Institute of Microwaves and Photonics SCHOOL OF ELECTRONIC AND ELECTRICAL ENGINEERING, FACULTY OF ENGINEERING Development of integrated QCLs for local oscillator applications in Earth observation and astronomy Y. J. Han, A. Valavanis, J. Zhu, L. H. Li, E. H. Linfield, A. G. Davies University of Leeds B. Ellison, M. Crook, T. Bradshaw, B. Swinyard (also UCL), D. Gerber RAL Space

Overview Introduction Development of QCLs for LO applications Waveguide block characterisation Summary Supplied by B. Ellison, RAL

The terahertz frequency region contains spectral signatures of ions, atoms, and molecules. By measuring them, we will: Increase understanding of natural and anthropogenic effects on climate change; Study the ‘gateway’ between the Earth’s atmosphere and the near space environment; Investigate the composition and origin of the Solar System; Explore star formation history and the evolution of Galaxy. Interests in Earth observation and astronomy in the THz region Small satellite sounder from LEO But, this requires terahertz instrumentation, suitable for operation on a satellite.

THz Instrumentation & the heterodyne approach Heterodyne receiver technology converts a THz input signal into a lower intermediate frequency (IF) signal – typically GHz; Key front-end components are the mixer and THz local oscillator (LO). e.g. RAL & STAR Dundee 350GHz Heterodyne Radiometer (previous CEOI funding)

Typical requirements of LO & Performance of THz QCLs Requirements of LOPerformance of THz QCLs Frequencies: 3.5 THz, 4.7 THz1– 5 THz Single mode operationDemonstrated using periodic gratings Frequency stability: ~ 1 MHz~ Hz (phase lock), linewidth < 20 kHz Operating temperature: > 50 K199.5/129 K (pulsed/CW) Out put power: 1.0 mW> 1 W/138 mW (pulsed/CW) Cooling power: < 5 Wseveral W CW operationdemonstrated Radiation hardnessdemonstrated compact and integratedSmall size, but need cooling system Can all the requirements be satisfied with one THz QCL?

THz QCL active region designs B-to-CResonant LO phonon (RP)B-to-C + LO phonon (Hybrid)

Comparison of different designs Single-metal waveguide (gold), with 2% duty cycle, pulsed operation. The hybrid design yields high output power and low current density, suitable for cw and high power emission. The RP structures have high dissipated power, limiting cw performance. The BTC structures have low maximum output power. Need to check the thickness

THz QCL waveguides DM waveguide is needed because of the better thermal performance.

DM waveguide results in a higher lasing temperature, but lower output power. The lasing frequency is similar (3.4 THz). 1000×150×14 μm, SISP-Au waveguide (SM) 1000×125×14 μm, Au-Au waveguide (DM) 3000×150×14 μm, SISP-Au waveguide (SM) 3000×125×14 μm, Au-Au waveguide (DM) Comparison of different waveguides L1152, 3.5 THz, B-to-C design

The frequency is 3.31 – 3.58 THz, and the maximum lasing temperature is 86 K. The maximum output power at 10 K is 0.41 mW with dissipated power of 3.10 W. Device dimensions: 1000 μm × 125 μm × 15 μm L1209, 3.5 THz, 9-well hybrid design CW characterisation of device 1 (DM)

The frequency is 4.63 – 4.84 THz, and the maximum lasing temperature is 60 K. The maximum output power at 10K is 0.22 mW with dissipated power of 4.49 W. Device dimensions: 1000μm×125μm×10μm L1169, 4.7 THz, 8-well hybrid design CW characterisation of device 1 (DM)

Summary of devices Device dimensions: SM (SISP-Au waveguide): 1000 μm × 150 μm DM(Au-Au waveguide): 1000 μm × 125 μm, * for L1169, SM: 3000 μm × 150 μm waferwave- guide f (THz) T_max (K) (pulsed/cw) Jth (A/cm (Pulsed/cw) P_max (pulsed/cw) P_dis (pulsed/cw) L1152SM3.4338/30171/1891.0/ L1152DM /8096/861.5/ L1209SMn. a.106/49170/18028/164.5 L1209DM /86134/1332.6/ L1169SM* /n. a.230/n. a.8.2/n. a.n. a. L1169DM /60157/1775.0/ LO requirements3.5 & 4.7> 50 Kn. a.1.0 mW< 5W

Next steps with THz QCLs Optimise epitaxial design of 4.7 THz QCLs; Investigate effect of ridge width on device performance; Improve heat sinking to give better thermal management; Use of back-facet coating, and silver waveguides.

Integrating Leeds THz QCLs into micro-machined blocks at RAL 13x QCL ridges (emission direction out of screen) [10x125 µm] Au waveguide layer [1 µm] n+ GaAs substrate [200 um x 3 mm] Ti/Au contact [~200 nm] Double-metal (DM) QCL structures have been processed at Leeds and bonded into waveguide blocks at RAL DM offers high-temperature performance & potentially good near- field waveguide coupling Device length ~1 mm

DM QCL mounting Ribbon bonding test undertaken at RAL Central ridge of sample device has been bonded into waveguide block All images supplied by B. Ellison, RAL

CW characterisation Mounted Unmounted Block integration concept works! No real change in threshold current at low temperature. ~5x reduction in power (no optimisation of output coupling, though), and no significant change in threshold current or temperature.

