R. Vijaya

Three-dimensionally ordered photonic crystal heterostructures with a double photonic stop band

The recently reported inward growing self-assembling method is used to fabricate photonic crystal heterostructures in less than 6h using polymeric colloidal suspensions. Structural characterization shows good ordering of the two types of photonic crystals constituting the heterostructure. The reflection/transmission spectra measured from the (111) plane of the heterostructure clearly demonstrate the signature of the double photonic stop band. The reduction in the reflectance values obtained for the heterostructure is explained in terms of the interface roughness. The double photonic stop band of the heterostructures exactly matches the stop bands of the individual photonic crystals.

Emission studies on photonic crystals fabricated using dyed polystyrene colloids

Three dimensionally ordered photonic crystals are fabricated with rhodamine B dyed polystyrene colloidal spheres using the inward growing self-assembling method in less than 3 h. This avoids the difficulties due to infiltration of active materials into passive photonic crystals. The superior optical quality of the photonic crystals fabricated using this method results in high reflectance values even at large angles of incidence. The study of emission characteristics on these functionalized photonic crystals shows a clear trend dependent on the angle of emission, wavelength, and the angle-resolved transmittance. The dip observed in the emission spectrum clearly matches the photonic stop band position at different angles of observation. The emission spectrum measured at different angles was found to follow a simple functional form related to the intrinsic emission of the dye and the stop band effect due to the photonic crystal environment.

Laser emission from self-assembled active photonic crystal matrix

Three-dimensionally ordered photonic crystals were grown using self-assembly technique from Rhodamine-B dye doped polystyrene micro-spheres resulting in a stop band at 611 nm overlapping the emission spectrum of the dye. When excited at a wavelength away from the stop band, using a frequency-doubled Nd:YAG laser, the crystal showed angle- dependent suppression of spontaneous emission of the dye in the wavelength range of the photonic stop band and enhancement at the band edge, in reflection and transmission geometries. Spectral narrowing, a sharp threshold and a highly directional emission, all indicative of stimulated emission, were observed from the active photonic crystal matrix.

Analysis of Lasing in Dye-Doped Photonic Crystals

Recently, we experimentally demonstrated room-temperature lasing of self-assembled opal photonic crystal (PhC) made of rhodamine-B-doped polystyrene colloids. Here, we explain this experimental observations by analyzing the phenomenon of light amplification in dye-activated PhCs via a complex-valued permittivity of the colloids. We show that the lasing is facilitated by the enhanced distributed feedback due to the reduced group velocity in the vicinity of the photonic band edge. This simple approach to the analysis of PhC lasing behavior allows us to calculate the lasing wavelength in close agreement with the experimental value. It also enables the estimation of gain coefficient required for lasing and may prove useful in design of compact PhC-based lasers.

Infiltration of ZnO in polymeric photonic crystal by the sol-gel process

Three-dimensionally ordered photonic crystals were fabricated using the vertical self-assembly method from colloidal polystyrene spheres of sub-micrometre diameters. Structural and optical characterization results indicate the extent of good ordering present in these samples. The photonic stop band was tuned by infiltrating two important oxide materials, namely, SiO2 and ZnO. Infiltration was done using a low cost room-temperature sol?gel process which does not require any sophisticated instrumentation. The optical measurements clearly demonstrate that the infiltration is significant. The experimentally measured values of wavelength shift and peak reflectance were compared with theoretically estimated values and found to be in good agreement.

Spatial and spectral distributions of emission from dye-doped photonic crystals in reflection and transmission geometries

Abstract. Spectral distribution of emission was measured in a large angular range (8 deg to 180 deg) around a self-assembled photonic crystal synthesized from colloids of Rhodamine-B dye-doped polystyrene. Its comparison with the emission from the same dye-doped colloids in a liquid suspension provides a better understanding of the anisotropic propagation of light within the structure due to its pseudo-gap properties. The spontaneous emission is suppressed by 40% in the presence of the stop band over a large bandwidth (∼50%) of the first-order bandgap in the ΓL direction, due to the appropriate choice of the colloidal diameter. Spectral shifts in the spontaneous emission spectrum occur with the variation in the detection angle. The inevitable disorder in the self-assembled crystals and the resultant effect on emission was modeled by comparing the experimentally obtained reflection spectrum with the band structure calculated using the Korringa–Kohn–Rostoker method to exclude finite-size effects. Reflection and transmission are complementary because of the absence of strong absorptive effects. The extent of redistribution in the emission from a photonic crystalline environment with respect to a homogeneous emitter is significant in the spectral and spatial domains.

Linewidth characteristics of a filterless tunable erbium doped fiber ring laser

The linewidth characteristics of a continuous wave erbium doped fiber ring laser whose tunability is controlled by intracavity loss are studied in this work. The spectral linewidth of such a filterless laser is measured experimentally, and its characteristics are analyzed using the existing analytical model of fiber laser. The long length of the cavity results in lasing into multiple longitudinal modes, thus leading to a broad linewidth. The extent of this broadening is found to depend on the intracavity attenuation and pump powers. Under the given experimental conditions, for a fiber of length 12m, the linewidth increases up to 4.5nm corresponding to an additional cavity loss of 5dB, while it increases up to 8nm for a loss of 1.2dB for a fiber of length 4.6m. The linewidth decreases with further increase in intracavity loss for both cases. In this work, the linewidth dependence on the intracavity loss is directly linked to the spectral dependence of the threshold power for each length. The linewidth incr...

Low-threshold lasing in active opal photonic crystals

We theoretically study a low-threshold band-edge lasing in three-dimensional photonic crystals (PhCs) with a face-centered cubic lattice structure, using a complex-valued permittivity approach combined with the Korringa-Kohn-Rostoker method. We show that the lasing threshold at the low-frequency band edge is smaller than that at the high-frequency band edge for the first-order stop band of the PhC. We also analyze the impact of the number of the PhCs layers on the frequency of band-edge lasing and the lasing threshold near the first-order stop band in the ΓL direction, and demonstrate a broad tunability of the lasing frequency with change in the emission collection angle. The obtained results are beneficial for the performance enhancement of tunable, PhC-based chip lasers.

Low-threshold lasing in photonic-crystal heterostructures.

We study a photonic crystal (PhC) heterostructure cavity consisting of gain medium in a three-dimensional (3D) PhC sandwiched between two identical passive multilayers. For this structure, based on Korringa-Kohn-Rostoker method, we observe a decrease in the lasing threshold of two orders of magnitude, as compared with a stand-alone 3D PhC. We attribute this remarkable decrease in threshold gain to the overlap of the defect cavity mode with the reduced group velocity region of the PhCs dispersion, and the associated enhancement in the distributed feedback from the ordered layers of the PhC. The obtained results show the potency for designing PhC-based, compact on-chip lasers with ultra-low thresholds.

Spectral characteristics of continuous wave broadband from a fiber laser with a low dispersion fiber in the cavity

The spectral width of the continuous wave broadband generated in an erbium doped fiber ring laser containing a dispersion shifted fiber (DSF) in a filterless cavity is found to be governed by the nature of the gain spectrum of the doped fiber, in addition to the nonlinear effects. This dependence is studied experimentally with different lengths of the doped fiber and the DSF. Spectral broadening is demonstrated in the conventional (C) and long wavelength (L) bands using pump powers less than 200 mW, with an appropriate choice of cavity parameters. The generated broadband is demultiplexed in the C-band to prove its utility in optical communication systems.