Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals

Kopp, Victor I.; Fan, Boyu; Vithana, Hemasiri; Genack, Azriel Z.

doi:10.1364/ol.23.001707

Cited by 748 publications

(424 citation statements)

References 12 publications

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“…3a. Lasing from the band-edge modes is well studied in the case of the ordered structures [12,20,21,22,23,24], with 1/L 3 dependence of the threshold [12] giving a good fit with our results. At the increased disorder, w n = 0.2, we observe the decrease of the threshold that is noticeably faster than exponential, expected for localized modes [6].…”

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confidence: 79%

Highest-quality modes in disordered photonic crystals

Yamilov

Cao

2004

Phys. Rev. A

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We studied lasing modes in a disordered photonic crystal. The scaling of the lasing threshold with the system size depends on the strength of disorder. For sufficiently large size, the minimum of the lasing threshold occurs at some finite value of disorder strength. The highest random cavity quality factor was comparable to that of an intentionally introduced single defect. At the minimum, the lasing threshold showed a super-exponential decrease with the size of the system. We explain it through a migration of the lasing mode frequencies toward the photonic bandgap center, where the localization length takes the minimum value. Random lasers with exponentially low thresholds are predicted.A finite open system of scattering particles can be characterized by a set of quasi-stationary (leaky) optical modes. When gain, sufficient to compensate the leakage in at least one mode, is introduced in such systemrandom laser is formed [1,2,3,4,5,6,7,8,9]. Unconventional feedback mechanism in a random laser leads to properties interesting from both fundamental and practical point of view [1,2,3,4,5,6,7,8,9,10,11,12,13]. While previous studies concentrated mainly on disordered systems, the resent experiments by Shkunov et al [14] demonstrated random lasing peaks associated with the bandgap in partially ordered system. In this letter, we obtain the lasing threshold for a random laser with different degree of ordering. Important information about these states can be extracted from the scaling of the average lasing threshold with the system size [12], L. For increasing values of the disorder parameter we find 5 scaling regimes: (a) photonic band-edge, 1/L 3 , (b) transitional super-exponential, (c) bandgap-related exponential, (d) diffusive, 1/L 2 , and (e) disorder-induced exponential, due to Anderson localized modes, regimes. In this letter we show that by optimizing the disorderness of the sample one can dramatically reduce the threshold of random laser, to the values comparable to those of photonic bandgap defect lasers [15]. This finding can open up a road to practical applications of random lasers.Finding the lasing threshold of a random laser theoretically is a difficult problem. In disordered photonic crystals one cannot take simplifying assumptions such as independent scattering approximation (low density limit), neglect the finite size of the scatterers, or the confined dimensions of the system. Random laser modes have the highest Q's and therefore weakly contribute to the transport properties of the system. Among other methods [16,17], finite difference time domain (FDTD) method has been shown to be a convenient tool in studying random laser modes in 1D [10] and 2D [11]. It was recently shown [10,11] that above the threshold, the shape of the lasing modes remain the same as in passive medium. In this work, we used FDTD method to find the modes with the smallest energy decay rate in an open passive 2D random medium system with different degree of order. These modes determine the value of the lasing threshold in an act...

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confidence: 79%

Highest-quality modes in disordered photonic crystals

Yamilov

Cao

2004

Phys. Rev. A

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“…In these chiral structures, a stop band exists for circularly polarized light that has the same sign of rotation as the CLC structure. Since the dwell time within the sample for emitted photons is enhanced near the band edge, the lasing threshold is also substantially reduced [7].In this Letter, we present an unexpected conjunction of strong expansion of the region of phase coherence within the medium and weak divergence of the output beam, which is made possible in a periodic structure possessing a photonic stop band. Our calculations show that a diffusionlike spreading of the coherent field leads to wide area lasing at the edge of a photonic stop band or at a localized state in the middle of the stop band.…”

