Band-edge Lasers Research Articles

Band-edge and random lasing in paintable liquid crystal emulsions

Lasing mechanisms within paintable dye-doped chiral liquid crystal emulsions are investigated. Evidence shows that by variation in liquid crystal droplet size, by simple control of mechanical mixing speeds, a change in the lasing mechanism from band-edge lasing (large droplets) to diffuse nonresonant random lasing (small droplets) can be facilitated. This approach represents a facile technique for the variation in lasing mechanism, within a self-organizing, flexible, and conformable system, and offers the opportunity of developing controllable linewidth laser sources.

Paintable band-edge liquid crystal lasers

In this paper we demonstrate photonic band-edge laser emission from emulsion-based polymer dispersed liquid crystals. The lasing medium consists of dye-doped chiral nematic droplets dispersed within a polymer matrix that spontaneously align as the film dries. Such lasers can be easily formed on single substrates with no alignment layers. The system combines the self-organizing periodic structure of chiral nematic liquid crystals with the simplicity of the emulsion procedure so as to produce a material that retains the emission characteristics of band-edge lasers yet can be readily coated. Sequential and stacked layers demonstrate the possibility of achieving simultaneous multi-wavelength laser output from glass, metallic, and flexible substrates.

Milliwatt-level fiber-coupled laser power from photonic crystal band-edge laser

We report unprecedentedly high output powers measured from large area two-dimensional square-lattice photonic-crystal band-edge lasers (BELs), patterned by holographic lithography. In order to ensure mechanical rigidity, the BELs were fabricated in an InP-based epilayer bonded onto a fused silica substrate beforehand. The BEL devices, employing the surface-emitting Γ-point monopole band-edge mode, provide a fiber-coupled single mode output power as high as 2.6 mW and an external differential quantum efficiency of ~4%. The results of a three-dimensional finite-difference time-domain simulation agree with the experimental observation that the large BELs are beneficial for achieving both high power output and high differential quantum efficiency.

Design and fabrication technology for high performance electrical pumped terahertz photonic crystal band edge lasers with complete photonic band gap

We detail the design and fabrication technology of two-dimensional photonic crystal (PhC) band edge emitting quantum cascade lasers (QCLs) operating at terahertz frequencies (∼3.3 THz). The entire QCL active layer has been deeply etched to form a triangular lattice PhC, in which a complete in-plane photonic band gap for TM polarized light exists. Strong vertical optical confinement is provided by metal-metal plasmon waveguide fabricated with thermocompression bonding and planarization. Benzocyclobutene is employed as a low loss medium to planarize the PhC structure. By tailoring the top metal contact on the planarized PhC structures, electrical current injection pad can also be defined. The lasing properties can be controlled by engineering the photonic band structure. Broad band continuous single mode tuning over 30 GHz was observed; while lower current threshold density and higher operation temperature compared to Fabry–Perot (FP) asers were obtained.

Band-edge lasing in Rh6G-doped dichromated gelatin at different excitations

One-dimensional photonic crystal band-edge lasing at different excitations was studiedexperimentally by altering the excitation angle. We considered almost every conditionincluding in-band, out-of-band and near the band edge while keeping the density of statesunchanged. Holographic rhodamin 6G-doped dichromated gelatin was used for creatinglow-threshold photonic band-edge lasing (PBEL). Lasing actions excited near thehigh-energy and low-energy band edges were observed simultaneously, and their full widthsat half maximum were different. The results show that the PBEL intensity and pumpefficiency are sensitive to the excitation angle, enhanced obviously at the excitation nearthe band edge and suppressed distinctly in the band which agreed well with thetheoretical prediction. We also demonstrated for the first time that active mattersexist not only in the air voids but also in the high-index regions of the gelatin.

