School of somethingFACULTY OF OTHER

T-rays in real life: development of THz lasers and applications at University of Leeds

Dragan Indjin

Institute of Microwaves and Photonics,

School of Electronic & Electrical Engineering,

University of Leeds, UK

About Leeds and University…

“LEEDS …the UK’s favourite destination”Good Britain Guide

• Lively & dynamic city!

• Commercial and Legal Centre

• Shopping Facilities

• Theatre and Music

• Sporting City

• Excellent Location

Leeds has…

• Simple transport links to other major cities – London 2.5 hours

• Attractive tourist destinations innearby countryside

• Low cost air travel to European destinations (Jet2, RyanAir,…)

• Population: ~400K (~750K with surrounding area)

• So much to see in a small space!

• An international university

• Established in 1904 (from 1874 beginnings)

• Quality in teaching & research

• 2nd largest university in UK

• Modern & innovative

• Very popular with UK students.

2nd highest number UK applications

• Friendly & welcoming

World Class

• 30,500 students

• 6,000 postgraduates

• 5000 students from outside UK

• Over 100 nationalities

• 8,800 staff

• 2,900 research, academic & teaching

• 1,500 academic related admin

• 500 technical staff

Students and Staff

Faculties

• Arts (English, History, Modern Languages, Humanities)

• Biological Sciences

• Business (including economics)

• ESSL (Education, Sociology & Social Policy, Politics & International Studies, Law)

• Engineering (Civil, Mechanical, Electrical, Process and Materials, Energy, Computing)

• Maths and Physical Sciences (including chemistry, colour chemistry, food science)

• Environment (Earth Sciences, Environment, Geography)

• Medicine & Heath (Clinical Med and Dentistry, Health Management & Policy Development (Nuffield), Nursing, Psychology)

• Performance, Visual Arts and Communications (Communication Studies, Fine Art, Design, Music, Performance etc)

Research Excellence

• 14th in Times Higher Education’s League Table of research quality

• 8th in UK ‘Research Power’ league

• More than half of schools or subjects ranked in top 10 in latest RAE rankings (2008)

• Research income > 100 million GBP

• Interdisciplinary and emerging fields encouraged

• 61% of Leeds research rated ‘World Leading / Internationally Excellent’with further 31% ‘recognised internationally’

Academic Facilities• All located on single vibrant city centre campus

• Outstanding laboratories

• Library 4th largest research library in the UK

– 2.8+ million items

– On-line access on and off campus

– Special collections

• Computing– 24 hr clusters

– wireless across campus

– Internet in all student rooms

Outstanding Opportunities

Building Futures

• £359m Campus Development Plan

• Sports facilities

• Accommodation/Conference Facilities

• Academic schools/faculties

• Enterprise and Innovation

• £860m economic contribution to the city and region

• 180,000 alumni in 130 countries

Dynamic Vision

“Leeds is a research intensive University which strives to create, advance and disseminate knowledge and develop outstanding graduates and scholars to make a major impact upon global society.”

Key Strategy Themes:

• Enhance our International Performance

and Standing

• Achieve an Influential world-leading

research profile

• Inspire our students to develop their

full potential

• Enhance enterprise and knowledge transfer

Development Strategy

FACULTY OF ENGINEERING

The Faculty of Engineering

FACULTY OF ENGINEERING

• Encourage, reward and promote excellence to achieve and sustain international standing in higher education teaching, learning and research

• Investment in staff and facilities to attract and retain high quality staff and to create impressive space for staff and students.

• Established centres of research excellence from which we grow and develop learning, teaching and world-class research

• Commitment to providing an excellent experience for all our students.

Our Strategy for Excellence

FACULTY OF ENGINEERING

• One of UK’s top Engineering and Computing Faculties, and one of the largest with approximately 700 staff and 3,000 students

• Top ratings for research. 2008 Research Assessment Exercise (RAE), Faculty is 7th in the UK with 75% of research activity rated ‘internationally excellent’ or ‘World leading’.

