Thermal Tuning Wanted and unwanted…

Origin of Temperature Dependence Temperature coefficient of the excitonic bandgap –Dominant effect (-4.6 x eV/ºC) –Electron-phonon interactions are main cause of shrinking energy gap with increasing temperature Thermal expansion coefficient –Secondary effect (2.5 x /ºC) Temperature dependence of silicon refractive index depends on wavelength –Near-IR: dn/dT~ 2 x K -1 –Visible: dn/dT ~ 4 x K -1 Consider silicon first…

Temperature Dependence of Various Materials in Near-IR dn/dT silica ~ 1 x K -1 dn/dT InP ~ 1 x K -1 dn/dT silicon ~ 1.83 x K -1 dn/dT GaAs ~ 2.6 x K -1 dn/dT polymer ~ 1 x K -1

Thermo-Optic Effect depends on Q-factor Transmission (%) Wavelength (nm) Q = 1250 n = 0.01 Si: 50ºC InP: 100ºC GaAs: 40ºC dB = 10.6 (in near IR)

Transmission (%) Wavelength (nm) Q = 2750 n = 0.01 Si: 50ºC InP: 100ºC GaAs: 40ºC dB = 10.6 dB = 17.3 Thermo-Optic Effect depends on Q-factor

The higher the Q-factor, the more sensitive the PBG device is to temperature variations n = n = 0.01 n = dB attenuation Q factor Silicon 500ºC 50ºC 5ºC Thermo-Optic Effect depends on Q-factor

Lets look at some examples

Si Photonic Crystal Waveguide Microcavity Fabricated in SOI with 340nm Si core and 3000nm silica cladding Design = 1.53 m (hole diameter = 250nm) Coupling to microcavity improved by size- graded holes along input and output channel WGs H.M.H. Chong and R.M. De La Rue, IEEE Photonics Technol. Lett. 16, 1528 (2004).

Integrated microheater –PECVD silica deposited on top of microcavity –Nichrome thin film heater evaporated on top and connected to two probe pads with nichrome bottom layer and gold top layer –Heater width = 300nm TE-pol light end fire coupled into WG using microscope objective Si Photonic Crystal Waveguide Microcavity

5nm shift, T=160ºC ~7 dB attenuation Q~500 Switching time expected to be submillisecond 9.2 mW Heat dissipation in silica core seems to be a problem

AlGaAs/GaAs heterostructure grown by MBE with three layers of InAs quantum dots in core as internal light source –Hole spacing = 220nm InP/InGaAsP heterostructure grown by MOVPE with two GaInAsP quantum wells in core layer as light source –Hole spacing = 440nm GaAs and InP Photonic Crystal Microcavity GaAs InP B. Wild et al., Appl. Phys. Lett. 84, 846 (2004)

Samples mounted on Peltier stage using silver paste (range: 20-76°C) GaAs photonic crystal PL peak at 1000nm with Q~900 showed 4.5nm red shift for T=56°C Measured d /dT=8 x nm/°C Calculated d /dT=9 x nm/°C (based on dn eff /dT=3.5 x /°C) GaAs and InP Photonic Crystal Microcavity

InP photonic crystal Fabry-Perot mode at 1564nm with Q~310 showed 5nm red shift for T=56°C Measured d /dT=9 x nm/°C Calculated d /dT=10 x nm/°C (based on dn eff /dT=2 x /°C) GaAs and InP Photonic Crystal Microcavity Due to large thermal conductivity of III-V semiconductors, difficult to exploit temperature tuning

Photonic Crystal Laser 19 air holes removed from 2D triangular photonic crystal lattice in InGaAsP Undercut (V-shaped groove in SEM) to form membrane Optically pumped with 865nm VCSEL Multimode fiber collects light and connects to optical spectrum analyzer P.T. Lee, et al., Appl. Phys. Lett. 81, 3311 (2002).

Photonic Crystal Laser Mounted on copper and fixed onto Peltier thermal electric cooler with heat sink Thermistor monitors temperature Emission at 1.55 m, FWHM =200nm Q ~ 10!!! Emission wavelength shifts ~ 0.5Å/K 1.5 nm

Photonic Crystal Laser Interesting to note that threshold pump power increases significantly as temperature increases Issue for practical applications

Thermal tuning is not always a desirable effect

Laser for WDM Laser Detector Transmission medium

Laser for WDM Laser Detector Transmission medium

Laser for WDM Laser Detector Transmission medium

The higher the Q-factor, the more sensitive the PBG device is to temperature variations Q factor Q = 6000 PC laser sensor Channel drop filter Q = 45,000 Nanocavity in 2-D PC slab Applications: n = n = 0.01 n = dB attenuation How Significant is the Thermal Drift?

