Band-edge Lasers Research Articles

Centrifugation-Mediated Crystal Growth of Attractive Colloids for Band Edge Lasing.

Attractive depletion interactions are utilized to organize colloidal particles into crystalline arrays with high crystallinity through spontaneous phase separation. However, uncontrolled nucleation frequently leads to the formation of crystalline grains with varied crystal orientations, which hampers the optical performance of photonic crystals. Here, colloidal crystals have been engineered with uniform orientation and high surface coverage by applying centrifugal force during the depletion-induced assembly of polystyrene particles. The centrifugal force encourages the particles to move toward the bottom surface, which fosters heterogeneous nucleation and supports rapid crystal growth, yielding densely-packed and uniformly-arranged crystal grains with high reflectivity. This study has observed that the nucleation and crystal growth behavior is significantly influenced by the salt concentration. Based on the pair potentials, the transition boundary has been quantitatively analyzed between fluid and crystal phases and identified the threshold for homogeneous nucleation. Utilizing the high-reflectivity colloidal crystals, band-edge lasing is achieved by dissolving the water-soluble dye into the aqueous suspensions. Upon optical excitation, a lasing emission characterized is observed by a narrow spectral width at the short-wavelength band edge. Notably, the laser wavelength can be adjusted by altering the salt concentration or particle diameter, offering a versatile approach to tuning the optical properties.

Bioinspired photonic crystal structures from Papilio Ulysses butterfly wings for versatile laser applications

Photonic crystals found within the wing of a Papilio Ulysses butterfly have demonstrated their efficacy in facilitating laser processes, particularly as a scattering material for imaging and illumination applications. This study delves into the investigation of the photonic crystal's band gap within the butterfly's wing, aiming to realize blue band-edge lasing using a newly developed gain medium comprising diphenylaminofluorene-based chromophores featuring a donor-acceptor-donor (D-A-D) motif. The versatility of the fabricated sample is highlighted, as it can function as both a single-mode laser and a random laser, with the operational mode determined by adjusting the excitation power and the direction of the pump source. Specifically, it exhibits a threshold of 2.5 μJ for random laser operation and 7.6 μJ for single-mode laser operation. The unique architectural elements found in the butterfly's wing, including the ridges on the wing scale and the photonic crystal within the scales, play distinct roles in enabling random laser and single-mode laser actions, respectively. This distinctive butterfly wing architecture holds promise for inspiring the development of future photoelectronic circuits requiring a hybrid design that incorporates both single-mode and comb-mode laser functionalities.

Thermal control band-edge lasing toward near IR from dye-doped cholesteric liquid crystal: Relatively stable operation at high temperatures

The exceptional band-edge lasing from dye-doped cholesteric liquid crystals (DD-CLCs), which can be easily fabricated by blending chiral molecules with nematic liquid crystals, showcases versatile tuning possibilities. In this work, we employed (ISO-(6OBA)2) with high helical twisting power (HTP) as the chiral molecule in the fabrication of DD-CLCs, ensuring superior tuning possibilities and operational stability at high temperatures. By thermally controlling DD-CLCs with a 5 wt% chiral molecule, the band-edge lasing peak exhibited a noticeable red shift with increasing temperature, demonstrating a tuning range of 106 nm as the temperature rose from 20 to 90 °C. In operation at 78 °C, the longest lasing peak of 692.7 nm was achieved from DD-CLCs with 4 wt% ISO-(6OBA)2. Furthermore, the long-term stability of band-edge lasing from DD-CLCs was proven during a three-hour monitoring period at a high operating temperature above 50 °C.

The enhancement and regulation of lasing in dye-doped multi-layer polymer film systems

As a new research hotspot in nano-materials and nano-structures, photoluminescence is widely used in medical imaging, biosensors, integrated devices and other fields because of its excellent optical performance and good stability. The design and fabrication of these nanostructures are expensive and complex, meanwhile, the influence of film structure change on lasing characteristics has not been systematically summarized. Thus, there are still challenges to realizing lasing regulation by using a multilayer film system. In this study, a dye-doped multilayer system was prepared by a convenient spin-coating method on polyethylene glycol terephthalate (PET) film. The effects of sandwich-like film and Au nanoparticles on the lasing characteristics were studied. The results show that the sandwich-like film can form a mirror symmetry plane and increase the photon scattering frequency. Furthermore, the band-gap edge effect can effectively improve the photon gain, enhancing the optical feedback and realizing band-edge lasing. The lasing intensity in the sandwich-like film is nearly 30% higher than that of the bilayer film system at the same pump energy. By adding Au nanoparticles as scatterers, the local electromagnetic field can be enhanced because of the surface plasmon resonance, improving the transition rate of dye molecules. Meanwhile, the photon scattering frequency among the scattering cross-section can also be increased, increasing the interaction between photons and dye. The lasing intensity can be enhanced by 160% at the same pump energy because of the interaction between the plasmon resonance effect and the scattering effect. The fabrication method in this study is convenient and easy to implement, which is conducive to improving the lasing intensity and provides an effective technical way for the realization of a modulated random laser.