• Strong links with industry

• First class facilities – millions of pounds invested in past two years. Investments include:

− New Undergraduate teaching laboratory with state of the art wireless communications and microwave circuit facilities provided by Agilent Technologies

− Refurbished ‘flexible study area for both private and group study

− Refurbished coffee bars and other social/leisure facilities.

FACULTY OF ENGINEERING

Cleanroom, Electronic and Electrical Engineering

Hip Simulators, Mechanical Engineering

Oscillatory Mixer/Reactor, Process Engineering

FACULTY OF ENGINEERING

Engineering Study Zone

Powerwall, Computing

Merlin Flight Simulator

FACULTY OF ENGINEERING

UG

PGR

PGT

UK

EU

International

UK

Over 3,000 students in 5 schools:

Civil Engineering

Computing

Electronic and Electrical Engineering

Mechanical Engineering

Process, Environmental & Materials Engineering

FACULTY OF ENGINEERING

UG

UK

UK

There is a good mix of students from 80 countries

Undergraduate Postgraduate Total

United Kingdom 1627 65 1692

International 674 345 1019

FACULTY OF ENGINEERING

Undergraduate Study

Faculty of Engineering

FACULTY OF ENGINEERINGUndergraduate Degrees

ComputingArtificial IntelligenceComputingComputer ScienceComputer Science with MathsInformation Technology

Civil EngineeringArchitectural EngineeringCivil & Environmental EngineeringCivil & Structural EngineeringCivil Engineering with Construction Management

Mechanical EngineeringAeronautical & Aerospace EngineeringAutomotive EngineeringMechanical Engineering Medical EngineeringMechanical with Nuclear Engineering

Electronic & Electrical EngineeringElectronic & Communications Electronic & ComputerElectronic & Electrical EngineeringElectronic EngineeringElectronics and NanotechnologyDigital Media EngineeringMusic Multimedia & Electronics

Chemical, Environmental, Energy and Materials EngineeringChemical Engineering (and Energy, Materials, Minerals, Nuclear, Pharmaceutical Engineering)Energy EngineeringMaterials Science and EngineeringPetroleum Engineering

Product Design

FACULTY OF ENGINEERING

MEng/BEng Majority of engineering degrees are integrated masters (MEng/BEng)

Options:

1. Graduate after 4 years with MEng

2. Leave after 3 years with BEng then take MSc or go directly into employment

Option 2 may be more attractive to international students

Microsoft Word 2007.lnk

1 2 3 4

Graduate with BEng Graduate with MEng

FACULTY OF ENGINEERING

Postgraduate Taught

FACULTY OF ENGINEERINGPostgraduate Taught Courses

Civil Engineering Engineering Project ManagementEnvironmental Engineering & Project ManagementInternational Construction Management & EngineeringStructural Engineering

ComputingArtificial IntelligenceComputing and ManagementAdvanced Computer Science

Computational Fluid Dynamics

Electronic & Electronic EngineeringBroadband Wireless and Optical Communications EngineeringElectrical Engineering & Renewable Energy SystemsEmbedded Systems EngineeringModern Digital & Radio Frequency Wireless CommunicationNanotechnology and Advanced Electronic DevicesRF & Microwave Design for Wireless Systems

Mechanical EngineeringAdvanced Mechanical EngineeringAutomotive EngineeringOilfield Corrosion EngineeringMedical EngineeringProcess EngineeringChemical EngineeringPharmaceutical Science and Engineering Environmental Engineering

Energy and EnvironmentFire and Explosion Engineering

FACULTY OF ENGINEERING

Research

Faculty of EngineeringResearch Assessment Exercise 2008

The headlines…• Civil: GPA=2.60 (10.0, 45.0, 40.0, 5.0, 0.0)

