Tunable degree of localization in random lasers with controlled interaction

Abstract

We show that the degree of localization for the modes of a random laser (RL) is affected by the inter mode interaction that is controlled by shaping the spot of the pump laser. By experimentally investigating the spatial properties of the lasing emission we infer that strongly localized modes are activated in the low interacting regime while in the strongly interacting one extended modes are found lasing. Thus we demonstrate that the degree o localization may be finely tuned at the micrometer level.PACS numbers: 42.25.Dd 07.05.Fb Keywords: Light localization, Random lasing RL are among the most complex systems in photonics, encompassing structural disorder and nonlinearity, 1 and ranging from micron sized optical cavities 2 to kilometerlong fibers 3 . Attention on this systems has been constantly growing as the number of potential applications, ranging from object coding 4 to speckle-free illumination 5 .First-principle time domain simulations show that modes of a RL arise from electromagnetic states [6][7][8] , that may appear in localized or extended fashion. Several experiments confirmed this view 2,9,10 , and tried to address the connection between the structure 11 pumping condition, or gain and the degree of localization 12-14 . The The pulsed laser (532 nm, repetition frequency 10 Hz and fluence 0.1nJ/µm 2 ), whose spot is shaped by a SLM in amplitude configuration (by using the two crossed polarizers P1 and P2), pumps a single titanium dioxide cluster (diameter between 5 and 12 µm). The RL emission is collected by a microscope objective (OBJ) to be imaged by using a beamsplitter (BS) in two different image planes. In one of them lies a fiber controlled by translators with nanometric resolution that allows the measurement of the spatio-spectral map. In this way it is possible to scan a magnified (50×) image of the sample and measure the spectra emitted from a single point. The fibre core (50 µm in diameter) collects spectra originating in an area of 1µm of diameter of the sample. The other light path allows imaging the sample on a CCD. presence of many modes may give rise to unique phenomena: in a linear system extended necklace states spread over the sample via multiple (localized) resonances 15 while in a system with gain the inter-mode coupling affects the whole spectrum generating mode repulsion 16 directly connected to the nonlinear interaction 17 .On the other hand it has been recently demonstrated that the inter-mode coupling plays a critical role in the onset of two fundamentally different RL regimes, distinguished by the shape of the emission: a "resonant feedback random laser" (RFRL) 18 , which appears as a set of sharp peaks oscillating independently at fixed spectral positions, and the "intensity feedback random laser" (IFRL) 19 , characterized by a smooth single line narrowed spectrum. By using a tailored spatial shape of the pump area 20,21 , a switching between the two can be achieved. In fact a RFRL is observed when activating a set of weakly interacting resonances while IFRL is produced un...

Random lasing of microporous surface of Cr2+:ZnSe crystal induced by femtosecond laser

“…Since then, random laser behaviors have been extensively studied with a variety of experiments [6][7][8][9][10] and theoretical calculations. [11][12][13] However, most of them are focused on the visible to near-infrared wavelength range.…”

Dynamics of phase-locking random lasers

“…Titanium particles have an average size of about 300 nm [titanium (IV) oxide, 89490 Sigma-Aldrich, particle size <1 μm] and the gain medium is a solution of ethanol and ethylene glycol doped with 1% (volume) of rhodamine B (Sigma-Aldrich 242425). The clusters are organized in a sparse arrangement on the bottom of the space defined by two microscopy coverslips separated by a plastic film [21], and it is well known that they are able to support modes with various degrees of interaction [26,27]. Moreover, this configuration allows for an easy manipulation of the number of activated modes (see below).…”

“…This approach, which has been well described in [20,21,27], allows to control, through the parameter, i.e., the angular aperture of the inverted area (see, for example, Fig the set of directions from which stimulated emission from the free dye pumps the cluster. In this configuration the pump selects only the modes that are well coupled with the input direction [28,29].…”

Non-locality and collective emission in disordered lasing resonators

“…Extended modes are favored in the highly interacting regime, while local resonances are activated in the barely interacting configuration. 25 Here, we will demonstrate that the change in the mode shape with H for a fixed disorder is due to a novel form of non-locality mediated by the mode-coupling between the open cavities constituting the set of involved resonators. 17,28 To date, spatial non-locality has been largely investigated in Hamiltonian nonlinear optical systems, but here we report the first evidence in a dissipative disordered medium.…”

“…By pumping the area surrounding the cluster, one can generate a flux of directional stimulated emission capable of pumping only modes that are well coupled with the pump shape, and the lasing threshold may be surpassed ( Figure 2). The number of activated modes is controlled by selecting, through a spatial light modulator, the angular aperture H of the wedge-shaped pumped area (see Figure 1b and studies [23][24][25] for further details).…”