CW characterisation Mounted Unmounted

Beam profile Obtained using linear scan of Golay cell ~1.5 mm aperture on detector (c.f., ~15 mm beam waist) Almost Gaussian profile FWHM = 17.1˚ (in-plane) and 19.7˚ (growth direction) Dramatic improvement over DM (~120˚)

Beam profile Reasonable match with simulated data from RAL; Electronics Letters article drafted on results.

Beam polarisation Obtained using wire grid polariser and Golay cell. The light intensity decreased to 27.5% when the polariser had its major E- field axis perpendicular to the substrate.

Next Steps – THz QCL micro- machined blocks Integrate THz QCL with Schottky diode for direct detection: Optimising the power-coupling from the QCL into the waveguide Test efficiency of power coupling in micro-machined waveguides, to start determining absolute cw power requirements. Integrate THz QCL with free-space feedhorn: Enabling characterisation of the metal micro-machined waveguide, and associated modelling. Optimise heat sinking of 4.7 THz QCLs within micro-machined blocks: Replicate in CW as closely as possible the pulsed performance.

Summary THz QCLs fabricated close to the required 3.5 THz and 4.7 THz lines Output powers (0.41 mW/0.22 mW) and cut-off temperatures (86K/60 K) achieved in cw mode. Waveguide block concept tested No change in threshold current/spectrum/operating temperature Significant improvement in beam pattern Optimisation of mode coupling to be undertaken

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Funding (with thanks) ESA In-orbit Demonstration Study Programme (SSTL PI): Science refinement; Payload concept definition; Spacecraft concept definition; Mission plan and cost estimate. NERC Critical Component Development (RAL PI): QCL development and waveguide demonstration; THz Schottky diode development; Integrated QCL & Schottky proof of concept. CEOI-ST Critical Payload Development (Leeds PI): 1.1 THz (Band 3) full development including: o Mixer, LO and spectrometer; QCL frequency stabilisation.

The LOCUS Team Members Thank you for your attention

So what is LOCUS? A breakthrough concept multi-terahertz remote sounder Compact payload to be flown on a ‘standard’ small satellite that will: Measure key species in the upper atmosphere, i.e. the mesosphere and lower thermosphere (MLT); Increase understanding of natural and anthropogenic effect on climate change; Allow study of the ‘gateway’ between the Earth’s atmosphere and near space environment. Small satellite sounder from LEO

Why use terahertz sounding? Terahertz (THz) frequencies (sub-mm-waves) penetrate dielectric media opaque at most other shorter wavelengths; Can detect and characterize molecular species through obscurants and located in a relatively low temperature environment; Offers higher spatial resolution than microwave/mm-wave range; Allows remote sounding of atmospheric constituents related to climate change on a local or global basis; Same technique provides information on the interstellar media, e.g. regions of star formation.

What science will be investigated? But, this requires terahertz instrumentation: LOCUS science achieved through: Tracing O, OH, NO, CO, O 3, H 2 O, HO 2, O 2 spectral emission signatures globally and from low Earth orbit (LEO); Using a limb sounding technique with cold space as a background to achieve height distribution; Provision of ultra-high spectral resolution (1MHz); Accurate spatial sampling with ~2km footprint at tangent heights from ~ 55km to 150km.

And therein lies one challenge… M. Tonouchi, Nature Photonics, 1, 97 (2007) IMPATT – Impact Ionization Avalanche Transit-Time diode HG – Harmonic Generation RTD – Resonant-Tunnelling Diode TPO – THz Parametric Oscillator PCS – Photoconductive Switch QCL – Quantum Cascade Laser

LOCUS Instrumentation To be based on heterodyne receiver technology, which converts THz input signal to a lower intermediate frequency (IF) – typically GHz; Provides low noise and high spectral resolving power - order >>10 4 ; Key front-end components are the mixer and THz local oscillator (LO).

The heterodyne approach Optimum system performance requires: o Efficient signal frequency translation, i.e. low conversion loss; o Minimal added system electrical noise; o Provision of adequate THz LO source power. e.g. RAL & STAR Dundee 350GHz Heterodyne Radiometer (previous CEOI funding)

The LOCUS payload concept System schematic

Key features: Highly integrated multi-channel THz radiometer system; Four separate bands identified that accommodate the required spectral windows; Schottky semiconductor diode mixer technology; Quantum Cascade Laser used a LOs for 1 and 2, harmonic up-conversion for 3 and 4; Fast Fourier Transform digital spectrometers provide 1MHz spectral resolution; Single primary ~ 40cm diameter and miniature coolers – 100K operational goal; UK sourced technology with critical elements support by the CEOI and NERC.