Rank=15/23 FTE=22.2• Computing: GPA=3.05 (25.0, 55.0, 20.0, 0.0, 0.0)

Rank=10/81 FTE=32.5• Electronic GPA=3.05 (30.0, 50.0, 15.0, 5.0, 0.0)

& Electrical Rank=1/34 FTE=22.0• Mechanical: GPA=2.90 (20.0, 55.0, 20.0, 5.0, 0.0)

Rank=7/33 FTE=40.5• SPEME: GPA=3.00 (20.0, 60.0, 20.0, 0.0, 0.0)

Rank=3/52 FTE=48.0

Faculty of EngineeringPublications

Thomson Reuters/ISI Web of Science 6th September 2010

30 academic staff, 30 PDRAs, 60 PhD students, 150 undergraduates.

Head of School: Professor Paul Harrison

RAE 2001 – one of only five UK universities with the maximum 5* award for research in electronic engineering.

RAE 2008 – recognized as the top electronic engineering department in UK.

Research is organized into three groups:

• Institute of Microwaves and Photonics (IMP) – UoL Golden peak of excellence

• Institute of Integrated Information Systems (I3S)

• Control and Power Applications Group

Over last RAE period we published 544 journal and 658 international conference papers; 31 patents; awarded 82 PhD degrees; and had total research expenditureof £13.9M.

School of Electronic and Electrical Engineering

Terahertz TechnologyINSTITUTE OF MICROWAVES AND PHOTONICS

THzregion

New imaging and spectroscopy technique across the physical, medical and biological sciences, eg pharma, security etc. Potential for fundamental investigations of condensed matter systems and nanotechnology. •Fundamental high-frequency investigations of nanostructures(EPSRC).

•On-chip THz microscopy (Agilent funding, and EPSRC Follow-on fund).

•Quantum cascade laser imaging system for security sensing (MOD funding, also HMGCC, DSTL, HOSDB, and New TeraTech funding).

•1.55 µm fibre laser THz generation (EPSRC ‘Portrait’, T-Ray, CIP, and Follow-on fund).

•Strong track record in EC Framework V, VI,VII programmes in this area.

• Significant theoretical activity

x-rayuvnear-IRmid-IR

microwaveradio

Largest terahertz laboratory in Europe/Asia; 160 m2, £3M facility.

Microwave Theory and MeasurementsINSTITUTE OF MICROWAVES AND PHOTONICS

We are the leading international group in microwave filters.Filtronic spun out from School We have demonstrated new network synthesis techniques for fixed frequency, tuneable, and self-adaptive microwave filters. (And their physical realizations).These are being exploited by BAe Systems, the MOD, ESA, etc. School research found in ALL mobile phones.

We have three well-equipped microwave laboratories:• Microwave circuits and systems;• Microwave measurements;• Microwave opto-electronics.

New laboratory completed in 2007.

Our new £1M MBE facility allows us to engineer opto-electronic devices with atomic-scale resolution.

III-V GaAs-AlGaAs materials system, incl InAs dots/rods.

We supply samples nationally and internationally(eg Harvard, Paris), with significant international collaboration and output.

Molecular Beam EpitaxyINSTITUTE OF MICROWAVES AND PHOTONICS

The School has a £2.5M state-of-the-art class 100 cleanroom, with photo- and electron-beam lithography. Multi-partner strategy to upgrade electron-beam this year.

Cleanroom used by 40 researchers across university.

Industrial projects undertaken.

BioelectronicsINSTITUTE OF MICROWAVES AND PHOTONICS

Integrating biological systems (proteins etc) with underlying electronic devices.Create hybrid molecular-electronic devices and sensors eg medical diagnosis.Use of molecular recognition to assemble nanoscopic systems on nanoscale.

Comprehensive bioelectronics suite, houses state-of-the-art molecular biology facilities. Cross-disciplinary collaboration across University and UK.

