Radiation Sources for Spectroscopy and Imaging in the Submillimeter/Terahertz

Frank C. De LuciaOhio State University

Advisory Group on Electron Devices

February 28, 2007

Arlington, VA

Terms of Reference

1. Assess state of R&D for compact SUBMM sources

2. Review DoD/Government needs and applications

3. Identify technical and operational limits for SUBMM source technology

4. Review foreign activities and programs

5. Determine commercial involvement in source technology

6. Identify opportunities in device design, fabrication and supporting technologies with potential for breakthroughs

7. Assess novel and hybrid approaches for THz generation/amplification

8. Create a THz source technology development roadmap

9. Understand the Signature Science of the targets of interest - The 2nd Gap in the Electromagnetic Spectrum

=>This 2nd gap negatively impacts our ability to develop APPROPRIATE technology

Attributes of the THzTechnology:

The region is very quiet and very sensitive detectors are possible

(Sources are very bright: 1 mW in 100 Hz corresponds to a temperature of 1018 K

Phenomenology/Signatures:

Penetration of dielectric materials (decreases rapidly with frequency - scatter and absorption)

Low pressure gases have strong and unique rotational signatures

Complex solids have low lying vibrational states in the THz, but these are much less studied and characterized

Active and Passive Images are complex and different from those in other spectral regions

ApplicationsEstablished Scientific Applications Clear Paths to Public Applications Widely Discussed Public Applications

_________________________________________________________________________________________________

Astronomy Imaging Through Obstructions Remote Explosive Detection

Atmospheric Science Dust, Clothing Remote Detection of Gases

Laboratory Spectroscopy Point Gas Sensors T-Ray Medical Imaging

Plasma Diagnostics Spectroscopic Imaging of Cancer

Physical Chemistry Imaging Through Obstructions

Walls

Remote Detection of Bio

Signature Science and Appropriate

Figures of Merit => Quantitative

end-to-end designs

Spectral Width/Frequency ReferenceAs a Basis for a Discussion of Matching and Developing Appropriate

Source Technology with Applications of Interest

<< 1 MHzFundamental Oscillators/Amplifiers (BWOs,TWTs, GUNNs, Klystrons)

Harmonic Generation

OPFIR

Femtosecond Demodulation

1 - 100 MHzQuantum Cascade Lasers

Cw/Mode Locked Laser Driven Photomixers

a few GHzPulsed Laser Driven Mixing

Broadband (resolution via FT detection)FTFIR

THz-TDS

> x 106

SMM/THz phenomena have a larger range of spectral widths (> x109)

We need appropriate Figures of Merit for sources, detectors, and systems

Solid-State THz Sources (CW)

0.001

0.01

0.1

1

10

100

1000

10000

10 100 1,000 10,000 100,000

Frequency (GHz)

Po

wer

(mW

)

1022

1021

1020

1019

1018

1017

1016

1015

100

Hz

- A

ctiv

e Im

ager

Source/Target Bandwidth Limited Brightness (K)

1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency.

2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz.

3. A typical heterodyne receiver will have a noise temperature of 3000 K

1 mW

Solid-State THz Sources (CW)

0.001

0.01

0.1

1

10

100

1000

10000

10 100 1,000 10,000 100,000

Frequency (GHz)

Po

wer

(mW

)

Source/Target Bandwidth Limited Brightness (K)

1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency.

2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz.

3. A typical heterodyne receiver will have a noise temperature of 3000 K

1 M

Hz

- S

pec

tro

sco

pic

Lin

e 1018

1017

1016

1015

1014

1013

1012

1011

1 mW

Solid-State THz Sources (CW)

0.001

0.01

0.1

1

10

100

1000

10000

10 100 1,000 10,000 100,000

Frequency (GHz)

Po

wer

(mW

)

Source/Target Bandwidth Limited Brightness (K)

1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency.

2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz.

3. A typical heterodyne receiver will have a noise temperature of 3000 K

10 G

Hz

- A

tmo

sph

eric

Lin

e

1014

1013

1012

1011

1010

109

108

107

1 mW

Solid-State THz Sources (CW)

0.001

0.01

0.1

1

10

100

1000

10000

10 100 1,000 10,000 100,000

Frequency (GHz)

Po

wer

(mW

)

Source/Target Bandwidth Limited Brightness (K)

1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency.

2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz.

3. A typical heterodyne receiver will have a noise temperature of 3000 K

100

GH

z -

So

lid

Res

on

ance

1013

1012

1011

1010

109

108

107

106

1 mW

Two SMM/THz Legacy ‘Public’ Applications -- Clear, but Challenging Paths to Success --

IMAGING ANALYTICAL CHEMISTRY

Engineering Progress and Signature Science R & D will Impact the Breadth of Applicability

Why is there a ‘Clear Path’ to Public Analytical Chemistry?

