Band-edge Lasers Research Articles

Photoresponsive Monodisperse Cholesteric Liquid Crystalline Microshells for Tunable Omnidirectional Lasing Enabled by a Visible Light‐Driven Chiral Molecular Switch

A self-organized, phototunable 3D photonic superstructure is fabricated from a photoresponsive 1D liquid crystal using microfluidics. The resulting microshells are water–oil–water double emulsions, in which the oil phase consists of the photoresponsive liquid crystals. The cholesteric microshells exhibit band-edge lasing in all directions, and the wavelength of the resultant band-edge laser is tuned via the pumping laser, causing photoisomerization of the chiral molecular switch.

Low-threshold photonic-band-edge laser using iron-nail-shaped rod array

We report the experimental demonstration of an optically pumped rod-type photonic-crystal band-edge laser. The structure consists of a 20 × 20 square lattice array of InGaAsP iron-nail-shaped rods. A single-mode lasing action is observed with a low threshold of ∼90 μW and a peak wavelength of 1451.5 nm at room temperature. Measurements of the polarization-resolved mode images and lasing wavelengths agree well with numerical simulations, which confirm that the observed lasing mode originates from the first Γ-point transverse-electric-like band-edge mode. We believe that this low-threshold band-edge laser will be useful for the practical implementation of nanolasers.

Subwavelength micropillar array terahertz lasers

We report on micropillar-based terahertz lasers with active pillars that are much smaller than the emission wavelength. These micropillar array lasers correspond to scaled-down band-edge photonic crystal lasers forming an active photonic metamaterial. In contrast to photonic crystal lasers which use significantly larger pillar structures, lasing emission is not observed close to high-symmetry points in the photonic band diagram, but in the effective medium regime. We measure stimulated emission at 4 THz for micropillar array lasers with pillar diameters of 5 µm. Our results not only demonstrate the integration of active subwavelength optics in a terahertz laser, but are also an important step towards the realization of nanowire-based terahertz lasers.

Effect of preparation parameters on the performance of polymer-stabilized cholesteric liquid crystals for laser emission

We examine the temperature-dependent reflection shifts, microscopic morphology, and laser emission of polymer-stabilized cholesteric liquid crystals. The preparation parameters, including the concentration of photo-initiator and laser dye, are evaluated and their influence on reflection band is considered not to be ignorable. Inadequate ultraviolet (UV) curing time less than the required value to fully photo-polymerize the monomer can also influence the spectral position and shape of the reflection band but still favor possible band-edge lasing, whereas extending UV curing duration can weaken and eventually eliminate the laser emis­sion. The behaviors are explained using the results derived from the mean field theory.

Distributed feedback lasing in cellulose films

Cellulose derivatives, because of their molecular structure and chirality, can self-assemble to form spatially periodic cholesteric liquid crystal phases. We have synthesized and produced solid cross-linked cholesteric cellulose based films optimized to provide high reflectivity. Since these films are self-assembled photonic bandgap materials, they may be expected to show distributed feedback lasing. By doping samples with fluorescent dyes and optically pumping thin films of these materials, we were able to demonstrate, to the best of our knowledge, for the first time, mirrorless band-edge lasing in cellulose derivatives.

Lasing action in strongly coupled plasmonic nanocavity arrays

Periodic dielectric structures are typically integrated with a planar waveguide to create photonic band-edge modes for feedback in one-dimensional distributed feedback lasers and two-dimensional photonic-crystal lasers. Although photonic band-edge lasers are widely used in optics and biological applications, drawbacks include low modulation speeds and diffraction-limited mode confinement. In contrast, plasmonic nanolasers can support ultrafast dynamics and ultrasmall mode volumes. However, because of the large momentum mismatch between their nanolocalized lasing fields and free-space light, they suffer from large radiative losses and lack beam directionality. Here, we report lasing action from band-edge lattice plasmons in arrays of plasmonic nanocavities in a homogeneous dielectric environment. We find that optically pumped, two-dimensional arrays of plasmonic Au or Ag nanoparticles surrounded by an organic gain medium show directional beam emission (divergence angle <1.5° and linewidth <1.3nm) characteristic of lasing action in the far-field, and behave as arrays of nanoscale light sources in the near-field. Using a semi-quantum electromagnetic approach to simulate the active optical responses, we show that lasing is achieved through stimulated energy transfer from the gain to the band-edge lattice plasmons in the deep subwavelength vicinity of the individual nanoparticles. Using femtosecond-transient absorption spectroscopy, we verified that lattice plasmons in plasmonic nanoparticle arrays could reach a 200-fold enhancement of the spontaneous emission rate of the dye because of their large local density of optical states.

