Abstract
The working threshold is an important parameter to assess the performance of cavity-free random lasers. Here, the temporal profile measurement is proposed as an alternative method to determine the thresholds of the surface plasmon based random lasers pumped by ns pulses based on analyzing the delay time (tDelay) and rising time (tR) of the emission signal. The obvious and slight inflection points of the curves of tDelay and tR varying with the pump power density are observed as indicators for the thresholds of random lasing and for the transition of lasing mode, respectively. The proposed method supplies consistent values to those supplied by traditional methods in frequency-domain for the random systems with different gain length. The demonstrated temporal profile approaches are free from the spectrometers and may be as a candidate for measuring the threshold of random lasers in ultrafast optics, nonlinear optics and bio-compatible optoelectronic probes.
Highlights
An alternative method is presented for determining the thresholds of surface plasmon based random lasers pumped by ns pulses based on analyzing the delay time and rising time of the temporal profiles varying with the pump power densities
A typical dual-regime random laser[36] is chosen and its emission characteristics are demonstrated in Fig. 3 under the condition of CR6G = 2.1 mM with ρAgNW = 7.3 × 107 mL−1
It should be pointed out that these random lasing peaks are located at 573.77 nm, 578.79 nm, 580.84 nm, 583.49 nm, which are in accordance with the Raman shift of Rhodamine 6 G (R6G) molecules at 1361 cm−1, 1509 cm−1, 1575 cm−1, 1651 cm−1 37
Summary
When a pumping beam is incident on a random system with gain materials, some of the photons are very rapidly (10−15 fs) absorbed. Few excited molecules can directly transits from vibrational level S1* to the ground level through a Raman scatter based radiative process[35] This process may shorten the pulse duration by decreasing the rising time of emission induced by the leaving out of σrlx. Based on the transition processes mentioned above, the non-radiative relaxation from the photo-excited state S1* to the emitting state S1 is measured through the rising time of the emission temporal profile[25]. The non-radiative relaxation from the emitting state S1 to the ground state S0 can be studied through the falling time, which reflects the lifetime tf Based on this physical mechanism, the temporal profile measurement technique is used to study the dynamic behaviors of RLs and analyze their threshold behaviors. Through the two-step measurement, the delay time between the pump source (purple line) and emission signal from the random system (pink line) can be measured and recorded as the delay time tdelay
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