Owen P. Marshall

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.

Super-narrow, extremely high quality collective plasmon resonances at telecom wavelengths and their application in a hybrid graphene-plasmonic modulator.

We present extremely narrow collective plasmon resonances observed in gold nanostripe arrays fabricated on a thin gold film, with the spectral line full width at half-maximum (fwhm) as low as 5 nm and quality factors Q reaching 300, at important fiber-optic telecommunication wavelengths around 1.5 μm. Using these resonances, we demonstrate a hybrid graphene-plasmonic modulator with the modulation depth of 20% in reflection operated by gating of a single layer graphene, the largest measured so far.

Electrically switchable emission in terahertz quantum cascade lasers.

Electrically switchable emission in a terahertz quantum cascade laser is demonstrated. Two active region designs are incorporated into the same waveguide, forming a heterogeneous cascade that lases at frequencies around 2.6 THz and 3.0 THz. We find that the position of the active regions within the waveguide does not effect the sequence in which the two colours reach laser threshold. The devices show good performance, with 2.6 THz and 3.0 THz modes operating up to 60K and 91K respectively and displaying thresholds as low as 79 Acm(-2) for the 2.6 THz mode.

Dual wavelength emission from a terahertz quantum cascade laser

We describe a heterogeneous terahertz (THz) quantum cascade laser that is composed of two different active region designs. This device emits simultaneously at around 2.5 and 2.9 THz with certain frequency tunability by applied current. We also investigate the spectral gain in the structure by THz time-domain spectroscopy and correlate the gain spectral bandwidth with the alignment and wavelength emission behavior of the two stack device.

Discrete mode tuning in terahertz quantum cascade lasers.

A holographically designed, aperiodic distributed feedback grating is used as a multi-resonance filter and embedded within an existing Fabry-Pérot (FP) terahertz (THz) quantum cascade laser (QCL) cavity. Balancing the feedback strengths of the filter resonances and the FP cavity creates a system capable of a high degree of single-mode selectivity, which is sensitive to changes in driving current. Multi-moded QCLs operating around 2.9 THz are thus modified to achieve purely electronic discrete tuning spanning over 160 GHz with an average tuning resolution of 30 GHz. Applying the same multi-resonance filter to QCLs with gain peaks around 2.65 and 2.9 THz leads to dual-mode lasing with an electrically controlled frequency separation of between 190 and 267 GHz. A phase sensitive mode selection mechanism is experimentally confirmed by the observation of divergent fine-tuning of the lasing modes.

Engineering optical properties of a graphene oxide metamaterial assembled in microfluidic channels

The dense packing of two dimensional flakes by van der Waals forces has enabled the creation of new metamaterials with desirable optical properties. Here we assemble graphene oxide sheets into a three dimensional metamaterial using a microfluidic technique and confirm their ordering via measurements of ellipsometric parameters, polarized optical microscopy, polarized transmission spectroscopy, infrared spectroscopy and scanning electron microscopy. We show that the produced metamaterials demonstrate strong in-plane optical anisotropy (Δn≈0.3 at n≈1.5-1.8) combined with low absorption (k<0.1) and compare them with as-synthesized samples of graphene oxide paper. Our results pave the way for engineered birefringent metamaterials on the basis of two dimensional atomic crystals including graphene and its derivatives.

Surface-emitting photonic crystal terahertz quantum cascade lasers

Surface-emitting terahertz quantum cascade lasers based on double-metal waveguides incorporating photonic crystal structures have been demonstrated. Far-field emission patterns are dominated by lobes close to the surface normal. In addition to modified emission profiles, enhanced output powers are also displayed in comparison to standard ridge waveguides, with over 2 mW peak power observed at a heat sink temperature of 10 K.

Longitudinal computer-generated holograms for digital frequency control in electronically tunable terahertz lasers

Longitudinal computer-generated holograms (LCGHs) can be used for the inverse design of aperiodic photonic lattices for customizable frequency control of light propagation. A one-dimensional binary LCGH, designed to harness the coarse gain tuning of a terahertz (THz) quantum cascade laser (QCL) operating at around 2.9 THz, is patterned directly by ion milling into the surface plasmon-based waveguides of pre-characterized QCLs. The initial multi-mode emission is suppressed in favour of electronically controlled, discretely tunable single-mode lasing spanning over 160 GHz. Side-mode suppression ratios of over 20 dB are also demonstrated in these tunable THz LCGH-QCLs.

Intensity detection of terahertz quantum cascade laser radiation using electro-optic sampling

We demonstrate intensity detection of terahertz radiation from a 2 THz quantum cascade laser with nonsynchronized electro-optic (EO) sampling under crossed polarizers. The detection sensitivity is limited by the residual birefringence of the EO detection crystal. With 100 fs fiber laser pulses and a CdTe EO crystal, terahertz radiation with power less than 100 μW was detected. A femtosecond gating pulse provides an ultrashort detection response that is potentially very useful for analyzing temporal performance of short pulsed radiation or observing fast phenomena probed by terahertz pulses.

Broadband photonic control for dual-mode terahertz laser emission

Short, holographically designed, aperiodic distributed feedback (ADFB) gratings are able to provide multi-band spectral filtering over arbitrarily wide bandwidths, offering a complimentary photonic technology to ultra-broadband terahertz quantum cascade lasers (THz QCLs). Using an ADFB grating, ion milled directly into the laser waveguide, high resolution spectral filtering is achieved in THz QCLs with heterogeneous active regions producing two distinct spectral gain peaks centred around 2.65 and 2.9 THz. Simultaneous dual-mode emission is achieved from a single section laser, with up to 20-dB side-mode suppression. Discrete electronic mode tuning occurs between ADFB bands, giving a switchable mode separation ranging from 163 to 267 GHz, along with continuous electronic and thermal tuning of up to ∼2 GHz.