Photonic Crystal Band-edge Laser Research Articles

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.

Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates

Highly integrated photonic crystal bandedge lasers monolithically grown on Si substrates

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.

Electrical modulation of a photonic crystal band-edge laser with a graphene monolayer.

The electrical control of photonic crystal (PhC) lasers has been an attractive but challenging issue. Laser operation by electrical injection is of key importance for the viability and applicability of the PhC lasers. Another key factor is the electrical modulation of the laser output. The Fermi level of a graphene monolayer can be controlled by electrical gating, which adjusts its optical absorption. In this study, a graphene monolayer sheet is integrated on top of a two-dimensional PhC structure composed of InGaAsP multiple-quantum-wells (MQWs) in order to demonstrate the electrical modulation of a high-power (microwatt-scale) PhC band-edge laser. The introduced dielectric spacer layer presets the delicate balance between the optical gain from the MQWs and optical loss at the graphene monolayer. The proposed device is covered by an ion-gel film, which enables a low-voltage laser modulation at |Vg|≤1 V. The modulation is extensively investigated experimentally, and the obtained results are confirmed by performing numerical simulations.

Efficient on-chip integration of a colloidal quantum dot photonic crystal band-edge laser with a coplanar waveguide

High-density photonic integrated circuits (PICs) are expected to replace their current electronic counterparts in the future. The most crucial prerequisite for realizing successful PICs is to develop a low-loss coupling technique between active and passive photonic components based on various nanoscale materials and devices. Here we propose and demonstrate an on-chip integration technique in which a high-refractive-index layer constitutes the coplanar structural backbone across the entire PIC chip. To prove the concept, patterns of a two-dimensional photonic crystal (PhC) band-edge laser and grating couplers are engraved into the backbone layer, and colloidal quantum dots (CQDs) for optical gain are selectively deposited in the PhC area by a conventional lift-off process. Using optical excitation, we observe that the CQD–PhC structure emits coherent single-mode laser light, which is subsequently coupled to and propagates through an adjacent slab waveguide in the well-defined directions corresponding to the selected band-edge point, finally emerging through the grating coupler. Our study demonstrates a simple but highly suggestive PIC platform that automatically guarantees high coupling efficiencies between the micro- and nanophotonic devices to be integrated (through high degrees of modal matching in the vertical direction) and will therefore advance the development of high-density PIC technologies.

Two-dimensional photonic crystal bandedge laser with hybrid perovskite thin film for optical gain

We report optically pumped room temperature single mode laser that contains a thin film of hybrid perovskite, an emerging photonic material, as gain medium. Two-dimensional square lattice photonic crystal (PhC) backbone structure enables single mode laser operation via a photonic bandedge mode, while a thin film of methyl-ammonium lead iodide (CH3NH3PbI3) spin-coated atop provides optical gain for lasing. Two kinds of bandedge modes, Γ and M, are employed, and both devices laser in single mode at similar laser thresholds of ∼200 μJ/cm2 in pulse energy density. Polarization dependence measurements reveal a clear difference between the two kinds of bandedge lasers: isotropic for the Γ-point laser and highly anisotropic for the M-point laser. These observations are consistent with expected modal properties, confirming that the lasing actions indeed originate from the corresponding PhC bandedge modes.

Surface passivation of a photonic crystal band-edge laser by atomic layer deposition of SiO2 and its application for biosensing.

We report on the conformal surface passivation of photonic crystal (PC) laser devices with an ultrathin dielectric layer. Air-bridge-type Γ-point band-edge lasers (BELs) are fabricated by forming a honeycomb lattice two-dimensional PC structure into an InGaAsP multiple-quantum-well epilayer. Atomic layer deposition (ALD) is employed for conformal deposition of a few-nanometer-thick SiO2 layer over the entire device surface, not only on the top and bottom surfaces of the air-bridge membrane but also on the air-hole sidewalls. Despite its extreme thinness, the ALD passivation layer is found to protect the InGaAsP BEL devices from harsh chemicals. In addition, the ALD-SiO2 is compatible with the silane-based surface chemistry, which allows us to use ALD-passivated BEL devices as label-free biosensors. The standard streptavidin-biotin interaction shifts the BEL lasing wavelength by ∼1 nm for the dipole-like Γ-point band-edge mode. A sharp lasing line (<0.2 nm, full width at half-maximum) and a large refractive index sensitivity (∼163 nm per RIU) produce a figure of merit as high as ∼800 for our BEL biosensor, which is at least an order of magnitude higher than those of more common biosensors that rely on a broad resonance peak, showing that our nanolaser structures are suitable for highly sensitive biosensor applications.

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.

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.

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.

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.

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.

Butt-end fiber coupling to a surface-emitting Γ-point photonic crystal band edge laser

The authors demonstrate a compact and efficient fiber coupling scheme for a surface-emitting photonic crystal laser by using a cleaved fiber tip connected to a wavelength-division-multiplexing 1×2 fiber coupler. Simultaneous optical excitation and laser light collection are achieved for a 1.55μm InGaAsP graphite-lattice Γ-point band edge laser. On comparing with the conventional microphotoluminescence setup based on an objective lens, their direct butt coupling setup is much simpler and yet offers the fiber-coupled output approximately ten times higher. Finite-difference time-domain simulations confirm that the Γ-point band edge laser exhibits the desired vertical emission property and that the fiber coupling efficiency can be as high as 30%.

In-Plane Coupling of Light From InP-Based Photonic Crystal Band-Edge Lasers Into Single-Mode Waveguides

In this paper, we analyze the coupling of light from photonic-crystal band-edge lasers into single-mode waveguides. Both active and passive devices lie in the same plane and coupling of light is achieved by using parabolic and nanotapers in InP based epitaxial structures. Two- and three-dimensional finite-difference time-domain methods are employed to analyze these devices. Coupling efficiencies higher than 80% can be obtained with parabolic couplers. We also present laser configurations that can reduce multiwavelength coupling of light into single-mode waveguides, using structures that are similar to coupled cavity Fabry-Peacuterot lasers

Two-dimensional AlGaInP∕GaInP photonic crystal membrane lasers operating in the visible regime at room temperature

The authors report on the fabrication and characterization of visible two-dimensional photonic crystal (PhC) band-edge lasers with AlGaInP∕GaInP multiple-quantum-well active layers. High-quality PhC air-bridge membranes are produced by optimizing the inductively coupled plasma etching process for AlGaInP materials. Room-temperature lasing operation at the wavelengths ranging from 630to690nm is obtained by tuning the PhC lattice parameters. The lasing action originates from high-symmetry band-edge modes, which is verified by three-dimensional finite difference time domain computation.