Conference-ISSS-5-An Aperture Probe of Near-Field Optical Microscope : Near-Field Tool to Modulate the Laser Diode Emissions

In situ emission profiles of AlGaInP multiple quantum well laser diodes (LD’s) observed by a near-field scanning optical microscope aperture probe shows a single broad elliptic profile similar to a far-field observation, whereas the local emission spectrum was different depending where the aperture probe was placed within the emission profile. This observation suggests the existence of a near-field coupling between the aperture and the LD cavity, which modulates the lasing wavelength among the longitudinal multimodes determined by the LD cavity length. [DOI: 10.1380/ejssnt.2009.757]


I. INTRODUCTION
Near-field scanning optical microscope (NSOM) is adequate to evaluate the optical property of quantum structures (QS) on nanometer scale [1]: photoluminescence (PL) from semiconductor QS with near-field excitation was extended to an evaluation of the diffusion length of electrons and holes [2][3][4][5]; exciton migration within a QS was traced based on a temporal change of PL excited by a femtosecond laser [6]; near-field excited photocurrent was also reported [7].However these previous studies focused only on its high spatial resolution, neglecting possible differences between the far-field observation and the nearfield one.Here we present a study concerning a possible optical coupling between the NSOM probe aperture and a laser diode's (LD's) ideally linearly polarized field of TE 00 lasing mode composed of longitudinal multimodes at different wavelengths competing for the optical gain of stimulated emission.

II. MATERIALS AND METHODS
Commercially available laser diodes were placed on a custom copper NSOM holder with a sufficient heat capacity to compare among emissions under different driving currents, with a sealed can opened to let the NSOM probe accessible to the end facet of the LD chip cavity [Fig.1].To reduce the polarization selection due to a non-circular aperture, the probe aperture was opened at a sufficient diameter of 300 nm to ensure a circular shape on the Au coat film of the probe.Local optical field picked up by the probe was monitored by an avalanche photodiode (PD) with a preamplifier (Analogue Modules Inc, 713A-4).To reduce the background noise, the LD driving current was modulated by a 1 kHz rectangular wave and the preamplifier output that was in phase with the driving current was detected by a lockin amplifier (Stanford Research Systems, SR830 was recorded by a spectrometer attached with a charge coupled device (CCD) camera with onchip multiplication capability (Roper, SP-2358 + PhotonMAX 97EMB).To obtain the far-field spectrum, the LD output was coupled to a multimode optical fiber and the spectrum was measured with an optical spectrum analyzer (Advantest, Q8384).

A. Scattered near-field at the NSOM probe tip
A broad elliptic emission [Fig.2(a)] was observed by a NSOM on the cleaved facet of AlGaInP multiple quantum well (QW) LD, similar to the far-field observation of the lasing TE 00 mode by a microscope [8], just where the active lasing region was located and along the electrode ridge.The emission region was elongated along the QW layer (a little slanted clockwise in Fig. 2(a) after manual adjustment of the LD chip position relative to the NSOM scanner piezo).The LD chip is tinned on a metal substrate with junction-down, that is, the boundary is just 5 µm in the left-hand side from the emission center.In spite of the existence of this boundary and the anode electrode far in the right-hand side, the emission profile was symmetrical across the QW layers as shown in Fig. 2(b).There was a strong light scattering by the Aucoated probe tip: the intensity of this scattered light was monitored by an underlying PD [Fig.1].The scattered light intensity showed a submicrometer scale dependence on the probe position [Fig.2(c)], which was not due to the intensity profile of the far-field laser emission because the scattered intensity was changed by a small position difference of the probe aperture as little as 160 nm, less than a half wavelength (WL): when the open space between the LD chip and the underlying PD was covered with black paint so as to block the scattered light from coming in, accepting only the far-field direct laser beam emitted downward from the lower endface of the LD chip, the detected intensity became independent of the probe position as expected [Fig.2(d)].The similarity with the NSOM-detected field profile of Fig. 2(b) indicates that the source of this scattered light was the near-field component of the lasing field that extended out of the LD cavity: the probe tip scattered this near-field partly into the probe aperture, converting it to a propagating mode inside the probe fiber, and partly outside of the probe fiber.The former light propagating inside the probe fiber is generally regarded to be proportional to the NSOM-detected field.The latter scattered light was detected by the underlying PD in the present measurement.The spatial profile similarity of the two suggests that the far-field laser emission scattered by the probe tip was negligible to the near-field scattering, and hence, that the intensity of the near-field was much more than that of the far-field laser emission at the proximity of the LD chip surface.The near-field may also be augmented by the conductive subwavelength aperture of the probe [9] (field enhancement due to a steep curvature of the tip).

