Sukhdeep Dhillon

13GHz direct modulation of terahertz quantum cascade lasers

By directly modulating the bias voltage of a double-metal waveguide, 2.8THz quantum cascade laser, we observe the appearance of multiple gigahertz sidebands in the emission spectrum, with a spacing that can be continuously tuned up to 13GHz. By using an upconversion technique, the terahertz spectrum is shifted at 1.57μm, and displayed on an optical spectrum analyzer. A marked increase in the number of sidebands is observed when the modulation frequency approaches the round-trip frequency (∼12.3GHz). The laser packaging high frequency response has been measured using a microwave rectification technique, and is limited by the bond-wire parasitic inductance.

Terahertz quantum cascade lasers with large wall-plug efficiency

Improved optical power performance of bound-to-continuum quantum-cascade lasers operating at 2.83THz is reported. Peak optical powers of 100mW at 4K and power conversion-efficiencies as high as ηw=(5.5±0.4)% in continuous wave at 40K were measured. The ηw values were assessed via an experimental method based on the analysis of the local lattice temperature as extracted by microprobe photoluminescence versus electrical power. From the measured ηw values they extracted a slope efficiency value 0.41±0.11W∕A.

Terahertz microcavity lasers with subwavelength mode volumes and thresholds in the milliampere range

The authors demonstrate terahertz microcavity lasers with ultralow current thresholds (Ith≈4mA) and with reduced mode volumes of ≈0.7(λeffective)3, i.e., less than one cubic wavelength. A double metal waveguide with reduced active core thickness (5.82μm) is used to achieve confinement in the vertical direction, without compromising the laser performances. Confinement in the longitudinal direction is obtained using microdisk resonators. The guiding properties of surface plasmons are exploited to guide the mode with the metal contact. This makes the use of a resonator with vertical and smooth sidewalls unnecessary. The emission wavelength is λ≈114μm. The devices lase up to 70K in pulsed mode, and they achieve continuous-wave operation up to 60K.

Surface plasmon quantum cascade lasers as terahertz local oscillators

We characterize a heterodyne receiver based on a surface-plasmon waveguide quantum cascade laser (QCL) emitting at 2.84 THz as a local oscillator, and an NbN hot electron bolometer as a mixer. We find that the envelope of the far-field pattern of the QCL is diffraction-limited and superimposed onto interference fringes, which are similar to those found in narrow double-metal waveguide QCLs. Compared to the latter, a more directional beam allows for better coupling of the radiation power to the mixer. We obtain a receiver noise temperature of 1050 K when the mixer is at 2 K, which, to our knowledge, is the highest sensitivity reported at frequencies beyond 2.5 THz.

Buried waveguides in terahertz quantum cascade lasers based on two-dimensional surface plasmon modes

A scheme for buried waveguides in terahertz quantum cascade lasers is demonstrated by combining a surface plasmon mode with ion implantation. The mode is bound to a metal strip deposited on the surface of the device that confines the light in both vertical and lateral directions without any etching requirements. Ion implantation is used to confine the current that selectively pumps the active region. Electrical and optical measurements demonstrate this two-dimensional confinement. Further, by taking advantage of the lower losses and a reduced thermal resistance, laser action is achieved in continuous wave up to 77K.

Integrated terahertz pulse generation and amplification in quantum cascade lasers

Terahertz pulse generation is demonstrated by a resonant femtosecond interband excitation of the miniband of a quantum-cascade-laser. The laser gain is subsequently used to amplify the terahertz pulse generated as it propagates through the cavity.

THz sideband generation at telecom wavelengths in a GaAs-based quantum cascade laser

Terahertz sideband generation is demonstrated by using the nonlinear intracavity interaction between far-infrared and near-infrared modes in a GaAs waveguide. A low power near-infrared beam at 1.32μm is coupled into a quantum cascade laser operating at 104μm (2.9 THz), which acts as both the THz source and the nonlinear medium. The conversion efficiency of the resulting sidebands is found to be approximately 4.5×10−6.

Ultra-low threshold THz microcavity lasers with sub-wavelength mode volumes

We demonstrate terahertz microcavity lasers at lambda=112 mum with ultra-low current thresholds of 4 mA and with mode volumes of less than one-cubic-wavelength. Confinement in the longitudinal direction is obtained using almost-circular micro-disk resonators. The guiding properties of surface-plasmons are exploited to guide the mode with the metal contact. The devices laser up to 70 K in pulsed mode, and up to 60 K in continuous-wave.

Continuous-wave THz generation through photomixing using a dual-frequency Yb3+:KGd(WO4)2 laser

We demonstrate a high-spectral-purity continuous-wave terahertz source, using a diode pumped Yb3+:KGd(WO4)2 dual frequency laser. THz radiation is generated by photomixing the two frequencies in a low temperature grown In:25Ga:75As photoconductor loading a dipole antenna. The frequency difference between the two optical modes is tuneable by step from d.c. to 3.1 THz. A maximum optical output power of 120 mW CW has been obtained with a beatnote-linewidth narrower than 30 kHz. Preliminary measurements show a tunable THz emission with a maximum output power in the order of a few tens of nW.

Monolithic echoless photoconductive switches for high-resolution terahertz time-domain spectroscopy (Conference Presentation)

Interdigitated photoconductive (iPC) switches [1] are powerful and convenient devices for time-resolved spectroscopy, with the ability to operate both as sources and detectors of terahertz (THz) frequency pulses. However, reflection of the emitted or detected radiation within the device substrate itself can lead to echoes that inherently limits the spectroscopic resolution achievable from their use in time-domain spectroscopy (TDS) systems. In this work, we demonstrate a design of low-temperature-grown-GaAs (LT-GaAs) iPC switches for THz pulse detection that suppresses such unwanted echoes [2]. This is realized through the growth of a buried multilayer LT-GaAs structure that retains its ultrafast properties, which after wafer bonding to a metal-coated host substrate, results in an iPC switch with a metal plane buried at a subwavelength depth below the LT-GaAs surface. Using this device as a detector, and coupling it to an echo-less iPC source [3], enables echo-free THz-TDS and high-resolution spectroscopy, with a resolution limited only by the temporal length of the measurement governed by the mechanical delay line used. As a proof-of-principle, the 2(12)-2(21) and the 1(01)-2(12) rotational lines of water vapor have been spectrally resolved, demonstrating a spectral resolution below 10 GHz. [1] A. Dreyhaupt, S. Winnerl, T. Dekorsy, M. Helm, Appl. Phys. Lett. 86, 121114 (2005) [2] K. Maussang, J. Palomo, J.-M. Manceau, R. Colombelli, I. Sagnes, L. H. Li, E. H. Linfield, A. G. Davies, J. Mangeney, J. Tignon, and S. S. Dhillon, Appl. Phys. Lett. 110, 141102 (2017). [3] K. Maussang, A. Brewer, J. Palomo, J.-M. Manceau, R. Colombelli, I. Sagnes, J. Mangeney, J. Tignon, S.S. Dhillon, IEEE Trans. Terahertz Sci. Technol. 6, 20 (2016)