Corrugated neat thin-film conjugated polymer distributed-feedback lasers

Wave-guided thin-film distributed-feedback (DFB) polymer lasers are fabricated by spin coating a PPV-derived semiconducting polymer, thianthrene-DOO-PPV, onto oxidised silicon wafers with corrugated second-order periodic gratings. The gratings are written by reactive ion beam etching. Laser action is achieved by transverse pumping with picosecond laser pulses (wavelength 347.15 nm, duration 35 ps). The DFB-laser surface emission and edge emission are analysed. Outside the grating region the polymer film is used for comparative wave-guided travelling wave laser (amplified spontaneous emission (ASE)) studies. The pump pulse threshold energy density for wave-guided DFB-laser action (4–9 μJ cm^-2) is found to be approximately a factor of two lower than the threshold for wave-guided travelling wave laser action. The spectral width of the DFB laser (down to Δλ_DFB≈0.07 nm) is considerably narrower than that of the travelling wave laser (Δλ_TWL≈14 nm). The DFB-laser emission is highly linearly polarised transverse to the grating axis (TE mode). Only at high pump pulse energy densities does an additional weak TM mode build up. The surface-emitted DFB-laser radiation has a low divergence along the grating direction. For both the DFB lasers and the travelling wave lasers, gain saturation occurs at high excitation energy densities. Received: 7 January 2002 / Revised version: 15 February 2002 / Published online: 14 March 2002     

Quasiparity‐Time Symmetric Microdisk Laser

Recent developments in parity‐time (PT) symmetric systems have ushered in unique photonic devices with enhanced functionalities. While single‐mode laser emission has been demonstrated in such systems, the current designs face severe challenges in applications, either due to their stringent requirement on fabrication precision or nonscalability to larger devices. Here, we demonstrate a general mechanism to achieve single‐mode lasing in coupled cavities, which relies on external mode coupling and overcomes these drawbacks. We find significant gain enhancement for selected modes by external coupling, and our experiments have confirmed the resulting single‐mode laser emission in size‐mismatched photonic molecules (PMs), when only one constituent cavity is pumped. This behavior persists for a wide range of pump power, from transparent threshold to gain saturation, and it is highly tolerant of fabrication imprecisions. In addition, the output intensity of such single‐mode lasers also displays enhancement when compared with the same PMs under uniformly pumping. We believe our results will both advance the understanding of different coupling scenarios in coupled cavities and improve the characteristics of onchip laser sources for practical applications.     

Cavity-Free Lasing and 2D Plasma Oscillations in Optically Excited InGaN Heterostructures

We demonstrate lasing emission of optically excited InGaN LD structures without intentional resonant cavity formation. We observe the equal mode-spacing character of this effect in the back-scattering geometry after exceeding the threshold excitation intensity. The homogeneity of the effect and stable mode spacing exclude participation of defects or wafer edges in lasing. We propose a lasing mechanism based on optically excited 2D electron–hole plasma oscillations, which act as a dynamical grating and resonantly couple the lasing modes separated by the plasma frequency, similar to the case of DFB lasers. The observed anomalous mode spacing is determined by the eigenfrequency of the plasma oscillations.     

Lasing action in two-dimensional organic photonic crystal lasers with hexagonal symmetry

We investigate fluorescence and lasing action from strongly modulated two-dimensional surface relief structures with hexagonal symmetry onto which thin films of optically active organic material have been deposited. As compared with second-order laser structures with square symmetry, these organic photonic crystal lasers exhibit unusual feedback mechanisms. As a result, we observe surface-emitting lasing action with a central beam normal to the surface and a hexagonal emission pattern of side-beams whose direction slightly deviates from the normal. A corresponding theoretical analysis allows us to determine the photonic bandstructure and the low-threshold laser modes in this system. These results agree very well with fluorescence data and confirm the hexagonal lasing pattern and the corresponding emission angles. PACS 42.55.Tv; 42.70.Jk; 42.70.Qs     

Sub-5-pm linewidth, 130-nm-tuning of a coupled-cavity Ti:sapphire oscillator via volume Bragg grating-based feedback

A novel coupled-cavity Ti:sapphire oscillator architecture featuring a volume Bragg grating as a feedback element is presented. The oscillator provides continuous wave lasing within a spectral linewidth as narrow as 5 pm. The output can be wavelength-tuned over an ultrabroad spectral range of 130 nm, extending from 714 to 842 nm. This unique combination of narrow spectral linewidth and wide tuning range makes the laser suitable for applications such as sensing and Raman and absorption spectroscopy. The laser also displays ideal TEM₀₀ mode operation throughout its tuning range with output powers beyond 300 mW. Detailed studies of the cw lasing dynamics across the wide tuning range are described. The general architecture of this design can be implemented for high resolution tuning across the broad spectral emission bands of other solid state lasers with single mode operation.     

