Control of Lasing from Bloch States in Microcavity Photonic Wires via Selective Excitation and Gain

A. Mischok, R. Brückner, H. Fröb, V. G. Lyssenko, K. Leo, and A. A. Zakhidov

Phys. Rev. Applied 3, 064016 – Published 22 June 2015

Abstract

By adding photonic wire structures to an organic microcavity, we create an additional confinement and a Bloch-like band structure in the dispersion of periodically structured cavities. We experimentally observe spontaneous and stimulated emission from the ground and different excited discrete modes at room temperature. By changing the spatial gain distribution via a two-beam interference, we are able to directly control the laser emission from both extended and confined modes of such organic photonic wires. Both spatial distribution and dispersion exhibit coherent emission from tunable modes, which we describe with an analytical model and numerical simulations, in agreement with our measurements.

Received 26 March 2015

DOI:https://doi.org/10.1103/PhysRevApplied.3.064016

Authors & Affiliations

A. Mischok^*, R. Brückner, H. Fröb, V. G. Lyssenko, and K. Leo

Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germany

A. A. Zakhidov^†

Fraunhofer COMEDD, 01109 Dresden, Germany

^*andreas.mischok@iapp.de
^†Present address: Texas State University, 601 University Drive, San Marcos, TX, USA.

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Vol. 3, Iss. 6 — June 2015

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Images

Figure 1
(a) Calculated absorption spectra of the organic emitter system for different fractions of excited DCM molecules. Apart from the absorption of $Al q_{3}$ centered at 400 nm and the absorption of DCM at 510 nm, we see the occurrence of gain (negative absorption) around 630 nm, for higher fractions of excited molecules. Dashed lines: Measured absorption of pure $Al q_{3}$ and DCM (green); photoluminescence intensity of 2 wt % DCM in $Al q_{3}$ (orange). (b) Sample schematic. DBR mirrors contain an organic cavity with ${SiO}_{2}$ wires of width $L_{W}$ and barriers of width $L_{B}$ . (c) Excitation geometry for selective excitation. The pump beam is split into two beams, which interfere under an oblique angle of incidence and create a periodic distribution of gain in the sample plane.
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Figure 2
(a),(b) Spatially resolved emission spectrum perpendicular to a photonic wire for above-threshold excitation localized on the wire [(a) $L_{W} = 3.7 μ m$ ] and of the barrier [(b) $L_{B} = 7.4 μ m$ ]. The potential induced by the ${SiO}_{2}$ wire laterally traps photons in discrete standing wave modes. Lasing takes place either from the lowest confined state (a) or the lowest extended above-barrier state (b) (deep red). The excitation spot position is indicated in pink. (c),(d) Angle-resolved emission corresponding to (a) and (b). The emission from the barrier exhibits a flat dispersion, while the above-barrier states show a Bloch-like dispersion with almost continuous spectra. The intensity is color coded in logarithmic scale in all figures. (e) Input-output curve for excitation of the barrier as in (b),(d) (red squares) with a lasing threshold of 0.15 nJ (approximately $20 μ J / {cm}^{2}$ ) and for the excitation of the wire as in (a),(c) (blue circles) with a threshold of 0.1 nJ. Dashed lines with a slope of 1 are added as a guide for the eye.
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Figure 3
Angle-resolved emission from photonic wire arrays of different $L_{W} = 1 μ m$ (a), $3.7 μ m$ (b), and $10.0 μ m$ (c) for excitation on the wire. The width of the photonic wire leads to a closer or wider spacing of the discrete modes below the barrier and a differing total number of confined states. Because of the flat dispersion of the confined states, lasing originates from the modes closest to the gain maximum of the organic emitter, even for different geometries. Intensity is color coded as in Fig. 2. Solid lines in (b) represent calculations of $k$ spacing according to Eq. (4)
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Figure 4
Lasing from a photonic wire of $L_{W} = 10.0 μ m$ for different gain distributions created via two excitation-beam interference. (a)–(d) show the angle-resolved dispersion, while (e)–(h) show the corresponding modes in the near field. Here, the $x$ coordinate is perpendicular, while the $y$ coordinate is parallel to the wire orientation. By varying the angle of two excitation beams and, thus, the spatial distribution of gain, modes with a corresponding field distribution are selectively excited from the ground state with one antinode (a) to higher excited states with $m + 1$ antinodes (b)–(d). Intensity in (a)–(d) is color coded as in Fig. 2.
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Figure 5
Calculated angle-resolved spectra of emission from photonic wires under different gain distributions. Intensity is color coded as in Fig. 2.
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