Color-Tunable Organic Microcavity Laser Array Using Distributed Feedback Giuseppe Strangi, * Valentin Barna, Roberto Caputo, Antonio De Luca, Carlo Versace, Nicola Scaramuzza, Cesare Umeton, and Roberto Bartolino LICRYL-INFM and Center of Excellence CEMIF.CAL, Department of Physics, University of Calabria, I-87036 Rende (CS), Italy Gabriel Noam Price Department of Physics and FLCMRC (Ferroelectric Liquid Crystal Materials Research Center), University of Colorado, Boulder, Colorado 80309-0390, USA (Received 30 August 2004; published 17 February 2005) Distributed feedback microstructures play a fundamental role in confining and manipulating light to obtain lasing in media with gain. Here, we present an innovative array of organic, color-tunable microlasers which are intrinsically phase locked. Dye-doped helixed liquid crystals were embedded within periodic, polymeric microchannels sculptured by light through a single-step process. The helical superstructure was oriented along the microchannels; the lasing was observed along the same direction at the red edge of the stop band. Several physical and technological advantages arise from this engineered heterostructure: a high quality factor of the cavity, ultralow lasing threshold, and thermal and electric control of the lasing wavelength and emission intensity. This level of integration of guest-host systems, embedded in artificially patterned small sized structures, might lead to new photonic chip architectures. DOI: 10.1103/PhysRevLett.94.063903 PACS numbers: 42.70.Qs, 42.70.Df Photonic crystals that strongly localize light are finding applications in many areas of physics and engineering, including coherent electrophoton interactions [1], photonic chips [2], nonlinear optics [3], low-threshold laser [4], and quantum information processing [5]. Photonic crystals are periodic dielectric structures that can selectively reflect light for any direction of propagation in specific wave- length ranges. This property, which can be used to confine, manipulate, and guide photons, should allow the creation of all-optical integrated circuits. In analogy with the energy gap in semiconductors, where a forbidden window opens up between the valence and the conduction bands, a di- electric periodic structure shows a stop band in the spec- trum of propagating electromagnetic modes, known as the photonic band gap (PBG). In PBG microstructures light can be confined and manipulated by engineering ideal dis- tributed feedback (DFB) microresonators. Confinement of light to small volumes has important implications for optical emission properties: it changes the probability of spontaneous emission from atoms, allowing both enhance- ment and inhibition [6,7]. Chiral liquid crystals (LCs) possess a helical superstructure which provides a 1D spa- tial modulation of the refractive index giving rise to Bragg selective reflection for circularly polarized light having the same handedness as the LC structure. Circular Bragg re- flection occurs between wavelengths  1  pn o and  2  pn e , where n o and n e are the ordinary and extraordinary refractive indices of the locally uniaxial structure, and p is the pitch of the helical structure. The optical reflection band is centered at  c  pn av with a bandwidth   n e  n o p, where n av is the average refractive index. In a medium with gain, such as a chiral LC doped with fluorescent guest molecules, a photonic band gap affects the emission spectrum. Within the band gap, where the wave is evanescent and decays exponentially, the sponta- neous emission is suppressed. This can be explained by taking into account that the photonic density of states (DOS) vanishes in large periodic structures and the rate of the spontaneous emission is proportional to the DOS [8]. The DOS diverges as the band edge is approached, owing to the fact that the group velocity approaches zero in proximity of the edges, and the resulting long dwell time of the emitted photons strongly supports stimulated emis- sion [9]. Kogelnik and Shank [10] were the first to report laser action in periodic DFB structures which do not utilize conventional cavity mirrors but provide optical feedback via backward Bragg scattering. Lasing has been demon- strated in dye-doped cholesteric liquid crystals [11], liquid crystals in polymer networks [12 –14], and also ferroelec- tric liquid crystals [15]. Here we report the fabrication of color-tunable, DFB microcavity lasers with a high ratio between the quality factor Q of the cavity and its volume V, the Purcell number [16]. The optical microcavities were obtained by embedding dye-doped helixed liquid crystals in holographically patterned polymeric microchannels (Fig. 1). The orientation of the LC helix axis along the microchannels offers several physical advantages: increase of the cavity length and thus the number of periods, small modal volume, directional control, improvement of the liquid crystal orientational order parameter, and wave- length tunability. The distributed feedback needed to ob- tain laser oscillations in the case of a small refractive index modulation requires thousands of periods in order to be- have as an optical cavity with a quite high quality factor, Q  280 [17]. The presented geometry allows obtaining a number of periods which is about 2 orders of magnitude larger than conventional systems because it exploits the PRL 94, 063903 (2005) PHYSICAL REVIEW LETTERS week ending 18 FEBRUARY 2005 0031-9007= 05=94(6)=063903(4)$23.00 063903-1  2005 The American Physical Society