Continuous‐wave vertically emitting photonic crystal terahertz laser

Compact semiconductor light sources with high performance continuous‐wave (CW) and single mode operation are highly demanded for many applications in the terahertz (THz) frequency range. Distributed feedback (DFB) and photonic crystal (PhC) quantum cascade (QC) lasers are amongst the leading candidates in this field. Absorbing boundary condition is a commonly used method to control the optical performance of a laser in double‐metal confinement. However, this approach increases the total loss in the device and results in a large threshold current density, limiting the CW maximum output power and operating temperature. In this letter, a robust surface emitting continuous‐wave terahertz QC laser is realized in a two‐dimensional PhC structure by a second order Bragg grating extractor that simultaneously provides the boundary condition necessary for mode selection. This results in a 3.12 THz single mode CW operation with a 3 mW output power and a maximum operation temperature (T_max) of 100 K. Also, a highly collimated far‐field pattern is demonstrated, which is an important step towards real world applications.     

ZnS:Cr and ZnSe:Cr thin‐film waveguide structures as electrically pumped laser media with an impact excitation mechanism

ZnS:Cr and ZnSe:Cr are the focus of studies as media for broadly tunable optically pumped lasers operating in the near‐ and mid‐IR regions. It is of great interest to obtain such lasers electrically pumped. The effective electrical excitation of Cr²⁺ ions was demonstrated only in the case of the impact mechanism for thin‐film structures with insulator layers between an electroluminescent film and electrodes, which prevent avalanche breakdown at high electric field (≥ 1 MV/cm). To obtain lasing, the waveguide electroluminescent structures were used. For the first time, the stimulated emission and the laser oscillation were demonstrated in the ZnS:Cr waveguide structures. Although the lasing is unstable as yet, these results lay the foundations of a new direction in laser physics aimed at creation of electrically pumped lasers by dint of impact excitation. Published results concerning various aspects of ZnS:Cr and ZnSe:Cr thin films and waveguide structures as promising electrically pumped laser media are reviewed. The causes of the instability of the laser oscillation and means of improving their characteristics are also considered.     

Room temperature operation of a deep etched buried heterostructure photonic crystal quantum cascade laser

High power single mode quantum cascade lasers with a narrow far field are important for several applications including surgery or military countermeasure. Existing technologies suffer from drawbacks such as operation temperature and scalability. In this paper we introduce a fabrication approach that potentially solves simultaneously these remaining limitations. We demonstrate and characterize deep etched, buried photonic crystal quantum cascade lasers emitting around a wavelength of 8.5 μm. The active region was dry etched before being regrown with semi‐insulating Fe:InP. This fabrication strategy results in a refractive index contrast of 10% allowing good photonic mode control, and simultaneously provides good thermal extraction during operation. Single mode emission with narrow far field pattern and peak powers up to 0.88 W at 263 K were recorded from the facet of the photonic crystal laser, and lasing operation was maintained up to room temperature. The lasing modes emitted from square photonic crystal mesas with a side length of 550μm, were identified as slow Bloch photonic crystal modes by means of three‐dimensional photonic simulations and measurements.

     

Optically pumped distributed feedback thin film waveguide lasers with multiwavelength and polarized emissions

Zirconia titania organically modified silicate (ZrO₂-TiO₂-ORMOSIL) thin film waveguides of thickness from 0.4 to 7.0 μm were synthesized using low temperature sol–gel method. Narrow linewidth distributed feedback (DFB) lasing was demonstrated in rhodamine 6G-doped ZrO₂-TiO₂-ORMOSIL waveguides. Simultaneous tuning of multiple-output wavelengths was achieved in the dye-doped waveguides by varying the period of the gain modulation generated by a nanosecond Nd:YAG laser at 532 nm. As many as eight separate output wavelengths were observed for a planar ZrO₂-TiO₂-ORMOSIL waveguide of thickness 7.0-μm. The output polarizations of the DFB waveguide lasers can be tuned by varying the polarization of the crossing pump beams. TE and TM optical waves belonging to the same propagation mode were generated by crossing two polarized pump beams, resulting in an effective double of the number of output wavelengths. Continuous tuning of the polarized laser outputs was also achieved by varying the crossing angle.     

