Thin film interference in ultra-thin layers: color coatings, tunable absorbers, and thermal emitters

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Thin film interference in ultra-thin layers:

color coatings, tunable absorbers, and thermal emitters

Mikhail A. Kats Harvard University

March 4, 2014 NanoLight [Benasque]

School of Engineering and Applied Sciences

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Overview

 Thin film interference in lossy films

 Ultra-thin color coatings

 Ultra-thin tunable perfect absorber in the infrared

 Anomalous thermal emitter

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Overview

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Anti-reflection (AR) coating

 One simple application: anti-reflective coatings

 Simplest/thinnest conventional AR coating: quarter-wave film

 (optimized for a particular wavelength)

𝑟₁₂ = 𝑛₁ − 𝑛₂ 𝑛₁ + 𝑛₂

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Anti-reflection (AR) coating

Phasor diagram

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Anti-reflection (AR) coating

Phasor diagram

Quarter-wavelength ≈ 50 nm to 150 nm in the visible

 We don’t expect to see strong interference effects for films much thinner than this

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Interference in lossy films

 What if the film becomes very lossy?

 Reflection coefficient from the 1-2 interface becomes complex-valued

 Phasor r₀ points away from the real axis

Im[r]

Re[r]

r₀

[1]

[2]

[3]

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Interference in lossy films

 The reflection coefficient is also complex-valued at the 2-3 interface, and even more-so if the substrate index is complex (but not ∞)

Im[r]

Re[r]

r₀

[1]

[2]

[3]

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Interference in lossy films

 The reflection coefficient is also complex-valued at the 2-3 interface, and even more-so if the substrate index is complex (but not ∞)

 Must account for both interface reflection phase shifts and gradual phase accumulation via propagation

 New interference condition  loss cannot be treated as a perturbation

Im[r]

Re[r]

r₀

[1]

[2]

[3]

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Lossy dielectrics + finite metals

 Metals with finite conductivity and lossy dielectrics have weird interface reflection phase shifts (i. e. not 0 or π)

 Different interference condition compared to the lossless case:

“resonance” can exist for films significantly thinner than λ/4

 Takeaway message: it is possible to suppress reflection by using interference in a system involving an ultra-thin, highly-lossy layer

Im[r]

Re[r]

h << λ/4n

^r⁰

r₁ r₂ r₃

M. A. Kats et al, APL 101, 221101 (2012)

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Overview

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Our first experimental system



Lossy dielectric  Amorphous germanium



Lossy metal  Gold

Ellipsometry Experiment

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Our first experimental system



Lossy dielectric  Amorphous germanium



Lossy metal  Gold

Calculation Calculation

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Our first experimental system



Lossy dielectric  Amorphous germanium



Lossy metal  Gold

Calculation Calculation

 60 - 90% absorption within a 10- 20 nm layer of semiconductor

 Continuous film; no patterning

 Immediate applications in mind

 Photodetectors

 Solar cells

 Localized heating

 Note: absorption enhancement without field enhancement

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Coloring gold



“Colored” gold films by coating with 5-20 nm germanium films

 much thinner than λ/4

Polished substrate

Rough substrate (still works!)

Bare Au

Au + 7nm Ge

Au + 15nm Ge

M. A. Kats et al, Nat. Materials 12, 20 (2013)

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Angle-dependent reflectivity spectra

Experiment

Experiment

Calculation

Calculation



Au + 15 nm of germanium

s-polarization p-polarization

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Nanometer thickness optical coatings



Patterning ultra-thin coatings to create images, labels, etc



The difference between blue and purple, and purple and pink is only ~4 nm continuous film of germanium (~ 8 atomic layers)!