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 PhC's 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.

Miniband lasing in a 1D dual-periodic photonic crystal

In this letter, we demonstrate miniband lasing in a 1D dual-periodic photonic crystal (PC). The lasing efficiency is dramatically enhanced by about eight times and meanwhile the threshold is decreased to about 1/6 of that of the band-edge lasing in a single-periodic PC. This high optical conversion efficiency can be attributed to the extremely flat dispersion and large mode volume of the miniband induced by dual-periodicity. Numerical studies are presented to explain the mechanism of the spontaneous emission enhancement induced by the miniband, which are in good accord with our experiments. This finding may provide potential applications to low-threshold lasers and the semiconductor laser community.

Lasing in nanoimprinted two-dimensional photonic crystal band-edge lasers

We demonstrate optically pumped polymer band-edge lasers based on a two-dimensional photonic crystal slab fabricated by nanoimprint lithography (NIL). Lasing was obtained at the photonic band-edge, where the light exhibits a low group velocity at the Γ point of the triangular lattice photonic crystal band structure. The active medium was composed of a dye chromophore-loaded polymer matrix directly patterned in a single step by nanoimprint lithography. Plane-wave and finite difference time domain algorithms were used to predict experimental lasing frequencies and the lasing thresholds obtained at different Γ points. A low laser threshold of 3 μJ/mm2 was achieved in a defect-free photonic crystal thus showing the suitability of nanoimprint lithography to produce cost-efficient optically pumped lasers.

Surface plasmon-enhanced lasing in dye-doped cholesteric liquid crystals

This study shows the results of a photonic band-edge laser using dye-doped cholesteric liquid crystals (CLCs) combined with silver (Ag) nanoparticles. When the Ag nanoparticle surface plasmon resonance wavelength matched the excitation source wavelength, the large optical fields provided by surface plasmons increased the fluorescence of dye molecules by enhancing the molecular excitation rate, achieving a low lasing threshold and high pumping efficiency.

Electrical fine tuning of liquid crystal lasers

We demonstrate high-precision, continuous, electrical tuning of a photonic band-edge laser based on a dye-doped cholesteric liquid crystal. A micro-patterned array of electrodes creates an electric field perpendicular to the cholesteric helix, which distorts the chiral order of the liquid crystal, thus shifting the resonant band edge modes. This configuration allows for smooth tuning of the laser emission in a wavelength range of about 3.5 nm, using low voltages (of the order of 10 V).

Multiwave out-of-normal band-edge lasing in cholesteric liquid crystals

We have investigated the lasing effect in dye-doped cholesteric liquid crystals, where the photonic stop-band for light propagating along the helical axis coinciding with the layer normal is strongly shifted to longer wavelengths with respect to the luminescence band of the dye. In the absence of overlapping of the photonic and luminescence bands, the well-known lasing effect along the normal at the stop-band edges is forbidden. However, the lasing is still allowed for out-of-normal directions, for which the photonic band is shifted to shorter wavelengths corresponding to the luminescence band. Despite of the out-of-normal light propagation, the positive feedback necessary for the lasing is still available in a case of the monodomain cholesteric layer, when its thickness is much less than the optical pumping aperture. We have observed a significantly increased energy threshold for the out-of-normal lasing modes despite they are characterized by lower threshold gain compared to the modes propagating along the helix axis. The increased energy threshold is explained in terms of continuous distribution of the lasing emission over a large angular sector and energy leakage caused by the leaky lasing into the substrates.

Finite-Difference Time-Domain Analysis of Twist-Defect-Mode Lasing Dynamics in Cholesteric Photonic Liquid Crystal

We have numerically investigated lasing dynamics from a twist defect in a cholesteric liquid crystal (CLC) by an auxiliary differential equation finite-difference time-domain (ADE-FDTD) method. As ADEs, the equation of motion of polarization described on the basis of the classical electron oscillator (Lorenz) model and the rate equation in a four-level energy structure are incorporated. A lower lasing threshold has been obtained from the twist-defect mode (TDM) than from band-edge lasing. Standing-wave-like electric fields are strongly localized only in the vicinity where a twist defect is introduced into a CLC, which works as a distributed feedback TDM laser source. The oscillation direction of a standing-wave electric field is not parallel or perpendicular to LC molecules, which is quite different from the bulk CLC case. Our results may be useful for creating more efficient TDM-based CLC lasers.