Raman fiber distributed feedback lasers

We demonstrate fiber distributed feedback (DFB) lasers using Raman gain in two germanosilicate fibers. Our DFB cavities were 124 mm uniform fiber Bragg gratings with a π phase shift offset from the grating center. Our pump was at 1480 nm and the DFB lasers operated on a single longitudinal mode near 1584 nm. In a commercial Raman gain fiber, the maximum output power, linewidth, and threshold were 150 mW, 7.5 MHz, and 39 W, respectively. In a commercial highly nonlinear fiber, these figures improved to 350 mW, 4 MHz, and 4.3 W, respectively. In both lasers, more than 75% of pump power was transmitted, allowing for the possibility of substantial amplification in subsequent Raman gain fiber.     

Whispering gallery mode microlasers and refractive index sensing based on single polymer fiber

The realization of whispering gallery mode (WGM) lasing in polymer fibers is hindered by an appropriate method to dissolve the polymer and the gain material. In this work, microfibers fabricated by directly drawing from a dye doped polymer solution are exhibited as high quality microlasers and microsensors. Multi‐mode and even single‐mode lasing is observed from the fiber under optical pumping at room temperature. The linewidth of lasing mode is narrower than 0.09 nm. The lasing mechanism is unambiguously verified by comprehensive spectroscopic analysis and ascribed to WGMs. Diameter‐ and polarization‐dependent lasing characteristics are systematically investigated, showing good agreement with the theoretical calculation. Particularly, application of the fiber laser for refractive index sensing based on resonant shift of lasing mode is demonstrated and the sensitivity up to about 300 nm/RIU is achieved. The promising potential of high quality polymer microfibers as optical sensors and multi‐function components for flexible photonic integrated systems is highly expected.     

Defect modes of chiral photonic crystals with an isotropic defect

Specific features of the defect modes of cholesteric liquid crystals (CLCs) with an isotropic defect, as well as their photonic density of states, Q factor, and emission, have been investigated. The effect of the thicknesses of the defect layer and the system as a whole, the position of the defect layer, and the dielectric boundaries on the features of the defect modes have been analyzed. It is shown that when the CLC layer is thin the density of states and emission intensity are maximum for the defect mode, whereas when the CLC layer is thick, these peaks are observed at the edges of the photonic band gap. Similarly, when the gain is low, the density of states and emission intensity are maximum for the defect mode, whereas at high gains these peaks are also observed at the edges of the photonic band gap. The possibilities of low-threshold lasing and obtaining high-Q microcavities have been investigated.     

Random lasing in an organic light‐emitting crystal and its interplay with vertical cavity feedback

The simultaneous vertical‐cavity and random lasing emission properties of a blue‐emitting molecular crystal are investigated. The 1,1,4,4‐tetraphenyl‐1,3‐butadiene samples, grown by physical vapour transport, feature room‐temperature stimulated emission peaked at about 430 nm. Fabry‐Pérot and random resonances are primed by the interfaces of the crystal with external media and by defect scatterers, respectively. The analysis of the resulting lasing spectra evidences the existence of narrow peaks due to both the built‐in vertical Fabry‐Pérot cavity and random lasing in a novel, surface‐emitting configuration and threshold around 500 μJ cm⁻². The anti‐correlation between different modes is also highlighted, due to competition for gain. Molecular crystals with optical gain candidate as promising photonic media inherently supporting multiple lasing mechanisms.     

Wavelength-tunable organic solid-state distributed-feedback laser

We report on a wavelength-tunable organic solid-state laser based on a second-order distributed-feedback (DFB) resonator design with the guest–host system Alq₃:DCM2 as the gain medium. The laser wavelength can be shifted from 608.9 to 646.5 nm by varying the grating period of the Bragg reflector. This allows the characterization of laser parameters for a large wavelength region on a single sample. The laser threshold and output characteristics are measured for different laser wavelengths. A minimum threshold for laser activity occurs at a lasing wavelength of 633 nm, giving the spectral position of the gain maximum. PACS 42.70.Jk; 78.45.+h; 78.66.Qn     

Two-photon pumping of random lasers by picosecond and nanosecond lasers

We experimentally demonstrated two-photon pumping of random lasers using picosecond and nanosecond pump lasers. The picosecond laser pumping experiment was performed with 400 ps laser pulses at 770 nm, and the gain media was a Coumarin 480D dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser pulse energy below 500 μJ. The nanosecond laser pumping experiment was performed with 7 ns laser pulses at 1064 nm, and the gain media was a Rhodamine 640 dye solution doped with TiO₂ nanoparticles. Onset of laser action was observed at a pump laser energy ∼18 mJ. Our results suggest that there exists an optimal pulse duration of the pumping laser in two-photon pumped random lasing that leads to minimum photodamage of the gain media and still keeps a high pumping efficiency. PACS 33.50.Dq; 42.55.Mv; 42.55.Zz     