Porous silicon 1-D PBG microcavities

Achieving Temperature Insensitivity Exploit mismatch of coefficient of thermal expansion between Si and oxide Coat silicon walls with oxide (thermal evaporation) Silicon ~ 2.5 x K -1 Oxide ~ 0.5 x K -1 Reflectance shift due to refractive index change ++/- How can a controlled pressure change be introduced?

Porous Silicon PBG Microcavity Investigate temperature dependence of two different size scale porous silicon microcavities Pore size ~ 20 nm Silicon walls ~ 5 nm Pore size ~ 150 nm Silicon walls ~ 50 nm MesoporesMacropores

Porous Silicon PBG Microcavity Investigate temperature dependence of two different size scale porous silicon microcavities MesoporesMacropores

Effective Medium Approximation Estimates refractive index of porous silicon M f(porosity, n Si, n pore ) For a given wavelength, an increase in porosity results in a decrease in refractive index Bruggeman approximation Porosity (%) Effective index Air in pores, = 1500 nm

Porous Silicon PBG Microcavity Investigate temperature dependence of two different size scale porous silicon microcavities MesoporesMacropores HP = 75%, n ~ 1.44 LP = 50%, n ~ 2.16 HP = 80%, n ~ 1.34 LP = 70%, n ~ 1.60

Porous Silicon PBG Microcavity Investigated temperature dependence of porous silicon microcavity (1-D PBG with defect) Reflectance (%) Wavelength (nm) Reflectance (%) Wavelength (nm) MesoporesMacropores visible near IR

How Serious is the Problem? Mesoporous silicon microcavity temperature dependence dn dT silicon cannot be neglected ~ 3 nm redshift for 100°C Resonance redshift (nm) Temperature (ºC) experiment simulation

Reflectance (%) Wavelength (nm) 25ºC Q ~ 1700 > 10 dB 80ºC ~ 2.8 nm How Serious is the Problem? Mesoporous silicon microcavity

How Serious is the Problem? Mesoporous and macroporous silicon microcavity temperature dependence Resonance shift (meV) Temperature (ºC) Mesopores (near IR) dn/dT ~ 2 x K -1 Macropores (visible) dn/dT ~ 4 x K -1 Simulation Experiment

Surface Treatment Outer layers of silicon rods converted to oxide by annealing ( °C) in O 2 Higher temperatures during anneal lead to thicker oxides

Passive Oxidation-Induced Shift Resonance permanently shifts to shorter wavelengths when silicon converted into silicon dioxide Reflectance (%) Wavelength (nm) Native oxide 400ºC 900ºC Active shift

Experimental Setup for Active Tuning Resistors = Heat Source Thermistor = Temperature Measurement Tool Al PSi backfront

Temperature Effect on Reflectance Oxide thickness ºC 400ºC 300ºC native oxide Reflectance shift (eV) Temperature (ºC) Mesoporous Silicon Microcavities Note: oxidation time ~ 10 min

native oxide Temperature Effect on Reflectance Oxide thickness º C Reflectance shift (eV) Temperature (ºC) Macroporous Silicon Microcavities 1000º C 1100º C Note: oxidation time ~ 10 min

Porous Silicon PBG Microcavity Pore size ~ 20 nm Silicon walls ~ 5 nm Pore size ~ 150 nm Silicon walls ~ 50 nm MesoporesMacropores

Understanding Temperature Insensitivity Reflectance shift due to refractive index change ++/- X-ray analysis to determine pressure change Pressure Increase Strain Increase For silicon: dn/dP = MPa -1

Unoxidized mesoporous silicon microcavity Increasing temperature Decreasing strain (decreasing pressure) (S) 85°C 45°C 25°C Intensity (a.u.) (degrees) (P) X-ray Analysis of Strain

Temperature Effect on Strain a/a ) ( x10 -4 ) Temperature (°C) Native oxide Mesoporous silicon microcavity

Intensity (a.u.) degrees) 25 C (P)(S) C 80 C X-ray Analysis of Strain Slightly oxidized mesoporous silicon microcavity Increasing temperature Increasing strain (increasing pressure)

Temperature Effect on Strain a/a ) ( x10 -4 ) Temperature (°C) Native oxide Temperature insensitive Slightly oxidized More heavily oxidized Mesoporous silicon microcavity

Pressure & Temperature Effect on Refractive Index Pressure effect compensates temperature effect on refractive index 400°C in O 2 leads to temperature insensitive mesoporous silicon PBG a/a ) ( x10 -4 ) Temperature (°C) as-anodized Temperature insensitive Slightly oxidized More heavily oxidized ºC 400ºC 300ºC native oxide Reflectance shift (meV) Temperature (ºC) Mesoporous silicon microcavity 1000°C in O 2 leads to temperature insensitive macroporous silicon PBG

Temperature Insensitivity – A General Method Extension to silicon-based 2-D and 3-D PBG structures Requires longer oxidation times at high temperatures Other materials systems Idea of using pressure as compensating effect still valid Application of method may be slightly more complicated