Joshua R. Freeman

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.

Direct intensity sampling of a modelocked terahertz quantum cascade laser

Pulses from an actively modelocked terahertz quantum cascade laser are fully characterized using an optical sampling technique to detect the total instantaneous terahertz intensity. By triggering the quantum cascade laser electronics with a femtosecond laser, we are able to measure both the formation of modelocked pulses and the quasi-steady state. The dependence of the pulse width on the modulation power and drive current are investigated. At low drive currents, we measure transform-limited gaussian-shaped pulses with a FWHM of 19 ps.

Mode-locking of a terahertz laser by direct phase synchronization.

A novel scheme to achieve mode-locking of a multimode laser is demonstrated. Traditional methods to produce ultrashort laser pulses are based on modulating the cavity gain or losses at the cavity roundtrip frequency, favoring the pulsed emission. Here, we rather directly act on the phases of the modes, resulting in constructive interference for the appropriated phase relationship. This was performed on a terahertz quantum cascade laser by multimode injection seeding with an external terahertz pulse, resulting in phase mode-locked terahertz laser pulses of 9 ps duration, characterized unambiguously in the time domain.

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.

Electric field sampling of modelocked pulses from a quantum cascade laser

We measure the electric field of a train of modelocked pulses from a quantum cascade laser in the time-domain by electro-optic sampling. The method relies on synchronizing the modelocked pulses to a reference laser and is applied to 15-ps pulses generated by a 2-THz quantum cascade laser. The pulses from the actively modelocked laser are completely characterized in field and in time with a sub-ps resolution, allowing us to determine the amplitude and phase of each cavity mode. The technique can also give access to the carrier-envelope phase of each pulse.

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.

Coherent detection of metal-metal terahertz quantum cascade lasers with improved emission characteristics

Coherent detection of emission from quantum cascade lasers with metal-metal waveguides is demonstrated through free-space coupling of a THz pulse to the sub-wavelength waveguide. We implement a simple, monolithic planar horn antenna design on the metal-metal waveguide that reduces the impedance mis-match to the waveguide. The resulting devices show up to 10 times more directed output power than conventional metal-metal waveguides. This enhanced coupling to free-space allows a more efficient injection of broad-band THz pulses into the waveguide. Through this, we are able to seed the laser emission and coherently detect the laser emission by electro-optic sampling.

Broad gain in a bound-to-continuum quantum cascade laser with heterogeneous active region

We demonstrate the operation of heterogeneous terahertz quantum cascade lasers with broadened gain by optimising the sub-stacks to align at the same field. In single plasmon waveguides, we find two-colour operation for nearly the entire dynamic range of the lasers with similar performance to homogeneous lasers. Time domain spectroscopy measurements confirm that a flat gain spectrum is present and the sub-stacks align at the same time. When incorporated into metal-metal waveguides, we find that performance is consistent with the constituent sub-stacks and there is broadband operation over 380 GHz.

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.

Near-Field Analysis of Terahertz Pulse Generation From Photo-Excited Charge Density Gradients

Excitation of photo-current transients at semiconductor surfaces by subpicosecond optical pulses gives rise to emission of electromagnetic pulses of terahertz (THz) frequency radiation. To correlate the THz emission with the photo-excited charge density distribution and the photo-current direction, we mapped near-field and far-field distributions of the generated THz waves from GaAs and Fe-doped InGaAs surfaces. The experimental results show that the charge dynamics in the plane of the surface can radiate substantially stronger THz pulses than the charge dynamics in the direction normal to the surface, which is generally regarded as the dominant origin of the emission.