressed in TiO2, and the results are shown in Figure 3. In Figure 3b, the gray curve is the spectrum measured by the spectrometer, the blue curve is the spectrum retrieved from the FROG result, and the orange dotted line is the spectrum phase retrieved.
In Figure 3c, the gray curve is the calculated transformation limit pulse, the blue curve and the orange dotted line are the inversion pulse envelope and phase, respectively. The pulse is close to the transformation limit, the half-width at half height is 7.7 fs, corresponding to an optical period, and the pulse energy is 16 nJ. The phase noise and intensity noise were measured by the author. As shown in Fig. 3d and e, the phase noise and relative intensity noise were 11 mrad and 0.036%, respectively, which were at very low levels, providing guarantee for the generation of the subsequent short-period mid-infrared pulse.
Figure 3 Pulse parameters before pulse auto-difference frequency [1]
As shown in Figure 2, the compressed single-cycle pulse is divided into three parts, one for CEO locking; One is used for electro-optical sampling and measurement of mid-infrared waveform; The last 9nJ is used as the pump light to generate a small period of mid-infrared pulse in ZGP crystal by self-differencing frequency within the pulse. The output spectrum after frequency difference is shown in Figure 4. Adjust the CEO of Pumlight Δφ＝ π. The output mid-infrared signal strength is the largest. The output spectrum is shown in the orange curve on Figure 4, with a spectral coverage of 0.9 μ m－12 μ m。 When adjusting Δφ＝ At 0, the spectrum is shown by the blue curve, and the depression of the spectrum disappears at 60THz, but the spectral range changes little. The upper right corner is the output flare shape. Figure 4 shows the corresponding simulation results, which show the same rule as the experimental results.
Figure 4 Intra-pulse auto-difference frequency output spectrum [1]
Figure 4 shows that the structure of the output spectrum can be changed by changing the CEP information of the differential pump light in the pulse. Similarly, the time-domain pulse waveform is also affected by the pump light CEP, as shown in Figure 5,
Figure 5 The influence of CEO of PumpLight on time-domain waveform [1]
Figure 5a shows the CEO output pulse waveform at different times, and Figure 5b shows the corresponding pulse envelope shape. When the CEO changes continuously from 0 to π, the pulse waveform changes significantly from Δφ＝ Cosine function structure at 0 to Δφ＝ The sine function structure at 3/6 π, and then to Δφ＝ Double pulse structure at π. When the pulse waveform changes, the envelope structure naturally changes, and the pulse width also changes between 0.7 and 1.2 cycles.
In short, Philipp Steinleitner et al. realized the output of short-period pulse in mid-infrared band by using the method of intra-pulse autodyne frequency, and realized the waveform control of mid-infrared single-period pulse by changing the autodyne frequency pump light, which is expected to provide flexible and reliable light sources for the fields of ultra-fast electronic dynamics, isolated attosecond pulse generation, etc.