Controlled tuning of the whispering gallery modes of GaN/InGaN {\mu}-disk
cavities is demonstrated. The whispering gallery mode (WGM) tuning is achieved
at room temperature by immersing the {\mu}-disks in water and irradiating with
ultraviolet (UV) laser. The tuning rate can be controlled by varying the laser
excitation power, with a nanometer precision accessible at low excitation power
(~ several {\mu}W). The selective oxidation mechanism is proposed to explain
the results and supported by theoretical analysis. The tuning of WGMs in
GaN/InGaN {\mu}-disk cavities may have important implication in cavity quantum
electrodynamics and the development of efficient light emitting devices

Aharonovich, Igor

El-Ella, Haitham A. R.

Hu, Evelyn L.

Kappers, Menno J.

Niu, Nan

Oliver, Rachel A

Rol, Fabian

Russell, Kasey J

Woolf, Alexander

Applied Physics Letters

English

arXiv

Igor Aharonovich

Nan Niu

Fabian Rol

Kasey J. Russell

Alexander Woolf

Haitham A. R. El-Ella

Menno J. Kappers

Rachel A Oliver

Evelyn L. Hu

Crossref

Controlled tuning of whispering gallery modes of GaN/InGaN microdisk cavities

Controlled tuning of the whispering gallery modes of GaN/InGaN microdisk cavities is demonstrated. The whispering gallery mode (WGM) tuning is achieved at room temperature by immersing the microdisks in water and irradiating with ultraviolet laser. The 

Aharonovich, I

Niu, N

Rol, F

Russell, K

Woolf, A

El-Ella, H

Kappers, M

Oliver, R

Hu, E

OPUS - University of Technology Sydney

Controlled tuning of the whispering gallery modes of GaN/InGaN microdisk cavities is demonstrated. The whispering gallery mode (WGM) tuning is achieved at room temperature by immersing the microdisks in water and irradiating with ultraviolet laser. The tuning rate can be controlled by varying the laser excitation power, with a nanometer precision accessible at low excitation power (approximately several W). The selective oxidation mechanism is proposed to explain the results and supported by theoretical analysis. The tuning of WGMs in GaN/InGaN microdisk cavities may have important implications in cavity quantum electrodynamics and the development of efficient light emitting devices. © 2011 American Institute of Physics

Russell, KJ

El-Ella, HAR

Kappers, MJ

Oliver, RA

Hu, EL

Controlled tuning of the whispering gallery modes of GaN/InGaN microdisk cavities is demonstrated. The whispering gallery mode (WGM) tuning is achieved at room temperature by immersing the microdisks in water and irradiating with ultraviolet laser. The tuning rate can be controlled by varying the laser excitation power, with a nanometer precision accessible at low excitation power (approximately several μW). The selective oxidation mechanism is proposed to explain the results and supported by theoretical analysis. The tuning of WGMs in GaN/InGaN microdisk cavities may have important implications in cavity quantum electrodynamics and the development of efficient light emitting devices.Engineering and Applied SciencesAccepted Manuscrip

Russell, Kasey Joe

Woolf, Alexander J

El-Ella, Haitham A.R.

