Pump-Controlled Directional Light Emission from Random Lasers

Hisch, Thomas; Liertzer, Matthias; Pogány, D.; Mintert, Florian; Rotter, Stefan

doi:10.1103/physrevlett.111.023902

Cited by 116 publications

(86 citation statements)

References 55 publications

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“…Recent experiments have shown lasing emission controlled by exploiting scattering dispersion via resonant scattering sustained by spherical particles [6,7] or by gain dispersion achieved by artificially increasing absorption in a spectral band [8]. Active tuning of the lasing properties has also been achieved by shaping the pump profile to selectively excite one or a few lasing modes [9][10][11].Different theoretical approaches to model random lasing action have been developed which combine multiple scattering and gain. For uncorrelated random systems in which interference between the scattered waves can be neglected, diffusive models are very accurate even in presence of optical gain [12,13] and they provide the time evolution of the lasing process and a smooth lasing spectrum with no spiking lasing behaviour [14].…”

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Tuning random lasing in photonic glasses

2015

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We present a detailed numerical investigation of the tunability of a diffusive random laser when Mie resonances are excited. We solve a multimode diffusion model and calculate multiple light scattering in presence of optical gain which includes dispersion in both scattering and gain, without any assumptions about the β parameter. This allows us to investigate a realistic photonic glass made of latex spheres and rhodamine and to quantify both the lasing wavelength tunability range and the lasing threshold. Beyond what is expected by diffusive monochromatic models, the highest threshold is found when the competition between the lasing modes is strongest and not when the lasing wavelength is furthest from the maximum of the gain curve.Random lasers (RL) are mirror-less lasing systems which have attracted a lot of interest due to their structural simplicity. Nowadays they have been studied in a vast variety of scattering systems ranging from semiconductor powder to biological tissue and biocompatible materials [1]. Random lasing originates from a complex out-of-equilibrium phenomenon with rich multimodes features [2] and surprising statistical features [3]. Despite its potential for practical applications [1], random lasing technology is still in its infancy with pioneering applications such as low coherence light source [4] and biosensing [5]. One of the factors that has limited practical applications is the difficulty of controlling the frequency and directionality of the emission. In conventional lasers the lasing emission can be tuned by engineering the high finesse cavity which provides the feedback and thus defines the lasing mode. Instead, feedback in RL is provided by multiple scattering and the lasing emission properties are determined by the complex interplay between gain and losses. Recent experiments have shown lasing emission controlled by exploiting scattering dispersion via resonant scattering sustained by spherical particles [6,7] or by gain dispersion achieved by artificially increasing absorption in a spectral band [8]. Active tuning of the lasing properties has also been achieved by shaping the pump profile to selectively excite one or a few lasing modes [9][10][11].Different theoretical approaches to model random lasing action have been developed which combine multiple scattering and gain. For uncorrelated random systems in which interference between the scattered waves can be neglected, diffusive models are very accurate even in presence of optical gain [12,13] and they provide the time evolution of the lasing process and a smooth lasing spectrum with no spiking lasing behaviour [14]. The radiative transport model with gain can also be solved for * Corresponding author: michele.gaio@kcl.ac.uk instance with Monte Carlo simulations which consider a random walk of photons [15,16] and in which amplification of single paths can be important in defining the spectral properties [17], and by solving the complete radiative transfer equations [18]. These approaches allow the study of large systems (> 100s...

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Tuning random lasing in photonic glasses

2015

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“…By evaluating the far-field properties of the laser, conclusions about the cavity modes can be drawn, which are usually difficult to access in actual experiments [28]. Furthermore, the THz spectral region that we operate in provides the advantage of a large emission wavelength, leading to conveniently adjustable cavity structures, which allows us to exactly determine the geometry of the random laser and thus to engineer its emission characteristics in even more detail [29]. Moreover, the electrically pumped nature of the used quantum cascade active region allows for highly controllable experimental conditions and is crucial for future applications of random lasers [30].…”

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Random lasers for broadband directional emission

