Plasmoniset infrapunailmaisimet

Transcription

1 VTT TECHNICAL RESEARCH CENTRE OF FINLAND LTD Plasmoniset infrapunailmaisimet Toteuttaja: Teknologian tutkimuskeskus VTT Oy MATINE-rahoitus: MATINEn tutkimusseminaari Kirsi Tappura, Tekn. Toht., Dos. VTT, Anturit ja integrointi

2 Sisältö Taustaa infrapunailmaisimien käytöstä ja haasteista Tieteellinen ongelma, tutkimuksen tavoite ja keinot Tuloksia Tulosten jatkokäyttö ja hyödyntäminen 17/11/2016 2

3 Infrared (IR) detectors in military and security applications IR detectors provide the means to detect objects under poor visibility conditions (dark, smoke, fog). Infrared radiation is emitted by all objects above absolute zero possible to see the environment without illumination by passive thermal imaging. The characteristic absorption frequencies of many chemical species are within the IR range remote sensing and stand-off detection of the critical chemical substances 17/11/2016 3

4 Transmission [%] The wavelength bands of most interest are the bands of high transparency, i.e. the so called atmospheric windows (e.g. 3-5 μm and 8-12 μm) where the absorption by H 2 O, CO 2 etc. is low. CO 2 H 2 O CO Wavelength [µm] 17/11/2016 4

5 The needs and the scientific challenge There is a continuous demand for higher performance: higher sensitivity, better thermal resolution (higher signal-tonoise ratio), fast response, increase in pixel density, multispectral imaging, and cost reduction especially in the IR imaging array systems. need to develop high-performance sensors with less cooling In spite of the widely available commercial photonic and thermal detectors, there is still a need especially for IR imaging array systems that combine: a reasonable price, convenient operation (i.e. less cooling), and still excellent performance. 17/11/2016 5

6 Objectives of the project To investigate the feasibility to improve the functionality of IR detectors by inclusion of plasmonic structures. enhanced photocurrent or photoresponsivity (signal/noise ratio) and sensitivity engineering the spectral response The problem was approached mainly by theoretical and computational means (2015: Tasks 1-3; 2016: Task 4). Tasks 1-3: Detector types selected to be as transferrable as possible Plasmonic structures for enhanced scattering into the detector Plasmonic structures to exploit the local field enhancements Task 4: Influence of plasmonic structures on the detector characteristics (S/N, spectral, pixel) Although the limited budget does not permit the clean room based fabrication of plasmonic IR detectors from scratch, it is investigated (considered optional in the project plan) whether experimental proof of principles for selected properties can be provided by applying commercial detector structures. 17/11/2016 6

7 So, what are the plasmons Plasmon: a harmonic oscillation of a free electron gas at a certain carrier density dependent frequency in a metal An electromagnetic field coupling to a plasmon hybirid excitation resonance plasmon polariton Such coupling occurs at the a metal-dielectric interface surface plasmon polariton (SPP) In a propagating surface plasmon polariton (PSPP) the electromagnetic field is confined in 1D or 2D. In the localized surface plasmon polaritons (LSPP) the electromagnetic field is confined in 3D, e.g. in nanoparticles. At a resonance the lifetime of photons is increased The local amplitude of the electromagnetic field is enhanced. ω p = ne2 mε 0 ω sp = ω p / 2 ω lsp = ω p / 3

8 How to enhance the absorption of electromagentic radiation in the detector absorber by plasmonics The methods applied in the project: Enhancing scattering cross-section: reduce the reflection losses ( antireflection coating ) enhance the optical path of photons in the active layer such that their probability for absorption is enhanced Light trapping Enhancing the electromagnetic near-field confined absorption in the desired volume The proposed methods provide efficient ways to optimize and improve the detector designs (size, speed, carrier collection efficiency, etc.) by giving more freedom.

9 The computational methods used for designing the plasmonic structures in the project Surface plasmons can be described by macroscopic electromagnetic theory, i.e. Maxwell s equations, if the electron mean free path in the plasmonic material << the plasmon wavelength (note: the quality of the materials and interfaces) Full wave solutions of Maxwell s equations with discretization of the fields on a grid Finite Element Method, FEM (frequency domain)

10 Detector materials of the systems studied Absorbers: PbS (short-to-mid IR) E g = 0.37eV (3.35 μm) Hg 1-x Cd x Te (long-wave IR) E g ~ 0.083eV (15 μm) for x = 0.2 (at 77 K) (An additional challenge: significant variation in the material parameters in the literature.) Materials for plasmonic structures: Ag, Au, Cu, heavily doped Al:ZnO 17/11/

11 Examples of plasmon-induced absorption enhancement in the absorber materials PbS HgCdTe The goals of the 1st year (2015) achieved in schedule.

12 The spectral response of a detector can be tuned by varying the surface density of the plasmonic structures (1) 17/11/

13 The spectral response of a detector can be tuned by varying the surface density of the plasmonic structures (2) 17/11/

14 The spectral response of the detector can be adjusted by varying the height of the plasmonic structures 17/11/

15 The potential of plasmonics for reducing the pixel size of a detector 17/11/

16 Influence of plasmonics on the signal/noise (S/N) ratio Increased absorption increased signal. Assuming that the fabrication of the plasmonics structures does not generate new noise sources or enhance the old ones S/N is increased Reduced pixel size decreases the noise of a detector. By applying plasmonics the decrease in signal can be kept smaller that that in noise. S/N is increased 17/11/

17 Spectral features are influenced by the interplay of the plasmonic structures with the detector designs 17/11/

18 Polarization response of a detector can be managed by plasmonics 17/11/

19 Conclusions 2015: About three-fold absorption enhancements were computationally demonstrated for PbS and HgCdTe detectors near the 3 µm and µm wavelength regions and tens to over 100-fold enhancements at narrow bands in Si detectors in the near-ir regions. 2016: Significant tuning possibilities for spectral features demonstrated by varying the details of the plasmonic structures and interplay with the detector. Simulations for designing specific plasmonics structures for a commercial detector showed % improvement compared to the plain chip without plasmonics. With proper designs plasmonic structures provide the means to increase the S/N ratio (or reduce cooling) and reduce the pixel size, and thus, increase the speed of the detector. 17/11/

20 Exploitation of the results The results are very promising suggesting that plasmonic nanostructures may be used in the development of future high-performance IR detectors with enhanced (pixel level) functionalities. The suggested methods also provide high potential for cost reduction in high-performance IR imaging systems less cooling smaller size/thickness: decrease in the material consumption and growth time of the absorber layer Considering the fast development of high-throughput fabrication techniques plasmonic structures in relatively low-cost devices seem feasible in the near future (note also the required size: IR vs. visible) can be introduced into a standard IR detector or imaging camera fabrication process with a single additional lithography step 17/11/

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