Photo: L. Liu

M. A. Kats et al, Nat. Materials 12, 20 (2013)

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Color gradients with ultra-thin coatings



Ge film of gradient thickness on gold (top section)



Overcoat with thin Al

₂

O

₃

layers



Increased color contrast



Protection from the elements



Can be replaced by transparent electrode (e.g. ITO)

No Al₂O₃ layer

14 nm Al₂O₃ layer

Photograph Calculation

M. A. Kats et al, APL 103, 101104 (2013)

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Additional layer of transparent dielectric



Au / 12 nm Ge / 44 nm Al

₂

O

₃

overcoat  R = 0 - 15% across the entire visible spectrum with most of the light absorbed in the germanium film

Experiment Calculation

M. A. Kats et al, APL 103, 101104 (2013)

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Gold + 15 nm of Ge Equivalent material

Structural color masquerading as a pigment

θ

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Gold + 15 nm of Ge

Structural color masquerading as a pigment

θ

Color by pigment

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Structural color masquerading as a pigment

θ

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Structural color masquerading as a pigment

θ = 0˚

θ = 40˚

θ = 80˚

θ = 40˚

θ



Optically it is impossible to determine if you’re looking at a solid material, a flat pigment, or a multi-layer system with ultra-thin films

Thin film

Glossy pigment

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Overview

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Lossy dielectrics + finite metals

 Metals with finite conductivity and lossy dielectrics have weird interface reflection phase shifts (i. e. not 0 or π)

 Resonance can exist for films significantly thinner than λ/4

 To achieve full suppression of a wavelength, the film should be very lossy

 Takeaway message: it is possible to achieve large (even perfect) absorption by using thin film interference in a system involving an ultra-thin, highly-lossy layer

Im[r]

Re[r]

h << λ/4n

^r⁰

r₁ r₂ r₃

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Making a tunable perfect absorber

 Our experimental system comprises a thin (180 nm vs. λ ~ 5-15 μm) film of vanadium dioxide (VO₂) on sapphire

 VO₂ serves as highly-absorbing layer (tunable)

 Sapphire is highly-reflecting due to phonon activity in the IR

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Why sapphire?

 In the IR, what reflectors give us non-trivial optical phase shifts upon reflection?

 At λ = 12 um, conventional metals do not work

 n_Au ~ 15 + 60i, n_Fe ~ 6 + 40i, etc  all pretty much PEC-like..

 Sapphire (crystalline Al₂O₃) fits the bill!

 Multi-phonon activity in Sapphire

 At 12 um, n ~ 0.1 + 0.8i

Gervais and Piriou, J. Phys. C (1974)

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Vanadium dioxide (VO

₂

)

 VO₂ is a correlated metal oxide which experiences phase change upon heating to past ~70 °C (reversible, but with hysteresis)

 Conductivity changes by >10,000 from the insulating to conducting state

Yang et al, Ann. Rev. Mat. Res. (2011)

VO₂ samples from Ramanathan group @ Harvard

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VO

₂

in the transition region

 What happens in the transition region of VO₂?

 Nanoscale islands of metal-phase VO₂ begin to form within a background of dielectric-phase VO₂, which then grow and connect

 The mixture can be viewed as a disordered, natural metamaterial

 The ratio of co-existing phases can be controlled  tunable medium

Qazilbash, Basov, et al, Science (2007)

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Tunable perfect absorber

 Temperature control of VO₂ allows significant tuning of its refractive index, and hence the sample reflectivity

 Reflectivity tuning from ~80% to 0.25% at 11.6um

 on/off ratio of more than 300

 entire structure is simply 180nm of VO₂ on sapphire

5 10 15

0 0.2 0.4 0.6 0.8 1

Wavelength (m)

Reflectivity

297K 343K 346K 347K 348K 351K 360K

M. A. Kats et al, APL 101, 221101 (2012)

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Changing the transition temperature

 Doping VO₂ with tungsten (W)  lower transition temperature

 1% W incorporated into VO₂ during growth  perfect absorption condition at ~50 °C rather than ~70 °C

2 4 6 8 10 12 14 16

0 0.2 0.4 0.6 0.8 1

Increasing Temperature (Sample1261, 1%W @ 580C)

Wavelength, (m)

Reflectance

30C + 35C + 40C + 40.5C + 41C + 41.5C + 42C + 42.5C + 43C +

43.5C + 44C + 44.5C + 45C + 45.5C + 46C + 46.5C + 47C + 47.5C +

48C + 48.5C + 49C + 49.5C + 50C + 50.5C + 51C + 51.5C + 52C +

52.5C + 53C + 53.5C + 54C + 54.5C + 55C + 55.5C + 56C + 56.5C +

57C + 57.5C + 58C + 58.5C + 59C + 59.5C + 60C + 60.5C + 61C +

61.5C + 62C + 62.5C + 63C + 63.5C + 64C + 64.5C + 65C + 65.5C +

66C + 66.5C + 67C + 67.5C + 68C + 68.5C + 69C + 69.5C + 70C +

75C + 80C + 85C + 90C + 95C + 100C +

In collaboration with:

C. Ronning, J. Rensberg H. Schmidt, I. Skorupa Wavelength (µm)

Reflectance

2 4 6 8 10 12 14 16 0

1

increasing T

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Overview

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IR absorber in reverse: thermal emitter

 Spectrum/intensity of thermal radiation emitted by an object is given by Planck’s law:

 Thermodynamic statement: Kirchhoff’s law of thermal radiation

Blackbody contribution

emissivity

spectroscopic

wavenumber ( = f/c)

absorptivity

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IR absorber in reverse: thermal emitter

 Spectrum/intensity of thermal radiation emitted by an object is given by Planck’s law:

 Thermodynamic statement: Kirchhoff’s law of thermal radiation

 Our VO₂/sapphire structure has temperature-dependent infrared absorptivity, and hence its emissivity is expected to also vary with temperature

 Anomalous behavior such as non-monotonic thermal emission with temperature

Blackbody contribution

emissivity

spectroscopic

wavenumber ( = f/c)

absorptivity

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Thermal emitter: experimental setup

 Data analysis must account for thermal emission from the detector and optics, and the frequency-dependent response of the instrument

 Can use a wafer blackened by candle soot as blackbody-like reference (ε ≈ 0.96 in the IR)

Fourier transform infrared spectrometer (FTIR)

DTGS detector sample

temp. controller

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Thermal emitter: experimental results

 Extracted spectral radiance (distribution of thermal emission)

 Thermal emission peak @ “perfect absorption” condition

 At some point emissivity surpasses our black soot reference

 Total emitted IR light goes up, then down with increasing temperature 6000 800 1000 1200 1400

1 2

3x 10^-3

wavenumber (cm^-1)

spectral radiance (a. u.) 50C black soot 50C VO

2/sapphire 74.5C black soot 74.5C VO

2/sapphire 100C black soot

100C VO

2/sapphire

M. A. Kats et al, Physical Review X 3, 041004 (2013)

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Thermal emitter: experimental results

 Integrated emitted power over the 8-14 μm atmospheric transparency region

 Multiple unusual features:

 Local maximum in the emitted power

 Negative differential thermal emittance over ~10°

 Hysteresis (intrinsic to VO₂)

40 60 80 100

0.4 0.6 0.8 1 1.2

Temperature (C) Emitted power (a. u.) heating

cooling black soot

M. A. Kats et al, PRX 3, 041004 (2013)

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Thermal emitter: experimental results

 Integrated emitted power over the 8-14 μm atmospheric transparency region

 Multiple unusual features:

 Local maximum in the emitted power

 Negative differential thermal emittance over ~10°

 Hysteresis (intrinsic to VO₂)

40 60 80 100

0.4 0.6 0.8 1 1.2

Temperature (C) Emitted power (a. u.) heating

cooling black soot

M. A. Kats et al, PRX 3, 041004 (2013)

 Design of IR emittance vs Temperature

 Efficient temperature regulation (passive or active)

 Infrared camouflage

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Summary

 Described existence of strong optical interference in ultra-thin, highly- absorbing films

 Demonstrated ultra-thin optical interference coatings comprising lossy semiconductor films on gold substrates

 Large optical absorption within nanometer-thick layers

 Thin film interference without iridescence

 Demonstrated a tunable absorber in the infrared based on VO₂ and sapphire

 Used the phase transition of VO₂ as a tunable, disordered, “natural metamaterial”

 Demonstrated an anomalous thermal emitter

 “Perfect” blackbody-like emission,

 Negative differential thermal emittance

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Acknowledgements and thanks!

 Federico Capasso

 Romain Blanchard

 Shuyan Zhang

 Patrice Genevet

 Steve Byrnes

 Deepika Sharma

 Jiao Lin

 Zheng Yang

 Changhyun Ko

 Shriram Ramanathan

 Matthias Kolle

 Joanna Aizenberg

 Mumtaz Qazilbash (UCSD, WM)

 Dmitri Basov (UCSD)

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Thin film interference in ultra-thin layers: color coatings, tunable absorbers, and thermal emitters