• Generic interfacing technology; directed assembly of metallic wires via proteins and viruses; soft-condensed matter (bilayer) systems (EPSRC Basic Technology).• Label-free protein chip (Wellcome, Yorkshire Concept, Abbott, SPD etc). Prize.

Microelectrodes coatedwith probe molecules

Electronic, label-free, detection of selective disease biomarkers

patent #1

patent #2

QCLs: design, fabrication & characterisation

Modelling and design personnel • P. Harrison, R. W. Kelsall, Z. Ikonic, C. A. Evans, N. Vukmirovic*, A.

Valavanis, L. Lever, G. Beji, N. Prodanovic, D. Indjin

• (cluster computer, *collaboration)

Experimental teamA. G. Davies, E. H Linfield, P. Steenson, S. Khanna, P. Dean, L. Li,…(160m2 clean room, THz Lab, MBE system, …)

Principles of QCLs

Interband laser• electron-hole recombination• wavelength is determined by material

bandgap

Intersubband (QC) laser• unipolar• electronic cascades• efficient injection into

upper laser level • fast extraction from

lower laser level• emission wavelength depends

on the designed layer thicknesses

Strategy

• Detailed electro-thermal modelling of QCL active regions and waveguides, including the study of new material systems;

• Extremely close theory/design-experiment interaction; experimental studies, based on a new MBE system, and a semiconductor cleanroom for sample processing (including electron and focused ion beam lithography);

• Experimental spectroscopy facilities, including a Bruker FTIR spectroscopy system and a (1.2 – 300) K optical access cryostat,

incorporating an 8 T superconducting magnet;

• Applications (particulary for THz)

• Strong national and international collboration

Rate-equation model of transport

( ) ( ), , , , , ,1 1 1 1 1

0N N M N N

fi i f f f i i i f kN i kN f f f i kN f kN i

i i k i i

dnnW n W n W W n W W

dt + + + += = = = =

⎡ ⎤= = − + + − +⎢ ⎥⎣ ⎦∑ ∑ ∑ ∑ ∑

+ the particle conservation law + Energy balance equation 1

N

i Si

n N=

=∑

CONTINUUM

D

fΨ

f N−ΨCentral period • System of non-

linear scattering rate equations

• Assume periodicity • ‘‘tight-binding’’

approximation • Energy balance

equation

Output characteristics

( ), ,1 1 1

N M N

i i f kN i kN fi k f

J q n k W W+ += = =

= −∑ ∑ ∑

• Current density is found as a sum of all scattering passing through a reference plane.

• Inter-period and cross-period current components included.

CONTINUUM

D

iΨi N−Ψ

ni

Reference plane

Central period

BOUND STATES

S2 simulation• Schrödinger equation

• Poisson equation

• Solve rate equations/balance

• Calculate scattering rates

Densities converge?

Band profile converges

no

yes

no

• Self-Self-consistent non-equilibrium electron distribution

yes

• Schrödinger equation solved in effective mass approximation

• Relevant scatterings taken into account:

– Electron-electron– Electron-LO phonon– Electron-ionised

impurities– Interface roughness

Inne

r sel

f-con

sist

ent l

oop

Out

er s

elf-c

onsi

sten

t loo

p

Inclusion of coherent transport

• Full density-matrix model • Application on modelling of THz QCL structures in

external magnetic field • Parasitic spikes in pure scattering transport I-V

curves eliminated• Better predictive performances/agreement with

measurements• CPU intensive for optimisation

CW analysis

Thermal modelling of surface-emitting THz QCL

Temperature measured in apertures using microprobe band-to-band PL technique

Temperature in central aperture plotted as a function of power -> extract thermal resistance of 26.2 K/W

What about the growth?What about the growth?

Typical THz QCL growths involve:

• Flux measurements;

• GaAs oscillations calibration (1 hour);

• AlAs oscillations (4 – 5 hours);

• QCL growth overnight (10 – 15 hours);

• AlAs oscillations (4 – 5 hours) – checking for drifts;

• GaAs oscillations (1 hours) – checking for drifts;

• Flux measurements.

