Experimental demonstration of frequency pulling in single-pass free-electron lasers

Abstract

Frequency pulling is a well-known phenomenon in standard laser physics, leading to a shift of the laser frequency when the cavity and maximum gain frequencies are detuned. In this letter we present the first experimental demonstration of frequency pulling in single-pass free-electron lasers. Measurements are performed using the single-pass free-electron laser installed on the Elettra storage ring. PACS numbers:Frequency pulling is a well-known phenomenon in standard lasers, taking place when the peak of the gain spectrum is slightly detuned with respect to the frequency of one of the modes selected by the laser cavity. When this occurs, the lasing frequency is close to the one of the selected mode, but slightly "pulled" towards the maximum of the gain curve [1]. While generally investigated in the case of the continuous-wave operation of laser oscillators close to threshold, frequency-pulling has more recently become of interest also to other laser configurations [2]. As an example, the importance of frequency pulling for a correct estimate of the laser-pulse parameters in the case of a mode-locked laser has been theoretically predicted and experimentally confirmed in [3]. Moreover, frequency pulling has been exploited for the fine tuning of the laser frequency [4] and for beat-stabilization in multimode laser operations [5].In a free-electron lasers (FEL) [6], the light amplification is not due to the stimulated emission of an atomic system (like in standard lasers), but is instead relying on the coherent emission of relativistic electrons oscillating into a periodic magnetic field generated by an undulator. In the early days of FEL's, the study of frequency pulling has been focused on (low-gain) oscillator systems [6]. For such FEL configuration, the effect of frequency pulling is similar to that taking place in standard lasers: the laser frequency is determined by the interplay between the proper frequencies of the cavity and that for which the gain is maximum [7,8]. In the case of highgain FEL's [9], the light amplification takes place during a single-pass of the electrons through undulators, without the use of a laser cavity. This motivates the strong interest in single-pass FEL's: being optics-free devices, they offer the unique opportunity to provide coherent and powerful emission in the spectral range from VUV to x-rays. In fact, the coherence of a single-pass FEL can be drastically improved if, before emitting, electrons are brought into interaction with an external coherent "seed" source (e.g., a laser). As described more in detail in the following, such a seed determines the resonant frequencies at which the lasing effect occurs. Therefore, its action is somehow similar to the one of an optical cavity in standard lasers and oscillators FEL's. In a recent work [10], we have theoretically predicted that, also in seeded FEL's, a mismatch between the frequencies "selected" by the seed and the peak of the FEL gain curve may lead to a frequency-pulling effect. Based on numerical simulations, we proposed an ...

Room temperature plasmonic lasing in a continuous wave operation mode from an InGaN/GaN single nanorod with a low threshold

“…2a) exhibits a 5 nm blue-shift compared to the bare nanorod sample (Fig. 3a), showing a frequency pulling effect24. From gain = loss = 2nπ/c × (Δν) under steady state conditions, where n and Δν are refractive index and FWHM, respectively25, the gain and loss can be estimated to 4.3 × 10 3  cm −1 , which is close to the gain stated above.…”

Generation and measurement of intense few-femtosecond superradiant extreme-ultraviolet free-electron laser pulses

“…Let us assume the amplification in the final amplifier as driven by a Gaussian equivalent seed pulse, centred at λ s and with relative spectral width σ s , and a Gaussian FEL gain function centred at the wavelength λ g and relative width σ g . In a linear amplifier, the relative detuning of the FEL emission central wavelength λ p , as a function of λ g would be given by 35 ,…”

“…Finally, we measured and compared the gain spectrum of the FEL in SRC and in a typical HGHG configuration. The analysis of the spectra in SRC has shown an unexpected behaviour consisting of a substantial 'frequency-pulling' 34 effect, which is normally barely observed in HGHG mode 35 .…”

Manipulable and Hybridized, Ultralow‐Threshold Lasing in a Plasmonic Laser Using Elliptical InGaN/GaN Nanorods

“…The schematic diagram and SEM images are shown in Figure a,b. At a low pumping power density, one broad emission appears at ≈510 nm, as shown by inset in Figure d; as the power density increases, the dominant peak shifts from 511 to 490 nm because of the frequency pulling effect . An obvious nonlinear “S” shape is evident in the L–L plot of the PL peak (inset in Figure d), indicating the transition from spontaneous emission to stimulated emission .…”