Electronic behaviour Peak performance corresponds to efficient injection of current Device dimensions are typically 1 mm × 150 μm × 10 μm 2 QCLs

Quantum Cascade Lasers (2) 1 W peak power is possible, and 10s of mW continuous- wave power, but cooled; QCLs have an intrinsically narrow linewidth (<20 kHz); Precise frequencies can be defined using periodic gratings defined into the ridge waveguides; Operation has been demonstrated over the 1 – 5 THz frequencies; Radiation hardness has been demonstrated. 2% duty cycle; ;asing up to 1.01 W (peak) at ~ 3.4 THz; > 400 µW at 77 K, T max = 118 K: L. Li et al, Electronics Letters 50, 309 (2014).

LOCUS Core Technology QCL Local Oscillator University of Leeds Schottky Barrier Diode & Space Coolers RAL Digital Spectrometer STAR-Dundee Small Satellite Surrey Satellites Lt d UK also leading LOCUS science definition via Leeds, UCL and RAL

In-orbit demonstration – satellite concept Objective: Prove core payload and platform technology in space: Polar sun synchronous orbit; Perform global species measurement; Novel approach to scene scanning via spacecraft nodding; Cold-space view and on-board c300K target provide payload calibration; Approx. total spacecraft volume, mass & power: 1m 3, 150kg, 70W. o Compare with NASA 43m 3, 3tonne, 4kW & MLS: ~8m 3, 500kg, 550W); IOD mission lifetime ~ 2 years, tbc.

Mission Concept Development Plan

Funding (with thanks) ESA In-orbit Demonstration Study Programme (SSTL PI): Science refinement; Payload concept definition; Spacecraft concept definition; Mission plan and cost estimate. NERC Critical Component Development (RAL PI): QCL development and waveguide demonstration; THz Schottky diode development; Integrated QCL & Schottky proof of concept. CEOI-ST Critical Payload Development (Leeds PI): 1.1 THz (Band 3) full development including: o Mixer, LO and spectrometer; QCL frequency stabilisation.

Summary THz remote sounding provides important information in relation to the Earth’s climate evolution and its monitoring; The THz detection method depends upon the nature of the defined science return; Where the science requires high spectral resolution and high sensitivity, THz heterodyne detection is the instrumentation of choice; In the 1 to 5 THz frequency range, novel heterodyne instrumentation is being conceived and developed that will allow novel scientific study; A UK initiated and presently majority UK funded instrument, LOCUS, is being developed to study the relatively unexplored supra-THz spectral range.

The LOCUS Team Members Thank you for your attention

Difficulties in THz QCLs THz QCLs have smaller laser subbands energy spacing (~meV), Strong non-radiative transitions make the realization of population inversion more difficult. Accurate carrier injection to upper laser levels is more difficult Inj.Upp. Low. J inj J par

B-to-C THz QCLs Efficient carrier injection into the upper state via resonant tunneling. Fast depopulation of the lower state via elastic scattering. The maximum output peak power at 10K is 9.65 mW. The current density at 10 K is 123A/cm 2.

Resonant-phonon THz QCLs A broad tunneling resonance of lower laser state 3 and extraction state 4. Strong LO phonon scattering from extraction state 4 to injection state 1’. The maximum output peak power at 10K is 15.3 mW. The current density at 10 K is 782A/cm 2.

Hybrid (B-to-C + RP) THz QCLs Narrow miniband leads to small parasitic current. Strong LO phonon scattering can reduce the thermal backfilling. The maximum output peak power at 10K is 27 mW. The current density at 10 K is 196 A/cm 2.

Summary of different designs StructureWafer IDT max [K]J th [Acm ‑ 2 ]P max [mW] A (3.1 THz BTC)L B (3.1 THz RP)L C (3.1 THz Hybrid)L Results are shown for devices with a single-metal gold waveguide, under 2% duty cycle, pulsed operation. The hybrid design yields high output power low current density, suitable for high power emission. The RP structure has high Joule heating, which will limit the performance. The BTC structures have low maximum output power.

L1142, 3.1 THz design – narrower ridges CW characterisation of device 1 (DM)

Layers: 3.5/9.0/0.6/16.3/0.9/16.0/1.0/13.8/1.2/12.0/1.5/11.0/2.4/11.0/3.2/12.1 nm Lasing frequency is 3.41 THz, and the maximum CW lasing temperature is 80 K. The maximum output power at 10 K is 0.12 mW with dissipated power of 1.79 W. Device: 1000 μm × 125 μm × 14 μm, Au-Au waveguide L1152, 3.5 THz, B-to-C design CW results of device 3 (DM)

And therein lies one challenge… M. Tonouchi, Nature Photonics, 1, 97 (2007) IMPATT – Impact Ionization Avalanche Transit-Time diode HG – Harmonic Generation RTD – Resonant-Tunnelling Diode TPO – THz Parametric Oscillator PCS – Photoconductive Switch QCL – Quantum Cascade Laser

THz QCL waveguides

Requirements for the QCLs as LO Local oscillators in terahertz heterodyne spectrometers have to be operated in continuous-wave mode; precisely defined target frequencies: 3.5 THz, 4.7 THz; For airborne instruments, operating temperature: as high as possible; output power: 50 K; dissipated power: <5 W; compact and integrated.