A high operation temperature THz QCLA high operation temperature THz QCL

A tunable THz QCL based on heterogeneous active region

Design and realisation of THz QCLs with heterogeneous active region –an idea transferred after successful realisation of GaAs-based mid-infrared (10-15µm) structure

• Attempt to modelling of heterogeneous cascade QCL

• 10 µm active region is changed systematically during the growth

• Growth rate was successively reduced from +6% to −4% of its nominal 1 µm/hr operating value.

• A gradual change of quantum-well layer thickness in conjunction with an increase of Al mole fraction in barrier layers

• To obtain a ~10% growth rate variation in L243, the active region was divided into 23 sets each of 10 repeated periods.

• Emission spectrum which can be tuned electrically – a necessary requirement for exploiting the THz spectrum.

• The single-plasmon waveguide is implemented as the high output powers and improved beam profile will be more appropriate for imaging applications.

0 200 400 600 800 1000 1200 1400 1600 18000

5

10

15

20

0

2

4

6

8

10

12

14

16

18

20

Vo

ltage

(V)

Current density (A/cm2)

4K 10K 20K 30K 40K 50K 60K 70K

Pow

er (m

W)

800 1000 1200 1400 1600 18000

5

10

Pow

er (μ

W)

Current density (A/cm2)

79K 80K 81K

0 200 400 600 800 1000 1200 1400 1600 18000

5

10

15

20

0

2

4

6

8

10

12

14

16

18

20

Vo

ltage

(V)

Current density (A/cm2)

4K 10K 20K 30K 40K 50K 60K 70K 80K 90K

Pow

er (m

W)

1000 1100 1200 1300 14000

5

10

15

20

25

30

35

Pow

er (μ

W)

Current density (A/cm2)

99K 100K

Device: 140µm x 1.603mmTmax : 81KPmax : ~ 8 mW (uncorrected for

the mirror and window losses)

Device: 140µm x 1.516mmTmax : 100KPmax : ~ 12 mW (uncorrected for

the mirror and window losses)

Electrical characterisation – pulsed operation

(homogeneous growth) (heterogeneous growth)

3.0 3.1 3.2 3.3 3.4 3.5

Inte

nsity

(a.u

)Frequency (THz)

18.2 kV/cm

18.0 kV/cm

17.4 kV/cm

17.0 kV/cm

16.4 kV/cm

15.8 kV/cm

15.4 kV/cm

15.1 kV/cm

14.4 kV/cm

3.0 3.1 3.2 3.3 3.4 3.5

18.0 kV/cm

17.2 kV/cm

17.0 kV/cm

16.6 kV/cm

15.8 kV/cm

15.4 kV/cm

15.2 kV/cm

14.8 kV/cm

Inte

nsity

(a.u

)

Frequency (THz)

14.4 kV/cm

device: 140µm x 1.603mmdevice: 140µm x 1.516mm

Measured ∆f = 0.02525 THzCalculated ∆f = 0.0282 THz

Measured ∆f = 0.02 and 0.05 THzCalculated ∆f = 0.0264 THz

Spectra

Both L234 and L243 exhibit multi-mode emission. The typical mode spacing in L234 was 0.025 THz inagreement with that expected (0.028 THz) for a 1.5 mm long Fabry-Perot cavity assuming a refractive index of 3.54. In contrast for L243 the measured mode spacing was 0.05 THz as compared to that expected (0.026 THz) for a 1.6 mm long Fabry-Perot cavity. This suggests that the measured spectra resulted from different sections of the structure lasing at different fields, rather than it represent different longitudinal lasing modes.