Large Area of Ultraviolet GaN-Based Photonic Quasicrystal Laser

In this study, large-area GaN-based photonic quasicrystal (PQC) nanopillars were fabricated on an n-GaN substrate using the nanoimprint lithography (NIL) technique. Under optical pumping condition, a high lasing action from the GaN photonic quasicrystals was observed. The lasing wavelength is at 366 nm with a low threshold power density of 0.009 kW/cm2. To confirm the band-edge lasing mode, the finite-element method (FEM) was used to perform the simulation for the 12-fold symmetry photonic quasicrystal lattices.

Photonic-band-edge-induced lasing in self-assembled dye-activated photonic crystals

We report experimental demonstration of photonic band-edge lasing in three-dimensionally ordered self-assembled photonic crystals consisting of rhodamine-B dye doped nanospheres of diameter 295 nm. Our all-solid photonic crystal shows a well-resolved photonic stop gap in the visible region. Laser-induced emission experiments reveal more than 51$%$ inhibition in spontaneous emission intensity of dye molecules within the stop gap and an enhancement near the blue side of the stop gap. With increase in incident pump energy, we achieve photonic-band-edge-induced lasing at room temperature with a lasing threshold of 0.7 mJ. We explain the origin of lasing as due to the enhancement of local density of states near the band edges and the light field distribution within the photonic crystal structure. Our results indicate that such all-solid, dye-activated self-assembled nanostructures are a promising candidate for realizing photonic devices.

Electrically switchable random to photonic band-edge laser emission in chiral nematic liquid crystals

Using a chiral nematic liquid crystal with a negative dielectric anisotropy, it is possible to switch between band-edge laser emission and random laser emission with an electric field. At low frequencies (&amp;lt;1 kHz), random laser emission is observed as a result of scattering due to electro-hydrodynamic instabilities. However, band-edge laser emission is found to occur at higher frequencies (&amp;gt;5 kHz), where the helix is stabilized due to dielectric coupling. These results demonstrate a method by which the linewidth of the laser source can be readily controlled externally (from 4 nm to 0.5 nm) using electric fields.

Printed photonic arrays from self-organized chiral nematic liquid crystals

Active laser arrays, of arbitrary pattern, were created by inkjet deposition of self-assembled photonic structures in the form of dye-doped chiral nematic liquid crystals. The print process itself achieves the requisite alignment for photonic band-edge lasing action normal to the substrate; laser threshold and linewidth characteristics are presented.

Nanobeam photonic bandedge lasers

We demonstrate one-dimensional nanobeam photonic bandedge lasers with InGaAsP quantum wells at room temperature from the lowest dielectric band of photonic crystal nanobeam waveguides. The incident optical power at threshold is 0.6 mW (effectively ~18 μW). To confirm the lasing from the dielectric bandedge, the polarization and the photoluminescent spectra are taken from nanobeams of varying lattice constants. The observed shift of the lasing wavelength agrees well with the computational prediction.

Free-Standing GaN-Based Photonic Crystal Band-Edge Laser

We report the fabrication of a GaN-based membrane-type photonic crystal (PC) band-edge laser (BEL) that requires a smaller PC active area than previous designs due to strong field confinement. A honeycomb-lattice PC was designed such that the Γ1 monopole band-edge mode fell within the emission band of InGaN quantum wells. The BEL exhibited pulsed lasing at room-temperature when optically pumped above its threshold pump energy density of ~ 15.5 mJ/cm2. Based on polarization angle analysis, we confirmed that the BEL indeed lased at the Γ1 monopole band-edge mode.

Complex-coupled photonic crystal THz lasers with independent loss and refractive index modulation

Compared to near infra-red photonic crystal (PhC) band-edge lasers, achieving vertical emission with quantum cascade (QC) material operating in the THz range needs dedicated engineering because the TM polarized emission of QCLs favors in-plane emitting schemes and the currently used double plasmon waveguide, prevents vertical light extraction. We present an approach with independent refractive index and extraction losses modulation. The extraction losses are obtained with small extracting holes located at appropriate positions. The modal operation of the PhC is shown to critically depend on the external losses introduced. Very high surface emission power for optimum loss extractor design is achieved.