Hu, Evelyn

Harvard University - DASH

 Controlled Tuning of Whispering Gallery Modes of GaN/InGaNMicrodisk Cavities  (Article begins on next page)The Harvard community has made this article openly available.Please share how this access benefits you. Your story matters.Citation Aharonovich, Igor, Nan Niu, Fabian Rol, Kasey J. Russell,Alexander Woolf, Haitham A. R. El-Ella, Menno J. Kappers,Rachel A Oliver, and Evelyn L. Hu. 2011. Controlled tuning ofwhispering gallery modes of GaN/InGaN microdisk cavities.Applied Physics Letters 99 (11): 111111.Published Version doi:10.1063/1.3640211Accessed February 19, 2015 1:12:10 PM ESTCitable Link http://nrs.harvard.edu/urn-3:HUL.InstRepos:11006840Terms of Use This article was downloaded from Harvard University's DASHrepository, and is made available under the terms and conditionsapplicable to Open Access Policy Articles, as set forth athttp://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAPControlled  tuning  of  whispering  gallery  modes  of  GaN/InGaN microdisk cavities  Igor Aharonovich1, Nan Niu1, Fabian Rol1, Kasey J Russell1, Alexander Woolf1, Haitham A. R. El-Ella2, Menno J. Kappers2, Rachel A Oliver2 and Evelyn L. Hu1 1School  of  Engineering  and  Applied  Sciences,  Harvard  University,  Cambridge,  MA, 02138, USA 2Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, United Kingdom  Email: igor@seas.harvard.edu  Abstract Controlled  tuning  of  the  whispering  gallery  modes  of  GaN/InGaN  µ-disk  cavities  is demonstrated.  The  whispering  gallery  mode  (WGM)  tuning  is  achieved  at  room temperature by immersing the µ-disks in water and irradiating with ultraviolet (UV) laser. The tuning rate can be controlled by varying the laser excitation power, with a nanometer precision  accessible  at  low  excitation  power  (~  several  µW).  The  selective  oxidation mechanism is proposed to explain the results and supported by theoretical analysis. The tuning of WGMs in GaN/InGaN µ-disk cavities may have important implication in cavity quantum electrodynamics and the development of efficient light emitting devices.    Optical microcavities are important components in studying solid state cavity quantum electrodynamics  (cQED),  developing  low  threshold  light  emitting  devices  and  ultra-sensitive sensors1-3. III-nitride materials are particularly interesting in this respect due their high potential in the realization of ultraviolet and visible micro- and nano-scale light sources operating at room temperature4-6. In addition, the strong exciton binding energy of  these  high  bandgap  materials  makes  them  attractive  candidates  for  the  study  of polariton dynamics and polariton lasing in the strong coupling regime7. Recent years have seen progress in the formation of III-nitride based cavities that incorporate quantum well (QW) or quantum dot (QD) active layers8-12. Nevertheless, an outstanding challenge in engineering  III  nitride  µ−disk  cavity  systems  is  the  tuning  of  the  cavity  modes  into resonance  with  the  emission  lines  of  embedded  QDs.  Such  tuning  is  essential  to demonstrate efficient coupling interaction between the cavity mode and the emitter.  A variety of methods have been employed to tune cavity modes in other semiconductor materials (e.g. GaAs) and are mostly based on the modification of the local refractive index of the cavity. Temperature tuning of photonic crystal cavities (PCCs) using gas condensation13 has been previously demonstrated. In a different approach, deposition of thin photosensitive layer on top of the cavity (such as chalcogenide14 or spiropyran15) is performed. Upon laser irradiation, the refractive index of the layers is modified, resulting in a tuning of the cavity modes. Alternatively, chemical digital etching which slightly modifies the cavity dimensions was also demonstrated16. Finally, employing heating pads positioned in a close proximity with the PCC, the QDs' resonance can be tuned17.  In  this  paper  we  demonstrate  tuning  of  the  InGaN/GaN  µ-disks  WGMs  through  a selective, in situ photo-enhanced process. The tuning is achieved by immersing the µ-disks in de-ionized water and exciting the embedded QDs with a UV laser (360 or 380 nm) excitation. The tuning of the WGMs continues as long as the laser is switched on and focused on the disk. In our approach, only the probed µ-disks undergo the tuning while other  components  on  the  chip  remain  unaffected.  By  measuring  cavity  luminescence during tuning, we are able to continuously monitor the spectral location of the modes and dynamically tune them. The  structure  of  the  µ-disks  consists  of  an  InGaN  QD  (~20%  In)  layer  sandwiched between two layers of GaN. A detailed description of the fabrication of these cavities and their  general  characteristics  will  be  published  elsewhere18.  