Band-edge lasers based on randomly mixed photonic crystals

By employing two-dimensional InGaAsP photonic band-edge lasers, we have experimentally demonstrated that a random mixture of two different photonic crystals (PCs) possesses a new band structure that is intermediate to that of the two host PCs. The photonic band-edges shift monotonically, but with a strong bowing effect, as the mixed PC system is systematically transformed from one PC to the other. The experimental observations are in excellent agreement with finite-difference time-domain simulations and model calculations based on virtual-crystal approximation with compositional disorder effect included.

Room temperature photonic crystal band-edge lasing from nanopillar array on GaN patterned by nanosphere lithography

An ordered GaN nanopillar array fabricated by nanosphere lithography exhibited room temperature photopumped lasing via the photonic crystal band-edge effect. With a monolayer of self-assembled nanospheres as hard mask, the ordered pattern was transferred to the sample to form nanopillars by inductively coupled plasma dry etch. Under pulsed optical excitation, room temperature lasing with a low lasing threshold of 30 mJ/cm2 was achieved. The dominant lasing peak, centered at 415.6 nm, corresponds to a band-edge mode at the Γ-point of the band diagram. A Q factor in the range of 600–700, and spontaneous emission coupling factor of 0.021 were evaluated.

Comparison of the performance of photonic band-edge liquid crystal lasers using different dyes as the gain medium

The primary concern of this work is to study the emission characteristics of a series of chiral nematic liquid crystal lasers doped with different laser dyes (DCM, pyrromethene 580, and pyrromethene 597) at varying concentrations by weight (0.5–2 wt %) when optically pumped at 532 nm. Long-wavelength photonic band-edge laser emission is characterized in terms of threshold energy and slope efficiency. At every dye concentration investigated, the pyrromethene 597-doped lasers exhibit the highest slope efficiency (ranging from 15% to 32%) and the DCM-doped lasers the lowest (ranging from 5% to 13%). Similarly, the threshold was found to be, in general, higher for the DCM-doped laser samples in comparison to the pyrromethene-doped laser samples. These results are then compared with the spectral properties, quantum efficiencies and, where possible, fluorescence lifetimes of the dyes dispersed in a common nematic host. In accordance with the low thresholds and high slope efficiencies, the results show that the molar extinction coefficients and quantum efficiencies are considerably larger for the pyrromethene dyes in comparison to DCM, when dispersed in the liquid crystal host.

Optical edge modes in photonic liquid crystals

An analytic theory of localized edge modes in chiral liquid crystals (CLCs) is developed. Equations determining the edge-mode frequencies are found and analytically solved in the case of low decaying modes and are solved numerically for the problem parameter values typical for the experiment. The discrete edge-mode frequencies specified by the integer numbers n are located close to the stop-band edge frequencies outside the band. The expressions for the spatial distribution of the n’s mode field in a CLC layer and for its temporal decay are presented. The possibilities of a reduction of the lasing threshold due to the anomalously strong absorption effect are theoretically investigated for a distributed feedback lasing in CLCs. It is shown that a minimum of the threshold pumping wave intensity may be reached, generally, for the pumping wave propagating at an angle to the helical axis. However, for lucky values of the related parameters, it may be reached for the pumping wave propagating along the helical axis. The lowest threshold pumping wave intensity occurs for the lasing at the first low-frequency band-edge lasing mode and the pumping wave propagating at an angle to the spiral axis corresponding to the first angular absorption maximum of the anomalously strong absorption effect at the high-frequency edge of the stop band. The study is performed in the case of the average dielectric constant of the liquid crystal coinciding with the dielectric constant of the ambient material. Numerical calculations of the distributed feedback lasing threshold at the edge-mode frequencies are performed for typical values of the relevant parameters.

