Photonic Crystal Quantum Cascade Lasers Research Articles

Surface-emitting photonic crystal terahertz quantum cascade laser adopting uniform triangular prism photonic crystal with a double-metal waveguide

Herein, we investigate surface-emitting photonic crystal terahertz quantum cascade lasers adopting double metal waveguide for high light extraction efficiency by utilizing finite-difference time-domain and filter diagonalization method. For the ease of fabrication and scalability of beam power, only a typical uniform 2D unit cell with triangular lattice and connected metal electrodes are considered in this study. Cylinder or uniform triangular prism photonic crystals are adopted as resonators within the selected unit cell structure. Candidate -point modes for efficient surface-normal emission are searched based on the unit cell analysis as a first step. After that, far-field, field distribution, and light-extraction efficiency are calculated to verify the candidate modes’ suitability by deploying 15(16) × 15 unit cells. Extracted results show that the surface-emitting quantum cascade lasers with triangular prism photonic crystal slabs have the possibility to attain high light-extraction efficiency with Gaussian beam shape even with compact chip size and high active region occupancy.

Large area photonic crystal quantum cascade laser with 5 W surface-emitting power.

Room temperature surface emission is realized on a large area (1.5 mm × 1.5 mm) photonic crystal quantum cascade laser (PhC-QCL) driven under pulsed mode, at the wavelength around 8.75 μm. By introducing in-plane asymmetry to the pillar shape and optimizing the current injection with a grid-like window contact, the maximum peak power of the PhC-QCL is up to 5 W. The surface emitting beam has a crossing shape with 10° divergence.

Room temperature surface emission on large-area photonic crystal quantum cascade lasers

We design and fabricate large-area (1.1 mm × 1.1 mm) photonic crystal quantum cascade lasers, enabling single-mode (wavelength ∼ 8.5 µm) surface emission at room temperature, with a maximum peak power up to 176 mW. The beam divergence is <1∘ and with no side-lobes. Moreover, by introducing asymmetry into the photonic crystal pillar shape, a single-lobed far-field pattern is realized. The photonic band structure is measured with high spectral (0.72 cm−1) and angular (0.1°) resolution by using the photonic crystal quantum cascade laser itself as a detector.

Analytical coupled-wave model for photonic crystal surface-emitting quantum cascade lasers.

An analytical coupled-wave model is developed for surface-emitting photonic-crystal quantum cascade lasers (PhC-QCLs). This model provides an accurate and efficient analysis of full three-dimensional device structure with large-area cavity size. Various laser properties of interest including the band structure, mode frequency, cavity loss, mode intensity profile, and far field pattern (FFP), as well as their dependence on PhC structures and cavity size, are investigated. Comparison with numerical simulations confirms the accuracy and validity of our model. The calculated FFP and polarization profile well explain the previously reported experimental results. In particular, we reveal the possibility of switching the lasing modes and generating single-lobed FFP by properly tuning PhC structures.

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

AbstractHigh 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. image

10-W pulsed operation of substrate emitting photonic-crystal quantum cascade laser with very small divergence.

High-power broad area substrate emitting photonic-crystal distributed feedback (DFB) quantum cascade lasers (QCLs) emitting around 4.73 μm is reported. Two-dimensional centered rectangular photonic-crystal (CRPC) grating is introduced to enhance optical coherence in large area device. Main lobe far-field radiation pattern with a very small divergence angle of about 0.65° × 0.31° is obtained. A record peak output power for vertical emitting QCLs exceeding 10 W is obtained with high reflectivity (HR) coating. Robust single longitudinal mode emission with a side mode suppression ratio (SMSR) of 30 dB is continuously tunable by the heat sink temperature up to 65°C.

Continuous‐wave vertically emitting photonic crystal terahertz laser

AbstractCompact 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 (Tmax) of 100 K. Also, a highly collimated far‐field pattern is demonstrated, which is an important step towards real world applications.

Monolithically integrated two-dimensional arrays of surface-emitting photonic-crystal terahertz lasers

We demonstrate two-dimensional arrays of surface-emitting THz frequency photonic-crystal quantum cascade lasers covering a broad spectral range. The arrays are made of 16 unit elements, each of them spectrally single mode and featuring a narrow divergence emission pattern. The array emission frequencies cover a range of ≈0.2 THz around an emission frequency of ≈2.75 THz. Large size and small size devices have been developed. The former devices operate up to a maximum temperature of ≈90K with 2 to 3 mW output power in continuous wave regime.

Low temperature near-field scanning optical microscopy on infrared and terahertz photonic-crystal quantum cascade lasers

We report the development of a scattering-type near-field scanning optical microscope (sNSOM) which operates at temperatures down to 100 K with a scanning range of up to 400 μm. We have used this sNSOM to map the electromagnetic near-field on mid-IR and terahertz (THz) surface emitting quantum cascade lasers with photonic-crystal resonators. Mid-IR devices operate at λ=7.5 μm (40 THz) while THz devices operate at λ≈110 μm (2.7 THz). The near-field images—in agreement with numerical calculations—demonstrate an instrument resolution of 100's nm.

