There is an increasing interest in using graphene , for optoelectronic applications. − However, because graphene is an inherently weak optical absorber (only ≈2.3% absorption), novel concepts need to ...be developed to increase the absorption and take full advantage of its unique optical properties. We demonstrate that by monolithically integrating graphene with a Fabry-Pérot microcavity, the optical absorption is 26-fold enhanced, reaching values >60%. We present a graphene-based microcavity photodetector with responsivity of 21 mA/W. Our approach can be applied to a variety of other graphene devices, such as electro-absorption modulators, variable optical attenuators, or light emitters, and provides a new route to graphene photonics with the potential for applications in communications, security, sensing and spectroscopy.
The terahertz (THz) spectral region, covering frequencies from 1 to 10 THz, is highly interesting for chemical sensing. The energy of rotational and vibrational transitions of molecules lies within ...this frequency range. Therefore, chemical fingerprints can be derived, allowing for a simple detection scheme. Here, we present an optical sensor based on active photonic crystals (PhCs), i.e., the pillars are fabricated directly from an active THz quantum-cascade laser medium. The individual pillars are pumped electrically leading to laser emission at cryogenic temperatures. There is no need to couple light into the resonant structure because the PhC itself is used as the light source. An injected gas changes the resonance condition of the PhC and thereby the laser emission frequency. We achieve an experimental frequency shift of 10(-3) times the center lasing frequency. The minimum detectable refractive index change is 1.6 × 10(-5) RIU.
Coupling of broadband terahertz pulses into metal-metal terahertz quantum cascade lasers is presented. Mode matched terahertz transients are generated on the quantum cascade laser facet of ...subwavelength dimension. This method provides a full overlap of optical mode and active laser medium. A longitudinal optical-phonon depletion based active region design is investigated in a coupled cavity configuration. Modulation experiments reveal spectral gain and (broadband) losses. The observed gain shows high dynamic behavior when switching from loss to gain around threshold and is clamped at total laser losses.
Quantum cascade lasers (QCLs) have been realized in several different material systems. In the mid‐infrared, active regions are predominantly based on In0.53Ga0.47As and InAs as quantum well ...material. Market‐ready devices routinely provide continuous‐wave operation at room temperature. For their THz counterparts, the situation is less clear. The most common material system for THz QCLs is the inherently lattice‐matched combination of GaAs with Al0.15Ga0.85As barriers. Yet, these devices still only reach a maximum operating temperature of 200 K with a lack of progress within the past years. Based on the identification of key parameters, this work reviews material systems for quantum cascade lasers with an emphasis on material and growth‐related aspects and the goal to identify promising candidates for future device generations. Similar active regions realized in different material systems allow to estimate the gain per unit thickness, as well as total growth times and relative thickness errors.
This work evaluates different material systems for THz quantum cascade laser active regions. Different combinations of quantum well and barrier materials are discussed with respect to growth and design‐related issues including practical aspects like a thickness error estimation. Experimental data from established materials furthermore allow to gauge the suitability of novel candidate materials.
A characteristic feature of quantum cascade lasers is their unipolar carrier transport. We exploit this feature and realize nominally symmetric active regions for terahertz quantum cascade lasers, ...which should yield equal performance with either bias polarity. However, symmetric devices exhibit a strongly bias polarity dependent performance due to growth direction asymmetries, making them an ideal tool to study the related scattering mechanisms. In the case of an InGaAs/GaAsSb heterostructure, the pronounced interface asymmetry leads to a significantly better performance with negative bias polarity and can even lead to unidirectionally working devices, although the nominal band structure is symmetric. The results are a direct experimental proof that interface roughness scattering has a major impact on transport/lasing performance.
Higher order modes in photonic crystal slabs Gansch, Roman; Kalchmair, Stefan; Detz, Hermann ...
Optics express,
2011-Aug-15, 2011-08-15, 20110815, Letnik:
19, Številka:
17
Journal Article
Recenzirano
Odprti dostop
We present a detailed investigation of higher order modes in photonic crystal slabs. In such structures the resonances exhibit a blue-shift compared to an ideal two-dimensional photonic crystal, ...which depends on the order of the slab mode and the polarization. By fabricating a series of photonic crystal slab photo detecting devices, with varying ratios of slab thickness to photonic crystal lattice constant, we are able to distinguish between 0th and 1st order slab modes as well as the polarization from the shift of resonances in the photocurrent spectra. This method complements the photonic band structure mapping technique for characterization of photonic crystal slabs.
Increased coupling is observed in distributed-feedback quantum cascade lasers when placing a shallow second order grating between a continuous surface-plasmon layer and the active region. The ...combined effect of an air cladding and a metallic layer on the opposite sides of the waveguide increases the overlap with the grating region resulting in calculated coupling coefficients up to 100 cm(-1). The waveguide design was implemented by Au thermo-compression bonding after grating formation and subsequent backside processing of ridges with air claddings. Lasers as short as 176 microm show single-mode behavior with a side-mode-suppression-ratio of 20 dB and thresholds (10 kA/cm(2)) as well as output powers (> 150 mW) close to Fabry-Pérot device performances are reached for 360 microm long devices.
The development of In
0.53
Ga
0.47
As/GaAs
0.51
Sb
0.49
terahertz quantum cascade lasers is reviewed, starting with the first demonstration, through growth direction dependent performance issues, to ...high performance devices. This InP-based material system is an attractive alternative to the almost exclusively used GaAs/Al
x
Ga
1-x
As. Devices achieve maximum operating temperatures of 142 K and exhibit broadband lasing over a range of 660 GHz. A special focus has to be put on the growth direction related interface asymmetry for this material system. Symmetric active region designs are an elegant technique to investigate such asymmetries. A significant impact on the device performance is observed and attributed to interface roughness scattering.
The authors use a quantum well intersubband photodetector fabricated into a two dimensional photonic crystal to investigate the optical defect modes of a single missing hole defect. The modes appear ...as a local enhancement in spectral photocurrent due to an increased in-coupling of surface incident light when a defect mode is present. The frequencies of these localized modes are tracked as they are varied by the defect geometry and compared to simulations.
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