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confidence: 79%

Large Coherence Area Thin-Film Photonic Stop-Band Lasers

2001

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We demonstrate that the shift of the stop-band position with increasing oblique angle in periodic structures results in a wide transverse exponential field distribution corresponding to strong angular confinement of the radiation. The beam expansion follows an effective diffusive equation depending only upon the spectral mode width. In the presence of gain, the beam cross section is limited only by the size of the gain area. As an example of an active periodic photonic medium, we calculate and measure laser emission from a dye-doped cholesteric liquid crystal film. DOI: 10.1103/PhysRevLett.86.1753 Photonic band-gap materials hold promise as a platform for a new generation of efficient, compact photonic devices. The initial excitement generated by these materials has been sustained by the discovery of new physical effects leading to new applications [1][2][3]. Yablonovitch has predicted that the lasing threshold can be reduced by introducing a defect into an otherwise periodic photonic band-gap structure. Since spontaneous emission is suppressed in the band gap, excitation will not then be drained by modes other than the lasing mode [2]. In addition, the long dwell time of such localized defect modes reduces the gain required to reach the lasing threshold. Lasing has recently been observed in 2D photonic crystals [4], and promising 3D photonic crystals have been fabricated [5]. The threshold for lasing may even be suppressed at defect modes in periodic structures that do not possess a full 3D photonic band gap, such as in 1D periodic samples, including vertical cavity surface emitting lasers (VCSELs) [6]. Recently, lasing at the band edge has been demonstrated in dye-doped cholesteric liquid crystals (CLCs). In these chiral structures, a stop band exists for circularly polarized light that has the same sign of rotation as the CLC structure. Since the dwell time within the sample for emitted photons is enhanced near the band edge, the lasing threshold is also substantially reduced [7].In this Letter, we present an unexpected conjunction of strong expansion of the region of phase coherence within the medium and weak divergence of the output beam, which is made possible in a periodic structure possessing a photonic stop band. Our calculations show that a diffusionlike spreading of the coherent field leads to wide area lasing at the edge of a photonic stop band or at a localized state in the middle of the stop band. These results are supported by measurements of the spatial profile of laser emission from a promising organic material -a dye-doped cholesteric liquid crystal. Since the maximum excitation energy is proportional to the laser area, large-area thin-film devices provide a new approach for high-power lasers. While liquid crystals are not stable at the elevated temperatures produced by intense laser excitation, polymeric CLCs may be promising materials for large-area high-power lasers.Though lasing has not yet been observed in 3D periodic structures, the enhanced photon dwell times and microcavity effects in l...

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“…As a consequence, the fluorescent emission in these materials is suppressed in the reflection band and is enhanced at the band edges. This gain enhancement and distributed feedback effect can give rise to low threshold mirrorless lasing at the band edge in a variety of liquid crystal materials [2][3][4][5][6][7][8][9][10][11][12][13][14][15].…”

Section: Introductionmentioning

confidence: 99%

Lasing Thresholds of Cholesteric Liquid Crystals Lasers

Cao

Palffy‐Muhoray

Taheri

et al. 2005

Molecular Crystals and Liquid Crystals

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Cholesteric liquid crystals (CLCs) are one dimensional photonic band-gap materials. Due to distributed feedback, low threshold mirrorless lasing can occur in dye-doped and pure CLCs. In order to optimize the lasing conditions, we have studied the dependence of the lasing threshold on dye concentration and sample thickness. In particular, we have studied dye concentrations in the range of 0.1-3.0wt%, and cell thicknesses are in the range of 5-50µm. We have found that the system has a shallow lasing threshold minimum and can operate efficiently in the range of dye concentrations of 0.3-2.4wt% and sample thicknesses of 10-50µm. We discuss the physical processes responsible for the observed behaviour.

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Low-threshold lasing at the edge of a photonic stop band in cholesteric liquid crystals

Cited by 748 publications

References 12 publications

Highest-quality modes in disordered photonic crystals

Highest-quality modes in disordered photonic crystals

Large Coherence Area Thin-Film Photonic Stop-Band Lasers

Lasing Thresholds of Cholesteric Liquid Crystals Lasers

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