Edge Modes in Chiral Liquid Crystals: Options for Low Threshold Lasing

An analytic theory of the localized edge modes (EM) in chiral liquid crystals (CLC) is developed. Equations determining the edge mode frequencies are found and analytically solved for the case of low decaying modes and solved numerically for the problem parameters values typical for the experiment. The discrete EM frequencies specified by the integer numbers n are located close to the stop band edge frequencies outside the band. The expressions for space distribution of the n's mode field in CLC layer and for its temporal decay are presented. The possibilities of reduction of the lasing threshold due to the anomalously strong absorption effect are theoretically investigated for a distributed feedback lasing in CLC. It is shown that a minimum of the threshold pumping wave intensity may be reached, generally, for the pumping wave propagating at an angle to the helical axes. However, for lucky values of the related parameters it may be reached for the pumping wave propagating along the helical axis. The lowest threshold pumping wave intensity occurs for the lasing at the first low frequency band-edge lasing mode and the pumping wave propagating at an angle to the spiral axes corresponding to the first absorption maximum of the anomalously strong absorption effect at the high frequency edge of stop band. The corresponding analytical study is performed for the case of the average dielectric constant of LC coinciding with the dielectric constant of the material limiting LC. Numerical calculations of the DFB lasing threshold at EM frequencies are performed for the typical values of the related parameters.

Removing the discontinuous shifts in emission wavelength of a chiral nematic liquid crystal laser

The laser wavelength of a band-edge liquid crystal laser is known to vary in a stepwise fashion with temperature. This report describes a method for removing the discontinuous shifts in the laser wavelength by doping a nematic sample with two different chiral dopants which exhibit opposing dependences of the natural pitch on temperature. On cooling from the isotropic phase to room temperature the total shift of the laser wavelength is 15 nm with no discontinuities. This is in contrast to that observed for samples consisting of only one chiral dopant whereby the wavelength shift was as large as 75 nm for the same temperature range.

High-power and large-alignment-tolerance fiber coupling of honeycomb-lattice photonic crystal Γ-point band-edge laser

Butt-end fiber coupling is a compact and efficient scheme especially suitable for a surface-emitting photonic crystal (PC) band-edge laser. The honeycomb-lattice PC offers a relatively large intact area without air-holes, and therefore, a high optical gain. The use of a Γ-point band-edge mode allows vertical laser emission, yielding a fiber-coupled output power of more than 30 μW. The system is virtually alignment-free in the lateral directions; however, it exhibits oscillatory decay in the vertical direction, which can be significantly suppressed by using an angle-cleaved fiber tip.

Fiber-coupled surface-emitting photonic crystal band edge laser for biochemical sensor applications

We report on a refractive index sensor platform based on a surface-emitting photonic crystal (PC) Γ-point band edge laser (BEL). A simple butt-end fiber coupling makes the BEL system compact, while the translational symmetry of the PC pattern removes the necessity of fiber alignment. The laser wavelength shifts linearly to longer wavelengths as the refractive index of the environmental medium increases. Our BEL system had an index resolution of ∼10−3. Taking into account the alignment-free nature, as well as its ultracompactness and high fiber-coupled laser output, the BEL system is well suited to a label-free sensor platform for biochemical applications.

Photonic band-edge micro lasers with quantum dot gain

We demonstrate optically pumped continuous-wave photonic band-edge microlasers on a two-dimensional photonic crystal slab. Lasing was observed at a photonic band-edge, where the group velocity was significantly small near the K point of the band structure having a triangular lattice. Lasing was achieved by using a quantum dot gain material, which resulted in a significant decrease in the laser threshold, compared with photonic band-edge lasers using quantum well gain material. Extremely low laser thresholds of approximately 80 nW at 6 K was achieved. Lasing was observed in a defect-free photonic crystal as small as approximately 7 microm square.

Photonic integration in k-space: Enhancing the performance of photonic crystal dye lasers

We demonstrate how two optical functionalities can be implemented in a single photonic crystal structure by carefully engineering dispersion in several different bands at several different wavelengths. We use the concept for optically pumped dye doped hybrid polymer band edge lasers and show how a rectangular photonic crystal lattice imprinted into the surface can provide both feedback for in-plane band edge lasing and couple pump light into the device plane, thus increasing the emitted intensity and lowering the lasing threshold by more than an order of magnitude.