Briefly,  the  disk  structure contains InGaN QDs clad by GaN layers, grown on a sapphire substrate. The growth of the InGaN QDs has been described previously19. 500 nm of SiO2 was deposited over the GaN-based structure; the entire sample was bonded onto a secondary sapphire substrate, and the original sapphire was removed by a laser lift-off process.  The overlying material, extraneous to the disk structure was removed by a dry etch process, and µ-disks were patterned  using  contact  lithography  and  a  post  formed  by  undercutting  the  SiO2  in hydrofluoric acid. Figure 1a shows a scanning electron microscope (SEM) of the µ-disk. The inset shows a schematic of the disk, displaying InGaN QD layer, ~ 2.5 nm thick, sandwiched between two GaN layers (60 nm each). The diameter of the disks is ~ 3 µm and the thickness is ~ 120 nm. The thin and smooth µ-disks support the propagation of WGMs in the periphery of the disks.  Optical  properties  of  the  µ-disks  were  assessed  using  a  frequency-doubled  titanium sapphire  laser  emitting  at  360  nm,  incident  on  the  sample  through  a  long  working distance objective (×100, numerical aperture (NA)=0.5). The diameter of the beam is approximately 500 nm. The emission from the µ-disks was collected through the same objective and directed into a spectrometer. Figure 1b shows a photoluminescence (PL) spectrum recorded at room temperature from the µ-disk. Modes with quality factor (Q) as high as 3500 were measured.   Fig. 1. (a) SEM image of the InGaN/GaN µ−disk. The inset shows the structure of the disk. (b) PL spectrum recorded from the disk showing the WGMs. Modes with Qs as high as 3500 were measured.   To  tune  the  WGMs,  the  disks  were  immersed  in  water  within  a  small  cell.  The  cell consists of a hollow rubber ring (1 mm height) in between two cover glass slides. The sample was positioned on the bottom cover slide, in the middle of the ring, and imaged using the objective through the top cover slide. Figures 2 (a,c) shows the tuning of the cavity  WGMs,  achieved  by  continuous  irradiation  of  the  disk  with  two  different excitation powers of 0.2 mW (a) and 0.9 mW (c) using the 360 nm excitation wavelength. It  is  noticeable  that  high  excitation  powers  cause  faster  WGMs  shifts,  while  lower excitation power results in a more moderate shift.    Figure  2.  Mode  tuning  of  InGaN/GaN  µ−disks  immersed  in  water  using  (a)  360  nm excitation  wavelength  with  0.2  mW  and  (c)  0.9  mW  excitation  power.  (b)  380  nm excitation wavelength with 0.3 mW and (d) 1 mW excitation power. The bright lines represent the PL intensity of the different modes.    We repeated the same tuning operation using a 380 nm excitation wavelength, which corresponds to an energy below the bandgap of the GaN. Figures 2 (b,d) show the results of the mode tuning employing 0.3 mW and 1 mW of excitation power. Comparing the two excitation wavelengths, it is clearly seen that tuning using the 380 nm excitation is considerably slower than that observed for the 360 nm excitation. Over the same period of time at excitation powers of ~1 mW, using 380 nm excitation, the modes shifted only by 1.9 nm, while shifts of up to 3.5 nm were observed under 360 nm excitation. The tuning  was  even  slower  employing  low  excitation  powers  (figure  2b)  and  was  not observed  when  the  power  dropped  to  ~50  µW.  These  data  suggest  the  possibility  of precise control of the tuning of the WGMs using low excitation power. Finally, we note that the mode shift is permanent and remains even after the water is removed and the disks are dried in air. Figures 3 (a, b) shows two PL spectra recorded dynamically during the tuning after 5 sec (black curves) and 90 sec (red curves) using 360 nm and 380 nm excitation, respectively. A comparison of these spectra reveals a degradation of Q for the 360 nm tuning process, which is minor for the 380 nm tuning, even for a comparable change in wavelength.    Figure 3. PL spectrum recorded after 5 sec (black curve) and 100 sec (red curve) during the tuning of the WGMs using (a) 360 nm and (b) 380 excitation wavelengths. The dark red arrows indicate the mode shift.   To further understand the mechanism underlying the tuning, we immersed the µ−disks in a non-polar solvent, toluene. The samples were subjected to 20 minutes of continuous irradiation at 80 µW power with 360 nm excitation laser. No tuning was observed.  