B. Local emission spectrum
While holding the NSOM probe at fixed lateral positions, labeled as "E" -"J" in Fig. 3(a), within the emission region, local emission spectrum was recorded [Figs.3(b) and 3(c)]: it was different depending on the lateral position of the aperture probe [8].Near the lasing threshold, the spectrum was a combination of longitudinal multimodes that corresponded to different resonant wavelength of the laser cavity.At the lasing threshold of driving current I LD = 25 mA [Fig.3(b)], the multimode envelope was similar independent of the aperture position.At I LD = 27 mA [Fig.3(c)], and when the aperture was at the emission center ("G" and "H"), however, the envelope maximum shifted toward the longer WL and was just at the same WL as that observed, in the absence of the aperture, by a fiber-coupled optical spectrum analyzer in the far-field.This shift may seem to be due to the heating of the LD chip surface by a light scattering at the probe tip, which is ruled out in the following discussion.
When the aperture was moved off the emission center by 160 nm [less than a quarter of wavelength, Figs.2(a) and 2(c)], the envelope maximum moved toward the shorter WL, further indicating the origin of this WL shift was the localized near-field.But this WL shift implies contradictory temperature decrease that is not realized by the heating of the LD chip: the strong near-field at the aperture may directly modulate the lasing, due to the optical coupling between the aperture and the LD cavity.This optical coupling model can explain the lasing WL modulation of the whole LD cavity field, even though the probe aperture was only at the top surface of the 300 µm length LD cavity.If the guide layer of the LD cavity, in which the lasing optical field is designed to be confined, has material inhomogeneity, a spatial shift of the optical field would result in an energy shift of the peak optical gain: the submicrometer aperture may force this spatial shift of the optical field, resulting in a different mode sehttp://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology lection among longitudinal multimodes resonant with the LD cavity.
To rule out the "heat-up" possibility, the LD was driven by a low duty cycle pulse current and the local emission spectrum change was traced: even when the LD was driven by I LD = 27 mA only for 5 µs and was hold at the lasing threshold (I LD = 25 mA) for 995 µs (at 1kHz current modulation), the envelope maximum was identical with that of Fig. 3(c), for each probe position "E" through "J".In contrast, a far-field measurement using an optical spectrum analyzer showed 0.15 nm/deg shift of the envelope maximum toward longer WL, in accordance with the general description of GaAs semiconductor property [10,11].
The WL of each of the multimode was identical irrespective of the probe position: in contrast in a far-field observation, the WL shifted to longer WL by 0.034 nm/deg when the LD substrate temperature was raised intentionally.These two observations further suggest that the emission WL was determined by the optical coupling between the LD cavity and the probe aperture, not due to the heat produced by the light scattering.Then even at the single mode operation of the LD, the lasing is expected to be tuned via this near-field optical coupling between the LD cavity and the probe aperture [12].

IV. CONCLUSION
An aperture NSOM probe showed a single broad elliptic profile of LD emission intensity, which was in agreement with the far-field observation of the lasing TE 00 mode, although the local emission spectrum, recorded with the aperture held at fixed lateral positions within this elliptic region, was different depending on the aperture position, when the LD driving current was above the lasing threshold.This observation suggests that, under the presence of aperture in the vicinity of the LD endface, lasing WL is modulated by the optical coupling in the near-field regime, implying a usefulness of the aperture probe as a novel tool to modulate the LD emission.

FIG. 2 :
FIG.2: Probe position dependence of optical fields.(a) Probe positions, each separated by 160 nm, at which the measurement was made was indicated across an elliptic region of laser emission (top view).(b) NSOM-detected optical field, (c) scattered light by the probe tip plus the direct incidence of laser emission, (d) the direct incidence of far-field laser emission to the PD.Probe position in (d) was estimated from the line profile of the NSOM-detected near-field after LD was covered with black paint and repositioned in the NSOM sample holder.Field intensity observed at the driving current ILD = 25 mA (lasing threshold), 26 mA, 27 mA and 0 mA, is marked by triangle, square, circle, and cross symbols in the graph, respectively.Scale bar in (a) represents 500 nm.

FIG. 3 :
FIG.3:(a) NSOM image of LD emission.White scale bar is 500 nm.(b, c) Local emission spectra observed at the driving current of threshold 25 mA and 27 mA above the threshold, respectively, when the NSOM probe was held at the designated positions, "E" through "J", as shown in (a).The longitudinal multimodes (indicated by equally spaced vertical lines) were barely resolved because the separation 0.2 nm is similar to the resolution of the highly sensitive spectrometer.