Experimental studies on the mode structure of random lasers

El-Dardiry, Ramy G. S.; Mosk, Allard; Muskens, Otto L.; Lagendijk, Ad

doi:10.1103/physreva.81.043830

Cited by 31 publications

(32 citation statements)

References 27 publications

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“…This kind of lasing emission has been observed in very strong scattering systems (Zinc Oxide or Gallium Phosphide) with very small spatial extension, very small pump spot, or also with stripe shaped spot [12]. In particular it has been shown [13] that by increasing the pump spot of a RFRL a smooth spectrum is recovered.Up to now different theoretical approaches have been suggested to account for the different aspects of RLs. The RL spectrum has been investigated through the strong nonlinear interaction of RL individual lossy modes [14], while an analogy with condensed matter physics a spinglass-like model [15] predicts the existence of various thermodynamic phases including a mode locked condition, in which lasing resonances are synchronized.…”

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Random laser tailored by directional stimulated emission

Leonetti¹,

Conti²,

López³

2012

Phys. Rev. A

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A disordered structure embedding an active gain material and able to lase is called random laser (RL). The RL spectrum may appear either like a set of sharp resonances or like a smooth line superimposed to the fluorescence. A recent letter [1] accounts for this duality with the onset of a mode locked regime in which increasing the number of activated modes results in an increased inter mode correlation and a pulse shortening ascribed to a synchronization phenomenon. An extended discussion of our experimental approach together with an original study of the spatial properties of the RL is reported here.PACS numbers: 42.55. Zz, 42.60.Fc The possibility of obtaining lasing into a scattering medium has been predicted in the sixties by Letokov [2] and experimentally realized for bulk lasing material and scattering particles dispersed in liquid dye in the last decades [3][4][5]. These pioneering experiments on random lasers (RL) have been characterized by a line narrowed frequency spectrum (from tens to some nanometers) on the top of the active medium fluorescence spectrum. This phenomenon, denoted as Intensity Feedback Random Lasers (IFRL), may be explained in the framework of diffusion approximation [6] in which all the properties of the photons propagation are defined by the transport mean free path ℓ. This approach, which neglects the wavelike nature of light, is particulary useful in different scattering regimes where it may be used to predict particular properties of RL, like fluctuations [7], or the appearance of spikes in the spectrum at random positions [8]. On the other hand, the diffusion approximation does not allow to predict the existence of RL peaks that have been more recently measured in numerous experiments [9]. These peaks, which appear at a fixed wavelength and whose light is emitted from localized portions of the disordered structure, may be associated with the presence of Anderson localized states [10] or with highly scattering resonances dwelling in the disordered structures [11], that are commonly referred to as Resonant Feedback Random Lasers (RFRL). This kind of lasing emission has been observed in very strong scattering systems (Zinc Oxide or Gallium Phosphide) with very small spatial extension, very small pump spot, or also with stripe shaped spot [12]. In particular it has been shown [13] that by increasing the pump spot of a RFRL a smooth spectrum is recovered.Up to now different theoretical approaches have been suggested to account for the different aspects of RLs. The RL spectrum has been investigated through the strong nonlinear interaction of RL individual lossy modes [14], while an analogy with condensed matter physics a spinglass-like model [15] predicts the existence of various thermodynamic phases including a mode locked condition, in which lasing resonances are synchronized. A different approach valid in the condition in which the This review follows a recent letter [1] in which it has been demonstrated that the particular regime (so far called either resonant or diffusive) in...

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confidence: 91%

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confidence: 91%

Random laser tailored by directional stimulated emission

Leonetti¹,

Conti²,

López³

2012

Phys. Rev. A

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“…3a), thus simple image analysis does not allow to obtain quantitative information about the spatial extent of every single mode; to this aim both spatial and spectral resolution is needed 22 . This is achieved by a suitably designed experimental setup: an image of the sample is formed trough a 50× magnification optics (microscopy objective and eyepiece) in an plane that is scanned by a motorized fiber connected to a spectrograph.…”

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Tunable degree of localization in random lasers with controlled interaction

2012

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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...

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“…In former experiments on RFRL a tightly focused pump spot was used to excite a limited number of modes thus obtaining a spectral emission displaying narrow spikes [15,22]. In our approach, for small Θ, we select modes that are strongly coupled with a directional input but dwell at distant positions (see figures 1c, 1d and 1e); in the absence of spatial overlap their mutual interaction is negligible, and the spectra obtained feature narrow peaks with limited correlation (figure 3e for low Θ).…”

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The mode-locking transition of random lasers

2011

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The discovery of the spontaneous mode-locking of lasers, i.e., the synchronous oscillation of electromagnetic modes in a cavity, has been a milestone of photonics allowing the realization of oscillators delivering ultra-short pulses. This process is so far known to occur only in standard ordered lasers with meter size length and only in the presence of a specific device (the saturable absorber). Here we demonstrate that mode-locking can spontaneously arise also in random lasers composed by micronsized laser resonances dwelling in intrinsically disordered, self-assembled clusters of nanometer-sized particles. Moreover by engineering a novel mode-selective pumping mechanism we show that it is possible to continuously drive the system from a configuration in which the various excited electromagnetic modes oscillate in the form of several, weakly interacting, resonances to a collective strongly interacting regime. By realizing the smallest mode-locking device ever fabricated, we open the way to novel generation of miniaturized and all-optically controlled light sources.Random lasers[1] (RLs) are made by disordered highly scattering materials able to amplify light when externally pumped. The simultaneous presence of structural disorder and nonlinearity makes these devices a fertile ground to connect photonics with advanced theoretical paradigms[2] like chaos [3], non Gaussian statistics[4], complexity [5] and also the physics of Bose Einstein condensation [6]. Historically there has been a bridge in the RL interpretation. In pioneering experiments a smooth, single-peaked emission was produced by pumping finely ground laser crystals [7], or titania particles dispersed in a dye-doped solution [8,9]. This phenomenon has been dubbed RL with incoherent feedback (IFRL) because it may be explained in the framework of the diffusion approximation [10] that neglects interference and treats light rays as the trajectories of random walking particles. However this theoretical framework does not explain another kind of RL exhibitting subnanometre sharp spectral peaks [11][12][13] associated with high-Q resonances[14-17] and labeled resonant feedback random laser(RFRL).Standard multimode lasers without disorder and characterized by equispaced resonances may be driven to a synchronous regime through the so called mode-locking transition [18,19], which so far has only been shown to occur spontaneously in the presence of a saturable absorber and allows to generate ultra-short light pulses [20,21]. We show that the same transition occurs in RLs and allows to lock modes of a RFRL casting its emission in the typical IFRL spectrum and demonstrating the inherently coherent nature of the random lasing phenomenon.The system we consider is an isolated micrometer sized cluster of titania nanoparticles immersed in a rhodamine dye solution (see supplementary information (SI)). In our novel setup we use the amplified spontaneous emission (ASE) from the surrounding dye to pump the cluster. The the ASE areas are defined by shaping the beam of an e...

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Experimental studies on the mode structure of random lasers

Cited by 31 publications

References 27 publications

Random laser tailored by directional stimulated emission

Random laser tailored by directional stimulated emission

Tunable degree of localization in random lasers with controlled interaction

The mode-locking transition of random lasers

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