We believe that the tuning mechanism results from a process similar to that seen in Photo Electro Chemical (PEC) etching of GaN8, 10, in which photo-generated holes enhance oxidation of the GaN, and where the oxide is subsequently dissolved in the electrolyte (water in our case). The consistent blue shift of the modes (several nanometers) with increased tuning power and time suggests a reduction in the size of the µ−disks. We note that typical PEC etching conditions employed for the smoothest-quality etching of GaN incorporates a dilute, 0.04 M HCl solution as the electrolyte10. In addition, there has been a previous report of photoelectrochemical oxidation of n-GaN in water, under an applied bias > 2V20. In that case, oxides of several 100 nm were formed, but no oxides were observed for biases less than 2V. We believe that in our case, photo-oxidation takes place, with a thin oxide that is subsequently dissolved in solution, resulting in a precise etch removal of the disk material that produces a shift in the cavity modes. The mechanism of the in situ PEC etch-tuning also accords well with the wavelength-dependence of the tuning. For excitation at 380 nm, below the bandgap of the GaN, electron hole pairs will only be generated within the InGaN QDs layer. At low incident powers (~ 50 µW), the photo-generated carriers would be confined in the InGaN region, and thus no PEC etching is observed. At higher powers, band-filling results, the electrons and holes may no longer be confined by the InGaN region, and will drift and diffuse to the  µ−disk  surfaces,  under  the  influence  of  the  built-in  electric  field  (Fig  S1a).  We believe that, given the direction of the internal fields, photo-oxidation/etching would take place predominantly on the top (Ga-face) surface, although this would need to be further verified. Excitation at 360 nm will generate electron-hole pairs throughout the volume of the disk. Again, the influence of the built-in electric fields will direct most of the photo-generated carriers to the top and bottom surfaces of the disk. However, because the photo-generated carriers can occupy the full volume of the disk, it is expected that carriers will readily diffuse to all the µ−disk surfaces, resulting in a fairly isotropic photo-oxidation/etching of the periphery of the disk, as well as the surfaces.  COMSOL  software  was  used  to  simulate  the  shift  in  wavelength  of  the  modes  with respect to change in dimensions of the µ−disk (Fig S1b). A change of 10 nm in the radius of the µ−disk produces only a 1 nm shift in the mode wavelength, while a 10 nm change in the thickness of the µ−disk, results in approximately 7 nm wavelength shift of the WGMs. Thus an ‘isotropic’ etch of the disk, with a reduction in both the thickness and diameter of the disk (as would apply to the 360 nm tuning excitation) would exhibit a mode  shift  very  nearly  equal  to  an  etch  that  affected  only  the  thickness  of  µ−disk (characteristic of the 380 nm tuning excitation). The WGMs are extremely sensitive to both changes in the disk diameter and to the quality of the etched sidewalls. Thus, for a similar  observed  mode  shift,  the  isotropic  etch  process  associated  with  the  360  nm excitation  could  have  a  greater  impact  on  the  dimensions  and  quality  of  the  µ−disk sidewalls.  Indeed,  a  larger  degradation  of  Q  seen  for  the  360  nm  tuning  process, compared with that undertaken using excitation at 380 nm. The COMSOL simulations also suggest that the ~ 5 nm tuning we observe corresponds to < 10 nm change in the thickness of the µ−disk.  To  summarize,  we  demonstrated  a  controllable  tuning  of  the  WGMs  of  InGaN/GaN µ−disk cavities. Our results indicate that the tuning occurs due to in-situ oxidation of the disk surface and oxide removal by the water. Theoretical analysis of the band diagram and simulations of the mode shifts are in a good agreement with our experimental data. Fine tuning of the cavity modes is accessible at low excitation powers, which can be beneficial  for  tuning  PCCs.  Tuning  availability  in  III-nitride  µ−disk  systems  would propel  the  use  of  III-nitrides  in  studying  cQED  systems  and  development  high performance optoelectronic devices.    Acknowledgments The authors thank Andrew Magyar for the help with the flow cell design.   References 1.  K. J. Vahala, Nature 424, 839 (2003). 2.  A.  M.  Armani,  R.  P.  Kulkarni,  S.  E.  Fraser,  R.  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Controlled Tuning of Whispering Gallery Modes of GaN/InGaN Microdisk Cavities

Controlled tuning of whispering gallery modes of GaN/InGaN microdisk
  cavities

Controlled tuning of whispering gallery modes of GaN/InGaN microdisk cavities

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OPUS - University of Technology Sydney

OPUS - University of Technology Sydney

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