Photonic crystal L3 cavity laser fabricated using maskless digital photolithography

Abstract Projection photolithography using an extreme-ultraviolet light source is the core technology that has enabled patterning on the scale of a few nanometers that is required for modern electronic chips. However, this high-end system is neither affordable nor needed for photonics where critical feature sizes are of 100s of nanometers (or of submicron). Although electron-beam lithography can provide a means for photonic device fabrication, it suffers from extremely low throughput. Therefore, a lithographic technique for submicron pattern generation at high throughput and low cost is in high demand. This group recently showed that maskless digital photolithography (MDPL), a convenient and versatile photolithographic technique that requires no photomask, could potentially address this demand by demonstrating photonic crystal (PhC) patterns with submicron periodicity and associated PhC band-edge lasers. In this paper, we report the fabrication of a PhC L3 cavity laser, which contains irregular air holes in terms of their positions and sizes, using the MDPL technique. Successful generation of such an aperiodic and nontrivial submicron pattern requires thorough understanding and scrupulous manipulation on light diffraction. Our achievements should provide the concrete foundation upon which compact, versatile, convenient, speedy, and economical lithographic tools for arbitrary submicron pattern generation can be developed.

Direct-focusing surface-emitting laser.

Focusing is a fundamental optical technique that has been widely implemented via lenses. Here, we demonstrate direct focusing from a band-edge surface-emitting laser, whose emission area is 200 µm × 200 µm, without any lenses. To achieve this, a phase-modulating layer is incorporated into the laser cavity. This layer acts simultaneously as a lasing cavity similar to that of a photonic crystal laser and as a holographic spatial-phase modulator, which transforms the output beam into a focusing beam by slightly shifting the positions of holes from a periodic square lattice. Beam profiles along the surface normal clearly show that direct focusing occurs with a focal length and focal spot size of 310 µm and 6.1 µm, respectively. The focal length agrees well with the theoretical value, and the focal spot size is 2.0 times the diffraction-limited size, which indicates that the higher transverse modes are sufficiently suppressed. In addition, the power density at the focus is 540 times higher than that at the near-field plane. Interestingly, a focus pattern is also observed in the opposite direction at the near-field plane, which indicates that a converging beam and a diverging beam are simultaneously emitted because of the nature of the in-plane band-edge laser. The conventional beam patterns of semiconductor laser cavities are limited to the regime of two-dimensional projection based on a Fourier hologram. In contrast, we demonstrate the simplest form of a three-dimensional point cloud based on a Fresnel hologram, which is quite useful for micro-sensing applications such as microfluidics, flow cytometry, blood sensors, and endoscopy.

Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates

Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates

The properties of mid infrared surface modes at the interface of air and one dimensional ternary photonic crystal

In the present manuscript, theoretical design of truncated one dimensional ternary photonic crystal is proposed for analyzing the properties of transverse electric (TE) surface modes viz. band structures, its localization in photonic band gaps (PBGs), field profiles and energy carried by these modes. The periodicity of the structure (comparable to the wavelength of incident electromagnetic waves) is chosen as such the modes are localized in mid-infrared frequency domain (600cm−1 to 1800cm−1 or 5μm to 16μm). The dispersion relation of surface modes is derived by using continuity condition of tangential components of electric and magnetic fields and transfer matrix method. It is found that by varying the cap layer thicknessdc and thickness of first layerdt of each unit cell, surface modes are either strongly (near center of PBG with decay length ~8.8μm) or weakly (near edge of PBG with decay length ~42.5μm) bounded to the interface. The energy carried by the surface modes along the interface plane is around 3.3mJ while across the interface plane it is of the order of10−14. It has been proved that the material rich modes (~2V/m) have high field enhancement compared to material deficient modes (~1V/m). The study stresses on different aspects of the proposed model that can be sought for suitable practicable applications in mid-infrared photonics, especially in design of mid-infrared bio-chemical sensors, mid-infrared temperature sensors, waveguides, omnidirectional reflectors, optical filters, band edge lasers, multiplexers, mitigators, compensators, etc.