Schönhuber

Brandstetter

Hisch

et al. 2016

Optica

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Broadband coherent light sources are becoming increasingly important for sensing and spectroscopic applications, especially in the mid-infrared and terahertz (THz) spectral regions, where the unique absorption characteristics of a whole host of molecules are located. The desire to miniaturize such light emitters has recently lead to spectacular advances with compact on-chip lasers that cover both of these spectral regions. The long wavelength and the small size of the sources result in a strongly diverging laser beam that is difficult to focus on the target that one aims to perform spectroscopy with. Here, we introduce an unconventional solution to this vexing problem relying on a random laser to produce coherent broadband THz radiation as well as an almost diffraction limited far-field emission profile. Our random lasers do not require any fine-tuning and thus constitute a promising example of practical device applications for random lasing.Various spectroscopic techniques rely on the specific absorption features of numerous molecules within the midinfrared and terahertz (THz) spectral regions, which allow their unambiguous identification. Although broadband coherent sources of radiation are already available within these frequency regions [1][2][3][4][5], a compact size and electrically pumped operation are additionally desired for actual applications. These criteria are fulfilled by quantum cascade lasers (QCLs), semiconductor sources which are able to provide broadband emission at mid-infrared [6,7] and THz [8,9] frequencies. However, for typically used THz QCL waveguides the output aperture is on the order of 10 µm, while the emission wavelength is about 100 µm. This, in fact, leads to a very divergent output beam, which is additionally distorted by interference effects [10,11]. Since external optical elements such as lenses or antennas are difficult to handle on a large scale [12,13], several monolithic concepts have been pursued to address this issue. To improve the far-field for facet emission, e.g., a 3rd order distributed feedback (DFB) grating can be used [14]. Another approach is to coherently emit from a large area on the laser surface. In contrast to interband semiconductor lasers, QCLs can only generate in-plane radiation due to intersubband selection rules, preventing the realization of VCSELtype resonators. Thus several concepts have been developed to couple out the in-plane cavity mode in vertical direction, including 2nd order DFB gratings [15,16] , and photonic crystal (PhC) cavities [17]. Recently, also non-periodic resonator structures such as graded photonic heterostructures [18], or quasicrystal cavities [19,20] have been developed. However, all on-chip techniques providing surface emission demonstrated so far are designed to support a single laser mode only, while for many spectroscopic applications a broadband coherent light source is necessary. To achieve the objective of a broadband and at the same time very collimated laser light emission in the THz regime, we propose here a radical ...

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“…Spatially nonuniform pumping schemes have previously been used to control various lasing characteristics, including emission directionality, thresholds, and frequencies in microdisks [8], spirals [9,21], asymmetric resonant cavities (ARC) [22,23], and random lasers [24][25][26][27][28][29], mostly in the single-mode regime and generally in a rather ad-hoc manner. To our knowledge, the present work is the first one to propose it as a scheme to systematically boost laser power in the multi-mode regime.…”

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Enhancement of laser power-efficiency by control of spatial hole burning interactions

Malik

Türeci

2014

Nature Photon

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The laser is an out-of-equilibrium nonlinear wave system where the interplay of the cavity geometry and nonlinear wave interactions, mediated by the gain medium, determines the self-organized oscillation frequencies and the associated spatial field patterns. In the steady state, a constant energy flux flows through the laser from the pump to the far field, with the ratio of the total output power to the input power determining the power-efficiency. While nonlinear wave interactions have been modelled and well understood since the early days of laser theory, their impact on the power-efficiency of a laser system is poorly understood. Here, we show that spatial hole burning interactions generally decrease the power efficiency. We then demonstrate how spatial hole burning interactions can be controlled by a spatially tailored pump profile, thereby boosting the power-efficiency, in some cases by orders of magnitude.Power-efficiency of lasers is a key property to be maximized to reduce energy requirements for on-chip applications, facilitate thermal management and pack lasers into smaller volumes. Earlier work on solid-state lasers addressed the quantum design of the gain medium and the optimization of carrier transport, demonstrating strong improvements in power-efficiency [1,2]. Far less systematic work has addressed the fundamental factors limiting the power efficiency of lasers and effective methods to overcome these, taking into account specific resonator properties. In particular, the impact of spatial hole burning interactions is poorly understood [3]. A case in point are the findings of Ref. [4], where the output power of microcavity quantum cascade lasers was found to increase exponentially with boundary deformation, resulting in a power enhancement over two orders of magnitude with respect to identical lasers of circular cross-section. After more than a decade of theoretical development, the fundamental factors behind this dramatic increase in power efficiency remain unknown. The goal of this paper is to provide a theoretical analysis of the mechanisms at work that enable such dramatic improvements in laser power efficiency.Typically, the pump power in a microlaser is deposited uniformly across the entire cavity area [ Fig. 1(a)] and lasing naturally occurs in cavity modes with the longest cavity lifetimes. Here we show the existence of a maximally power-efficient "optimally out-coupled mode" that may never turn on in a uniformly pumped cavity due to spatial-hole burning interactions. We further show that a non-uniform pump distribution designed to selectively pump the optimally out-coupled mode can lead to strong power enhancements.Spatially non-uniform pump distributions can be realized by fabricating patterned contacts [5], by spatially non-uniform doping in the case of current injection lasers [6,7], or by using spatial light modulators or special lenses for optically pumped lasers [8,9] (see Fig. 1(b)). We discuss the optimal spatial profile of the pump for a given resonator to maximize the power effic...

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Pump-Controlled Directional Light Emission from Random Lasers

Cited by 116 publications

References 55 publications

Tuning random lasing in photonic glasses

Tuning random lasing in photonic glasses

Random lasers for broadband directional emission

Enhancement of laser power-efficiency by control of spatial hole burning interactions

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