Signatures: A well understood spectroscopic signature science foundation is in place

False Alarms: False alarm rates in complex environments have been studied and can be shown to be low because of the number of resolution elements and ‘complex redundancy’ of molecular fingerprints

Clutter: Background clutter/interference at trace levels have been studied and can be shown to be low

Appropriate Technology Developed: Compact, high resolution solid state sources based on diode harmonic generation technology have been developed

Potential for Low Cost: Rapid expansion of wireless communication technology to higher frequency is rapidly reducing the cost of the power amplifiers to drive this diode harmonic generation technology

Sensor System Figures of Merit Sensitivity - ‘Dynamic Range’ is widely abused

1. Only source power in the signature bandwidth (Brightness - W/Hz) is useful

- the rest often causes additional noise (a fundamental limit for FTFIR)

2. Noise and dynamic range example:

- 1 mW in a 100 Hz bandwidth, 3000K noise temperature =>dynamic range of >140 db

- in ideal noise limited spectrometer, the minimum detectable absorption with 1 second of integration time is only - 90 db

Psys noise PthermalPcarrier

This is good for the imager because the bandwidth of the receiver can be matched to the source and frame rate of the imager

But people who build spectrometers should never discuss dynamic range because the detection of a small amount of power in a narrow bandwidth is fundamentally different than the detection of a small change in a large amount of power.

Solid-State THz Sources (CW)

0.001

0.01

0.1

1

10

100

1000

10000

10 100 1,000 10,000 100,000

Frequency (GHz)

Po

wer

(mW

)

Source Brightness vs System Noise (K)

1. To keep graph simple an integration time (1 microsecond) that corresponds to the spectral linewidth is used at 1 mW this provides a S/N of ~106. In a more optimized system, an integration time of ~ 1 second might be used, and a S/N of ~109 results.

2. Unless the noise temperature of the receiver is higher than the system noise (which results from the addition of the thermal noise voltage to the carrier signal), it is not important.

1 M

Hz

- S

pec

tro

sco

pic

Lin

e 1018

1017

1016

1015

1014

1013

1012

1011

1.7 x 1010

1.7 x 109

1.7 x 108

1.7 x 107

Source Brightness (K) System Noise (K)

1 mW

Why is there a ‘Clear Path’ to Public Imaging?Heritage: Many special purpose, single pixel, imagers have been built over the last 40+ years

Detectors:

- scientifically we understand

- in single element receivers we can approach well understood fundamental limits

Transmit power:

- acceptable solid state sources for some applications exist

Propagation:

-overall absorption generally known

-impact of fluctuations noise less clear

Signatures/targets/clutter:

-nature of active images complex, but large contrast in images provides opportunities

-strategies to minimize impact of obscuration needed

Practicality:

Where can we get to on sensitivity-speed-size-cost tradeoff in a FPA?

These are not show stoppers, but the answers will determine the Breath of Application

640 GHz

But no extra time/power required

Angular Diversity

To Average Away Speckle: Move Imaging Mirror by its Diameter (Independent of distance)

sp ~D / 2

N 2 /sp2 ~ 8 2

D2~ 1000

D

ssD

path ssD

Some TIFT illumination scheme are multimode and do this automatically with a very large number (10000?) of modes

Specular Reflection:

Number of Illumination angles to insure that one is normal:

Modes and Angles:Active and Passive Imaging in the THz

For a single mode, 100 Hz bandwidth, 300 K, the thermal power/noise is ~4 x 10 -19 W

1 mW in 100 Hz corresponds to a noise temperature of ~1018 K A reasonable receiver noise temperature is 3000 K

For diffuse target, the number of return modes is NAD = (spot size/wavelength)2 ~ 100 (our system in portrait mode)For a specular target, the number of return modes is 1

Floodlight limit: If an illuminator of power PI is used to flood light (i.e. fill all modes) of an object whose scale is l, in a 100 Hz bandwidth the temperature/mode is

With l = 1 m, = 0.5 mm TI ~2 x 1011 K

Random illumination limit: A practical way to get spotlight illumination would be to illuminate the whole room or ‘urban canyon' assume a 10% reflection, and let the target come into equilibrium with the room. If we let l = 100 m, then TI ~2.5 x 106 K.

This is a very bright light bulb for a focal plane array

like TIFT and the angular diversity will largely eliminate

coherent effects and the need for ‘strategic angles’

TI PIk

l

2

What is so favorable about the SMM/THz?What are the Opportunities?

The SMM/THz combines penetrability with -a reasonable diffraction limit -a spectroscopic capability -low pressure gases have strong, redundant, unique signatures

-solids can have low lying vibrational modes, especially at high THz frequencies

Rotational transition strengths peak in the SMM/THz

The SMM/THz is very quiet: 1 mW/MHz => 1014 K

The commercial wireless market will provide us with a cheap technology

Favorable Underlying Physics: It should be possible to engineer small (because of the short wavelength), high spectral purity (because we can derive via multiplication from rf reference) and low power (because the background is quiet/the quanta is small) devices and systems

What is so Challenging about the SMM/THz?

Efficient generation of significant tunable, spectrally pure power levels

Practical broadband frequency control and measurement

The need to develop systems without knowledge of the phenomenology

Impact of the atmosphere

What Needs to be Done to Enable the SMM/THz Spectral Region?

1. Source and detector figures of merit appropriate for different classes of applications. A better match between technologies and applications.

2. What are the signatures of solids? Distribution in frequency relative to penetration?

3. Classical penetrability, scatter, and specular reflection as a function of frequency and material.

4. What are the signatures of clutter for scenarios of interest? With this knowledge can we develop strategies to overcome related limits?

Meaningful decisions about source development directions require quantitative and comprehensive understanding not only of the sources, but also of their interactions with detectors (noise), target signatures, and clutter and their respective figures of merit.