Controlling electric field distribution to enhance laser emission with low-refractive-index separation layer in sparse resonant grating

Guided-mode resonance (GMR) displays a wide range of optical properties that could prove useful for many applications in photonics. We report the enhancement of laser emission from the GMR sparse grating-based dye laser. The enhancement is due to an introduction of the low-refractive-index (low-n) separation layer, which causes a strong interaction between the local electric field and dye molecules. Finite-difference time-domain method is used to study how the structure’s geometry, including the thickness of the separation layer and the fill factor of the grating, affects the electric field and laser intensity. The results show that the emission can be enhanced by a factor of up to 8 compared with high-refractive-index SU-8 as the separation layer. The laser intensity reaches to the maximum value when the fill factor of the grating is 0.2, i.e., the sparse grating is used. The threshold of the low-n GMR dye laser is also examined. This design can effectively improve the performance of pumped photonic band-edge lasers.

Effect of Hole Shift on Threshold Characteristics of GaSb-Based Double-Hole Photonic-Crystal Surface-Emitting Lasers.

Photonic-crystal (PC) surface-emitting lasers (SELs) with double-hole structure in the square-lattice unit cell were fabricated on GaSb-based type-I InGaAsSb/AlGaAsSb heterostructures. The relative shift of two holes was varied within one half of the lattice period. We measured the lasing wavelengths and threshold pumping densities of 16 PC-SELs and investigated their dependence on the double-hole shift. The experimental results were compared to the simulated wavelengths and threshold gains of four band-edge modes. The measured lasing wavelength did not exhibit switching of band-edge mode; however, the calculated lowest threshold mode switched as the double-hole shift exceeded one quarter of the lattice period. The identification of band-edge lasing mode revealed that modal gain discrimination was dominated over by its mode wavelength separation.

Diode pumping of liquid crystal lasers

Liquid crystal lasers are typically pumped with Q-switched lasers, which provide the short pulse durations necessary to produce band-edge liquid crystal laser emission. However, the current cost and bulk of the pump laser, relative to the liquid crystal component, limit the application potential of liquid crystal laser technology. A low-cost and compact alternative pump source has often been discussed within the liquid crystal laser community but has not yet been demonstrated. In this paper, a 445 nm laser diode was integrated with carefully selected electronics in an attempt to reproduce equivalent pump conditions to that achieved from a Q-switched laser, and then used to pump a liquid crystal laser. Successful band-edge lasing was achieved, demonstrating, for the first time, a clear threshold and single mode, narrow linewidth emission. Red, green and blue outputs from different liquid crystal lasers were also realised when pumped with the same laser diode, demonstrating the broad wavelength selectivity of liquid crystal lasers using a single low-cost, compact device.

Electrically controlled switching of mixed mode laser within the band-gap of cholesteric liquid crystal

ABSTRACT Both random laser and multimode band-edge laser have been found in cholesteric liquid crystals (CLCs). In this work, the laser emission in CLC with negative dielectric anisotropy under electric fields with different frequencies is investigated. Awide multi-wavelength lasing is achieved via the formation of multiple domains induced by low-frequency alternating current (AC) in the CLC system. The photonic bandgap with aweak reflection for aspecific band can also be observed in the multiple domains structure. Furthermore, two types of liquid crystal hosts with different birefringences are used to obtain different lasing cover ranges. The range of the multi-wavelength lasing is within the CLC bandgap and positively correlated to the bandwidth of the bandgap. In addition, ahigh-frequency AC or direct current (DC) can switch it to aband-edge laser based on the regular planar helical structure. The wide range switchable multi-wavelength laser may provide various applications, such as sensing and communication.

Submicrometer-scale pattern generation via maskless digital photolithography

Maskless photolithography based on digital micromirror devices (DMDs) is considered the next-generation low-cost lithographic technology. However, DMD-based digital photolithography has been implemented only for micrometer-scale pattern generation, whereas sophisticated photonic devices require feature sizes of approximately 100 nm. In this study, we adopt a high-magnification objective lens ( 200 × ) for a custom-built digital photolithography system to generate submicrometer-scale patterns. We also improvise techniques to augment the digital photolithography, pattern tilting, and grayscale exposure. We demonstrate that photonic crystal band-edge lasers of various lattice structures and periods can be quality-assessment testbeds.

PH-sensitive GaInAsP photonic crystal fractal band-edge laser.

The light emission characteristics of GaInAsP semiconductors in aqueous solutions are modified by the surface charge, depending on the balance between radiative and nonradiative recombination. This Letter demonstrates the application of a GaInAsP photonic crystal band-edge laser in sensing a solution pH, which reflects the surface charge. The sensitivity is enhanced by a hybrid of fractal honeycomb and close-packed structures with a large surface-to-volume ratio and a high quality factor (Q), which allows a low threshold carrier density. We experimentally obtained a maximum pH sensitivity of 1.9 dB/pH near the threshold and a signal-to-noise ratio of 52/pH (pH resolution of 0.019) at a pump power 2.4 times the threshold where the intensity noise diminished.