Plasmonic leaky‐mode lasing in active semiconductor nanowires

Leaky modes are below‐cutoff waveguide modes that lose part of their energy to the continuum of radiation modes during propagation. In photonic nanowire lasers, leaky modes have to compete with almost lossless above‐cutoff modes and are therefore usually prevented from crossing the lasing threshold. The situation is drastically different in plasmonic nanowire systems where the above‐cutoff plasmonic modes are very lossy because of their strong confinement to the metal surface. Due to gain guiding, the threshold gain of the hybrid electric leaky mode does not increase strongly with reduced wire diameter and stays below that of all other modes, making it possible to observe leaky‐mode lasing. Plasmonic ZnO nanowire lasers operating in the gain‐guided regime could be used as coherent sources of surface plasmon polaritons at the nanoscale or as surface plasmon emitting diodes with an emission angle that depends on the nanowire diameter and the color of the surface plasmon polariton.

     

Lasing mechanism in two-dimensional photonic crystal lasers

We conduct a comprehensive investigation of the lasing mechanism in a photonic crystal slab laser with a refractive index that is periodic in two dimensions. Experimental spectra of laser structures fabricated with organic gain media are presented. It is found that lasing frequencies can be explained in terms of Van Hove singularities in the density of modes. We also observe lasing spectra that cannot be obtained from structures with one-dimensional periodicity, such as traditional distributed feedback lasers. Lasing frequencies are computed using numerical techniques. Received: 7 April 1999 / Accepted: 12 April 1999 / Published online: 19 May 1999     

Nanoimprinted distributed feedback lasers comprising TiO2 thin films: Design guidelines for high performance sensing

Design guidelines for optimizing the sensing performance of nanoimprinted second order distributed feedback dye lasers are presented. The guidelines are verified by experiments and simulations. The lasers, fabricated by UV‐nanoimprint lithography into Pyrromethene doped Ormocomp thin films on glass, have their sensor sensitivity enhanced by a factor of up to five via the evaporation of a titanium dioxide (TiO₂) waveguiding layer. The influence of the TiO₂ layer thickness on the device sensitivity is analyzed with a simple model that accurately predicts experimentally measured wavelength shifts induced by varied superstrate refractive indices. The superstrate refractive index is additionally shown to determine which of the possible waveguiding modes dominates for lasing, indicating a method to flexibly select the polarization of the laser. The detection limit of the sensor system is further discussed, finding an optimum at 7.5· 10⁻⁶ RIU. Wavelength changes caused by dye bleaching must be taken into account for long‐term measurements.     

A 795 nm gain coupled distributed feedback semiconductor laser based on tilted waveguides

The 795 nm distributed feedback lasers have great application in pumping the Rb D1 transition. In this paper, in order to realize specific 795 nm lasing, we designed tilted ridge distributed feedback lasers based on purely gain coupled effect induced by periodic current injection windows through changing the angle of the tilted ridge. The fabricated devices were cleaved into 2 mm-cavity-length, including 5 tilted angles. The peak output powers of all devices were above 30 m W.Single longitudinal mode lasing was realized in all tilted Fabry–Perot cavities using periodic current injection windows,with side mode suppression ratio over 30 d B. The total wavelength range covered 8.656 nm at 20℃. It was disclosed theoretically and experimentally that the output powers, threshold currents, and central wavelengths of the tilted ridge purely gain coupled DFB lasers were relevant to the tilted angles. The results will be instructive for future design of DFB laser arrays with different central wavelengths.     

Effect of particle size and shape on nonresonant random laser action of dye-doped polymer random media

We present an experimental study of the light emission from dye-doped polymer random media dispersed with TiO₂ particles of various sizes, shapes, and structures. Random lasing with nonresonant feedback, similar to that for spherically shaped particles that are used for conventional random lasers, is observed for almost all types of particles and aggregates. The efficiency of random lasing for each medium is analyzed using the relationship between the emission spectrum and the transport mean free path (TMFP), which is measured by enhanced backscattering experiments. Results show that the peak emission intensity depends strongly on the particle shape and structure, whereas the spectral linewidth is governed by the TMFP.