Wideband Electrically Controlled Vernier Frequency Tunable Terahertz Quantum Cascade Laser

Frequency tuning in terahertz frequency quantum cascade lasers is challenging because of low thermal and current tuning coefficients. Moreover, photonic designs like Vernier selection based sampled gratings, used in telecom lasers to tune emission frequency, are unsuitable due to the long terahertz wavelengths and will require impractically long cavities (>15 mm). Here, we report the first wideband frequency tuning from a monolithic device exploiting Vernier selection rules using a coupled-cavity laser with a defect engineered photonic lattice. A precisely positioned defect lattice allows us to engineer the free spectral range and finesse of one of the cavities, similar to a sampled grating but using shorter cavity lengths (<4 mm). A coupled-cavity was used to tune the emission frequency. We achieve frequency tuning over 209 GHz, including mode hop-free continuous tuning of ∼6–21 GHz across six frequency bands, controlled through Stark shift, cavity-pulling, localized Joule heating, and thermal effects

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

Pump-probe measurements of gain in a terahertz quantum cascade laser

The gain recovery time of a bound-to-continuum terahertz frequency quantum cascade laser, operating at 1.98 THz, has been measured using broadband terahertz-pump-terahertz-probe spectroscopy. The recovery time is found to reduce as a function of current density, reaching a value of 18 ps as the laser is brought close to threshold. We attribute this reduction to improved coupling efficiency between the injector state and the upper lasing level as the active region aligns

Characterization of the THz quasi-optical channel for the measurement of the power radiated by photoconductive antennas

In this paper a rigorous electromagnetic characterization of the setup for measuring the THz power radiated by pulsed photoconductive antenna is discussed. Such characterization is expressed in terms of efficiencies which quantify how much power is lost in the coupling between the various components involved in the measurement setup. The conducted analysis highlights how such efficiencies affect the energy spectrum of the measured pulsed signal. Measurement results with two different detectors will be shown during the conference and will be compared against the power estimation obtained by a recently developed equivalent circuit model for photoconductive antennas. The proposed electromagnetic modeling allows us to effectively improve the design of THz time domain systems

THz quantum cascade lasers with output power over 1 W

Terahertz (THz) frequency radiation has many potential applications, ranging from imaging and chemical sensing through to telecommunications. However, one of the principal challenges is to develop compact, low-cost, efficient THz sources. In this respect, the development of the THz quantum cascade laser (QCL) has provided a potential solid-state solution. Nonetheless, for many remote sensing and imaging applications, high THz powers are desirable, in part owing to the significant attenuation of THz radiation by water vapour in the atmosphere. THz QCLs have been demonstrated with peak pulsed output powers (Ppeak) of up to 470 mW per facet, using a direct wafer-bonding technique to stack two separate THz QCLs together. This approach, however, requires the QCL to have a symmetric active region, limiting widespread applicability of the technique. In general, increased output powers in semiconductor lasers can be obtained by using longer and/or broader area cavities. Indeed, in long-cavity 4.7-THz QCLs, a Ppeak of up to 875 W (from both facets) was recently achieved. Here, we demonstrate THz QCLs with a Ppeak of 1.01 W from a single facet at 10 K by using a broader area device. The devices operate in pulsed mode with an emission frequency of around 3.4 THz. To the best of our knowledge, this is the first demonstration of THz QCLs with Ppeak exceeding 1 W

Selective Deuterium Ion Acceleration Using the Vulcan PW Laser

We report on the successful demonstration of selective acceleration of deuterium ions by target-normal sheath acceleration (TNSA) with a high-energy petawatt laser. TNSA typically produces a multi-species ion beam that originates from the intrinsic hydrocarbon and water vapor contaminants on the target surface. Using the method first developed by Morrison, et al.,$^{1}$ an ion beam with $>$99$\%$ deuterium ions and peak energy 14 MeV/nucleon is produced with a 200 J, 700 fs, $>10^{20} W/cm^{2}$ laser pulse by cryogenically freezing heavy water (D$_{2}$O) vapor onto the rear surface of the target prior to the shot. Within the range of our detectors (0-8.5$^{\circ}$), we find laser-to-deuterium-ion energy conversion efficiency of 4.3$\%$ above 0.7 MeV/nucleon while a conservative estimate of the total beam gives a conversion efficiency of 9.4$\%$.Comment: 5 pages, 5 figure

Silver-based surface plasmon waveguide for terahertz quantum cascade lasers

Terahertz quantum cascade lasers (THz QCLs) have undergone rapid developments since their first demonstration in 2002. Presently, the wide spectral range (1.2–5.2 THz) and high output power (1 W) make THz QCLs promising sources for applications in high-resolution spectroscopy and THz imaging. However, their maximum operating temperature is only 199.5 K and therefore cryogenic cooling is still needed. Improving the thermal performance of THz QCLs is a key challenge for their practical usage. The waveguide loss is closely related with the device thermal performance. To lower the loss, copper has been used to replace the gold in the standard metal–metal waveguide scheme, and around 10 K increase in the maximum lasing temperature has been achieved. Here, we employ silver as the waveguide metal and investigate its effects on devices with a single surface-plasmon waveguide configuration

Injection locking of a terahertz quantum cascade laser to a telecommunications wavelength frequency comb

High resolution spectroscopy can not only identify atoms and molecules, but can also provide detailed information on their chemical and physical environment, and relative motion. In the terahertz frequency region of the electromagnetic spectrum, where many molecules have fundamental vibrational modes, there is a lack of powerful sources with narrow linewidths that can be used for absorption measurements or as local oscillators in heterodyne detectors. The most promising solid-state source is the THz frequency quantum cascade laser (QCL), however, the linewidth of this compact semiconductor laser is typically too broad for many applications and its frequency is not directly referenced to primary frequency standards. In this work we injection lock a QCL operating at 2 THz to a compact fibre-based telecommunications wavelength frequency comb, where the comb line spacing is referenced to a microwave frequency reference. This results in the QCL frequency locking to an integer harmonic of the microwave reference, and the QCL linewidth reducing to the multiplied linewidth of the microwave reference, < 100 Hz. Furthermore, we perform phase-resolved detection of the locked QCL and measure the phase noise of the locked system to be –75 dBc/Hz at 10 kHz offset from 2 THz carrier

Photoconductive Arrays on Insulating Substrates for High-Field Terahertz Generation

We report on the design, fabrication and characterisation of large-area photoconductive THz array structures, consisting of a thin LT-GaAs active region transferred to an insulating substrate using a wafer-scale bonding process. The electrically insulating, transparent substrate reduces the parasitic currents in the devices, allowing peak THz-fields as high as 120 kV cm−1 to be generated over a bandwidth >5 THz. These results are achieved using lower pulse energies than demanded by conventional photoconductive arrays and other popular methods of generating high-field THz radiation. Two device sizes are fully characterised and the emission properties are compared to generation by optical rectification in ZnTe. The device can be operated in an optically saturated regime in order to suppress laser noise