Electrically switchable band-edge laser based on polymer-stabilized blue phase liquid crystal

ABSTRACT An electrically switchable band-edge laser has been demonstrated in a dye-doped polymer-stabilised blue phase liquid crystal. The light intensity of the laser can be tuned in three-dimensional space with a switching time of submillisecond, which promotes this band-edge laser widespread use in displays, point-of-care biomedical analysis, and optical security coatings.

Low threshold random lasing in dye-doped and strongly disordered chiral liquid crystals

Random lasing was experimentally investigated in pyrromethene 597-doped strongly disordered chiral liquid crystals (CLCs) composed of the nematic liquid crystal SLC1717 and the chiral agent CB15. The concentration of the chiral agent tuned the bandgap, and disordered CLC microdomains were achieved by fast quenching of the mixture from the isotropic to the cholesteric phase. Random lasing and band edge lasing were observed synchronously, and their behavior changed with the spectral location of the bandgap. The emission band for band edge lasing shifted with the change of the bandgap, while the emission band for random lasing remained practically constant. The results show that the threshold for random lasing sharply decreases if the CLC selective reflection band overlaps with the fluorescence peak of the dye molecules and if the band edge coincides at the same time with the excitation wavelength.

Threshold current temperature dependence of quantum-dot photonic crystal surface-emitting lasers with respect to gain-cavity detuning.

We investigate threshold current temperature dependence of electrically injected quantum-dot (QD) photonic crystal (PC) surface-emitting lasers (SELs) with respect to wavelength detuning between QD gain peak and PC cavity resonance. The lasing emissions cover wavelengths from 1283 nm to 1318 nm. Almost infinite characteristic temperature is realized at certain temperature range for PCSEL with large negative gain-cavity detuning. Moreover, band-edge lasing mode is identified in our "PC slab-on-substrate" structure, and its far-field distribution is characterized as doughnut-shaped beam with azimuthal polarization.

Degenerate band edge laser

We propose a class of lasers based on a fourth-order exceptional point of degeneracy (EPD) referred to as the degenerate band edge (DBE). EPDs have been found in parity-time-symmetric photonic structures that require loss and/or gain; here we show that the DBE is a different kind of EPD since it occurs in periodic structures that are lossless and gainless. Because of this property, a small level of gain is sufficient to induce single-frequency lasing based on a synchronous operation of four degenerate Floquet-Bloch eigenwaves. This lasing scheme constitutes a light-matter interaction mechanism that leads also to a unique scaling law of the laser threshold with the inverse of the fifth power of the laser-cavity length. The DBE laser has the lowest lasing threshold in comparison to a regular band edge laser and to a conventional laser in cavities with the same loaded quality $(Q)$ factor and length. In particular, even without mirror reflectors the DBE laser exhibits a lasing threshold which is an order of magnitude lower than that of a uniform cavity laser of the same length and with very high mirror reflectivity. Importantly, this novel DBE lasing regime enforces mode selectivity and coherent single-frequency operation even for pumping rates well beyond the lasing threshold, in contrast to the multifrequency nature of conventional uniform cavity lasers.

Thermally switchable photonic band-edge to random laser emission in dye-doped cholesteric liquid crystals

By changing the doping concentration of the chiral agent to adjust the relative position of the reflection band of cholesteric liquid crystals and the fluorescence emission spectrum of the dye, photonic band-edge and random lasing were observed, respectively. The reflection band of the cholesteric phase liquid crystal can also be controlled by adjusting the temperature: the reflection band is blue-shifted with increasing temperature, and a reversible switch from photonic band-edge to random lasing is obtained. Furthermore, the laser line width can be thermally adjusted from 1.1 nm (at 27 °C) to 4.6 nm (at 32.1 °C). A thermally tunable polarization state of a random laser from dual cells was observed, broadening the field of application liquid crystal random lasers.

Electrically assisted bandedge mode selection of photonic crystal lasing in chiral nematic liquid crystals

Selection of the bandedge lasing mode of a photonic crystal laser has been realized in a fluorescent dye doped chiral nematic liquid crystal by exerting electrical control over the mode competition. The bandedge lasing can be reversibly switched from the short-wavelength edge mode to the long-wavelength edge mode by applying a voltage of only 20 V, without tuning the bandgap. The underlying mechanism is the field-induced change in the order parameter of the fluorescent dye in the liquid crystal. The orientation of the transition dipole moment determines the polarization state of the dye emission, thereby promoting lasing in the bandedge mode that favors the emission polarization. Moreover, the dynamic mode-selection capability is retained upon polymer-stabilizing the chiral nematic liquid crystal laser. In the polymer-stabilized system, greatly improved stability and lasing performance are observed.