Thomas G. Folland

Gain Modulation by Graphene Plasmons in Aperiodic Lattice Lasers

Tunable lasers Lasers emit coherent light at wavelengths that are well defined. These wavelengths are usually fixed once the device has been fabricated. Now, Chakraborty et al. have combined an atomically thin graphene sheet with terahertz quantum cascade lasers to realize a terahertz laser that can be tuned via the carrier doping level of the graphene layer (see the Perspective by Polini). The demonstration opens up the possibility of reversible control over the laser emission through the integration of graphene waveguides. Science, this issue p. 246; see also p. 229 A layer of graphene is used to control the emission spectrum of a laser. [Also see Perspective by Polini] Two-dimensional graphene plasmon-based technologies will enable the development of fast, compact, and inexpensive active photonic elements because, unlike plasmons in other materials, graphene plasmons can be tuned via the doping level. Such tuning is harnessed within terahertz quantum cascade lasers to reversibly alter their emission. This is achieved in two key steps: first, by exciting graphene plasmons within an aperiodic lattice laser and, second, by engineering photon lifetimes, linking graphene’s Fermi energy with the round-trip gain. Modal gain and hence laser spectra are highly sensitive to the doping of an integrated, electrically controllable, graphene layer. Demonstration of the integrated graphene plasmon laser principle lays the foundation for a new generation of active, programmable plasmonic metamaterials with major implications across photonics, material sciences, and nanotechnology.

Coherent detection of THz laser signals in optical fiber systems

Terahertz (THz) coherent detectors are crucial for the stabilization and measurement of the properties of quantum cascade lasers (QCLs). This paper describes the exploitation of intra-cavity sum frequency generation to up-convert the emission of a THz QCL to the near infrared for detection with fiber optic coupled components alone. Specifically, a low cost infrared photodiode is used to detect a radio frequency (RF) signal with a signal-to-noise ratio of approximately 20dB, generated by beating the up-converted THz wave and a near infrared local oscillator. This RF beat note allows direct analysis of the THz QCL emission in time and frequency domains. The application of this technique for QCL characterization is demonstrated by analyzing the continuous tuning of the RF signal over 2 GHz, which arises from mode tuning across the QCLs operational current range.

Threshold gain in aperiodic lattice lasers.

Aperiodic lattices are a promising route to achieving tunable or multi-frequency lasing, but their threshold spectrum remains largely unstudied. We find that holographically designed aperiodic lattices can possess a multimode spectral response, containing both defect and band-edge photonic states. Under the influence of facet feedback the aperiodic lattice maintains remarkable spectral control at multiple frequencies over a wide bandwidth. This control arises from enhancement to the photon density of states at the designed frequencies, reducing the threshold of modes in the Fabry Perot coupled aperiodic lattice laser. Our results suggest that aperiodic lattice lasers are robust against fabrication imperfections, as exemplified by experimental demonstrations in prior work.

Dual-Frequency Defect-Mode Lasing in Aperiodic Distributed Feedback Cavities

In this letter, we aim to provide the first in-depth study of a multiband hologram filter response under the influence of gain. We find that these aperiodic distributed feedback (ADFB) gratings, which are essentially computer optimized digital holograms, can be inverse designed to possess multiple defectlike modes analogous to those present in quarter-wave phase shifted gratings. We attribute this phenomenon to the interaction between multiple photonic band gaps in ADFB cavities. Further examination of the ADFB gratings reveal that the defect-mode lasing solutions are insensitive to the variations of the grating feedback strength (κ). This result in particular, which has important significance to minimize error in fabrication, is confirmed against published experimental data for an ADFB laser.