Low Threshold DFB Lasing at the Edge and Defect Modes in Chiral Liquid Crystals

A brief survey of the recent experimental and theoretical results on the low threshold distributed feedback (DFB) lasing in chiral liquid crystals (CLC) as well as new original theoretical results in the field are presented. The presentation is concentrated on the lasing thresholds at frequencies corresponding to the stop band edge lasing modes and defect modes in CLC. It is demonstrated that the analytic approach reproduces all features of the DFB lasing in CLC obtained by the traditional numerical approach and, more over, allows to reveal some qualitative effects escaped from the researchers employing the numerical methods. Namely, the effect of anomalously strong absorption at the defect mode frequency, a direct analogue of the corresponding effect at the stop band edge mode frequency, is predicted in the framework of the analytic approach. It is proposed also to reduce the lasing threshold by adjusting the pumping wave to the conditions of the anomalously strong absorption effect at the defect mode frequency. The developed approach helps to clarify the physics of lasing at the stop band edge and defect modes in CLC and manifests an agreement with the corresponding results of the previous investigations obtained by a numerical approach.

Fabrication of Defect-Free Nanoimprinted Photonic Crystals for Laser Applications

The fabrication of two-dimensional photonic crystals by nanoimprint lithography is established to realize lasing oscillations at three band edge frequencies. The optically pumped lasers are based on a dye-chromophore-loaded polymer matrix on a transparent substrate. A two-dimensional plane-wave algorithm was used to design the band-edge lasers showing a good agreement with the experimental lasing frequencies. These results show that nanoimprint lithography is well-suited to the one-step fabrication of two-dimensional active-polymer photonic crystals.

Low Threshold DFB Lasing in Chiral Liquid Crystals

A brief survey of the recent experimental and theoretical results on the low threshold distributed feedback (DFB) lasing in chiral liquid crystals (CLC) as well as new original theoretical results in the field are presented. The presentation is concentrated on the lasing thresholds at frequencies corresponding to the stop band edge lasing modes and defect modes in CLC. It is demonstrated that the analytic approach reproduces all features of the DFB lasing in CLC obtained by the traditional numerical approach and, more over, allows to reveal some qualitative effects escaped from the researchers employing the numerical methods. Namely, the effect of anomalously strong absorption at the defect mode frequency, a direct analogue of the corresponding effect at the stop band edge mode frequency, is predicted in the framework of the analytic approach. It is proposed also to reduce the lasing threshold by adjusting the pumping wave to the conditions of the anomalously strong absorption effect at the defect mode frequency. The developed approach helps to clarify the physics of lasing at the stop band edge and defect modes in CLC and manifests an agreement with the corresponding results of the previous investigations obtained by a numerical approach.

Optofluidic tuning of photonic crystal band edge lasers

We demonstrate optofluidic tuning of polymer photonic crystal band edge lasers with an imposed rectangular symmetry. The emission wavelength depends on both lattice constant and cladding refractive index. The emission wavelength is shown to change 1nm with a cladding refractive index change of 10−2. The rectangular symmetry modification alters the emission characteristics of the devices and the relative emission intensities along the symmetry axes depend on cladding refractive index, suggesting a sensor concept based on detection of intensity rather than wavelength.

Two-dimensional polymer photonic crystal band-edge lasers fabricated by nanoimprint lithography

We report on the fabrication and characterization of two-dimensional polymer photonic crystal band-edge lasers operating in the visible range. The components have been fabricated in a dye chromophore-loaded polymer matrix by nanoimprint lithography. High-symmetry band-edge modes are used to generate laser emission. The experimental lasing frequencies are in good agreement with those calculated using a two-dimensional plane wave algorithm. These results demonstrate the potential of nanoimprint lithography for the fabrication of two-dimensional planar photonic crystal structures in an active medium in a one-step process.