(homogeneous growth – L234) (heterogeneous growth –L243)

Design: layer thicknesses

I

V1 V2 Vn-1 Vn

• The stack of dissimilar active regions can be thought of a number of non-linear resistors in series• Same current through each set of active regions• Each has its own I-V curve so means that each stack will have different field for given current•Nonlinear system of equations

Modelling: I-V characteristics

• Redistribution of the carriers across different stacks is expected to keep current constant

Tuneability

2.5 3 3.5 4 4.5Frequency [THz]

-100

-50

0

50

100

150

200

Gai

n [c

m-1

]

T = 80 K

12.5 V

13 V13.5 V

14 V

Calculated tuneability (red) and measured tuneability (blue)

The envelope gain spectra of the whole device at 80K plotted for values of external applied bias at which lasing is expected.

Tuneability

2.85-3.50 THz - calculated

3.07-3.45THz - measured

Applications: Diffuse and reflection imaging with Applications: Diffuse and reflection imaging with QCLsQCLs

• Diffuse reflection measured with bolometer (D2), specularreflection (D1) with pyroelectric detector. 2.8 THz QCL used as a source.

Exemplar image (5p coin concealed in envelope)Exemplar image (5p coin concealed in envelope)

• Diffuse reflection (b) is less sensitive to precise alignment compared with specular reflection (a), making it more suitable for stand-off detection.

Diffuse imaging of powdersDiffuse imaging of powders

• Diffuse imaging of powders (in perspex box) is sensitive not only to THz reflection/absorption, but also particle size (particle diameters in µm shown).

DualDual--frequency imaging using an electrically tunable frequency imaging using an electrically tunable terahertz quantum cascade laserterahertz quantum cascade laser

-Dual-frequency imaging of polycrystalline samples using an electrically tunable THz QCL

-Obtained images at 3.05 THz and 3.24 THz in a single raster scan of a sample.

Transmission images of a pellet containing 26-vol % PETN taken at (a) 3.05 THz and (b) 3.24 THz. Both images are normalised to the incident power. Difference transmission T3.24–T3.05 image of (c) the PETN pellet and (d) a pure lactose monohydrate pellet. Images (a) and (b), and (c) and (d), are shown on the same colour scales.

Example: International Collaboration with Belgrade

Funded grants:- Royal Society Collaborative grant 2002-2004 (Leeds-Belgrade)- EPSRC Grant 2004-2005 (Leeds-Belgrade)- NATO Collaborative Research Grant 2008-2010 (Belgrade-Bari-Leeds) - Number of PhD ORSAS (UK) awards (Belgrade ->Leeds)

Ongoing applications:- NATO Science for Peace Project (Leeds-Belgrade-Delft-Moscow-Nhizny

Novgorod)- EC Framework 7 (Leeds-Belgrade-Wurzburg-Paris-Bari)

Example: International Collaboration with Belgrade 2 –FP7

Figure: Thethree objectives for Terahertz at room temperature.

TeraArt : Terahertz at room temperature - FP7

Example: International Collaboration with Belgrade 2 –NATO SfP

Terahertz QCL based spectrometer for rapid detection of chemicalagents and explosives – NATO SfP

NPDDragan Indjin

University of Leeds

PPD Vladimir Vaks

IPM

Jian-Rong GaoJelenaRadovanovićUniversity of

Belgrade

Ekaterina Orlova Gregory Gol’tsman

Example: International Collaboration with Belgrade 3 – PhD students training

Dr Vladimir Jovanovic

PhD Leeds 2002-2005

-Lehman Brothers Bank,UK

-Nomura Bank,UK

20 (23)

Dr Ivana Savic

PhD Leeds 2003-2006

-Grenoble, France

-Univ Davies CA, USA

13(17)

Dr Nenad Vukmirovic

PhD Leeds 2004-2007

-Berkley, USA

22 (34)

Goran Isic

PhD Leeds 2007-

8

Nikola Prodanovic

PhD Leeds 2010-

2

Acknowledgments

EPSRC (UK)

ORSAS (UK)

EC Framework

British Council

Royal Society

NATO