Chapter 12 Semiconductor Nanophotonics Using Surface Polaritons

The properties of crystalline materials are dictated by the physical arrangement and behaviour of their constituent atoms. The behaviour of electrons and phonons (lattice vibrations), generally dominate the optoelectronic properties of a material. Electrons typically occupy either bound states, where they are unable to move and contribute to charge transfer, or in a delocalised state, where they can propagate through the lattice and carry electric current. These states form energy bands, called valence and conduction bands (see Fig. 12.1a), and the energies of these bands are one of the key ways of classifying materials. In a metal, these bands of energies overlap, and as a result, electrons are always able to freely propagate throughout the material. On the other hand, in an insulator these bands are separated by an energy difference (the band gap) and there is a small electronic density of states near the valence and conduction band edges. Semiconductors exist in-between these two extremes. They possess a band gap, but charge carriers can be moved between bands through either external stimuli or during the growth process via doping. This means that whilst semiconductors intrinsically behave as insulators, perturbations induced by thermal energy, light, dopants, or an electric field can switch them into acting as a conductor. Furthermore, in polar semiconductors the charge separation between the ionic lattice sites allow for crystalline vibrations (phonons) to couple with infrared to terahertz light. This broad range of interactions is what has made semiconductors an integral part of electronics, light emitting diodes, lasers, detectors and photovoltaics.

Precise control of infrared polarization using crystal vibrations

A natural material has been discovered that exhibits an extreme optical property known as in-plane hyperbolicity. The finding could lead to infrared optical components that are much smaller than those now available.In-plane hyperbolicity observed in a natural material.

Localized phonon-polariton modes in periodic GaN nanowire arrays grown by selective area epitaxy (Conference Presentation)

Localized surface phonon-polariton (SPhP) resonances in polar semiconductor nanostructures can provide highly sub-diffractional electromagnetic fields. Furthermore, SPhP resonances offer enhanced resonant quality factors when compared to plasmon-polariton based systems. The various material platforms and nanostructure geometries achievable in polar semiconductors suggest they would be ideal platforms for tunable, long-wavelength photonics applications. Moreover, the constituent atomic basis defines the operating frequency regime for SPhP resonances; tunable from the mid-infrared to THz. Here, we investigate Raman active aspects of SPhP modes in GaN nanowire arrays that are grown via selective area molecular beam epitaxy. We detect strong Raman peaks within the Reststrahlen band of GaN that are not found in the bulk GaN Raman spectrum. These SPhP modes occur around 700 cm^-1 (~ 14.3 microns), offering a spectral region for device applications which is currently not accessible by plasmonic based systems or other SPhP enabled materials. Utilizing selective area epitaxy, we created GaN nanowire arrays with various diameters and pitches, from which the Raman spectra showed tuning of the apparent SPhP resonances. Infrared reflectance measurements were also performed with an FTIR microscope to further establish the physical properties of the resonances. Finally, computational studies of the structures’ reflectance were used to solidify our understanding of the geometry/SPhP-resonance-tuning relationship.

Optical side-band generation in THz Fabry-Perot laser cavities

Optical nonlinearities in semiconductor laser cavities can be exploited to characterize the properties of laser radiation or perform high speed frequency conversion operations. For example, nonlinear up-conversion inside the cavity of quantum cascade lasers allows the use of near infrared optical components to measure high-speed terahertz or mid-infrared optical effects. This letter investigates two aspects of cavity up-conversion which control both the bandwidth and up-converted power: waveguide dispersion and cavity feedback. Specifically, we up-convert multi-mode Fabry Perot terahertz laser emission and detect each THz mode as a sideband signal on an optical carrier in the near infrared. Analysis of these results shows that a single frequency near infrared laser can up-convert terahertz modes spanning a bandwidth of approximately 220 GHz, limited by the group index mismatch between the near infrared and terahertz waves. Second, transfer matrix techniques are used to study strong cavity feedback on all ...

Gain control using graphene plasmons in aperiodic DFB lasers

We integrate tunable graphene plasmons into a THz laser waveguide with an aperiodic hologram lattice. This allows us to control the cavity photon lifetime and hence modulate round trip gain by doping the graphene layer.