Loss-reduction in midinfrared photonic crystal quantum cascade lasers using metallic waveguides

We describe a mechanism for plasmonic loss reduction in midinfrared metallic photonic crystals and apply it to surface-plasmon quantum cascade lasers. We obtain pulsed, room-temperature operation of surface-emitting photonic crystal quantum cascade lasers operating at λ≈7.4 μm. The photonic crystal resonator is patterned in the de- vice top metallization, and laser operation is obtained on a band-edge mode of the photonic band structure. The emission is spectrally single mode, with a side-mode suppression ratio of 20 dB, and on-chip tun- ability is obtained over a wavelength range of ≈0.52 μm. Simulations based on a finite elements approach and on the finite-difference time- domain method allow us to study the photonic-band structure, the elec- tromagnetic field distributions, and especially, the influence of the device parameters on the losses. The comparison between the measured and simulated far-field emission patterns and polarization proves the lasers operate on a monopolar-symmetry mode C 2010 Society of Photo-Optical Instru-

Optimized surface-emitting photonic-crystal terahertz quantum cascade lasers with reduced resonator dimensions

Terahertz frequency photonic-crystal quantum cascade lasers allow directional and controllable surface emission while at the same time functioning efficiently well above liquid nitrogen temperature. Through an in-depth understanding of the mechanism underlying surface emission, we demonstrate optimized devices with significantly reduced absolute threshold currents. We are able to reduce the device surface area by more than a factor of two, while maintaining angularly narrow, single-lobed surface emission with a divergence of ≈10°×10°. The devices emit at ∼2.8–2.9 THz, with maximum operating temperatures in the range 80–150 K.

Continuous-wave operation of 2.7 THz photonic crystal quantum cascade lasers

Continuous-wave laser action has been achieved in a 2.7 THz quantum cascade device operating on the modes of a two-dimensional photonic crystal resonator. The photonic structure is patterned into the device top metal contact layer. The emission is obtained from the device surface. It is single lobed and highly directional. At cryogenic temperatures the output power is ≃300 μW.

Bringing THz into focus

In this paer, a continuous-wave surface-emitting photonic crystal THz quantum cascade laser (QCL) with a monochromatic and low divergence output beam was demonstrated. Researchers at the University Paris Sud in France in collaboration with Cambridge University in the UK combined their recently designed efficient photonic crystal resonator with a low-threshold quantum cascade material to produce an optimised 2.7 THz QCL which could be used for heterodyne detection applications such as in astronomy.

Surface-emitting quantum cascade lasers with metallic photonic-crystal resonators

Surface emitting photonic-crystal quantum cascade lasers operating at λ≈7.3 μm are demonstrated. The photonic crystal resonator is written solely on the top metallization layer. The mismatch between the modes supported by metallized and nonmetallized regions yields enough optical feedback to achieve laser action. The devices exhibit single-mode emission with a side mode suppression ratio of ≈20 dB, the wavelength is lithographically tunable across a range of almost 70 cm−1, and the radiation is emitted from the surface. The maximum operating temperature is 220 K. The divergence of the output beam, which is doughnut-shaped, is approximately 9°.

Predictable surface emission patterns in terahertz photonic-crystal quantum cascade lasers

We demonstrate a framework to understand and predict the far-field emission in terahertz frequency photonic-crystal quantum cascade lasers. The devices, which employ a high-performance three-well active region, are lithographically tunable and emit in the 104-120 microm wavelength range. A peak output power of 7 mW in pulsed mode is obtained at 10 K, and the typical device maximum operating temperature is 136 K. We identify the photonic-crystal band-edge states involved in the lasing process as originating from the hexapole and monopole modes at the G point of the photonic band structure, as designed. The theoretical far-field patterns, obtained via finite-difference time-domain simulations, are in excellent agreement with experiment. Polarization measurements further support the theory, and the role of the bonding wires in the emission process is elucidated.

Design and fabrication of photonic crystal quantum cascade lasers for optofluidics

We present novel designs and demonstrate a fabrication platform for electrically driven lasers based on high quality-factor photonic crystal cavities realized in mid-infrared quantum cascade laser material. The structures are based on deep-etched ridges with their sides perforated with photonic crystal lattice, using focused ion beam milling. In this way, a photonic gap is opened for the emitted TM polarized light. Detailed modeling and optimization of the optical properties of the lasers are presented, and their application in optofluidics is investigated. Porous photonic crystal quantum cascade lasers have potential for on-chip, intracavity chemical and biological sensing in fluids using mid infrared spectroscopy. These lasers can also be frequency tuned over a large spectral range by introducing transparent liquid in the photonic crystal holes.

Photonic crystal quantum cascade lasers with improved threshold characteristics operating at room temperature

The authors have fabricated GaAs∕Al0.45Ga0.55As quantum cascade lasers (QCLs) with two-dimensional photonic crystal (PhC) mirrors. The monolithically integrated PhC mirrors consist of a triangular lattice of ∼14μm deeply etched semiconductor pillars. The PhC is designed to exhibit a photonic band gap for both the ΓM and the ΓK crystal orientation and provides reflectivities of >80%. Compared to a reference laser with the same cavity length of 600μm but as-cleaved facets, the QCLs with ΓM and ΓK oriented PhCs show reductions of the threshold current by 34% and 30%, respectively.

Progress towards photonic crystal quantum cascade laser

The paper describes recent progress in the design, simulation, implementation and characterisation of photonic crystal (PhC) GaAs-based quantum cascade lasers (QCLs). The benefits of applying active PhC confinement around a QCL cavity are explained, highlighting a route to reduced threshold current operation. Design of a suitable PhC has been performed using published bandgap maps; simulation results of this PhC show a wide, high reflectivity stopband. Implementation of the PhC for the device is particularly difficult, requiring a very durable metallic dry etch mask, high performance dry etching and a low damage epilayer-down device mounting technique. Preliminary shallow etched PhC QCLs demonstrated the viability of current injection through the metal etch mask and the device mounting technique. Development of the etch mask and dry etching have demonstrated a process suitable for the manufacture of deep etched PhC structures. All the necessary elements for implementing deep etched PhC QCLs have now been demonstrated, allowing for the development of high performance devices.